A discussion of the test pressure of collapsible coefficient for Q₂ loess

Collapsible coefficient is an index to evaluate loess collapsibility, which is measured by laboratory test with collected intact samples. Collapsible coefficient varies with normal pressure loaded in test. It is better to use the real normal pressure in foundation when measuring the collapsible coefficient, but for reasons of convenience, the pressures are always designated as definite values in the codes or standards of construction in collapsible loess areas. However, the designated pressure for Q₃ (Malan) loess in the standards is reasonable and applicable, while that for Q₂ loess is still necessary to discuss. In this paper, the collapsible loess foundation of the Huaneng Electric Plant in Zhengning, Gansu Province is used as a case, the Q₂-Q₄ loess samples were collected in a 35.0 m deep shaft in 1 m intervals and the basic physical properties were measured in laboratory first. The collapsible coefficient is measured with the double oedometer method under low to high pressure for all the samples. Based on the test results, the collapsible coefficient with respect to depth determined with the present used GB 50025—2018 standard and the real pressure in foundation in the cases of various basement widths and basement pressures are compared. The results show that (1) the coefficient of collapsibility has a peak value. The initial collapse pressure and peak collapse pressure of Q₃ loess are higher than those of Q₂ loess. The peak coefficient of collapsibility of Q₃ loess is higher than that of Q₂ loess, which decreases with depth. (2) For Q₃ loess, the collapsible coefficient and subsidence determined by the pressure of the present standard are close to those determined by the real pressure in foundation, while for Q₂ loess, those determined by the pressure of the present standard in foundation deviates increasingly with depth to those determined by the real pressure. (3) Therefore, a modified test pressure for measuring collapsible coefficient of loess is proposed. It is demonstrated that the collapsible coefficients determined by the modified test pressure agree well with those by the real soil pressure.