STUDY OF SWELLING BEHAVIOUR OF BLACK COTTON SOIL IMPROVED WITH SAND COLUMN

Transcription

1 STUDY OF SWELLING BEHAVIOUR OF BLACK COTTON SOIL IMPROVED WITH SAND COLUMN Aparna 1, P.K. Jain 2, and Rakesh Kumar 3 1 M. Tech. Student, Department of Civil Engineering M.A.N.I.T. Bhopal, (M.P) India. 2 Professor, Department of Civil Engineering, M.A.N.I.T. Bhopal, (M.P) India. 3 Assistant Professor, Department of Civil Engineering, M.A.N.I.T. Bhopal, (M.P) India. ABSTRACT This paper presents the result of an experimental study conducted for evaluating the effect of size of the sand column on swelling of expansive soil. The sand columns of diameters25mm, 37.5mm and 50mm were made in black cotton soil test beds in a cylindrical mould of diameter 100mm. The test beds were prepared at different water contents (14, 18, 22, 26,30,36,40 and 44% by weight of dry soil) keeping the dry density of the soil as constant. The soil with sand column was submerged and the swelling of the composite material was observed. The test results show that the presence of sand column in the expansive black cotton soil reduces the swelling. The reduction in swelling depends on the size of the sand column and the initial moisture content in the soil. A column of diameter 50mm reduces swelling more than the smaller ones. For 14% initial moisture content in the black cotton soil, the stone columns of diameters 25mm, 37.5mm and 50mm have shown reduction in swelling by 11.5%, 23% and 42% respectively in comparison to that exhibited by the raw soil. The soil with high initial moisture content shows less swelling than those with low moisture content. Thus, by manipulating the initial moisture content and the diameter of the sand column, the expansive soil reinforced with sand columns can be made volumetrically stable. KEYWORDS: Sand column, swelling, expansive soil, composite ground, ground improvement. I. INTRODUCTION Expansive soils are encountered in arid and semi-arid regions of the world, where annual evaporation exceeds annual precipitation. In India, expansive soils cover about 20% of the total land area (Ranjan and Rao 2005, Shelke and Murthy 2010). These soils increase in volume on absorbing water during rainy seasons and decrease in volume when the water evaporates from them (Chen, 1988). The volume increase (swell) if resisted by any structure resting on it; then vertical swelling pressure is exerted by the soil on the structure. This pressure if not controlled, may cause uplifting and distress in the structure (Shelke and Murthy 2010). The strength loss on wetting is another severe problem with such soils. Due to this peculiar behaviour many civil engineering structures constructed on expansive soils get severely distressed. Pavements are in particular, susceptible to damage by expansive soils because they are lightweight and extend over large areas. Dwelling houses transferring light loads to such soils are also subjected to severe distress. Similarly, earth structures such as embankments, canals built with these soils suffer slips and damages (Mishra et al., 2008). Soil stabilization techniques are widely used for stabilizing expansive soils. Physico-chemical stabilization using lime, cement, fly ash, enzymes, and other chemicals control the swelling in expansive soil (Lopez-Lara et al., 1999). In these techniques, uniform mixing of the stabilizers in soil must be ensured else erratic results may come. Mechanical stabilization of soil (without altering chemical properties) includes controlling compaction (Sridharan and Gurtug, 2004), pre-wetting (Chen, 1988), mixing with sand (Sridharan and Gurtug, 2004; Mishra et al., 2008), using cohesive non-swelling soil (Katti et al., 1983), EPS geofoam.(shelke and Murthy 2010), reinforcing the soil 905 Vol. 7, Issue 3, pp

2 using geosynthetics (Sharma and Phanikumar, 2005; Ikizler et al., 2009) and by using polypropylene fiber ( Muthukumar 2012) are used. Special foundation techniques such as lime under-reamed piles, belled piers and granular pile anchors are also suggested (Phanikumar 1997). Recently Kumar and Jain 2013 and Kumar 2014 have demonstrated that the concept of granular pile, that is popular in improving the weak marine clays, could be utilized to improve the load carrying capacity of soft expansive black cotton soils too. In the granular pile, also known as stone column, technique about 10 to 35% weak soil is removed and replaced with granular material in the form of piles. Kumar (2014) performed model test in the laboratory on granular piles of sand constructed in soft expansive soil of different UCS values. End bearing granular Piles were casted in the soil and the load test were performed. The test results showed that load carrying capacity of a footing resting on granular pile is significantly more than the corresponding value for the footing placed directly on the soft soil bed. The increase in load carrying capacity is observed for all soil consistencies of the expansive soil. It is concluded from his study that loss in strength and excessive settlement of the expansive soil due to wetting could be minimized to a large extent by installation of granular piles in the soil. As reported above, besides the strength loss, the swelling and volume instability are the other severe problems with these soils. The present work is an attempt to fill this gap. The size of the sand column and the consistency of the soil play an important role in changing the behaviour of composite ground. Hence these two aspects are varied in the present work and the influence of the sand column diameter and the initial moisture content on the swelling of an expansive black cotton soil has been studied. The details of the experimental program, results of the tests and the conclusions drawn from the study are described below. II. EXPERIMENTAL PROGRAM The experiments were carried out in a cylindrical mould. The soil beds of black cotton soil were prepared in the mould at a dry density of 15kN/m 3. The initial mixing water content in the soil and the diameter of the sand columns were the variable of the study. The swelling of composite soil (black cotton soil reinforced with a sand column) on wetting was recorded. Soil beds were prepared with 14%, 18%, 22%, 26%, 30%, 36%, 40%, 44% of water content and for each test bed three diameters of sand columns (25 mm, 37.5 mm, 50 mm) were installed. One series of swell measurements were taken for black cotton soil beds prepared at the above water contents i.e. without the sand columns. III. MATERIAL PROPERTIES Two basic materials used for this study are: the black cotton soil representing the soft soil to be improved and, the fine river sand as sand column forming material. The properties of these materials are as follows: (i) Black cotton soil: The black cotton soil was taken from NIT Bhopal campus. Its properties are given in Table -1. Table -1: Properties of Black Cotton Soil Properties Values Liquid limit (L.L.), % 54 Plastic limit (P.L.)% 29 Plasticity index (P.I.), % 25 Maximum dry density (MDD), kn/m 3 15 Optimum water content (OMC), % 23.5 Differential free swell (DFS), % 40 Specific gravity (G) 2.64 Clay and silt content, % 95.0 Soil Classification (IS: ) CH, Clay of high plasticity (ii) Sand: Properties of the river sand used in the sand column are listed in Table Vol. 7, Issue 3, pp

3 Table -2: Properties of Sand Properties Values Particle size corresponding to 10% finer, D 10, mm 0.32 Particle size corresponding to 20% finer, D 20, mm 0.41 Particle size corresponding to 30% finer, D 30, mm 0.44 Particle size corresponding to 50% finer, D 50, mm 0.51 Particle size corresponding to 60% finer, D 60, mm 0.54 Coefficient of curvature, C C 1.12 Coefficient of uniformity C U 1.69 Minimum dry density, ρ min, kn/m Maximum dry density, ρ max, kn/m Specific gravity, G 2.76 Brown s suitability Number (Brown 1976) 8.87 Soil Classification (IS: ) SP (Poorly graded sand) IV. TEST SETUP AND PROCEDURE A typical test arrangement with a single column of sand is shown in Fig. 1. A swelling mould of 100 mm diameter and 128 mm height with one collar and two porous plates was used. The soil was oven dried and a predetermined amount of water was mixed and compacted in three layers to attain a dry density of 15kN/m 3. A porous plate with filter paper was placed below the soil sample. Then a hole of required diameter was formed by using an auger and a casing pipe. The granular material (fine river sand) was filled in the hole and compacted in layers to get the required density. One porous plate with a filter paper is placed above the soil sample and the collar is fitted to the mould. Heave stake was placed on the soil sample inside the swelling mould and a dial gauge was fixed on the top of the heave stake to measure the swelling. This entire arrangement is placed inside a tank filled with water. The swelling was monitored continuously by taking the dial gauge readings from time to time till dial reading ceases to change. Tests were conducted for all the specimens prepared with different water contents and with different sizes of the sand columns. Fig. 1 Experimental setup V. RESULTS AND DISCUSSIONS The variation of swelling with respect to the time, for 14% water content in the soil, is plotted in Fig. 2. It was observed that swelling continue to occur nearly for about 96 hours, beyond which there is no change in swelling. Similar trend is obtained for test beds of other initial water content in the soil. 907 Vol. 7, Issue 3, pp

4 Fig 2 Variation of swelling of soil with time at 14% water content The maximum swelling in each case was noted and its variation with the diameter of the sand column is plotted (Fig. 3). It can be observed from this figure, that there is a decrement in swelling with the increase in the diameter of the sand column. The sharp decrement in swelling is observed when the test bed is prepared at lowest water content of 14%. In this case the 50mm diameter stone column reduces swelling by 42% in comparison to the raw soil. The corresponding values for 37.5mm and 25mm diameter stone columns are 23% and 11.5% respectively. The reason for observing reduction in swelling with installation of the sand columns is mainly due to the replacement of expansive soil by non expansive sand. A large diameter column replaces more soil, hence reduction in swelling is observed more than that by the smaller one for a particular value of the initial water content in the soil. Fig. 3 Effect of sand column diameter on swelling of expansive soil Further, there is significant reduction in swelling with increase in the initial water content in the black cotton soil. To show it, Fig. 4 is plotted. It can be noted from this figure that as moulding water content (i.e. the initial water content) in the black cotton soil increases, the swelling decreases in all 908 Vol. 7, Issue 3, pp

5 the cases. This observation is obvious because an expansive soil containing high initial water content has little scope for imbibing more water on submergence as all its mineral particles are already saturated and therefore very small, practically nil, swelling is observed at high initial moisture content of 44%. Fig 4 Variation of swelling with water content for raw soil and sand column reinforced soil VI. CONCLUSIONS The results of the testing program show that installation of sand column in expansive soil controls the swelling effectively. The size of sand column and initial water content in the black cotton soil affect the swelling behaviour. A large size sand column reduces swelling more than by a smaller one. Swelling is also reduced with increase in water content. At 44% water content there is no swelling found in the soil. The reduction in swelling is mainly due to replacement of expansive soil by nonexpansive sand and also because of presence of water in the soil. Thus if sand columns are installed in expansive soils in wet condition maximum benefit in terms of volume stability can be achieved. VII. SCOPE FOR FUTURE WORK Installations of granular pile in soil are easy in comparison to other methods of soil improvements such as lime or cement stabilization, where proper mixing of stabilizer with the soil, the depth of soil to be treated poses practical difficulty. The use of sand column/ granular pile technique in swelling soil is a relatively new area. The density, mineral characteristics of the expansive soil, and the properties of the granular pile forming material are expected to influence performance of sand columns in expansive soils. Future research in this area will pave the way to develop a design methodology for mitigating the problems of expansive soil by sand columns with confidence. REFERENCES [1]. Brown, R.E. (1976). Vibration compaction of granular hydraulic fills. ASCE, National Water Resources And Ocean Engineering Convention, pp [2]. Chen, F.H. (1988). Foundations on Expansive Soils. Elsevier Scientific Publishing Co., Amsterdam. [3]. Ikizler, S. B., Aytekin, M. and Vekli, M. (2009). Reductions in swelling pressure of expansive soil stabilized using EPS geofoam and sand. Geosynthetics International, 16(3), Vol. 7, Issue 3, pp

6 [4]. I.S: Classification and Identification of soils for General Engineering Purposes. [5]. Harishkumar. K and Muthukkumaran, K (2011). Study on swelling soil behaviour and its improvements. International Journal of Earth Sciences and Engineering, ISSN , Volume 04, No 06 SPL, October 2011, pp [6]. Katti, R. K., Bhangle, E. S. and Moza, K. K. (1983). Lateral pressure of expansive soil with and without cohesive non-swelling soil layer applications to earth pressures of cross drainage structures of canals and key walls of dams (studies of K 0 condition). Central Board of Irrigation and Power. Technical Report 32, New Delhi, India. [7]. Kumar, R. and Jain P. K. (2013). Expansive Soft Soil Improvement by Geogrid Encased Granular Pile. Int. J. on Emerging Technologies, 4(1): 55-61(2013). [8]. Kumar, R. (2014). A Study on soft ground improvement using fiber-reinforced granular piles, Ph. D. thesis submitted to MANIT, Bhopal (India) [9]. Lopez-Lara, T., Zepeta- Garrido, J. A. and Castario, V. M. (1999). A comparative study of the effectiveness of different additives on the expansion behavior of clays. Electronic Journal of Geotechnical Engineering, 4(5), paper [10]. Mishra, A. K., Dhawan, S. and Rao, M. S. (2008). Analysis of swelling and shrinkage behavior of compacted clays. Geotechnical and Geological Engineering, 26(3), [11]. Muthukumar. M (2012). Swelling pattern of polypropylene fiber reinforced expansive soils. International Journal of Engineering Research and Applications, Vol. 2, Issue 3, May-Jun 2012, pp [12]. Phanikumar, B.R.(1997), A study of swelling characteristics of and granular pile anchor foundation system in expansive soils. Ph.D. thesis, Jawaharlal Nehru Technological Univ., Hyderabad, India. [13]. Ranjan, G and Rao, A.S.R. (2005), Basic and applied soil mechanics, New Age International (P) Ltd, New Delhi pp 753. [14]. Sharma, R. S. and Phanikumar, B. R. (2005). Laboratory study of heave behaviour of expansive clay reinforced with geopiles. Journal of Geotechnical and Geoenvironmental Engineering, 131 (4), [15]. Shelke, A.P. and Murthy, D.S. (2010). Reduction of Swelling Pressure of Expansive Soils Using EPS Geofoam. Indian Geotechnical Conference (2010). [16]. Sridharan, A. and Gurtug, Y. (2004). Swelling behavior of compacted fine-grained soils. Engineering Geology, 72(1-2), AUTHORS Aparna was born in Lucknow (U.P.), India, in She received the Bachelor degree in civil engineering from Babu Banarasi Das National Institute of Technology & Management, in 2012 and she is currently pursuing the Master degree in geotechnical engineering from Maulana Azad National Institute of Technology, Bhopal, which will complete in July, Her research interests include geotechnical engineering. Pradeep Kumar Jain was born in Jhansi (U.P), India, in He received the Bachelor in civil engineering degree from the MITS, Gwalior, in 1986 and Master degree in construction technology and management from MITS, Gwalior, in He has completed his PhD from IIT Roorkee, Roorkee, in His research interests include geotechnical engineering. Rakesh Kumar was born in Gajrola (U.P), India, in He received the Bachelor degree in civil engineering from the MMMEC, Gorakhpur, in 1999 and Master degree in geotechnical engineering from IIT Roorkee, Haridwar, in He has completed his PhD from Maulana Azad National Institute of Technology, Bhopal, in His research interests include geotechnical engineering. 910 Vol. 7, Issue 3, pp