Volume 34, Issue 1 (January 2011)
Stress Concentration Ratio of Model Stone Columns in Soft Clays
In this work, laboratory experiments have been carried out to study the value of the stress concentration ratio, n, which is defined as the ratio of vertical stress acting on the stone column to that acting on the surrounding soil. A laboratory setup was manufactured in which two proving rings are used to measure the total load applied to the soil-stone column system and the individual load carried directly by the stone column. The foundation steel plates have 220 mm diameter and 5 mm thickness. These plates contain 1, 2, 3, and 4 holes. The spacing between all the holes equals twice the stone column diameter, D, center to center. The stone columns made of crushed stone were installed in very soft clays having undrained shear strength ranging between 6 and 12 kPa. Two length to diameter ratios L/D were tried, namely, L/D=6 and 8. The testing program consists of 30 tests on single, two, three, and four columns to study the stress concentration ratio and the bearing improvement ratio (qtreared/ q untreated) of stone columns. The experimental tests showed that the stone columns with L/D=8 provided a stress concentration ratio n of 1.4, 2.4, 2.7, and 3.1 for the soil having a shear strength cu=6 kPa, treated with single, two, three, and four columns, respectively. The values of n were decreased to 1.2, 2.2, 2.5, and 2.8 when the L/D=6. The values of n increase when the shear strength of the treated soil was increased to 9 and 12 kPa.