黄土的结构屈服及湿陷变形的分析

黄土是一种典型的结构性土,其湿陷性是浸水后黄土结构破坏的反应。黄土的结构性可用构度指标定量地描述,它既包含了土的物质组成,还包含了不同沉积年代黄土的结构特征。加载和浸水均可引起黄土结构性衰减,加载结构性衰减可由一定含水率黄土的压缩结构屈服应力和压缩变形曲线表征;浸水结构性衰减可由浸水的压缩结构屈服应力减小和压缩变形曲线的变化表征。针对不同场地、不同沉积年代的Q_3黄土和Q_2黄土,依据不同含水率黄土的压缩曲线,分析了不同黄土构度指标与其压缩结构屈服应力之间的关系,以及黄土压缩变形过程中孔隙比和初始孔隙比比值与压缩应力和压缩结构屈服应力比值对数之间的关系。表明不同沉积年代黄土的构度随其反映基本物性的综合物理特征量单调变化,压缩结构屈服应力与构度之间近似呈线性关系,压缩结构屈服前后的孔隙比比值和压缩应力比值对数之间服从近似一致的变化规律。建立了由黄土沉积年代和基本物性指标确定构度,进而确定压缩结构屈服应力;依据孔隙比比值与压缩应力比比值对数的关系,分别描述天然含水率黄土和浸水饱和黄土的压缩曲线;进而,确定饱和自重作用下黄土的自重湿陷系数,计算场地自重湿陷变形的评价方法。该方法应用于西安地区黄土场地自重湿陷评价,得到了与现场浸水试坑试验实测自重湿陷量比较一致的结果,论证了考虑黄土结构性的湿陷性评价方法的合理性和准确性,为建筑黄土地基湿陷变形和大厚度湿陷性黄土地下结构地基湿陷变形评价提供了一种新途径。

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.