Initial excess pore water pressures induced by tunnelling in soft ground

Tunnelling-induced long-term consolidation settlement attracts a great interest of engineering practice. The distribution and magnitude of tunnelling-induced initial excess pore water pressure have significant effects on the long-term consolidation settlement. A simple and reliable method for predicting the tunnel-induced initial excess pore water pressure calculation in soft clay is proposed. This method is based on the theory of elasticity and SKEMPTON's excess pore water pressure theory. Compared with the previously published field measurements and the finite-element modelling results, it is found that the suggested initial excess pore water pressure theory is in a good agreement with the measurements and the FE results. A series of parametric analyses are also carried out to investigate the influences of different factors on the distribution and magnitude of the initial excess pore water pressure in soft ground.

Analytical solution of vacuum preloading technology combined with electroosmosis coupling considering impacts of distribution of soil’s electrical potential

Combining vacuum preloading technology and electroosmosis can improve the treatment effect of soft soil foundation by utilizing the advantages of both methods.Many studies indicate that the soil electrical potential is non-linearly distributed in the treatment process by the combined method.However,in the previous theoretical study,the non-linear-distribution impacts of soil’s electrical potential on soft soil foundation treatment have not been considered.It is always assumed to be linear distribution,which is different from the experimental results.In this paper,the coupling consolidation model of this technology under the two-dimensional plane strain condition is initially established;and the well resistance effect,the vacuum load decreasing along the soil depth and the non-linear variation of electrical potential in the soil are considered.Then,the analytical solutions of the average excess pore water pressure and soil’s consolidation degree in the anode affected area are acquired based on the soil’s electrical potential distribution.Finally,the rationality of the analytical solution is testified by conducting an experimental model test,which proves the scientificity of the analytical solution.The analytical solution is adopted to better predict the dissipation of excess pore water pressure and soil consolidation degree when using the combined technology.This study can provide a reference with more accuracy for the engineering practices of this combined technology in the future.

典型平推式滑坡水力分布规律及启动机理研究

平推式滑坡形成于近水平岩层中,地下水对平推式滑坡的启动和运动过程影响非常显著。为揭示平推式滑坡滑动面上水力分布与滑坡启动的相关性,以成都市狮子山滑坡为地质原型,自主研制试验设备,进行水力测试试验,测试近水平倾角(0°~10°)条件下,底滑面两种不同连通条件下不同位置的水压力,研究底滑面承压水压力分布规律及启动机理。结果表明:①底滑面渗流条件较差或滑坡前缘堵塞条件下,水压力分布呈现梯形分布;流通条件下,水压力在底滑面方向上呈曲线下降,自后缘至剪出口下降速率先增大后减小,底滑面倾角越增大,下降速率变化越显著。②层间裂隙水扬压力值比传统采用三角分布形式的扬压力值小约27.6%。③薄层状泥岩的软化、变形是滑坡启动的根本原因;暴雨情况下底滑面扬压力和后缘裂隙中高水压下的水平推力联合作用将滑体推出是滑坡启动的直接原因。研究结果对平推式滑坡的稳定性分析评价和治理方案确定具有一定参考和借鉴价值。

考虑初始孔隙率的非饱和土土 -水特征曲线模型

为了构建考虑初始孔隙率的非饱和土土-水特征曲线模型,从土体的孔隙分布规律出发,通过对参考状态下孔隙尺寸分布曲线的平移缩放得到任意荷载状态下的土体孔隙尺寸分布曲线,并结合Yong-Laplace方程与VG模型推导得到了可考虑初始孔隙率的土水特征曲线模型。该模型仅包含5个参数且其物理意义明确,且仅需通过参考状态的土水特征曲线确定模型参数,即可用得到的参数预测任意初始孔隙比的土水特征曲线。通过5组试验数据对所提出的模型进行验证,结果表明该模型可较好地模拟不同初始孔隙率下的非饱和土持水特征。

CONSOLIDATION THEORY OF UNSATURATED SOIL BASED ON THE THEORY OF MIXTURE(Ⅱ)

The present paper uses the mathematics model for consolidation of unsaturated soil developed in ref.[I]to solve boundary value problems.The analytical solutions for one-dimensional consolidation problem are gained by making use of Laplace transform and finite Fourier transform.The displacement and the pore water pressure as well as the pore gas pressure are found from governing equations simultaneously.The theoretical formulae of coefficient and degree of consolidation are also given in the paper.With the help of the method of Galerkin Weighted Residuals,the finite element equations for two-dimensional consolidation problem are derived.A FORTRAN program named CSU8 using 8-node isoparameter element is designed.A plane strain consolidation problem is solved using the program,and some distinguishing features on consolidation of unsaturated soil and certain peculiarities on numerical analysis are revealed.These achievements make it convenient to apply the theory proposed by the author in engineering practice.

Understanding the microscopic moisture migration in pore space using DEM simulation

The deformation of soil skeleton and migration of pore fluid are the major factors relevant to the triggeringof and damages by liquefaction. The influence of pore fluid migration during earthquake has beendemonstrated from recent model experiments and field case studies. Most of the current liquefactionassessment models are based on testing of isotropic liquefiable materials. However the recent NewZealand earthquake shows much severer damages than those predicted by existing models. A fundamentalcause has been contributed to the embedded layers of low permeability silts. The existence ofthese silt layers inhibits water migration under seismic loads, which accelerated liquefaction and causeda much larger settlement than that predicted by existing theories. This study intends to understand theprocess of moisture migration in the pore space of sand using discrete element method （DEM） simulation.Simulations were conducted on consolidated undrained triaxial testing of sand where a cylindersample of sand was built and subjected to a constant confining pressure and axial loading. The porositydistribution was monitored during the axial loading process. The spatial distribution of porosity changewas determined, which had a direct relationship with the distribution of excess pore water pressure. Thenon-uniform distribution of excess pore water pressure causes moisture migration. From this, themigration of pore water during the loading process can be estimated. The results of DEM simulationshow a few important observations： （1） External forces are mainly carried and transmitted by the particlechains of the soil sample; （2） Porosity distribution during loading is not uniform due to nonhomogeneoussoil fabric （i.e. the initial particle arrangement and existence of particle chains）; （3）Excess pore water pressure develops differently at different loading stages. At the early stage of loading,zones with a high initial porosity feature higher porosity changes under the influence of external loading,which leads to a larger pore pressure variation （increase or decrease） in such zones. As the axial strainincreases, particle rearrangement occurs and final porosity distribution has minor relationship with theinitial condition, and the pore pressure distribution becomes irregular. The differences in the porepressure development imply that water will migrate in the pore space in order to balance the pore waterpressure distribution. The results of this simulation offer an insight on the microscale water migration inthe soil pore space, which is important for holistic description of the triggering of soil liquefaction in lightof its microstructure. 2015 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting byElsevier B.V. All rights reserved.

考虑温度和初始孔压非均布的竖井地基固结解

针对目前竖井地基热固结理论的不足,建立了基于轴对称下竖井地基固结计算模型,采用解析解法推导了考虑温度和初始孔压非均布影响的竖井地基固结一般解;给出了初始孔压呈梯形分布、矩形分布、正三角形分布和倒三角形分布下的超静孔压和固结度的计算表达式。根据所得解析解,编制计算程序,通过参数敏感性分析对竖井地基的固结特性进行研究。结果表明:在温度不变时,初始孔压的分布形式对固结特性有较大影响;初始孔压呈矩形和正三角形分布时,温度越高,超静孔压的消散速率越快;竖井地基底部的孔压值在固结过程中总是最大的,当初始孔压呈倒三角和梯形分布时,随着温度的升高,地基底部孔压值呈现由小变大的特点。

复杂初始应力状态下松砂多向循环单剪特性

松砂极易液化,微小的应力状态变化也会影响其液化特性。基于多向循环单剪试验,采用松砂作为试验材料,开展不同静剪应力大小、方向和复杂剪切路径下的循环单剪试验,研究复杂初始应力状态下松砂循环单剪特性,得到以下主要结论:(1)随静剪应力比增大,试样剪应力峰值增大,第1个循环内孔隙水压力增量变大,试样更易液化;初始静剪应力大小对孔隙水压力的影响在剪切初期更显著。(2)随初始静剪应力和动剪应力主轴之间夹角增大,试样在X方向的剪应力峰值减小,试样孔隙水压力加速增长,在第1个循环以及最后1个循环内孔隙水压力增量变大,循环间差值增大,试样更易发生瞬时液化。(3)8字形剪切路径试样的应力-应变滞回圈面积最大,每个循环消耗能量最多,圆形剪切路径次之,直线剪切路径最小。复杂剪切路径会在剪切开始时诱发孔隙水压力的突然增加,加大各循环中孔隙水压力的增量,试样更易液化。(4)影响松砂液化因素的排序为:初始静剪应力与动剪应力夹角、剪切路径、初始静剪应力大小。

复杂荷载作用下饱和钙质砂孔压特性及损耗能试验研究

钙质砂是一种由海洋生物残骸沉积而成的特殊岩土体,为我国南海岛礁主要的吹填材料,研究其动力特性意义重大。通过一系列不排水循环剪切三轴试验,分析饱和钙质砂孔压特性发展规律;引入能量法,建立钙质砂孔压和损耗能的内在联系。结果表明,饱和钙质砂的极限残余孔压比随固结应力比呈先增大后减小的变化趋势,循环加载过程所累积的损耗能与试样初始静剪应力比及相对密实度有关,受循环应力比影响极小,可通过构建的能量模型较好地预测不同试验条件下饱和钙质砂的损耗能。研究成果能为钙质砂地基的稳定性分析提供理论依据和技术支撑。

基于核磁共振技术探究细粒土中孔隙水的分布

细粒土中孔隙水分布特征影响其物理力学性质,厘清细粒土的孔隙水分布特征及其影响因素对深刻认识细粒土的力学特性尤为重要。为探究细粒土颗粒孔隙水水分布机制,利用核磁共振技术进行了一系列试验,研究了细粒土颗粒物理化学性质和初始含水率对孔隙水分布的影响。随着含水率的增加核磁共振分布曲线向右侧移动,而高岭土表现得最为明显;对比不同细粒土饱和后发现,高岭土的弛豫时间要大于膨胀土和红黏土,饱和后高岭土的孔径较大。

Influence of changes in extreme daily rainfall distribution on the stability of residual soil slopes

Many landslides triggered by intense rainfall have occurred in moun-tainous areas in Thailand,causing major economic losses and infra-structure damage.Extreme daily rainfall is a significant trigger for hillslope instability.Increases in extreme daily rainfall intensity due to climate change may be one of the key factors responsible for the increased landslides.Thus,in this context,changes in the intensity of extreme daily rainfall in Chiang Mai Province in North Thailand and their effects on hillslope stability are analyzed.Extreme rainfall is modeled using a generalized extreme value distribution and esti-mated for various return periods.A numerical analysis of seepage and an infinite slope stability model are combined to understand the hillslope response under extreme rainfall conditions.The analysis period is divided into two periods of 34 years:1952 to 1985 and 1986 to 2019.According to the analysis results,the distribution of extreme daily rainfall changes in terms of location.The average annual daily maximum rainfall increased by approximately 11.13%.The maximum decrease in the safety factor is approximately 4.5%;therefore,these changes in extreme daily rainfall should be consid-ered in future landslide prevention policies.

基于BP神经网络的涤纶水刺非织造布孔径及其分布预测

以涤纶短纤维为原料,以水刺压力和水针作用距离的不同组合制备30种涤纶水刺非织造布,采用泡点法测取其孔径,探讨制备工艺参数对涤纶水刺非织造布的孔径及其分布的影响规律;以水刺压力和水针作用距离为输入,建立基于BP神经网络的模型对涤纶水刺非织造布的孔径及孔径变异系数进行预测。结果表明:随着水刺压力的增大,孔径及其变异系数均呈减小趋势;随着水针作用距离的增大,孔径及其变异系数均呈增大趋势;BP神经网络经过2000次迭代运算后,其性能函数值最小误差达0.010563,测试样本的网络输出值与网络目标值的相关系数达到0.99478;基于BP神经网络的预测,涤纶水刺非织造布的孔径预测值与实测值之间的绝对百分比误差的平均值仅为2.78%,孔径变异系数预测值与实测值之间的绝对百分比误差的平均值仅为3.14%;BP神经网络模型的预测准确度非常高,可以用于涤纶水刺非织造布孔径及其分布的预测。

盾构隧道施工引起的土体初始超孔隙水压力分布研究

盾构施工会对周围土体产生扰动,形成超孔隙水压力,引起工后固结沉降。运用应力释放理论推导与衬砌相邻的土体初始超孔隙水压力计算公式。假定扰动范围边界呈圆弧状,确定初始超孔隙水压力的分布范围;同时运用应力传递理论,推导分布范围内任一点土体的初始超孔隙水压力计算公式。通过对实测资料的分析可知,计算值与实测值吻合较好。算例分析表明,与衬砌相邻的土体初始超孔隙水压力呈近似圆形(顶部小、底部大);随着到衬砌的径向距离增加,土体初始超孔隙水压力呈凹曲线形状;隧道底部的等值线最密,即变化最快;隧道顶部上方土体、不同深度处土体初始超孔隙水压力,以隧道轴线处为最大,呈现类似Peck曲线形状。

初始孔压非均布考虑起始比降的一维固结解

研究了考虑起始比降的一维固结问题,获得了初始孔压非均布条件下考虑起始比降的一维固结解答,并与已有文献解答进行了对比,讨论了考虑起始比降后土体的一维固结特性。计算分析表明,当初始孔压降小于起始比降时,渗流并不会在整个土层中瞬间发生,而是逐渐向下发展的,起始比降越大,渗流边界移动速度越慢;固结结束时,土体中的孔隙水压力并不会完全消散,且起始比降越大,残余孔隙水压力越大。与不考虑起始比降不同,均质地基平均固结度按沉降定义和按孔压定义是不同的,随着起始比降的增大,按沉降定义的固结度随之增大,而按孔压定义的固结度则随之减小。

盾构施工引起土体超孔隙水压力峰值的计算及影响因素分析

运用应力释放理论及应力传递理论,推导了盾构施工引起周边土体任一点的超孔隙水压力峰值的计算公式。并通过算例分析表明:与衬砌相邻的土体超孔隙水压力峰值呈近似圆形(底部大于顶部);随着离隧道中心距离的增加,土体超孔隙水压力峰值呈凹曲线衰减。同时发现盾构直径及埋深对土体超孔隙水压力峰值的影响是相反的。当直径减小或是埋深增大,均会使得与衬砌相邻周边土体超孔隙水压力峰值的底部与隧道中心水平线处的差异更加明显,反之亦然。隧道底部的等值线最密,即变化最快;隧道上方区域的等值线间距逐渐变大,即变化变缓。在一定深度处,超孔隙水压力峰值在隧道轴线上方为最大,远离隧道轴线则减小;随深度增大,其最大值有增大趋势。

不同颗粒级配条件下堰塞坝溃决特征试验研究

为研究坝体颗粒级配对堰塞坝溃决特征的影响,引入平均粒径D50作为主要指标,以不同均值粒径的土体作为研究对象,通过开展不同颗粒级配条件下堰塞坝溃决特征试验,分析堰塞坝体在3种不同工况下的破坏模式和溃决特征。结果表明,1工况3条件下坝体溃决历时是工况1的2.5倍,而溃决洪峰流量减小了约60%,说明坝体平均粒径对坝体的溃决特征有显著影响,平均粒径越大,坝体整体抗冲刷能力越强,溃口发展速度越慢,坝体越稳定,溃决洪峰流量就越小,达到洪峰流量的时间也相对滞后;2平均粒径和渗透系数可拟合成二次函数曲线,随着平均粒径的增大,渗透系数也随着增大;以孔隙水压力的变化特征为依据,把溃决过程分为3个阶段,即上游库水位增加阶段、水流漫顶下渗阶段、坝体溃决阶段。

恒定渗流作用下泥石流起动过程冲刷试验分析

渗流是泥石流水动力条件主要来源之一,不同渗流流量具有不同的渗流力和冲刷力,从而可引起不同规模泥石流。通过开展室内水槽试验,利用测压管量测渗流过程中的孔隙水压力,并结合高清摄像技术从微观角度记录堆积土体内部细颗粒的运移、骨架颗粒的坍塌现象,以此分析研究土体渗透破坏、起动形成泥石流过程中的渗流和冲刷作用。在此基础上设定水槽坡度为7°,调节恒定渗流流量分别为120、170、265、320 ml/s,分析不同恒定渗流流量对固体堆积物失稳、泥石流起动过程中流态变化的影响。分析结果表明,在恒定渗流流量作用下,堆积土体内部细颗粒迁移、骨架颗粒坍塌造成土体颗粒重排列、孔隙水压力上升进而导致土体抗力降低是泥石流土体颗粒失稳、起动、冲刷的重要原因;随着渗流流量增加,流速迅速上升,土体内孔隙水压力逐步增大,骨架颗粒的失稳、移动主要受渗流及水流冲刷两方面共同作用,堆积土体颗粒的移动分别表现出缓慢小幅滑动后稳定、过渡型滑动和快速流滑现象。

初始孔压非均布条件下涂抹区渗透系数变化的砂井地基固结解

考虑初始孔压非均布条件和涂抹区渗透系数变化的砂井地基固结问题,推导了基于等应变假设的固结控制方程,得到一般解。然后根据实际工程中可能出现的情况建立了5种渗透系数变化模式,同时考虑初始孔压非均布条件的4种分布,在一般解基础上得到这20个问题相应的固结解,现有等应变条件下的砂井地基固结解是本文的特例。通过计算研究这两种因素共同作用下砂井地基固结性状的情况,结果表明:固结最快的是初始孔压为倒三角形分布、涂抹区渗透系数变化为模式2的情况;固结最慢的是初始孔压为正三角形分布、涂抹区渗透系数变化为模式4的情况。

初始孔压非均布竖向排水井地基固结方程的解析解

推导了等应变条件下初始孔压任意分布的竖向排水井地基固结一般解,并具体给出了初始孔压为梯形、正三角形、倒三角形分布时的相应解。现有等应变条件下打穿竖向排水井地基固结理论解为本文解的特例。根据所得一般解编制了计算程序,以分析初始孔压非均布条件下的竖向排水井地基的固结性状,并结合工程实例进行计算分析,与实测数据作了比较。结果表明,初始孔压分布形式对竖向排水井地基固结性状有重要的影响,并且对地基中超静孔压的影响比对固结度的影响更为显著;在单面排水条件下,初始孔压为倒三角形时竖向排水井地基固结最快,初始孔压为正三角形分布时固结最慢;在双面排水条件下,竖向排水井地基固结速率不受初始孔压分布形式的影响;考虑初始孔压非均布的本文理论与实际接近。

循环交通荷载下软土路基长期沉降理论解

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