【作者】 李玲玲；

【导师】 王立忠；

【作者基本信息】 浙江大学 ， 岩土工程， 2007， 博士

【摘要】 我国东南沿海地区广泛分布着深厚的软弱粘土层。这些天然软粘土由于结构性的存在表现出与重塑土截然不同的性状。土体的结构性性状研究是目前岩土工程学术界研究及工程实践中的热点和难点问题,如何考虑和描述土体的结构性以及进一步应用于实际具有重要意义,而本文正是针对此问题开展讨论和研究的。天然土体结构性的形成机理不同,会相应表现出不同的性质。不同于次压缩的因素,胶结作用为主形成的结构性土体拥有一定的结构强度,使土体不需要进一步压密便能支承上覆荷载重,因而保持有大孔隙比状态;这类结构性土体是本文的主要研究对象。论文立足于这类结构性土的主要性状分析,并着重于其在解决实际工程中问题的应用性研究。论文首先从土结构性的基本概念和结构性土的基本性状出发,结合天然软土的一维压缩特性及其变形机理,系统提出了原位压缩曲线的确定、验证以及应用。在土体的侧向变形性状方面,指出了超固结比OCR和屈服应力比YSR的不同,探讨了结构性土体的K₀值及增量侧压力系数的变化规律;结合增量侧压力系数和切线泊松比的联系,引入状态转折面的概念,构建了非线性弹性模型中泊松比的计算模式。然后基于传统邓肯张双曲线模型的思路,考虑土体结构性的作用,建立了一种考虑结构性的非线性弹性模型,并结合有限元程序VFEAP进行了初步的数值模拟。综上所述,本文在分析软粘土结构性和构建非线性弹性模型的过程中主要完成了以下一些工作;1.提出了结构性软粘土原位压缩曲线的确定方法。针对天然土体的压缩性状以及已有确定方法的优点和不足,在温州软土的室内试验结果基础上,结合系统收集的国际上典型结构性软粘土的试验结果,提出了对应于转折应力的关键点和原位压缩曲线的确定方法,并在此基础上,多种角度验证了该方法的适用性与合理性。从研究思路、确定步骤、变形机理、试验验证、工程应用等多方面完善了原位压缩曲线的构建体系。2.揭示了原位施工扰动的机理及原位扰动压缩曲线的校正方法。从土体微观结构角度看,原位施工扰动主要是使土体颗粒间的胶结作用减弱,宏观表现出结构屈服应力减小,结构强度降低,土体压缩性增大,不排水强度降低等性状。在此基础之上,提出了原位扰动压缩曲线的校正方法,并探讨了原位扰动的定量评价方法,结合实例给出了考虑扰动的地基沉降计算方法。3.构建了天然结构性土体非线性弹性模型中切线泊松比的计算模式。从土体结构性形成机理着手,指出软土沉积后结构性的形成抑制了次固结的发展,原位K₀值建议仍取为K_0NC;并探讨了土体在加载过程中侧压力系数的变化规律。在结构性土体侧限压缩变形规律的基础上,结合K₀固结试验结果,引入状态转折面的概念,采用修正剑桥模型的椭圆形屈服面形式,计算天然结构性土体非线性弹性模型中的切线泊松比,并推广到更符合结构性土屈服特性的倾斜椭圆屈服面的情况。4.提出了一种考虑结构性影响的非线性弹性模型（NLS Model）。基于传统双曲线模型的思路,结合天然土体变形机理以及基本弹塑性理论,模型的计算参数包括YSR、CIc2、G_s、φ’、e₀、v以及峰值应变ε_1p等七个参数。该模型能较好地模拟土体应力应变在不同阶段的发展规律,描述土体的实际应力应变性状;且参数通过简单的土工试验均可获得,便于应用。并将该模型在有限元程序中实现,最后,结合工程算例对该模型的有效性和实用性进行验证。更多还原

【Abstract】 Soft clays are widely distributed in the coastal area of the southeast China. These natural clays usually have high water content,extensive compressibility and low strength just as other soft clays all around the world.The most important is that the properties of natural clay differ from its reconstituted properties in a number of important ways due to the soil structure.Geotechnical designers commonly use test data of disturbed clays in their calculation and designing.Therefore,investigation of the effect of structure on properties of natural clays becomes an important issue for understanding the clay behavior and reliable design in practice.The compressibility and K₀ behaviour of natural structured clay is studied firstly in detail.Based on these basic charateristics,a non-linear elastic constitutive model considering the soil structure is developed in this paper.Finally the model is introduced to the finite element program to solve practical problems.In general, some developments for soft clays are involved in this paper as below.（1）The new predicting method of field compression curve is proposed.It is important to describe the one-dimensional compression of soils,one of the most popular problems in geotechnical engineering.Based on the multi-linear relationship of the void ratio and the consolidation pressure in semi-log scale,the FCC of natural clays is determined by the exact location of the two key stresses（yield stress and transition stress）.For a case history,comparisons between the predictions and the observed results in situ verify the validity and applicability of this new procedure.（2）The mechanism of field disturbance and a new method for modifying the compression curve of disturbed soil are presented.Natural soil is prone to be disturbed.The field disturbance,distinct from the sample disturbance,usually results in the degradtion of bonding,reduction of structural strength and increase of compressibility.Then the evaluation of the field disturbance degree is discussed associated with the results of the vane test in situ.Furthermore,the additional settlements due to the field disturbance are analyzed.（3）The computation pattern to determine the value of tangent Poisson ratio is suggested.The faster formation of bonding inhibits the development of secondary compression in the process of soil deposition.K₀ value in situ,therefore,is still equal to that of the normal consolidated soil,i.e.the K_0NC.Besides,according to the laboratory test of natural clay in many regions,the behavior of tangent Poisson ratio is three-phase pattern with the turning point of the yield stress and transition stress. Then,the concept of phase transition curves is introduced and the value of tangent Poisson ratio is computed,which is applied to non-linear elastic model.（4）A new non-linear elastic constitutive model considering soil structure,NLS model,is developed.According to the theory of Duncan-chang model and basic elastoplastic theory,this model is developed based on the deformation mechanism for typical natural clays and.The seven parameters of the NLS model,including the yield stress ratio YSR,compression index after yielding C_c2,swelling index C_s,initial void ratio e₀,Poisson ratio v and strain at peakε_1p,are easier to obtained from the common labotary tests.Finally,this model is introduced into the finite element program.The computed results for a case history indicate that the NLS model is available in practice and the behaviour of structured clay can be simulated well.（5）Reseach in this paper can be used to the deformation and stability analysis of building and construction,such as the embankment or the dam on soft ground.更多还原