R G Robinson

The use of stone columns is one of the proven ground improvement techniques for improving the load carrying capacity of soft clay deposits. When the stone columns are installed in soft clays, there is a possibility of intrusion of fine clay particles into the column which reduces the shear strength of the stones resulting in reduction in capacity. Geosynthetic encasement has been adopted by several investigators in order to enhance the load carrying capacity. The capacity of the stone column may be further increased if the undrained shear strength of the surrounding soft soil is improved. In the present study, the possibility of applying the vacuum through encased stone column for improving the strength of soft clay soil is explored through laboratory model tests. It was observed that both stiffness and load carrying capacity of the column increased significantly after the application of vacuum suggesting that the proposed method is a viable ground improvement option. © 2013 Indian Geotechnical Society.

Vacuum preloading of clay deposits is becoming an increasingly popular ground improvement technique. Although many studies have been reported in literature, the mechanism of vacuum preloading is still not properly understood. Soil under vacuum preloading is expected to undergo an inward lateral deformation, therefore the influence of lateral deformation on volumetric strain requires further analysis. This paper addresses this aspect through laboratory studies on reconstituted samples of kaolinite. The Rowe cell apparatus was modified to measure lateral deformation under various states of stress because lateral and vertical deformation of soil depends on its state of stress and associated lateral pressure. A method for predicting the volumetric and lateral strains under vacuum consolidation is proposed and then applied to two case studies in China.

Preloading coupled with vertical drains is one of the commonly used methods of ground improvement technique, especially for soft clayey soils. During the installation of prefabricated vertical drains (PVD), the mandrel is pushed with static force until the required installation depth is reached. After reaching the installation depth, the mandrel is withdrawn, and the PVD is held in place by an anchor. This installation and taking out of the mandrel result in remolding of soils around the mandrel which undergoes high shear deformation and resulting in a reduction in permeability. This zone of remolding with reduced permeability is called as the smear zone. The soil properties within this smear zone are not the same as in the intact zone. The reduced permeability delays the radial consolidation process and needs to be accounted while implementing radial consolidation theory. A proper knowledge of the soil properties in the remolded zone and the development of precise forecasts of the influence of radial drainage during consolidation are of paramount importance. Several studies on preloading with vertical drains considering smear effect have been reported in the literature. This study presents a critical review on the experimental, theoretical, analytical, field and numerical studies pertaining to the extent of smear zone and permeability variations within the zone. The content of the paper will bring out the effect of smear on the consolidation rate which needs to be accounted for in the design.

Stone columns are often considered as an excellent ground improvement technique adopted to improve the load carrying capacity and reduces the settlement characteristics. However the load carrying capacity mainly depends on the undrained shear strength of surrounding soils. Reinforcement in the form of encasement is usually provided around the stone columns which imparts lateral confinement, resulted in improved load carrying capacity. This geosynthetic encasement also acts as an excellent filter, prevents the entry of finer clay particles and offers better drainage path for pore water dissipation. Vacuum consolidation is a type of preconsolidation technique in which vaccum is applied through PVD's in a sealed membrane system. The applied vacuum creates negative pore pressure which increases the effective stress without altering the total stresses. The improvement in effective stress thus leads to increase in undrained shear strength of clay soils. The research work involves application of vacuum through encased stone columns and tests done to study the improvement in load carrying capacity of encased stone column after vacuum application. Soil samples after the vacuum consolidation were also taken to study the increase in undrained shear strength after vacuum application.

Vertical drains are used widely to accelerate the consolidation of soft clay deposits when preloading is used as a ground improvement technique. One of the essential input parameters required in Barron's theory is the coefficient of horizontal consolidation, ch. The values of ch can be determined by the radial consolidation test, using either a central sand drain or a porous plastic peripheral drain. This paper presents the laboratory tests carried out to understand the reason for the difference in values of ch determined from inward and outward radial flow consolidations tests. A 150 mm diameter instrumented consolidation cell was used to perform the inward or outward radial consolidation tests. The total stress measurements during consolidation showed non-uniform stress distribution in clay with higher effective stress values close to the drainage boundary. This stiffening of the clay close to the drain retards the consolidation rate resulting in reduced values of ch. As a result, the ch values determined by radially outward consolidation tests with larger drainage boundary area are lower to those obtained by the inward radial flow test. The pore water pressure measurements showed significantly higher undissipated pore water pressure away from the drainage boundary for the outward flow test.

Preloading coupled with vertical drains is a successful ground improvement technique for soft clay deposits. Though surcharge in the form of sand pile is conventionally used, vacuum consolidation is also an option. This paper presents a case study on ground improvement using vacuum consolidation technique carried out near the shoreline in the city of Kakinada, Andhra Pradesh. Detailed methodology, method of installation of PVD, settlements observed during the consolidation process and results from light cone penetration test performed to quantify the ground improvement and the insights gained from the study are described.