GEOTEXTILE-REINFORCED EMBANKMENTS ON SOFT CLAYS - EFFECTS OF A FOUNDATION SOIL CRUST STRENGTHENED BY LIME DIFFUSION

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1 Technical Paper by A. Edinçliler and E. Güler GEOTEXTILE-REINFORCED EMBANKMENTS ON SOFT CLAYS - EFFECTS OF A FOUNDATION SOIL CRUST STRENGTHENED BY LIME DIFFUSION ABSTRACT: This paper presents the results of a study of the effects of a lime crust in the foundation soil, obtained by lime diffusion, on the performance of nonwoven geotextile-reinforced embankments. The study consists of laboratory model tests to simulate failure mechanisms during the construction and lifetime of embankments. A 1/1-scale model 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 reduced the water content of the clay in the crust layer. It was observed that crust formation using 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 greater than that of the untreated soil. It was found that the shear strength increase is dependent on the quantity of lime added, temperature, and curing time. KEYWORDS: Reinforced embankment, Geosynthetic, Geotextile, Lime diffusion, Soft clay, Shear strength. AUTHORS: A. Edinçliler, Projects Technical Coordinator, International Union of Local Authorities-Section for the Eastern Mediterranean and Middle East Region (IULA-EMME), Sultanahmet, Yerebatan Cad. 2, 344 Istanbul, Turkey, Telephone: 9/ , Telefax: 9/ ; and E. Güler, Professor, Department of Civil Engineering, Bogaziçi University, 8815 Bebek, Istanbul, Turkey, Telephone: 9/ /1452, Telefax: 9/ , eguler@boun.edu.tr. PUBLICATION: Geosynthetics International is published by the Industrial Fabrics Association International, 181 County Road B West, Roseville, Minnesota , USA, Telephone: 1/ , Telefax: 1/ Geosynthetics International is registered under ISSN DATES: Original manuscript received 15 January 1998, revised version received 18 February 1999 and accepted 2 February Discussion open until 1 November REFERENCE: Edinçliler, A. and Güler, E., 1999, Geotextile-Reinforced Embankments on Soft Clays - Effects of a Foundation Soil Crust Strengthened by Lime Diffusion, Geosynthetics International, Vol. 6, No. 2, pp

2 1 INTRODUCTION The safe construction and operation of embankments over soft foundations are still major problems for engineers, despite the widespread use of reinforcing materials such as geotextiles. Soft foundations are usually characterized by a high water content, which appreciably decreases their load carrying capacity. The soft soil on which the embankment is constructed must have sufficient strength to support the weight of the embankment and any live loads likely to occur. In addition, the soft soil must also be sufficiently strong to carry construction equipment. There are a number of available methods to increase the bearing capacity of soft foundation soils. Among these, reinforcement materials such as geotextiles and geogrids are widely used. These reinforcing materials can significantly improve the performance 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 as temperature 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 called the 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 geosynthetic reinforcement was investigated. In small-scale models, the soft foundation soil was modeled using kaolin clay, and the nonwoven geotextile reinforcement was placed between 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 and without 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 was constructed 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 be achieved before failure. Indraratna et al. (1991) concluded that this result would encourage the use of chemical additives for surface stabilization. Based on finite element analyses, Rowe andmylleville (199) discussed theeffect of a higher strength surface crust. The soft clay foundation was modeled using an undrained shear strength and undrained modulus that increased linearly with depth from a given surface value. Geosynthetic reinforcement was placed at the clay surface. Rowe and Mylleville (199) examined numerical results for undrained strength profiles with and without a high strength surface crust. Based on the presence of the higher strength crust, Rowe and Mylleville (199) made the following observations: 72

3 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 (199) 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 capacity 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 geosynthetic modulus on the behavior of reinforced embankments over soft brittle clay deposits 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 magnitude of 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 dominates, even in the presence of a very high modulus geosynthetic. Using a finite element model, Humphrey and Holtz (1989) showed that the properties and the thickness of a surface crust can significantly influence reinforced embankment 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 at failure. The reinforcement becomes more effective as the crust strength increases. The crust strength has a large effect on failure height of both reinforced and unreinforced 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 appreciable relationship between the reinforcement tensile load at failure and crust strength. S The effect of crust 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 increases as the crust thickness decreases. S For a given embankment height, the maximum tensile load in the reinforcement increases as the crust compressibility increases. Conversely, as the crust compressibility decreases the reinforcement tensile load increases. 73

4 3 PHYSICAL MODELS 3.1 Laboratory Modeling General An objective of this study was to observe the effects of lime placed on the surface of soft clay soil layers. Based on undrained short-term analyses, a laboratory model was constructed and experiments conducted. A 1/1-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.75 m, internal width of.38 m, and internal depth of.5 m. All three sides of the tank are glass to enable the observation of soil movements (Edinçliler 1995) 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 kaolin clay was mixed at a water content of 5%, which was approximately 1.5 times the liquid limit of the soil. Before filling the tank with the foundation soil, the inner glass surfaces of the tank were greased to ensure frictionless surfaces. At the bottom of the tank, a 5 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 4 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 71 and 1,754 g/m 2. Time periods ranging from one to three months were allowed to pass for the diffusion of 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. Before embankment construction, vane shear tests were carried out at four points on the surface.1 m Surcharge load, q Nonwoven geotextile Crust.4 m Kaolin foundation soil.75 m Figure 1. Schematic cross section of the embankment laboratory model. 74

5 and at various depths to measure the change in the foundation soil shear strength with the quantity of lime and the elapsed time. A Typar 327 geotextile (Table 3) was used as the reinforcement. This low strength geotextile was chosen because the model-scale stresses were expected to be low. Wires attached 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. γ dry (kn/m 3 ) w opt (%) G s Physical property Activity w L (%) w P (%) Value Note: γ dry = dry unit weight, w opt = optimum water content, G s = specific gravity,w L = liquid limit,w P = plastic limit, and PI = plasticity index. PI (%) Table 2. Chemical and mineral properties of the kaolin clay. Chemical analysis (%) SiO 2 Al 2 O 3 Fe 2 O 3 TiO 2 CaO MgO K 2 O Na 2 O SO Mineral content (%) Kaolin Potassium feldspar Sodium feldspar Free quartz Others Table 3. Measured properties of the nonwoven geotextile reinforcement. Physical property Test method Value Mass per unit area - 68 g/m 2 Thickness under 2 kpa (average value) -.36 mm Strength (2 mm wide specimen) 3.3 kn/m Elongation at maximum load 35% BS 696 Tensile strength at 5% elongation 1.5 kn/m Tensile modulus 3 kn/m Grab strength (2 mm wide specimen) ASTM D kn Elongation at maximum load > 6% Burst strength ASTM D kpa Polymer type - Polypropylene Polymer specific gravity -.91 Melting point - 165_C Service temperature range - -4 to 1_C Fibre diameter - 4 to 55 µm Type of fibre bonding - Thermal bonding 75

6 55 mm 125 mm 225 mm 35 mm Zone 1 Wire 1 Wire 2 Zone 2 Wire 3 Nonwoven geotextile Figure 2. Schematic plan view of a model showing the location of the horizontal deformation gauges and the nonwoven geotextile. Fine-grained sand, with the properties shown in Table 4, was used to construct the model embankments. Amodel embankment height of.1mwasselected. Thegeometry of the model embankment is given in Figure 1. Due to symmetry about the vertical axis, onlyonehalfofafullsizeembankment 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 recorded by measurements taken through the glass sides of the tank. After the loading stage, specimens were taken from three locations at different foundation depths: surface, middle, and bottom. The water content of these specimens was then measured 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 (Edinçliler 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, G s 2.67 Coefficient of permeability, k m/s Dry unit weight, γ dry 17.8 kn/m 3 Void ratio, e.45 Fraction passing: 2. mm.425 mm.75 mm 1% 87% 5% 76

7 In order to determine the advantages of foundation improvement techniques, an untreated 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 model was also used as a basis for comparison of the 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. Models 4, 5, 6, 7, 8, 9, and 1 had lime diffusion treated crusts in addition to geosynthetic reinforcement. In each experiment, the quantity of unslaked quicklime (i.e. lime) that was spread over the soil was chosen as 2, 3, or 5 grams per surface area, which corresponds to 71, 1,52, and 1,754 g/m 2, respectively. The following amounts of lime were used for the different models: 1,754 g/m 2 for Models 4 and 7; 1,52 g/m 2 for Models 5 and 8; and 71 g/m 2 for Models 6 and 9. The tests were grouped into two sets: Set I model tests (Models 4, 5, and 6) were conducted in January when the average laboratory temperature 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 the lime was spread over the foundation soil; for Models 4, 5, 6, 7, 8, and 9 a one-month period elapsed and for Model 1 a three-month period elapsed before the geotextile was placed and the embankment constructed. For Model 1, 1,754 g/m 2 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 load was defined as the load that produces a sudden increase in the foundation soil displacement. At each loading stage, horizontal and vertical deformations were recorded. To maintain undrained loading of the foundation soil, the time interval between the application of load increments was approximately 1 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, excessive vertical settlements occurred. Because of the embankment self-weight, sinking of the foundation soil occurred even before subsequent load increments were applied (Figure 3). The first load increment was the self-weight of the embankment followed by two surcharge increments corresponding to 1.47 and 3.17 kpa, respectively, which induced catastrophic failure. 77

8 Table 5. Shear strength values for the embankment models. Model number Crust type Amount of lime (g/m 2 ) Curing time (month) Curing Shear strength values temperature with depth (kpa) (_C) 25 mm 5 mm 75 mm 135 mm 1 No crust No crust Desiccated crust Set I 4 Lime crust 1, Lime crust 1, Lime crust Set II 7 Lime crust 1, Lime crust 1, Lime crust Lime crust 1, Settlement and heave (mm) q =1.47kPa q =3.17kPa q =1.47kPa q =kpa q =3.17kPa Unreinforced (Model 1) (Fill weight + surcharge, q) Reinforced (Model 2) (Fill weight + surcharge, q) Distance from embankment center (mm) Figure 3. Comparison between reinforced (Model 2) and unreinforced (Model 1) model embankments. For Model 2, a geotextile was placed over the foundation soil and the same loading procedure as for Model 1 was applied. For this case, it was observed that the use of geotextile reinforcement results in a significant strength improvement during fill placement. The embankment fill did not cause any vertical settlement; however, only two load surcharges, q = 1.47 and 3.17 kpa, could be applied before failure. 78

9 The measured foundation soil settlement and heave values for Models 1 and 2 are plotted 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 also decreased. It was noted that, for this case, the reinforcement reduces settlements from to 37.2 mm, at a location of 1 mm from the center of the embankment. Thus, the reinforcement resulted in a 3% reduction in surface settlement. This clearly reflects the benefit of using geotextile reinforcement in embankment model stabilization. The horizontal geotextile elongations measured by the three dial gauges were recorded during the loading stages. The strain values at two intervals can be calculated using the measured horizontal elongation of the geotextiles at 5, 125, and 225 mm intervals from the center of the embankment. In Table 6, the tensile stress in 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 is a high shear strength in the upper crust region, which decreases from a surface value of 2.4 kpa at a depth of 25 mm to a minimum value of 1.6 kpa under the crust at a depth of 5 mm from the surface. It then increases to 2.4 kpa at a depth of 135 mm. Below this depth, the shear strength is assumed to increase linearly with depth. The tensile load in the reinforcement was calculated using the strain values calculated from 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, the tensile load in the reinforcement, at the location where larger soil settlements occur (Zone 1), is clearly larger than the tensile load in the reinforcement in Zone 2. The maximum reinforcement tensile load value of 2. kn/m was measured during the application of the 7.3 kpa surcharge load, at which point a circular slip failure through the foundation 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) ε T = J ε (dimensionless) (kn/m).43 (1) (2) (1) (2).87 Notes: Superscript (1) and (2) = zone numbers shown in Figure 2a. J = tensile stiffness, ε = tensile strain, T = tensile load. 79

10 (a) 6 Shear strength (kpa) (b) Model 3 (desiccated crust) Model 4 Model 5 Model 6 Shear strength (kpa) 4 2 Model 7 Model 8 Model 9 Model Depth from foundation surface (mm) Figure 4. Shear strength profiles for models: (a) Models 3, 4, 5, and 6; (b) Models 7, 8, 9, and 1. Table 7. Collapse load for each model. Model number Model collapse load (kpa) The variation of vertical settlement and heave values for Model 3 (desiccated crust model) are plotted in Figure 6. A total of 1 loading stages were applied. In the first two loading stages, no settlement was observed. Appreciable settlement begins at an ap- 8

11 Tensile load (kn/m) 4 2 Zone 1 (see Figure 2) Zone 2 (see Figure 2) Vertical foundation pressure (kpa) 2 Figure 5. Reinforcement tensile load versus vertical foundation pressure for Model 3 (desiccated crust). 8 Settlement and heave (mm) Foundation pressure = 4.92 kpa Foundation pressure = 6.85 kpa Foundation pressure = 7.33 kpa Foundation pressure = 9.96 kpa Foundation pressure = 13.2 kpa Foundation pressure = 15.3 kpa Foundation pressure = 16.1 kpa Foundation pressure = 17.7 kpa Distance from embankment center (mm) Figure 6. Settlement and heave values for Model 3 (desiccated crust). 81

12 plied cumulative foundation pressure = 4.92 kpa. A maximum settlement of 73 mm occurs 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 loading stage. It can be observed that a sudden increase in vertical settlement occurred at a foundation pressure = 15.3 kpa. Accordingly, a foundation pressure = 15.3 kpa was considered 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), the embankments 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 15.3 kpa, 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 the load carrying capacity. This increase in load carrying capacity may be attributed to the loss ofwater byevaporation. The water content fell from 5 to 35% at a depth of 5 mm, leading to an increase 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 1 were constructed to examine the effects of a lime diffusion crust on the stability of the model embankments 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/m 2 of lime), the soil shear strength at a depth of 25 mm is 3. 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. kpa at a depth of 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 increases in the upper region of the foundation soil due to the addition of lime (Figure 4a). In Model 5 (1,52 g/m 2 of lime), the corresponding shear strength values with depth are 4.7, 3.1, and 3.6 kpa, at depths of 25, 75, and 135 mm, respectively. For Model 6 (71 g/m 2 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 decreases with increasing lime content; however, this trend is not as obvious in the lower layers. 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, reduces the water content. The water content values are given in Table 8. 82

13 Table 8. Water content values for Set I embankment models. Depth from Water content, w (%) foundation surface (mm) Model 4 Model 5 Model As observed in Table 8, the water content in the foundation soil decreases at the surface because of the lime added to the surface. This decreased water content leads to an increase in the shear strength and the load carrying capacity. Tensile load in the geotextile reinforcement for Set I models (Models 4, 5, and 6 which 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 maximum tensile loads were measured at locations where the failure surface through the foundation soil intersected the reinforcement layer. In Figure 7a (Model 4), the tensile load at the intersection of the observed failure surface reaches a maximum value (2.3 kn/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 vertical pressure of 16. 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 between Models 4 and 5. Figure 7c illustrates the tensile load values for Model 6, which has the minimum amount of lime. The tensile loads at the critical slip surface are lower than those for Models 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 is used. 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. 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 foundation of Model 4 was weaker than the foundations of Models 5 and 6. This is in accordance with 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 can be observed for Models 5 (Figure 8b) and 6 (Figure 8c). Therefore, the increase in crust strength is inversely proportional to the lime content. 83

14 (a) Tensile load (kn/m) 4 2 Zone 2 Zone 1 Model 4 (b) Tensile load (kn/m) 4 2 Zone 2 Zone 1 Model 5 (c) Tensile load (kn/m) 4 2 Zone 1 Zone 2 Model Vertical foundation pressure (kpa) 24 Figure 7. Tensile load versus vertical foundation pressure: (a) Model 4; (b) Model 5; (c) Model 6. 84

15 (a) Settlement and heave (mm) Foundation pressure = 17. kpa Model 4 Model 5 Model 6 (b) Settlement and heave (mm) Foundation pressure = 18.1 kpa Model 5 Model 4 Model 6 (c) Settlement and heave (mm) Foundation pressure = 17.4 kpa Model 6 Model 4 Model Distance from embankment center (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. 85

16 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 Figure 4b. As seen with Models 4, 5, and 6, the effect of the lime addition on the shear strength is 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 increases 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 in the 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 Models 8 and 9 showed any significant amount of settlement (Figure 9a). The critical load for Model 8 occurs at a foundation pressure = 22.1 kpa. How- (a) (b) Settlement and heave (mm) Settlement and heave (mm) Foundation pressure = 19.8 kpa Model 7 Model 8 Model 9 Foundation pressure = 22.1 kpa Model 8 Model Distance from embankment center (mm) 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. 86

17 ever, Model 7 failed before this load level could be achieved (Figure 9b). The critical load 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 given in Figures 1a, 1b, and 1c. It can be seen that the tensile load values for Set II models are 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 reinforcement strains 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. Set I was conducted during the winter at an average temperature of 17_C andsetiiwas conducted during the summer at an average temperature of 23_C. As a result, Set II models performed better than Set I models. As indicated by the strength profiles for each model shown in Figures 4a and 4b, there is a significant difference in bearing capacity. As evident in the model deformation profiles shown in Figures 11a, 11b, and 11c, 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 with increasing temperature. 4.5 The Effect of Curing An analysis was conducted to investigate the effect of curing time on embankment stabilization. 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 profiles for each model at collapse are shown in Figure 12. As evident in Figure 12, Model 1 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 in the literature. The purpose of the current study was to investigate the quantitative benefits of nonwoven geotextile reinforcement in combination with lime stabilization. Model test results show that the inclusion of geotextile reinforcement results in a considerable reduction in settlement and heave. The addition of geotextile reinforcement resulted in a 3% 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 and settlement 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 small changes in crust strength. The degree of improvement obtained through lime stabilization depends on many factors. 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. 87

18 (a) Tensile load (kn/m) 2 1 Zone 1 Zone 2 (b) Tensile load (kn/m) Zone 1 Zone (c) Tensile load (kn/m) Zone 1 Zone Vertical foundation pressure (kpa) Figure 1. Tensile load versus vertical foundation pressure: (a) Model 7; (b) Model 8; (c) Model 9. 88

19 (a) Settlement and heave (mm) Model 4 Model 7 Foundation pressure = 17. kpa (b) (c) Settlement and heave (mm) Settlement and heave (mm) Foundation pressure = 18.1 kpa Model 5 Model 8 Model 6, foundation pressure = 17.4 kpa Model 9, foundation pressure = 41.2 kpa Distance from embankment center (mm) 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 5 collapse load; (c) Model 6 collapse load. 89

20 Settlement and heave (mm) Model 1 (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. kpa) Distance from embankment center (mm) Figure 12. A comparison of settlement and heave profiles for experiments carried out at different temperatures (Models 4 and 7) and curing times (Model 1). 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 reduce the maximum benefit that can be derived from the lime stabilization technique. A 1.2 fold improvement in Models 7, 8, and 9 was obtained by increasing the temperature from 17 to 23_C, compared to Models 4, 5, and 6, respectively. A practical consequence of this observation is that greater improvement can be obtained if the technique is applied during summer months. It can be stated that the increase in temperature increases the effectiveness of the lime diffusion technique. Also, as evident from the experimental results, increasing the curing time positively affects lime stabilization. In conclusion it can be stated that the lime diffusion stabilization technique can be successively used in combination with geosynthetic reinforcement to improve foundation soil behavior for embankments constructed on soft clay soils. REFERENCES ASTM D 4632, Standard Test Method for Breaking Load and Elongation of Geotextiles (Grab Method), American Society for Testing and Materials, West Conshohocken, Pennsylvania, USA. ASTM D 3786, Test Method for Hydraulic Bursting Strength of Knitted Goods and Nonwoven Fabrics: Diaphragm Bursting Strength Tester Method, American Society for Testing and Materials, West Conshohocken, Pennsylvania, USA. BS 696, Methods of Test for Geotextiles: Determination of Tensile Properties Using a Wide-Width Strip, British Standards Institute, London, UK. 9

21 Edinçliler, A., 1995, Effect of Crust for Embankments Constructed on Soft Clays, Ph.D. Dissertation, Department of Civil Engineering, Bogaziçi University, Istanbul, Turkey, 167 p. Humphrey, D.N. and Holtz, R.D., 1989, Effect of Surface Crust on Reinforced Embankment, Geosynthetics 89, IFAI, Vol. 1, San Diego, California, USA, February 1989, pp Indraratna, B., Balasubramaniam, A.S. and Balackandran, S., 1991, Performance of Test Embankment Constructed to Failure on Soft Marine Clays, Journal of Geotechnical Engineering, Vol. 118, No. 11, pp Michalowski, R.L., 1992, Bearing Capacity of Nonhomogeneous Cohesive Soils Under Embankments, Journal of Geotechnical Engineering, Vol. 118, No. 7, pp Mylleville, B.L.J. and Rowe, R.K., 1991, On the Design of Reinforced Embankments on Soft Brittle Clays, Geosynthetics 91,IFAI,Vol.1,Atlanta, Georgia, USA,February 1991, pp Rowe, R.K. and Mylleville, B.L.J., 199, Implications of Adopting an Allowable Geosynthetic Strain in Estimating Stability, Proceedings of the Fourth International Conference on Geotextiles, Geomembranes and Related Products, Balkema, Vol. 1, The Hague, Netherlands, May 199, pp NOTATIONS Basic SI units are given in parentheses. e = void ratio (dimensionless) G s = 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 (%) w L = liquid limit (%) wopt = optimum moisture content (%) w P = plastic limit (%) ε = tensile strain (dimensionless) γ dry = dry unit weight of soil (N/m 3 ) 91