Structural yield and collapse deformation of loess

The loess is a typically structural soil, thus its collapsibility is actually the response of damaged structure under water immersion. The structure of loess, which can reflect the physical composition and the structure-deposited characteristics of different geological ages for loess, can be quantitatively described by the structural index. Both loading and water immersion can result in the attenuation of the structure of loess. However, the representative indexes of the two attenuation actions are different, i.e., the loading-induced attenuation can be represented by the compressive yield stress of structure and the compression curve of loess under certain water content; while the water immersion-induced attenuation can be illustrated by the decrease of the compressive yield stress of structure and the variation of compression curve. Two kinds of compression tests and structural index tests are performed on a series of loess specimens with different water contents, which are sampled from several sites deposited in the Middle and Upper Pleistocenes. According to the test results, the relationship between the structural index and the compressive yield stress of structure is analyzed, as well as the logarithmic relationship between the ratio of the pore ratio of loess under compressive deformation to initial pore ratio and the ratio of compression stress to the compressive yield stress of structure. The analysis indicates that the structural index of loess has a monotonic relation with the comprehensively physical quantity representing the fundamental physical property. The compressive yield stress of structure is approximatively linear with the structural index. The variation law of the ratio of pore ratio is approximatively consistent with that of the logarithmic value of the ratio of compressive yield stress. Then, the method for the calculation of structural index is proposed that the structural index can be derived from several fundamental physical parameters, moreover, the compressive yield stress of structure can be determined according to the linear relation with structural index. The compression curves of natural loess and saturated loess can be respectively described in accordance with the logarithmic relationship between the ratio of pore ratio and the ratio of compressive stress, thereby, the coefficient of collapsibility under overburden pressure of saturated loess can be obtained. This method has been used in several loess sites in Xi'an district to calculate the collapse deformation under overburden pressure. The computed results are in good agreement with the measured ones of the filed immersion tests, demonstrating the rationality and accuracy of the collapsibility evaluation method for the structural loess. It can be applied as a new method, by which the collapse deformation evaluation of loess foundation can be calculated for the construction engineering and the underground structure embedded in the loess layer with a larger thickness.