【作者】 郭林；

【导师】 蔡袁强；

【作者基本信息】 浙江大学 ， 岩土工程， 2013， 博士

【摘要】 我国东南沿海地区广泛分布着深厚饱和软粘土,软粘土地基上已建和在建大量高速公路、铁路、机场跑道等重大工程。路堤静荷载和交通动荷载长期循环往复作用引起地基土应变累积、强度降低,导致重大工程过大变形和失稳等灾变,造成巨大经济损失,甚至威胁生命安全。因此,高速交通重大工程长期服役性能亟需提升,迫切需要发展有效的软粘土地基灾变控制技术。开展复杂应力路径的饱和软粘土静动力特性研究,则是解决这一问题的关键所在。本研究采用薄壁切土法获得了高质量的温州原状软粘土试样,通过GDS动三轴仪和空心圆柱系统对其开展了一系列复杂应力路径下的动静力特性试验研究,主要完成了以下工作：1.针对路堤静荷载下天然软粘土地基真实应力状态,开展各向同性固结试样的不排水剪切试验及Ko固结试样的应力路径三轴试验和不同大主应力方向角下的剪切试验研究了原状饱和软粘土的结构性、应力路径依赖性及各向异性。研究了结构性的存在对软粘土试样剪切强度、应力-应变关系及有效应力路径发展的影响；建立了p-q平面上不同应力路径下K0固结软粘土试样的孔压计算公式,验证了正常固结粘土p-q-e的唯一性关系；分析了不同大主应力方向角下K0固结软粘土试样的应力-应变关系和孔压发展规律,得到了τZθ-(σZ-σθ)/2平面上破坏强度包络线,揭示了大主应力增量和大主应变增量之间的不共轴现象。2.针对交通荷载作用的长期性特征,开展了不同围压和不同循环应力比下次数达50000次的单向激振三轴试验。研究了饱和软粘土在长期循环加载下的孔压发展、应力-应变滞回曲线演化、模量软化及应变累积特性。基于有效路径峰值点到临界状态线(CSL)的距离与轴向总应变的关系建立了长期循环荷载作用下孔压-应变模型；建立了长期循环荷载作用下饱和软粘土试样回弹模量经验方程,为数值分析中模量的选取提供了指导；建立了长期循环荷载作用下累积应变经验方程,给出了方程中两个参数εp,1000和λ的物理意义和确定方法,进一步通过对不同循环应力比下的方程参数进行回归分析,得到了饱和软粘土的长期累积应变预测模型。3.通过对不同围压和不同循环应力比下经50000次循环后的稳定回弹模量和累积应变进行分析,发现当循环应力比CSR<0.32时,稳定回弹模量随循环应力比的增加逐渐降低,此时累积应变随循环应力比增长相对缓慢,近似呈线性；而当CSR>0.32时,稳定回弹模量随循环应力比保持不变(Mr,50000/p’0=90),达到“渐近线强度”,此时累积应变随循环次数增加开始迅速增长,线性关系不再存在,随循环应力比的增大,累积应变呈指数增长。表明对温州原状饱和软粘土而言,CSR=0.32是一个动应力水平的临界值,可称之为“容许循环应力比”,当循环应力比小于该值时,饱和软粘土试样的长期累积应变可控制在2%以下。容许循环应力比提出,克服了在软粘土地区路基设计时采用门槛循环应力比过于保守,而采用临界循环应力比又难以控制过大沉降的难题。4.针对长期交通荷载作用的振动特性、软粘土结构性及地基排水条件,开展了各向同性固结温州原状饱和软粘土试样在不同影响因素下的常围压循环三轴试验。分析了结构性、振动频率、排水条件和间歇循环加载对饱和软粘土孔压和应变发展、应力-应变滞回曲线演化、模量软化及应变累积特性的影响并解释其原因,得到的一些结论对于深入了解真实工程条件下土体的动力特性提供了丰富的数据支持。5.针对交通荷载下竖向动应力与水平动应力祸合的特征,利用GDS双向振动三轴仪对Ko固结试样进行了不同循环应力比下循环围压和循环偏应力耦合的循环三轴试验。基于Yasuhara提出的孔压公式及Skempton理论得出了变围压循环三轴试验条件下的总孔压(弹性孔压、残余孔压和传递孔压)计算公式。研究了围压的循环变化对饱和软粘土的动应变、动孔压和有效应力路径的影响。结果表明,在分析交通荷载作用下饱和软粘土的动力特性时,水平应力循环变化的影响是不可忽略的。6.针对交通荷载作用下饱和软粘土地基上大主应力的大小和方向同时变化的特点,利用GDS空心圆柱系统对K0固结试样进行了循环扭剪(CTS)和循环三轴(CT)应力下的对比试验研究。采用自定义波形控制循环竖向应力和循环扭剪应力实现了2τZθ-(σz-σθ)平面中交通荷载心脏形应力路径的模拟,加载过程中动偏应力发生改变的同时大主应力方向角也由-90°变为90°。研究发现相同竖向动应力水平下,CTS试验中由于主应力轴的循环旋转加速了饱和软粘土试样的孔压累积和软化,进而加速了回弹和累积应变的产生。随着循环应力比的增加,CTS与CT试验之间的区别也越来越大。通过在长期循环三轴试验下建立的累积应变预测模型基础上,通过修正循环应力比引入考虑主应力轴旋转的η参数,建立了考虑主应力轴循环旋转的累积应变计算模型。更多还原

【Abstract】 Thick soft clay layers are widely distributed in the southeast of China. A lot of important projects, such as motorways, high-speed railways, airport runways, have been or will be constructed on the soft clay subgrade. Strain accumulation and strength degradation of the subgrade clays are induced by the static load of embankment and the cyclic load of traffic loading, leading to large deformation and unstability. These types of catastrophe cause great economic loss and even threat to the safety of humans. Hence, long-term service performance of important high-speed traffic engineering is urgently needed to be improved and it is necessary to develop effective catastrophe control technology for the soft clay subgrade. The key to solve this problem is to study the static and cyclic behavior of saturated soft clay under complex stress paths.In this study, high quality samples of natural Wenzhou soft clay are firstly obtained by thin-walled tube cutting method. A series of experimental studies are then conducted on them through GDS triaxial apparatus and hollow cylinder system. The detailed research contents are as follows:1. According to the actual stress conditions of natural soft clay subgrade induced by the static load of embankment, undrained shear tests on isotropic consolidated specimens and stress paths triaxial tests or shear tests under different major principal stress angles on K0consolidated specimens have been conducted. In this section, the characteristics of soil structure, dependency of stress path, and anisotropy of soft clay are researched. The influence of soil structure on the shear strength, stress-strain relationship and development of effective stress path is investigated. A formula for pore pressure of K0consolidated soft clay specimens under different stress paths in p-q plane is established. The uniqueness relationships of p-q-e are verified. The stress-strain relationship and pore water pressure evolution of K0consolidated soft clay specimens under different major principal stress angles are analyzed. The failure envelop in the τzθ-(σz-σθ)/2plane is obtained. The non-coaxial phenomenon between the increment of major principal stress and major principal strain is unveiled.2. One-way cyclic triaxial tests with number of50000under different confining pressures and cyclic stress ratios have been conducted to study the long-term cyclic behavior of soft clay induced by traffic loading. The pore pressure evolution, stress-strain hysteresis loops, resilient modulus degradation and permanent strain under large number of cycles are investigated. The long-term pore pressure-strain model is established based on the relationship between the distance of peak point in effective stress path to the CSL and the axial strain. An empirical equation for long-term resilient modulus of saturated soft clay is established, providing guidance for the chosen of the value of modulus in numerical analysis. Besides, an empirical equation for long-term permanent strain is established. The physical meanings and the determination methods of the two parameters, εp,1000and λ, in this equation are given. The long-term permanent strain prediction model is obtained by further regression analysis on the equation parameters.3. The steady resilient modulus and permanent strain after50000under different confining pressure and cyclic stress ratio have been researched. Results show that for CSR<0.32, the values of steady resilient modulus decrease with the increase of CSR. At the same time, the values of permanent strain increase slowly and almost linearly with the increase of CSR. While for CSR>0.32, the values of steady resilient modulus almost remain unchanged (Mr,50000/P0=90) and reach the "asymptote strength". At the same time, the values of permanent strain begin increasing rapidly. The linear relationship no longer exists and the values of permanent strain increase exponentially. Thus CSR=0.32is a critical value for Wenzhou saturated soft clay, which can be called "allowable cyclic stress ratio". When CSR is below this value, the long-term permanent strain of saturated soft clay specimens can be controlled below2%. The proposed allowable cyclic stress ratio can solve the problem that the threshold cyclic stress ratio is too conservative while the critical cyclic stress ratio may lead to large settlement when they are used in pavement engineering.4. According to the vibration performance of traffic loading, soil structure of soft clay and drained condition of subgrade, cyclic triaxial tests with constant confining pressure under different influence factors on isotropic consolidated natural Wenzhou soft clay have been conducted. The influence of soil structure, cyclic frequency, drained condition and intermittent cyclic loading on the pore pressure and strain evolution, stress-strain hysteresis loop development, modulus degradation and strain accumulation are investigated. The conclusions provide a lot of data for deeply understanding the cyclic behavior of soft clay under actual engineering conditions.5. Cyclic triaxial tests with the coupling of cyclic confining pressure and cyclic deviator stress have been conducted through GDS dynamic triaxial apparatus to simulate the characteristic of coupling cyclic deviator stress and cyclic horizontal stress in traffic loading. A computational formula for the total pore pressure (elastic pore pressue, residual pore pressure and transmit pore pressure) in VCP tests is obtained based on the pore pressure formula proposed by Yasuhara and Skempton theory. The influence of cyclic confining pressure on the cyclic strain, pore pressure and effective stress path are researched. Results show that the influence of cyclic horizontal stress can not be ignored when the cyclic behavior of saturated soft clay are investigated under traffic loading.6. Comparative experimental researches between cyclic torsional shear tests (CST) and cyclic triaxial tests (CT) have been conducted by GDS hollow cylinder system to simulate the characteristic of variation for both amplitude and direction of major principal stress in subgrade soft clay under traffic loading. The cardioid stress path in the2τzθ-(σz-σθ) plane induced by traffic loading is simulated by user-defined waveforms of cyclic vertical stresses and cyclic torsional stresses. Under a single cycle, the major principal stress angle is changed form-90°to90°. Results show that under the same vertical stress level, the cyclic rotation of principal stress axle in CTS test can accelerate the pore pressure accumulation and the specimen degradation. Furthermore, the generation of resilient and permanent strain is accelerated. With the increase of CSR, the differences between CTS and CT are widened. A permanent strain equation considering principle stress rotation is established by introducing the parameter of η to modify the CSR based on the permanent strain prediction model obtained by the long-term cyclic triaxial tests.更多还原