WO2018116313A1

WO2018116313A1 - Method of reducing the swelling pressure of the expansive soils by reinforcing it with the granular pile

Info

Publication number: WO2018116313A1
Application number: PCT/IN2017/050072
Authority: WO
Inventors: Kundan MESHRAM; Pradeep Kumar JAIN
Original assignee: Meshram Kundan; Jain Pradeep Kumar
Priority date: 2016-12-20
Filing date: 2017-02-22
Publication date: 2018-06-28

Abstract

The present invention relates to the method of reducing the swelling pressure of the expansive soils by reinforcing it with the granular piles. A method of reducing the swelling pressure (S_p) of the expansive soils such as clay by providing the granular pile structure in the expansive soil comprising the steps of separating type, gradation and the density of the granular pile material, installation of piles, managing spacing and arrangement of piles, identifying and preparing specified area ratio, calculating required diameter and length of piles wherein the initial moisture content of the expansive clay soil is varied and the granular pile structure has a varied relative density.

Description

METHOD OF REDUCING THE SWELLING PRESSURE OF THE EXPANSIVE SOILS BY REINFORCING IT WITH THE GRANULAR PILE FIELD OF THE INVENTION

The present invention relates to the method of reducing the swelling pressure of the expansive soils by reinforcing it with the granular piles. Another objective of the present invention is to determine the expansive pressure of the soil with or without using the granular pile structure.

BACKGROUND OF THE INVENTION

Expansive soil deposits occur in the arid and semi arid region of the world. They cover a major portion of the geographical area of the world and about one fifth the area of India (approximately 3, 00,000 sq. km.). In India, such soils are popularly recognized as black cotton soils and are found extensively in Madhya Pradesh, Maharashtra, Andhra Pradesh, Karnataka and Tamil Nadu. These are problematic to engineering structures because of their tendency to heave during wet season and shrink during dry season. They lie in top about 4.0 meter depth in most places and exhibit swell-shrink behavior owing to fluctuating water content. During swelling phase, if expansion of soil is prevented, it causes tremendous pressure and may severely damage the structure particularly the lightly loaded one. The expansive soils can swell by over 10 and exhibit swelling pressure in the general range of 10,000 psf to 30,000 psf (480 kPa to 1440 kPa). Values of swelling pressure as high as 58,000 psf (2780 kPa) have also been measured. The swelling pressure exerted by the soil depends on the type and amount of clay mineral, degree of saturation, initial void ratio and conduciveness for ingress of water. During swelling phase the soil looses strength too and behaves like soft material. Due to the above reasons construction on or using expansive soils is considered to be unsafe. Many techniques and methods have been developed to improve engineering properties of such soils. These include soil stabilization using lime, cement and other admixtures, use of Cohesive Non-Swelling (CNS) layer etc.The available solutions have limited applicability and a single versatile solution is yet to be developed.

The technique of soft soil improvement by installation of granular piles (also known as stone columns) has become popular in recent past. The concept has successfully been applied to improve the soft soils such as 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. In saturated state expansive soil also behaves like soft soil. Considering this fact, it has been attempted to apply the technique of granular pile to strengthen the soft expansive soil beds. The model study conducted in the laboratory in which a granular pile was installed in expansive soil bed by method of removal. For different clay consistency and pile material properties load tests were performed on the pile installed in the clay. The sand with and without nylon fibre and geo-grid encasement has been taken to act as pile forming material. It is reported that a footing resting on granular pile exhibited higher load carrying capacity than that directly resting on soil bed alone. The consistency of the soil also found influencing the load carrying capacity of the footing on granular pile. Stiffer the clay, greater is the load carrying capacity. Inclusion of fiber in sand and encasement of sand by geogrid has positive effect in increasing the load carrying capacity of the granular pile. The test results have been modeled numerically by FEM software Plaxis too. As this study has been conducted on fully saturated clay bed and has inspected only the strength aspect due to installation of granular pile, the another serious problem with expansive soils i.e. swelling and shrinkage has remain untouched. A few studies reported in literature address the aspect of swelling of expansive soils reinforced with granular material. Harishkumar and Muthukkumar (2011) studied influence of (i) stone column, (ii) stone column and a horizontal layer of geotextile, (iii) layers of geotextile - on swelling pressure of an expansive soil and concluded that due to the above swelling pressure gets reduced. Aparna et al. (2014) reported that initial moisture content and the size of sand column influences swelling of the expansive soils. The granular pile in a soil may be installed at any time (or in any season) of the year. In casethey are installed in partially saturated condition of the soil, which is a quite obvious situation, their influence on the swelling of the soil is yet not very clear. The initial moisture content in the soil and the property of the granular pile material will certainly influence the swelling of the compound ground. Further, the spacing between the piles is expected to influence the pile behavior, hence a systematic study on the influence of above factors on swelling of the composite ground needs to be carried out.

Expansivity of the Soil: Expansive soils have predominantly clay mineral montmorillonite. The other clay minerals in expansive soils are kaolinite and the illite. The behavior of these minerals are widely different, hence expansive soils of different origins behave differently. The unit cell of a montmorillonite mineral consists of an alumina octahedral sheet sandwiched between two silicate sheets. The bonding between the adjacent unit cells of montmorillonite mineral is through very weak van der Waals' force which absorb huge amount of water on wetting and hence the soil loses its strength and shows large swelling. On the other hand the constituent sheets in clay minerals kaolinite and illite are bonded by strong Hydrogen bond and non exchangeable potassium bond respectively and as such they do not absorb large amount of water on wetting resulting in relatively less strength reduction and swelling of the soils containing them.

The degree of expansivity of a fine-grained soil can be known by a term Free Swell Index (FSI) defined by

(vd-vfckiec

F^SI = ¾ ^

Where Vd is the equilibrium sediment volume of 10 g of oven dry soil passing 425 μπι sieve placed in a 100 ml graduated measuring jar containing distilled water, and Vk is the equilibrium sediment volume of lOg of oven-dried soil passing through a 425 μπι sieve placed in a 100ml graduated measuring jar containing kerosene.

However, this method gives negative free-swell indices for kaolinite-rich soils and may underestimate the expansivity of montmorillonite soils if the soils contain a significant amount of kaolinite clay material. To overcome this difficulty, Sridharan and Prakash (2000) used another term, Free Swell Ratio (FSR), which is the ratio of equilibrium sediment volume of 10 g oven-dried soil passing through a 425 μπι sieve in distilled water to that in carbon tetra chloride.

FSR is given by

FSR =^ (2.2)

The free- swell ratio approach predicts soil expansivity more realistically and satisfactorily.

Table 2.1 shows the classification of expansive soils based on various index properties as per IS: 1498-1970 and based on the free-swell ratio of Sridharan and Prakash (2000).

Table 2.1

Classification of expansive soils based on index properties LL PI Free Swell Free- Swell Degree of Degree of Index Swell Potential Expansion Severity

(%) ratio (%)

Non-

20-35 <12 <50 1-1.5 1-5 Low

critical

35-50 12-23 50-100 1.5-2 5-15 Medium Marginal

50-70 23-32 100-200 2-4 15-25 High Critical

70-90 >32 >200 >4 >25 Very High Severe

Determination of sediment volume of highly expansive soils in distilled water can be problematic; as such soils take a prohibitively long time to settle. This difficulty can be overcome by using a 0.025% NaCl solution instead of distilled water, which reduces the settling time markedly without affecting the final equilibrium sediment volume. It has been observed that soils can take about 24—216 h to reach equilibrium in water depending upon their type. However, the same soils take about 4—24 h to reach equilibrium in a 0.025% NaCl solution. Prakash and Sridharan 2004 proposed FSR to predict the soil clay mineralogical composition in a soil. Table 2.2 and Fig. 2.1 may be used to know dominant mineral type in a fine-grained soil.

Table 2.2

Dominant clay mineral type in a soil based on FSR

Both the black cotton soils and the bentonite have predominant clay mineral montmorrilonite. Whereas most of the black cotton soils of India fall in soil group III A and IIIB, bentonite comes in soil group IIIC.

The swelling characteristic of a soil is also expressed by a term swelling potential and by swelling pressure. Swelling potential is defined as a percentage of swell of a laterally confined sample in an odometer test which is soaked under surcharge load of 7kPa after being compacted to maximum dry density at optimum moisture content (IS: 2720-1977 (PartXLI). By the same test swelling pressure can also be known. It is defined as the pressure required for preventing volume expansion in the odometer.

The range of values describing physical and engineering properties of expansive soils, as reported by Solanki (2009) are presented in Table 2.3.

Table 2.3

Typical Engineering Properties of Expansive Soil

Solanki (2009) reports that Indian black cotton soils have clay content values ranging from 30 to 45%, silt content 40 to 60%, sand content 5 to 20%, liquid limit 40 to 85%, plastic limit ranging from 20 to 40%, plasticity index 15 to 45, shrinkage limit from 10 to 15% and free swell index from 30 to 130%, and the swelling pressure from 50 to 200 kN/m². Factors affecting swelling: The swelling pressure of an expansive soil is not a unique property but is influenced by a number of factors, such as, the initial density and water content, method of compaction, soil structure, availability of water, electrolyte concentration in the water, confining surcharge and the specimen size. The higher the initial dry density, the greater is the swelling pressure. The drier the soil before the start of expansion, the higher will be the swelling pressure. The initial moisture variations within the shrinkage limit of a soil do not seem to significantly influence the final swelling pressure on saturation. As regards the upper limit, there is little or no swelling of a highly plastic clay, if its initial water content corresponds to a liquidity index of 0.2 or more. If the area surrounding the test area is covered with surcharge, the swelling pressure increases as the area of confinement (surcharged area) increases. The swelling pressure is observed to vary directly with the height and inversely with the diameter of the test specimen. However, if the skin friction is eliminated, the swelling pressure is found to be independent of the size of the test specimen. Many investigations have been carried out to analyze the factors affecting the swelling of clayey soils. However, they have been put in two categories:

(i) Soil properties related factors

(ii) Factors related to environment

Soil Properties related factors

(a) Clay Mineralogy

(b) Soil Water Chemistry

(c) Soil Suction

(d) Plasticity

(e) Soil Structure and Fabric

(f) Dry Density

Environmental Factors

(a) Initial Moisture Content

(b) Moisture Variations

(c) Climate

(d) Groundwater

(e) Drainage

(f) Vegetation

(g) Permeability

(h) Temperature

Brief mention of the above factors is as follows: As already discussed that soils that contains active clay minerals like montmoriUonite and vermiculate show high swelling. They have more capacity of cation exchange and specific surface and therefore exhibit high swelling. Larger ion concentration in pore water suppresses diffuse double layers, reducing the heave. Soil suction, is related to saturation, pore size and shape, surface tension, electrical and chemical characteristics of soil particles and water, represented by negative pore pressure in unsaturated soils and results in swelling.

Expansive soils show plastic nature; have high liquid limit and plasticity index and reflect greater swelling potential.

Compaction changes soil fabric and structure. A soil compacted at high water content has dispersed structure has low swell potential than the one compacted at lower water contents. Higher density indicates closer particle spacing, greater repulsive forces between particle and larger swelling potential.

Change in moisture due to variation of precipitation and evapotranspiration is responsible for heave. Variation in moisture through vegetation, fluctuation of water tables and surface drainage also affect the swelling potential. Soil having higher permeability allow faster migration of water and promote faster rates of swell. At high temperatures, the swelling capacity of clay decreases, although the influence of temperature is less evident when the applied stress is high.

The swelling pressure is also dependent on the method of testing. Two methods namely constant volume method and consolidometer method are suggested in IS 2720 (Part XLI) - 1977. Both the methods use consolidometer. Further, the field swelling pressure may significantly be different than that obtained from laboratory test (IS: 2720-1977 (PartXLI)).

Estimation of Swelling and Swelling Pressure of Soils: Empirical models for estimating swelling and swelling Pressure of soils are developed by many researchers. They are compiled and presented in Table 2.4.

Table 2.4

Empirical Models for estimating Swelling and Swelling Pressure of soils

(afterEriz and Gunes 2013)

S= Swelling, Sp= Swelling Pressure, LL= Liquid Limit, PI= Plasticity Index, J_c/= dry density of soil, w= water/moisture content, CEC=Cation Exchange Capacity, C= clay percent, LI = liquidity index, FS= Free Swell

In the present work, since the swelling of expansive soils due to installation of granular pile is of concern, hence swelling pressure of test beds reinforced with pile and without pile has been measured. The swelling pressure was determined by restraining the soil to change in volume on wetting. Remedial measures: There are many methods/techniques for ground improvement. They are removal and replacement of soil, mixing of sand dust, fly ash, chemical change, thermal, bio enzymes, under-reamed pile, CNS layer, sand cushion method etc.

The traditional techniques of ground improvement like lime stabilization, cement stabilization; fly ash stabilization, etc. are not suitable because the additives (lime, cement etc.) are difficult to mix with soil properly and are limited to treat the soil to shallow depth.

In view of the above, the search for developing a new technique for expansive soil improvement still remains alive and relevant. Granular pile technique: The granular piles are nothing but vertical pile i.e. columnar elements formed below the ground level with compacted and un-cemented granular material (i.e. stone fragments or gravels or sand). These load bearing piles usually penetrate through the weak strata. The construction of granular pile involves partial replacement or lateral compaction of unsuitable or loose subsurface soils with a compacted vertical pile of granular materials such as sand, stone, stone chips with sand etc. The presence of the piles creates a composite material which is stiffer and stronger than the original soil. They improve the performance of foundation by reducing the settlement and increasing the load carrying capacity. They also increase the time rate of consolidation.

Granular pile is now well established ground improvement technique and is widely practiced since last four decades to improve the soft and compressible soil deposits ranging from soft clay to loose silty sand. They are reported to be most effective in clayey soils with undrained shear strength ranging from 7-50 kPa.

Installation of Granular Pile: Rao (1982);Ranjan and Rao (1983) developed a simple method, particularly useful in developing countries. A spiral auger is used to make the borehole utilizing manual labor. After reaching the desired depth, the borehole is thoroughly cleaned. After that the stone aggregate is placed in the borehole in layers of 300 - 500 mm followed by sand layer of 50 - 100 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 by the hammer 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. Schematic illustration with diagram is shown in Fig. 2.2.

Installation/construction of granular piles has become more advanced in recent years. It is performed with a vibrating poker device which can penetrate to the required treatment depth under the action of its own vibrations. The vibrations imparted to the ground are predominantly horizontal and will increase the relative density of soil if the granular content is greater than 90%. This process is referred to as Vibro-Compaction, and has been used to compact loose sands to depths of 30m.

Unit Cell Concept : According to this concept, in a ground reinforced with granular piles, it is not necessary to analyse the group of piles, as all the piles behave in similar manner (except those which are near the loaded area). It means if the behavior of a single pile is known, the load carrying capacity of the entire area can be known. Each pile acts as unit cell; where the influence of one pile ceases that of surrounding pile starts.

As per IS 15284(Part-I):2003, the pile spacing may broadly range from 2 to 3 times the granular pile diameter depending upon the site conditions, loading pattern, the installation technique, settlement tolerances, etc. For large projects, it is desirable to carry out field trials to determine the most optimum spacing of granular piles taking into consideration the required bearing capacity of the soil and permissible settlement of the foundation.

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. 2.3, IS 15284(Part-

I):2003.

For an equilateral triangular pattern of granular piles the equivalent circle has an effective diameter of D_e =1.05 s and similarly for a square grid D_e =1.13 s where "s" is the spacing of granular piles IS 15284(Part-I) : 2003.

The resulting equivalent cylinder of material, having a diameter D_e enclosing the tributary soil and one granular pile, is known as unit cell (Fig 2.4). The granular pile is concentric to the exterior boundary of the unit cell (Fig. 2.5).

Effective s/d ratio: In 2007, Ambily and Gandhi carried out model test by loading the column area alone as well as entire unit cell area to study the limiting axial stress on the stone column and stiffness of improved ground by varying parameters like spacing between the columns and shear strength of soft clay. The soft soil used was CH, i.e. clay of high plasticity and compressibility, having following properties: LL=52, PL=21, PI=31, G=2.6, OMC = 19.26%, MDD =16.63kN/m³. Stone column material used was crushed stone, particle size varying from 2 mm to 10 mm,

17.3 kN/m , y_mi_n = 15.0 kN/m 3 , angle of internal friction at dry density 16.62 kN/m 3 =430. It was concluded that columns arranged with spacing more than 3 times the diameter of the columns do not give any significant improvement as shown in Fig. 2.6 (a). They have also conducted that the limiting axial stress on single column increases as the soil strength increases as shown in Fig. 2.6(b).

Experiments were also carried out by loading the entire unit cell area to study stiffness of improved ground. The load- settlement behavior of a unit cell with an entire area loaded is almost linear and it is possible to find the stiffness of improved ground. Finite element analysis using software package PLAXIS was also carried out. The numerical results have been compared with the experimental results and agreement between the two is observed. IS 15284-2003 (Part 1) also specifies that the column spacing may broadly range from 2 to 3.

Failure Mechanisms: IS 15284(Part-I): 2003, discusses failure mechanism of a single granular pile loaded over its area. It states that the failure of a granular pile depends primarily upon its length. Granular piles usually extend through the most significant compressible strata that contribute to the settlement of the foundation. For piles having length greater than its critical length (that is about 4 times the pile diameter) and irrespective whether it is end bearing or floating, it fails by bulging (see Fig. 2.7(a)). However, piles shorter than the critical length are likely to fail in general shear if it is end bearing on a rigid base (see Fig. 2.7 (b)) and in end bearing if it is a floating column as shown in Fig. 2.7 (c).

Granular Blanket: Irrespective of the method used to construct the granular pile, the blanket laid over the top of the piles should consist of clean medium to coarse sand compacted in layers to a relative density of 75 to 80 percent.

As per IS 15284 (Part-I):2003, minimum thickness of the compacted sand blanket should be 0.5 m. This blanket should be exposed to atmosphere at its periphery for pore water pressure dissipation.

Code also specifies that after ensuring complete removal of slush deposited during boring operations, a minimum depth of 0.5 m, preferably 0.75 m below the granular blanket should be compacted by other suitable means, such as rolling/tamping to the specified densification criteria.

Application of granular piles in expansive soils :Field application of granular piles in expansive soils has not been reported. However, some of the laboratory studies reported in the literature is as follows;

Sharma and Phanikumar (2005) reported heave behavior of soft expansive clay reinforced with geopiles, which are vertical cylindrical cells made of geogrid and filled with sand. The effects of diameter of the geopile and the type of the fill material on heave response have been investigated. The results indicated that heave decreased with increasing diameter of the geopile and increase in the particle size of the fill material. In the case of a group of geopiles, spacing between the geopiles was varied and its effect on heave was also studied (Fig. 2.8). It is observed that heave decreased with closer spacing between the geopiles. No group effect of geopiles was observed when the spacing was more than four times the diameter of the geopile. They stated that a geopile controls heave of an expansive soil because of the friction mobilized at the interface formed by the fill material, geogrid-expansive soil.

Harishkumar and Muthukkumaran (2011) studied the swelling behavior of expansive soil with inclusion of stone column and geotextile. Moulds were prepared at single moisture content of 25% and at dry density of 1.65g/cc, the values corresponding to OMC and MDD obtained from standard proctor test. Pile material filled was 6mm size gravel. The diameter of stone column was 3.5 cm and height of column was 6.25 cm from the top of mould. The properties of soil taken by them are given in Table 2.5 and different cases studied are presented in Fig. 2.9. The test results are shown in Fig. 2.10. It may be inferred from test results that stone columns in expansive soil reduce the swelling pressure. With single layer of geotextile the stone column further reduces swelling pressure.

Table 2.5 Properties of expansive soil

Kumar and Jain (2013) reported results of an experimental study conducted on expansive soft soil improved by granular pile without and with geogrid encasement. The load- settlement behavior was observed for granular piles of different diameters. They concluded that:

(i) Inclusion of granular pile considerably improves the load-settlement characteristics of expansive clay. The ultimate load of clay bed reinforced with granular pile is increased about three times the ultimate load in plane clay bed.

(ii) The load capacity of the granular pile further increases by encasing the granular pile by geogrid. The ultimate load of clay bed reinforced with encased granular pile is increased about 4.5 to 4.8 times the ultimate load in plane clay bed.

(iii) The load capacity increases as the diameter of the granular pile increases.

Greater details of Kumar and Jain (2013) study are reported by Kumar (2014).

The various case discussed in this study are shown in Fig. 2.11.

The experimental results were modelled numerically using software PLAXIS (V8). The load-settlement behavior of footing resting on soil of different UCS value and corresponding behavior of footing on granular pile is shown in Fig. 2.12. The effect of different pile characteristics on load settlement behavior is shown in Fig.2.13. The experimental and numerical results are shown in Fig. 2.14.

Arora et al. (2014)discussed load settlement behavior of floating granular piles. Tests were conducted in a model tank, having a granular pile installed in soft black cotton soil, with varying length to diameter ratio (L/d ratio) situation of the pile. Ordinary granular piles (OGP test series) and geogrid encased granular piles (EGP test series) were made. The L/d ratio was varied as 1, 3, 5, 7, 9 and 11. It is reported that with increase in L/d, the load carrying capacity of the granular pile increases in both the cases, more in geogrid encased pile compare to the plain pile. The effect of pile inclusion on load carrying capacity was expressed by a ratio Quit pile /Q, where Quit pile is the ultimate load carrying capacity in case of pile and Q is the ultimate load carrying capacity of the soil without pile. Test results are reproduced in Table 2.6.

Table 2.6

Effect of L/d ratio on pile load carrying capacity

L/d

Qult.pile/Q (O.G.P. Test Series) Qult.pile/Q (E.G.P. Test Series) ratio 1 1.93 3.15

3 4.95 6.24

5 6.24 7.54

7 6.99 8.69

9 7.83 9.99

11 8.77 11.49

Dehariya et al. (2014) reported effect of initial moisture content in expansive soil on load carrying capacity of granular pile. The test beds prepared at different initial water contents were at 14%, 18%, 26% and 35%. The corresponding dry densities of soil were 16.67 kN/m³, 16.10 kN/m³, 15.08 kN/m³, and 14.07 kN/m³. The load -settlement response of soil without granular pile and with granular pile in unsaturated and saturated condition were determined. The saturation period was taken four days. Their test results are mentioned in Table 2.7 and Table 2.8.

Table 2.7

Effect of molding water content on load carrying capacity of a footing on expansive soil in unsaturated and saturated condition (without pile)

Table 2.8

Effect of molding water content on load carrying capacity of a footing on granular pile made in expansive soil in unsaturated and saturated condition

Aparna et al. (2014) have observed reduction in swelling by using granular pile/sand column at different water content. They conducted tests to study the effect of sand column size on swelling of expansive soil. The sand columns of diameters 25mm, 37.5mm and 50mm were made in black cotton soil test beds in a cylindrical mould of diameter 100mm and height 125mm. 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 that with low initial moisture content. At 44% water content, no swelling was observed in the soil. 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. The reason for reduction in swelling was reported mainly due to replacement of expansive soil by non-expansive sand and saturation of soil. Thus if sand columns are installed in expansive soils in wet condition maximum benefit in terms of volume stability can be achieved. Effect of sand column diameter on swelling of expansive soil is shown in Fig. 2.15.

SUMMARY OF THE INVENTION

In this method it has been found that with increase in initial water content, the swelling pressure of expansive soil with granular pile decreases. Further, for a particular s/d ratio; increase in the relative density of the sand lowers the swelling pressure. However, at a given relative density of the sand; increase in s/d ratio results in increase in swelling pressure of the composite soil.

BRIEF DESCRIPTION OF THE ACCOMPANIYING DRAWINGS

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

FIG. 2.1 is the classification of soils as montmorillonitic and kaolinitic types

FIG. 2.2 is the Installation of stone column by simple auger boring method (after Rao

1982) FIG. 2.3 is the typical granular piles (i.e. stone columns) arrangement

FIG. 2.4 is the Idealization of unit cell

FIG. 2.5 is the Stress on interface between unit cells

FIG. 2.6 is the comparison of model and numerical results (after Ambily and Gandhi 2007)

FIG. 2.7 is the failure mechanisms of a granular pile in non-homogeneous cohesive soil (after IS 15284(Part-I): 2003)

FIG. 2.8 is the illustrative use of geopiles for embankment and building foundations (after Sharma and Phanikumar 2005)

Fig. 2.9 is the various cases of swelling pressure study with and without stone column

FIG. 2.10 is the graph showing swell Pressure studies in expansive soil using stone column and geotextile layers

FIG. 2.11 is the various cases of granular pile load-settlement study in soft expansive soil FIG. 2.12 is the graph showing Effect of UCS on load- settlement behavior of footing on soil alone and on granular pile (after Kumar 2014)

FIG. 2.13 is the graph showing Granular Pile: load-settlement curve for geogrid encased fiber mixed sand depth as a variable (d=50mm) (after Kumar 2014)

FIG. 2.14 is the graph showing Comparison of numerical and experimental load- settlement curve for varying UCS of soft soil (d=50mm) (after Kumar 2014)

FIG. 2.15 is the graph showing Effect of sand column diameter on swelling of expansive soil

FIG. 3.1 is the graph showing Particle size distribution for different soils

FIG. 3.2 is the graph showing Particle size distribution of the sand

FIG. 4.1 is the graph showing swelling pressure of Soil A at different initial water content for s/d ratio 2

FIG. 4.2 is the graph showing swelling pressure of Soil B at different initial water content for s/d ratio 2

FIG. 4.3 is the graph showing Swelling pressure of Soil C at different initial water content for s/d ratio 2

FIG. 4.4 is the graph showing Swelling pressure of Soil D at different initial water content for s/d ratio 2 FIG. 4.5 is the graph showing Swelling pressure of Soil E at different initial water content for s/d ratio 2

FIG. 4.6 is the graph showing Swelling pressure of Soil F at different initial water content for s/d ratio 2

FIG. 4.7 is the graph showing Swelling pressure of Soil A at different initial water content for s/d ratio 3

FIG. 4.8 is the graph showing Swelling pressure of Soil B at different initial water content for s/d ratio 3

FIG. 4.9 is the graph showing Swelling pressure of Soil C at different initial water content for s/d ratio 3

FIG. 4.10 is the graph showing Swelling pressure of Soil D at different initial water content for s/d ratio 3

FIG. 4.11 is the graph showing Swelling pressure of Soil E at different initial water content for s/d ratio 3

FIG. 4.12 is the graph showing Swelling pressure of Soil F at different initial water content for s/d ratio 3

FIG. 4.13 is the graph showing Swelling pressure of Soil A at different initial water content for s/d ratio 4

FIG. 4.14 is the graph showing Swelling pressure of Soil B at different initial water content for s/d ratio 4

FIG. 4.15 is the graph showing Swelling pressure of Soil C at different initial water content for s/d ratio 4

FIG. 4.16 is the graph showing Swelling pressure of Soil D at different initial water content for s/d ratio 4

FIG. 4.17 is the graph showing Swelling pressure of Soil E at different initial water content for s/d ratio 4

FIG. 4.18 is the graph showing Swelling pressure of Soil F at different initial water content for s/d ratio 4 DETAILED DESCRIPTION OF THE INVENTION

The swelling pressure of expansive soil depends on various factors such as the dry density, degree of saturation, overburden pressure, type of clay mineral, equilibrium moisture content etc. In field, granular piles may be installed during any time of the year and this will affect swelling of the composite ground i.e. the natural expansive soil reinforced with granular pile. Further, the spacing between the piles and the relative density of the granular pile material may also vary, hence the variables of study selected were the initial moisture content in the soil, s/d ratio (where s = spacing between the piles and d= diameter of granular pile), and the relative density of granular pile forming material. The tests were planned in steel moulds of different sizes. The details of the experimental work are as follows.

The materials used in the investigation consist of six soils having different swelling behavior and the sand.

Soils

One natural soil and five artificial soils have been taken for experimentation. The natural soil is the black cotton soil available in MANITcampus, Bhopal. To this soil commercially available bentonite is mixed in different proportions to obtain artificial soils. Table 3.1 describes designation of these soils and the amount of bentonite mixed in each of them.

Table 3.1

Expansive soils used in the present work

The physical properties of the above soils are determined by conducting following tests as per relevant IS codes.

Hydrometer analysis (IS: 2720 (Part 4)-1985), tests for Atterberg limits (IS: 2720 (Part 5)1985), Specific gravity test (IS: 2720 (Part III/Secl)-1980), Standard compaction test (IS: 2720 (Part VII)-1980), free swell index test, and the swelling pressure test. The particle size distribution curves for these soils are shown in Fig. 3.1.

The soils are classified as per (IS: 1498-1970). They all fall in CH i.e. clay of high plasticity and compressibility group. They are also classified according to FSR, as suggested by Sridharan and Prakash (2000). The basic properties of the soils are presented in Table 3.2 and classification as per FSR is given in Table 3.3.

Table 3.2

Properties of the Soils

Table 3.3

Expansive soil classification based on FSR

Sand: The sand used in the present study has been taken from the river Narmada, passing through a nearby town,Hoshangabad (M.P., India). It was dried in sun and sieved through 4.75 mm IS sieve.The particles larger than 4.75 mm size had been removed. The tests conducted on the sand in the laboratory were sieve analysis, specific gravity, minimum and maximum dry unit weight test (IS: 2720 (Part 14)-1983). The grain size distribution curve of the sand is shown in Fig. 3.2and other relevant properties are given in Table 3.4. The sand is identified as SP i.e. poorly graded sand asper (IS: 1498- 1970).

Table 3.4

Properties of thesand

Test preparation and setup

The test setup consists of a loading frame, having arrangement for applying load on the soil sample manually through a proving ring, test mould and other accessories for preparing the test sample as shown in Fig. 3.3. The accessories were the hollow cylindrical pipes (steel pipes), rammer, tamping rod, perforated plates, dial gauge, porous plates, filter papers, andwater bath.

Mould preparation

In this test, three moulds have been used to prepare granular piles of different diameters. The moulds were designated as Mj, M₂, and M3.

The diameters of the moulds were selected in such a way that the requirement of s/d ratio equal to 2, 3 and 4 is complied with for triangular pattern of granular pile installation as per the criteria specified in unit cell concept.

The three pile diameters, 48 mm, 32mm and 25mm were used in the investigation, accordingly the requirement of mould diameter for different s/d ratio for these pile sizes are as given in Table 3.5. In field the arrangement of piles for different s/d ratio is represented schematically by Fig. 3.4.

The height of a mould was fixed as five times the diameter of the pile to satisfy the critical length criteria. As per IS: 15284 (Part-I)-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 four times the pile diameter. However, Mitra and Chattopadhyay (1999) suggested it to be 4.5 times the pile diameter.

Hollow Cylindrical Pipes (Steel Pipes)

Steel pipeswereused to construct granular piles of different diameters. The outer diameters of steel pipes were 25 mm, 32 mm, and 48mm and thickness of the pipes was 2 mm. Two steel rods have been welded at top of a pipe for its easy handling for making hole in the clay bed.

Rammer

A steel rammer of mass 2.6 kg, which is used in standard compaction test having provision of dropping it through a height of 310 mm, was used for compacting the soils.

Tamping Rod

A steel rod was used for compaction of the sand in the sand column. The diameter of the rod is 15 mm and length 750 mm.

Perforated Plate

The load on entire area of specimen was applied through a mild steel circular plate having perforations in it. The diameter of the plate was equal to the diameter of mould and it was 20mm thick.

Proving Ring

The proving ring of capacity 2.5 kN was used. Top portion of proving ring was attached with the moving rod that was attached to the loading frame. The other end of the proving ring was attached to the perforated plate by a rod.

Dial Gauge

A dial gauge was used to measure movement of the soil. The least count of it was 0.01 mm.

Filter Papers Two filter papers were used for every sample, one at the top of the soil sample and the other at the bottom.

Porous Plates

Two porous plates were used in the swelling mould. They were placed after the filter paper. Theplates were 8mm thick.

Water bath

A water bath was used for submerging the test mould. PREPARATION OF THE TEST BED

As the relationship between moisture content and dry unit weight is different for different soils, hence the quantity of dry soil to be taken, for given moisture content, is also different for different soils. Three moisture content values have been selected for each soil for the preparation of test beds. These are 15%, 17% and 20% by weight of the dry soil. Corresponding to these moisture contents, the dry unit weight to be achieved for each soil is different and is given in Table 3.6.

For preparing the test beds, the required quantity of dry soil (clay) and the water was taken. The water was thoroughly mixed to the soil and it was left covered for four days so that the water was uniformly distributed in the entire mass. A thin coat of grease was applied along the inner surface of mould wall to reduce the friction between clay and the tank wall. The moist soil was then filled in the mould in layers of about 40 mm thick. Each layer was compacted uniformly using steel rammer. Care was taken to ensure that no significant air voids were left out in the test bed.

Granular Pile Construction

Once the clay bed was prepared, its centre point was marked at the top surface. A thin open-ended steel pipe of diameter equal to that of the pile to be constructed was pushed into the soil by the pressure of the hand up to the bottom. Slight grease was applied on both inner and outer surfaces of the pipe for easy penetration and withdrawal. The clay within the pipe was scooped out using the auger. Maximum height of soil removed from the pipe at a time was 30 mm so as to avoid suction effect. When all the soil from the pipe was removed, required quantity of nearly dry sand ~ (5% water) was filled into the pipe in layers of 40 mm compacted thickness. The compaction was done by the tamping rod giving 10 blows of 100 mm lift. It resulted in to a dry unit weight of 16.7 kN/m and 17.2kN/m³, the value corresponding to 50% and 70% relative density of the sand (D_r). The pipe was simultaneously raised in stages ensuring a minimum of 5 mm penetration below the top level of the placed sand.

At the top and bottom of the mould porous plates with filter papers were placed.

TEST PROCEDURE

In the present study, swelling pressure was measured by restraining the soil to expand during submergence. The prepared mould was placed in swelling pressure apparatus within the water bath. Initially the water bath was kept empty i.e. without water. Proving ring with their accessories was placed on the mould and adjusted to zero for both the ring and the settlement recording dial gauge. Then water was added in water bath to submerge the mould. After a few minutes soil starts swelling and load and settlement dials showed some reading. When settlement measuring dial shows a value nearly 0.1mm, it was noted and to keep the volume constant, force was applied through the loading arrangement through the proving ring till settlement dial reading came to zero. After an elapse of certain period the soil again swells and settlement dial gives some reading, the loading step was repeated. The process was continued till no further swelling and change in settlement dial was noted. The total load applied through proving ring divided by the area of the mould gives swelling pressure value. Tests were conducted for all the test specimens prepared with different water contents, sizes of the granular columns and relative densities of the granular soil.

TESTS CONDUCTED

As mentioned earlier initial water content in soil, s/d ratio for granular pile and the relative density of sand were the variables of the study for observing the swelling pressure due to installation of pile. Three series of tests named as TS1, TS2 and TS3 have been conducted by varyinginitial water content and relative density of sand for s/d ratio 2, 3, 4 on different soils reinforced with granular pile of diameter 48 mm, 32 mm, and 25mm, respectively shown in Table 3.7. In order to study the swelling behaviour of the expansive soil reinforced with granular piles, under variable situation of moisture content, spacing between the piles and the denseness of the pile material, in all 162 tests had been conducted in three test series. Test Results

In order to accomplish the desired study, three series of tests were carried out to study the swelling pressure of soft expansive clays reinforced with granular piles. In a test series, the s/d ratio was kept constant but the other parameters were variables. These are named as TS1, TS2 and TS3 for s/d values of 2, 3 and 4 respectively. To compare the change in swelling pressure due to installation of pile, swelling pressure of soils without pile were also noted.

Test Series TS1:

In this test series granular pile of diameter 48 mm was constructed in prepared test bed of clay in a mould of diameter 100 mm and height 240mm giving s/d ratio as 2. The moulding moisture content of clay and the relative density of sand were the test variables. The swelling pressure values, measured for all six types of soils are given in Table 4.1. The test results are shown through Fig. 4.1 to 4.6.

Table 4.1

Swelling Pressure of soils for s/d = 2

Test Series TS2:

The s/d ratio in this test series was 3. The granular pile of diameter 35 mm was constructed in a mould of diameter 100 mm and height equal to 160mm. Corresponding to three initial moisture contents in clays i.e. 15%, 17% and 20% and two relative densities of the pile material; 50% and 70% , the swelling pressure of the composite soil bed and that without pile is given in Table 4.2 for all the six soils. Test results for each soil are depicted separately through Figs 4.7 to 4.12.

Table 4.2

Test series TS2, Swelling Pressure of soils for s/d = 3

Test Series TS3:

In this test series s/d ratio was kept as 4. Making of the granular pile of diameter 35 mm in a mould of diameter 105 mm, 125 mm high gave s/d ratio as 4 for triangular pattern of pile arrangement as per unit cell concept. The test variables i.e. type of soil, initial moisture content in clays and the relative density of the sand were varied in the same way as in TSl and TS2. The test results of this test series are given in Table 4.2 and expressed pictorially through Figs 4.13 to 4.18 for different soils. Table 4.3

Swelling Pressure of soils for s/d = 4

Claims

1. A method of reducing the swelling pressure (S_p) of the expansive soils such as clay by providing the granular pile structure in the expansive soil comprising the steps of separating type, gradation and the density of the granular pile material, installation of piles, managing spacing and arrangement of piles, identifying and preparing specified area ratio, calculating required diameter and length of piles wherein the initial moisture content of the expansive clay soil is varied and the granular pile structure has a varied relative density (D,.) of about 50% - 70% of the sand which reduces swelling pressure of a soil due to the construction of granular pile, removed some of the expansive clay soil and replaced by non swelling material.

2. A method as claimed in claim 1 wherein the initial swelling pressure is provided by giving the moisture content to the clay at the initial position.

3. A method as claimed in any of the preceding claims wherein the initial variation in the moisture content of the clay can be from 15% to 21%.

4. A method as claimed in any of the preceding claims, wherein the relative density D_r) of the sand is varied from 50% to 70%.

5. A method as claimed in any of the preceding claims wherein the s/d ratio of the granular pile can be varied from 2-4.

6. A method as claimed in any of the preceding claims wherein the swelling pressure (S_p) is reduced with the increase in the relative density of the sand (D_r)

7. A method as claimed in any of the preceding claims wherein with a constant s/d ratio and the constant relative density (D,.) of the fill, the swelling pressure decreases with the increase in the initial moisture content of the clay.

8. A method a claimed in any of the preceding claims wherein the swelling pressure decreases with the increase in the initial water content of the clay.

9. A method as claimed in any of the preceding claims wherein the granular piles can be installed in the square pattern or any other pattern also.

10. A method as claimed in any of the preceding claims wherein the granular piles can be installed preferably in an equilateral triangle pattern wherein the densest packing is achieved.