摘要
通过对国内外学者关于基坑开挖卸荷过程中及交通荷载、地震荷载、波浪荷载作用下地基土单元应力状态变化研究成果的总结,发现主应力轴旋转是土单元应力状态变化的显著特征。显然,主应力轴旋转路径下土体的剪切破坏特性及破坏标准是实际工程设计关注的重要问题之一。基于此,根据我国东南沿海广泛分布软粘土地区工程建设的需要,采用空心圆柱扭剪仪、常规试验三轴仪、SEM电镜扫描仪,以典型的杭州原状软粘土为研究对象,进行了考虑初始应力状态变化的主应力轴旋转系列宏微观试验。主要工作及成果如下:
1.开展了主应力轴单向、正逆向、连续循环旋转及剪应力幅值与主应力轴方向同时变化路径下的固结不排水试验。试验结果表明:(1)主应力轴旋转阶段的应力应变开展水平很小,主应力轴旋转阶段赋予后期定向剪切阶段的大主应力方向角对整个剪切阶段应力应变开展特征起控制作用。(2)主应力轴单向旋转对整个剪切阶段孔压开展特征影响很小,但是,主应力轴正逆向、连续旋转及剪应力幅值与主应力轴方向同时变化对整个剪切阶段孔压开展特征影响较大。(3)主应力轴旋转路径对整个剪切阶段原状软粘土的强度变化特征影响较小,其强度的各向异性受后期定向剪切阶段的大主应力方向角控制。
2.开展了考虑初始主应力方向角、中主应力系数、初始剪应力、超固结比变化的主应力轴旋转系列宏微观试验。试验结果表明:(1)初始主应力方向角决定了试样剪切变形状态,以初始主应力方向角等于45°为界。(2)随着中主应力系数的增大,产生的孔压增量先减小后增大,但粘土的归一化强度没有呈规律性的变化,这是因为中主应力系数不同时影响原状软粘土归一化强度各向异性的微观变化因素不同。(3)初始剪应力与超固结比变化对剪切过程中孔压的开展特征影响较大,从孔压角度建立破坏标准时应当考虑初始剪应力与超固结比变化的影响。
3.在分析主应力轴旋转与初始应力状态变化对原状软粘土剪切破坏特性影响的基础上,对复杂应力路径下原状软粘土的剪切破坏标准进行了研究,得到以下结论:(1)对Lade-Duncan破坏准则做了进一步修改,使其适用本文试验控制体系下原状软粘土剪切破坏特征的分析。(2)采用修正Lade-Duncan破坏准则计算不同剪切方向的粘土参数,并对极限孔隙水压力、有效主应力比峰值、给定广义剪应变、主应力差峰值剪切破坏标准进行了对比分析,发现复杂应力路径下原状软粘土剪切试验应采用主应力差峰值作为剪切破坏标准。
Through a summary of research results belonging to domestic and foreign scholars on stress state change for foundation soil unit in the process of excavation unloading of pit and under traffic loads, seismic loads and wave loads, I found that rotation of principal stress axis was salient features in stress state change for foundation soil unit. Clearly, characteristics of shear failure and failure criterion in stress path for rotation of principal stress axis were one of the important issues related to practical engineering design. Based on this, according to the needs of construction projects in southeastern coastal areas of widely distributed soft clay, Hollow cylinder apparatus, Instrument for triaxial test and Scanning electron microscopy were used to carry out series of macro-micro tests for Hangzhou intact soft clay including the rotation of principal stress axis and the initial stress state changes. The main work and achievements were as follows:
1. Undrained tests including one-way. forward& reverse, continuous cycle for rotation of principal stress axis and meanwhile changes for amplitude of shear stress and direction of principal stress axes had been done, and it was found that(1) the level of stress and strain was very small during rotation of principal stress axis. and major principal stress direction angle of late shear stage with fixed principal stress direction that was given by early rotation of principal stress axis controlled stress-strain characteristics for the complete shear stage.(2)one-way rotation of principal stress axes carried out little impact for pore pressure characteristics of the complete shear stage, but forward& reverse, continuous cycle for rotation of principal stress axis and meanwhile changes for amplitude of shear stress and direction of principal stress axes carried out greater impact for that.(3)rotation of principal stress axis carried out little impact for shear strength characteristics of intact soft clay in the complete shear stage, and the anisotropy of shear strength was controlled by major principal stress direction angle in late shear stage with fixed principal stress direction.
2. Series of macro-micro tests for rotation of principal stress axis considering changes that included the initial principal stress direction angle, coefficient of intermediate principal stress, initial shear stress and over-consolidation ratio had been done, and it was found that(1)the initial principal stress direction angle determined the characteristics of shear deformation, and the initial principal stress direction angle equal to 45 degrees was the cut-off point.(2)when coefficient of intermediate principal stress was increasing, the increment of pore pressure generated was increased after the first decrease, but normalized shear strength of clay had not reduce or increase regularly, because micro-change factors which influenced anisotropy of normalized shear strength of intact soft clay were different in different coefficients of intermediate principal stress.(3)the changes of initial shear stress and over-consolidation ratio had a greater impact to pore pressure characteristics, so the establishment of failure criterion from the perspective of pore water pressure should consider the effects of changes of initial shear stress and over-consolidation ratio.
3. Based on the analysis of impacts of rotation of principal stress axis and the initial stress state changes to shear characteristics of intact soft clay, failure criterion for intact soft clay under complex stress path was researched, the conclusions was that(1)Lade-Duncan failure guidelines after the authors had done a further revision might analysis characteristics of shear failure in stress control system of this test.(2)revised Lade-Duncan failure guidelines was used to calculate clay parameters in different shear directions, and the limit pore water pressure, the peak of effective principal stress ratio, the generalized shear strain and the peak of principal stress difference as failure criterion was carried out a comparative analysis, then the shear test for intact soft clay under complex stress path should use the peak of principal stress difference as failure criterion.
引文
[1]王常晶,陈云敏.移动荷载引起的地基应力状态变化及主应力轴旋转.岩石力学与工程学报,2007,26(8):1698-1704.
[2]Ishihara K,Towhata I.Sand Response to Cyclic Rotation of Principal Stress Direction as Induced by Waved Loads,Soils and Foundation,1983,23(4):11-26.
[3]Wijewickreme D,Vaid Y P.Behavior of loose sand under simultaneous increase in stress ratio and principal stress rotation.Canadian Geotechnical Journal,1993,30:953-964.
[4]Lade P.V,Kirkgard M M.Effects of stress rotation and changes of b-values on Cross-anisotropic behavior of nalural K_0-consolidated soft,Soils and Foundation,2000,40(6):93-105.
[5]Zdravkovic L,Jardine R J.The effect on anisotropy of rotating the Principal stress axes during consolidation.Geotechnique,2001,51(1):69-83.
[6]Sivathayalan S,Vaid Y P.Influence of generalized initial state and principal stress rotation on the undrained response of sands.Canadian Geotechnical Joumal,2002,39:63-76.
[7]沈扬,周建,张金良,等.考虑主应力方向变化的原状软黏土强度及超静孔压特性研究.岩土工程学报,2007,29(6):843-847.
[8]Symes M J,Gens A,Hight D W.Drained principal stress rotation in saturated sand.Geotechnique,1988,38(1):59-81.
[9]Hight D W,Gens A and Symes M J.The development ofa new hollow cylinder apparatus for investigating the effects of principal stress rotation in soils.Geotechnique,1983,33(4):355-384.
[10]Towhata I,Ishihara K.Undrained strength of sand undergoing cyclic rotation of principal stress axes.Soils and Foundation.JSSMFE,1985,25(2):137-147.
[11]Towhata I,Ishihara K.Shear work and pore water pressure in undrained shear,Soils and Foundation.JSSMFE,1985,25(3):73-84.
[12]Symes M J,Gens A and Hight D W.Undrained anisotropy and principal srtess rotation in saturated sand.Geotechnique,1984,34(1):11-27.
[13]Wong R K S,Arthur J R F.Sand sheared by stresses with cyclic variations in direction.Geotechnique,1986,3(2):215-226.
[14]Sayao & Vaid Y P.Drained Principal stress rotation in saturated sand.Geotechnique,1989,39(3):549-552.
[15]沈瑞福,王洪瑾,周景星.动主应力轴连续旋转下砂土的动强度.水利学报,1996,(1):27-33.
[16]王平安,于澍.主应力轴旋转下饱和砂土振动孔隙水压力发展和变化的研究.西安建筑科技大学学报,1996,28(4):433-437.
[17]李志星,张季如.饱和软粘土在循环荷载下的孔压数值分析.土工基础,2007,21(2):51-54.
[18]虞海珍,汪稔,赵文光,等.波浪荷载下钙质砂孔压增长特性试验研究.武汉理工大学学报,2006,28(11):86-89.
[19]Yoshimine M,Ishihara K,Vargas W.Effects of principal stress direction and intermediate principal stress on undrained shear behavior of sand,Soils and Foundation.JGS,1998,38(3):179-188.
[20]Uthayakumar M,Vaid Y P.Static liquefaction of sands under multiaxial loading.Canadian Geotechnical Journal,1998,35:273-283.
[21]Sato K,Yasuhara K,Higuchi T.Effect of principal stress direction on undrained cyclic shear behavior of dens sand.Journal of Geotechnical Engineering,JSCE,1996,41/35:199-213.
[22]Nakata Y,Hyodo M,Murata H.Flow deformation of sands subjected to principal stress rotation.Soils and Foundation.JGS,1998,38(2):115-128.
[23]李万明,周景星.初始主应力轴偏转对粉土动力特性的影响.见:第四届全国土动力学学术会议论文集,杭州,1994.47-50.
[24]李万明,周克骥,周景星.用扭剪仪研究主应力轴偏转的影响.见:第三届全国土动力学学术会议,上海,1990.239-242.
[25]王洪瑾,马齐国,周景星,等.土在复杂应力状态下的动力特性研究.水利学报,1996,(4):57-64.
[26]付磊,王洪瑾,周景星.主应力偏转角对砂砾料动力特性影响的试验研究.岩土工程学报,2000,22(4):435-440.
[27]郭莹,栾茂田,许成顺,等.主应力方向变化对松砂不排水动强度特性的影响.岩土工程学报,2003,25(6):666-670.
[28]郭莹.复杂应力条件下饱和松砂的不排水动力特性试验研究.大连理工大学博士论文,2003.
[29]李建国,汪稔,虞海珍,等.初始主应力方向对钙质砂动力特性影响的试验研究.岩土力学,2006,25(6):723-726.
[30]杨春秋,周健.初始剪应力下砂土的动力特性预测模型.大坝观测与土工测试,1997,21(6):25-26.
[31]齐剑锋,栾茂田,聂影,等.饱和粘土循环剪切强度与变形特性的试验研究.水利学报,2008,39(7):822-828.
[32]Yamada Y,Ishihara K.Undrained deformation characteristics of loose sand under three-dimensional stress condiitons.Soils and Foundations,1981,21(1):97-107.
[33]Yoshimine M,Ishihara K,Vargas W.Elects of principal stress direction and intermediate principal stress on unrained shear behavior of sand.Soils and Foundations,1998,38(3):179-188.
[34]Hong W P,Lade P V.Elasto-plastic behavior of K_0-consolidated clay in tortion shear tests,soils and Foundations,1989,29(2):127-140.
[35]栾茂田,许成顺,刘占阁,等.一般应力条件下土的抗剪强度参数讨论.大连理工大学学报,2004,44(2):271-276.
[36]刘金龙,栾茂田,袁凡凡,等.中主应力对砂土抗剪强度的影响.岩土力学,2005,26(12):1931-1935.
[37]许成顺,栾茂田,何杨,等.中主应力对饱和松砂不排水单调剪切特性的影响.岩土力学,2006,27(5):689-693.
[38]Sivathayalan S,Vaid Y P.lnfluence of generalized initial state and principal stress rotation on the undrained response of sands.Canadian Geotechnical Journal,2002,39:63-76.
[39]Ishihara K,Yamazaki A,Haga K.Liquefaction of K-consolidation sand under cyclic rotation of principal stress direction with lateral constraint.Soils and Foundations,1985,25(4):63-74.
[40]陈存礼,何军芳,胡再强,等.动荷作用下饱和尾矿砂的孔压和参与应变演化特性.岩石力学与工程学报,2006,25(2):4034-4038.
[41]张茹,何昌荣,费文平,等.固结应力比对土样动强度和动孔压发展规律的影响.岩土工程学报,2006,28(1):101-105.
[42]朱小可,胡敏云,朱烨.循环荷载作用下近海粉土工作性状的试验研究.浙江工业大学学报,2008,36(2):214-220.
[43]陈颖平,黄博,陈云敏.循环荷载作用下结构性软粘土的变形和强度特性.岩土工程学报,2005,27(9):1065-1071.
[44]杨雪强,朱志政,韩高升,等.对土体强度破坏标准的探讨.湖北工业大学学报,2006,21(5):1-5.
[45]赵慧.循环荷载作用下粉土的破坏标准和动力特性的试验研究.河海大学硕士论文,2006.
[46]Andersen K H,Kleven A,Heien D.Cyclic soil data for design of gravity structure.Journal of Geotechnical Engineering,ASCE,1988,114(5):517-539.
[47]栾茂田,齐剑锋,聂影,等.循环应力下饱和粘土剪切变形特性试验研究.海洋工程,2007,25(1):43-49.
[48]刘胜群,吴建奇.循环荷载作用下饱和软粘土孔隙水压力变化规律的试验研究.铁道建筑,2007,(1):68-70.
[49]郑晓,郭玺.超固结软粘土动力特性试验研究.路基工程,2008,(3):45-47.
[50]王军,陈春雷,丁光亚.循环荷载下温州超固结软土动强度与变形分析.2008,18(4):125-134.
[51]沈杨.考虑主应力方向变化的原状软粘土试验研究.浙江大学博士论文,2007.
[52]张坤勇,殷宗泽,梅国雄.土体各向异性研究进展.岩土力学,2004,25(9):1503-1509.
[53]李振泽,唐晓武.土破坏准则的推导与验证.岩土力学,2007,28(6):1247-1249.
[54]曾长女,刘汉龙,陈玉民.细粒含量对粉土动孔压发展模式影响的试验研究.岩土力学,2008,29(8):2193-2198.
[55]Skempton A W.The pore pressure coefficients A and B.Geotechnique,1954,4(4):143-147.
[56]Henkel D J.The relationship between the strength,pore water pressure and volume change characteristics of saturated clays.Geotechnique,1960,10(1):41-49.
[57]李锦坤,张清慧.应力劳台角对孔隙压力发展的影响.岩土工程学报,1994,16(4):17-23.
[58]唐益群,张曦,赵书凯,等.地铁振动荷载下隧道周围饱和软粘土的孔压发展模型.土木工程学报,2007,40(4):82-86.
[59]殷宗泽.土工原理.北京:中国水利水电出版社,2007.
[60]朱思哲,刘虔,包承纲,等.三轴试验原理与应用技术.北京:中国电力出版社,2003.
[61]施斌.粘性土微观结构简易定量分析法.水文地质工程地质,1997,(1):7-10.
[62]施斌.粘性土击实过程中微观结构的定量评价.岩土工程学报,1996,18(4):57-62.
[63]王清,王凤艳,肖树芳.土微观结构特征的定量研究及其在工程中的应用.成都理工学院学报,2001,28(2):148-152.
[64]王清,王剑平.土孔隙的分形几何研究.岩土工程学报,2000,22(4):496-498.
[65]毛灵涛,薛茹,安里千,等.软土孔隙微观结构的分形研究.中国矿业大学学报,2005,34(5):600-604.
[66]张敏江,阎婧,初红霞.结构性软土微观结构定量化参数的研究.沈阳建筑大学学报,2005,21(5):455-459.
[67]王宝军,施斌,刘志彬,等.基于GIS的黏性土微观结构的分形研究.岩土 工程学报,2004,26(2):244-247.
[68]施斌.粘性土微观结构研究回顾与展望.工程地质学报,1996,4(1):39-44.
[69]张礼中,胡瑞林,李向全,等.土体微观结构定量分析系统及应用.地质科技情报,2008,27(1):108-102.
[70]姚仰平,谢定义.砂土的形变能破坏标准.西安理工大学学报,1994,10(1):42-46.
[71]龚晓南.高等土力学.浙江大学出版社,1996.
[72]龚晓南.土塑性力学.浙江大学出版社,1990.
[73]中华人民共和国国家标准土工试验方法标准(GB/T50123-1999).中国计划出版社,1999.