Geotextile-reinforced Embankments On Soft Clays

8/2/2019 Geotextile-reinforced Embankments On Soft Clays

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71GEOSYNTHETICS INTERNATIONAL S 1999, VOL. 6, NO. 2

Technical Paper by A. Edinliler and E. Gler

GEOTEXTILE-REINFORCED EMBANKMENTS ONSOFT CLAYS - EFFECTS OF A FOUNDATION SOILCRUST STRENGTHENED BY LIME DIFFUSION

ABSTRACT: This paper presents the results of a study of the effects of a lime crust inthe foundation soil, obtained by lime diffusion, on the performance of nonwoven geotex-

tile-reinforced embankments. The study consists of laboratory model tests to simulate fail-ure mechanisms during the construction and lifetime of embankments. A 1/100-scalemodel of the embankment was constructed. Lime was spread over the foundation soils to

increase the shear strength of the soil through lime diffusion. In the laboratory experiments,vertical and horizontal deformations of the geotextile were recorded. Spreading of lime re-

duced the water content of the clay in the crust layer. It was observed that crust formationusing lime diffusion increased the shear strength of the foundation soil thereby allowing the

soil to carry larger loads. The soil was capable of carrying loads up to five times greaterthan that of the untreated soil. It was found that the shear strength increase is dependenton the quantity of lime added, temperature, and curing time.

KEYWORDS: Reinforced embankment, Geosynthetic, Geotextile, Lime diffusion,Soft clay, Shear strength.

AUTHORS: A. Edinliler, Projects Technical Coordinator, International Union of

Local Authorities-Section for the Eastern Mediterranean and Middle East Region

(IULA-EMME), Sultanahmet, Yerebatan Cad. 2, 34400 Istanbul, Turkey, Telephone:90/212-511-1010, Telefax: 90/212-519-0060; and E. Gler, Professor, Department of

Civil Engineering, Bogazii University, 80815 Bebek, Istanbul, Turkey, Telephone:90/212-263-1540/1452, Telefax: 090/212-287-2463, E-mail: [email protected].

PUBLICATION: Geosynthetics International is published by the Industrial FabricsAssociation International, 1801 County Road B West, Roseville, Minnesota

55113-4061, USA, Telephone: 1/651-222-2508, Telefax: 1/651-631-9334.Geosynthetics International is registered under ISSN 1072-6349.

DATES: Original manuscript received 15 January 1998, revised version received 18February 1999and accepted 20 February 1999. Discussion open until 1 November 1999.

REFERENCE: Edinliler, A. and Gler, E., 1999, Geotextile-Reinforced

Embankments on Soft Clays - Effects of a Foundation Soil Crust Strengthened by LimeDiffusion, Geosynthetics International, Vol. 6, No. 2, pp. 71-91.


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EDINLILER & GLER D Embankments on Soft Clay - Lime Diffusion Strengthened Foundation

72 GEOSYNTHETICS INTERNATIONAL S 1999, VOL. 6, NO. 2

1 INTRODUCTION

The safe construction and operation of embankments over soft foundations are stillmajor problems for engineers, despite the widespread use of reinforcing materials suchas geotextiles. Soft foundations are usually characterized by a high water content,

which appreciably decreases their load carrying capacity. The soft soil on which theem-bankment is constructed must have sufficient strength to support the weight of the em-

bankment and any live loads likely to occur. In addition, the soft soil must also besufficiently strong to carry construction equipment.

There are a number of available methods to increase the bearing capacity of softfoundation soils. Among these, reinforcement materials such as geotextiles and geo-grids are widely used. These reinforcing materials can significantly improve the perfor-

mance of the foundation and increase the factor of safety.A widely used alternative method for soil improvement is the addition of various

chemical agents. Lime has been found to be a good stabilizing agent. The reaction of

lime with the soil is dependent on the type of soil and environmental factors, such astemperature and humidity. The reaction of lime with soil is also strongly dependent on

time. The time interval that elapses while lime is left to diffuse within the soil is calledthe curing time. Also, lime stabilization reduces the water content and increases the

strength of the soil.For the current study, the combination of lime stabilization in addition to geosyn-

thetic reinforcement was investigated. In small-scale models, the soft foundation soilwas modeled using kaolin clay, and the nonwoven geotextile reinforcement was placedbetween the embankment and the foundation. Lime was spread over the foundation soil

to increase the shear strength of the surface. The effects of lime stabilization with andwithout geotextile reinforcement were investigated.

2 EFFECT OF SURFACE CRUST ON REINFORCED EMBANKMENTS

2.1 Background

Indraratna et al. (1991) examined the performance of a test embankment that wasconstructed on soft marine clays. The embankment was built over a soft silty clay layer,

which had a weathered crust layer. Using finite element analyses, Indraratna et al.(1991) found that the presence of the crust beneath the embankment resulted in greater

resistance to lateral displacements, thus allowing greater embankment heights to beachieved before failure. Indraratna et al. (1991) concluded that this result would en-courage the use of chemical additives for surface stabilization.

Based on finite element analyses, Rowe and Mylleville (1990) discussed the effectof a higher strength surface crust. The soft clay foundation was modeled using an un-

drained shear strength and undrained modulus that increased linearly with depth froma given surface value. Geosynthetic reinforcement was placed at the clay surface. Rowe

and Mylleville (1990) examined numerical results for undrained strength profiles withand without a high strength surface crust.

Based on the presence of the higher strength crust, Rowe and Mylleville (1990)

made the following observations:


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S the embankment fill thickness increases;

S the maximum strain in the geosynthetic reduces; and

S the magnitude of maximum shear strains in the foundation soil at failure reduces.

Rowe and Mylleville (1990) concluded that a surface crust ,which commonly overlays

soft cohesive deposits, improves the performance of embankments.Using limit equilibrium analyses, Michalowski (1992) investigated the bearing ca-

pacity of cohesive soils under embankments, for cases of strength increase with depth

and a strong surface crust. Michalowski (1992) concluded that the strength increase

with depth influences embankment reinforcement. Michalowski (1992) also stated that

the effect of embankment reinforcement on the bearing capacity of cohesive soils with

a strong crust is not pronounced.A finite element study by Mylleville and Rowe (1991) considered the effects of geo-

synthetic modulus on the behavior of reinforced embankments over soft brittle clay de-posits with and without a higher strength surface crust. Mylleville and Rowe (1991)

suggested that the modulus of the geosynthetic had very little effect on the magnitudeof calculated foundation soil shear strains. Also, Mylleville and Rowe (1991) noted

that, for soft brittle soils with a high strength surface crust, the effect of the crust domi-nates, even in the presence of a very high modulus geosynthetic.

Using a finite element model, Humphrey and Holtz (1989) showed that the proper-

ties and the thickness of a surface crust can significantly influence reinforced embank-ment behavior. They also concluded that crust compressibility is an important factor.

Humphrey and Holtz (1989) examined the following factors:

S The crust strength has a large effect on displacements and embankment height atfailure. The reinforcement becomes more effective as the crust strength increases.

The crust strength has a large effect on failure height of both reinforced and unrein-

forced embankments. It was also found that the width of the embankment and the

overall foundation thickness have comparably smaller influences compared to the

effect of crust strength. Finite element analyses indicated that there is no appreciablerelationship between the reinforcement tensile load at failure and crust strength.

S The effect ofcrust thickness was assessed by comparing displacements at the toe of

reinforced and unreinforced embankments on foundations with and without a crust.The study indicated that the failure height decreases as the crust thickness decreases

and that, for a given embankment height, the reinforcement tensile load increasesas the crust thickness decreases.

S For a given embankment height, the maximum tensile load in the reinforcement in-

creases as the crust compressibility increases. Conversely, as the crust compressibil-ity decreases the reinforcement tensile load increases.


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3 PHYSICAL MODELS

3.1 Laboratory Modeling

3.1.1 General

An objective of this study was to observe the effects of lime placed on the surfaceof soft clay soil layers. Based on undrained short-term analyses, a laboratory model was

constructed and experiments conducted.A 1/100-scale model of an embankment was constructed for laboratory modeling

(Figure 1). The tank, which served as a container for the clay foundation, is rectangular

with an internal length of 0.75 m, internal width of 0.38 m, and internal depth of 0.50m. All three sides of the tank are glass to enable the observation of soil movements

(Edinliler 1995).

3.1.2 Procedure

A kaolin clay was used as the foundation soil (see Tables 1 and 2 for the physical

and chemical properties of the clay). During preparation of the foundation soil, the ka-olin clay was mixed at a water content of 50%, which was approximately 1.5 times the

liquid limit of the soil. Before filling the tank with the foundation soil, the inner glasssurfaces of the tank were greased to ensure frictionless surfaces. At the bottom of thetank, a 50 mm thick sand layer was placed to drain excess water and air bubbles in the

foundation soil. The tank was then filled with kaolin clay to a depth of 400 mm.For the lime crust to form over the foundation soil, unslaked quicklime was spread

over the surface of the kaolin. The amount of lime varied between 701 and 1,754 g/m2.Time periods ranging from one to three months were allowed to pass for the diffusionof lime toward the lower layers of the foundation soil. This time lapse was necessary

for the clay and lime to react and increase the shear strength of the surface soil. Beforeembankment construction, vane shear tests were carried out at four pointson thesurface

Figure 1. Schematic cross section of the embankment laboratory model.

Surcharge load, q

Nonwoven geotextile

Crust

0.75 m

0.40 m

0.10 m

Kaolin foundation soil


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and at various depths to measure the change in the foundation soil shear strength withthe quantity of lime and the elapsed time.

A Typar 3207 geotextile (Table 3) was used as the reinforcement. This low strengthgeotextile was chosen because the model-scale stresses were expected to be low. Wiresattached to the surface of the geotextile were connected to three horizontal deformation

gauges to measure the horizontal deformations of the reinforcement layer (Figure 2).

Table 1. Geotechnical properties of the kaolin clay.

Physical property

dry

(kN/m3)

wopt(%)

Gs ActivitywL

(%)

wP(%)

PI

(%)

Value 13.5 24.0 2.6 0.37 32.5 22.0 10.5

Note: dry = dryunitweight, wopt = optimum watercontent, Gs = specific gravity,wL = liquid limit,wP =plasticlimit, and PI= plasticity index.

Table 2. Chemical and mineral properties of the kaolin clay.

Chemical analysis (%)

SiO2 Al2O3 Fe2O3 TiO2 CaO MgO K2O Na2O SO3

78.0 15.0 0.5 0.3 0.2 0.1 0.1 0.1 0.5

Mineral content (%)

Kaolin Potassium feldspar Sodium feldspar Free quartz Others

37.2 0.6 0.8 59.7 1.6

Table 3. Measured properties of the nonwoven geotextile reinforcement.

Physical property Test method ValueMass per unit area -- 68 g/m2

Thickness under 2 kPa (average value) -- 0.36 mm

Strength (200 mm wide specimen)

Elongation at maximum load

3.3 kN/m

35%

Tensile strength at 5% elongation

Tensile modulus

BS 69061.5 kN/m

30 kN/m

Grab strength

(200 mm wide specimen)ASTM D 4632 310 kN

Elongation at maximum load > 60%

Burst strength ASTM D 3786 580 kPa

Polymer type -- Polypropylene

Polymer specific gravity -- 0.91

Melting point -- 165_C

Service temperature range -- --40 to 100_C

Fibre diameter -- 40 to 55 m

Type of fibre bonding -- Thermal bonding


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Figure 2. Schematic plan view of a model showing the location of the horizontal

deformation gauges and the nonwoven geotextile.

Wire 1

Wire 2

Wire 3

Zone 1

Zone 2

0 55 mm 125 mm 225 mm 350 mm

Nonwoven geotextile

Fine-grained sand, with the properties shown in Table 4, was used to construct the

modelembankments. A modelembankment height of0.10 m wasselected. Thegeometryof the model embankment is given in Figure 1. Due to symmetry about the vertical axis,onlyone halfof a fullsize embankment wasconsidered. Duringthetests, theembankment

surcharge load was applied by placing iron plates on the crest of the embankment.During each loading stage, the settlement of the original ground surface was re-

corded by measurements taken through the glass sides of the tank. After the loadingstage, specimens were taken from three locations at different foundation depths: sur-

face, middle, and bottom. The water content of these specimens was then measured.

3.1.3 Models

Unreinforced and reinforced models with and without a crust were constructed to

observe the effects of the crust layer on the bearing capacity of the foundation (Edinlil-er 1995). To compare the effect of lime diffusion, three control tests were conducted.

Table 4. Properties of the uniform-size fine sand (embankment fill).

Physical property Value

Specific gravity, Gs 2.67

Coefficient of permeability, k 3.5 10--5 m/s

Dry unit weight, dry 17.8 kN/m3

Void ratio, e 0.45

Fraction passing:

2.00 mm

0.425 mm

0.075 mm

100%

87%

5%


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In order to determine the advantages of foundation improvement techniques, an un-treated model was tested. In the untreated and unreinforced model, the embankment

was constructed directly over the foundation soil (Model 1).To determine the effect of geotextile reinforcement, a model without lime treatment

was constructed (Model 2). Once the foundation soil was prepared, the geotextile was

placed on top, and the embankment was then constructed on the geotextile. This modelwas also used as a basis forcomparison ofthe improvement caused by the lime diffusion

treatment.In addition to the experiments conducted with a lime crust, a separate experiment

was conducted with a desiccated crust (Model 3). This model was not treated with lime.

A crust was formed simply by allowing the foundation soil to stand for a period of one

month. A crust formed because of the loss of water by evaporation from the surface,

and the reinforced embankment was then constructed over the desiccated crust.

Seven tests were designed to investigate the effects of several parameters. Models4, 5, 6, 7, 8, 9, and 10 had lime diffusion treated crusts in addition to geosynthetic rein-

forcement. In each experiment, the quantity of unslaked quicklime (i.e. lime) that wasspread over the soil was chosen as 200, 300, or 500 grams per surface area, which corre-

spondsto 701, 1,052, and 1,754 g/m2, respectively. The following amounts of lime wereused for the different models: 1,754 g/m2 for Models 4 and 7; 1,052 g/m2 for Models

5 and 8; and 701 g/m2 for Models 6 and 9. The tests were grouped into two sets: Set Imodel tests (Models 4, 5, and6) wereconducted in January whenthe average laboratorytemperature was 17_C; and Set II model tests (Models 7, 8, and 9) were conducted in

July when the average laboratory temperature was 23_C.Another parameter was the elapsed time before embankment construction over the

treated foundation soil. The embankment was not constructed immediately after thelime was spread over the foundation soil; for Models 4, 5, 6, 7, 8, and 9 a one-monthperiod elapsed and for Model 10 a three-month period elapsed before the geotextile was

placed and the embankment constructed. For Model 10, 1,754 g/m2 of lime was used.A summary of test conditions and measured vane shear strength values are given in

Table 5. The models were loaded until the critical load was reached; the critical loadwas defined as the load that produces a sudden increase in the foundation soil displace-

ment. At each loading stage, horizontal and vertical deformations were recorded. Tomaintain undrained loading of the foundation soil, the time interval between the ap-plication of load increments was approximately 10 minutes, which was a sufficient time

to obtain steady readings from the deformation gauges.

4 RESULTS

4.1 Models Without a Lime Treated Crust (Models 1 and 2)

For Model 1, the embankment was unreinforced and, during fill placement, exces-

sive vertical settlements occurred. Because of the embankment self-weight, sinking ofthe foundation soil occurred even before subsequent load increments were applied (Fig-

ure 3). The first load increment was the self-weight of the embankment followed bytwosurcharge increments corresponding to 1.47 and 3.17 kPa, respectively, which induced

catastrophic failure.


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Table 5. Shear strength values for the embankment models.

Model CrustAmount

of lime

Curing

time

Curing

temperature

Shear strength valueswith depth (kPa)

number type(g/m2) (month) (_C) 25 mm 50 mm 75 mm 135 mm

1 No crust -- -- 17 -- -- -- --

2 No crust -- -- 17 -- -- -- --

3Desiccated

crust-- 1 17 2.4 1.6 1.7 2.4

Set I 4 Lime crust 1,754 1 17 3.0 2.5 2.1 3.1

5 Lime crust 1,052 1 17 4.7 2.3 3.1 3.6

6 Lime crust 701 1 17 5.6 4.3 3.3 3.8

Set II 7 Lime crust 1,754 1 23 4.4 3.5 2.9 3.9

8 Lime crust 1,052 1 23 4.9 3.8 2..4 3.3

9 Lime crust 701 1 23 5.4 2.3 3.6 4.8

10 Lime crust 1,754 3 17 6.5 4.2 5.1 6.3

Figure 3. Comparison between reinforced (Model 2) and unreinforced (Model 1) model

embankments.

100

0

---100

---200

0 200 400 600 800

Distance from embankment center (mm)

Settlementandheave(mm)

Unreinforced (Model 1)

(Fill weight + surcharge, q)

Reinforced (Model 2)(Fill weight + surcharge, q)

q = 1.47 kPa

q = 3.17 kPa

q = 1.47 kPa

q = 3.17 kPaq = 0 kPa

For Model 2, a geotextile was placed over the foundation soil and the same loadingprocedure as for Model 1 was applied. For this case, it was observed that the use of geo-

textile reinforcement results in a significant strength improvement during fill place-

ment. The embankment fill did not cause any vertical settlement; however, only twoload surcharges, q = 1.47 and 3.17 kPa, could be applied before failure.


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The measured foundation soil settlement and heave values for Models 1 and 2 areplotted in Figure 3. It can be clearly seen that the inclusion of geotextile reinforcement

leads to a considerable reduction in the settlement and, consequently, heave was alsodecreased. It was noted that, for this case, the reinforcement reduces settlements from113.5 to 37.2 mm, at a location of 100 mm from the center of the embankment. Thus,

the reinforcement resulted in a 30% reduction in surface settlement. This clearly reflectsthe benefit of using geotextile reinforcement in embankment model stabilization.

The horizontal geotextile elongations measured by the three dial gauges were re-corded during the loading stages. The strain values at two intervals can be calculated

using the measured horizontal elongation of the geotextiles at 50, 125, and 225 mm in-

tervals from the center of the embankment. In Table 6, the tensile stressin the geotextile

for Model 2 was calculated based on the measured reinforcement strains. The measured

tensile load, T, is less than the tensile load at 5% elongation (T= 1.5 kN/m), which was

provided by the manufacturer.

4.2 Model with a Desiccated Crust (Model 3)

For Model 3, a desiccated crust was allowed to develop over a period of one month.The effect of the desiccated crust was to improve the strength of the clay foundation,

especially in the upper sections. In Figures 4a and 4b, it can be observed that there isa high shear strength in the upper crust region, which decreases from a surface valueof 2.4 kPa at a depth of 25 mm to a minimum value of 1.6 kPa under the crust at a depth

of 50 mm from the surface. It then increases to 2.4 kPa at a depth of 135 mm. Belowthis depth, the shear strength is assumed to increase linearly with depth.

The tensile load in the reinforcement was calculated usingthe strain values calculatedfrom the elongation in the wires connected to the dial gauges and the geotextile. Figure

5 presents the reinforcement tensile load at each loading stage. As can be seen in Figure

5, thetensile load inthe reinforcement, at thelocation where largersoil settlements occur

(Zone 1), is clearly larger than the tensile load in the reinforcement in Zone 2. The maxi-

mum reinforcement tensile load value of 2.0 kN/m was measured during the applicationof the7.3 kPasurcharge load, at which point a circular slip failure through thefoundation

soil was observed (see Table 7 for the collapse/failure loads of each model).

Table 6. Tensile load in the nonwoven geotextile reinforcement (Model 2).

Surcharge load, q

(kPa)

J

(kN/m)

(dimensionless)

T= J

(kN/m)

1.47 300.043 (1)

0.012 (2)

1.29

0.36

3.17 300.049 (1)

0.029 (2)

1.46

0.87

Notes: Superscript (1) and (2) = zone numbers shown in Figure 2a. J= tensile stiffness, = tensile strain, T= tensile load.


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6

4

2

0

0 50 100 150 200

Depth from foundation surface (mm)

Shearstrength(kPa)

250 300 350

Model 7

Model 8

Model 9

Model 10

Figure 4. Shear strength profiles for models: (a) Models 3, 4, 5, and 6; (b) Models 7, 8, 9,

and 10.

Model 3 (desiccated crust)

Model 4

Model 5

Model 6

6

4

2

0

Shearstrength(kPa)

(a)

(b)

Table 7. Collapse load for each model.

Model number

1 2 3 4 5 6 7 8 9 10

Model collapse load (kPa) 3.17 3.17 15.3 17.0 18.1 17.4 19.8 22.1 41.2 47.7

The variation of vertical settlement and heave values for Model 3 (desiccated crust

model) are plotted in Figure 6. A total of 10 loading stages were applied. In the first twoloading stages, no settlement was observed. Appreciable settlement begins at an ap-


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4

2

0

0 4 8 12 16Vertical foundation pressure (kPa)

Tensileload(kN/m)

20

Figure 5. Reinforcement tensile load versus vertical foundation pressure for Model 3(desiccated crust).

Zone 1 (see Figure 2)

Zone 2 (see Figure 2)

80

40

0

0 200 400 600 800


Settlementandhea

ve(mm)

Foundation pressure = 4.92 kPa

---40

---80








Figure 6. Settlement and heave values for Model 3 (desiccated crust).


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plied cumulative foundation pressure = 4.92 kPa. A maximum settlement of 73 mm oc-curs at the last loading stage (foundation pressure = 17.7 kPa). Heave begins at a

foundation pressure = 13.2 kPa, and the maximum heave is 49 mm at the last loadingstage. It can be observed that a sudden increase in vertical settlement occurred at afoundation pressure = 15.3 kPa. Accordingly, a foundation pressure = 15.3 kPa wascon-

sidered to be the critical loading pressure.If the magnitudes of the loads are examined, the data shows that the desiccated crust

considerably reduces settlements. For the models with no crust (Models 1 and 2), theembankments failed at loads of approximately 3.17 kPa; the embankment constructed

over the soft soil with a desiccated crust did not show any settlement at these loads. In

fact, the embankment constructed over the foundation with a desiccated crust failed at

a load value of approximately 15.3kPa, an almost five-fold increase in the load carrying

capacity as compared to the non-crust model. As a result, the presence of a desiccated

crust beneath the embankment resisted the vertical displacements and increased theload carrying capacity.

This increase in load carrying capacity may be attributed to the loss of water by evap-oration. The water content fell from 50 to 35% at a depth of 50 mm, leading to an in-

crease in shear strength of the soil, which was measured using vane shear tests.

4.3 Models with a Lime Crust

Models 4, 5, 6, 7, 8, 9, and 10 were constructed to examine the effects ofa lime diffu-

sion crust on the stability of the model embankments.

4.3.1 Models with a Lime Treated Surface at 17_C

The shear strength profiles for this group of models (Models 4,5, and 6; Set I) are

given in Figure 4a. In Model 4 (1,754 g/m2 of lime), the soil shear strength at a depthof 25 mm is 3.0 kPa. As in the case with a desiccated crust, the shear strength decreases

with depth to a minimum value of 2.1 kPa at 75 mm, then, increases to 3.0kPa at a depthof 135 mm. Below this depth it is assumed that the shear strength increases linearly with

depth. It is evident in the remaining experiments of Set I that the soil shear strength in-creases in the upper region of the foundation soil due to the addition of lime (Figure 4a).

In Model 5 (1,052 g/m2 of lime), the corresponding shear strength values with depth

are 4.7, 3.1, and 3.6 kPa, atdepthsof 25, 75, and 135 mm, respectively. ForModel 6 (701g/m2 of lime), the undrained shear strength values at depths of 25, 75, and 135 mm are

5.6, 3.3, and 3.8 kPa, respectively. In the surface layer, the undrained shear strength de-creases with increasing lime content; however, this trend is not as obvious in the lowerlayers. This unexpected trend in shear strength values can be attributed to the formation

of a thin crust over the lime-treated layer that prevents desiccation and, hence, reducesthe water content. The water content values are given in Table 8.


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Table 8. Water content values for Set I embankment models.

Depth from Water content, w (%)foundation surface

(mm) Model 4 Model 5 Model 6

50 37.3 33.2 32.8

150 38.1 34.2 33.5

300 39.4 35.3 34.7

As observed in Table 8, the water content in the foundation soil decreases at the sur-

face because of the lime added to the surface. This decreased water content leads to anincrease in the shear strength and the load carrying capacity.

Tensile load in the geotextile reinforcement for Set I models (Models 4, 5, and 6which have a lime crust formed at a temperature of 17_C) are given in Figures 7a, 7b,and 7c, respectively. For these models with a lime crust, the tensile loads are similar

to the tensile loads for Model 3 (desiccated crust at a temperature of 17_C): the maxi-mum tensile loads were measured at locations where the failure surface through the

foundation soil intersected the reinforcement layer. In Figure 7a (Model 4), the tensileload at the intersection of the observed failure surface reaches a maximum value (2.3kN/m) at a vertical pressure of 18.5 kPa. The corresponding tensile load for Model 5

(Figure 7b) reaches the maximum value (2.3 kN/m) during the application of a verticalpressure of 16.0 kPa. Comparing Figures 7a and 7b, it is observed that there is a slight

difference in the plot of tensile loads versus applied foundation surface pressure be-tween Models 4 and 5.

Figure 7c illustrates the tensile load values for Model 6, which has the minimumamount of lime. The tensile loads at the critical slip surface are lower than those forModels 4 and 5. For Model 6, the tensile load values even at the critical applied load

are smaller than the tensile load value, T = 1.5 kN/m, which was supplied by the

manufacturer for the geotextile stress at 5% strain.Therefore, for the cases considered, it has been demonstrated that for soft clays with

a higher strength surface crust, the effect of the crust dominates even if a geotextile isused. In addition, the horizontal displacements were reduced by using a higher strength

surface crust.For Model 4, the critical foundation pressure occurs when the applied embankment

load reaches 17.0 kPa. If the corresponding stages for Models 5 and 6 are compared(Figure 8a) to Model 4, it can be seen that even before the other embankments displayed

heave in the unloaded zone, Model 4 had failed. This demonstrates that the foundationof Model 4 was weaker than the foundations of Models 5 and 6. This is in accordancewith the reduced shear strength in the surface region due to the addition of excess lime,

which, as mentioned before, prevents a reduction in water content. The same trend canbe observed for Models 5 (Figure 8b) and 6 (Figure 8c). Therefore, the increase in crust

strength is inversely proportional to the lime content.


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4

2

0

Tensileload(kN/m)

Zone 2

Zone 1

Figure 7. Tensile load versus vertical foundation pressure: (a) Model 4; (b) Model 5;

(c) Model 6.

4

2

0

Tensileload(kN/m)

Zone 2

Zone 1

4

2

0

4 8 12 16Vertical foundation pressure (kPa)

Tensileload(kN/m)

20

Zone 1

Zone 2

24

(a)

(b)

(c)

Model 4

Model 5

Model 6


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100

0

---100

0 200 400 600 800


Settlementandheave(mm

)

Figure 8. Comparison of foundation settlement and heave profiles for Set I models at the

collapse load of the selected model: (a) Model 4 collapse load; (b) Model 5 collapse load; (c)Model 6 collapse load.

200

0

---200Settlementandheave(mm

)

200

0

---200Settlementandheave(mm)

Model 4

Model 5

Model 6

Model 5

Model 4

Model 6

Model 6

Model 4

Model 5

(a)

(b)

(c)





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4.3.2 Models with a Lime Treated Surface at 23_C

The shear strength profiles for Set II models (Models 7, 8, and 9) are given in Figure4b. As seen with Models 4, 5, and 6, the effect of the lime addition on the shear strengthis the same. The shear strength is relatively high at the foundation surface due to lime

diffusion. The undrained shear strength decreases to a minimum value and then in-creases linearly with depth. The negative influence of the increased amount of lime on

the shear strength is clearly evident in Figure 4b. There are significant differences inthe measured soil shear strengths for models with a lime crust depending on the amount

of lime and the curing temperature.The same argument can be applied for Model 7 when considering the amount of

settlement. The critical load for Model 7 occurs at a foundation pressure = 19.8 kPa.

Model 7 failed before Models8 and 9 showed any significant amount of settlement (Fig-ure 9a). The critical load for Model 8 occurs at a foundation pressure = 22.1 kPa. How-

100

0


100

0


Model 7

Model 8

Model 9

Model 8

Model 9

(a)

(b)

Figure 9. Comparison of foundation settlement and heave profiles for Set II models at the

collapse load of the selected model: (a) Model 7 collapse load; (b) Model 8 collapse load.

0 200 400 600 800





17/21



ever, Model 7 failed before this load level could be achieved (Figure 9b). The criticalload for Model 8 occurs at a foundation pressure of 41.2 kPa.

The tensile load values in the reinforcement for Set II (Models 7, 8, and 9) are givenin Figures 10a, 10b, and 10c. It can be seen that the tensile load values for Set II modelsare lower than the tensile load values for Set I models. As a result, the tensile load in

the geotextile decreases as the crust strength increases. It was found that reinforcementstrains are also sensitive to small changes in crust strength.

4.4 The Effect of Temperature

The only difference between the Set I and Set II models was the temperature. SetI was conducted during the winter at an average temperature of 17_C and Set II was

conducted during the summer at an average temperature of 23_C. As a result, Set IImodels performed better than Set I models. As indicated by the strength profiles foreach model shown in Figures 4a and 4b, there is a significant difference in bearing ca-

pacity. As evident in the model deformation profiles shown in Figures 11a, 11b, and11c,the settlement values are lower for high strength lime crust models. Higher crust

strengths occur because of the increased diffusion and reaction of lime in the soil withincreasing temperature.

4.5 The Effect of Curing

An analysis was conducted to investigate the effect of curing time on embankmentstabilization. A curing time of three months was chosen. The amount of lime used was

the same as for Models 4 (cured at 17_C) and 7 (cured at 23_C). The deformation pro-files for each model at collapse are shown in Figure 12. As evident in Figure 12, Model10 with a three-month curing time has lower settlement values than Models 4 and 7,

both of which have a curing time of one month.

5 CONCLUSIONS

The benefit of embankment reinforcement using geotextiles has been identified inthe literature. The purpose of the current study was to investigate the quantitative bene-

fits of nonwoven geotextile reinforcement in combination with lime stabilization.Model test results show that the inclusion of geotextile reinforcement results in a con-

siderable reduction in settlement and heave. The addition of geotextile reinforcement re-sulted in a 30%reduction in surface settlement when compared to the unreinforced case.

It was further found that a lime stabilized crust, created by spreading lime over the

soft clay foundation, results in a significant improvement of the load carrying andsettlement behaviour of geotextile-reinforced embankments on soft soil. It has been

shown experimentally that the tensile load in the geotextile decreases as the crust

strength increases and that the geotextile reinforcement strains are sensitive to smallchanges in crust strength.

The degree of improvement obtained through lime stabilization depends on manyfactors. In the current study, it was determined that lime diffusion stabilization depends

on the amount of lime introduced, temperature, curing time, and type of soil.


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Tensileload(kN/m) Zone 1Zone 2

Figure 10. Tensile load versus vertical foundation pressure: (a) Model 7; (b) Model 8;

(c) Model 9.

Tensileload(kN/m)

Vertical foundation pressure (kPa)

Tensileload(kN/m)

(a)

(b)

(c)

10 20 30 40 50

2

1

0

3

2

1

0

2

1

0

Zone 1

Zone 2

Zone 1

Zone 2

8 12 16 20 24

8 12 16 20 24


19/21



50

0

---100

0 200 400 600 800


Settlementandheave(m

m)

Figure 11. Comparison of foundation settlement and heave profiles for Set I and Set II

models at the collapse load of the selected model: (a) Model 4 collapse load; (b) Model 5collapse load; (c) Model 6 collapse load.

100

0

---100Settlementandheave(m

m)

100

0

---100Settlementandheave(mm

)

Model 4

Model 7

(a)

(b)

(c)

---50

Model 5

Model 8



Model 6, foundation pressure = 17.4 kPaModel 9, foundation pressure = 41.2 kPa


20/21



Figure 12. A comparison of settlement and heave profiles for experiments carried out at

different temperatures (Models 4 and 7) and curing times (Model 10).

100

0

---100

0 200 400 600 800


Settlementandheave(

mm)

Model 10 (17_C, 3 months, foundation pressure = 47.7 kPa)

Model 7 (23_

C, 1 month, foundation pressure = 19.8 kPa)Model 4 (17_C, 1 month, foundation pressure = 17.0 kPa)

The amount of lime spread over the soil should be carefully estimated because there

is an optimum lime content. A lime content in excess of the optimum value will reducethe maximum benefit that can be derived from the lime stabilization technique.

A 1.2 fold improvement in Models 7, 8, and 9 wasobtained by increasing the temper-ature from 17 to 23_C, compared to Models 4, 5, and 6, respectively. A practical conse-

quence of this observation is that greater improvement can be obtained if the techniqueis applied during summer months. It can be stated that the increase in temperature in-

creases the effectiveness of the lime diffusion technique. Also, as evident from the ex-perimental results, increasing the curing time positively affects lime stabilization.

In conclusion it can be stated that the lime diffusion stabilization technique can besuccessively used in combination with geosynthetic reinforcement to improve founda-tion soil behavior for embankments constructed on soft clay soils.

REFERENCES

ASTM D 4632, Standard Test Method for Breaking Load and Elongation of Geotex-

tiles (Grab Method), American Society for Testing and Materials, West Consho-

hocken, Pennsylvania, USA.

ASTM D 3786, Test Method for Hydraulic Bursting Strength of Knitted Goods and

Nonwoven Fabrics: Diaphragm Bursting Strength Tester Method, American Societyfor Testing and Materials, West Conshohocken, Pennsylvania, USA.

BS 6906, Methods of Test for Geotextiles: Determination of Tensile Properties Usinga Wide-Width Strip, British Standards Institute, London, UK.


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Edinliler, A., 1995, Effect of Crust for Embankments Constructed on Soft Clays,Ph.D. Dissertation, Department of Civil Engineering, Bogazii University, Istanbul,

Turkey, 167 p.

Humphrey, D.N. and Holtz, R.D., 1989, Effect of Surface Crust on Reinforced Em-

bankment, Geosynthetics 89, IFAI, Vol. 1, San Diego, California, USA, February

1989, pp. 136-147.

Indraratna, B., Balasubramaniam, A.S. and Balackandran, S., 1991,Performance ofTest Embankment Constructed to Failure on Soft Marine Clays, Journal of Geo-

technical Engineering, Vol. 118, No. 11, pp. 12-33.

Michalowski, R.L., 1992, Bearing Capacity of Nonhomogeneous Cohesive Soils Un-der Embankments, Journal of Geotechnical Engineering, Vol. 118, No. 7, pp.

1098-1119.

Mylleville, B.L.J. and Rowe, R.K., 1991, On the Design of Reinforced Embankmentson Soft Brittle Clays, Geosynthetics 91, IFAI, Vol. 1, Atlanta, Georgia, USA, Febru-

ary 1991, pp. 395-408.Rowe, R.K. and Mylleville, B.L.J., 1990, Implications of Adopting an Allowable Geo-

synthetic Strain in Estimating Stability, Proceedings of the Fourth International

Conference on Geotextiles, Geomembranes and Related Products, Balkema, Vol. 1,The Hague, Netherlands, May 1990, pp. 131-136.

NOTATIONS

Basic SI units are given in parentheses.

e = void ratio (dimensionless)

Gs = specific gravity (dimensionless)

J = tensile stiffness (N/m)

k = coefficient of permeability (m/s)

PI = plasticity index (%)

q = surcharge load (Pa)

T = tensile load (N/m)

w = water content (%)

wL = liquid limit (%)

wopt = optimum moisture content (%)

wP = plastic limit (%)

= tensile strain (dimensionless)

dry = dry unit weight of soil (N/m3)

Documents

Geotextile-reinforced Embankments On Soft Clays