Estimating the Coefficient of Subgrade Reaction in the Laboratory Using the CBR Testing Apparatus

The coefficient of subgrade reaction (k) is used in the analysis and design of substructures, and there is a need to estimate this coefficient in a laboratory setting. This paper aims to explore the feasibility of using the California bearing ratio (CBR) testing apparatus to determine k in the laboratory. The methodology includes an experimental investigation involving CBR tests on a variety of soils under different conditions. The tested soils include sand, marl, and clay soils, at different densities and at soaked and unsoaked conditions. Instead of using a single value from the CBR test data, the entire set of data was utilized to estimate the values of k for the tested soils. The results are presented as a complete spectra of the secant (k_s) and tangent (k_t) coefficients, as functions of penetration and pressure. The spectrum for k_s is more appropriate than that for k_t. The spectrum for k_s plotted against penetration is more appropriate than the spectrum plotted against pressure. The maximum value for k_s is 1,162 MN/m³ for soaked dense sand, and the minimum is 6.8 MN/m³ for soaked loose clay. The maximum value for k_t is 1,531 MN/m³ for soaked dense sand, and the minimum is 6.6 MN/m³ for soaked loose clay. The results are compared with those found in the literature. The outcomes indicate that the CBR apparatus can be used to estimate k in the laboratory from the entire spectrum of the CBR results.