Screw plate load test in the estimation of allowable bearing capacity in cohesive soil deposit

Abstract

Allowable bearing capacity is often indirectly estimated from the measurements of field tests without conducting further laboratory tests on undisturbed soil samples. Standard penetration resistance (SPT N-value), widely practiced all over the world, can give reliable indication about the strength or bearing capacity of cohesion-less soil strata only. However, in clay strata, SPT N-values obtained may be too small (e.g., 1 or 2), though the strata is not actually as weak as the degree indicated in SPT. In this respect, screw plate load test (SPLT) can be considered an advantageous means for obtaining load-settlement curve in undisturbed field condition at any depth below the ground. In this study, SPLTs at three different depths and unconfined compressive strength tests conducted on the undisturbed samples collected from the cohesive strata under consideration were conducted. This chapter presents the performance evaluation of SPLT in determining allowable bearing capacity as compared to the ones obtained by using undrained cohesion in the bearing capacity equation. As compared to the SPLT results, the bearing capacity equations are found in close agreement for stiff clay (Location-1), overestimating for firm clay (Location-2) and slightly underestimating for very stiff clay (Location-3). SPLT is found to indicate more representative qa values, as the strengths of the soils below the test depth are reflected in the result.