非共轴本构模型的数值应用及离心机试验研究

发布时间：2018-04-05 09:10

本文选题：主应力方向旋转　切入点：非共轴本构模型　出处：《大连理工大学》2013年博士论文

【摘要】：在许多实际岩土工程问题中,例如基坑开挖、浅基础地基承载力与变形特性、地震或波浪荷载对海床的作用等,主应力方向的旋转所引起的初始主应力状态的改变是不容忽视的。在主应力方向的旋转过程中,主应力方向与塑性主应变增量方向的旋转变化趋势不是一致的,即存在着非共轴现象。对于非共轴现象的描述及分析是当前土力学研究中的热点问题之一。采用合理的本构模型对非共轴现象及其影响进行研究,通过离心机试验结果对非共轴模型的数值计算结果的合理性进行验证,能够完善传统的岩土材料弹塑性本构模型,也能够为重大工程结构和地基设计提供依据,具有重要的理论价值和实际工程意义。本文围绕着非共轴现象及其对应力-应变关系的影响进行了系统的研究,通过砂土单剪试验的数值模拟研究了非共轴现象及其影响因素,针对浅基础地基承载力与变形特性进行数值计算分析和离心机模型试验研究,通过离心机试验结果对非共轴模型计算结果的合理性进行了验证。主要研究内容和结论如下： 1.将角点结构非共轴理论应用到传统弹塑性共轴本构模型中,建立了一种非共轴本构模型。对砂土单剪试验进行有限元数值模拟,研究了非共轴现象及其对应力-应变关系的影响。研究结果表明：非共轴模型得到的主应力方向和塑性主应变增量方向在剪切变形的初期是不重合的,随着剪切变形的深入发展,两者逐渐趋于一致。非共轴现象对应力-应变关系具有“软化”作用,即非共轴模型计算得到的剪应力比的增长速度滞后于共轴模型计算结果的增长速度。与理想弹塑性情况下的计算结果相比,非共轴现象对应力-应变关系的影响在应变硬化情况下更加明显。当采用应力峰值作为密砂的抗剪强度时,需要考虑非共轴模型与共轴模型计算结果之间的差异；当采用临界状态的残余强度作为抗剪强度时,非共轴模型与共轴模型计算结果之间没有差异。静止侧压力系数、竖向应力和流动法则等因素对非共轴现象具有显著的影响。 2.在理想弹塑性、应变硬化和应变软化情况下,采用共轴和非共轴模型对浅基础地基荷载-变形问题进行数值分析,研究了非共轴现象对地基荷载-位移关系的影响。研究结果表明：非共轴现象对地基荷载-位移曲线具有“软化”作用,即非共轴模型得到的地基荷载的增长速度要滞后于共轴模型计算结果的增长速度。随着浅基础下方土体单元水平位置的增加,主应力方向的旋转越来越显著,非共轴现象也越来越明显。非共轴现象在浅基础边缘下方深度较浅处比较明显。与理想弹塑性情况下的计算结果相对比,非共轴现象对地基荷载-位移关系的影响在应变硬化和应变软化情况下更加明显,在应变硬化和软化模型的基础上考虑非共轴现象的影响具有实际意义。研究非共轴现象对数值计算结果的影响时,需要考虑基础形式、基础埋深和偏心距的变化。 3.对浅基础地基承载力与变形特性进行了离心机模型试验研究,分析了地基荷载-沉降关系的特点、地基承载力系数Nγ与基础宽度的关系、以及基础尺寸效应。研究结果表明：对于中密砂地基而言,试验所得到的地基竖向荷载-位移关系曲线没有显著的拐点或曲线斜率突变阶段；随着竖向位移的增加,竖向荷载-位移曲线的初始斜率随着基础宽度的增加而增加；将确定地基承载力特征值fak时所采用的V/D提高到0.015～0.05时,试验结果与理论公式的结果比较接近；基础尺寸效应在基础沉降较大时比较明显,试验所得到的Nγ与理论公式结果比较接近；当基础沉降较小时,基础尺寸效应不明显,试验所得到的Nγ低于理论公式结果。对于密砂地基而言,离心机试验得到的竖向荷载-位移关系曲线具有明显的拐点；地基承载力随着基础宽度的增加而增加；地基承载力随着基础埋深的增加而增加,该结论在基础宽度较小时比较明显；达到地基承载力时的基础沉降δ对于Nq的影响不明显,δ对Nγ的影响比较明显,考虑δ时的Nγ低于不考虑δ时的Nγ；基础形式的变化对Nγ的影响比较明显,其对Nq的影响不明显。与方形浅基础的试验结果相比,圆形浅基础试验结果中的基础尺寸效应更加明显。 4.对浅基础离心机试验进行数值模拟,将数值计算结果与试验结果进行对比分析。从地基荷载-位移关系和地基承载力两个方面,验证了非共轴模型计算结果的合理性。研究结果表明：当基础沉降较小时,数值计算得到的竖向荷载-位移关系曲线的斜率大于离心机试验结果的斜率,共轴模型与非共轴模型的计算结果之间的差异不明显；随着基础沉降的增加,非共轴模型与共轴模型计算得到的荷载-位移曲线之间的差异越来越明显,离心机试验结果分布在非共轴模型结果中间。共轴模型计算得到的地基承载力系数高于离心机试验和非共轴模型的结果,离心机试验与非共轴模型所得到的地基承载力系数比较接近。
[Abstract]:In many practical geotechnical engineering problems , such as foundation pit excavation , shallow foundation foundation bearing capacity and deformation characteristic , earthquake or wave load , the change of initial principal stress state caused by rotation of main stress direction is not to be ignored . In the course of rotation of the main stress direction , the main stress direction and the plastic main strain increment direction are not consistent , that is , there is a non - coaxial phenomenon .

In this paper , the non - coaxial phenomenon and its influence on stress - strain relationship are studied . The non - coaxial phenomenon and its influencing factors are studied by numerical simulation of single shear test of sand and soil . The rationality of non - coaxial model calculation results is verified by the results of centrifugal test . The main research contents and conclusions are as follows :

1 . A non - coaxial constitutive model is established by applying the non - coaxial theory of corner structure to the traditional elastic - plastic co - axial constitutive model . The influence of non - coaxial phenomenon and its effect on stress - strain relationship is studied .
When the residual strength of the critical state is adopted as the shear strength , there is no difference between the non - coaxial model and the calculation result of the coaxial model . The static side pressure coefficient , the vertical stress and the flow rule have significant influence on the non - coaxial phenomenon .

2 . Under the condition of ideal elastic - plastic , strain hardening and strain softening , the influence of non - coaxial phenomenon on the load - displacement relationship of foundation is studied . The results show that the non - coaxial phenomenon has a ' softening ' effect on the foundation load - displacement curve .

3 . The characteristics of foundation load - settlement relations , the relationship between foundation bearing capacity coefficient N 纬 and foundation width , and the basic dimension effect are analyzed . The results show that the vertical load - displacement relation curve of the foundation obtained by the test has no obvious inflection point or curve slope mutation stage for medium - dense sand foundation .
As the vertical displacement increases , the initial slope of the vertical load - displacement curve increases with the increase of the base width ;
When the V / D used in determining the characteristic value fak of the ground - bearing capacity is increased to 0.015 - 0.05 , the experimental results are close to the results of the theoretical formula .
The basic dimension effect is obvious when the foundation settlement is large , and the N 纬 obtained by the experiment is close to the result of the theoretical formula .
When the foundation settlement is small , the foundation size effect is not obvious , the N 纬 obtained by the experiment is lower than the theoretical formula result . For the dense sand foundation , the vertical load - displacement relation curve obtained by the centrifuge test has obvious inflection point ;
the bearing capacity of the ground is increased along with the increase of the basic width ;
The bearing capacity of the foundation increases with the increase of the buried depth of the foundation , and the conclusion is more obvious at the base width .
The effect of 未 on Nq is not obvious . The effect of 未 on N 纬 is obvious , and the N 纬 at 未 is lower than that when 未 is not considered .
The effect of the change of the basic form on the Nq is not obvious . Compared with the experimental results of the square shallow foundation , the basic dimension effect in the results of the circular shallow foundation test is more obvious .

4 . The numerical simulation of shallow foundation centrifuge is carried out , and the numerical results are compared with the experimental results . The results show that the slope of the vertical load - displacement relation curve obtained by numerical calculation is greater than that of the centrifuge test results when the foundation settlement is small , and the difference between the coaxial model and the non - coaxial model is not obvious ;
With the increase of the foundation settlement , the difference between the non - coaxial model and the load - displacement curve calculated by the coaxial model is more and more obvious , and the results of the centrifuge test are distributed among the non - coaxial model results . The coefficient of the ground - bearing capacity calculated by the coaxial model is higher than that of the centrifuge test and the non - coaxial model , and the centrifuge test is close to the ground - bearing capacity coefficient obtained by the non - coaxial model .

【学位授予单位】：大连理工大学
【学位级别】：博士
【学位授予年份】：2013
【分类号】：TU41

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