Rama Subbarao G.V.
, Associate Professor, Department of Civil Engineering, S.R.K. Institute of Technology, Enikepadu, Vijayawada.
Expansive soils popularly known as Black cotton soils in India are highly problematic, as they swell on absorption of water and shrink on evaporation thereof. Because of this alternate swell and shrinkage, distress is caused to the foundations of structures laid on such soils. Extensive research is going on to find the solutions to black cotton soils. The present paper reviews innovative solutions along with conventional foundation practices to counteract the dual problem of swelling and shrinkage posed by expansive soils. Besides, the present paper throws a light on causes of distress in lightly loaded structures founded on expansive soils and also various measures to rehabilitate the distressed structure founded on them.
Expansive soil is commonly known as black cotton soils, because of their color and their suitability for growing cotton. Black cotton soil is one of the major regional soil deposits in India, covering an area of about 3.0 lakh sq.km. Expansive soils are problematic soils because of their inherent potential to undergo volume changes corresponding to changes in the moisture regime. When they imbibe water during monsoon, they expand and on evaporation there of in summer, they shrink. Because of this alternate swelling and shrinkage, structures founded on them are severally damaged. The annul cost of damage to the civil engineering structures is estimated at £150 million in the UK, $1000 million in the USA and many billion of pounds worldwide (Gourley et al. 1993).
In India, black cotton soils have liquid limit values ranging from 50 to 100%, plasticity index ranging from 20 to 65% and shrinkage limit from 9 to 14%. The amount of swell generally increases with increase in the plasticity index. The swelling potential depends on the type of clay mineral, crystal lattice structure, cation exchange capacity, ability of water absorption, density and water content. Swell in the vertical direction is called heave. Among the illite, kaolinite and montmorillinite clay minerals, the montmorillinite possesses the greatest ability to swell by illite. The Kaolinite does not swell. Black cotton soils are very hard in dry state and possess high bearing capacity. In summer, it is very common to see shrinkage cracks with hexagonal columnar structure, with vertical cracks as wide as 10mm extending up to a depth of 3m or more. Soils containing expansive clays become very sticky when wet and usually are characterized by surface cracks or a “popcorn” texture (Fig.1) when dry. Therefore, the presence of surface cracks (Fig.2) is usually an indication of an expansive soil.
|Figure 1: Expansive soil with “popcorn” texture
||Figure 2: Expansive soil showing cracks
Problems With Expansive Soils
The problem is more in case of light structures; those cannot counteract the upward thrust posed by expansive soils. The damage will be apparent, usually, several years after construction. The soil below will exert swelling pressure both upwards and laterally. As a result, the floor slab is lifted up, leading to cracking of floor. Cracking is normally evident at the corners of window and door openings. These usually assume in the form of diagonal cracks-a consequence of differential settlement in the wall (Fig. 3 & 4). Often, utilities buried in soil as the water pipelines and sewage lines, get damaged due to displacement in the soil in which they are buried. The ensuing leakage further aggravate the situation. Roads that pass through expansive soil sub-grade are subjected to heaving and shrinkage settlement of these treacherous soils. Both the lined and unlined canals are subjected to the vagaries of expansive soils. The unlined canal slopes erode and become soft. Canal beds heave up obstructing the functioning of the canal. The concrete linings splinter like glass pieces on account of deleterious cyclic movement of background swelling clay. This heavy results in seepage losses.
|Figure 3: Cracks in exterior walls, as a result of upward soil expansion
||Figure 4: Major cracks in exterior walls at doors and windows
Foundation Practices on Expansive Soils
The following conventional foundation practices and innovative techniques can provide solutions to problematic soils.
Sub excavating or replacing the Expansive Soil by Cushions
|Figure 5: Subexcavating or Replacing the Expansive Soil by Cushions
In this technique, the expansive soil is replaced either in part or full (Fig. 5) with a material that doesn’t undergo swell. The load of the cushion provides the load necessary to counter heave.
Sand Cushion Method
Satyanarayana (1969) has suggested that the entire depth of the expansive soil stratum or a part there of may be removed and replaced with a sand cushion, compacted to the desired density and thickness. Swelling pressure varies inversely as the thickness of the sand layer and directly as its density. Therefore, generally sand cushions are formed in their loosest possible state without, however, violating the bearing capacity criterion. 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. Most of the foundation engineers often suggest some arbitrary thickness for the sand cushion without consideration to the depth of the zone of potential volume change which itself is difficult to determine. The high permeability of sand creates conditions conducive to easy ingress and accumulation of water from surface runoff.
CNS Layer Method
Replacement by soils with relatively impervious material may, to a great extent offset the disadvantages of sand cushion method. Katti (1978) has developed a technique where by removal of about 1m of expansive soil and replacement by cohesive non-swelling soils (CNS) layer beneath foundations has yielded satisfactory results. Katti has successfully adopted it for prevention of heave and resultant cracking of canal beds and linings and recommends it for use in foundations of residential buildings also. According to Katti cohesive forces of significant magnitude are developed with depth in an expansive soil system during saturation which is responsible for reducing heave and counteracting swelling pressure. The behaviour is mainly attributed to the influence of electrical charges present on the surface of clay particles on the dipolar nature of water molecules, producing absorbed water bonds that give rise to cohesion.
Moorum is a typical example of CNS material. The cohesive bonds develop around the particles at a faster rate than the ingress of water molecules into the interlayer of the expanding lattices of montmorillinite, thereby reducing heave. 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 1.0m.The shear strength of the underlying expansive soil at the interface and below increases with the thickness of CNS layer. The ultimate bearing capacity after saturation at the interface and 1m below interface have been found to increase compared with the value of expansive soil in winter. Thus the expansive soil should be excavated up to of 1m below the footing level and replaced with CNS layer, compacted to modified AASHO specifications, projecting up to 1m beyond the foundations.
However, studies conducted later (Subba Rao et al., 1995) indicated that CNS Cushion was effective in arresting heave only during the first cycle of seasonal moisture fluctuations and, during the subsequent cycles, the heave may be more than that recorded by a black cotton soil without cushion. Besides, a soil conforming to the specifications suggested by Katti (1978) for suitability as CNS material is difficult to find.
Fly Ash Cushion
Each one of the above methods has one limitation or the other, in terms of its efficacy or economy. The studies have been carried out using fly ash as a cushioning material (Sree Ramarao et al., 2005). Developments of cohesive bonds in a lime-stabilized fly ash cushion, when stabilized with lime, 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 heave of expansive soil 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 get the heave reduced 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.
Deep Foundation Techniques
In this case, the foundation is made to rest at some depth by passing the soil in the active zone, i.e. the zone within which volume changes in the soil occur due to seasonal moisture changes.
Under-reamed bored piles were introduced in India by Central Building Research Institute (C.B.R.I), Roorkee. In India, at about 3.5m below the ground, movements are negligible and if foundations are anchored at that depth, they will remain stable. Based on this principle, under-reamed piles (Sharma et al, 1978) were adopted for foundations in expansive soils in India. The bulbs are provided generally in the inactive zone where sufficient anchorage is available. The diameter of the stem of the under-reamed pile ranges from 20-50cms and the diameter of the bulb is normally 2 ½ times the diameter of the stem. The spacing of the bulbs, in the case of multi under-reamed pile, should not exceed 1 ½ times the bulb diameter. The Bureau of Indian Standards has also brought a code IS 2911: Part III-1980 on under-reammed piles (Fig. 6).
|Figure 6: Under-Reammed Pile Foundation
||Figure 7: Granular Pile-Anchor Foundation System
Granular Pile-Anchor Foundation (GPAF) System
It has been observed that under-reamed pile foundations suffer from the difficulty of both formation upon which the whole mechanism of anchoring depends. Phanikumar et al (1996) felt that the cost of under-reamed pile foundation is more for light structures where the cost of structure itself is very low. In this technique, the foundation is anchored at the bottom a granular pile to mild steel anchor plate with the help of a mild steel anchor rod. This is called a granular pile-anchor (Fig. 7) also counteracts the problem of shrinkage acting as a storage medium. As the granular pile is a particulate medium, it cannot resist the tensile uplift force on the foundation, and as such needs to be modified into a pile-anchor by the above mechanism. As the expansive soil absorbs water, it swells and uplifts the foundation. But, an enormous resistance to uplift is mobilized along the cylindrical pile–soil interface because of the shear parameters of the Pile-soil Interface, and the shear resistance augmented by the lateral swelling pressure. Model tests conducted in the laboratory revealed that heave and swell potential are enormously reduced by the installation of granular pile anchors. The % reduction was about 90 to 95. It has also been observed that the strength characteristics of the ambient soil surrounding the granular pile-anchor showed a large improvement and that the composite ground showed improved bearing capacity.
Chemical stabilization of expansive soils can be adopted to alleviate the problems posed by these soils to civil engineering structures. Chemical stabilization of expansive clays consists of changing the physico-chemical around and inside of clay particles where by the clay requires less water to satisfy the static imbalance and making it difficult for water that moves into and out of the system. The most common chemical admixtures used in soil stabilization are lime and cement.
Lime stabilization has been used successfully on major projects to minimize swelling of the expansive soil. Generally, 3 to 8% by weight hydrated lime is added to the top several inches of the soil (John et al). 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 is sparingly soluble in exchange reactions are less. Further, the lime diffusion into soil either from lime piles or lime slurry pressure injection is hardly 38 to 50mm in 1 to 4 years unless extensive fissure and crack system is present. 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. For highly plastic clay, it is not effective like lime in stabilization. Addition of 2 to 6% cement content can produce a soil that acts as a semi rigid slab (John). Some investigators have tried and succeeded in minimizing the swelling of expansive soil using chemicals like calcium chloride (CaCl2
), calcium sulfate (CaSo4
), potassium chloride (Kcl), aluminum chloride (AlCl3
Stabilization by Industrial Wastes
Utilization of industrial wastes like fly ash, quarry dust, silica fume, copper slag, tannery sludge, etc (Sabat et al, Stalin et al) in the geotechnical engineering field will solve the problem of disposal of these wastes. Extensive research is carried and carrying by the geotechnical investigators to reduce the swelling of expansive soils by using industrial wastes. Fly ash is a waste material produced due to burning of coal for thermal power industries. It is a hazardous material causing environmental pollution degradation. Fly ash is added to soils treated with lime to increase the pozzolanic reaction and improve the gradation of granular soils. The pozzolanic activity of silt soils has been improved by using a lime-fly ash ratio of 1:2. Liquid limit decreases and plastic limit increases with increase in the percentage of fly ash. Generally, the plasticity index reduces by about 50% when 20% of fly ash added. The optimum moisture content decreases and maximum dry unit weight increase with increase in fly ash content. When the non–plastic fly ash particles are added to the expansive clay the water content required for the reorientation of the particles will be less (Pandian et al., 2004).
Stabilization by Reinforcement
Using fibers like jute fabrics, coir ropes, rubber tire chips, waste plastics, synthetic fibre etc can successfully stabilize the expansive soils. The work reported by Raid R. Al-Omari and Faris J. Hamodi (1991) showed the feasibility of using tensile geogrid for the purpose of controlling the swell of plastic soils. Swelling tests using an enlarged oedometer revealed promising results. The reinforcements were cylindrical geogrid of varying stiffness values embedded in clays of different plasticity indices. The reduction in swell increased with increasing the geogrid stiffness, apparently due to a strong ‘interference’ bond restricting the relative movement between clay and the grid. A footings model test confirmed the effectiveness of the proposed technique.
Horizontal Moisture Barriers
Horizontal moisture barriers can be installed around buildings in the form of membranes or paving, both flexible and rigid. Horizontal barriers are meant to prevent excessive intake of moisture. Considerable success has been achieved with asphaltic membranes-catalytically blown asphalt membranes or prefabricated sheets. Asphalt membranes can be used to cover the surface of expansive soils so that non-expansive fill can be placed on top of the membranes. This minimizes infiltration of surface water into the under slab soils.
Vertical Moisture Barriers
Vertical moisture barriers using concrete, ferrocement or any other impervious material around the perimeter of the building, to cut off the source of water, can be very useful in minimizing seasonal drying and shrinkage of the perimeter foundation soils and also in maintaining long-term uniform moisture conditions beneath covered areas. Vertical moisture barriers should be provided to a depth greater than the depth of seasonal moisture changes.
SERC Roorkee / Ghaziabad have developed technology for ferrocement waterproofing and water barriers. Construction and same has been successfully used in field. Distress in Lightly Loaded Structures Founded on Expansive Soils If the load is placed on the expansive soil is more, the selling is arrested. When the imposed loads are light, the swelling is more pronounced. It is interesting to note that it is rare that heavily loaded structures have problem with swelling soils while it is the lightly loaded single and two storeyed buildings which experience maximum distress.
Causes for Distress in Lightly Loaded Structures
The following are the causes for distress in lightly loaded structures founded on expansive soils:
- The construction of building on marshy area and water table is observed at a shallow depth below the ground level.
- There is no flagging/plinth protection around the building.
- Growth of vegetation is observed around the building.
- Sump tank and sewage pipes are very close to the foundation.
- Wastewater and rainwater are disposed directly on the ground very close to the foundation.
- Cracks at plinth, sill, lintel levels and differential heaving of flooring, shifting of walls, extensive cracks are observed in internal and external walls of the building. It is due to the high swelling and shrinking characteristics of expansive black cotton soil in the foundation region.
- The presence of chloride and sulphate contents in fine aggregate are very high compared to the permissible values aggregate could have affect the concrete durability, which in turn have results severe corrosion of reinforcement in various members.
Measures for Rehabilitation of Distressed Structures Founded on Expansive Soils
The following restoration measures as suggested below to counteract the dual problem of swelling and shrinking behavior of expansive clay (Rama Rao M, et al (2004), Sivapullaiah, et al (2005), Prabhakar, et al (2005):
- Construction of additional one or two floors above the existing building should be done so that the loading on the foundation would be more than the existing swelling pressure.
- The plinth beam should be separated from the natural ground by leaving an air gap of 8 to 10cm between the plinth beam bottom and natural ground. If the gap is not provided the plinth beam have at least to be designed for upward pressure due to soil swelling.
- A flexible water proof apron (plinth protection) of about width 2.0m shall be provided all round the building.
- Installation of horizontal/ vertical moisture barriers around the perimeter of the building.
- The internal non-load bearing walls with wide multiple cracks and dislocations shall be removing completely and rebuilt. Before dismantling, the complete roof should be supported by either steel or timber props.
- Flooring shall be redone after removing existing filled up soil up to about 1.5 m from the floor level and replacing the same with well-compacted non-expansive materials placed in layers not exceeding 30cm thickness.
- The sewer pipes with leak proof joints close to the foundation shall be beyond the foundation media.
- Providing sump tank far from foundation region.
- Plantation of trees, plants and hedges within 3m distance around the building should be avoided. This because of extensive watering of plants close to the building contributes to swelling.
- Discharging rainwater collected from roof at a distance from the structure.
Adequate geotechnical investigations are imperative for the characterization of expansive soil. By evaluating the properties of expansive soils accurately, it is feasible to choose the proper foundation technique with a good constriction quality. The distress in the lightly loaded structures is essentially due to high swelling and shrinking characteristics of expansive black cotton soil in the foundation media. The light loaded structures founded on expansive soils must be designed in such way to observe that the load coming on the structure is sufficiently more than the swelling pressure of the expansive soil. It should be ensured that there is no presence of high level of chloride and sulphate contents in fine aggregate using during construction, if not that may lead to the corrosion of reinforcement.
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