Strong Correlation between the Bearing Capacity and CBR of a Gypsiferous Subgrade Soil Subjected to Long-Term Soaking

Abstract

Presented in this paper is a correlation between the bearing capacity of a gypsiferous subgrade soil with the corresponding CBR value. The geotechnical tests carried out on this soil indicate that the soil is sandy lean clay of CL group according to the USCS and A-6 (6) group according to AASHTO Soil Classification System. The soil contains about 33% gypsum content. For this purpose, thirty six CBR samples were prepared at optimum moisture content (of the modified AASHTO compaction test) namely 11.5% at compactive efforts of 1, 2 and 4.56 times that of the Proctor compaction (standard AASHTO). These
samples were soaked for 0, 4, 7, 15, 30, and 120 days under the effect of 40 lbs (178 N) surcharge load. For each soaking period, three pairs of CBR soil samples were prepared, one for each compactive effort. The first CBR sample from each pair was used to determine the CBR value while the second CBR sample was used to obtain triaxial soil samples for unconsolidated undrained tests to arrive at the corresponding shear strength parameters. The present paper reveals that for each soaking period, there is a linear increase of the ultimate bearing capacity with increasing compactive effort, while a significant drop in the ultimate bearing capacity of the tested soil took place with increasing soaking period. The paper
reveals also that there is a strong linear correlation between the estimated ultimate bearing capacity and the corresponding CBR value indicating that the Rosenak's equation correlating the bearing capacity with CBR value is very conservative.