模拟基坑开挖过程的试验研究

采用三轴剪切试验对基坑开挖过程中土体卸荷的应力路径进行模拟，研究了由于侧向卸荷对土体抗剪强度指标产生的影响，并从土的微观结构角度分析了产生这一现象的原因。试验结果表明，经过侧向卸荷过程土样的抗剪强度指标与常规三轴试验明显不同。     

An oil sand pseudo-elastic model for determining ground deformation under large mobile mining equipment

Strain softening of oil sand under dynamic loading from large mining equipment inhibits the ability of the equipment to function at optimal design performance. This paper looks at the findings of dynamic plate load tests, which effectively mimick the loading and unloading action of a shovel track pad. A pseudo-elastic model was proposed based on the results of the dynamic plate load testing to predict the deformation of oil sand under cyclic loading. Both field and laboratory cyclic plate load tests were performed on oil sand materials. The field tests were performed with different plate sizes, or footprints. The load was normalized based on the pressure stiffness concept in units of pressure per unit deformation. FLAC was used to model the field plate load test deformation with the elastic concept. The laboratory tests were performed at room temperature with more control on the load, loading rate, and cycles than possible during the field testing. Tests were conducted using a circular plate of 14.9 cm diameter, at stress magnitudes of 200, 400, 500 and 600 kPa. The plate load tests were conducted for varying loads, holding, and relaxation times of 0, 2, 5 and 10 min respectively for each magnitude of stress. The outcome of laboratory plate load tests show that after frequent cycles, the pressure stiffness (ratio of stress to deformation) converges on a plateau value of 8 kPa/mm. The proposed approach can be used to evaluate oil sands ground performance, enhancing the prediction process for ground deformation under the operation of ultra-class mining equipment.     

杭州湾浅层储气砂土应力路径试验研究

利用GDS非饱和三轴试验系统开展了杭州湾储气砂土主动压缩、被动压缩、主动伸长与被动伸长等4种应力路径试验研究,以论证应力路径对非饱和砂土的变形性状与强度特性影响。结果表明,在卸荷应力路径被动压缩与被动伸长下非饱和砂土的应力-应变曲线呈明显的应变软化性状,而加荷应力路径主动压缩与主动伸长则分别表现为轻度应变软化及显著应变硬化变形特征;非饱和砂土的强度参数在卸荷与加荷应力路径中也存在差异,前者的有效凝聚力高于后者,而内摩擦角则相反;但其表观凝聚力与基质吸力关系在不同应力路径下均可用乘幂函数 有效描述,而试验参数a、b值不同,公式 可合理地用于表述非饱和砂土的强度特性。     

泥炭质土K0固结不同开挖路径应力-应变关系研究

基坑开挖过程中不同部位的土体会因不同的卸荷力学行为而表现出动态的破坏特性。为研究基坑土体开挖过程中复杂的卸荷应力路径,利用TSZ-1S应力控制式三轴仪分别对湖相沉积的泥炭质土进行固结不排水及K₀固结下的加、卸荷试验,并按侧向、轴向、轴侧向同时卸荷等不同卸荷条件制定试验方案,模拟基坑开挖过程中不同部位土体卸荷路径下的应力-应变曲线、卸荷剪切破坏时的强度及初始切线模量等的变化规律。试验结果表明：土体的应力-应变特性与应力路径密切相关,各路径下应力-应变曲线都近似呈双曲线型;卸荷剪切破坏时强度明显低于加荷破坏。对不同卸荷路径下初始切线模量(E_i)的研究发现,E_i受侧向卸荷影响较大,卸荷后E_i有所提高,轴向卸荷对其影响较小。对各应力-应变曲线进行归一化处理,构建了考虑不同归一化因子的归一化方程,以该方程为基础对不同应力路径下的泥炭质土进行归一化处理,并对结果进行了验证,效果良好。本研究可为泥炭质土场地上基坑在不同卸荷路径下的变形参数和本构关系的研究提供参考。     

花岗岩三轴循环加卸载条件下的气体渗透率

为研究花岗岩在三轴循环加卸载条件下气体渗透率的演化规律,利用岩石多场耦合三轴试验仪分别对花岗岩进行三轴恒下限分级循环加卸载试验和气体渗透试验。试验结果表明：加载曲线与上一次循环卸载曲线形成塑性滞回环,随着循环次数的增加,每个循环应力下限的轴向应变也随之增加;前期几个循环试样体积压缩明显,以轴向压缩变形为主,后期轴向应力达到一定数值时体积由压缩转为扩容,裂纹发展方向偏向于轴向方向,均呈剪切脆性破坏;气体渗透率变化分为稳定下降阶段、缓慢增加阶段、急剧上升阶段3个阶段,试样脆性破坏后气体渗透率均上升2～3个数量级;体积应变曲线拐点与横向应变曲线结合起来可以作为研究岩石渗透率变化规律的一个重要参考因素。     

基坑工程侧向卸、加载应力路径试验及模量计算

分析认为,基坑支护外侧的土体有可能产生侧向卸载和加载两种应力路径,因此进行了侧向卸载、加载的应力路径试验,并根据邓肯-张模型的思路和试验结果,推导了侧向卸载、加载的切线弹性模量公式。通过实例计算发现,用推导的切线弹性模量公式,可以得到很好的计算结果,这说明考虑侧向卸载、加载的应力路径的模量公式能够更好地模拟基坑问题。     

循环与单调加载作用下冻结黄土的变形与损伤特性

针对冻土工程中地基冻土体受力形式复杂、常处于变应力路径、反复加卸载作用问题, 开展了冻结黄土静力条件下的三轴加卸载试验与单调加载对比试验, 研究了两种应力路径下冻结黄土的变形和损伤特性. 通过对比发现, 加载方式对冻结黄土变形特性随围压的变化规律影响不大. 循环加卸载作用下, 冻结黄土的压密变硬增强了其抵抗变形的能力. 卸载阶段, 冻结黄土表现出卸载体胀的弹性现象, 呈现出与融土不同的体变特征. 基于弹性模量的劣化定义了冻结黄土的损伤变量, 并根据试验结果得出了冻结黄土的损伤演化规律可用双曲线函数来描述. 在较低的围压下, 围压增加对冻土损伤的发展有抑制作用; 而当围压足够大时, 由于冰的压碎和压融的出现, 围压增大加剧了损伤的发展.     

K0应力路径试验在上海地区工程勘察中的应用

上海地区在进行软粘性土的变形特性试验时,勘察过程中极少进行对原状土在不同应力路径下的变形及强度试验研究,这使得报告中提供的参数不能很好地和基坑开挖卸荷过程中土体地层的实际相吻合。本文以上海市徐家汇某深大基坑项目为例,在勘察阶段进行K0应力路径三轴试验,试验内容为对选取的代表性土样进行加荷、卸荷及再加荷,并记录各阶段的应力应变及变形模量。对试验结果的分析表明,应力路径对软粘性土的影响较明显,K0应力路径可以较为真实地反映土体在天然状态下进行基坑开挖过程中的变形特性。建议以后在对变形敏感的深大基坑工程中加强推广应用。     

两种卸荷路径下泥岩流变特性试验研究

分析岩石流变特性对边坡长期变形稳定性具有十分重要的意义。以贵州省某高速公路为背景,在施工现场取样,设计了恒轴压卸围压与等比例卸围压和轴压两种应力路径下的分级卸荷流变试验。试验结果表明,两种应力路径下泥岩岩样卸荷流变趋势大体一致,破坏应力下岩样的侧向变形迅速增长并超过了轴向变形,侧向扩容较显著;对比恒轴压卸荷,等比例卸荷历时较长,岩样轴向变形虽出现了回弹,但轴向变形量和侧向变形量所占全变形的比例较大,约为恒轴压卸荷的2倍,说明等比例卸载围压和卸轴压应力路径下岩样破坏可预见性更小,但其破坏影响更大。等比例卸荷实验参照实际边坡开挖坡度选取比例设计实验,对实际边坡工程长期稳定性分析极具参考价值。     

黄土边坡开挖卸荷力学响应与破坏机理研究

开挖黄土边坡诱发的黄土滑坡频繁发生, 对黄土地区的工程建设产生了极大的威胁, 严重阻碍了各种基础设施建设的推进, 认识开挖卸荷过程中黄土边坡的力学响应机制及变形破坏过程, 是对此类型滑坡进行防治的前提条件和重要依据。基于此, 开展饱和黄土常规三轴剪切试验和卸荷路径下的三轴剪切试验(卸荷三轴试验), 着重分析在两种应力路径下饱和黄土的力学响应及变形特征, 并以兰州上洼子滑坡为例对其形成机制进行分析。研究表明:(1)在三轴试验中, 相比于常规条件, 侧向卸荷条件下, 虽然孔压增长幅度较低, 但卸荷导致侧向压力大大降低, 因此孔压比快速增长, 有效应力持续降低, 对边坡的稳定更为不利; (2)侧向卸荷条件下试样达到峰值强度时所需应变更小(1%~2%), 表明卸荷条件下边坡更易失稳破坏且具有突发性; (3)两种应力路径下饱和土体抗剪强度参数有显著差异, 与标准三轴剪切试验相比, 卸载三轴剪切条件下, c'值降低62.32%~76.92%, 且接近于零, 有效内摩擦角φ'值增大26.92%~29.77%;(4)采用FLAC-3D对上洼子滑坡进行数值模拟分析, 选取侧向卸荷三轴剪切试验结果更符合土体真实应力路径, 揭示了上洼子滑坡的破坏模式为“后退渐进式”。     

锦屏二级水电站大理岩复杂加卸载应力路径力学特性研究

地下洞室开挖围岩经历典型径向卸载、环向加载应力路径,由此引起的岩体强度、变形特征和破坏机制也不尽相同。针对锦屏二级水电站高地应力赋存环境,对施工排水洞大理岩开展常规单轴全应变、三轴压缩、卸围压、卸围压-加载轴压等4种不同应力路径力学试验,得到应力-应变全过程曲线、变形破坏特征和极限储能变化规律。试验研究结果表明, （1）锦屏二级水电站大理岩破坏时轴向应变一般较小,为硬脆性材料,卸荷应力路径下该脆性特征更为明显;（2）卸围压同时加载轴压试验峰值强度对应轴向应变、环向应变及体积应变值一般高于单纯的卸围压值,而对应峰值强度则一般低于卸围压值;（3）卸荷速率较大时,变形模量越大,大理岩峰值强度越低。加载速率越大,变形模量越小,峰值强度越高。初始围压越高,变形模量值越低,峰值强度越高;（4）无侧限作用时试件主要为张拉破坏,低侧限作用时为剪切破坏为主,局部存在张拉破坏,较高侧限时,剪切面为典型X或Y型;（5）岩石试件具有极限储能值,该值受多种因素的影响。一般情况下试件破坏对应围压越高,极限储能值越高,卸载速率越大,极限储能值越小。研究结果对于岩爆孕育发生机制解释以及工程实际问题的解决均有参考价值。     

An AI-based model for describing cyclic characteristics of granular materials

Modelling cyclic behaviour of granular soils under both drained and undrained conditions with a good performance is still a challenge. This study presents a new way of modelling the cyclic behaviour of granular materials using deep learning. To capture the continuous cyclic behaviour in time dimension, the long short-term memory (LSTM) neural network is adopted, which is characterised by the prediction of sequential data, meaning that it provides a novel means of predicting the continuous behaviour of soils under various loading paths. Synthetic datasets of cyclic loading under drained and undrained conditions generated by an advanced soil constitutive model are first employed to explore an appropriate framework for the LSTM-based model. Then the LSTM-based model is used to estimate the cyclic behaviour of real sands, ie, the Toyoura sand under the undrained condition and the Fontainebleau sand under both undrained and drained conditions. The estimates are compared with actual experimental results, which indicates that the LSTM-based model can simultaneously simulate the cyclic behaviour of sand under both drained and undrained conditions, ie, (a) the cyclic mobility mechanism, the degradation of effective stress and large deformation under the undrained condition, and (b) shear strain accumulation and densification under the drained condition.     

考虑基坑卸载影响的改进弹性抗力法及其应用

弹性抗力法是目前实际工程中基坑支护结构变形内力计算的一种主要方法,该方法借鉴的是计算水平荷载作用下竖直放置弹性桩的地基反力系数法,但没有考虑开挖卸载对土抗力系数和坑内土体的变形影响,因而导致变形计算值通常小于实测值。首先在开挖面施加反向自重应力模拟基坑的开挖卸荷,利用Mindlin解得到坑内土体的卸荷应力分布;在前人的实验研究基础上,推导了不同卸荷比下考虑卸荷影响的土体水平抗力系数;将支护结构的变形分解为开挖瞬时的卸载作用和基坑内外水土压力差作用2个阶段,考虑卸载应力路径对坑内土体水平抗力系数以及开挖瞬时变形的影响,对传统的弹性抗力法进行了改进,最后将该方法应用于典型三维基坑算例。研究发现,改进法得到的支护结构水平变形大于传统方法,而对内力结果影响不大;基坑平面尺寸越大,两者的变形结果差别越明显,改进法更能反映平面尺寸对基坑变形的影响     

Monotonic and cyclic tests on kaolin: a database for the development,calibration and verification of constitutive models for cohesive soils with focus to cyclic loading

A database with about 60 undrained monotonic and cyclic triaxial tests on kaolin is presented. In the monotonic tests, the influences of consolidation pressure, overconsolidation ratio, displacement rate and sample cutting direction have been studied. In the cyclic tests, the stress amplitude, the initial stress ratio and the control (stress vs. strain cycles) have been additionally varied. Isotropic consolidation leads to a failure due to large strain amplitudes with eight-shaped effective stress paths in the final phase of the cyclic tests, while a failure due to an excessive accumulation of axial strain and lens-shaped effective stress paths was observed in the case of anisotropic consolidation with \(q^{\text{ ampl }}< |q^{\text{ av }}|\). The rate of pore pressure accumulation grew with increasing amplitude and void ratio (i.e. decreasing consolidation pressure and overconsolidation ratio). The “cyclic flow rule” well known for sand has been confirmed also for kaolin: With increasing value of the average stress ratio \(|\eta ^{\text{ av }}| = |q^{\text{ av }}|/p^{\text{ av }}, \) the accumulation of deviatoric strain becomes predominant over the accumulation of pore water pressure. The tests on the samples cut out either horizontally or vertically revealed a significant effect of anisotropy. In the cyclic tests, the two kinds of samples exhibited an opposite inclination of the effective stress path. Furthermore, the horizontal samples showed a higher stiffness and could sustain a much larger number of cycles to failure. All data of the present study are available from the homepage of the first author. They may serve for the examination, calibration or improvement in constitutive models dedicated to cohesive soils under cyclic loading, or for the development of new models.     

侧向卸荷条件下冻结砂岩变形特性

新庄煤矿立井采用冻结法施工技术,在井筒开挖的过程中,由于侧向卸荷作用导致围岩产生卸荷变形。从新庄煤矿立井现场采集白垩系中粒砂岩,对加工后的岩样进行饱水处理,然后利用GCTS电液伺服控制高低温高压岩石三轴测试系统进行冻结（-10 ℃）条件下的恒轴压、卸围压三轴试验,模拟在井筒开挖过程中围岩的应力变化路径,探索冻结砂岩的变形特性。研究表明：侧向卸荷条件下冻结砂岩表现出弹-脆性特征,轴向表现为压缩变形,径向表现为膨胀变形,径向变形量约为轴向变形量的2倍;当卸荷速率一定时,岩样的卸荷变形随初始围压的增大而增大,尤其是径向变形最为显著,这可能与卸荷回弹变形及岩样内部聚集的能量大小有关;围压卸荷到同一应力水平时,高卸荷速率下岩样的卸荷变形量较小,而变形速率较大;卸荷作用导致岩样变形模量减小,横向应变与纵向应变之比增大,卸荷速率越小,初始围压越大,应变之比变化越大。     

考虑土体硬化的基坑开挖性状及隆起稳定性分析

基坑开挖过程中,土体应力路径、卸载回弹再压缩特性与简单加载或卸载不同,采用常规的理想弹塑性模型模拟基坑开挖,得到的围护墙位移、坑内土体回弹以及坑外沉降较大。分析了基坑开挖不同区域土体的性状,采用土体硬化模型模拟基坑开挖的卸载与土体硬化行为,结合工程算例,对比土体硬化模型和理想弹塑性模拟以及实测的围护结构土压力、围护墙水平位移和坑外土体沉降,并利用强度折减法分析基坑的稳定性。计算结果表明,考虑土体硬化的HS模型有限元方法能体现土体卸载再加载与开挖的特性,所得土压力、围护结构水平位移以及基坑抗隆起稳定性符合软土地区基坑工程的实践。     

砂岩分级加卸载蠕变试验过程能量演化分析

对陕西柠条塔砂岩样品进行单轴分级加卸载蠕变试验，分析样品变形破坏过程中的能量演化规律。结果表明：随循环级数增加，各级耗散能非线性增长，各级塑性应变能相对稳定；耗散能超过塑性应变能可作为样品破坏的先兆能量特征。通过定义相关系数，建立能量与变形之间的关系后发现：随循环级数增加，各级塑性应变能与各级新增塑性应变呈正相关性，各级耗散能与塑性应变累积量呈正相关性。分别对加载、蠕变、卸载及恢复等4个阶段的能量计算后得出：各阶段能量演化速率均随循环级数的增加而增大。加、卸载速率恒定时，加载段能量变化率大于同级卸载段能量变化率，蠕变段能量变化率大于同级恢复段能量变化率。通过分析各级循环加载段能量变化规律，提出能量衰减系数 ，该值随应力水平增长呈幂函数形式降低，以此提出一种可以有效预测破坏应力的方法。     

峰前卸荷对节理岩体力学特性影响试验研究

实际工程中,岩体的应力变化状态非常复杂,并不只是简单的加载或者卸荷,如水利工程中的大坝,当坝基开挖时其力学条件主要表现为开挖方向的卸荷(其他方向有可能卸荷,也有可能加载),而坝体修筑以后,力学条件又主要表现为加载。因此,对卸荷损伤后的岩体再次加载时的力学性质进行研究具有十分重要的工程意义。通过三轴卸荷及卸荷损伤后加载破坏试验,研究了不同围压下,含断续预制节理岩体经过不同程度卸荷损伤后再次加载时的力学特性。研究结果表明:围压对于含断续节理岩体变形模量影响不明显,岩体弹性模量与围压没有明显关系,相同围压下节理岩体弹性模量和变形模量均显著低于完整岩体:节理岩样峰值应变随着围压的增大线性增大,二者之间有较好的相关性,相比于完整岩样,节理岩体峰值应变随围压增大增长更快:围压越低,卸荷对岩体强度的影响越大,随着围压增加,卸荷量不同造成岩样的损伤差异逐渐减小。     

粉质黏土卸荷变形特性试验研究

依托武汉长江隧道江北明挖段基坑工程,开展了一系列的K0固结状态下原状粉质黏土卸荷应力路径排水剪三轴试验,并对卸荷变形特性进行了深入分析。分析表明：卸荷应力路径下原状粉质黏土的应力-应变关系与应力路径有着密切的关系,近似呈双曲线型。根据应力-应变曲线表现,可将卸荷应力路径分为A、B两组,A组应力路径试样表现为轴向压缩;B组应力路径试样表现为轴向伸长。卸荷应力路径下粉质黏土的应力-应变关系具有良好的归一化特性,同一应力路径下不同固结压力的应力-应变曲线可采用双曲线函数用平均固结压力归一化,但A、B两组应力路径下的归一化方程有所差别。给出了各种应力路径下的归一化方程,可为卸荷土体的变形参数及本构关系研究提供参考。     

A multi‐mechanism constitutive model for plastic adaption under cyclic loading

As is well known, granular soils under cyclic loading dissipate a large amount of energy and accumulate large irreversible strains. Usually, with time, this second effect reduces and the accumulation rate decreases with the number of cycles until obtaining a sort of ideal stationary cyclic state at which ratcheting disappears. In this paper, only this ideal state is taken into consideration and simulated by means of a multi‐mechanism constitutive model for plastic adaptation. For this purpose, the concept of cycle is discussed, many different categories of cyclic stress/strain paths are considered and some theoretical issues concerning both the flow and the strain‐hardening rules are tackled. Even though the paper focuses on soil behaviour, the conclusions can be extended to all materials exhibiting ratcheting due to volumetric behaviour.Copyright © 2013 John Wiley & Sons, Ltd.