南阳原状膨胀土不排水剪切性状时效性试验研究

Experimental study of ageing effect on mechanical properties of Nanyang undisturbed expansive soil

针对南阳原状膨胀土不同卸荷间歇时间后的三轴不排水剪切特性进行了试验研究。将削好的原状膨胀土样,在常规三轴仪上首先固结至先期固结应力,进行不同卸荷比(0.00、0.50、0.75)与卸荷间歇时间(0、1、10、20、30d)的三轴不排水剪切试验,最后对部分土样进行了压汞试验。试验结果表明,南阳膨胀土力学特性与卸荷间歇时间和卸荷幅度均相关。相同轴向应变时,经历卸荷过程土样偏应力随卸荷间歇时间减小,未经历卸荷过程则表现为增大。卸荷间歇时间增加,孔隙水压力峰值应变umax?呈减小趋势。未经历卸荷过程的膨胀土强度随间歇时间增加而线性(对数坐标)增大,而经历卸荷过程的膨胀土强度随卸荷间歇时间的增加而减小,且以初期(0~1d)降幅最大,后期变缓并最终趋于稳定状态。其中,卸荷幅度越大,膨胀土强度及模量越小,达到稳定状态所需时间越长。建议边坡开挖后及时支护。在边坡稳定性分析中建议采用umaxq进行计算。结合压汞试验与吸(排)水量结果,卸荷间歇时间对原状膨胀土剪切特性的影响受微裂隙发展与颗粒间胶结强化的耦合作用。

The ageing effect of the triaxial undrained shear properties of Nanyang undisturbed expansive soil was studied.Firstly,the undisturbed expansive soil sample was consolidated under the pre-consolidation stress with the conventional triaxial apparatus.The triaxial undrained shear tests were conducted with different unloading ratios (R=0.00,0.50,0.75) and unloading time intervals (0,1, 10,20,30 d),respectively.The microstructure features of soil sample was characterized by mercury injection test.Test results show that the mechanical properties of Nanyang expansive soil are closely related to the unloading interval time and unloading amplitude. Under the same axial strain,the deviatoric stress of the soil sample after the unloading process decreases,when the unloading interval (R=0.5 and 0.75) increases.While the deviatoric stress without unloading process increases when R=0.The peak strain of the pore water pressure decreases with the increase of unloading interval.The shear strength of the expansive soil increases linearly with the increase of the intermittent time (logarithmic coordinates) for R=0.However,the strength decreases with the increase of the unloading time interval for R=0.5 and 0.75.Furthermore,it declines fast at the first day (0-1 d) and then becomes slow until stable. Besides,the greater the unloading amplitude is,the smaller the shear strength and modulus are,and the longer it takes to reach a stable state for expansive soil.It is suggested that the slope excavation should be supported timely,and umax q is recommended in the slope stability analysis.The results of mercury test and absorption (discharge) show that the effect of the unloading interval on the shear characteristics of the undisturbed expansive soil is affected by the coupling effect of microfracture development and intergranular cementation.