patra sir term paper

8/6/2019 Patra Sir Term Paper

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1. INTRODUCTION

The increasing demand for sustainable transport infrastructure, construction materials and

redevelopment of soft marshy or derelict land necessitates special ground improvement

techniques. Often good-quality materials are either too costly or not feasible as construction backfill for road embankments. Embankments made of locally available materials or resting

on soft soil often faces large settlement and difficult construction.

Uncontrolled movement of subsurface water may lead to reduction of shear strength in soils,

development of horizontally inclined forces which increase overturning forces and moments

leading to the possibility of a structural failure and increased lateral and upward pressure of

groundwater. (4)

Traditionally such problems have been addressed using vertical drainage pipes, sand or

prefabricated vertical drains (PVD).The search for better and alternate solutions paved theway for soil improvement techniques using Quicklime and filter fabric.

The mechanism of soil improvement by Quicklime is either by chemical action, when it is

used as an additive to the soil, or by providing an alternate drainage path, or it could even

give mechanical support when used as columns though this is only secondary to the former.

Permeable geosynthetics (filter fabrics) are used in order to accelerate time-dependent

settlement for any construction with/on soft soil, by draining water from the soil during the

consolidation process. (3)

2. BRIEF REVIEW

The method of introducing dry quick lime into the ground was first put into use by the

Swedish Geotechnical Institute in 1965. The lime columns technique was introduced in

Sweden in 1967 as a new type of foundation for light buildings (Bredenberg and Broms,

1984) and this is frequently used by the Scandinavian countries. Initially only quicklime was

used as a bonding agent for improving the load bearing properties of cohesive soils. However

since 1980¶s lime has been used in combination with cement, gypsum, fly ash etc. Chida in

1982 proposed the Dry Jet mixing method (DJM) which injects quicklime or cement powder

deep into the ground through a nozzle pipe with the help of compressed air. This increased

the depth of access as well as the efficiency of the process. (2)

Inclusions of different sorts mixed with soil have been used for thousands of years. They

were used in roadway construction in Roman days to stabilize roadways and their edges.



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These early attempts were made of natural fibres, fabrics or vegetation mixed with soil to

improve road quality, particularly when roads were built on unstable soil which in a buried

environment is susceptible to biodegradation that occurs from microorganisms in the soil.

With the advent of polymers in the middle of the 20th Century much more stable materials

became available.

The first papers on geosynthetics in the 1960s were as filters in the United States and as

reinforcement in Europe. The 1977 conference in Paris brought together many of the early

manufacturers and practitioners. The International Geosynthetics Society (IGS) founded in

1982 has subsequently organized worldwide conference every four years and its numerous

chapters have additional conferences. (3)

Nowadays advances in technology in material science have produced geosynthetic materials

such as geocomposites. Geocomposites also known as hybrid materials, combine the bestattributes of two or more geosynthetic materials so as to provide optimum performance

and/or minimum cost.

3. QUICKLIME

The objective of soil µImprovement¶ with quicklime is to achieve an immediate reaction,

which significantly strengthens the soil due to the removal of moisture and a chemical change

in clays. It is not intended to induce any long-term strength gain, which occurs when soils are

µStabilised¶. However where clay soils are present even modest additions of lime may cause

some long-term strength improvement. (4)

The long term alteration of the properties of lime treated soil is due to the formation of

cementated hydrated calcium silicates, resulting in improved strength and deformation

characteristics of the soil. The extent of change in soil characteristics depends on the type and

amount of clay, the quantity of lime added and the duration of treatment. (2) With cohesive

soils there is a simultaneous improvement in the condition of the clay particles due to a

chemical change brought about by the calcium ions. This chemical change is known as

µModification¶ and is additional to the drying process. This reaction essentially transforms the

material into a different soil with enhanced geotechnical properties. Compaction

characteristics, MCV (moisture condition value) results, plastic limit and bearing capacity are

all changed beneficially in this reaction. These reactions occur immediately the quicklime is



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dispersed into the soil. Stiff clays will lose plasticity and become more friable due to the

change in soil properties. (6)

Quick lime is found to have the following specific uses:

1) Treatment of expansive soils with quicklime decreases their plasticity and hence

increases their strength rapidly and effectively. (4)

2) Deep Lime Mixing Method (DLM) or Lime columns strive to improve the bearing

capacity, prevent slope failure, reduced liquefaction potential during earthquakes,

protection of structures in the vicinity of excavations, and encapsulation of

contaminated soils.(5)

3) Lime slurry pressure injection (LSPI) is a stabilization operation that has been used by

transportation industries such as road and railways to improve the geotechnical

performance of problematic subgrades and embankments that persistently fail to meet

serviceability requirements. The method involves the use of a hirail rig fitted with 3

vertical probes forced to target depths in problematic subgrade soils, usually within

the seasonal moisture fluctuation zone at approximately 2±4 m. A cementitious slurry

of lime and fly ash agents is injected under a typical hydraulic pressure of 800±1,000

kPa and ceased when slurry breaks out at the surface, or when a maximum pressure of

1,450 kPa is reached Kayes et al. 2000_. This procedure is repeated at upstaged

intervals of typically 1±2 m to achieve a complex network of chemically active slurry

seams intersecting subgrade strata.(7)

4. FABRICS

The basic engineering functions of Geotextile fabrics are drainage, filtration, sediment

control, separation, erosion control, moisture barriers, and reinforcements. Filtration is the

oldest and the most common use of geotextile fabrics. In this application, the Fabric to soil

system allows for free liquid flow (but no soil loss) across or through the plane of the fabric

over an indefinitely long period of time. Most geotextiles can perform this function except

the slit tape because its openings vary in size.



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Fig 1: Filtration Function of Geotextiles

Filt Fabr i are found to have the following specif ic uses:

The use of a permeable geosynthetic layer in the form of geocomposite may hel p to

lower phreatic surface developed in slopes constructed with marginal soils. The

problem of drainage is more severe in soils having relatively high percentage of f ines

(referred herein as marginal soils) having poor draining quality, leading to

development of excess pore pressure causing instabilityto the structure. FIG. 3 shows

a typical cross-section of a geosynthetic-reinforced slope sub jected to raising

groundwater table and traff ic load, and FIG. 2 depicts an element in which a fabr ic

layer is sandwiched in between 2 impermeable soil layers. (3)

FIG 2: A Filter fabric layer sandwiched between marginal soil subjected to a load and

in-plane flow.



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Fi 3 Cross-section of a geosynthetic reinforced slope 2) During an earthquake, loose sandy soils have the potential to liquefy, causing great

damage to structures supported by the soil. One solution is to densify the loose soils

and/or provide a drainage path for the dissipation of pore pressures before they reach

dangerous levels. In many cases Earthquake Drains can provide the necessary

liquefaction mitigation. Earthquake drains consist of high flow capacity prefabricated

vertical drain wrapped with a geotextile fabric. Typically the diameter is about 75mm

(3 inches). The core is tightly wrapped with geotextile filter fabric, selected for its

filtration properties, allowing free access of pore water into the drain, while

preventing the piping of fines from adjacent soils.

3) Other successful applications of filter fabrics are in a pavement and behind a retaining

wall. In both the cases, the fabric was used in preventing the water table from rising as

a result of rainfall. (1)



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5. CASE STUDY:

Objective

To construct a ground for a sanitary fill in Shimonoseki, a large scale project of cutting and

filling in a mountain slope is required. The height of the embankment is generally high. Thematerial for the embankment was cohesive soil of strongly weathered tuff unsuitable for

constructing the embankment. Multiple strip-sandwich method was required, using quicklime

without any mixing to construct embankments with cohesive soil. A case study for the

construction of a high embankment using this method with a cohesive soil of strongly

weathered tuff. (8)

Multiple Strip Sandwich Method

A sandwich layer is placed on each cohesive soil layer of about 1 m thick. Sandwich layer is

made of granular quicklime of 5 cm thick between two sheets of filters as shown in figure 1.

FIG. 1 Diagram of proposed embankment method.

Functions of quicklime are:

(i) Consolidative dewatering due to the additional pressure caused by the expansive of

quicklime during the early stage of quicklime hydration.

(ii)Suction of water from the cohesive soil due to further hydration of quicklime in the area

near the sandwich layer. A part of water from the soil is drain out of the embankment through

filters.



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(iii)Soil temperature is increased due to the heat discharge from the quicklime during

hydration. This increases the coefficient of permeability in the cohesive soil, making drainage

easier, especially in the cold season.

(iv)Carbon dioxide included in soil air changes the slaked lime to calcium carbonate after a

certain period, and the resulting compound gives a kind of structural reinforcement to the

whole embankment.

Two series of test embankments with alluvial soft clay of high water content was performed.

In field experiments, the four sections of different sandwich systems were investigated as

shown in figure 2. In the B and C sections the sandwich layers were placed over the top

surfaces of all the piled clay layers. In the section B and C, the expansion of quicklime will

not cause any increase in the vertical stress, and absorption of water into sandwich layer due

to quicklime hydration causes a dewatering of soil. On the other hand, in the section D the

sandwich layer was laid in strips, in order to cause a three dimensional expansion of

quicklime when it changes to slaked lime in the clay mass resulting in a larger consolidation

pressure than that in the case of the continuous spreading of quicklime, as in the sections B

and C.

FIG. 2 Depth wise distribution of cone bearing capacity of test embankments



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Specif ications

Properties of cohesive soil:

Excavated soil was cohesive soil with a natural water content of about 30%, liquid limit

40%, plastic limit 28%, specific gravity 2.74, 10% diameter 2 µm, 60 % diameter 9 m. The

rate of strength increases with consolidation of the cohesive soil, Cu/P, was ranged

between 0.5 and 0.7.Coefficient of consolidation Cv and compression index Cc of the soil

were (0.6-2.5)*10-2

cm2/s and 0.19-0.28 respectively.

FIG.3 Rate of strength increase with consolidation for the cohesive soil

Specifications for embankment works:

The sandwich layer is made of granular quicklime of 5 cm thick, sandwiched between two

sheets of filter fabric, 30 cm wide and 0.3 cm thick made from a synthetic material of

nonwoven fabric. The filter fabric serves as a drain path even in a compressed state under the

overburden pressure and the additional pressure caused by quicklime hydration. Sandwich

layers were laid at a spacing of 2.1 m between the centers in the horizontal direction and with

a pitch of 1.8 m in the vertical direction. The equivalent diameter of a fabric filter is

dw= (2(a+b))/ ««««««««««««««««««««««.(i)



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Where dw= equivalent diameter of filter, a and b= width and thickness of filter respectively

and = the shape factor.

Substitute a=30 cm, b=0 and =0.75 in equation (i), dw becomes 15 cm. The effective

diameter of the zone of influence of filter fabric is

de=(4*A/)0.5

««««««««««««««««««««««...(ii)

Where A= effective area of the filter and substitute A=2.1 m*0.9 m in equation (ii), de

becomes 1.55 m. The average degree of consolidation is

U=1-exp(-8Tn/F(n)) «««««««««««««««««««««... (iii)

Where n= de/dw

F (n) = (n2/n2-1) ln (n) - ((3n2-1)/4n2)

Tn=Cvt/de2

t=- (de2F (n))/ (8Cv) log (1-U) ««..«...««««««««««««««««. (iv)

Substituting the values n=10.3, F (n) =1.61 and CV=6*10-3 cm2/s in equation (iv)

t = - 9.3log (1-U) (in days)

FIG.4 change the average degree of consolidation with increase in time on embankment

soil



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Improvement of cohesive soil:

Cu = Cuo + (Cu/P)*Pv*U «««««««««««««««««««.«..(v)

Where Cuo = initial undrained shear strength (=50 KN/m2) and Pv = overburden pressure.

Assume Cu/P = 0.5. At an intermediate degree of consolidation, Cu =50+0.5*Pv*U

(KN/m2).

The expansion pressure caused by the hydration of quicklime effectively acts on the soil

layers as an additional consolidation pressure when the sandwich layers are placed providing

spaces in the embankment. The additional consolidation pressure could not be evaluated

quantitatively, thus this pressure was not taken into consideration in the calculation of Cu.

The shear strength of the cohesive soil under consolidation by equation (v), a relationship

was examined between the height of improved soil layer h and the safety factor for slide

failure was evaluated by the method of slices assuming circular failure as shown in figure 5.

Factor of safety was obtained 1.2.

FIG.5 Relationship between height of improved soil layer and safety factor for slide

failure



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Test embankment and earthwork:

Test embankment:

A test embankment was constructed in a scale 2.5 m high and 3 m* 2 m wide at the top

surface. Two layers of cohesive soil 1 m thick were piled on each sandwich layer. Multiple

strip-sandwich method gives an increase of the penetration resistance to the cohesive soil.

The N value of the non improved soil layer was zero. On the other hand, the N values of the

improved soil layers were in the range of 6 to 36.

Earthwork:

The soil was then spread at a depth of about 0.3 m and the compaction of the soil layers were

performed by running a 20 t motor scraper taking care not to break the sandwich layers.

Average moist unit weight of the compacted soil was 18.1 KN/m3. Earth pressure cells were

installed in the embankment to measure the vertical pressure. Measured vertical pressure with

increasing in the embankment height compared with the overburden pressure Pv=*h. The

ratio of measured vertical pressure to calculated vertical pressure Pm/Pv reaches upto 1.6 over

the early period of about 20 days reducing toward 1.0 after the period as shown in figure 6.

An excess vertical pressure was actually caused by the expansion pressure of the sandwich

layer due to hydration of quicklime. This excess vertical pressure would result in larger shear

strength of the cohesive soil.

Boring investigations:

To study how the cohesive soil was improved by the multiple strip-sandwich method boring

investigations were made at two points in the embankment. The N value of the improved

part of the embankment were in the range of from 6 to 18 but on the other hand the N value

of the non improved part were in the range of from 9 to 13. Water content of cohesive soil

changed from initial value of 30-33 % to 20-22 %. The moist unit weight of the cohesive soil

changed from 18.1 KN/m3

to 18.6-19.6KN/m3. Change in the water content or the moist unit

weight resulted from the consolidation of the soil by virtue of the overburden pressure and

the additional pressure caused by the three dimensional expansion of quicklime.

The shear strength Cu from UU test is

Cu = 50+1.0* *h (KN/m2) ««««««««««««««««««««(vi)



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The shear strength estimated from the laboratory shear test is

Cu =50+0.5* *h (KN/m2) «««««««««««««««..««.(vii)

FIG.6 Comparison of measured vertical pressure with calculated overburden pressure

The vertical pressure measured by the earth pressure cell was 1.6 times that of the calculated

overburden pressure *h. Assuming the effect of the additional vertical pressure contributes

fully to the consolidation, equation (vii) was modified as Cu =50+0.8* *h (KN/m2) which

becomes similar to equation (vi).Figure 7 shows the relation between Cu and N suggesting an

approximate relationship of Cu= 10*N (KN/m2). The undrained shear strength of the

embankment soil is estimated to be Cu = 90-160 KN/m2.

Horizontal movement:

To measure the horizontal movement a jointed aluminium pipe of 5 cm diameter was inserted

into the borehole and was secured with cement mortar at the bottom of it to reach a hard

stratum in the ground. By moving an inclinometer of the pendulum type up and down in the

aluminium pipe the continuous change of the depth wise inclination could be measured, from

which the horizontal displacement of the borehole was estimated by integrating the

inclination. Inclinometer was capable of measuring inclinations in the range of ± 30 to +30

degrees with an accuracy of 4 sec. The maximum value of the horizontal movement was



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found about 0.5 cm from the surface of the embankment. Shimonoseki city area suffered

from heavy rains several times but no instability features in the embankment were observed.

FIG.7 Relationship between undrained shear strength and standard penetration

resistance.

Conclusions from the Case study

For embankment work, the material was a more difficult cohesive soil than had beenexpected at the time of planning. Due to this fact, several methods for the embankment work

were searched and the multiple strip-sandwich method described to solve the problem.

Multiple strip-sandwich method using quicklime without any mixing with cohesive soil.

After completing the embankment works a few boring investigations were carried out on the

embankment to study the effect of the multiple strip-sandwich method. It was found that the

cohesive soil could be improved resulting a safety of factor of 1.33 for the failure of the

whole embankment when only the improved soil strength was incorporated into the

calculation of the slope stability. Several depth wise distributions of the horizontal movement

of the embankment were investigated in the borehole by using an inclinometer and no

problem was found even during heavy rains which occurred several times in the area after the

completion of embankment. Therefore, multiple strip-sandwich method proved its usefulness

for the construction of high embankments with difficult cohesive soils.



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6. CONCLUSION

y The addition of quicklime to cohesive, granular and chalk materials causes a

reduction in moisture content as water is used in the hydration process. This could

produce a material with acceptable engineering properties. The various forms in

which Quicklime is used for Soil improvement are Lime columns, Lime slurry

injections and as dry additives in powder form.

y Filter fabrics are used as inclusions in between soil layers which promote the

permeability of the soil system and aids in its quick consolidation, thereby making the

soil stable. Filter fabrics find various areas of applications in railways, roads, retaining

walls, and embankments etc where drainage without soil loss is essential.

7. SCOPE FOR FUTURE DEVELOPMENTS

y Thick geotextiles can be used as permeable layers for draining water expelled from

the soil by the consolidation process. There are certainly economic as well as design

limitations in manufacturing thicker geotextiles. However, a combination of a thin

granular blanket covering the permeable geosynthetic might be an apt alternative. (3)

y Use of non-woven geosynthetics having good permeability can be an alternative as a

drainage element with tensile reinforcement for reinforced soil walls and slopes with

backfills having higher fines. However clogging of non-woven geosynthetic and

attainment of wet soil conditions along soil±geosynthetic interface can inhibit its

performance as a drainage system. This can be overcome by providing a thin sand

cushion surrounding the geosynthetic layer. The provision of sand cushion along the

geosynthetic layer further enhances the in-plane permeability of soil±geosynthetic

system and may further reduce the chances of geosynthetic clogging with fines.(1)



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8. REFERENCES

1. Ghosh, C and Yasuhara, K (2004); ³Clogging and flow characteristics of a

geosynthetic drain confined in soils undergoing consolidation´, Geosyntheics

International, 11(1), 19-34.2. Hewayde, E; Naggar,H El and Khorshid,N (2005); ³ Reinforced lime columns: A new

technique for heave control´, Ground Improvement, 9(2), 79-87.

3. Raisinghani, D.V and Viswanadham, B.V.S (2010); ³Evaluation of permeability

characteristics of a geosynthetic-reinforced soil through laboratory tests´, Geotextiles

and Geomembranes, 28, 579-588.

4. Rao, S.M and Thyagaraj, T (2003); ³Lime slurry stabilization of an expansive soil´,

Geotechnical Engineering, 156(3), 139-146.

5. Rogers, C.D.F;Glendinning, S and Holt, C.C(2000); ³ Slope stabilization using lime

piles ³, Ground Improvement, 4, 165-176.

6. Sivapullaiah, P.V; Sridhran, A and Raju, K.V.B (2000); ³ Role of amount and type of

clay in the lime stabilization of soils´, Ground Improvement, 4, 37-45.

7. Wilkinson, A; Haque, A; Kodikara, J; Adamson, A and Christe, D(2010);

³Improvement on problematic soils by lime slurry pressure injection´, Journal of

Geotechnical and Geoenvironmental Engineering, 36(10), 1459-1468.

8. Yamanouchi, T; Miura, N; Matsubayashi, N and Fukuda, N (1982); ³Soil

improvement with quicklime and filter fabric´, Geotechnical engineering, 108(7),953-

955



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