摘要
我国东南沿海地区广泛分布着深厚的软弱粘土层,随着经济社会的迅猛发展,越来越多大型的建(构)物选择修建于其上,这也为岩土工程界的设计和研究人员提出了更高地要求和挑战。天然软粘土的流变特性和循环剪切特性是决定软弱地基及其上部结构工后沉降和稳定性的主要因素,对其进行系统的研究将具有重要的工程实际意义,本文的研究和讨论也是围绕这一主题而展开的。
论文首先总结了前人的工作,并采用室内土工试验的手段,对K_0固结原状温州粘土的流变性状展开研究,以期对天然软土的流变性状及机理有更为全面的认识。在此基础上,建立了一个能反映K_0固结软粘土流变性状的弹粘塑性本构模型,通过进一步细化分析粘塑性软土各向异性的演化,引入了与模型相匹配的旋转硬化规律,提高了模型的适用性。然后采用此流变模型,分析了粘土屈服性状和强度的应变率效应,给出了相关应变率效应参数的解析表达。最后对K_0固结原状温州粘土进行了不排水的循环剪切试验,考察了粘土的循环累积性状和强度弱化效应,并采用本文流变模型对其循环剪切性状进行了模拟。
综上所述,本文在分析天然软粘土的流变性状的过程中,主要完成了以下一些工作:
1.开展了针对原状软粘土流变性的试验研究。通过一系列针对性的一维和三轴室内试验,对原状温州粘土的蠕变性状以及压缩性和剪切性状的率相关性进行了系统的考察,补充完善了温州粘土工程性状的试验成果,同时为本文建立的弹粘塑性本构模型提供必要的计算参数。
2.建立了K_0固结软粘土的弹粘塑性本构模型。借鉴过应力理论的建模思想,引入了对应于土体正常固结状态的参考面(f),在此基础上构建了一般应力条件下的弹粘塑性本构模型。考虑原位土体K_0固结历史诱发的各向异性影响,模型的加载面和参考面均为倾斜椭圆。通过与试验结果对比,验证了模型的有效性。
3.研究了K_0固结粘塑性软土的旋转硬化规律。考虑到土体粘性的影响,在本文流变模型的基础上引入了与之相匹配的旋转硬化规律,并通过考察等向固结过程中土体的体积变形,推求了旋转硬化参数b的解析表达式。通过计算对比和参数b的敏感性分析,探讨了引入旋转硬化规律的必要性。
4.分析了K_0固结软粘土的应变率效应。利用本文建立的弹粘塑性本构模型,定量考察了表观先期固结压力和三轴不排水强度的应变率效应,得到了各自的应变率效应参数的解析表达式。采用不同的强度取值标准,由本文模型分别获得了K_0固结软粘土的不排水压缩和拉伸强度的解析解。本文关系的计算结果与现有多数应变率效应试验结果均吻合较好。
5.开展了针对原状软土循环剪切性状的试验研究。通过对K_0固结原状温州粘土进行不同幅值、不同频率条件下的三轴不排水循环剪切试验,系统分析了土体在循环剪切过程中的应变累积、残余孔压和动强度等特性,详细探讨了荷载频率对土体循环剪切性状的影响。同时对粘土在循环加载后的静力抗剪强度弱化进行了定量的试验研究,得到规律性的结论。
6.对天然软土的循环剪切性状进行了流变学模拟。详细讨论了粘土不排水循环剪切的累积效应与静力蠕变特性的相似性,采用流变学思路,将其应力等效为蠕变过程来求解。利用反分析,得到了等效次固结系数(?)_α的计算式,并与本文弹粘塑性模型相结合,模拟分析了K_0固结原状温州粘土的不排水循环剪切的破坏性状和强度弱化,验证了本文流变等效计算方法的适用性。
Soft clays,which have been commonly encountered in real construction projects,are widely distributed in the coastal area of the southeast China.Rheological property and cyclic shearing behavior are the most important factors of natural clays that may lead to large settlements and unstability.Therefore,investigation of the effect of rheology and cyclic shearing on properties of natural clays is an important issue for understanding the behavior of clay and reliable design in practice,and this is just the object of this thesis.
Based on the summary of related work,the rheological properties of K_0-consolidated natural Whenzhou clay are studied in detail from series of laboratory tests,and then an general elastic viscoplastic constitutive model is developed in this paper.The presented model is introduced to investigate the strain-rate dependency on yielding and strength behavior of clays,and the theoretical expressions of their strain rate parameters are obtained.Finally,the undrained cyclic shearing behavior of K_0-consolidated natural Whenzhou clay is observed in triaxial cyclic shear tests,and the results are fairly simulated by this rheologic constitutive model.
In general,some developments for soft clays are involved in this dissertation as below:
1.Experimental researches are conducted to study the rheological properties of natural Whenzhou clay.Creep and the strain rate dependency,which are the most two common behavior of clay rheology,are investigated systematically.Some features of the rheology of Whenzhou clay are obtained.The values of the model parameters can also be determined by parts of these experimental results.
2.A new anisotropic elastic viscoplastic(EVP) constitutive model are presented for K_0-consolidated clays in general stress space.Just similar to the original overstress theory,two surfaces is assumed to exist for any loading history:loading surface f and reference surface(?),which are inclined ellipses in the p~'～q plane due to the effects of the stress induced anisotropy.Both of these two surfaces are constant curves of the flow functionφ, so the exact expression of flow fuctionφcan be established by the viscoplastic volumetric strain rate under one-dimensional condition.With comparisons between the simulations and the observed results of some representative clays,the validity and applicability of this model is verified.
3.A rotational hardening law accounting for changes in anisotropy due to viscous strains is adopted into the presented model.By analyzing the soil's volumetric changes during isotropic loading,the expression of the new rotational parameter b is presented. Followed by comparisons with experimental data,the proposed model is capable of capturing key viscous features of soft clays,and the importance of the evolution of anisotropy is indicated.
4.The strain rate dependent behavior of K_0-consolidated clays is investigated.Based on the developed anisotropic EVP model,the expressions of strain rate parameters of preconsolidation stress and undrained triaxial strength are presented,separately.The analytical solutions for undrained triaxial compression and extension strength of K_0-consolidation clays are deduced.The coincidence of the predicted and measured results shows the advantages of the new anisotropic EVP model.
5.Experimental researches are conducted to study the undrained cyclic shearing behavior of natural Whenzhou clay.Considering the effects of anisotropy,all of the soil specimens are K_0-consolidated,which is very different from many existed tests.The deformation、pore pressure and cyclic strength behavior of natural clay is studied,and the effects of the loading frequency are analyzed.In addition,postcyclic degradation of strength is studied by another series of undrained triaxial tests,and some rewarding conclusions can be obtained.
6.The cyclic shearing behavior is simulated by the presented rheological model.Based on their similarity,the accumulation during cyclic shearing is equivalent to a static creep process with the condition that the static incremental loadΔq=q_(cyc).Then,a simple expression of the equivalent secondary compression coefficient(?)_αis adopted into the presented model which is used to simulate the accumulated cyclic shearing behavior of clays.The predicted results indicate that the creep equivalent simplification with the presented anisotropic EVP model is suitable in practice,the accumulation behavior and the postcyclic degradation of strength of natural clays can be fairly simulated.
引文
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