Stabilization of Risalpur Soil to Mitigate Collapse Suscepibility

Abstract

Collapsible soils have caused major structural damages all over the World. These soils show excellent strength and stiffness properties in dry conditions but undergo a sudden reduction in volume when exposed to water. Most of the buildings in Risalpur region show severe signs of distresses in the form of extensive cracking and differential settlements. Geological evidences show that the soil deposits in Risalpur region are mostly alluvial and loess with traces of carbonates that can potentially act as cementing agents to impart stiffness to the soil. Previous research has shown that the soil up to depth of 20 feet is collapsible with most severe collapse potential at a depth of 4 feet. The present study focused on mitigation of collapse susceptibility using chemical and mechanical means. Lime and cement were used as additives to impart volume stability whereas stone columns were used as reinforcements to improve the settlement behavior of soil. The target soil layer for improvement was between the depths of 4 to 8 feet which corresponds to the typical depth of shallow foundation and its zone of influence. The detailed program of investigations included laboratory as well as field tests; including basic tests to estimate index properties and classification of soil, pH Test, Standard Proctor Test, Unconfined Compression Test, XRD, and Plate Load Tests under different loading conditions. Both the chemical and mechanical techniques showed promising results. Test results show that the minimum strength gain by lime and cement after soaking is greater than 1 tsf which is more than the in-situ soil strength before treatment. Volume stability was observed with both cement and lime only after two days of curing which further increased with time. Plate load test performed on in-situ soil under normal and flooded conditions confirmed the severity of collapse as settlements are instantaneous and abnormal (more than 2 inches). The use of stone columns improved the load carrying capacity and reduced the settlement from 2” to less than 0.25” inches under flooded conditions. It is recommended that soil stabilization be considered as an integral part of new construction at Risalpur. A study may also be carried out to develop the procedures for injecting lime and / or cement in the foundations of existing damaged structures at the garrison.