Modified Methods for Relative Density of Deep Covered Sand Based on Standard Penetration Blow Number

Relative density is an important index to evaluate the properties of deep covered sand, and also the key to the laboratory sample preparation and simulation test. Relative density can be obtained by in-situ tests such as standard penetration test (SPT). Based on the existing achievements of determining the relative density of sand by SPT, this paper estimates the relative density of deep-covered sand by empirical formulas and empirical parameters, but the result is obviously smaller. So a correction methods are proposed, I.e. method 1^st : Use constant c=a/b to transform the empirical formula, reduce the number of constants, and determine the constant c and the grain size coefficient C_d based on the physical properties of the sand. The relative density calculated by method 1^st is generally close to the in-situ test results, but there is still a problem of low relative density of deep sand. For this reason, on the basis of method 1^st, the overburden pressure correction is also required for the deep-covered sand, namely method 2^nd: correcting the effective stress of the deep soil layer based on the critical depth theory. Method 2^nd avoid the unreasonable phenomenon that the relative density becomes smaller as the depth of the soil layer is more deeply buried. After comparative analysis, the calculation results of the method in this paper are very close to the in-situ test of boreholes. It solves the problem of the difficulty in sampling deep-covered sandy soil in its original state, and the small number of samples leads to low accuracy of the relative density results.