粗粒土在三向卸載時的強度特性試驗研究

施維成 朱俊高 胡錫鵬 代國忠 李雄威 李書進

摘 要： 卸載是引起巖土材料破壞的常見工況，為了研究粗粒土在不同中主應力系數b情況下三向卸載時的強度特性，使用真三軸儀對粗粒土進行偏應力q保持300 kPa不變，球應力p不斷減小的等q、等b三向等量卸載試驗，分析三向卸載條件下粗粒土的強度特性，研究常用強度準則對卸載條件粗粒土的適用性。試驗結果表明：在三向等量卸載條件下，粗粒土的強度參數與中主應力系數b有關;對于不同的b值，b=0時的內摩擦角φb最小，破壞應力比Mb最大;破壞應力比Mb隨著b的增大而減小，且隨著b的增大，減小的梯度在逐漸減小;內摩擦角φb在b值較小時隨著b的增大而增大，在b值較大時隨著b的增大而減小，3個方向剪切的共同影響可以解釋這一變化規律;Matsuoka-Nakai強度準則、粗粒土應力不變量強度準則和Lade-Duncan強度準則這3個強度準則都能反映內摩擦角φb與b之間的這一規律，其中，Matsuoka-Nakai強度準則在b值較小時與試驗結果較為接近，粗粒土應力不變量強度準則在b值較大時與試驗結果較為接近;粗粒土角隅函數強度準則與等q、等b試驗結果較為吻合。

關鍵詞： 強度準則;粗粒土;強度參數;卸載;加載

中圖分類號：TU447? ? 文獻標志碼：A? ?文章編號：2096-6717（2020）02-0010-07

Experimental study on the strength characteristics of coarse-grained soil under three-dimensional unloading

Shi Weicheng1， Zhu Jungao2， Hu Xipeng3， Dai Guozhong1， Li Xiongwei1， Li Shujin1

（1.School of Civil Engineering & Architecture， Changzhou Institute of Technology， Changzhou 213032， Jiangsu， P.R.China;2.Key Laboratory of Ministry of Education for Geomechanics and Embankment Engineering，Hohai University，Nanjing 210098， P.R.China;3.CCCC Third Highway Engineering CO.， LTD， Beijing 101300， P.R.China）

Abstract：? Unloading is a common condition that causes damage to geotechnical materials.In order to investigate the strength characteristics of the coarse-grained soil under three-dimensional unloading in different cases of the coefficient of intermediate principal stress b， several three-dimensional equal unloading tests in which the spherical stress p decreases with constant b and constant deviatoric stress q （300 kPa） were performed on coarse-grained soil by using true triaxial apparatus.The strength characteristics of coarse-grained soil were analyzed and the applicability of the commonly used strength criterions was studied.The test result shows that under the condition of three dimensional equal unloading， the strength parameter of coarse-grained soil is related to the coefficient of intermediate principal stress b.When b equals to 0， the internal friction angle φb is the minimum and the failure stress ratio Mb is the maximum.The stress ratio at failure decreases with the increase of b and， as b increases the gradient decreases gradually.The internal friction angle increases with b for small b values while decreases with the increase of b for large values.This can be explained by the co-impact of the shearing in three directions.All of the three strength criterions， including the Matsuoka-Nakai criterion， the criterion expressed by stress invariants of coarse-grained soil and Lade-Duncan criterion， can response the law between φb and b.It is found that the Matsuoka-Nakai criterion is the closest to the test results for small b values while the strength criterion expressed by stress invariants of coarse-grained soil is the closest to the test results for large b values.It is also found that the corner function strength criterion of coarse-grained soil is consistent with the experimental results.

當b繼續增大，σ2-σ3繼續增大，“因素2”使剪切強度參數減小的影響也繼續增大;而σ1-σ2繼續減小，在σ3側向約束下，σ1、σ2間的剪切作用繼續減小，“因素3”繼續使剪切強度參數增大;“因素1”中的σ2側向約束繼續增大，也是使剪切強度參數增大的因素。……