IGC. 50 th INDIAN GEOTECHNICAL CONFERENCE

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1 BEARING CAPACITY IMPROVEMENT OF CONFINED FOOTINGS IN EXPANSIVE SOILS Sareesh Chandrwanshi 1, Rakesh Kumar 2 and P.K. Jain 3 The civil structures constructed on expansive soils encounters severe problems like excessive displacement and instability in vertical and lateral directions. The common adopted remedial measure to effectively deal with these problems is to improve the bearing capacity of expansive soil with the application of stone/sand columns. The construction of stone/sand column involves partial replacement of unstable or loose soils in a column type structure laid out in a particular pattern with a compacted vertical column of stone aggregate or sand at higher relative density to form a compacted bed. The load carrying capacity of improved soil after stone/sand columns is installed is dependent on the confinement offered by the surrounding soil as it bulges and surrounding soil supports by applying earth pressure.the application of geosynthetics on the entire surface is one way of further improving the performance of stone/sand columns as it resists the bulge formation, but even this technique has limitations of improving the load carrying capacity. Confined footings are made by applying confining cylinders made of strong material than can withstand large hoop stresses. After inserting the confining cylinder, expansive soil is replaced with stone aggregates or granular material. The special advantage of this technique over geosynthetic encasement stone/sand column is the ease of application along with higher bearing capacities at same displacement. This paper presents the results of small-scale laboratory model tests conducted on circular footing resting inexpansive soil after the application of confined footing and elaborates properties. The increase in load carrying capacity due to the application of confined footing is investigated by conducting tests on leveled surface formed by installing a confined footing of diameter bigger than that of footing in expansive soil bed prepared in stable condition in a testing tank. Circular footing of 50 mm diameter is kept constant for all the tests conducted. Diameter and depth of confining 1 Research Scholar, Civil Engg. Dept, M.A.N.I.T., Bhopal, India, sareesh23@gmail.com 2 Asst. Professor, Civil Engg. Dept, M.A.N.I.T., Bhopal, India, rakesh20777@gmail.com 3 Professor, Civil Engg. Dept, M.A.N.I.T., Bhopal, India, pkjain10@rediffmail.com

2 Sareesh Chandrawanshi, Rakesh Kumar and P.K. Jain cylinder applied is varied to arrive at the optimum dimensions of it. Expansive soil bed was prepared at constant water content for yielding the same bearing capacity each and every time for different tests. The floating type sand column, sand column penetrating to complete depth and confined footing were compared. The results show improvement in bearing capacity after the construction of sand column i.e. both floating type and completely penetrated type. Keywords: Expansive soil, Stone column, Floating type stone column, Confined footing.

3 BEARING CAPACITY IMPROVEMENT OF CONFINED FOOTINGS IN EXPANSIVE SOILS Sareesh Chandrawanshi, Research Scholar, M.A.N.I.T., Bhopal, India, Rakesh Kumar, Asst. Professor, Civil Engg. Dept, M.A.N.I.T., Bhopal, India, P.K. Jain, Professor, Civil Engg. Dept, M.A.N.I.T., Bhopal, India, ABSTRACT: The civil structures constructed on expansive soils encounters severe problems like excessive displacement and instability in vertical and lateral directions. The common adopted remedial measure to effectively deal with these problems is to improve the bearing capacity of expansive soil with the application of stone/sand columns. The construction of stone/sand column involves partial replacement of unstable or loose soils in a column type structure laid out in a particular pattern with a compacted vertical column of stone aggregate or sand at higher relative density to form a compacted bed. The load carrying capacity of improved soil after stone/sand columns is installed is dependent on the confinement offered by the surrounding soil as it bulges and surrounding soil supports by applying earth pressure. The application of geosynthetics on the entire surface is one way of further improving the performance of stone/sand columns as it resists the bulge formation, but even this technique has limitations of improving the load carrying capacity. Confined footings are made by applying confining cylinders made of strong material than can withstand large hoop stresses. After inserting the confining cylinder, expansive soil is replaced with stone aggregates or granular material. The special advantage of this technique over geosynthetic encasement stone/sand column is the ease of application along with higher bearing capacities at same displacement. This paper presents the results of small-scale laboratory model tests conducted on circular footing resting in expansive soil after the application of confined footing and elaborates properties. The increase in load carrying capacity due to the application of confined footing is investigated by conducting tests on leveled surface formed by installing a confined footing of diameter bigger than that of footing in expansive soil bed prepared in stable condition in a testing tank. Circular footing of 50 mm diameter is kept constant for all the tests conducted. Diameter and depth of confining cylinder applied is varied to arrive at the optimum dimensions of it. Expansive soil bed was prepared at constant water content for yielding the same bearing capacity each and every time for different tests. The floating type sand column, sand column penetrating to complete depth and confined footing were compared. The results show improvement in bearing capacity after the construction of sand column i.e. both floating type and completely penetrated type. INTRODUCTION Infrastructure development is the prime requirementfor the development of any nation. Due to rapid increase in constructional activities majorly in metropolitan cities, there is shortage of suitable sites with good bearing capacity at shallow depth from the surface. Therefore, in this situation construction is also being carried out on sites having average or even poor bearing capacity like expansive soils, which covers large parts along the Indian coast and major part of Indo Gangetic plains. Cases where soils having inappropriate bearing capacity for construction is dealt by

4 Sareesh Chandrawanshi, Rakesh Kumar and P.K. Jain applying ground improvement techniques. For lowheight residential buildings and the light structures, parks and grounds etc. that can sustain minor settlements, ground improvement techniques are normally considered best because of economical consideration and stone/sand column is one of the most commonly adopted technique used worldwide. The stone/sand columns are vertical cylindrical elements formed by partial/full replacement of poor or undesirable soil by coarse granular material, such as stone aggregates or sand. If the total depth of application of stone aggregate or sand is equal to the thickness of inferior layer than it is termed as end bearing stone/sand column otherwise it is known as floating stone/sand column. The presence of the stone/sand column creates a ground profile which is stronger and stiffer than the original expansive soil. When the stone/sand column reinforced ground is loaded, initially the column deforms by bulging and expands laterally into the surrounding expansive soil. For stone/sand column having extended lengths greater than critical length (i.e., about four times the diameter of the column), it is identified that the bulging failure is governing factor for the load carrying capacity whether they bear on stiff layer or penetrate partially into the medium stiff soil (Madhav et al. 1994). Load carrying capacity of stone/sand column can be increased manifold further by restricting the bulging by the application of horizontal reinforcement (geogrid) at upper portion of granular column (Sharma et al. 2004) or by applying circumferential reinforcement like geogrid casing (Rao 1995).The soil is confined by the application of vertical reinforcement in the form of geogrid. The improved behavior of circular foundation resting on confined subgrade soil by confining cell was first investigated by El Sawwaf and Nazer (2005) and later by Singh and Prasad (2007). The previous investigators carried out experimental studies of applying confining cylinder only in sandy soil because it cannot be efficiently applied to expansive soils. The novel ground improvement technique presented in this research paper is a combination of two techniques. It involves using confined foundation and local replacement of expansive soil inside the confining cylinder. In this technique there is complete replacement of expansive soil with well compacted sand only within confining cylinder beneath the footing. This replacement of expansive soil is done only inside the confining cylinder and that is 1.4 times the diameter of the footing without whole site stabilization. Adopted dimensions of confining cylinder is 1.4 times the diameter of footing on the basis of extensive research done on confined footings (Sawwaf et al. 2005). This localized stabilization is adopted to decrease both the quantity of replaced soil and also reduce the compaction effort involved in the complete replacement process. Therefore, the main objective of this research is to assess the aspect of improvement in the bearing load through local soil replacement of expansive soil overlying a compacted sand bed with confining cylinder and comparing it with conventional stone/sand column and floating stone/sand column. LABORATORY MODEL TEST The objectives of the experimental investigation in the present work are to study the effect of sand column on expansive soil and comparing it with the effect of confining cell of different diameters and heights. Small scale model circular footing of 50 mm diameter and 10 mm thickness resting on expansive soil is used in the present study. To achieve these, a detailed experimental program has been carried out. Model Circular Box Fig. 1. showsthe detailed view of the experimental model apparatus used in this investigation. The model test box is of cylinder-shape, having inside diameter of 30 cm and 30 cm in depth, the walls of tank are of uniform thickness of 2.5 mm. The test box was built sufficiently rigid to maintain plain strain conditions all the time in all directions during the various loading conditions. The inner walls of the test tank are made smooth to reduce friction by using oil coating every time before the tank is filled.

5 Expansive soil used The clay used in the investigation is expansive soil picked from MANIT Bhopal campus. The soil was air dried, thoroughly pulverized and sample passing through 0.075mm sieve is taken. The normally consolidated soft clay bed was prepared by filling the clay in layers, at a particular water content. The shear strength for the soil was obtained through laboratory vane shear test. The soft clay bed was thoroughly mixed and placed by hand into the model tank and mild tamping is done by applying uniform pressure of 20 kn/m 2 and kept for 5 min to get uniform surface. Its detailed properties are listed in Table 1. Test Setup Fig. 1. Load Setup The load frame used to the one that is used in performing California bearing ratio test. It has a platform that can move up and down, at controlled strain rates, by a motorized arrangement and the load and settlement were measured with the help of proving ring and dial gauge respectively. The capacity of the loading frame is in accordance with the maximum load that has been applied throughout the experiment. A typical test arrangement is shown in Fig. 1 in which a test tank filled with expansive soil at a constant density and moisture content yielding a particular unconfined compressive strength in all the tests. Figure also shows the confining cell inserted in position and the replaced sand inside the cell filled at a particular density index. And the test box is placed on the loading platform for the application of load. Material Used The materials used in the investigation consist of expansive soil, sand and confining cell (i.e. upvc pipe). The source and relevant properties of these materials are as follows. Sand Used Table. 1 Properties of Expansive soil Properties Values Liquid Limit (%) 57 Plastic Limit (%) 34 Plasticity Index (%) 23 Specific Gravity 2.69 Optimum Moisture Content (%) 22.4 Maximum Dry Density (kn/m 3 ) 15.3 Differential Free Swell (%) 52 Classification (IS: ) CH Degree of Saturation (%) 100 Unconfined Comp. St. (kn/m 2 ) at 27% Water 40 Content The sand used in the study was taken from the river Narmada flowing by a nearby town Hoshangabad (India). It was dried in sun and sieved through 4.75 mm IS sieve. The particles larger than 4.75 mm size were discarded and only those passing were selected for model test. The tests conducted on the sand in the laboratory were sieve analysis, specific gravity, minimum and maximum dry unit weight test. After drawing particle size analysis curve of the sand it was observed that soil was only coarse

6 Sareesh Chandrawanshi, Rakesh Kumar and P.K. Jain grained and there was no fraction of gravel at all. No plasticity was observed and C U and C C values were calculated and given in the Table. 2. Sand was classified as SP, i.e. poorly graded sand. Table. 2 Properties of Expansive soil Properties Values Silt and clay content 1.42% Sand content 98.58% Gravel content Effective Size, D 10 Effective Size, D 30 Effective Size, D 60 Coefficient of Uniformity, C U Nil mm mm mm Coefficient of Curvature, C C Soil classification SP Maximum Density, γ max kn/m 3 Minimum Density, γ min kn/m 3 Maximum Void Ratio, e max Minimum Void Ratio, e min Confining Cell Confining Cell were used to laterally confine the sand. The rigid unplasticized polyvinyl chloride pipes (upvc pipes as per IS 4985:2000 specifications of make Kisan) of different diameters were used as confining cell. The pipes of diameters (d) 75 and 90 were taken. These were cut in lengths i.e. height (h) of 50 mm, 100 mm and 150 mm to act as a confining cell in the present study. The interior and exterior surfaces of the skirts were made smooth by oiling. The thickness of the cylinder wall is dependent on its diameter and varies from 2 to 2.5 mm to withstand maximum pressure of 8.6 MPa as mentioned in IS code. Fig. 2. Designation of terminology w.r.t to confining cell Table. 3 Dimensions and Properties of Confining Outer Dia, mm Inner Dia, mm Cell Dimension of Pipes Thickness, mm Max. Safe Working Pressure (as per IS 4985:2000) MPa MPa TESTING PROGRAM AND METHODOLOGY Preparation of soft clay bed The overall dimensions of the test tank are so selected that the loading on the clay bed and stone/sand column would not be affected by the boundaries. All the tests have been conducted in expansive soil bed prepared at unconfined compressive strength (U.C.S.) of 40 kn/m 2 at 27% water content. A thin coat of oil was applied along the inner surface of tank wall to minimize friction between clay and tank wall. Expansive clay was filled in the tank in layers with measured quantity by weight. The surface of each layer was provided with uniform compaction with a tamper to achieve a 50 mm height and uniform bulk unit weight of 18.7 kn/m 3.

7 Construction of Sand Column After the expansive soil bed was prepared sand column was constructed by a replacement method. Thin hollow steel pipe of 50mm outer diameters and wall thickness 1 mm were used to construct the stone/sand column. The pipe was pushed into the soil up to the bottom of the tank for the completely penetrated case and partially for floating column. Only manual force is required to push the pipe into the soil, therefore the disturbance in the clay soil was also minimal. The displaced clay during the installation of granular columns was taken out and the clay bed surface in the tank was trimmed level. Outer surface of the pipe was lubricated by applying a thin layer of grease for easy withdrawal without any significant disturbance to the surrounding soil. Sand was moistened (with 2% of water) and from absorbing the moisture from the surrounding clay soil. Sand was poured in the hole in layers in measured quantities to achieve a compacted height equal to the depth of stone/sand column. For achieving a uniform unit weight for relative density of 60%, compaction was done with a 2 kg circular steel tamper with 10 blows of 100 mm drop to each layer and for relative density of 70% numbers of blows were increased by 5.. This light compaction effort was adopted to ensure that there is no significant lateral bulging of the column which creates disturbance to the surrounding soft clay. For the unit weight of sand was found to be kn/m 3 and15.71 kn/m 3 that corresponds to relative density of 60% and 70% was for different test combinations the tests. Construction of confined footing Steps were same as discussed in the previous step. The confining cell of appropriate diameter is first oiled from outside and inserted to a limited depth Table. 4 Tests Program for Experimental Models of 5 cm and extracted. Then the inside clay is expelled out and it is again oiled and reinserted to further 5 cm. This steps continues till the complete depth of clay is cleared and then finally it is put in place. Then the sand is backfilled and compacted to achieve the relative density of 60% or 70% inside to confining cylinder. The chosen outer diameters of confining cells were 75 mm and 90 mm. Length of the confining cells were 50 mm, 100 mm and 150 mm as per the ratio of one, two and three times the diameter of circular model footing. Test procedure The load-settlement behavior of the stone/sand column has been studied by applying vertical load with the help of a loading frame. To load the stone/sand column area alone, a loading plate of diameter equal to the diameter of the column was placed over the granular column. The load was applied through a proving ring of appropriate least count and capacity at a constant strain rate of 1.25 mm/min. Settlements were monitored for a total settlement equal to 20% of the diameter of model circular footing. As the loading is quick it is essentially undrained loading which simulates the loading condition immediately after the construction. A series of laboratory model tests were performed i.e. load test on the clay bed, load test on ordinary sand columns and load tests on confined footing with varying diameter(1.4 times and 1.7 times the diameter of model circular footing) and varying heights (50, 100 and 150 mm). Testing Parameters Testing parameters are given in tabular form in Table No. 4. Test Variable Parameters No. of Tests Description Series D d h R.D. A Test on Clay bed B Sand column (end bearing) %, 70% 2 C Sand column (floating) , 100, %, 70% 6

8 Sareesh Chandrawanshi, Rakesh Kumar and P.K. Jain D E Confined (dia=1.4d) Confined (dia=1.7d) footing footing RESULTS AND DISCUSSIONS Reading corresponding to 5 mm settlement or 10% S/D ratio were taken for calculation of bearing capacity. S/D Ratio is equal to settlement of confined footing/floating sand column/end bearing sand column in terms of diameter of model test footing. 50, 100, %, 70% %, 70% 2 Fig. 4. Load settlement curve for floating sand column of varying depths (R.D.=60%) Effect of floating sand column Construction of floating sand column increases the load carrying capacity of the soil. With the increase of depth the load carrying capacity also increase. The reason attributed to this phenomenon is that more clay is being replaced by more stable and compressed sand. Increase was of the order of 10%, 45% and 85% with respect to clay bed bearing capacity with depth D, 2D and 3D depths of floating sand column. Results were shown in Fig. 3 for clay bed only and in Fig. 4 for floating sand column. Fig. 5. Load settlement curve for floating sand column of varying depths (R.D.=70%) Fig. 3. Load settlement curve for clay bed Effect of confined footing Confined footing of two different diameters were used. The confined footing of 1.4 times the diameter of model test footing shows considerable in increases the load carrying capacity of the soil. The reason attributed to this phenomenon is that clay is being replaced by compressed sand and its bulging is restricted due the presence of confining cell. Increase was of the order of 80%, 105% and 125% with respect to clay bed bearing capacity

9 with depth D, 2D and 3D depths of confined footing. Results were shown in Fig. 5. And for the confinement cell of 1.7D no considerable increment of bearing capacity is observed because the sand is getting no lateral confinement due to the larger diameter of confinement cylinder. Fig. 6. Load settlement curve for confined footing (dia=1.4d) for varying depths (R.D.=60%) Comparison of various cases Comparison of all the above cases were shown in Fig. 8 and Fig. 9. Fig. 8. Comparison of load settlement curve of clay bed, floating sand column, confined footing (dia=1.4d) and end bearing sand column (R.D.=60%) Fig. 7. Load settlement curve for confined footing (dia=1.4d) for varying depths (R.D.=70%) Fig. 9. Comparison of load settlement curve of clay bed, floating sand column, confined footing (dia=1.4d) and end bearing sand column (R.D.=70%) Effect of end bearing sand column The maximum gain in bearing capacity with respect to clay bed is achieved in end bearing case because the in this case because here is maximum replacement of clay with sand and it also derives strength from the vertical confinement apart from lateral confinement in previous cases.

10 Sareesh Chandrawanshi, Rakesh Kumar and P.K. Jain Fig. 10. Comparison of load settlement curve of clay bed, confined footing (dia=1.7d) and end bearing sand column (R.D.=60%) Fig. 11. Comparison of load settlement curve of clay bed, confined footing (dia=1.7d) and end bearing sand column (R.D.=60%) CONCLUSIONS In the present study model tests have been conducted extensively on sand column in Geotechnical Engineering Laboratory, Department of Civil Engineering, MANIT Bhopal. The aim of the experimental investigation program was to investigate the feasibility of floating sand column and confined footing to strengthen the soft clay. Analysis of experiment test data indicates that with the use floating sand column and confined footing, the load carrying capacity increases. The trends obtained in these laboratory tests are in good agreement with the results reported in the literature. The major conclusions that can be drawn from the present study are as follows: a. Inclusion of floating sand column considerably improves the load settlement characteristics of soft clay. b. The load carrying capacity of floating sand column increases with the increase in depth of column. c. The load carrying capacity of floating sand column increases with the increase in relative density of sand column. d. Construction of confined footing considerably improves the load settlement characteristics of soft clay. e. The load carrying capacity of confined footing increases with the increase in depth of column. f. The maximum capacity is achieved in the case of end bearing sand column. g. The confined footing is better than the floating sand column for the same depth. h. Confined footing of 1.4 times the diameter of model test footing should be preferred over floatable sand column and recommendable depth is twice the diameter model test footing as it would restrict the bulging completely thus giving optimum results. REFERENCES [1]. Binquet, J., and Lee, K.L. (1975). "Bearing capacity tests on reinforced earth slabs." Journal of Geotechnical Engineering Division, ASCE, 101, [2]. Eid, Hisham T. (2012). "Bearing capacity and settlement of skirted shallow foundations on sand." International Journal of Geomechanics, ASCE. 13, [3]. Gupta, Rajeev and Trivedi, Ashutosh (2009)." Bearing capacity and settlement of footing resting on confined lossesilty sands." Electronic Journal of Geotechnical Engineering, 14, [4]. Mandal, J.N., and Manjunath, V.R. (1995). "Bearing capacity of strip footing resting on reinforced sand subgrades." Construction and Building Material, 9(1), [5]. Madhav MR and Miura N (1994) "Soil ImprovementPanel report on Stone Columns". In: Proceedings of the 13th international

11 conference on soil mechanics and foundation engineering, New Delhi, India, pp [6]. Nazir, Ashraf K. and Azzam, Wasim R. (2010). Improving the bearing capacity of footing on soft clay with sand pile with/without skirts. Alexandria Engineering Journal, 49, [7]. Rao, N.B.S. and Nayak, G. (1995) "Model Studies on Partially Confined Sand Column using Geo-grid tube". Indian Geotechnical Journal, Vol. 25, No. 3, pp [8]. Sawwaf, M.EI., and Nazer, A. (2005). "Behavior of circular footings resting on confined granular soil." Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 131(3), [9]. Sharma et.al. (2004) "Compressive Load Response of Granular Piles Reinforced with Geogrids". Canadian Geotechnical Journal, Vol. 41, No. 1, pp