a highlight on stability of road embankments ...the main section details the design of floating...

A HIGHLIGHT ON STABILITY OF ROAD EMBANKMENTS

ON SOIL REINFORCED BY FLOATING STONE COLUMNS: A

TUNISIAN CASE STUDY

Mounir BOUASSIDA *1 JM DEBATS

2

ABSTRACT

Nowadays, the use of floating stone columns to reinforce soft/compressible soils is becoming

more common. In fact the optimization of the length of the floating columns will make this

improvement technique much more cost-effective. Adopting such reinforcement scheme

relies on the verification of the residual settlements within the unreinforced soil layers which

should remain within an admissible range. The financial harbor at Raoued (Ariana

governorate) located north of Tunis City is a large project where the first step in progress is to

build several kilometers of roads over 5000 Acres. The geotechnical profile of this area

neighboring the Mediterranean Sea is essentially composed of soft soil layers up to 15 m

depth overlying compressible clays horizons extending to30 m depth. Two options of soil

improvement techniques were studied: preloading embankments associated to geodrains of 30

m length and reinforcement by floating stone columns of 14-17 m length. Due to the

significant savings in time provided by the stone column reinforcement compared to the use

of geodrains with preloading the suggested paper details the design of the stone columns,

taking into account the requirement of an allowable residual differential settlement of 2 cm

within the unreinforced compressible clays having good homogeneity and also an OCR of 1.3

as predicted from CPTu tests. The design of floating column reinforced foundation was

carried out by using the methodology suggested by Bouassida and Carter (2014) and inherent

verifications detailed by Bouassida and Hazzar (2015).

1. INTRODUCTION

Presently a wide spectrum of improvement techniques is affordable for the treatment of weak

and highly compressible soils like soft clays. Prefabricated vertical drains, vacuum

consolidation, stone columns, sand compaction piles and deep soil mixing are among the most

widely used techniques to enhance bearing capacity, to mitigate settlements and accelerate the

consolidation of soft soils. Choosing the suitable solution among these techniques depends on

several factors like the ground conditions to ensure suitable installation. This is often checked

by performing ”trial zones” which also permit to quantify the expected benefits. In this

context the Project of Financial Harbor at Raoued (PFHR) at Ariana (Tunisia) is addressed for

the construction, in a first step, of roads on embankments with variable height up to 4m. The

*1 Université de Tunis El Manar, Ecole Nationale d'ingénieurs de Tunis. Ingénierie Géotechnique, LR14ES03, BP 37 Le Belvédère 1002

Tunis, Tunisia. [email protected] 2 Eguilles, Aix en Provence, France.

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7. Geoteknik Sempozyumu 22-23-24 Kasım 2017, İstanbul

mailto:[email protected]

PFHR is located in a coastal area close to the Mediterranean sea. In such area recent

sedimentary deposits, e.g. compressible soft soils, are often encountered.

This paper, first, briefly introduces the consistency of the geotechnical surveys performed at

site and the synthesis of the geotechnical data to be considered for the design. Especially the

overconsolidation of the compressible clay layers encountered is investigated from oedometer

and CPTu test results. The main section details the design of floating stone column

reinforcement as a foundation of road embankments. This alternative raises the problem of

long term settlement in the unreinforced clay layers: are those settlements admissible in

magnitude and in time as a function of embankment height? The design of stone columns

reinforcement is carried out by duly taking all those considerations into account.

2. THE PROGRAMME OF GEOTECHNICAL SURVEYS

As reported in [1] and [2], two main geotechnical surveys were carried out in 2014 and in

2017 for the construction of road embankments the geotechnical surveys comprised:

- Eighteen pressuremeter profiles performed to various depths (81 to 96 m);

- Three boreholes performed up to 40 m depth below ground surface. From these boreholes

sixteen intact soil specimens were extracted to be used for identification and mechanical

laboratory tests. Laboratory tests included grain size distribution, Atterberg limits, unit

weight and water content determination and essentially oedometer tests.

- Seven SPT tests were performed.

- Five profiles of CPTu tests were performed to refusal depths varying from 19 to 35 m

depth (first survey).

- Eight profiles of CPTu tests were performed to refusal depths varying from 24.9 to 30.3m

depth (second survey).

Detailed experimental results of in-situ and laboratory tests are given in the reports [1] and

[2].

3.SYNTHESIS OF GEOTECHNICAL INVESTIGATIONS

From the existing geotechnical reports two soil profiles were considered for the design as

summarized in Table 1.

Table 1. Typical geotechnical profiles

Profile n° 1 (unfavored data between I-2 &

III-4)

Profil n° 2 (unfavored data between I-1, III-1

& III-2)

Horizons Thickness

(m)

Em (MPa) Pl* (MPa) Horizons Thickness

(m)

Em (MPa) Pl* (MPa)

I 6 0.9 0.21 II 4 3.5 0.68

II 6 3.5 0.68 III 38 3 0.45

III 18 3 0.45 IV 15 5.3 0.9

IV 20 5.3 0.9 V 21 13 1.55

V 34 13 1.55 - - - -

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Along the main direction 1-A of road embankments the soil profiles given in Table 1

indicates homogeneous formations interrupted by some indurated layers or lenses. Herein,

focus is given on clayey layers of horizon III, in particular the evaluation of their over-

consolidation ratio (OCR), compression index and estimates of vertical and horizontal

coefficient of consolidation. Those parameters significantly affect the estimation of primary

consolidation (or long term) settlement and also its evolution in time.

3.1. Evaluation of OCR of clayey layers (horizon III)

From results obtained from oedometer tests carried out on several intact soil specimens it was

recommended by [3] to consider that clay layers of horizon III are, at least, normally

consolidated. Such recommendation was checked by analyzing the profiles of CPTu tests in

layer III which look quite homogenous, but sometimes are interrupted by thin hard layers or

lenses. The recorded results from CPTu tests are helpful to learn much better about the OCR

of layers in horizon III. First, the advantage of CPTu test is to record in a direct manner the

the tip resistance without any disturbance of the in-situ soil. Whilst from oedometer tests the

results can be seriously affected by the disturbance of specimens during their extraction from

sampling tubes, transportation and preparation steps before performing the oedometer test

itself. Second, since the seventies in Sweden, several correlations were proposed for

estimating the preconsolidation pressure 'p of clays from the tip resistance qc or from the

corrected tip resistance qt recorded during the cone penetration test. Due to this the correlation

expressed by Eq (1) and shown in Fig. 1 was agreed because its correlation coefficient is quite

good as R2 = 0.884 (Bowles, 1982):

'p = 0.5439 qt0.8635 (1)

Figure 1. Correlated pre-consolidation pressure vs corrected tip resistance from CPT-u tests

(Bowles, 1982)

The evaluation of OCR

From CPTu test results given in [1] the profiles PZ EX2 et PZ EX6 were compiled from

which the tip resistance qc was considered as a safe alternative since qc is lower than qt and

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therefore, the estimated pre-consolidation pressure from correlation (1) will not be

overestimated. Detailed discussion on the method of determination of the OCR of clay layers

in horizon III is available in [4]. It was agreed that the minimum and safe OCR value is 1.3

for clay layers encountered between 17 and 28 m depth. This over-consolidation was

confirmed from identification laboratory tests, in particular by a consistency index of 0.75,

and by the geological era (10,000 years) of the Raoued area that served as the basin of

Meliane river.

Overconsolidation of clay layers from -14 m to -28 m depth is essentially attributed to the

overlaid formation composed by silt sand horizons having low consistency. For such

condition the following explanation might prevail.

Step 1: the deposit of sub clay layers on the prior river bed (e.g. from -30 to -13 m depth for

instance) the sea level was zero. At this stage the effective stress at elevation -21 equals (21-

13)m x 17 kN/m3 -21x10 kN/m

3 = -74 kPa (under-consolidated stage, obviously).

Step 2: Deposit of alternated sand/clay formation over the sub-clay layer, e.g. from -13 up to

zero level, the sea level is always located at zero level. At this stage the effective stress at

elevation -21 becomes: (21-0)x 17 kN/m3 - 21x10 kN/m

3 = 147 kPa. Hence, the sub clay

layers, after consolidation over some thousands years, might be over-consolidated

approximately in a ratio as (147+74)/147 = 1.5 which would confirm the magnitude of

expected over consolidation.

For those arguments, from CPTu data available in reports [1] and [2], an over-consolidation

ratio OCR = 1.3 can be adopted. This value is rather conservative for clay layers located from

14 m to 28 m depth in view of estimating their long term settlement.

3.2. Evaluation of vertical and horizontal coefficients of consolidation and compression

index of clay layers

Figure 2. Withman and Lambe's (1969) chart

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4. DESİGN OF EMBANKMENT FOUNDATION ON REINFORCED

SOİL BY FLOATİNG COLUMNS

The adopted methodology, as detailed in Bouassida & Carter (2014) and Bouassida (2016),

relies on two verifications: the bearing capacity and settlement. Implementation of this

methodology in Columns 1.01 software (Bouassida & Hazzar, 2012) enables the prediction of

optimized area ratio on the basis of given short term settlement (end of construction). This

settlement is estimated using the linear elastic behavior by adopting Young modulus of

column material equals 10 times that of the initial soil after the French recommendation

revised in (2011). The first design step aims at the estimation of Young modulus of all

crossed layers from the surface up to 30 m depth where the effect of surcharge load due to

embankments vanishes and, in addition, a stratum level appears (horizon IV in Table 1).

4.1 Estimation of modulus of deformation

From the performed CPTu and SPT tests executed in clay layers and sand layers, respectively

the Young modulus is deduced for each layer from the following correlations suggested by

Bowles (1996) and Das (2014):

clayey formation: E = (3 to6)* qc (2)

Silt-saturated fine sand E = 300 * (N+6) (3)

Or:

E = 320 * (N+15) (4)

N denotes the SPT number measured from the SPT test.

From correlations given by Eqs (2), (3) and (4) and considering safe estimations the adopted

modulus of deformation are the following:

For clays: E = 4 * Qc (5)

For sands the correlation given by Eq (3) was agreed. Hence, the adopted modulus of

deformations are summarized in Tables 2 and 3.

Table 2. Modulus of deformation of sand layers

Depth (m) SPT E (kPa)

1 to 5 N = 8 4200

5 to 12 N = 15 6300

Table 3. Modulus of deformation of clay layers.

Depth (m) CPT E (MPa)

12 to 23 qc = 1.25 MPa 5

23 to 27 qc = 1.6 MPa 6.4

27 to 30 qc = 2.5 MPa 10

≥ 30 m qc = 10 MPa 40

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4.2 Road embankments: geometry and associated networks

Those embankments have a variable height from 0.5 to 4 m and different cross section with a

slope 2 on vertical over 3 on horizontal directions. Over their length profile these

embankments have progressive slopes each 80 m to 100 m distance. At 0.5 m depth below the

basis of those embankments gravity hydraulic networks will be installed with very low slope

of 0.2 percent. For such networks the requirement is to not exceed a long term residual

settlement of 2 cm. The biggest cross section of embankment has by 4 m height, 62 m of

width base and 52 m at width crest with 80 kPa uniformly distributed load. Due the

compressibility of clay layers in horizon III and transmitted excess of vertical stress within

those layers long term settlements are expected so that functioning of the hydraulic gravity

networks will be affected.

4.3 Why was the reinforcement by stone columns contemplated?

It is noted that the verification of bearing capacity for the embankments' foundation is

warranted so that the 80 kPa surcharge load is admissible. In fact, based on the safe

determination of undrained cohesion equal to 35 kPa of superficial layer there is no risk in

regard to the admissible bearing capacity. In turn, the settlement verification requires a

comprehensive analysis as explained above.

As a first option, it was judged that the long term settlement of embankments can be

accelerated, prior to their construction by a preloading associated to vertical drains of length

extending up to 28 to 30 m. Although this option offers a safe solution to avoid any surprise

for the functioning of hydraulic networks it will take a long time for the preloading over an

approximated area of 5000 Acres. Also the installation of prefabricated vertical drains (PVD),

however already experienced in Tunisia for infrastructure projects, but with length less than

18 m, presents the risk on inclined installation of the PVD from which the full depth of

treatment will not be covered also needs to be checked.

Therefore, the second suitable solution consists in both reducing and accelerating the

consolidation settlement essentially over the first 14 m depth of sand silt formation. Such

solution obviously offers the benefit in saving the preloading time. This alternative is feasible

when floating stone column reinforcement is targeted. Whilst the length of the stone columns

should be optimized taking into account the over-consolidation of clay layers (horizon III) and

the decrease of induced excess vertical stress due to the surcharge embankment.

For the purpose of suitable and easy execution the optimized length of stone columns was

decided as 14m. Beyond 14 m depth the treatment by stone columns in the clay layer will be

time consuming but it remains effective.

4.4 Design using floating stone columns -Settlement reduction - Long term settlement.

4.4.1 Short term settlement

Consider a total short term settlement of 16 cm which is quite admissible for 4 m

embankment height, on the basis of soil layers and column material properties the optimized

improvement area ratio as determined by the software Columns 1.01 is found by 16%. The

settlement of reinforced soil layers over 14m depth equals 8.1 cm. Since stone columns

behave like vertical drains the settlement of reinforced soil will occur at the end of

embankment construction. It is proposed that stone columns of diameter 1m are installed in

triangular pattern with an axis to axis spacing equals 2.46 m. Assuming that the consolidation

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of reinforced soil only occurs horizontally the use of Barron's chart leads that 90% of

consolidation is expected in three months approximately. This duration coincides almost with

one stage construction of 4m embankment height. The primary consolidation settlement of

unreinforced layers over 14 m in such duration can be neglected so that the residual settlement

of unreinforced clay layers corresponds to the residual settlement to be induced under the road

embankments.

4.4.2 Estimation of long term settlement

The oedometer and CPTu methods are both considered for estimating the consolidation

settlement of unreinforced clay layers. Comparison between the predictions of these methods

is then discussed.

Estimation of long term settlement using the oedometer method

The prediction of long term settlement over 14 m thickness of clay layers by the oedometer

method is 18.5 cm for a surcharge load of 80 kPa. This prediction is obtained by an Excel

sheet calculation by subdividing the 14 m in sub-layers of thickness 3 m. Excess of vertical

stress due to embankment load, vertical effective stress and preconsolidation stresses were

calculated at mid thickness of each sub layer. The averaged long term settlement per meter of

embankment height and per meter of compressible layer is sketched in Figure 3.

Figure 3. Variation of normalized long term settlement per meter of compressible clay later

and per meter of embankment height.

Estimation of long term settlement using the CPTu test results.

The long term settlement can be estimated from the correlation between oedometer modulus

Eoedand the tip resistance qc:

Eoed = 5 qc (6)

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Considering each CPT profile (in total there are eight profiles), by adopting the correlation in

Eq (5) for each 50 mm thickness the oedometer modulus is calculated and the long term

settlement is predicted. The averaged settlement per one kPa and per meter of clay layer is

0.185 mm/kPa/m as detailed from the Excel sheet calculation given in Table 4.

Table 4. Estimation of long term settlement from CPTu data

It is noted that the estimation of long term settlement from the CPTu data exceeds by 10%

that estimated from the oedometer method. Hence, this confirms that adopted OCR = 1.3 for

the compressible clay layers is secured for the prediction of long term settlement. The

duration of long term settlement, calculated for consolidation ratio = 50%, by assuming one

drainage path (from the tip of stone columns) of the unreinforced layers over 14 m thickness,

is about 16 years. Therefore, the variation of primary consolidation settlement evolves slowly.

Verification of residual differential settlement

Predicted long term settlement shown in Figure 3 corresponds to uniform settlement induced

by uniform surcharge load which is not the case of embankment having variable height.

Along a given section of road embankment of typical length equals 100 m the height is

variable and so is the induced settlement. Figure 4 clearly shows quite variable absolute

settlement along the main section 1-A of the PFHR of total length approximating 1,180 m. In

this figure the variation of differential settlement is also shown. Thus, it is the differential

settlement: s = si -si-1 between two successive points "i-1" and "i" almost distanced by 100 m within a section of road embankment, rather that the absolute settlement, that will affect the

functioning of hydraulic networks which will be installed with a quite low slope of 0.2 %.

Such analysis can be highlighted from Figure 5 that shows the evolution of residual

differential settlement along the first sub-section (0 to 180 m) of main axis 1-A compared to

the allowable residual long settlement of "+" or "-" 2 cm.

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Figure 4. Variation of absolute and differential long term settlement along axis 1-A

Figure 5. Variation of differential long term settlement along the first section of axis 1-A

From Figure 5 the most unfavorable situation corresponds to the sub-section between points

0 and 1 where the differential settlement equals 8.1 cm. The admissible differential

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settlement equals 2 cm that corresponds to a vertical degree of consolidation calculated from

one-dimensional Terzaghi's theory:

U(Tv) = 2 cm/6.1 cm = 0.32786.

From the equation of time factor: Tv = cv*t/H2 one can determine from which time the

differential settlement will exceed the admissible limit of 2 cm, it is: t = 14 years. It is the

soonest time from which the residual differential settlement might affect the functioning of

hydraulic gravity networks.

From Figure 5 three critical sub-sections (e.g. where the residual differential settlement

exceeds its admissible limit) are expected. For those sub-sections it recommended to proceed

for a reinforcement with 17 m length of floating stone columns for reducing and accelerating

the consolidation settlement of clay layers (horizon III). Nevertheless, the more likely,

expected differential settlements would not affect the functioning of hydraulic gravity

network because those settlements will only take place with a long passage of time.

It is noted that all predictions on the basis of suggested design should be validated by

scheduling a loading test with recorded load-settlement curves.

4. CONCLUSIONS

This paper addressed the design of floating stone column foundations for embankments road

scheduled for the ongoing Tunisian project "Project Financial Harbour of Raoued". The

geotechnical investigations conducted for this project have been synthesized with special

focus on the over consolidation of clay layers overlaid by silt sand formation both having 14

m thickness. In this regard the use of CPTu data revealed more suitable than data recorded

during oedometer tests. Two ground improvement options were decided for this project: first

option was a preloading associated with prefabricated vertical drains, and the second

suggested to install floating stone columns of 14 m length crossing the homogenized silt sand

formation.

The study of behavior of road embankments having variable height is tributary of the residual

differential settlement rather than the absolute consolidation settlement. From this analysis it

is proposed to focus on the differential consolidation settlement to occur within the

unreinforced clay layers starting from 14 m depth, below the ground surface, and should

affect the functioning of the gravity hydraulic networks.

The suggested design consists in stone columns of 14 m length crossing the sand silt

formation installed by the vibrocompation method to improve the stiffness of treated silt sand

layers. In turn, over limited sub-sections of the road embankments it was recommended to

install stone columns of 17 m in length to fulfill the required residual settlement of 2 cm.

For the road embankments foundation of the PFHR a technical-economical comparison will

be carried out to state about the suitable improvement technique to be executed.

REFERENCES

[1] Hydrosol-Fondations. Campagne de reconnaissance géotechnique. Phase d'exécution du

port financier à Raoued.

[2] [1] Hydrosol-Fondations. Tunis Bay Project (Raoued-Tunis). Geotechnicalreport.

[3]Terrasol-Tunisie. Rapport géotechnique de Raoued Port financier.

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[4]Africa Engineering (2017). Variante de renforcement par colonnes ballastées:

Dimensionnement et modalités d'exécution, Mai.

[5] Bouassida M., Hazzar L. (2008). Comparison between Stone Columns and Vertical

Geodrains with Preloading Embankment Techniques. Proc. 6th Int. Conf. On Case

Histories in Geotechnical Engineering. Arlington VA (USA), 11-18 August, Paper

No. 7.18a.

[6] Bouassida, M. and Carter, J. P. (2014). “Optimization of Design of Column-reinforced

Foundations”. Int. J. Geomech., Volume 14, Issue 6 (December 2014),

04014031-1-10.

[7] Bouassida M. (2016); Design of Column-Reinforced Foundations. J. Ross Publishing

(FL, USA), August. 224 pages. ISBN: 978-1-60427-072-3.

[8]. M. Bouassida, and L. Hazzar (2012). Novel tool for optimised design of reinforced soils

by columns. Ground Improvement: Proc. ICE, London 165, Issue 1, pp 31 –40.

[9]. Document français (2011). Recommandations sur la conception, le calcul, l’exécution et

le contrôle des colonnes ballastées sous bâtiments et sous ouvrage sensible au

tassement (in French). Comité Français de Mécanique des Sols, Version n°2,

March 16, 32 pages.

[10]. Bowles, J. E. (1996). Foundation Analysis and Design, 5th. edition, McGraw-Hill, New

York.

[11]. Das M.B. (2014). Principles of Foundation Engineering. Cengage Learning. USA

[12]. Lambe T.W. and Withman R.V. (1969). Soil mechanics. M.I.T.

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