AN EXPERIMENTAL INVESTIGATION ON THE ENGINEERING BEHAVIOUR OF CHEMICALLY STABILISED EXPANSIVE SOILS

Abstract

Expansive soils are popularly known in India as black cotton soils. They have an inherent nature to experience changes in volume depending upon changes in water content. Hence, they heave or increase in their volume when they absorb water in rainy seasons and shrink or decrease in their volume when they lose water on evaporation in summer seasons. This propensity of expansive soils to respond to changes in water content makes them highly problematic. Owing to this alternate swelling and shrinkage, lightly loaded infrastructure like residential buildings, canal linings and pavements constructed upon them are damaged. Structural members such as flooring, walls and columns develop severe cracking. Diagonal tension cracks are observed above doors and windows and below window-sills in the case of lightly loaded buildings. To control these volume expansions and volume reductions of these soils, innovative foundation techniques have been invented. The broad classification of these techniques is as follows: i) mechanical modification (Sand cushion and CNS layer), ii) physical modification, iii) chemical modification and iv) foundations counteracting tension. In mechanical modification, the top few layers of the expansive clay layer are removed and replaced by layers of non-expansive material such as sand and gravel. Sand cushion is a classic example of mechanical modification. Cohesive non-swelling (CNS) layer technique is another example of mechanical modification. In physical modification, the top layers of the soil stratum are excavated, pulverised and mixed with non-expansive materials like sands and gravels and compacted back in layers. As the soil layers are replaced by non-swelling materials, swelling is reduced. Foundations counteracting tension are special foundation techniques. The examples are under reamed piles, large-bottomed piers or belled piers, straight-shafted piers or drilled piers, and granular pile-anchors (GPAs). The swelling pressure of the soil causes the uplift force on these foundations. And skin friction causes the force resisting this uplift. Granular pile-anchors (GPAs) are a recent innovation over conventional granular piles. In a GPA, an anchor is created for the foundation using a mild steel anchor rod and a mild steel anchor plate placed at the bottom of the granular pile. When the expansive clay bed swells by imbibing water, swelling pressure uplifts the foundation. But, the anchorage in the GPA develops resistance to this uplift which is mobilised over the cylindrical GPA-clay interface. The resistance mobilised acts in the downward direction; and this resistance can be attributed to the weight of the GPA and the GPA-clay interface shear parameters, namely, c’ and φ’. In chemical modification, various chemicals and industrial byproducts such as lime, silica fume (SF), fly ash (FA), cement, rice husk ash (RHA), pond ash (PA), calcium chloride (CaCl2) and GGBS are added to expansive clay for controlling their volume changes, reducing their plasticity and improving their engineering behaviour. Chemical stabilisation of expansive clays basically works through flocculation and cementation which are the two important reactions of which one is an immediate reaction and the other is a time-bound reaction. Flocculation reduces plasticity and swelling. And cementation causes cementitious products which harden the blends. This thesis is a focused research on chemical stabilisation of a remoulded expansive clay. Lime, cement, fly ash and GGBS were used as the additives to the clay. Free swell index (FSI), liquid limit (LL), plastic limit (PL) and plasticity index (PI) were the index properties determined at varied amounts of the above additives. Compaction characteristics, hydraulic conductivity, unconfined compressive strength corresponding to varied curing periods and soaked CBR were the engineering properties studied. Further, to study the load-settlement characteristics, plate load tests were performed on clay beds stabilised with varied amounts of the above additives. Moreover, in another series of tests, the plain clay bed and the clay beds stabilised with 6% lime, 20% cement, 20% fly ash and 20% GGBS were subjected to five swell-shrink cycles. In each of the swell-shrink cycles, swelling was observed for 10 days and shrinkage was observed for 50 days. Thus, each clay bed was continuously monitored for 300 days for swelling and shrinkage. Furthermore, shrinkage/desiccation cracks developed in the untreated and the treated clay specimens were quantified by image analysis to determine the crack area, the crack density factor and the crack intensity factor.