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The present invention relates to method of reducing swelling of expansive soils without and with granular pile.


Expansive soil deposits occur in the arid and semi arid region of the world and are problematic to engineering structures because of their tendency to heave during wet season and shrink during dry season. These soils are prone to increase in volume on absorption of water, resulting in great volume expansion and reduction in strength (Holtz and Gibbs 1956; Grim 1968; Jones and Holtz 1973; Chen 1988). Several nations in the world face construction problems associated with expansive grounds (Gourley et al. 1993; Puppala and Cerato 2009; John and Ian 2012). Light structures such as highways, railroads, runways, and other lifeline structures, constructed over such soils often get severely damaged due to high swell-shrinkage behaviour and loss of strength owing to fluctuating water content (Zheng et al. 2009). Due to the above reasons construction on or using expansive soils is considered to be unsafe.

The traditional techniques of ground improvement, like cohesive non -swelling soil layer, sand cushion method, chemical stabilization, deep soil mixing method, mixing of different cementitious and non-cementitious materials like lime, cement, fly ash, stone dust etc. (Katti et al. 1983: Satyanarayana 1969; Estabragh et al. 2014; Madhyannapu et al. 2009: Cokca 2001 ; Zha 2008; Mathur et al. 2012; Phantkumar and Sharma 2004: Sridharan et al. 2006; Eme et al. 2016) etc. are available. These have difficult of mixing the stabilizer to the soil properly and limited to treat the soil to shallow depth only. Hence the search for developing a new technique for expansive soil improvement still remains alive and relevant.

The technique of soft soil improvement by installation of granular piles (also known as stone columns) is popular to marshy lands, marine clays, loose sand, silty or clayey sand, and compressible soils. Granular piles not only strengthen the load bearing capacity of the ground but reduce the settlement, improve the drainage and overall stability. They are most effective in clayey soils having undrained shear strength ranging from 7-50 kPa (IS: 15284 (Part-1):2003).

Many investigators have reported that installation of granular pile in expansive soils have positive effect in enhancing the load carrying capacity (Hughes and Withers 1974; Priebe 1995; Black et al. 2006; Guetifet al. 2007; Stucdlein and Holtz 2012; Hanna 2013: Zhang et al. 2013; Rangeard et al. 2016). However, detailed and systematic invention on the influence of granular pile on swelling of ihe expansive soil has needs to be carried out.

Rao and Padmavalhi (2010) studied heave behaviour of expansive clay reinforced with granular pile anchors (GPA). In that, the foundation was anchored to a mild steel plate provided at the bottoin of the granular pile. That became tension - resistant and prevented the particulate medium from getting sheared away by the swelling soil surrounding it. The various forces acted on the granular pile-anchor is shown in Fig. I . The swelling potential of the untreated soil with an initial moisture content of 14% at dry unit weights yj of 13.0, 14.0 and 15.0 kN/m3 were found to be 6%. 9% and 14% respectively. The swelling potential of the soil was increased with an increase in its dry unit weight. This is because, as yj is high, the number of panicles in the expansive soil is more. Greater surface area was thus available which gives greater scope for accumulation of water around the soil particles and a consequential increase in swelling.

Aparna et al. (2014) have observed reduction in swelling of black cotton soil by using granular pile/sand column at different water content. The black cotton soil used was CH, i.e. clay of high plasticity and compressibility, having following properties: Clay and silt content” 95%, LL=54, PL=29. PI-25. G=2.64, OMC = 23.5%, MDD =I 5kN/m\ Differential tree swell =40%. Diameters of sand columns were of 25mm, 37.5rnm, 50mm respectively used and mould size was used i00mmXl25mm. The test beds were prepared at different water contents 14. 18, 22.26,30,36,40 and 44%. The results indicated that higher diameter of sand column reduced more swelling as compare to lower diameter. Soil with high initial moisture content showed less swelling than that with low initial moisture content. At 44% water content, no swelling was observed in the soil.

Granular pile technique

A vertical sub-structural element, installed in-situ by ground improvement techniques (replacement, displacement, and/or mixture with chemical agents), that carries the load of the super-structure or earth structure with surrounding soil and transmits it to geo-media around and/or below, through compression, shear or rotation (Han 2015). it is known as granular piles or stone columns. Granular piles are the most natural and ecologically neutral

foundation system in existence. Granular piles consist natural material like river sand, gravel, stone or stone chips with sand etc.

Installation of Granular Pile

The construction of granular pile is done by two methods namely displacement method, in which the soil is laterally pushed or shifted while making the hole and the non- displacement method in which the in place soil is taken out during boring to make the hole. The non displacement method is also known as replacement method. These methods have been discussed in IS 15284(Part-l): 2003.

The improvement of a soft soil with granular pile or stone column can be accomplished using various excavation, replacement and compaction techniques. Some of these techniques have proved their applicability whereas others have yet to confirm so. Vibroflotation (Greenwood and Kriseh 1983), rammed stone column (Datye and Nagaraju 1981 ) and simple boring technique (Ranjan and Rao 1983) are some of these common techniques.

Unit Cell Concept

Granular piles should be installed preferably in an equilateral triangular pattern which gives the densest packing, although a square pattern may also be used. A typical layout in an equilateral triangular pattern and square pattern are shown in Fig.2. IS 15284(Part-l):2003.

For an equilateral triangular pattern of granular piles the equivalent circle has an effective diameter of De ::: l .05 s and similarly for a square grid De1.13 s where“s” is the spacing of granular piles IS 15284(Part-l): 2003.


In this method it has been found that for a particular s/d ratio; increase in initial water content, and relative density of pile material, the swelling of expansive soils decreases.


The subject matter is better understood when read in conjunction with the accompanying drawings in which:

Fig. 1 is Concept of granular pile anchor and forces acting on a granular pile anchor (After Rao and Padmavathi 2010)

Fig.2 is Arrangements of granular piles (After IS 15284(Part-l):2003)

Fig. 3 is Experimental setup of without pile

Fig. 4 is Experimental setup of with granular pile


Swelling is governed by various factors such as variation in moisture content, type of soil, type of clay mineral, dry density etc. Granular pile may be construct in any season at field. The variable parameters were: (i) Initial moisture content in soil soil ( Wi) (ii) s/d ratio (V is die spacing between piles, and‘<f is the effective influence diameter of the granular pile) (iii) Relative density of the granular soil (Dr ).

There are six expansive soils used for determination of swelling and sand used as pile material.

( i)Sand : Properties of sand is presented in Table I .

(ii)Suil: Six expansive soils i.e. black cotton soil (natural soil) and others are artificial soils were used. The artificial soils have made by mixing of black cotton (BC) soil and different percentage of bentonite (BT) clay shown in Table 2.

The basic properties of the expansive soils are determined by conducting following tests as per relevant IS codes and given in Table 3. All expansive soils were classified, Cl-1 i.e. clay of high plasticity and compressibility group, as per (IS: 1498-1970).


FSl::: Free Swell Index. OMC- Optimum Moisture Content, MDD- Maximum Dry Density

Test Moulds

Three moulds have been used to prepare granular piles of different diameters accordingly the requirement of mould diameter (or different s/d ratio for these pile sizes are as given in Table 4.

The height of a mould was fixed as 5 d (d=diameter of the pile) to satisfy the critical length criteria. As per IS: 15284 (Part-1 j-2003, a granular pile should have length equal to or more than critical length for developing full limiting axial stress in it and as per this code this value should be nearly 4 d (d:::diameter of the pile).

Preparation of Clay Bed

Quantity of soil was taken corresponding to initial moisture content i.e. 15%. 17% and 20%, and their dry unit weight for different expansive soils as given in Table 5.

Table 5 Dry unit weight of expansive soils at different moulding moisture contents

A thin coat of grease was applied along the inner surface of mould wall. Water was mixed to the soil and fill into the mould. Soil was compacted uniformly in three layers by using steel rammer. Test set up for without pile and with granular pile were shown in Fig. 3 and Fig. 4.

Construction of Granular Pile

After the clay soil bed was prepared, a casing pipe having an outer diameter equal to the diameter of the granular pile and an auger were used to install the granular pile at the center of top surface by replacement method. Thin open-ended seamless steel pipes of 25mm. 32mm, and 48mm outer diameters were used. While inserting the auger or pipe the disturbance should not be caused in the clay bed. Outer surface of the pipe and auger was lubricated by applying a thin layer of grease for easy withdrawal without any significant disturbance to the surrounding soil. Granular material i.e. sand is filled into the hole in layers in measured quantities and then compacted by tamping rod. It resulted in to a dry unit weight of 16.15 kN/m3 and 16.80 kN/m3, the value corresponding to 50% and 70% relative density of the sand (D,). The porous plate with a filter paper placed above and below the soil sample.


After preparing soil sample, collar fitted to top of the mould and placed into water tank. The heave stake was placed on the top of porous plate and a dial gauge was fixed on the top of the heave stake to measure the swelling. The dial gauge was set on zero. Then water was added in water tank to submerge the mould. Swelling was monitored continuously by taking the dial gauge readings from time to time till its reading ceases to change. The total reading of dial gauge gives swelling value.

Tests were conducted for all the test mould prepared with different water contents, s/d ratio and relative densities of the granular material.


Three test series were conducted for swelling of expansive soils reinforced with granular pile. These are TS I , TS2 and TS3 for s/d values of 2, 3 and 4 respectively.

Test Series TSI :

In Test series TSI, s/d ratio = 2, swelling for six expansive soils was determined as given in

Table 6.

Test Series TS2:

The s/d ratio in this test series was 3 and result was shown in Table 7.

Test Series TS3:

In this test series s/d ratio was 4 and swelling of expansive soils is given Table 8.

f rom test results, the effects of variables of present invention on swelling of expansive soils are discussed below:

Effect of Variables on Swelling of expansive soils

In order to study the influence of installing granular pile on swelling of expansive soils, the test variable related to soil was (a) initial moisture content and those related to pile were (b) s/d ratio, and (c) relative density of pile material. The swelling of plain soil test bed or the composite material i.e. the soil reinforced with granular pile was noted by replacement method. The discussion on the results of the tests is presented in following manner:

(a) Effect of" initial moisture content on swelling of expansive soils

(b) Effect of s/d ratio of pile on swelling of expansive soils

(c) Effect of relative density of pile material on swelling of expansive soils (a) Effect of initial moisture content on swelling of expansive soils

It is observed that for test beds without granular pile and those with granular pile; (he swelling is found decreased as the initial moisture content in the soil increased. This is observed for all soils and mould sizes. Because dry or partial saturated soil has water absorption capacity more than fully saturated soil.

(b) Effect of s/d ratio of pile on swelling of expansive soils

For given moisture content, as s/d ratio increases, swelling of raw soil increases but inclusion of granular pile lowers the swelling.

The reason for reduction in swelling may be attributed to

(a) removal of some of expansive soil and replacement of it, by granular material and.

(b) friction between the expansive clay and the granular material.

(c) Effect of relative density of pile material on swelling of expansive soils

Under a given case of W, and s/d ratio, Dr of sand affect the reduction in swelling of expansive soils. The granular pile at high Dt, produces more reduction in swelling than that at low Dr. It may be explained due to the fact that sand ofhigher relative density develops, large friction with the surrounding soil than that of the lower relative density.

On comparing the results of different soils, t.e. from soil SSI to soil SS6. it may be noted that

1. All the expansive soils exhibit reduction in swelling due lo the inclusion of pile.

2. The trend of variation of swelling with ski ratio or with W] or with Dr is similar for all soils SS7, SS2. SS3, SS-i, SS5, and SS6.

3. The percentage decease in swelling is different for different soils.


Recent invention revealed that installation of granular piles in expansive soils reduces swelling of the composite ground. However, no systematic study on the influence of pile installation on swelling of expansive soil was known. Swelling of expansive soils is serious concern to the field engineers as many lightly loaded structures, building components, roads etc. constructed on expansive soils fiul due to significant up lift pressure exerted by the soil. Present invention has clearly established that swelling reduces significantly by the installation of granular piles. Under different initial moisture conditions, spacing between the piles and densities of the pile material; a soil bed reinforced with granular pile reduces swelling; the magnitude of which may depend on soil and pile characteristics. Thus the results of the present invention will build confidence in practicing engineers and researchers to use and recommend the technique of granular piles to mitigate the problems associated with construction in expansive soil areas.