Influence of type of drainage boundary on coefficient of horizontal consolidation

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.