黑棉土力学特性和单桩承载特性试验研究

Experimental research on mechanics properties of black cotton soil and bearing features of single pile

黑棉土是一种特殊的强膨胀土,为避免其胀缩导致的基础和结构损坏,需研究其力学特性及桩基承载特性。首先通过室内试验测定黑棉土的有荷膨胀特性、侧限压缩特性和剪切特性。试验结果表明,当上覆压力由0k Pa增加到300k Pa时,土体的有荷膨胀率从36.6%减小至3.3%;含水率增大时,土体平均侧限压缩模量和抗剪强度均减小;含水率为52.0%的黑棉土在较大剪切位移(6mm)时的黏聚力较峰值强度下的黏聚力下降70%。之后通过离心机试验比较不同含水率的黑棉土地基中单桩的承载特性发现,相比含水率为23.0%的黑棉土地基,含水率为52.0%的黑棉土地基中单桩的刚度和承载力均明显降低,桩端阻力的下降尤为明显。建议在工程桩基设计中,湿化下黑棉土的摩阻力宜采用大剪切位移下土体抗剪强度。

Black cotton soil is one special type of strong expansive clay. The mechanics properties of black cotton soil and bearing features of single pile in this soil were studied to avoid the damage of foundations and structures due to the swellshrink characteristic of this expansive clay. Firstly,loaded swelling ratio,confining compression feature and shear strength of black cotton soil were measured by indoor laboratory tests. Experimental results indicate that loaded swelling ratio reduces from 36. 6% to 3. 3% when overburden pressure increases from 0k Pa to 300 k Pa. Confining compression modulus and shear strength of soil decrease with increase of soil water content（ ω）. Besides,the cohesion force of black cotton soil with ω = 52. 0% reduces 70% of that under peak shear stress when shear displacement increases up to 6mm. Then bearing features of single piles in black cotton soil foundation with different water content were analyzed by centrifuge tests. Compared with black cotton soil foundation with ω = 23. 0%,stiffness and bearing capacity of single pile with ω = 52. 0% significantly decrease and there is obvious decrease in pile end resistance. In pile engineering design,it is recommended to use the soil shear strength under the large shear displacement for determination of friction resistance of wet black cotton soil.