【作者】 周秋娟；

【导师】 陈晓平；

【作者基本信息】 暨南大学 ， 工程力学， 2009， 博士

【摘要】 随着我国大中城市的发展,城市能够利用的土地资源迅速减少,迫使城市开始往三维空间发展,城市地下空间的开挖利用越来越受到重视,城市轨道交通及城市地下空间综合开发利用项目在国内各大城市开展。而我国大中城市主要分布在沿海和河流中下游及湖泊附近地区,主要地基基础为软土,因此,开展各种开挖条件下软土力学特性研究对城市的地下空间开发利用具有十分重要的意义。其次,基坑工程作为地下空间开发的一个重要手段,对软弱地层中基坑工程的稳定性深入开展研究迫在眉睫,因此,进行软土基坑试验研究以及数值分析,探讨基坑工程的变形、安全、稳定,同样具有十分重要的理论和实际意义。鉴于此,本文定义土体只要有一个方向的应力减小就为卸荷,针对珠三角州相沉积的典型灰黑色淤泥质软土及软土开挖工程进行了系统研究,主要研究内容与成果如下:1.为分析开挖工程中土体的侧向卸荷行为,对原状土样进行了侧向卸荷条件下的三轴剪切试验,结果表明:(1)侧向卸荷下土体的强度明显小于轴向加荷下的土体强度,即土体在侧向约束减小后所能承受的剪应力将降低;(2)在侧向卸荷时由于土体产生负孔隙水压力,使得有效应力强度反而低于总应力强度,与轴向加荷条件下的强度规律相反;(3)侧向卸荷条件下软土在p-q平面上的屈服条件满足Drucker-Prager模型的屈服条件;(4)建立侧向卸荷条件下的卸荷变形模量计算公式。2.为分析开挖工程中施工过程或施工间歇对基坑稳定的影响,进行了不同卸荷路径下三轴卸荷流变试验,结果表明:(1)若土体为侧向卸荷而轴向荷载不变,此时土体的卸荷流变变形表现为压缩变形;即使在偏应力较小时,土体的总变形较小,但蠕变变形仍不可忽略;随着偏应力的进一步增大,应变随着偏应力的施加迅速发展直至土样产生鼓型破坏;(2)若土体为轴向卸荷或轴向侧向同时卸荷,则土体在仅发生轴向卸荷时产生明显的滞后回弹变形,且随平均固结压力的增大愈显著;而土体在轴向与侧向均卸荷路径下,土体产生剪切变形,无明显的流变特征,但在该路径下土体出现了负孔压,且卸荷压力越大,负孔压极值越大;(3)不论是何种卸荷路径,应变率与时间的双对数关系均为线性关系,因此,可建立软土卸荷流变模型为(?)=bt~m,其中b与m为流变参数,与卸荷路径、应力水平相关。3.为分析开挖工程中土体的回弹行为,进行了不同卸荷路径、不同预压荷载条件下的一维压缩回弹试验,结果表明:(1)含水量、卸荷路径下对软土回弹模量的影响不大,而卸荷比、预压荷载越大,土体回弹将越明显;若卸荷比一定,则回弹变形与预压荷载成反比;(2)孔隙比、预压荷载与荷载之间存在相应的经验关系;(3)土体的回弹变形存在临界卸荷比R=0.8,当R＜0.8,土体的回弹率很小;(4)次回弹系数随卸荷比增大而增大;(5)土体的回弹模量具有时效性,随卸荷比、时间、预压荷载而不断变化,回弹模量随着时间的增长而迅速减小,约在10min处达到稳定值。4.为分析开挖工程中土体的渗透特性,进行了不同卸荷条件下的渗透试验,结果表明:(1)土体在不同初始固结压力下的逐级卸荷时,渗透系数将随卸荷比不同而随之变化,当卸荷比R＜0.8时,渗透系数变化不大,当R＞0.8时,渗透系数开始随着土体的卸荷比增大而迅速增大;(2)卸荷路径对饱和软粘土的渗透特性基本没影响,但渗透系数受初始固结压力影响较大,初始固结压力越大,卸至同一级固结压力下的渗透系数反而越小。5.为分析开挖工程的破坏特征,针对一软弱基坑进行了模拟开挖及支护全过程的离心模型试验,试验中分别考虑了基坑的不同支护方式以及软土的强度等影响因素,获得了各种条件下的变形和破坏的最直观描述、以及深厚软土层中基坑工程稳定的基本条件。试验表明:(1)在有挡土墙无支撑支护情况下,随着基坑开挖深度的加大,基坑将由于挡土墙与坑侧土体变形过大而发生倾覆破坏,此时,坑侧土体大面积开裂,严重影响基坑的周边环境;挡土墙向坑内倾斜与坑侧土体发生侧向卸荷是坑侧土体发生沉降的主要原因;基坑开挖而引起的土体卸荷回弹变形是造成坑底土体隆起的主要原因;(2)在有挡土墙有支撑支护情况下,软土的强度对基坑的稳定性影响很大;随着基坑开挖深度的加大,基坑将由于坑底土体隆起变形过大、挡土墙向坑外倾斜、第1排支撑失去作用而使得基坑整个支护结构失稳破坏;造成基坑坑侧土体沉降的主要原因是挡土墙底部土体向坑内流动,同时这也是坑底土体产生隆起变形的主要原因之一;土体产生卸荷回弹变形是坑底土体发生隆起变形的另一主要原因。6.为了分析基坑开挖卸荷造成的周边土体应力偏转问题,对应力偏转造成的变形、强度问题进行探讨:(1)对开挖卸荷后基坑周边土体应力场进行理论分析,通过引入应力矢量比概念,建立考虑主应力轴偏转的应力矢量本构关系;(2)通过对不同大主应力方位角下软粘土剪切试验可知,大主应力发生偏转后,土体的应力-应变仍为双曲线型,可进行归一化处理;土体的强度将降低,且变形模量减小,表明在基坑工程中若不考虑应力的偏转,将降低工程的安全性;(3)建立土体初始变形模量随大主应力方位角变化的折减公式。7.基于前述室内试验结果和土工离心模型试验结果,对土工离心模型试验的原型基坑进行数值计算,结果表明:(1)不考虑应力偏转对土体变形与强度的影响下,计算所得到的变形规律与实测变形规律有相似性,但变形较小;在数值计算中,最大回弹位移发生在整个基坑的中部,而离心试验中,坑底最大回弹位移发生在所进行试验的坑底的中部;坑侧土体的流变效应明显于坑底土体;软土基坑开挖卸荷后,基坑周边土体将发生不同程度的应力偏转;(2)为考虑应力偏转对土体变形的影响,对每一计算步后的变形模量进行折减,结果表明,考虑应力偏转效应后,基坑周边土体变形明显增大,且流变效应相对明显。更多还原

【Abstract】 Available land in cities decreases rapidly with the development of cities,which forces the cities to develop toward three-dimensional spaces.The development of underground space becomes more and more important.The exploiture of railway traffic and underground space of big cities have been carried out.The big cities of our nation mainly distributed at coastland, middle or downward position of rivers and lake land,where soft soil is the main foundation subsoil.Therefore,the research of soft soil unloading behavior has a significant meaning to the exploiture of subterranean space in big cities.Besides,foundation ditch project is a key tool in underground space exploiture,and further research to the stability of foundation ditch project in weak underground becomes urgent.Therefore,it is important both in theoretical and practical meaning that we take experiments and numerical analysis to soft soil,discuss the deformation, safety and stability of foundation ditch project.This thesis carries out a systematic study on the typical grayer silt-like soft soil at triangle area and soft soil excavation.The main results are listed as follows:1.In order to analyze the lateral unloading behavior of soil excavating,triaxial-shear tests were taken to the intact sample.The results indicated that:(1) Soil that unloading a lateral load obviously had a lower strength than that undertaken a axial load,that is to say,the shear stress of soil decreased with the decrease of lateral restriction;(2) The effective stress strength was lower than the total stress strength because minus pore water pressure were produced under lateral unloading,which was opposite to the stress law under axial-loading condition;(3) Under the condition of lateral unloading,yielding condition of soil at p-q plane meet the yielding condition of Drucker-Prager model;(4) Calculation expressions of unloading deformation modulus under lateral unloading was established.2.In order to analyze the influence of construction process or intermission to the stability of foundation,triaxial unloading rheological tests under different unloading paths were carried out. The results indicated that:(1) The unloading rheological deformation of the soil was shown to be compress deformation if the soil undergone a lateral unloading while the axial loading was kept as a constant;The total deformation of the soil was slight under a small deviator stress,but the creep could not be ignored;The deformation developed rapidly when further increase the deviator stress,and drum-like destroy occurred;(2) Obvious hysteresis rebound deformation was observed,which increased obviously with the increase of average consolidation pressure;Shear deformation of the soil was observed if the soil was undergone both axial and lateral unloading; Without obvious rheological characters,minus pore pressure appeared which increased with the increase of unloading pressure;(3) There was a linear relationship between deformation ratio and logarithm of time whatever the unloading path was.A rheology model to the unloading of soft soil was built to be(?)=bt~m,where b and m are rheological parameters in relationship with unloading path and stress.3.In order to analyze the rebound behavior in excavating project,tests under different unloading paths and various pre-loading conditions were carried out.The results indicated that: (1) Water content and unloading path had little influence to the rebound modulus of soft soil; Rebound was obvious with the increase of unloading ratio and pre-loading;(2) Experiential relation was found between void ratio,pre-loading and loading;(3) There was a critical unloading ratio R=0.8 in soil rebound deformation.When R＜0.8,the soil rebound ratio was low; (4) Secondary rebound coefficient increased with the increase of unloading ratio;(5) The soil rebound modulus was time dependent which varied with unloading ratio,time and pre-loading; Rebound modulus decreased with the increase of time,reaching a stable value at about 10 min.4.In order to analyze the permeability character of soil in excavating project,permeability test under various unloading conditions were carried out.The results indicated that:(1)When soil was unloaded at different pre-solidified pressure,permeability coefficient varied with the change of unloading ratio;When unloading ratio R＜0.8,permeability coefficient varied slightly;when R＞0.8,permeability coefficient increased rapidly with the increase of unloading ratio;(2) The unloading path had little influence to the permeability character of saturated soft clay.The permeability character was greatly influenced by initial consolidation pressure.The greater the pre-solidified pressure,the smaller the permeability coefficient under the same solidified pressure.5.In order to analyze the destroy character in excavating project,centrifugal model tests in the whole simulative excavation and support process were carried out.Factors such as support mode and soft soil strength were considered.Intuitionistic description of deformation and destruction under various conditions and basic conditions in the stability of foundation project in deep soft soil clay were obtained.The results indicated that:(1) With the increased depth of excavation,retaining wall and lateral soils deformation of the foundation ditch were too much, so the foundation ditch had overturn failure;At this time,and also cause a huge crack of the soil around the excavation,which would influence the surroundings of the foundation ditch;The retaining wall inclined toward the foundation ditch and soil around the foundation ditch had a lateral unloading were the main reasons for the settlement occurs on lateral soils;The basal heave deformation of soil under the foundation ditch were main caused by the soil unloading rebound deformation;(2) In the retaining wall supporting with bracing,strength of soft soil on the foundation ditch of a great influence on the stability;With the increased depth of excavation, basal heave had excessive deformation,retaining wall to the excavation outside the tilt,No.1 platoon support lost its effect,so the foundation ditch was damage caused by the entire supporting structure failure;the main factor that caused the settlement of the foundation ditch lateral soils was the flow of soil under the retaining wall toward the excavation,which was also one of the reasons that cause the basal heave deformation under the excavation;Another reason which caused the basal heave deformation under the excavation was the unloading rebound deformation of the soil after the excavating of the soil above the excavation.6.The deformation caused by stress deflection and strength problems were discussed in order to analyze the deflection of surrounding soil caused by foundation excavating unloading: (1) Theoretical analysis was carried out to the surrounding soil after foundation excavating unloading;Through the introduction of stress vector ratio,stress vector constitutive relationship was built considering primary stress axial deflection;(2) Through the shear tests of soft soil under different primary stress azimuth angles,we could get the conclusion that after the primary stress deflection,the relationship between stress and strain was hyperbola,which was unitary; The strength and the deformation modulus of the soil decreased,which indicated that the security of the project was decreased if the stress deflection of the foundation project was neglected;(3) The reduction formula on the relation between soil initial deformation modulus and the primary stress azimuth angles was built.7.Numeric simulation was carried out to archetypal foundation in centrifugal model tests based to the test results and centrifugal model results obtain above.The results indicated that:(1) Without the influence of stress deflection to soil deformation and strength,the deformation law calculated was similar to the results measured,but the deformation was small;The biggest displacement happen in the middle of the whole foundation in numeric simulation,while in centrifugal tests the biggest displacement happen in the middle of the foundation measured; Rheology effect of the soil on the side of the foundation was more obvious than the soil under the foundation;The surrounding soil of the foundation had various deformation deflection after foundation excavating unloading;(2) The reduction of deformation modulus was carried out aider every calculation in order to consider the influence of stress deflection to soil deformation; The results indicated that when the stress deflection effect was considered,the soil deformation around foundation increased greatly and rheology effect became obvious.更多还原