Systematization for Evaluating Ultimate Bearing Capacity of Drilled Shaft Socketed in Weathered Zone

Abstract

Most of the drilled shafts in Korea are constructed with socketed into weathered rock or soft rock. Axial load that applied on the drilled shafts is mostly resisted by bearing capacity of weathered zone which is composed of residual soil and weathered rock. Therefore, evaluating appropriately the bearing capacity of the weathered zone based on quantitative subsurface information is very important. However, as the weathered zone is transition state between soil and rock, boundaries of the residual soil and the weathered rock could not be clearly determined. And, because of difficulty for obtaining core sample of weathered rock, socket depth of pile and strength parameters of weathered rock could not be objectively evaluated, which is very important for evaluating an ultimate bearing capacity of drilled shaft. Furthermore, designers are evaluating the bearing capacity by confusedly using different methods, such as an upper limit of soils, various equations which are suggested by foreign specifications. Therefore, a study about systematization for evaluating the bearing capacity of drilled shaft is required. This study intended to objectively determine design parameters based on subsurface investigate reports, and suggest a systematized method for evaluating the bearing capacity which improved drawbacks of existing methods. Firstly, quantitative classifications criteria of weathered zone and evaluation methods for the bearing capacity which were suggested by several institutions in Korea were reviewed in order to analyze drawbacks of the existing methods. And then, improved methods about the analyzed drawbacks were suggested for more reasonable evaluation of the bearing capacity. For verifying effects of the improved methods, bearing capacities of drilled shafts which were socketed in weathered rock mass were evaluated by applying the improved methods, and then were compared to pile load test results. As the result of comparing the evaluated bearing capacities to the load test results, in case of obtaining an unconfined uniaxial strength of weathered rock, applying an AASHTO (2010) method for evaluating the bearing capacity was the most appropriate, which evaluated the bearing capacity conservatively and the most closely to the pile load test result. When an unconfined uniaxial strength of weathered rock could not be obtained, applying an upper limit of soil bearing capacity was appropriate for a conservative evaluation of the bearing capacity.