饱和土体原位大型剪切试验方法与实践

The method for large scale in-situ shear test of saturated soils and its application

由于室内剪切试验结果存在试样结构性扰动和尺寸效应等带来的不确定性问题,且以往的常规原位大型剪切试验是针对天然状态土体,不能获得饱和土体的强度参数,导致岩土结构稳定性分析和工程设计中大多采用反演方法求取强度参数,而很少直接运用这些试验参数。在本次试验方法研究中,试验仪器仍然采用常规的天然土体原位大型剪切试验仪器,针对土体饱和问题,设计了"制样后饱和法"和"饱和后制样法"2种浸水饱和装置,并采用这2套装置在甘肃省黑方台地区进行了7个饱和黄土原位大型剪切试验。结果表明,2种饱和方法都能够使土样饱和,"饱和后制样法"容易达到饱和,试验历时短,但对试样扰动大,甚至导致试样在制样时破坏,试验成功率低;"制样后饱和法"达到饱和历时稍长,对试样扰动小,试验成功率高。与原位天然大剪试验相比,粘聚力c值由44.65kPa下降至17.35kPa,Φ值由14.18°降至11.95°;与室内饱和固结快剪相比,饱和黄土原位大剪的c值增加约4kPa,Ф值增加3°左右。

Due to inherent uncertainties associated with laboratory testing, such as sampling disturbance and size effect, and the limitation that conventional in-situ large scale shear tests are generally carried out for soil masses of natural water content, researchers always use soil mass strength parameters determined by inversion methods instead of testing for geotechnical structural stability analyses and engineering designs. In this study, based on conventional in-situ large scale shear test apparatus, the authors developed two modes of apparatus, i.e., the vertical soaking mode and the lateral soaking mode, to accommodate saturated testing techniques. With the apparatus, seven in-situ large scale shear tests were carried out on saturated loess masses in Heifangtai, Gansu Province. The results show that the loess masses could be saturated by both modes. Compared with the lateral soaking technique, vertical soaking often causes more sample disturbance or even leads to failure although it takes less time to saturate. A comparison of parameters from in-situ large scale shear tests of the loess masses with natural water content shows that cohesion from in-situ large scale shear tests of the saturated loess masses decreased sharply from 44.65kPa to 17.35kPa, while internal friction angle decreased marginally from 14.18° to 11.95°. A comparison with parameters obtained through consolidated quick direct shear tests on the saturated loess shows that the increments of cohesion and the internal friction angle from in-situ large scale shear tests of the saturated loess masses were about 4kPa and 3° respectively.