高等土力学-第三章强度课件.pptVIP

* * * * * * * * * * * * * * * * * * * * * 2） 的计算 偏应力增量（　　　　）产生了相应的孔隙水压力增量 ，在常规三周压缩试验中 ＝0，大主应力方向有效应力增量为 ， 而小主应力有效应力增量为 。 对于弹性材料， 不引起体变，由于 但土不是理想弹性体，剪应力变化也可引起体变，英国学者司开普顿（A.W.skempton）引入孔隙压力系数A代替系数 ，则 孔隙系数A主要反映剪应力对土的体积变化影响，它在剪切过程中不是常数，它取决于施加应力水平，应变速率，是加载还是卸载。此外，还取决于排水条件和应力历史。 4.4 Pore water pressure due to uniaxial loading A saturated soil element under a uniaxial stress increment is shown in Figure 4.8. Let the increase of pore water pressure be equal to u. As explained in the previous section, the change in the volume of the pore water is Again, as pointed out previously, Cp is much smaller than Cc. So Cp/Cc≈0, which gives A = 1/3. However, in reality, this is not the case, i.e., soil is not a perfectly elastic material, and the actual value of A varies widely. The magnitude of A for a given soil is not a constant and depends on the stress level. If a consolidated drained triaxial test is conducted on a saturated clay soil, the general nature of variation of A with axial strain will be as shown in Figure 4.10. For highly overconsolidated clay soils, the magnitude of A at failure (i.e., Af ) may be negative. Table 4.2 gives the typical values of A at failure =Af for some normally consolidated clay soils. Figure 4.11 shows the variation of Af with overconsolidation ratio for Weald clay. Table 4.3 gives the typical range of A values at failure for various soils. 4.5 Directional variation of Af Owing to the nature of deposition of cohesive soils and subsequent consolidation, clay particles tend to become oriented perpendicular to the direction of the major principal stress. Parallel orientation of clay particles could cause the strength of clay and thus Af to vary with direction. Kurukulasuriya et al. (1999) conducted undrained triaxial tests on kaolin clay specimens obtained at various inclinations i as shown in Figure 4.12. Figure 4.13 3）三轴应力状态下孔隙压力的一般表达式 对于三轴应力状