Development of calibration chamber for cone penetration test in sand
Date of Issue2008
School of Civil and Environmental Engineering
In-situ penetration test such as the cone penetration test (CPT) is increasingly being used in the characterization of sand fill that covers most recent reclaimed land space in Singapore. The usefulness of in-situ penetration test results for engineering applications relies heavily on the development of interpretation procedure in a calibration chamber (CC). A reliable and practical chamber for such purpose is yet to be developed. The chamber wall is the crucial element in the design of a calibration chamber due to the induced boundary effects. The objective of this investigation is to explore an appropriate chamber wall design for a calibration chamber that can minimize the boundary effect for the calibration of cone penetration tests in sand. It is postulated that such a chamber would incorporate a suitable compressible layer inside the rigid wall. In this study, a theoretical analysis based on cavity expansion theory has been developed to analyze the stress and displacement in the soil under the semi-infinite elastic half space and in a semi-confined cylindrical boundary. The analysis has subsequently been extended to arrive at a suitable criterion for the selection of the elastic stiffness and thickness of the compressible layer. Results of this analysis indicate that a close simulation of the field boundary condition as prevailed during cone penetration in a semi-infinite soil mass in a calibration chamber is possible if the induced radial stress can be stipulated at the soil-wall boundary. This can be accomplished by the incorporation of a compressible layer on the inner face of the rigid wall in a conventional chamber. A series of CPT tests were carried out in the developed calibration chamber on Changi sand using a standard cone penetrometer. Results obtained from these calibration chamber tests were verified and interpreted to evaluate the performance of the developed calibration chamber with buffer wall that is capable of reducing and minimizing the boundary effect. The boundary effects of the calibration chamber with rigid wall for CPT tests are obvious and significant for medium dense and dense sand samples. For very loose sand, the boundary effects may be still existed but it is small enough and can be neglected. The CC tests results show that the boundary effects of the chamber can be reduced and minimized by increasing the thickness of the buffer layer to a certain value for a selected material and the qc measured in chamber converges to value close to the in-situ qc. A comparison of the relationship of qc vs Dr of the sand between existing correlations proposed by other researchers and this research has also been made. The correlations proposed by Jamiolkowski et al. (1985) and Baldi et al. (1986) are applicable to Changi sand although some scatter exists.