LOESS COLLAPSIBILITY ASSESSMENT BASED ON STRUCTURAL YIELD PRESSURE

With promotion of the level and standard of engineering construction in loess region,more strict demands are arisen to loess collapsibility evaluation. In this paper,collapsibility of the loess on the railway line from Xi'an to Hancheng was investigated. A sampling shaft of 30 m depth was excavated on the site of Heyang Loess Platform where the railway passes through. The samples were collected at 1 m interval. The basic physical indexes of the samples at each depth were measured first,and the collapsibility of them was measured with double-odometer test. The compression of the intact loess samples with both original water content and saturated were tested under the pressures of 12.5~4000 kPa respectively. The void ratios and the equivalent coefficients of collapsibility vs logarithmic pressure curves were obtained with the test results. The curves show that the coefficient of the loess collapsibility increases first and then decreases with the pressure increment. The coefficient of collapsibility exhibits a peak value with respect to pressure. The corresponding pressure to the peak collapsibility coefficient represents the structural yield pressure of natural loess,while the initial collapse pressure corresponds to the structural yield pressure of saturated loess. Therefore,the peak coefficient of collapsibility is proposed to evaluate the collapsibility of loess foundation. Meanwhile,it is found that the collapsibility of loess varies greatly with depth,collapsible loess,non-collapsible loess and weak expansive loess all exist on a profile. When a small amount of collapsible loess is interbedded in non-collapsible or weak expansive loess,the collapsible deformation may be balanced by the surrounding loess. Therefore,the depth of the lower boundary of collapsible loess layer should not be determined according to a single value of a loess sample,it is suggested that the base of the loess with obvious and continuous collapsibility should be recognized as the lower boundary of collapsible loess.