【作者】 汪东林；

【导师】 栾茂田；

【作者基本信息】 大连理工大学 ， 岩土工程， 2007， 博士

【摘要】 非饱和土是一种由固相、液相和气相所组成的三相土。非饱和土在实际工程中分布十分广泛，其工程特性相对于饱和土要更为复杂，理论尚不完善。在非饱和土力学中，非饱和土体变理论一直是非饱和土理论研究的一个重要研究内容，体积变化的本构方程将变形状态变量与应力状态变量联系起来。由于非饱和土含有固相、液相和气相，使得非饱和土的力学性质相对于饱和土要更为复杂。在试验中，试样孔隙水压和气压的控制和测量较为复杂，非饱和土试验资料仍然十分缺乏。并且，非饱和土力学在实际工程的应用中还明显落后于理论的发展，许多工程师仍然采用传统的饱和土理论来解决非饱和工程问题。如何进一步完善非饱和土理论，将传统的土力学理论发展为广义的土力学理论，并在实际工程中得到应用和推广是非饱和土力学领域工作者的一个重大挑战。因此，本文利用GDS非饱和三轴仪在分别控制试样净平均应力和基质吸力条件下开展了大量试验研究。基于试验研究结果，根据非饱和土力学理论提出了可以全面考虑非饱和区和饱和区沉降变形的数学模型，并对地下水位的降低引起地面沉降问题进行了详细分析和研究。本文的工作主要包括下列内容。（1）利用高进气值压力板仪对非饱和重塑砂土、粉土、黏土和粉质黏土开展了土水特征曲线试验，并结合试样收缩曲线，考察了四种类型土体干燥收缩过程中在净平均应力σ_m-u_a=0 kPa条件下基质吸力和孔隙比关系。一般认为，屈服吸力s₀将土体收缩过程分为弹性和弹塑性两个阶段。但试验结果表明，当试样基质吸力达到某一特定值后，基质吸力的增大不引起试样的进一步收缩变形，本文称此吸力为缩限吸力s_s。因此，屈服吸力和缩限吸力可将整个收缩过程分为三个阶段，即弹性阶段、弹塑性阶段和缩限阶段。通过理论分析可知缩限吸力s_s可假定为试样的缩限含水率在土水特征曲线中所对应的基质吸力，试验结果也验证了假设的正确性。试验结果表明，不同类型土的缩限吸力大小并不相等，试样的塑性指数越大，缩限吸力也越大。并且在干燥收缩过程中，当饱和度在减小至0.90过程中试样孔隙比迅速减小，试样的收缩变形基本在此阶段完成；当试样饱和度减小到0.70时，试样的孔隙比基本保持不变。（2）本文采用GDS非饱和土三轴仪，在分别控制试样基质吸力和净平均应力条件下对非饱和重塑黏土的体积变化和含水率变化特性进行了详细试验研究。在非饱和三轴试验研究中，本文开展了2组不同应力路径试验：①各向同性压缩试验：在控制基质吸力u_s=u_a-u_w的条件下施加不同的净平均应力p=σ_m-u_a；②三轴收缩试验：在控制净平均应力条件下施加不同的基质吸力。在控制非饱和黏土吸力条件下的压缩试验中，根据所测得各个试样的总体积变化变化考察了试样的压缩指数和屈服应力与基质吸力的相关性，当试样的净平均应力加载到设定的最高值后，在保持基质吸力不变条件下，将净平均应力直接卸载到0，来考察不同基质吸力下试样的回弹指数；在给定的基质吸力条件下，随着净平均应力的增加，水不断从试样中排出，通过水比容概念考察了水体积变化特性。在控制试样净平均应力条件下的干湿循环试验中，分别考察了土水特征曲线和收缩特性与净平均应力的相关性；当试样加载到设定的最大基质吸力值并达到吸力平衡后，将试样重新饱和后（即基质吸力卸载到0 kPa），来考察试样干湿循环特性。（3）在地面沉降研究中，人们常采用饱和土有效应力原理来计算沉降量。实际上，地下水位的降低也会引起非饱和区域土体饱和度的减小，导致净平均应力降低和基质吸力增大。随着非饱和土力学的发展，人们逐渐认识到基质吸力的增大也可造成土体的压缩变形。结合饱和-非饱和渗流与非饱和土体变本构模型，本文提出了一种可以全面考虑非饱和区域与饱和区域沉降的数学模型。由于地下水位下降引起的饱和土区域和非饱和土区域的沉降变形有着本质的区别，因此需分别估算饱和土区域（即u_a-u_w≥0）和非饱和土区域（即u_a-u_w＜0）的沉降变形量。对于饱和区，利用饱和土的有效应力原理来计算土体有效应力增大而引起的压缩变形量；对于非饱和区，采用非饱和土的体变本构模型可分别计算土体由于基质吸力增大而引起的收缩变形量和净平均应力减小而引起的回弹量。（4）根据饱和-非饱和沉降数学模型，对Kai-Yuan Ke试验模型进行模拟分析，并与实测数据进行了对比分析，验证了本文提出的饱和一非饱和沉降数学模型的合理性和可行性。模型试验结果表明在某些情况下非饱和区域的沉降变形量是不可忽略的，如果按照传统的计算方法仅仅考虑饱和区的变形量，则计算结果与实际情况相差较大。基于非饱和土层基质吸力分布为静水平衡的假设，针对不同的地下水位降低，本文根据饱和-非饱和沉降数学模型分别估算了非饱和区域和饱和区域的沉降量。通过非饱和区沉降量和地表总沉降量的对比，验证了考虑非饱和区沉降变形量的重要性。对于非饱和区，本文分别估算了由于净平均应力的降低引起的地面回弹量和基质吸力的增大引起的地面沉降量。计算结果表明，在这两种变形中收缩变形量在非饱和区变形中占有主导地位，净平均应力的减小而引起的地面回弹量可以忽略不计。（5）在基坑开挖过程中，为防止基坑渗透破坏，往往采用井点降水等措施来降低地下水位，但地下水位的降低会引起周围建筑物和地下管线附加沉降而产生不良影响。基于井点降水形成的二维饱和-非饱和稳态渗流模拟分析，本文对某一基坑分别估算了饱和土区域和非饱和土区域沉降变形量。计算结果表明，在基坑降水时非饱和区的变形量往往是不可忽略的。并对降水深度、初始水位高度、土体压缩指数、收缩指数、渗透系数各向异性、土层初始吸力大小、净平均应力和基质吸力耦合关系各种影响因素进行了详细分析。基于饱和-非饱和非稳态渗流，本文对基坑降水后引起周围土层沉降随时间不断发展的过程进行了动态模拟，详细分析和探讨了沉降变形与时间和空间的相关性。更多还原

【Abstract】 Unsaturated soil is a three-phase media and is composed of soil gain, water and air. And, unsaturated soils are widely distributed and display more complex behaviour compared with saturated soils in engineering practice. The theory of volume change is an important topic in unsaturated soils mechanics, state variables of strain and stress are related by constitutive equation of unsaturated soils. Unsaturated soil is composed of solid phase, fluid phase and gas phase, the pore water pressure and pore air pressure of samples is hard to be controlled and measured in tests, so it is serious lack of testing data of unsaturated soils. Besides, the application of unsaturated soils to practice fall behind of its theory development, many engineers are like to utilize conventional method to solve the unsaturated soil problems. How to perfect the theory of unsaturated soil and develope conventional soil mechanics to generalized soil mechanics is a big challenge for researchers. So, using GDS unsaturated triaxial apparatus, the volume change tests of unsaturated soils are conducted under the condition of controlling matric suction and net mean stress respectively in a predetermined stress path. Based on experimental results and the theory of unsaturated soil, the saturated-unsaturated settlement model is established, this model can be used to consider the settlement taken place in zone of saturated soils and in zone of unsaturated soils. Land subsidence due to drawdown of groundwater table is analyzed by using of the new model. The following portions are included in this dissertation.（1） The empirical relationship between matric suction and void ratio on drying path are established from four types of soils including sand, silt, clay and silty clay by utilizing the shrinkage curves and the soil-water characteristic curves which are determined through pressure-plate tests. It is indicated that during the process of drying path, the soil sample continuously shrinks with increase of matric suction. However, increase of suction will display no effect on the shrinkage deformation of the soil sample once the matric suction increases to a given value which is defined as the shrinkage limit suction s_s. The values of s_s are dependent on plasticityindex of soil. The process of shrinkage of soil is divided by yielding suction s₀ and shrinkage limit suction s_s into three stages, i.e., elastic stage, elasto-plastic stage and stage of shrinkage limit. Moreover, in the process of drying-shrinkage of soil sample, shrinkage of the soil sample is almost fulfilled when degree of saturation of soil sample decreases to 90% and void ratio of the soil sample almost keeps unchanged while degree saturation of soil is reduced to 70%.（2） By using the GDS unsaturated triaxial apparatus, this paper study the change of total volume change and the change of water content under controlled matric suction and net mean stress condition in a predetermined stress path. Two groupe of tests with different stress path are conducted in unsaturated triaxial tests.①Isotropic compression tests. Different net mean stress is applied under the condition that matic suction is controlled;②Triaxial shrinkage tests. Different matric suction is applied under under the condition that net mean stress is controlled. The results show that the parameter related to compression is suction-dependent and rebound coefficient is dependent on matric suction in a certain degree, shrinkage index and the soil-water characteristic curve are also dependent on net mean stress. At a given matric suction, With the increase of net mean stress, the void ratio and water content of soil sample is decrease, but thedegree of saturation is increased. There is about linear relationship between water content of soil and net mean stress for the soil samples under higher suction. At a given net mean stress, the soil sample will shrinkage in accordance with increase of matric suction. In wetting stage, i.e., zero suction is applied to soil samples in saturated state, the circulating character be analysed.（3） People often like to utilize the theory of effective stress of saturated soil to calculate land subsidence. In fact, Drawdown of groundwater table will cause the change of net mean stress and matric suction in unsaturated soils and then induces volume change of the soil mass. Combined with saturated-unsaturated seepage flow and the constitutive model of volume change, this paper present a new settlement model which can consider settlement in both of saturated zone and unsaturated zone There is an essential distinction between settlement occured in saturated zone and unsaturated zone, so this model utilize effective stress theory to calculate settlement of saturated zone; for unsaturated zone, this model can calculate shrinkage of soil because of increase of matric suction and rebound of soil because of decrease of net mean stress.（4） Based on saturated-unsaturated settlement model, simulation analysis is conducted for Kai-Yuan Ke test model. Through the compare of analysis results and test data, this paper certificates the rationality and feasibility of saturated-unsaturated settlement model. The results of model test indicate that the settlement of unsaturated zone can not be omitted in some case. If we use conventional model to calculate the settlement of ground, the results will not coincidence with facts.Based on the assumed that pore air pressure equal to zero consistently and the magnitude of matric suction in unsaturated stratum is equilibrium with water table, using the saturated-unsaturated model, subsidence occurred in unsaturated soil zone and saturated zone are estimated respectively for different drawdown depth. Computed results indicate that in total amount of subsidence, the portion of subsidence of unsaturated soil zone cannot be overlooked. For unsaturated zone, the shrinkage of soil because of increase of suction and rebound of soil because of decrease of net mean stress are calculated. Also, the result show that shrinkage of soil has the leading position in deformation of unsaturated zone and the rebound of soil can be omitted.（5） During cutting of foundation ditch stage, well-point dewatering is often used to drawdown of groundwater table to avoid the seepage damage of foundation ditch. But, drawdown of groundwater table maybe causes the damage of surrounding building and underground pipeline. Based on simulation of two dimensional steady-state flows induced by foundation pit dewatering, this paper calculate the subsiedce occurred in saturated zone and unsaturated zone respectively. Computed results indicate that in total amount of subsidence, the portion of subsidence of unsaturated soil zone cannot be overlooked. The analyse of influencing factors are conducted, including drawdown depth, initial groundwater table, compression index and shrinkage index of soil, anisotropic permeability, initial matric suction in soil layer, coupIing relationship between suction and net mean stress. Base on simulation of two dimensional saturated-unsaturated unsteady flows, this paper analyse the dynamic process of settlement of ground caused by foundation pit dewatering, and the dependency relation of subsidence and time-space is discussed in detail.更多还原