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Journal of Construction Engineering
Volume 2013 (2013), Article ID 293809, 10 pages
Improvement of Bearing Capacity of Shallow Foundation on Geogrid Reinforced Silty Clay and Sand
Department of Civil and Environmental Engineering, Southern Illinois University Carbondale, 1230 Lincoln Drive, MC 6603, Carbondale, IL 62901, USA
Received 6 December 2012; Revised 12 May 2013; Accepted 26 May 2013
Academic Editor: Mohammed Sonebi
Copyright © 2013 P. K. Kolay et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The present study investigates the improvement in the bearing capacity of silty clay soil with thin sand layer on top and placing geogrids at different depths. Model tests were performed for a rectangular footing resting on top of the soil to establish the load versus settlement curves of unreinforced and reinforced soil system. The test results focus on the improvement in bearing capacity of silty clay and sand on unreinforced and reinforced soil system in non-dimensional form, that is, BCR. The results show that bearing capacity increases significantly with the increased number of geogrid layers. The bearing capacity for the soil increases with an average of 16.67% using one geogrid layer at interface of soils with equal to 0.667 and the bearing capacity increases with an average of 33.33% while using one geogrid in middle of sand layer with equal to 0.33. The improvement in bearing capacity for sand underlain silty clay maintaining and equal to 0.33; for two, three and four number geogrid layer were 44.44%, 61.11%, 72.22%, respectively. The finding of this research work may be useful to improve the bearing capacity of soil for shallow foundation and pavement design for similar type of soil available elsewhere.
The use of geosynthetic materials to improve the bearing capacity and settlement performance of shallow foundation has gained attention in the field of geotechnical engineering. For the last three decades, several studies have been conducted based on the laboratory model and field tests, related to the beneficial effects of the geosynthetic materials, on the load bearing capacity of soils in the road pavements, shallow foundations, and slope stabilizations. The first systematic study to improve the bearing capacity of strip footing by using metallic strip was by Binquet and Lee [1, 2]. After Binquet and Lee’s work, several studies have been conducted on the improvement of load bearing capacity of shallow foundations supported by sand reinforced with various reinforcing materials such as geogrids [3–9], geotextile [10–12], fibers [13, 14], metal strips [15, 16], and geocell [17, 18].
Several researches have demonstrated that the ultimate bearing capacity and the settlement characteristics of the foundation can be improved by the inclusion of reinforcements in the ground. The findings from several laboratory model tests and a limited number of field tests have been reported in the literature [19–25] which relates the ultimate bearing capacity of shallow foundations supported by sand reinforced with multiple layers of geogrid. Recently, Yin  compiled extensive literature in the handbook of geosynthetic engineering on reinforced soil for shallow foundation. For the design of shallow foundations in the field, the settlement becomes the controlling criteria rather than the bearing capacity. Hence, it is important to evaluate the improvement in the bearing capacity of foundations at particular settlement () level. From the finding of numerous researchers, it can be concluded that the bearing capacity of soil also changed with various factors like type of reinforcing materials, number of reinforcement layers, ratios of different parameters of reinforcing materials, and foundations such as (footing width), (location of the 1st layer of reinforcement to width of footing), (vertical spacing between consecutive geogrid layer to width of footing), (width of the geogrid layer to width of footing), (depth of footing to width of footing), type of soil, texture, and unit weight or density of soil, [6, 7].
Out of several studies, very few studies are available on the two-layer soils. Generally, all the studies are ultimately related to improvement in the bearing capacity of soil using reinforcing materials and related to the effect of various parameters on bearing capacity. The ratio of improvement in the bearing capacity can be expressed in a nondimensional form as bearing capacity ratio (BCR) which is the ratio of bearing capacity of reinforced soil to bearing capacity of unreinforced soil. Several studies [5, 6, 26] show the effect of various parameters (e.g., , , , and ), types of geosynthetic materials (e.g., geogrid, geotextiles, and geocell), effects of footing width , types of soils, layer of soils, and so forth. But no studies are available on silty clay soil of Carbondale, Illinois, related to the improvement in bearing capacity of rectangular footing by placing sand layer on top of silty clay soil (i.e., two-layered soil) and geogrid system. Most of the studies either used sand or clay only and used geogrid as the reinforcing material. The present study investigates the bearing capacity of two layers of soil (i.e., a thin sand layer underlain by silty clay) and also of single-layer silty clay soil (for comparison purpose) with varying the number of biaxial geogrid at different layers and by keeping other properties constant.
2. Experimental Study
2.1. Materials Used
Two types of soils were used to conduct the experimental study, that is, silty clay soil and sand.
2.2. Silty Clay Soil and Sand
The silty clay soil sample was collected from New Era Road in Carbondale, Illinois. The collected soil was sun-dried, pulverized, and passed through US sieve # 10 (i.e., 2 mm) for different physical, engineering properties and bearing capacity test. The properties of the silty clay soil were determined in the laboratory by performing several tests using respective ASTM standard. A thin layer of sand was placed on top of silty clay soil (two-layer soil system) to evaluate the improvement on load bearing capacity of the silty clay soil.
Biaxial geogrid was used in the present experimental study. Biaxial geogrid has tensile strength in two mutually perpendicular directions so that it gives more strength to the soil. Different properties of the biaxial geogrid are presented in Table 1.
2.4. Model Test Tank
A model test tank with the dimensions having length () 762.0 mm, width () 304.8 mm, and depth () 749.3 mm was designed and fabricated to perform the test. The horizontal and vertical sides of the model tank are stiffened by using steel angle sections at the top, bottom, and middle of the tank to avoid any lateral yielding during soil compaction in the tank and also while applying load at model footing during the experiment. Two side walls of the tank were made of 25.4 mm thick Plexiglas plates, and the other two side walls of the tank were made of 12.7 mm thick Plexiglas plates, and these were also supported by 19.05 mm wooden plates. The inside walls of the tank were smooth to reduce the side friction.
2.5. Model Footing
A model footing, with the dimensions of length equal to 284.48 mm, width equal to 114.3 mm, and thickness equal to 48.26 mm, was used in the experimental study. The footing dimensions were selected based on the model tank’s dimension. The model footing was designed in such a way that its width is less than 6.5 times the depth of the model tank so that the effect of the load could not reach the bottom of tank. The bottom surface of the model footing was made rough by cementing a layer of sand with epoxy glue to increase the friction between the footing base and the top soil layer. Also a 12.7 mm thick steel plate was used at the top of the model footing to reduce bending while applying the load.
2.6. Laboratory Model Tests
In the present study the silty clay soil was used at the bottom part of the model tank overlaid by a small thickness of sand layer at the top. The criterion of selection of the thickness of the top sand layer is based on the studies by previous researchers . In the geogrid reinforced model tests, the optimum values related to the reinforcement arrangement, such as the location of the first layer of reinforcement , the vertical spacing between consecutive reinforcement layers , and the length of each reinforcement layer , were adopted based on the model tank size and findings from the previous researchers.
Figure 1 shows the cross sectional view of the model tank and the model footing with two-layer soil system having different reinforcement layers. The model rectangular footing with width is supported by sand at the top layer and silty clay soil at the bottom layer reinforced with number of geogrid layers having a width “”. The vertical spacing between consecutive geogrid layers is “”. The top layer of geogrid is located at a depth “” measured from the base of the model footing. The depth of reinforcement, , below the bottom of the foundation can be calculated by using the following: The magnitude of the bearing capacity ratios (BCR) for a given rectangular footing, silty clay soil, sand, and geogrid will depend on different parameters like , , , and ratios. In order to conduct model tests with geogrid reinforcement in two-layer soil system, that is, silty clay soil and sand, it is important to decide the magnitude of and to get the improvement of the bearing capacity for a particular footing. Earlier researchers [10, 13, 14] found that, for a model footing resting on surface (i.e., ) having multiple layers of reinforcements for given values of , , and , the magnitude of BCRu (for unreinforced case) increases with and attains a maximum value at . If is greater than , the magnitude of BCRu decreases. By analyzing several test results, Shin et al.  determined that for strip footing can vary between 0.25 and 0.5. Similarly, for given , , and values, the optimum value of for surface foundation condition to get the maximum increase in BCRu with using reinforcement can vary from 6 to 8 for strip foundations . By considering the previous findings, it was decided to adopt the following parameters for the present study: , 0.67; ; , number of geogrid layers : 0, 1, 2, 3, 4, length of each reinforcement layer : 73.66 cm.
The specific gravity () of silty clay soil and sand sample was determined by using the ASTM D 854 method. For the sake of accuracy, the average specific gravity is obtained from the results of three tests. The standard Proctor compaction test was conducted as per ASTM D 698 method to determine the maximum dry density and optimum moisture content (OMC). The particle size distribution of the silty clay soil and sand samples was obtained by using dry sieve as well as hydrometer analyses according to ASTM D 422. ASTM D 4318 method was used to determine the liquid limit and plastic limit of the silty clay soil, and ASTM D 2166 method has been used for the unconfined compression strength (UCS) test to determine the cohesion of the silty clay soil. The maximum index density (i.e., minimum void ratio) and the minimum index density (i.e., maximum void ratio) of the sand samples were obtained according to ASTM D 4253 and ASTM D 4254 methods, respectively. For the minimum index unit weight, a small funnel was used to pour the sand in mold from a small height (i.e., 25.4 mm) and for the maximum index unit weight; the sand was vibrated for 10 minutes. Direct shear test has been conducted to determine the friction angle of the sand sample by using the method mentioned in ASTM D 3080.
The processed silty clay soil sample was kept in a big container, and then 19% water (i.e., OMC of the silty clay soil) was added to the soil and mixed thoroughly to make a uniform homogeneous mixture. Before running the tests in the model tank, the moisture content was checked for soil water mixture. To obtain a uniform density, the silty clay soil was compacted in 13 layers up to an approximately 673.1 mm depth of the model test tank. An approximately 12.25 kg flat round hammer was used to compact the silty clay soil in each layer.
In the model test tank, the unit weight of the silty clay soil was 86.8% of the maximum dry unit weight at its optimum moisture content (OMC). After compaction of the silty clay soil in the model tank up to 673.1 mm, a 76.2 mm thick sand layer was placed above the compacted silty clay. For the bearing capacity tests, sand sample was compacted in two layers with a thickness of 76.2 mm in each layer. Biaxial geogrid reinforcements were placed at pre-determined depths below the base of the model footing. The model footing was placed at the top of sand layer. All tests were conducted at a constant relative density of sand, , equal to 96% of sand and relative compaction of silty clay soil, that is, 86.8% of the maximum dry unit weight of silty clay. The load was applied to the model footing by using a manual hydraulic pump system with a capacity of an approximately 44.48 kN. The loading rate was kept constant in every test. The load and corresponding foundation settlement were measured by using a load cell and a dial gauge, respectively. In the present study, different tests that were conducted for silty clay soil, sand, and two-layer soil system with varying numbers of geogrid layers are presented in Table 2.
4. Results and Discussion
4.1. Physical and Engineering Properties of Silty Clay Soil and Sand
The results of various physical and engineering properties of silty clay and sand are presented here. The results of specific gravity () test for the silty clay and sand were measured to be 2.67 and 2.64, respectively.
The particle size distribution curve for the silty clay soil obtained from sieve analysis and hydrometer tests is presented in Figure 2. From Figure 2, it is clear that 97.9% of the soil passed through the US sieve # 200. The soil consists of 30% clay-sized particles (<2 μm), 67.9% silt-sized particles (2 μm to 75 μm), and 2.1% sand-sized particles (75 μm to 2 mm).
The liquid limit and the plastic limit for the silty clay soil sample were measured to be 42% and 19%, respectively. The particle size distribution of sand sample used in the present study was also presented in Figure 3. Uniformity coefficient () and coefficient of curvature () calculated to be 1.83 and 1.89, respectively, and the effective particle size () is calculated to be 0.18 mm. Hence, the sand is classified as poorly graded sand (SP) according to unified soil classified system (USCS).
The results of standard Proctor compaction test for silty clay soil is presented in Figure 3. From Figure 3 it is observed that the maximum dry unit weight and optimum moisture content (OMC) of the silty clay soil are 16.73 kN/m3 and 19%, respectively.
Based on the two UCS tests, the average value of unconfined compressive strength is equal to 90.32 kN/m2, and undrained cohesion is calculated to be 45.16 kN/m2. The physical and engineering properties of the sand tested are presented in Table 4.
4.2. Determination of Ultimate Bearing Capacity
Figure 4 shows the bearing pressure versus settlement curves obtained from all the tests conducted in this study. From Figure 4, it is noticed that no distinctive failure point has been observed in bearing capacity tests. Several methods are available to estimate the ultimate bearing capacity (UBC, i.e., ) from the bearing pressure versus the settlement curve. Each method gives a different value of the ultimate bearing capacity is and it hard to decide which method is more accurate. Currently, four methods are available to estimate the failure of a shallow foundation, based on load settlement curves, but if there is no distinct failure pattern of the foundation/soil system available, the values obtained by using different methods have the following order [27, 28]: log-log method < tangent intersection method (TIM) < 0.1 B method < hyperbolic method. Out of all available methods, we used the 10% width of footing method (i.e., 0.1 B method), and tangent intersection method (TIM) to find the ultimate bearing capacity for each case in our experimental study.
4.3. Ultimate Bearing Capacity of the Silty Clay Soil
At first, the bearing capacity test was performed on the silty clay soil and the settlement is expressed into a nondimensional form by dividing the width of footing . The bearing pressure versus settlement/width ratios (i.e., ) is shown in Figure 5. By analyzing the load settlement curve, no distinct failure point has been observed for a rectangular footing in the silty clay soil. From Figure 5, it can be estimated that the ultimate bearing capacity () for silty clay soil is about 172.37 kN/m2.
The bearing capacity test conducted on only sand layer compacted at 97% of its maximum density is presented in Figure 6. From Figure 6, it can be calculated that the average ultimate bearing capacity () of sand is about 174.76 kN/m2.
4.4. Theoretical Ultimate Bearing Capacity
The theoretical ultimate bearing capacity for double-layer soil system is calculated by using Meyerhof and Hanna’s  equation as follows. They assumed that the top layer is strong sand and bottom layer is saturated soft clay.
The ultimate bearing capacity for top layer can be calculated by using (2). The ultimate bearing capacity for bottom layer can be calculated by using the following: Hence, the ultimate bearing capacity, , for the double layer system can be calculated by using the following: where is the undrained cohesion for silty clay soil and is the punching shear coefficient which depends on ratio of where .
In the present study, the top layer is poorly graded sand (SP) with an effective particle size () equal to 0.18 mm. With an angle of internal friction, , the bearing capacity factor, , , , can be obtained as 46.12, 33.30, and 48.03, respectively. The bottom layer is local silty clay (CL) with 19% water content and the angle of internal friction, . For the bearing capacity factors can be obtained as , and , .
From (4), the ultimate bearing capacity () for double-layer soil system can be obtained as 250.59 kN/m2. Also from (4), the bearing capacity of the top layer can be calculated as 43.31 kN/m2, which is quite low because the model width of the footing is only 114.3 mm as compared to the real foundation size.
4.5. Ultimate Bearing Capacity of Two-Layered Soil System Using Geogrid
Five tests were conducted on double- or two-layer soil system by placing geogrid at different depths from the base of the footing and also varying the number of geogrid layers. Figure 4 shows the bearing pressure versus settlement curves for all tests. There is no distinct failure point observed on bearing capacity versus settlement curve.
The 10% width of footing method and, tangent intersection method are used to estimate the ultimate bearing capacity for shallow foundation which is shown in Figures 7 and 8, respectively. From Figure 7, it is clear that the bearing capacity increases with the increase in the number of geogrid layers. Out of five tests, two tests were conducted by using one geogrid layer but at various positions, that is, the depth of geogrid from the base of footing is different. This is the case of varying (i.e., depth of first layer of geogrid/width of footing) ratio by keeping the number of geogrid layer constant. While in other tests, the ratio (depth of first layer of geogrid/width of footing) and (consecutive height of two geogrids layers) ratio were kept constant but varying the number of geogrid layer. 10% (width of footing) method is used to find out the ultimate bearing capacity for all these cases. The ultimate bearing capacity values with geogrid layer can be compared with unreinforced soil condition for single layer and also for two-layer system. The results of various tests conducted on two-layer soil system with and without geogrids are presented in Table 5.
4.6. Improvement in Ultimate Bearing Capacity of Silty Clay Soil Using Sand and Geogrid
The present experimental study investigates the effect of reinforcement in the load bearing capacity of the rectangular footing in silty clay soil. Two tests were performed without using the geogrid for a comparison purpose to see the effect of geogrid. The ultimate bearing capacity obtained from the experimental investigations for reinforced cases was compared with the ultimate bearing capacity of the unreinforced case, that is, silty clay soil only. The bearing capacity of the silty clay soil only is considered as the reference value to be compared with the bearing capacities of all other geogrid reinforced soil system. In all these investigations, only one type of biaxial geogrid was used. In these tests the ratio is equal to 0.33 (the depth of 1st layer of geogrid from the footing to the width of footing) and (depth of consecutive layer of geogrid to the width of footing), ratio remains the same except in one test where only one geogrid was used at the interface of sand layer and silty clay soil with ratio of 0.667. The results of ultimate bearing capacity based on 10% method, percentage improvement in bearing capacity with respect to silty clay soil only, and bearing capacity ratio (BCR) obtained from all test series are summarized in Table 6. The results show that for the same quantity settlement the ultimate carrying capacity increases with the inclusion of sand and geogrids layers. Sitharam and Sireesh  have conducted bearing capacity test of circular footing on base geogrid with geocell-reinforced sand overlying soft clay (CL), and they also observed similar test results. Khing et al.  have conducted model test to determine the bearing capacity of strip footing, and they found that the maximum bearing capacity increased when geogrid has been placed at the interface between two different layers of soil; the present study also observed similar trend of results. Omar et al.  studied the bearing capacity of strip footing with geogrid-reinforced sand with and equal to 0.33, and they found that ultimate load per unit area with 1, 2, 3, and 4 number of geogrid was approximately 150, 200, 300, 315 kN/m2, respectively. In the present study with the same and ratios, the ultimate bearing capacity varies from 201.10 to 296.86 kN/m2 with the same number of geogrids used, when ultimate bearing capacity has been calculated by using 10% BM method. Kumar et al.  have studied the bearing capacity of strip footing resting on two-layer sand, and they also found similar trend with the present study. Demir et al.  have conducted model studies of circular footing resting on soft soil, and they also observed a similar trend of (settlement/diameter of the footing) versus pressure diagram.
As we can see from the result that, when a small thickness of sand layer is placed on the top of silty clay soil layer, the bearing capacity increases in small magnitude (i.e., 7%) because the sand has more strength and slightly more unit weight as compared to the silty clay soil. After putting the geogrids in the two-layer system the load-carrying capacity is significantly increased as compared to the bearing capacity of silty clay soil and silty clay soil with the top layer of sand; hence it can be concluded that the bearing capacity mainly increased from the geogrid soil interaction. The result proved that the placement of geogrid also affects the bearing capacity in the two-layer soil system; that is, ratio also affects the bearing capacity.
The experimental study was performed in the two layer of soil system; that is, a portion of the silty clay soil was replaced by a 76.2 mm thick layer of sand at the top. Five tests were performed to evaluate the effect of geogrid layer on the same kind of soil system. The BCR value is assumed one for the sand overlying the silty clay soil without using geogrid. It can be taken as a reference value for the comparison purpose in the same arrangement; hence, it is possible to observe the improvement in the bearing capacity after using the geogrid. The results are also presented in Table 5. From Table 5 it is concluded that there is a significant increase in the bearing capacity after increasing the number of geogrid layers. Therefore, geogrid can be considered as a good reinforcing material.
Two tests were performed with the same number of geogrids to evaluate the effect of the distance between the base of footing and geogrid, that is, the distance of the 1st geogrid from the base of the footing. Generally the distance is expressed in the form of nondimensional unit as , where is depth of first layer of geogrid from the base of footing and is width of footing. The ultimate bearing capacity calculated based on ratio is presented in Table 7. In one test the was kept 0.33; that is, geogrid was placed at 38.1 mm from the base of footing and the ultimate bearing capacity of the footing, supported by the two-layer of soil is measured to be 229.83 kN/m2. In the other test the was 0.667; that is, the geogrid was placed at 76.2 mm from the base of footing in the two-layer system of soil, and the bearing capacity was measured to be 248.98 kN/m2. These results show that, if the increases, the bearing capacity increases. These results are consistent with other studies which show the effect of ratio on the bearing capacity of various footing supported on different kinds of soils. It has been observed that the bearing capacity increases as the ratio increases, and the present study has also shown the similar trend in case of two-layer soil system.
Figure 9 shows the effect of number of the geogrid layers on the two-layer soil system. The ultimate bearing capacity increases with the increase in the number of geogrid layers. In the beginning the improvement is more significant as compared to last stage so that it can be concluded that the top layer of geogrid has a more contribution to the improvement of bearing capacity of silty clay soil. Omar et al.  also observed similar trend with BCR approximately equal to 3.8 and with equal to 0.33, whereas in present study with same ratio BCR is approximately 1.61 with the same number of geogrid layers.
The present study investigates the effect of geogrids on sand layer underlain by silty clay soil towards the improvement of bearing capacity of rectangular footing. The silty clay soil and sand used are classified as CL and SP, respectively, based on the Unified Soil Classification System (USCS).
A number of model tests were performed to evaluate the load-carrying capacity of a rectangular model footing supported on the silty clay soil overlaid with small thickness of sand and with inclusion of geogrids at different depths from the base of the footing. Based on the model tests, the following conclusions are drawn.(i)The load-carrying capacity of the silty clay soil obtained from Carbondale, Illinois, increased by 7% when the top of the silty clay soil was replaced with 76.2 mm thick layer of sand.(ii)The bearing capacity for the two-layer soil increases with an average of 16.67% using one geogrid layer at interface of soil (i.e., silty clay soil and sand) with the equal to 0.667. The bearing capacity for the two-layer soil increases with an average of 33.33% while using one geogrid in the middle of sand layer having the equal to 0.33. (iii)The improvement in bearing capacity for double layer soil maintaining and equal to 0.33; for two, three and four number geogrid layer were 44.44%, 61.11%, 72.22%, respectively.(iv)Bearing capacity is also dependent on ratio; that is, bearing capacity is higher if the is higher.
Based on the results of this study, it is concluded that the bearing capacity of the silty clay soil can be improved by using the geogrid. The finding of this research work may be useful to improve the strength of soil for foundation and pavement design for a specific area or similar types of soils available elsewhere.
The authors would like to acknowledge Professor V. K. Puri for his guidance on experimentation and critical comments throughout the study. The authors also would like to thank Mr. John Hester of Geotech laboratory for fabricating the test tank and instrumentation.
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