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1 Research Paper PROSPECT OF USING GRANULAR PILES FOR IMPROVEMENT OF EXPANSIVE SOIL Kumar Rakesh. 1 and Jain P.K. 2 Address for Correspondence 1 Asst. Professor 2 Professor, Department of Civil Engineering, M.A.N.I.T. Bhopal , India ABSTRACT An expansive soil is treacherous from civil engineering construction point of view as it is volumetrically unstable due to seasonal moisture variation. Its strength decreases and compressibility increases tremendously on wetting. Different ground improvement techniques have been proposed in the literature to counteract the ill effects of soil instability. The construction of granular piles/stone columns has been proved successful in improving soft marine clays, which are very poor from strength and compressibility criteria. The expansive soil may also be considered as soft soil under wet condition. The technique of granular pile may be applied in expansive soil too to have its improved behaviour. The paper discusses the outcome of one such attempt made at MANIT, Bhopal wherein Rathod (2012) has carried a model study on granular pile made in expansive black cotton soil. The load settlement behavior of the soil was determined for different size of the granular pile. Geo-grid encased granular piles were also installed in the soil. It is observed that significant improvement in load carrying capacity is obtained with installation of granular pile without and with geo-grid encasement. For a given value of applied load, the settlement of the soil with granular pile is found to reduce significantly in comparison to that without the pile. KEYWORDS: Granular Pile/Stone column, Bearing Capacity, Settlement, Soft Soil (Expansive Soil) 1. INTRODUCTION Expansive soils are found in many parts of the world. Argentina, Australia, Burma, Canada, Cuba, Ethiopia, Ghana, India, Iran, Mexico, Morocco, Rhodesia, South Africa, Spain, Turkey, U.S.A., Venezuela and Israel are reported to have significant area having expansive soil regions Donaldson, (1969). In India about one -fifth of the area is covered by expansive soil. The soil is popularly known as black cotton soil as it is black in color and is good for growing the cotton. The soil behaves like a soft soil under wet/saturated condition. There are a number of methods available to stabilize expansive soil such as soil replacement, sand cushion, cohesive nonswelling layer, mechanical, chemical and thermal stabilization etc. Where the construction on a large area is to be carried, such as the construction of highways over the expansive soil belts, these techniques are not very much effective or become costly. The concept of granular pile, used to improve weak marine clays, have prospect of being utilized in improving the behaviour of expansive soils in such regions. Granular piles/stone columns are constructed in soft soils by making circular holes and filling them with granular materials such as natural stone, sand or stone chips. When about 10 to 35% weak soil is removed and replaced with granular material in the form of piles, the load carrying capacity of the ground increases and settlement decreases significantly and the ground becomes useable to support the structure. With this in view, an attempt has been made at MANIT Bhopal to study the behaviour of saturated expansive black cotton soil installed with granular piles. A review of literature on granular piles installed in soft clays suggest that the failure of granular piles takes place by bulging of the granular pile material near the upper portion of the pile, hence geo-grid encased granular piles have also been installed in the saturated expansive soils and the load settlement behaviour of the soil was studied. The paper reviews the various ground improvement techniques commonly employed in expansive soils, discusses their merits and demerits and briefly describes the concept of granular pile in soft soils. The process of installation of granular piles in soft soils and failure modes of the granular piles are also discussed. The model study carried by Rathode (2012) is then presented. 2. COMMON METHODS OF EXPANSIVE SOIL IMPROVEMENT 2.1. Soil Replacement In this method the poor soil is excavated up to certain depth and is replaced by good soil which is not expansive. This is possible only where the non problematic soil is easily and cheaply available nearby. Removal and replacement is generally practical only above ground water table. Earthwork operation is difficult when the soil is wet or submerged Sand Cushion Method When the entire depth of the expansive soil stratum or a part there of is removed and replaced with the sand; compacted to the desired density and thickness, the ill effects of poor soil are minimized Satyanarayana, (1969). The basic advantage of the sand cushion method is its ability to adapt itself to volume changes in the soil. However, the sand cushion method has several limitations particularly when it is adopted in deep strata. The high permeability of sand creates conditions conducive to easy ingress and accumulation of water from surface runoff Cohesive Non Swelling Layer Replacement by soils with relatively impervious material may, to a great extent offset the disadvantages of sand cushion method. The method proposed by Katti (1979) uses cohesive non-swelling (CNS) layer to reduce the effects of swelling. The heave of expansive soil underlying a CNS layer reduces exponentially with increase in thickness of the CNS layer and attains a value of no heave around a depth of 1m.The shear strength of the underlying expansive soil at the interface and below increases with the thickness of CNS layer. The method is recommended for construction of canals in black cotton soil area.

2 2.4 Mechanical Stabilization Mechanical stabilization is the process of improving the properties of soil by changing its gradation. Two or more type of natural soils is mixed to obtain composite material which has better strength. Generally coarse grained materials such as sand, crusher dust, moorum (a soil predominantly coarse grained, red in color having fine silt and clay) etc are mixed with fine grained soil. 2.5 Chemical Stabilization Mixing of lime, cement, fly ash and combination of these in small quantities changes the physicochemical characteristics around and inside of clay particles and the soil gives improved behaviour. A brief description of these methods is as follows Stabilization with Lime Lime stabilization has been used successfully on major projects to minimize swelling of the expansive soil. In general all lime treated fine grained soil exhibit decreased plasticity, improved workability and reduced volume change characteristics. Generally, 3 to 8% by weight of hydrated lime is added to the top several inches of the soil. Lime continues to be widely used additive for modification of expansive clays in view of its cost-effectiveness although limited success in many instances. Lime diffusion into soil either from lime piles or lime slurry pressure injection is also used. It is reported that hardly 38 to 50mm diffusion of lime in to soil takes place in 1 to 4 years unless extensive fissure and crack system is present. Venkataswamy et.al.(2003) studied the improvement of expansive clay by deep in-situ technique. They concluded that the pozzolonic reaction due to presence of lime has shown marked increase in unconfined compressive strength and reduction in swelling pressure as well as plasticity index. They made a hole of 150mm diameter by pushing a steel pipe in to the soft ground up to a depth of 5.5 m and poured a mixture of sand and calcium in1:1 proportion. All the area covered for 21 days by gunny bags then undisturbed samples were collected at different radial distances. The improvement in properties was found up to radial influence zone is of 750 mm at 3.5m, 4.5m and 5.5m depths Stabilization with Cement Portland cement can be used either to modify or improve the quality of the soil or to transform the soil in to a cemented mass with increased strength and durability. The amount of cement used depends upon whether the soil is to be modified or stabilized Kowasliki, et. al.( 2007).The hydration of Portland cement is a complex pozzolanic reaction that produces a variety of different compounds and gels. The results of mixing cement with clay soil are similar to that of lime. It reduces liquid limit, the plastic index and the potential of volume change. It increases the shrinkage limit and shear strength. Addition of 2 to 6% cement by weight of soil can produce a soil that acts as a semi rigid slab. Some investigators have tried and succeeded in minimizing the swelling of expansive soil using chemicals like calcium chloride (CaCl 2 ), calcium sulphate (CaSo 4 ), potassium chloride (KCL), aluminium chloride etc Stabilization with Fly ash As fly ash is freely available, for projects in the vicinity of thermal power plants, it can be used for stabilization of expansive soil. Phanikumar and Sharma (2004) studied the effect of mixing fly ash (content 5, 10, 15 and 20% by dry weight of soil) on engineering properties of expansive soil through an experimental investigation. Free swell index was found to reduce by 50% on addition of 20% fly ash. The hydraulic conductivity of expansive soil decreases with increase in fly ash content. The undrained shear strength increases with increase in the ash content. White et al reported that addition of fly ash changes soil compaction characteristics, compressive strength, wet/dry durability, freeze/thaw durability, hydration characteristics, and rate of strength gain and plasticity characteristics. Ramarao et al., (2005) studied the developments of cohesive bonds in a lime-stabilized fly ash cushion. The combination of lime and fly ash is expected to produce an environment similar to the one obtained in CNS material following saturation and consequently arrest heave. The results of the study showed a new solution to the problem of heave of expansive soil could be in the form of Fly ash cushion method. It also solves the problem of fly ash utilization and disposal to some extent. If at a site containing black cotton soil, the depth of the active zone is 3m, it would be sufficient if 1.5m of expansive clay is removed and replaced with fly ash cushion to reduce heave significantly. With the superstructure load causing further reduction of heave, the amount of sub-excavation and replacement with lime stabilized fly ash cushion can be further reduced. 2.6 Stabilization of Expansive Soil using Reinforcement Reinforcing the soil is usually accomplished by one of the following methods: Soil nailing, Soil anchoring, Micro piles, Stone columns and Fiber reinforcement. Using fibres like jute fabrics, coir ropes, rubber tire chips, waste plastics, synthetic fibres etc one can stabilize the expansive soils. The soil and its reinforcing elements act in combination and increase the shear strength of the soil mass, reduce its settlement under the load, and improve its resistance to liquefaction. The work reported by Al-Omari and Hamodi (1991) showed the feasibility of using tensile geo grid for the purpose of controlling the swell of plastic soils. Swelling tests using an enlarged odometer revealed promising results. The reinforcements were cylindrical geo grid of varying stiffness values embedded in clays of different plasticity indices. The reduction in swell increased with increasing the geo grid stiffness.relative merits and demerits of different methods discussed above are compared in Table1. Table.1 Merits and Demerits of Commonly used Expansive Soil Improvement Methods Ground Improvement Method Soil Replacement Merits When the soil to be replaced is up to shallow depth and good soil is easily/cheaply available. Demerits If good soil is not cheaply available, this method can be expansive.

3 Sand Cushion CNS Method Mechanical Stabilization Chemical Stabilization Soil Reinforcement This is simple method and need no special methods/equipment. The CNS concept is useful for canal lining. If area is small the use of admixtures is good. If area and depth of treatment is small the use of admixtures is good. Useful for shallow depth. High permeability of sand facilitates easy ingress of water from the surface runoff and the swelling process accelerates. The CNS concept for buildings has been very limited. If area is large the cost will be more. If the soil is to be treated for greater depth, the method is cumbersome. Mixing/laying is major problem. 3. GRANULAR PILE IN SOFT CLAYS Weak soil, which has very low shear strength and high compressibility to support structures require strengthening to be capable of carrying loads from structures. Granular piles when installed in soft clays they improve the load carrying capacity and reduce settlement. They act as vertical drains and thus speeding up the process of consolidation. The replacement of the soft soil by a stronger material and initial compaction of soil during the process of installation increases the unit weight of the soil. Granular piles are ideally suited for structures, because of reduction of total and differential settlements, increased bearing capacity of the site to make it possible to use shallow foundation. Granular pile have provided an economical method of support in compressible and fine-grained soils for low-rise buildings and structures such as liquid storage tanks, abutments, embankments, and factories that can tolerate some settlement. 3.1 Construction of Granular Pile The improvement of a soft soil with granular pile can be accomplished using various techniques that involve excavation, replacement and compaction. Rao (1982) and Ranjan and Rao (1983) developed a simple method, particularly useful in developing countries, which is technically viable and uses indigenously developed equipment. A spiral auger is used to make the borehole utilizing manual labor. After reaching the desired depth, the borehole is thoroughly cleaned and the stone aggregate is placed in the borehole in layers of mm followed by sand layer of mm. A cast iron hammer weighing 125 kg and diameter less than the diameter of the borehole, operated by a power winch having a fall of 750 mm is used to compact the sand/stone aggregate layer. During the course of compaction, the sand fills the voids of the stone aggregates followed by the lateral and downward displacement of the charged material till full compaction of the surrounding soil is reached. Various stages of installation procedure of a granular pile are shown in Fig.1. No skilled labor is required. Hence the technique is economical where man power is cheaply available such as in the developing countries like India FAILURE MODES OF GRANULAR PILE Three cases are discussed here Single Granular Pile in Homogeneous Soft Layer Granular pile may be constructed as either end bearing on a firm stratum underlying soft soil, or as floating columns with the tip of the column embedded within the soft layer. Fig.1.Granular Pile Installation by Simple Auger Boring Method (After Rao, 1982) The failure of a granular pile may take place in one of the following three modes. Either end bearing or frees floating granular piles having length greater than about three diameters in length fail in bulging as illustrated in Fig.2(a). A very short column bearing on a firm support will undergo either general or local shear (bearing capacity) type failure at the surface [Fig.2(b)]. Finally, a floating granular pile less than about 2 to 3 diameters in length may fail in end bearing in the weak underlying layer before a bulging failure can develop [Fig.2(c)]. For the subsurface conditions generally encountered in practice, however, bulging is usually the controlling failure mode. Fig.2 Failure Modes of a Single Stone Column in Homogeneous Cohesive Soil (After U.S. Department of Transportation, Report No. FHWA/RD-83/026, 1983) Single Granular pile in Non-Homogeneous Cohesive Soil Fig.4 shows a failure mechanism of a single Granular pile in non-homogeneous cohesive soils. A very soft zone at the surface, l-3 m thick, has a dominating influence on the settlement and ultimate strength of either granular pile/stone column groups or single columns. Further, field observations have indicated that the presence of a very weak layer such as peat, greater than about one column diameter in thickness, may also seriously affect granular pile performance [Fig.3 (b) and (c)]. Fig.3 Failure Mechanisms of a Single Stone Column in Non-Homogeneous Cohesive Soil

4 Group Granular Pile Homogeneous Soft Layer The ultimate load carrying capacity of Granular pile in group is slightly more than an isolated single granular pile due to the confinement of interior column by surrounding columns. As a result, interior columns are somewhat stiffened and give rise to slight increase in the ultimate load carrying capacity per column. A wide flexible loading such as due to construction of an embankment over a stone column improved ground is illustrated in Fig.4(a).The soil beneath and to the sides of the foundation move laterally outward due to the construction of the embankment over the weak foundation as illustrated in Fig.4(a) and (b). This phenomenon is called spreading. Lateral spreading also slightly increases the bulging, the granular pile undergoes, compared to the condition of no spreading. A group of stone columns in a soft soil probably undergoes a combined bulging and local bearing type failure as illustrated in Fig.4(c). A local bearing failure is the punching of a relatively rigid granular pile (or group) into the surrounding soft soil. Granular pile groups having short column lengths may fail in end bearing [Fig.4 (d)] or perhaps undergo a bearing capacity failure of individual granular pile similar to the failure mode of short single granular pile. Fig.4 Failure Modes of Granular Pile in Group (After U.S. department of Transportation, Report No. FHWA/RD-83/026, 1983) Mugerson,S. et al (2006), have brought out conceptual performance of granular piles encased in geosynthetic material and recommended it for supporting a wide variety of structures including buildings and flexible structures. The stone columns derive their load capacity from the confinement offered by the surrounding soil. In very soft soils this lateral confinement may not be adequate and the formation of the stone column itself may be doubtful. Wrapping the individual stone columns with suitable geotextile/geosynthetic is one of the ideal forms of improving the performance as it makes the stone columns stiffer and stronger. In addition, encasement prevents the lateral squeezing of stones in to the surrounding clay soil and vice versa, preserves drainage function of the stone column and frictional properties of the aggregates. 4. MODEL STUDIES ON GRANULAR PILES IN EXPANSIVE SOIL Rathod (2012) has carried experimental investigation to study the behavior of granular pile in expansive soil (Fig.5). The experiments were carried out in a cylindrical tank of diameter 300mm and height 750mm. Tests were carried out with two types of loading 1. Only granular pile/stone column loaded to estimate the limiting axial capacity. 2. The entire area of tank loaded Fig.5 Test Setup used in Model study by Rathod(2012) The black cotton soil was taken from Raisen road Bhopal, India. The soil is clay of high compressibility. The properties of the soil are given in Table 2. Table.2 Properties of Expansive Clay used in the Study Property Specific Gravity Clay Content Silt Content Liquid Limit Plastic Limit Plasticity Index Value % 52 % 60 % 28 % 32 % Granular materials consisting of crushed stone having size between 2mm to 10mm have been used to form the stone column. To load the stone column area alone, a loading plate equal to the diameter of the column was placed over the stone column. The load was applied through a proving ring up to15 minutes. Settlements were monitored for equal intervals of loads up to failure. In the case where load is applied over the entire area, a 30 mm thick sand layer was placed over the entire surface. A steel plate of 12 mm thickness and 10 mm diameter less than the inside diameter of the tank was placed over the sand blanket. The loading was applied in the similar way until the settlement exceeded 10 mm. The granular piles of diameter 40mm, 60mm and 80mm were taken in the study. The load-settlement behaviour of soil with granular pile of different diameters is shown in Fig.6 and Fig. 7. Fig. 6. Load-Settlement Curve for Granular Pile of Different Diameters (Only Stone Column Loaded) Fig.7. Load-Settlement Curve for Granular Pile Encased with Geo grid for Different Depths (Only Stone Column Loaded)

5 The load settlement behaviour of geo grid encased stone columns with different lengths of geo grids is shown in Fig8 and Fig. 9. Fig.8. Load-Settlement Curve for of Granular Pile of Different Diameter (Entire Area Loaded) Fig.9. Load-Settlement Curve for of Geo grid Encased Granular Pile to Different Depths (Entire Area Loaded) The findings of the studies are summarized as given below: Stone columns play an effective role in reducing the settlements of expansive soil and increase the bearing capacity. As the diameter stone columns increases the bearing capacity increases and settlement decreases. The bearing capacity of the stone column increases by introducing circumferential geogrid reinforcement. And as the depth of circumferential reinforcement increases settlement decreases and the bearing capacity increases. Further reduction in settlements is noticed with the increasing depth of geo grid-encasement. On full depth encasing, reduction in total settlements of up to 79.13% is noticed. Smaller diameter geo grid-encased stone columns show better performance than large diameter geo grid-encased stone columns. The tests also reveal that the larger diameters of ordinary stone columns can be replaced by smaller diameters of geo grid-encased stone column. 5. CONCLUSIONS The unsuitable and unfavorable sites can be utilized by using ground improvement techniques. A variety of ground improvement techniques are available and have to be adopted according to necessity of structure and economy. The use of granular pile as a technique of soil reinforcement is frequently implemented in soft cohesive soils and has been successfully used to support isolated footings, large raft foundations and embankment. Their use in soft clays has been found to provide increases in load carrying capacity accompanied by significant reduction in settlement. Being granular and freely drained material, consolidation settlement is accelerated and post construction settlement is minimized. The possibility of using them for improving behavior of expansive black cotton soil has been explored through a model study conducted art MANIT, Bhopal (INDIA). The results of model study carried by Rathod (2012) showed that granular pile/ stone column increases the load bearing capacity and decreases the settlement of soft expansive soil. Further by using geogrid as a circumferential reinforcement to stone column, added improvement in bearing capacity and reduction in settlement can be obtained. Besides the strength, the aspect of swelling and shrinkage of granular pile improved expansive soil need to be studied in detail. REFERENCES 1. Al-Omari, R.R. and Hamodi, F.J., 1991, Swelling Resistant Geo grid - A New Approach for the Treatment of Expansive Soils, Geotextiles and Geomembranes, Vol.10, No.4, pp Donaldson, G.W. (1969) The Occurrence of Problems of Heave and the Factors Affecting its Nature Second International Research and Engineering Conference on Expansive Clay soils, Texas A&P Press Hughes, J.M.O., Withers, N.J., Greenwood, D.A., Field trial of the reinforcing effect of a stone column in soil. Geotechnique 25 (1), Kowalski, T.E. et al. (2007) Modern Soil Stabilization Techniques. Annual Conference of the Transportation Association of Canada, Saskatoon, Saskatchewan, Oct.14-17, p.p Murugesan, S. and Rajagopal, K. (2006) Geosynthetic-encased stone columns: numerical evaluation. Geotextiles and Geomembranes, Vol. 24, pp Phanikumar B.R., &Radhey S. Sharma (2004)" Effect of fly ash on Engg. Properties of Expansive Soil" Journal of Geotechnical and Geoenvironmental Engineering Vol. 130,no 7,July,pp Rama Rao M., Sridevi G., (2004) Failure of a lightly loaded structure on Expansive soil - a case study, IGC-2004, Warangal, pp Ranjan, G. and B.G. Rao (1983) Skirted Granular Piles for Ground Improvement, Proceeding VIII European Conference on Soil Mechanics and Foundation Engineering, Halainki. 9. Rathod, A. (2012) Behaviour of Stone Columns in Expansive Soil M. Tech. Thesis, MANIT Bhopal, India. 10. Saran, S. (2006) Analysis and design of substructures - Limit state design. Second Edition, Oxford and IBH publishing Co. Pvt. Ltd., New Delhi, India. 11. Sree Ramarao, A. et al. (2003), Use of Cement-Stabilized Fly Ash Cushion in minimizing swell of Expansive clays, IGC- 2003, Roorkee, Vol.2, pp Satyanarayana, B. (1969), Behaviour of expansive soil treated or cushioned with sand, Proceedings of 2nd National Conference on Expansive soils, Texas, pp U.S. Department of Transportation (1983) Design and Construction of Stone Columns, Vol. 1, Report No. FHWA/RD-83/ U.S. Department of Transportation (1983) Design and Construction of Stone Columns. Vol. 2, Report No. FHWA/RD-83/ Venktaswamy, et. Al. (2003) Improvement of Expansive Clay by Deep in situ Technique Indian Geotechnical Conference, IIT Roorkee., pp White, D.J.2005, Fly ash Stabilization for Non Uniform subgrade soils, IOWA State University. Volume I: Engineering Properties and construction Guidelines 461FHWA Project 4). (IHRB Project TR-