lessloss deliverable 012

8/10/2019 LESSLOSS Deliverable 012

1/107

Priority 1.1.6.3

Global Change andEcosystems

Project No.: GOCE-CT-2003-505488

LESSLOSS

Risk Mitigation for Earthquakes and LandslidesIntegrated Project

Sixth Framework Programme

Priority 1.1.6.3 Global Change and Ecosystems

Deliverable Report

Deliverable 12 Reports on methods of slope stabilisation

Sub-Project 3 Innovative approaches for landslide assessment and slope stabilization

Deliverable/Task Leader: GDS Revision: Final

September, 2005

Project co-funded by the European Commission within the Sixth Framework Programme (2002-2006)

Dissemination Level

PU Public X

PP Restricted to other programme participants (including the Commission Services)

RE Restricted to a group specified by the consortium (including the Commission Services)

CO Confidential, only for members of the consortium (including the Commission Services)


2/107


3/107

PREFACE

This report contains the contribution of GDS and AUTH to Deliverable D12 : Reports

on methods of slope stabilisation. This Deliverable is intended to present withconventional methods of slope stabilisation that have already been implemented inpractice, whether the method was successful or failed. A short description of eachmethod is given, followed by an outline of the design methodology; then cases historiesare described and assessed with respect to success or failure. The report contains twoparts. Part A (chapters 1 to 5) deals with methods for conventional problems: rainfall,earthquake.... Part B (chapters 6 to 9) deals with the response of piles against liquefactioninduced lateral spreading.


4/107


5/107

TABLE OF CONTENTS

PREFACE.......................................................................................................................................................i

TABLE OF CONTENTS..........................................................................................................................iii

LIST OF TABLES......................................................................................................................................vii

LIST OF FIGURES .................................................................................................................................... ix

LIST OF SYMBOLS AND ABBREVIATIONS .................................................................................xiii

1. INTRODUCTION.................................................................................................................................1

2. RESHAPING............................................................................................................................................3

2.1SOIL NAILED WALLS (CLOUTERRE,1991) .......................................................................................3

2.1.1 Description of the technique................................................................................................3

2.1.2 Field of application................................................................................................................5

2.1.3 Design method.......................................................................................................................6

2.1.3.1 Mechanism and behaviour.....................................................................................6

2.1.3.2 Conception and design .........................................................................................12

2.1.4 Case study .............................................................................................................................14

2.2MIXED STRUCTURES .........................................................................................................................16

2.3REINFORCED EARTH WALL .............................................................................................................18

2.3.1 Description of the technique..............................................................................................18

2.3.2 Field of application (Schlosser and Guilloux, 1982) .......................................................18


6/107

LESSLOSS Risk Mitigation for Earthquakes and Landslidesiv

2.3.3 Recommendations for design of Reinforced Earth in mountain areas ....................... 18

2.3.4 Case study............................................................................................................................. 21

3. INTERNAL REINFORCEMENT.................................................................................................... 25

3.1SOIL NAILING ................................................................................................................................... 25

3.1.1 description of the technique .............................................................................................. 25

3.1.2 Field of application ............................................................................................................. 26

3.1.3 Design method.................................................................................................................... 26

3.1.4 Case study (Guilloux, 1993)............................................................................................... 29

3.1.4.1 Old Railway embankment over the Paris-Lyon line : micropiles................... 29

3.1.4.2 Hambach embankment: sheetpiles .................................................................... 30

3.1.4.3 A41 Highway : steel sections............................................................................... 31

3.1.4.4 Bousst Saint Antoine railway embankment: bored concrete piles................. 33

3.1.4.5 Cut slope reinforcement by stabilising piles in Korea (Hong et al, 1997) .... 34

3.2SOIL DOWELLING ............................................................................................................................ 38

3.2.1 Description of the technique............................................................................................. 38


3.2.3 Design method.................................................................................................................... 39

3.2.4 Case study............................................................................................................................. 41

3.3GEOGRID REINFORCEMENT........................................................................................................... 41

3.3.1 Description of the technique............................................................................................. 41


3.3.3 Design method.................................................................................................................... 42

3.3.4 Case study............................................................................................................................. 42


7/107

Sub-Project 3 Innovative approaches for landslide assessment and slope stabilisation v

4. DRAINAGE ...........................................................................................................................................47

4.1DESCRIPTION OF THE TECHNIQUE (SEMBENELLI,1988) ...........................................................47

4.2FIELD OF APPLICATION ...................................................................................................................49

4.3CASE STUDY.......................................................................................................................................49

5. CONCLUSIONS....................................................................................................................................51

Soil nailed walls ................................................................................................................................51Reinforced Earth..............................................................................................................................52

Soil nailing.........................................................................................................................................52

Soil dowelling ...................................................................................................................................53

Drainage ............................................................................................................................................53

6. OVERVIEW: LIQUEFACTION AND LATERAL SPREADING.............................................55

7. RECORDED INSTANCES OF PILE FAILURES IN SOILS UNDER LATERAL

SPREADING..............................................................................................................................................59

7.111-STOREY R/CBUILDINGS -KOBE 1995 ....................................................................................59

7.2OIL TANK INMIKAGEHAMA ISLANDKOBE 1995...................................................................61

7.3R/CBRIDGE IN NEW ZEALAND -EDGECUMBE 1987................................................................63

7.4CONCLUSIONS ON SEISMIC RESPONSE OF PILES IN SOILS SUBJECTED TO EARTHQUAKE-INDUCED HORIZONTAL DISPLACEMENT .............................................................................................66

8. EXPERIMENTAL INVESTIGATION OF PILE RESPONSE IN SOILS SUBJECTED TOLATERAL SPREADING.........................................................................................................................67

9. PILE DESIGN METHODOLOGIES AGAINST LATERAL SPREADING..........................75

9.1DESIGN PARAMETERS......................................................................................................................75

9.2ESTIMATION OF PERMANENT GROUND DISPLACEMENT ...........................................................75

9.3DETERMINING P-Y CURVES FOR THE LIQUEFIABLE SOIL LAYER..............................................78

9.4JAPANESE DESIGN CODES (JRA1996)...........................................................................................82


8/107

LESSLOSS Risk Mitigation for Earthquakes and Landslidesvi

9.5LIMIT EQUILIBRIUM METHOD........................................................................................................ 84

9.6SEISMIC DEFORMATION METHOD (RAILWAY CODE,JAPAN) .................................................. 85

REFERENCES .......................................................................................................................................... 87


9/107

LIST OF TABLES

Table 9-1 Values of coefficient CLbased on depth and cyclic shear stress and stress-to-strength

ratio ........................................................................................................................................83


10/107


11/107

LIST OF FIGURES

Figure 2.1 Soil nailed wall. Construction phases.......................................................................................4

Figure 2.2 Progressive loading in tension of nail ......................................................................................7

Figure 2.3 Behaviour of a soil nailed wall: passive and active zones, maximum tension line .............8

Figure 2.4 Internal failure ...........................................................................................................................11

Figure 2.5 External failure ..........................................................................................................................11

Figure 2.6 Mixed failure ..............................................................................................................................12

Figure 2.7 Cross section of the proposed solution .................................................................................16

Figure 2.8 Soil nailed wall with prestressed anchors at the upper part ................................................16

Figure 2.9 Nailed Tervoile wall..................................................................................................................17

Figure 2.10 Nailed Berlin wall....................................................................................................................17

Figure 2.11 Geometrical alternatives for Reinforced Earth walls .........................................................19

Figure 2.12 Potential failure surfaces ........................................................................................................20

Figure 2.13 Design of profiles ...................................................................................................................22

Figure 2.14 Safety factors ...........................................................................................................................23

Figure 3.1 Behaviour of soil nailing for slope stabilisation ....................................................................27

Figure 3.2 Effect of soil nail orientation on slope stabilsation..............................................................27

Figure 3.3 Railway embankement in Paris-Lyon line. Soil nailing design............................................30

Figure 3.4 Railway embankment in Paris-Lyon line. Stabilisation Movements ..................................30

Figure 3.5 Hambach embankment............................................................................................................31


12/107

LESSLOSS Risk Mitigation for Earthquakes and Landslidesx

Figure 3.6 A41 Highway. Cross section................................................................................................... 32

Figure 3.7 A41 Highway. Slope movements and stability analysis....................................................... 33

Figure 3.8 Boussy Saint Antoinerailway embankment........................................................................... 34

Figure 3.9 Cut slope reinforced by stabilising piles. Design to control the landslide....................... 35

Figure 3.10 Cut slope reinforced by stabilising piles. Construction stages ......................................... 36

Figure 3.11 Cut slope reinforced by stabilising piles. Pile deflection................................................... 37

Figure 3.12 Cut slope reinforced by stabilising piles. Bending stresses in piles. ................................ 37

Figure 3.13 Cut slope reinforced by stabilising piles. Soil deformation .............................................. 38

Figure 3.14 Creeping slope with dowels.................................................................................................. 40

Figure 3.15 Dowelling at Dautenheim..................................................................................................... 41

Figure 3.16 Schematic plan view (a) and cross section (b).................................................................... 44

.Figure 4.1 Cross section of the slope. .................................................................................................... 50

Figure 6.1 Stages of critical pile loading during lateral spreading of soil (Berill & Yasuda, 2002). .. 56

Figure 6.2 Development of soil displacements in conjunction with soil-foundation-structureinteraction ............................................................................................................................. 56

Figure 7.1Typical failure modes of piles due to lateral spreading (Tokimatsu 1999) ........................ 59

Figure 7.2 (a) Location of the three buildings and spatial distribution of maximum grounddisplacements (b) Plan view and cross-section of the three buildings foundation(Sotetsu, 1996)...................................................................................................................... 60

Figure 7.3 Pile failure in building B (Tokimatsu 1997) .......................................................................... 61

Figure 7.4 Plan view and cross-section of the pile grid, with soil profile............................................ 62

Figure 7.5 View of the area near the tank and dimensions of the ground displacements in thevicinity (Ishihara et. al. 1997).............................................................................................. 62

Figure 7.6 Permanent lateral displacement of pile head in plan (above) and distribution of cracksalong piles (below). .............................................................................................................. 63

Figure 7.7 Sketch of foundation along Landing Road Bridge (Berill et. al. 2001) ............................. 64


13/107

Sub-Project 3 Innovative approaches for landslide assessment and slope stabilisation xi

Figure 7.8 Soil profile and SPT results along the bridge........................................................................65

Figure 7.9 Failure mechanism of a typical pier due to passive earth pressures by non-liquefiablesurface layer (J.B.Berill et. al. 2001). ...................................................................................65

Figure 8.1 Ground displacement profiles with depth at the end of the excitation for each of thetwo soil profiles (single- and double-layered). ..................................................................68

Figure 8.2 Bending moment profiles with depth for the two soil profiles (single- and double-layered)...................................................................................................................................69

Figure 8.3 Large scale shaking table model (Tsukamoto et. al. 1997). .................................................69

Figure 8.4 (a) Distribution of ground displacement with depth (b) Distribution of momentswith depth..............................................................................................................................70

Figure 8.5 Device used in the experiment and simultaneous plots of soil displacement and pileresponse plots derived (Abdoun and Dobry, 2003). .......................................................71

Figure 8.6 (a) Experimental model used to study the effects of non-liquefiable layers (b) Bendingmoment distribution along pile ..........................................................................................72

Figure 8.7 Layout used in the field test (N. Kamijo et. al. 2004). .........................................................72

Figure 8.8 Distribution of maximum acceleration within the soil, the pile and the superstructureaccording to the level of excitation: (a)0.02g (b) 0.14g (c) 0.58g....................................73

Figure 9.1 Correlation of range of influence of lateral spreading (L) with maximum observedground displacement (D0) (Tokimatsu et. al.,1997 and Shamoto and Hotta, 1996)....76

Figure 9.2 Illustration of the geometric parameters involved ...............................................................76

Figure 9.3 Detailed model of the loading of a pile group in which each pile sustains differentpermanent ground displacement. .......................................................................................78

Figure 9.4 Coefficient of subgrade reaction khand maximum reaction pressure pyo.........................79

Figure 9.5 Variation of scaling factors and with Nspt profile (AIJ 2001).....................................80

Figure 9.6 The effect of scaling factors and on pile deformation (Tokimatsu et. al. 1998)........80

Figure 9.7 Modification of the non-linear p-y curve for a liquefiable soil (Tokimatsu, 1998)..........81

Figure 9.8 Trilinear p-y curve model (Goh and ORourke, 2000). .......................................................81

Figure 9.9 (a) Bilinear p-y model (Meyersohn, 1994). (b) Calibration of suggested model.............82


14/107

LESSLOSS Risk Mitigation for Earthquakes and Landslidesxii

Figure 9.10 Distribution of earth pressures in the analytical model according to the kind of soilgenerating them (JRA 1996). .............................................................................................. 83

Figure 9.11 (a) Theoretical loading model - three-layer soil profile (b) Theoretical loading model- the free body diagram....................................................................................................... 84

Figure 9.12 Analytical model used in the Seismic Deformation Method (Railway Code, Japan1999) ...................................................................................................................................... 85

Figure 9.13 Diagram for estimating correcting factors and . ........................................................... 86


15/107

LIST OF SYMBOLS AND ABBREVIATIONS

k = Coefficient of subgrade reaction

E = Young's modulus

D = Diameter

L = Length

I = Bending inertia

p = Soil pressure

V = Shear force

I = Viscosity

= Creep velocity

= Slope angle

H = Slope height

= Soil unit weight

' = Soil friction angle

m = Mass

T = Period of vibration

g = Acceleration of gravity = 9.81 m/s2

x = Distance

y = Displacement

z = Depth


16/107


17/107

1. INTRODUCTION

Slopes either occur naturally or are engineered by humans. Slope stability problems havebeen faced throughout history when men or nature have disrupted the delicate balance of

natural soil slopes.

Slope stabilization methods involve specialized construction techniques that must beunderstood and modelled in realistic ways.

Different methods can be used in slope stability analyses: limit equilibrium methods suchas the ordinary method of slices, simplified Janbu method, simplified Bishop method,Spencers method, performed either under a total stress assumption, or an effective stressassumption, and the use of computer programs.

The analysis of slopes takes into account a variety of factors relating to topography,

geology, hydrogeology and material properties, often relating to whether the slope wasnaturally formed or engineered. Therefore, all this factors must be taken intoconsideration when choosing a stabilization method.

The aim of this bibliographic research is the description and analysis of those mostcommonly used in latest years. These methods can be classified into four large categories:

Reshaping

Internal reinforcement

External stabilization

Drainage

Slope reshaping consists in changing the slope geometry, by adding or removing weight,that is, by excavation at the top and/or backfilling at the toe to reduce the driving forcesand/or to increase the resisting ones. It can also involve in some cases the constructionof external stabilizing structures like berms or retaining walls.

Internal reinforcement consists in the introduction of some stabilizing forces in the slopeequilibrium equations; these stabilization forces are provided by inclusion of externalelements like anchors, nails, deep foundations, etc. or by modifying the soil properties


18/107

LESSLOSS Risk Mitigation for Earthquakes and Landslides2

(soil improvement). In soil improvement, the soil strengthening is achieved by theincorporation of various materials into the soil. These materials change the properties ofthe soil by chemical or mechanical strengthening. In mechanical strengthening, the mostcommon reinforcement materials are synthetic polymers (geogrids).

Examples of external stabilization methods are buttressing, surface slope protection orvegetation.

In most cases, the use of one of those methods individually is not sufficient to provide

the required stabilization result; therefore, it may be necessary to combine several of themas it will be illustrated by some case studies included in this document.


19/107

2. RESHAPING

2.1

SOIL NAILED WALLS (CLOUTERRE,1991)

2.1.1

Description of the technique

Soil nailed walls are primarily used during excavation to provide slope support to limit theslope angle. Construction of a soil nailed wall consists in reinforcing the soil as workprogresses in the area being excavated by the introduction of passive bars (nails), whichessentially work in tension. These bars are usually parallel to one another and slightlyinclined downward. They can also work partially in bending and by shear. The skinfriction between the soil and the nails puts the latter in tension.

The work progresses from the top downward. In order to keep the soil from caving inbetween the bars, some sort of facing needs to be installed. This is generally made withsome shotcrete reinforced by a welded wire mesh. This facing can be vertical, battered toa wide variety of angles, or made up of a series of benches.

A soil nailed wall is difficult to build under a water table. If the soil nailed wall has to benecessarily built under a water table, special procedures will need to be introduced, suchas pumping operations to lower the groundwater levels, drainage, etc.

Soil nailed wall and Reinforced Earth walls are very similar. However, several differencescan be observed. A soil nailed wall is built downward with the soil being reinforced insitu, and a Reinforced Earth wall is constructed by building an embankment that is then

strengthened as the work progresses. In addition, unlike the Reinforced Earth technique,the construction of a soil nailed wall involves a critical phase with respect to local oroverall stability. The latter can be more critical during the construction phase than whenthe wall is completed. Local excavation stability during the earthwork phase dependsdirectly on the height of excavated soil.

Soil nailed walls are constructed in successive phases from the top to the bottom:


20/107


Excavation, generally limited to 1 or 2 meters deep and possibly limited in lengthdepending on the type of ground being stabilized. During this phase the soil must remainstable, which calls for some degree of short-term cohesion in the soil, although notnecessarily very high.

Introduction of subhorizontal or inclined nails into the in situ soil.

Construction of a facing wall on site (shotcrete over a welded wire mesh or fibrousconcrete) or installation of precast elements (or panels) that can be architecturally treated

in various ways.

If the soil cohesion is low, it is possible to carry out the excavation in slots, and phases 2and 3 can be carried out in that order or they can be reversed, that is, the shotcrete can beapplied before the nails are introduced.

Figure 2.1 Soil nailed wall. Construction phases.

The most commonly used techniques for installing the nails are percussion orvibrationof the nail into the soil, or grouting the nails into a hole that has been made in advance,

usually by drilling.

i) Driving (mainly percussion) is particularly suitable for soft grounds containing nohard obstacles or too many large blocks, and for lighter nails of average length, notexceeding eight or so meters. For reasons linked to their installation, these nails musthave some stiffness; they are therefore made from bars having a fairly high mechanicalefficiency (steel angle, metallic tubes, etc.)


21/107

Sub-Project 3 Innovative approaches for landslide assessment and slope stabilisation 5

Special care must be taken when driven reinforcement bars are used in either medium orlong-term structures in aggressive soils, in order to protect them against corrosion.

ii) As regards the drillingprocess, it is possible to use nails of any length in practicallyany type of soil. It is the only technique possible for very long nails and in soils wherethey cannot be driven.

Other techniques can be used, by combining vibration driving with grouting processes.

The choice of the technique depends on economic criteria and other technical factors,particularly on the type of nail and on any potential difficulty in using that nail (forexample, stability of the borehole walls), on the efficiency of the nail in certain types ofterrain, and on the height of the excavation phases which can be made.

The nails are generally made of steel, although other materials have been used, inparticular glassfibers.

Regarding both short and long-term structures, particular attention should be paid at alltimes to the durability of any nail used in corrosive soils and to long-term movements,particularly those caused by creep in clays.

In the case of reinforced shotcrete, the facing wall is constructed to a calculated thicknessthat depends, mainly, on the grid layout of the nails. However, the actual volume ofshotcrete is often higher, because of over excavation of the planned cross-section used.

Weepholes must always be provided through the facing so that any water infiltrating thestructure can drain away. In areas subject to internal hydraulic flows of water, it isappropriate to install drainage measures, such as subhorizontal drains or drainage details(for example, geocomposites installed before the facing wall is constructed).

2.1.2

Field of application

The most common nailed wall applications are the following:

Temporary and permanent walls for building excavations.

Cut slope retention for roadway widening and depressed roadways.

Bridge abutments: addition of traffic lanes by removing end slopes from in front ofexisting bridge abutments.

Slope stabilization.

Repair or reconstruction of existing structures.


22/107


As soil nailed walls are difficult to build under a water table, their use is limited to soilsthat have no water table or that are protected by a reduction in the water table level.

The majority of soil nailing studies has been limited to homogeneous soils, but thetechnique can as well apply to heterogeneous soils insofar as the density of nails can beadapted to the type and the resistance of soil. Grouted nails can pass easily throughlocally heterogeneous soils with occasional boulders; it is also possible, if space allows, tolocally modify the orientation, length or density of the nails.

The following types of ground are considered favourable to soil nailing:

Naturally cohesive materials: silts and low plasticity clays that are not prone to creep.

Naturally cemented or dense sands and gravels with some real cohesion (due to fines)or apparent cohesion (due to natural moisture).

Weathered rock.

On the other hand, soil nailing technique is generally not successful in the following soilconditions:

Sands which have no apparent cohesion and where the stability of the excavation

cannot be guaranteed. Very plastic, clayey soils and very sensitive soils, particularly where there is a relatively

low unit skin friction value between the soil and the inclusion.

In swelling clays or soils that are frost susceptible, since in these cases, high forcescould develop in the nails and the facing.

Aggressive soils with respect to the nails and facing materials, particularly in long-term structures.

2.1.3

Design method

2.1.3.1 Mechanism and behaviour

Principal of structural behavior

The construction method has an important influence on the distribution of displacementsand deformations, as well as on stresses in the soil and reinforcements. During successiveexcavations, the soil is subjected simultaneously to lateral decompression and tosettlement. As a result, at the end of construction a slight tilting of the facing occurs

where horizontal and vertical displacements are at their maximum at the wall top.


23/107


As regards the loading in tension of the reinforcements, the horizontal decompression ofthe mass during the successive excavations results in preferential tensile stresses in thesubhorizontal nails. Tension in a row of nails starts only when the lower levels are beingexcavated. In addition to this, the rate of increase in tension in a nail due to a certainexcavation decreases as the excavation proceeds. More precisely, the tension of a nail idepends mainly on the three following excavations phases, i+1, i+2 and i+3. The result isthat the lowest rows of nails are the least subjected to tensile stresses at the end of anexcavation phase. However, at the end of construction, progressive tension of these rowsdevelops due to long-term deformations.

Figure 2.2 Progressive loading in tension of nail

The maximum tensions in the nails are inside the soil nailed mass, not at the facing. Thisis a characteristic of reinforcement techniques in which the interaction with the soil iscontinuous along the whole length of the reinforcements. The geometric location of thepoints of maximum tension Tmax makes it possible to separate the soil mass into twozones:

Active zone, behind the facing. In this zone the skin friction stresses applied by the soilon the nails are directed outward.

Passive zone, where skin friction stresses are directed inward and oppose the lateraldisplacement of the active zone.

Because of the large number of parameters coming into play, it is not very easy todetermine the location of Tmax. Generally, the shape and the position of the line of


24/107


maximum tension, which can be considered as a potential failure surface, are verydifferent from the Coulomb straight line.

Figure 2.3 Behaviour of a soil nailed wall: passive and active zones, maximum tension line

Nails often have some stiffness that enables them to work not only in tension but also inbending and shear. However, nails working at service loads in a structure are, in practice,not subjected to bending nor shear, except sometimes locally, near the facing. The facingmay sometimes hang on the nails during the first phases of construction when it is verythick or when the short-term adherence of the ground is small.

There are basically two techniques of soil nailing:

Hurpin's method, which uses low-capacity nails of short length and fairly close toeach other (Sv, Sh 1 m). Nails are usually driven.

Grouting bars into the soil, generally of high capacity, of longer length and morewidely spaced (Sv, Sh > 1m).

The overall behaviour in the second case is considered to be similar to the first one aslong as SvSh 6 m2.

The two main differences between both methods are the stresses taken up by the facing

and the forces and bending moment mobilized in the nails. In the first method, thestresses taken up by the facing are much lower, so the facing can be thinner, and neithershear force nor bending moments are mobilized in the nails because of their smallmoments of inertia.

Soil-nail interaction

Two types of interaction develop in nailing used in retaining structures:


25/107


Shear stress (skin friction) applied by the soil along the nail, which induces tension inthe nails. That is the most important interaction.

Passive pressure of the earth along the nail during the displacement of the latter. Thispressure makes possible the bending moment and shear force to be mobilized in thenails.

In this case, nails deformations are calculated like piles subjected to a lateral load by usingthe simplified method of the coefficient of subgrade reaction, which leads to theequation:

4

40

s

d yEI k D y

dz+ = (0.1)

where:

ks: coefficient of subgrade reaction,

y: lateral displacement of the nail,

z: coordinate along the nail,

p=ksy: lateral pressure on the nail,

D: nail diameter.

Types of failure of soil nailed walls

A. Internal failure

A.1. Failure by breakage of the nails

The failure surface that develops in the soil is very close to the line of maximum tension,which can, therefore, be considered as a potential failure surface. The bending resistanceof the nails prevents the development of a clear-cut failure surface, as it allows greaterdeformations before failure. With flexible nails, failure is sudden and without warning.

This type of failure can occur in the following cases:

Underdesign of the cross section of nails.

Corrosion of the steel bars in the nails.


26/107


Surcharge on top of the wall, if the wall has not been designed to resist it.

Saturation of the wall under the effects of water infiltrations (rain or thaw).

Ice lenses in frost-susceptible soils. This phenomenon induces tension in the nailsnext to the facing when the frost front forms within the soil. When the facing is veryresistant, nails may break in tension and the wall may fail with the facing beingdisconnected.

A.2. Failure by lack of adherence

This type of failure is more frequent than the previous one and results from a poorestimation of the unit skin friction of the nails and/or construction mistakes.

This failure is due to the insufficient length of the nails in the passive zone to balance themaximum tensions. The nails are then pulled out of the soil. The failure is not usuallysudden and large deformations develop.

It can occur in fine-grained soils under the effect of saturation or increase in moisturecontent, or during construction, if the length of the nails at the head of the wall isinsufficient.

A.3. Failure during excavation phases

A.3.1. Failure due to excessive height of continuous excavation

In this case, a fairly sudden failure can occur through local instability and propagation tothe top of the wall. The soil flows behind the facing due to successive elimination of thearch effects. The facing drops as one block until stopped by the foundation soil, and thenails deform through bending but may not break.

This type of failure is more frequent than the previous ones. To prevent it, the excavationheight must be kept lower than the critical height or excavation in slots must be used.

A.3.2. Failure by piping of the soil

The cause of this type of failure is the existence of a pocket of water in the soil due to benailed. During excavation, pore water pressure in this pocket and the resulting water flowforces, destabilizes the soil locally in the zone being excavated. Rapid and regressivefailures cause the soil to flow behind the facing. A sudden subsidence of the facing occursthat can have repercussions on both sides of the pocket of water.


27/107


This type of failure is frequent and results either from the heterogeneity of the soil orfrom the lack of drainage during construction.

Figure 2.4 Internal failure

B. External failure

External failure occurs generally by sliding along a failure surface, affecting the wholestructure and going through the foundations. The wall behaves like a monolithic block.

It is due to either poor quality foundation soils or to insufficient length of the nails.

Figure 2.5 External failure

C. Mixed failure

This type of failure combines both internal and external instability; the failure surface isboth in the wall and outside the wall.

It is generally due to nails being of insufficient length, associated with a defect in strengthof the nails or in the unit skin friction.


28/107


Figure 2.6 Mixed failure

2.1.3.2 Conception and design

The design of soil nailed walls is limited to ultimate limit state (ULS) stability calculations.

2.1.3.2.1. Analysis of stability

Stability of a soil nailed wall can be analyzed either by calculating the deformations or byusing limit equilibrium analysis.

A. Calculations of deformations

This type of calculation is usually done by using the finite element method, but thismethod is not used in practice to study the stability and design of soil nailed structures.

B. Limit equilibrium methods

These methods examine the equilibrium of a volume of soil at failure by taking intoaccount the strength of the materials used. That is, they analyze the internal and externalstability of the structure by verifying the static equilibrium of a part of the system limitedby a potential failure surface. Stability is defined in relation to the most critical potential

failure surface.

There are two types of methods:

Classical limit equilibrium methods. These consist in verifying the equilibrium of apart of the soil mass limited by a potential failure surface and subjected to externalforces and to stresses or forces mobilized respectively in the soil and nails. The latterare determined based on the failure criteria of the used materials making severalassumptions. Fellenius and Bishop methods of slices or the perturbation method


29/107


(Raulin et al, 1974) are examples. With these methods, stability along internal, mixed,and external failure surfaces can be analyzed.

Methods based on limit analysis and more recently yield design theory (Salenon,1983). These methods study the static equilibrium of one part of the system limitedby a potential failure surface. Like in classical methods, this part is subjected to theeffects of external forces and to the resistance stresses that can be mobilized in thesoil and in the nails along the potential failure surface, according to the failure criteriaof the materials involved. These methods, however, are mechanically more rigorousthan classical limit equilibrium methods because they do not require any additional

assumption, besides the strength of the materials.

In both types of limit equilibrium methods, the effects of soil nailing soil are taken intoaccount in the form of vector forces applied to the points where the nails intersect withthe potential failure surface. The forces in the nails are determined considering the

various modes of failure of the soil nailed structure and the corresponding failure criteriafor the soil and nail materials, as well as the soil-nail interaction.

The practical validity of these methods assumes the possibility of simultaneouslymobilizing the limit states of the soil and the various nail rows. This implies the straincompatibility at failure of both the nails and the soil, the ductility of the nails, and gradual

yielding of the soil.

In addition, these methods assume that the displacements and deformations will be smallenough not to have to take account of any geometric changes in the structure prior tofailure. In the case of ductile nails, their reorientation during movement along the failuresurface is beneficial to the structure stability.

The methods of homogenization (De Buhan and Salenon, 1987) can also be mentioned.This principle consists of replacing the heterogeneous medium made by the soil and thenails (assumed to be regularly distributed) by a homogeneous medium that is equivalentfrom the point of view of limit loading at a macroscopic scale. This approach, which canbe applied to more general problems, has proved to be very difficult to use for usual

practical applications.

C. Limit equilibrium methods with relative displacements

These are based on classical limit equilibrium methods and consider that forces in thenails are dependent on the relative displacements along the potential failure surface.

This approach was initially developed for studying the stabilization of unstable slopesusing soil nailing (Delmas et al, 1986). It requires assumptions on the behaviour in order


30/107


to link together the forces mobilized in the nails and the displacements along thepotential failure surfaces. This method examines stability conditions in terms of bothdisplacements and the strength criteria of the materials. These displacements are assumedto be concentrated along and around the potential failure surface, so it seems unsuitablefor retaining structures since there is no pre-existing failure surface, but it is suitable inthe case of natural unstable slopes.

Limit equilibrium methods using potential failure surfaces are the approachrecommended for designing and justifying soil nailed structures (Clouterre, 1991).

Calculations relating to the stability of the finished structure cannot be separated fromthose linked to the various construction phases, some of which can be the most criticalones. In fact, stability calculations must be ongoing from the very first excavation phaseto the completed structure.

2.1.3.2.2. Some design methods

In France, the first specific method for designing soil nailed walls appeared in 1980 withthe initial TALREN software package (Blondeau et al, 1984). This package, thanks to theperturbation method, allows any failure surface to be taken into account, and it is alsopossible to work in heterogeneous soils with or without the presence of water.

Other two programs were developed in France later: PROSPER and STAR programs.The PROSPER program takes displacements into account, but it is more suitable fordealing with displacements in unstable slopes reinforced by soil nailing than for retaining

walls, since displacements are assumed to be concentrated along and around the potentialfailure surface. The STAR program uses logarithmic spirals as potential failures surfacesand it is based on the yield design theory.

As regards the international context, several methods have been developed, byconsidering different failure surfaces, such as bilinear failure surfaces (German method,Stocker et al., 1979) or vertical axis parabolas (Shen method, USA, 1978). The Juran

design method (1990) uses logarithmic spiral as failure surfaces, (Clouterre, 1991).

2.1.4

Case study

Landslide stabilization using soil nail and mechanically stabilized: earth walls (Turner andJensen , 2005).

U.S. Highway 26-89 through Snake River Canyon in northwest Wyoming is a two-laneroad constructed in 1947. The 39 km section through the canyon has been rebuilt to


31/107


current design standards. One of the narrowest portions of the route is a sectionapproximately 300 m long near a point where the Snake River makes a nearly 90 bend,locally termed the Elbow.

Prior to construction in 1999, the existing roadway through the Elbow section consistedof two 3.4 m wide travel lanes with virtually no shoulders. The new roadway incorporatestwo 3.7 m travel lanes and 2.4 m wide shoulders. To widen the right-of-way, significantamounts of cut and fill would be required, but site conditions limited the amount of cutand fill that was possible: 20 m high unstable fill slope with a resulting landslide, which

lies adjacent to a slowly creeping large landslide which extends approximately 450 mabove the roadway.

The final solution was to move the roadway as far towards the river as possible, to avoidall cutting through this area and to create a toe berm to resist movement of the slide. Thisproposal required installation of some type of retaining structure to provide the additionalroadway width.

The proposed design incorporates two soil nailed walls (tiered) and a mechanicallystabilized earth wall (MSE). A lower soil nailed wall varying in height from 1.8 to 3.0 m

with 12.2 m long soil nails would reinforce the existing embankment and provide asuitable foundation for the mechanically stabilized earth wall. An upper soil nail wall

varying in height from 4.0 to 7.6 m with soil nails extending 10 m into the slope wouldprovide support for the existing roadway during and after construction of the MSE wall.

The MSE wall would consist of reinforced backfill and modular block facing and wouldprovide the additional width needed for the new roadway. The final configuration resultsin the roadway being supported partly by the upper soil nail wall and partly by the MSE

wall and with soil nails crossing the failure plane of the active slide.

Slope inclinometer readings taken during and after construction show that the lower soilnail wall has been effective in controlling ground movements along the Elbow fill slide.Deformations in the MSE wall are consistent with expected lateral deformations for thistype of flexible structure and occurred mostly during the first six months after

construction. Deformations over the last six months of monitoring show only minormovement which appears to be diminishing over time.

This project demonstrates the feasibility of utilizing soil nailed walls for stabilization ofactive landslides, extending the application of soil nailing beyond its traditional scope ofstabilization of cut slopes or for potentially unstable slopes.


32/107


Figure 2.7 Cross section of the proposed solution

2.2 MIXED STRUCTURES

These are retaining structures in which the reinforcement of in situ soil combines thenailing technique and other retaining methods (prestressed anchors, bracing system).

In general, the aim of a mixed structure is to limit the lateral displacements of the

structure or to prevent instability problems from developing (blocking the displacementsat the top of the very high wall). Mixed structures are used as well to obtain higherexcavation phases or when confronted with problems of instability due to flow or water.

Soil nailed wall with a row of prestressed anchors at the upper part. It is importantfor the grouted anchorage zone of the prestressed ground anchors to be separatedfrom the soil nailed wall and placed behind the latter.

Figure 2.8 Soil nailed wall with prestressed anchors at the upper part


33/107


Nailed Tervoile. The wall is built in successive excavations, placing the nails and theactive ground anchors as work progresses. The facing is placed as excavation progressesand comprises lengths of prefabricates posts, assembled as the work progresses.

Figure 2.9 Nailed Tervoile wall

Nailed Berlin wall. The installation of posts is made before excavation. Nailing makes itpossible to increase the distance between the posts by reducing the moments in the

facing and the stresses in the posts.

Figure 2.10 Nailed Berlin wall


34/107


2.3 REINFORCED EARTH WALL

2.3.1


The Reinforced Earth was invented in France by Henry Vidal in the early sixties, and thefirst full scale wall was constructed in the Pyrnes Mountains in 1965. A cohesivematerial of great strength and stability is formed by the association of granular soil andreinforcements. Through friction, the mechanical properties of soil were improved byreinforcement with steel strips placed in the fill during construction from base to top (from nailed wall).

The main property which makes the use of Reinforced Earth successful for manyhighways on unstable slopes is its deformability. These structures are often designed

when the slope is in a state of limiting stability, since when movements of the slopes canbe expected it is not possible to design rigid structures.

The main factors involved in the design and construction of Reinforced Earth structuresin mountain areas are:

the geometry of the wall,

the geotechnical data,

the short-term stability of the excavations,

the long-term stability of the slope.

2.3.2 Field of application (Schlosser and Guilloux, 1982)

Reinforced Earth walls are successfully used in highways on unstable slopes, especiallywhen the safety factor of the natural slope is critical.

In general, it is used in both mountain and urban highways projects for earth-retainingstructures and bridge abutments.

2.3.3

Recommendations for design of Reinforced Earth in mountain areas

In general, there are two conflicting aims to be pursued when designing a ReinforcedEarth wall on a slope:

Minimization of the volume of excavations to ensure their short-term stability.

Improvement of general stability.


35/107


The first one can be obtained by reducing the embedment depth and the length of strips.However, the failure surface will be deeper and less critical if the wall has deepembedment and long strips. It is necessary, therefore, to find the best compromise in theearly phase of the project.

Usually, Reinforced Earth walls have a rectangular shape, with embedment equal to onetenth of the height and with strip length equal to 0.7-0.8 times the height.

There are other geometrical alternatives for the wall:

On a slope it is possible to design a wall with shorter strips in the lower part than inthe upper part. In this case, the minimum length of the strips must be equal to 0.4times the wall height and the difference between the length of two adjacent layers ofstrips must be limited to 1 or 2 m. In this way, the size of excavations can be limited.

It is possible also to design a retaining structure made of two or more superposedwalls. This is an interesting solution for a highway on two levels, but it can be usedfor any retaining structure with a reduction in the volume of excavation.

Figure 2.11 Geometrical alternatives for Reinforced Earth walls

It is important to consider the general stability when choosing the solution, as it variesconsiderably in the different cases.


36/107


Long-term stability

It is necessary to go through a complete stability analysis of the slope with the retainingstructure. This analysis is done with classical methods, but certain aspects regarding thefailure surfaces passing through the Reinforced Earth mass must be considered. Failuresurfaces cutting some layers of strips have generally a higher safety factor than the onespassing outside the Reinforced Earth mass.

As regards failure surfaces passing through Reinforced Earth, computer calculations with

methods, such as Bishops method of slices, can be used by adding to the shear strengthalong the failure surface a contribution due to the force in the strips (either failureresistance or limiting adherence). This method enables stability estimation for structuresof complex geometry, especially those made of several walls, for which it may be difficultto judge whether a potential failure surface is a critical one or not.

With the deepening of the potential failure surfaces caused by the strips, the long termsafety factor of the slope after construction of the wall is frequently equal or even largerthan the one of natural slope.

In the case of water circulations, it is necessary to use a draining material for theReinforced Earth structure or to design drainage behind and under the wall, with drainagedevices for the water.

Figure 2.12 Potential failure surfaces

Short-term stability of the excavations

Short-term stability is the main problem when the slope is in a state of limitingequilibrium and when there are water circulations in the surface layers.


37/107


If the short-term stability is only required for some days or weeks, the Reinforced Earthstructure can be built by plots, so that the excavations have a length approximately equalto twice their height. This construction method allows a good mobilization of the archingeffect.

When this solution is not sufficient, the stabilization of the excavations is necessary. Thatcan be assured by different methods:

Grouting, if the nature of the soil allows it.

Nailing or root-piling. Anchored structures, such as beams, along the slope as the excavation progresses.

These solutions must be avoided when possible, as they are quite expensive.

2.3.4

Case study

Access road to the Frejus tunnel (Schlosser and Guilloux 1982)

On the French side of the Franco-Italian Frejus tunnel, the access road rises for 4.5 kmalong the slopes of the Arc valley. The platform is 13.50 m wide, and the natural slope is

very steep, up to 42.

The subsoil consists of rock (Quartzites) and shale, but almost everywhere covered withheterogeneous and thick layers of debris: gravels and blocks in a clayey-sandy matrix.

Their thickness may reach several dozen of meters and many localized water circulationshave been observed.

The slope was in a state of limiting equilibrium, so it was impossible to get undisturbedsamples of the soils. That is why the shearing strength was calculated a posteriori byassuming a safety factor of 1 for these landslides.

A careful monitoring of the displacements was implemented, due to the natural instability

of some slopes and to the difficulty of applying classical methods for the analysis. It wasestimated, by means of topographical measures and the implementation of inclinometers,that surface displacements rates could be as much as 10 cm per year.

The choice of the stabilizing structure was governed by two main criteria:

The slope being in a state of limiting equilibrium, it was essential to avoid anyadditional disturbance to its stability during construction and final state. This wasachieved by a mixed cut-embankment profile.


38/107


The slope being subjected to possible displacements, the structure had to be quitedeformable.

Reinforced Earth was chosen for the lower embankment wall. In order to minimize theheight of excavations, crib-walls or anchor-walls were chosen for the upper wall,according to its height. These choices have a tendency to limit the volume of excavations,because of the difficulty of ensuring their short term stability. That is why strips length inthe lower part of Reinforced Earth walls was lower than current length.

The internal stability of the Reinforced Earth walls was analysed with the classicalmethods for Reinforced Earth.

External stability investigations were carried out with regard to the bearing capacity of thewalls foundations, sliding on the base and general sliding along a slip surface passingoutside the wall.

Figure 2.13 Design of profiles

The safety factor for a failure surface that cuts one or two layers of strips is practicallynever smaller than the safety factor for a surface outside the reinforced mass.

The analysis was made in terms of relative variation of the safety factor with respect tothe safety factor in the natural state, and all the profiles were found to have a long termstability factor practically equal or higher than the safety factor of the natural slope.


39/107


Figure 2.14 Safety factors

However, the short term stability of the required excavations was more critical. Differentsolutions were proposed: previous grouting of the slope, stabilization by root-piles or byprestressed anchors connected to concrete beams. The chosen solution was the last one.


40/107


41/107

3. INTERNAL REINFORCEMENT

3.1

SOIL NAILING

3.1.1

description of the technique

Soil nailing is a technique used to reinforce and to strengthen existing ground. It consistsin the transmission, by means of resistant elements (piles, steel bars, etc.), of a part of theshear forces created by the sliding mass to the substratum.

Nails can withstand normal forces both in tension and in compression, shear stresses andbending moments. Prestressing is not necessary; forces develop only due to the slopemovements.

There are different kinds of nails, depending on the site conditions. The most commonlyused are:

Micropiles. These are steel bars or tubes of small diameter (40-150 mm) with lowbending stiffness. They can be placed in any direction and, as a consequence of theirlow inertia, the density of nail per m2is high (usually one nail per 4 m2).

Sheetpiles or H sections. Their dimensions are from 0.1 to 0.30 m (maximumdimension in section) and their inertia is larger than that of micropiles. They areusually driven vertically and placed in one or few lines.

Large diameter piles or diaphragm walls. Their diameter or width is from 0.5 to 3 mand they have a very high bending stiffness. They are placed vertically in one or twolines. Their behaviour and design methods are very similar to those of soil nailing butin fact they cannot be considered strictly as soil nailing.

Nails of low inertia, which essentially work in tension, are usually placed subhorizontally,and nails which can work in shear are generally placed vertically or perpendicularly to theslip surface.


42/107


3.1.2 Field of application

Soil nailing is generally used for slope stabilization when other methods, such as changingthe geometry of the slope or drainage, are inapplicable, mainly because of accessconditions. This is due to the reduced equipment and materials needed for soil nailing.

That is the case of natural or excavated slopes near constructions (buildings, highways,railways).

This technique is frequently used with two different aims, according to the configuration:

Slopes where a slide already occurred, in order to stop the slide or to reduce thesliding rate.

Potentially unstable slopes, both natural and cut slopes, in order to increase theirsafety factor.

3.1.3 Design method

When analysing the general behaviour of a nail crossing a shear surface in a soil, becauseof the stressespexerted by the soil on the nail above the slip surface (and by the nail onthe soil below it), the nail suffers shearing and tensile deformations. These deformationsinduce forces at the point where the nail crosses the slip surface, and these forces must be

taken into account in the equilibrium equations of the sliding mass. These are thefollowing:

tensile, or sometimes compressive, force N,

shearing force V,

bending moment M.

The effect of these forces, when considering the reinforced soil mass as a whole, is toincrease the shear strength, mainly the cohesion (Juran et al, 1983).

There are two important points to consider when designing slope stabilization by nailing:

Nails would be preferably placed with a small angle with respect to the normal to theslip surface, preferably 30, particularly for rigid nails. That is, it is better to place thenails so that they mobilize mainly tensile force. Nevertheless, this will not be alwayspossible, because of the installation procedure, for instance in driven or bored piles,

which are commonly vertical, at least in France.

Significant displacements are required to reach the peak strength, mainly with flexiblenails, so maximum stabilization will be effective only after some slope movementshave taken place after nails placing.


43/107


Figure 3.1 Behaviour of soil nailing for slope stabilisation

Figure 3.2 Effect of soil nail orientation on slope stabilsation

The first projects of slope stabilisation by soil nailing were designed with the assumptionof limit equilibrium state of soil around the nails (pressure distribution of Brinch-Hansen,


44/107


1961 (Brinch-Hansen, 1961), but this approach proved inadequate for modelling the soil-nail interaction (Carter, 1986).

In the stability analysis it is necessary to assess the relative displacements between the nailand the soil surrounding it, in order to evaluate the mobilized forces in the nails andtherefore the stabilizing forces in the sliding soil mass.

Ideally, the design is done in two steps:

1) Determine, for each one of the inclusions, the forces mobilized along the slip surfacewith respect to soil displacements. In France this is done by using the method proposedby Baguelin et al (Baguelin et al, 1976) for piles subject to horizontal soil movements, byimproving the Winkler beam model and solving the differential equation:

[ ]4

4

d yp( z ) EI kD y( z ) g( z )

dz= = (3.1)

where:

p (z): stress between the soil and the nail

EI: rigidity of the nail

k: modulus of subgrade reaction

D:width of the nail

y (z):horizontal displacement of the nail

g (z): soil displacement without nail

g (z) is known when inclinometers are placed in the slope. In other case, its shape mustbe assumed, this assumption being very important.

The forces N, V and M at the cross-point between nail and slip surface can be known bysolving this equation.

The calculation is done for increasing values of g (z), and a criterion must be chosen tostop the calculation. This criterion may depend on the project:

yielding of the soil along the nail, either at one point or along a given length,

yielding of the nail material, or

maximum value of soil displacement g (z) or of pile y (z).


45/107


2) Introduce these forces in a classical slope stability analysis method, such as Felleniusor Bishop, in which external forces are added to the static equilibrium equations

In the case of rigid nails, it is easy to use this method, but in the case of numerousmicropiles type nails, the method is not so straightforward. This problem is solved withthe development of the TALREN computer program (Blondeau et al, 1984), whichintegrates directly the two steps with some assumptions, such as the symmetry of the naildeformation. The PROSPER program (Delmas et al, 1986) is based on the same concept(it takes into account the displacements in the soil mass as well), but soil-nail interaction

modelling is more detailed.

The STAR software package (Anthoine, 1990), which uses logarithmic spirals as potentialfailure surfaces and which is based on the yield design theory is also worth mentioning.By using this theory and homogeneous soil, the logarithmic spiral results in a very simplecalculation and gives a good estimation of the structures safety compared to any othertype of potential failure surfaces.

These programs can be used as well for the design of soil nailed walls (cf. 2.1.3.2.2.).

3.1.4

Case study (Guilloux, 1993)

3.1.4.1 Old Railway embankment over the Paris-Lyon line : micropiles

A railway embankment over the Paris-Lyon line, more than one century old, was wideneda few decades ago, by a new embankment made of plastic clays and marls, the whole lyingover clayey soils. A slide occurred including the new fill and the foundation soilsdownstream.

It was reinforced by placing 50 mm diameter steel tubes including a 16 mm diameter barinside, grouted in 150 mm boreholes, at a 23 m mesh.

Soil characteristics were determined by back analysis (safety factor SF=1) and the nailing

design was done in order to get a 30% increase of this safety factor.

The movements measured in the inclinometer showed that, after a rapid increase duringthe works due to the effect of the equipment traffic on the slope, the stabilization waseffective although some slow movements were carrying on some months after the works.


46/107


Figure 3.3 Railway embankement in Paris-Lyon line. Soil nailing design

Figure 3.4 Railway embankment in Paris-Lyon line. Stabilisation Movements

3.1.4.2 Hambach embankment: sheetpiles

The highway marly embankment 12 m high proved to be unstable two years after itsconstruction (apparition of cracks on the pavement and superficial slides on the slope).


47/107


Inclinometers revealed actual movements over 3 m depth and creep movements down to6 m.

Shear strength of the soil fill was back analysed, and it was decided to improve theembankment slope with sheetpiles SL3 placed at mid-slope on two lines, with 0.65 mspacing in each line. They were placed in a discontinuous way in order to avoid a barriereffect for the water seepage inside the embankment.

The stabilization was monitored with inclinometers, and the sheetpiles deformations

measured, when the pavement movements stopped, proved to correspond to an increasein the safety factor of 13%.

Figure 3.5 Hambach embankment

3.1.4.3 A41 Highway : steel sections

For the construction of the A41 highway in the Alps, a small cut about 4 m high had tobe done 85 m downstream a railways over a gentle slope of silty clays. It appeared thatthe slope was in a state of limit equilibrium, as the small cut quickly initiated movementsof the railways, which were unacceptable (150 mm in one month).


48/107


It was decided to try to stop or to reduce the sliding movements by driving two lines ofsteel inclusions E140 with an horizontal spacing of 0.5 m in each line, down to 9 m depthfor a 6 m deep slip surface.

Movements began to decrease with the first line, and were almost stopped after drivingthe second line, which required about a month and a half.

The TALREN program was used for the analysis, and it showed that for an initial safetyfactor of 1.08, the cut induced a decrease to approximately 1.0, and each steel inclusionss

line increased the safety factor by about 3%. That was effective to reduce the movements.Permanent stabilization was obtained by drainage, which allowed the safety factor toreach values higher than 1.3.

Figure 3.6 A41 Highway. Cross section


49/107


Figure 3.7 A41 Highway. Slope movements and stability analysis

3.1.4.4 Bousst Saint Antoine railway embankment: bored concrete piles

The embankment rested over a slope of plastic clayey and marly debris and it sufferedslow but continuous movements down to 10 m deep.

It was decided to stabilize the slope by placing 800 mm diameter bored piles in two rows,with an average spacing of 1.5 m in each row. Moreover, an intensive monitoring wasimplemented in order to better understand the stabilization behaviour.

The stabilization was globally effective, as the rate of sliding was reduced from 3-4cm/year before to 0.2-0.4 cm after the works. However, the monitoring showed that thestabilization required almost two years before being effective and that movements at theground level were going on up to four years after the works. Moreover, the measureddeformations when stabilization was obtained proved to correspond to a very lowincrease of the safety factor (about 3%).


50/107


Figure 3.8 Boussy Saint Antoinerailway embankment

3.1.4.5

Cut slope reinforcement by stabilising piles in Korea (Hong et al, 1997)

A cut slope of 1H:1V was constructed on a construction site of apartment buildings inPusan, situated about 432 km southeast of Seoul, Korea. The cut slope was originallysupported by an anchored retaining wall with height of 12.5 m. During heavy rain forseveral days, however, landslides took place in 1993, following partial collapses of theanchored retaining wall.

The soil layer just below the ground surface is composed of mainly weathered soils,approximately 10.3 m thick, with some colluvial soils. The soils can be described as sandysilts. A bedrock composed of weathered rock and soft rock is found below the weatheredsoils.

To improve the stability of the failed slope, a row of stabilizing piles was constructed inthe slope. After installation of stabilizing piles, the slope was modified to an inclination of1.5H:1V with two soil berms. For an installation of stabilizing piles directly below thesecond soil berm, the holes, 450 mm in diameter, were drilled to the depth of 1.5 m intothe soft rock layer. Steel H-piles (H-3003001015) were inserted into the boreholesand the left spaces around H-piles in the holes were filled by cement grouting to preventcorrosion of the steel piles. The piles are installed in a row at centre-to-centre intervals of1.5 m. Pile heads were connected by a wale and reinforced concrete capping.


51/107


Figure 3.9 Cut slope reinforced by stabilising piles. Design to control the landslide.

A retaining wall, 8.4 m high, was constructed to cut the toe of the slope. It is composedof an anchored retaining wall and a concrete retaining wall.

The anchored retaining wall consists of soldier piles with concrete lagging and anchors.The soldier piles (H-250250914) installed into the bored holes at intervals of 2 m

were surrounded by cement grouting to prevent corrosion. Four rows of anchors, havingdifferent free lengths and bond lengths, were designed to have sufficient resistanceagainst pull-out or fracture. Permanent anchors were used to maintain the retentioneffect. Based on the concept that half of the initial anchor load previously determined inanchored retention wall section would be lost with time, installation of the additionalconcrete retaining wall of about 8.4 in height directly at the front face of the anchoredretention wall was proposed to ensure the long-term stability of the slope.

Construction works for stabilization of the cut slope on hillside can be divided into threestages as follows:

Slope modification. The slope was modified to an inclination of 1.5H:1V with a soilberm after installation of stabilizing piles below a second soil berm.

Toe excavation. Anchored retention wall was constructed by excavating the toe ofthe slope after installation of soldier piles with lagging.

Underground excavation. It was performed in front of the anchored retention wall toprovide underground parking facilities. Concrete retaining wall was constructed at theend of this stage.


52/107


Figure 3.10 Cut slope reinforced by stabilising piles. Construction stages

After construction, to ensure the stability of the failed slope, an instrumentation includingstrain gages, inclinometers and standpipe piezometers was designed.

Rainfall is the most significant factor which influences landslides in Korea. According toprevious investigations, the groundwater level did not increase significantly during heavyrain. Therefore, the groundwater level in this slope could not affect the behaviour of piles

and slope. The wetting front, rather than the groundwater level, may affect the behaviourof piles and slope, since the driving force of the slope is increased by increase in the

weight of soil above the wetting front.

Behaviour of piles

Piles horizontal deflection was measured with the inclinometer installed in them.

The maximum deflection was measured at pile head and the deflection angle at pile headwas zero, that is, the pile is under a fixed head condition. The peculiarity of this resultmust be emphasized, since in this type of stabilization, piles are subjected to shearstresses and therefore to bending moments.

Piles deflection grew gradually with the lateral earth pressure which was developed onpiles due to cutting and excavation (Figure 3.11(a)). Deflection increased also duringheavy rain (Figure 3.11(b)), since the lateral earth pressure increased with increase in the

weight of soil above the wetting front due to infiltration of rain. It decreased again,however, when the soil above the wetting front dried.


53/107


Figure 3.11 Cut slope reinforced by stabilising piles. Pile deflection

As regards bending stresses developed in piles, the pile above the weathered rock layer issubjected to lateral earth pressure due to driving force of slope, while the pile in theground below the soil layer is subjected to subgrade reaction against pile deflection. Inaddition, bending stresses in piles increased during heavy rain, but they decreased again

when the soil above the wetting front dried.

Figure 3.12 Cut slope reinforced by stabilising piles. Bending stresses in piles.


54/107


Therefore, all these results illustrate that the piles exhibit an elastic behaviour duringheavy rain.

Behaviour of slope

An inclinometer was installed in the potential plastic zone between piles. The resultsillustrate that the soil between the piles was restrained by the arching action of the soiland its elastic behaviour during heavy rain.

Figure 3.13 Cut slope reinforced by stabilising piles. Soil deformation

Effects of piles on slope stability

Judging from the measured movements of piles and soil, it is supposed that the behaviourof the stabilizing piles and the slope is elastic. The slope movement could be restrained toquiet small movements by the effect of the stabilizing piles, so it can be said thatstabilizing piles are effective to control landslides in this site.

3.2 SOIL DOWELLING

Soil dowelling can be studied as a particular case of soil nailing in slope stabilization.

3.2.1


The dowelling technique consists in reinforcing the creeping slope by inserting piles sothat the prevailing loading of the piles is transverse shearing. Dowels can be made ofconcrete or steel, and they connect a downsliding earth block to the solid underground.


55/107


Creeping slopes in stiff clay can be stabilized by using dowels made of concrete or steel.The stabilization results in a reduction of the creep rate to a level which is harmless tosuperstructures. The dowels transmit the stabilizing force from the substratum to thecreeping soil.

In a creeping slope the soil moves slowly downhill, with typical velocities from 0.1 mm to5 cm per month, and usually the groundwater table lies near the surface. The creep

velocity decreases and sometimes the creep nearly stops with lowering of thegroundwater table. When the water table is raised, creeping begins again.

Usually a creeping slope consists of, nearly saturated, stiff clay to a depth of 5 to 15 m, oreven deeper. In the transition zone (the thin shear zone) between the moving soil and thestable layer, the water content is usually higher than in the surrounding soil. Often theclay is fissured and therefore the permeability is quite uneven.

It usually requires enormous strength, and therefore high cost, to stabilize a creepingslope totally. In this context stabilization of a creeping slope means reducing the velocityto a level which is harmless to structures.

3.2.2

Field of application

This technique is used usually for the stabilization of creeping slopes in stiff clay.

3.2.3

Design method

The Department of Soil Mechanics and Foundation Engineering of Karlsruhe Universitydeveloped a new design method for dowelling (Leinenkugel, 1976). Design ofstabilization is carried out assuming the dowels as elastic beams with a constantcoefficient of lateral subgrade reaction.

This method assumes the following hypotheses:

The creeping slope is considered as a rigid body of weight W which slides on aninclined surface.

The shear force V (V=W sin) is decreased by the forces Qscarried by the dowels, socreep velocity decreases from v0 to v1.


56/107


Figure 3.14 Creeping slope with dowels

Soil is a viscous fluid with a strongly non-linear viscosity which obeys Leinenkugelslaw (Leinenkugel, 1976):

( )1 0 1 01 vV V I ln v / v

= +

(3.2)

where V: shear force

Iv: viscosity index

v: creep velocity

The viscosity indexIvvaries between 0.01 and 0.06 and can be obtained from triaxial testswith variable rates of deformation.

The number of dowels can be obtained from the previous equation:

0 1D v

W sinn I ln( v / v )

Qs

= (3.3)

For design, the values Iv and W sin are given and the ratio v0/v1 must be chosenaccording to the requirements for the structures on the slope. The objective is thereforeto determine the resulting dowel force nDQsfor the most economical and safe stabilizingeffect, that is, dowels should not be damaged during the design life of the structure andthey need not be stronger than necessary.

Regarding the mechanical behaviour of a dowel, the relationship between thedisplacement uand the horizontal load His given by the following equation:

Hdz

duIE DD =4

4

(3.4)


57/107


This is the same equation which has been already described in 3.1.3. It is assumed thatthe displacement within the creeping part of the slope is constant, that is, it behaves as arigid body.

As regards the dowel diameter, the optimum one is roughly 5% of the depth to the slipsurface.

It should be noted that the dowel action only occurs after a time which is needed for thedisplacement to mobilize a sufficiently high lateral force.

3.2.4 Case study

Landslide at Dautenheim (Germany) (Gudehus and Schwarz, 1985)

An 8 m high fill embankment was erected on a slope with an angle of 5. After someyears significant creeping began. The position of the sliding surface was identified bymeans of inclinometer measurements and was found to be located in stiff tertiary clay.

The landslide was stabilized with two rows of 1.5 m diameter dowels, so that the formercreep rate of 0.1 to 0.15 cm/day was reduced to a rate which was not measurable over theperiod of a few months. It is possible as well that the lowering of the groundwater table

contributed to this result.

Figure 3.15 Dowelling at Dautenheim

3.3 GEOGRID REINFORCEMENT

3.3.1


This technique consists in strengthening the soil by introducing synthetic polymers in it.Usually these polymers are placed in layers into the soil mass to be reinforced.


58/107


3.3.2 Field of application

The most common application of Geogrid reinforcement is the construction of retainingwalls. Geogrid reinforced slopes can be built at any angle. Therefore, this technique couldbe considered as well as a slope reshaping method, specifically a mechanically stabilizedearth wall (retaining structure) (2).

3.3.3 Design method

The design method for geogrid reinforced steep slopes described by Jewell et al. (Jewel et

al, 1984) is based on extensive stability analyses with the two-part wedge mechanism. Itallows obtaining design charts which give the coefficient of earth pressure, K, and theratio between the geogrid length and the slope height (L/H) as a function of the slopeangle . A family of curves is provided for various friction angles . To achieve arequired global factor of safety (SF), before entering the charts the effective friction angleis reduced according to the equation:

* tan ' ' atanSF

=

(3.5)

The global outward force which must be resisted by the tensile forces mobilized in the

geogrids is then given by:

2

1

2

qF K H

= +

(3.6)

where qis the surcharge and the soil unit weight.

It is then possible to determine the number, length and position of the geogrid layersneeded to resist the force F, i

lessloss deliverable 012

Documents