不排水条件下非饱和土水力–力学耦合特性数值模拟

非饱和土即便在不排水、不排气条件下也将发生体变,这为预测非饱和土不排水、不排气力学特性带来了困难。基于三相孔隙介质理论,结合非饱和土的弹塑性本构模型推导了不排水、不排气条件下非饱和土三轴剪切三相耦合控制方程,对非饱和土的不排水、不排气三轴剪切特性进行预测。数值模拟结果表明,此方法能够描述非饱和土在不排水、不排气条件下的水力–力学特性,如体缩特性、孔隙水压力和孔隙气压力增长以及饱和度增大等特性。数值计算结果与室内试验结果一致,证实了所提数值分析方法的合理性。     

重塑粘性土的基质吸力与土水分及密度状态的关系

非饱和土的基质吸力与其中水一气体系的温度、孔隙水的密度和孔隙气的相对湿度有关。测试了23组不同水分状态和密度状态的重塑非饱和粘性土三轴试验土样的温湿度，计算了相应的基质吸力。数据分析表明：基质吸力与干密度的关系在不同的含水量段具有不同的特点；基质吸力与含水量呈强非线性关系；在土的水分状态和密度状态空间，基质吸力的分布为一变形的W型折曲面。该曲面左翼陡立上翘，位于低水分状态，表征强非饱和土，右翼水平伸展，位于高水分状态，表征弱非饱和土，中心部位是一个“A”型折面，位于中等水分状态，表征典型非饱和土。首次提出了孔隙充水结构的概念，藉以从孔隙结构和毛细凝结两方面来准确理解非饱和土的水气界面效应，从而利于认识和把握非饱和土中基质吸力的变化规律。     

连续孔隙介质土力学及其在非饱和土本构关系中的应用

首先论述了土力学的理论基础仍然很不完善,仍处于半理论、半经验的发展阶段,它还缺少统一的理论基础。指出:连续孔隙介质土力学是以多相孔隙介质理论为基础描述自然界中土的行为的一门科学,它(或多相孔隙介质理论)可以作为土力学统一的理论基础。然后介绍了连续孔隙介质土力学的理论基础,讨论了多相孔隙介质理论的整体平衡方程的建立以及土力学中控制方程的选择和确定。针对非饱和土问题,利用多相孔隙介质理论(包括热力学和内变量理论),基于3种广义应力即非饱和土的有效应力(式(22))、基质吸力和气相压力作为3个独立的应力状态变量,提出并详细地论证和推导了考虑气相的能量耗散和塑性体积变形的非饱和土弹塑性本构方程。     

考虑孔隙比和水力路径影响的非饱和土土水特性研究

以非膨胀性黏土为试验研究的对象,利用压力板法研究了初始孔隙比对不同水力路径下非饱和压实土土水特性的影响。试验结果表明,不同初始孔隙比土水特征曲线均存在明显的滞洄现象;当吸力大于某一值时,以含水率与吸力关系表示的话,不同初始孔隙比主脱湿土水曲线几乎重叠;以饱和度与吸力关系表示,初始孔隙比对土水特征曲线存在较大的影响。还提出归一化处理全吸力范围内不同初始孔隙比土水特征曲线的方法。最后,在归一化的处理方法基础上,提出了考虑初始孔隙比影响的非饱和土滞洄特性的模拟方法,并利用相关实测数据加以验证。     

非饱和重塑弱膨胀土孔隙结构与土–水特征曲线试验研究

为研究压实程度对非饱和重塑弱膨胀土孔隙结构的影响,探讨膨胀土孔隙结构改变对土–水特征曲线的影响。以新疆哈密地区重塑膨胀土为研究对象,采用压汞试验测定压实程度对土样的微观孔隙结构影响,并利用热力学关系模型对膨胀土分形维数进行研究;采用滤纸法试验测量不同初始干密度下重塑土样的土–水特征曲线,提出可考虑压实作用影响的膨胀土土–水特征曲线双参数拟合模型,最后,基于土体孔隙特征,借助毛细管原理计算并结合滤纸法试验数据修正其土–水特征曲线。研究结果表明：随压实程度的增加,新疆哈密地区非饱和弱膨胀土孔隙呈初始三峰状并逐渐趋于双峰状分布,其在微观上表现为大孔隙的压缩引起的大孔隙向小孔隙的转化,孔隙内壁的粗糙程度及孔隙结构的复杂程度提高;土体基质吸力随含水率的增加而降低,相同体积含水率下,土体初始干密度越大,基质吸力越大,双参数模型可较好反映土体初始干密度对膨胀土土–水特征曲线发展规律的影响;基于毛细管原理的土–水特征曲线计算值同滤纸法实测曲线有相同函数模型发展规律,且随试样初始干密度的增加两者呈逐渐“远离”趋势发展;土–水特征曲线修正模型可统一描述非饱和弱膨胀土基质吸力随孔隙结构、含水率及密度状态的发...     

非饱和粘弹性地基在加荷随时间指数变化作用下的一维固结半解析解

研究了非饱和粘弹性地基在加荷随时间指数变化作用下的一维固结特性。针对粘弹性地基土最基本的merchant模型,采用李氏比拟法,依据Fredlund固结理论及Cayley-Hamilton数学方法得到Laplace变换域内的超孔隙压力的解;再根据Crump方法编制程序进行Laplace逆变换得到了时间域内的超孔隙压力及土层沉降的半解析解。最后,本文给出了一个典型算例,揭示了在加荷随时间指数变化下非饱和粘弹性地基的固结特性,扩展了非饱和土固结理论在工程实际中的适用性。     

非饱和土应力状态变量试验验证研究

为验证非饱和土两个应力状态变量的合理性,对常规非饱和土三轴仪进行了改进,采用3个压力–体积控制器分别控制孔隙水压力、内室压力和外室压力,提高了量测精度。使用改进的非饱和土三轴仪对描述非饱和土应力状态的两个应力状态变量进行系统的试验验证,做了不同初始干密度、不同初始饱和度、不同初始吸力的3组重塑黄土及1组原状Q_3黄土的各向等压试验,试验中等值改变(增加或减少)总围压、孔隙水压力、孔隙气压力而保持应力状态变量本身不发生变化,通过量测试样体积和水量变化分析检验两个非饱和土应力状态变量的合理性。研究结果表明:对于原状Q_3黄土和各种起始条件的重塑土(包括不同密度、不同饱和度及不同吸力的土样),当只改变应力状态变量的个别组成部分而保持两个应力状态变量本身不发生变化时,则土样的体积变化和含水率变化均很小,可忽略不计。研究结果说明描述非饱和土的两个应力状态变量是合理的,使非饱和土的两个应力状态变量理论有了牢靠的试验依据。     

气体压力和孔隙对垃圾土体气体渗透系数影响的研究

垃圾土的非饱和气体渗透系数是填埋场气体运移分析和抽气井设计的重要参数,孔隙比、有机物含量、饱和度等是影响垃圾土渗透特性的主要因素。通过研制的垃圾土非饱和渗透试验仪,进行了不同影响因素下的非饱和垃圾土气体渗透试验,发现渗透系数随渗透压力呈现非线性特性,可以用Forchheimer非达西渗流方程较好地拟合。孔隙比增加,渗透系数增大,非达西系数Ba逐渐减小,分界点的气体压力减小;有机物含量越高,渗透系数越小,分界点气体压力越大;片状分布有机物使有效连通的渗透路径减少是垃圾体气体渗透特性降低的原因;饱和度越高,垃圾土的渗透系数越小,分界点气体压力越大;在试验的有效孔隙范围内,渗透系数、分界点气体压力与有效孔隙呈现较好的相关关系、非达西系数变化不大。     

瀑布沟堆石坝砾石土心墙施工期孔隙水压力特征与分析

 土石坝施工期孔隙水压力长消过程十分复杂,以瀑布沟心墙监测资料为基础,分析施工期砾石土心墙的孔隙水压力特征及其形成机制,并与小浪底坝和鲁布革坝监测资料进行对比分析。研究结果表明：瀑布沟心墙施工期孔隙水压力长消主要有4种模式,即滞后响应型、即时响应型I、即时响应型II、不响应型;即时响应型II是最主要的长消模式;渗压计响应滞后的实质是渗压计周围土料从非饱和状态过渡到饱和状态的过程;渗透系数是影响渗压计是否滞后的决定性因素;即时响应型I长消模式是心墙局部含水量偏高的结果,施工过程中应尽量使心墙含水量均匀,避免产生即时响应型I长消模式;填筑速率对滞后响应型和即时响应型II孔隙水压力的长消影响显著,而对即时响应型I孔隙水压力长消影响不显著。因此,设计和研究砾石土心墙坝应高度重视砾石土的不均质性。     

非饱和粉砂临界状态的试验研究

非饱和土临界状态给出了土变形过程的终点,对于分析土的基本力学性质和建立相应的本构模型十分重要。以非饱和粉砂为研究对象,利用非饱和土双压力室三轴试验系统(GDS)开展相关试验研究,对不同干密度粉砂在不同净应力和吸力条件下剪切至临界状态过程中,分别测定了强度、体变、含水率及饱和度等相关状态参量的变化规律。结果表明:剪切至临界状态过程中,不同干密度非饱和粉砂的强度和变形特性差异明显;随轴向应变的不断增加,不同初始干密度粉砂试样的偏应力、体应变以及饱和度曲线最终趋于稳定值,试样剪切达到临界状态。基于试验结果,揭示了临界状态条件下非饱和粉砂强度、孔隙比、孔隙水比体积随吸力的变化规律,并建立了q–p?、ν–ln p?、νw–ln p?平面内的临界状态线方程,提出了从强度、变形和孔隙水三个方面对非饱和粉砂临界状态进行综合描述的方法。     

非饱和土一维固结简化计算

提出了一种简化的非饱和土一维固结计算方法。加荷初期,水、气、土骨架共同承担荷载。假定土体瞬时压缩,利用非饱和土有效应力原理、土骨架压密方程、气体压密方程、水分特征曲线,可解得三者各自分担的应力和土体体积压缩量。在其后的固结过程中,假定水的流出仅仅取决于水压力,不受气的影响,单独建立水的连续性方程,解水压力随时间的变化;又将水和气看成混合的可压缩流体,建立混合流体的连续性,解混合流体压力随时间的变化。可进而求气压力。吸力和土体的压缩量。由于本文只研究一维问题,解连续性方程用差分方法。     

Monotonic and Cyclic Behavior of Unsaturated Sandy Soil Under Drained and Fully Undrained Conditions

Changes in air pressure during monotonic and cyclic loading are in some cases important for the behavior of unsaturated soil. For example, in order to investigate the stability of embankments and slope failure during earthquakes, it is necessary to consider the effect of the pore air or the pore gas pressure as well as the pore water pressure and the interaction between the soil and the pore fluids. In the present study, we carried out a series of monotonic and cyclic loading tests on sandy soil used for the improvement of river embankments. The effects of the initial suction, the confining pressure, and the degree of compaction under fully undrained conditions, namely, constant water and constant air shearing tests, as well as under drained conditions for both air and water, were studied. For the stress variables of the unsaturated soil, the skeleton stress was used to describe the experimental results and was defined as the difference between the total stress tensor and the average pore pressure of water and gas (Oka et al., 2010). From the monotonic and cyclic test results, we found that the stress-strain behavior of unsaturated sandy soil strongly depends on the initial suction, especially under fully undrained conditions, due to the difference in pore pressures. In the cyclic loading tests under fully undrained conditions, the mean skeleton stress decreased due to the increase in air pressure and led to the failure of the specimen in the case of a lower level of initial suction. In addition, the test results exhibited the strain rate effect on the stress-strain behavior during cyclic loading under fully undrained conditions.     

非饱和土受压变形的简化计算研究

非饱和土的固结问题是岩土工程研究中非常复杂的热门课题。本文拟将非饱和土受压后的固结过程简单地分为压密和固结两个阶段。在压密过程中,与饱和土固结不同是,由于非饱和土的孔隙水、气来不及排出,与骨架共同承担荷载,产生了超孔隙气压和超孔隙水压,这个阶段土体变形主要是压缩孔隙气产生的压缩变形;固结阶段,土体在恒定荷载作用下,超孔隙气压和超孔隙水压逐渐消散而固结。对于饱和度较高的地基土,将水、气可看作混合流体,并考虑残留混合流体的压缩性,建立平衡微分方程和混合流体的连续方程,求出有效应力和混合流体压力;土体中水压力可通过水连续方程求出,继而求出气压力、吸力等。最后算例表明:加载过程中,有效应力、混合流体压力、水压力不断增大,吸力不断减小;竣工后,混合流体压力、水压力还在增长;由加载产生变形以荷载压密变形为主,之后变形增量逐渐减小。这与路堤填筑过程中的监测结果相一致,说明本文的简化方法是合理的。因此本文的非饱和土固结研究对于促进了非饱和土固结变形计算走向实用化,具有重要的工程意义。     

Liquefaction of Unsaturated Sand Considering the Pore Air Pressure and Volume Compressibility of the Soil Particle Skeleton

A series of cyclic triaxial tests of unsaturated soils was conducted to get a better understanding of the general liquefaction state of unsaturated soils. In the tests, cyclic shear strain was applied to fine clean sand with the same dry density but different initial suction states under the undrained condition. During cyclic shear, the volume change of the soil particle skeleton, the pore air pressure and the pore water pressure were measured continuously. Having used the effective stress defined by Bishop (Bishop et al., 1963), where the net stress and suction contribute to the effective stress, our test results showed that unsaturated sand specimens with quite a low degree of saturation lose their effective stress due to cyclic shear. At a zero effective stress state, unsaturated specimens behaved similarly to liquids in much the same way as saturated specimens. From experimental and theoretical considerations, the zero effective stress state (i.e., liquefaction) for unsaturated sand was found to have been established when both the pore air and water pressures build up to the point where it is equal to the initial total pressure. A volume change of pore air under the undrained condition, if a volume change of pore water is negligible, is equal to that of the soil particle skeleton. Therefore, it can be concluded that the liquefaction of unsaturated soil generally depends on the volume compressibility of the soil particle skeleton and the degree of saturation. On the other hand, according to the ideal gas equation of Boyle-Charles law, the volume change required to bring about a zero effective stress state can be calculated from the initial pore air pressure (usually the atmospheric pressure) and the final pore air pressure (the initial confining pressure). Therefore, the liquefaction of unsaturated soils also depends on the initial confining pressure. Based on this concept, the liquefaction potential of unsaturated soil can be evaluated by comparing the volume compressibility of the soil particle skeleton and the volume change of the pore air required to bring about a zero effective stress state.     

三轴试验中非饱和土试样吸力的量测

本文作者改装了常规三轴仪使之能量测非饱和土试样的孔隙水压力和控制孔隙气压力，并与轴移技术所测得的吸力进行了对比验证。利用这种改装的仪器和轴移技术，进行了一组非饱和土试样的排气常含水量三轴剪切试验，量测出剪切过程中的吸力变化值，试验中发现围压和剪切应变对试样的吸力均有较大影响。     

从变形、水量变化和强度三方面验证非饱和土的两个应力状态变量

为了验证非饱和土两个应力状态变量的合理性,对常规非饱和土三轴仪进行了改进,采用3个压力–体积控制器分别控制孔隙水压力、内室压力和外室压力,提高了量测体变和含水率的精度;用加荷器施加轴向荷载,可以方便地控制偏应力。在笔者前期用各向等压试验验证非饱和土的两个应力状态变量的基础上,使用改进的非饱和土三轴仪和重塑Q_3黄土做了两类验证试验:第一类是两组控制净围压、偏应力和吸力的固结不排水剪切试验,第二类是3组控制净围压和吸力的固结排水剪切试验。在第一类试验中,等值改变(增加或减少)总围压、孔隙水压力和孔隙气压力而保持净围压、吸力和偏应力本身不发生变化,发现试验过程中土样的体积变化和含水率变化均非常小,可以忽略不计。第二类试验的每一组包含两个控制净围压和吸力分别相同的固结排水剪切试验,其中一个试样在固结完成后直接进行排水剪切试验,另一个试样在固结完成后等值增加总围压、孔隙水压力和孔隙气压力而保持净围压和吸力本身不发生变化的条件下进行排水剪切试验,发现二者的抗剪强度、剪切过程中的体积变化、排水量和广义剪应变都分别十分接近,可以认为两两相等。研究结果从变形(包括体应变和剪应变)、水量变化和强度3个方面说明描述非饱和土的两个应力状态变量是合理的,进一步夯实了非饱和土力学的应力理论基础,为非饱和土的两个应力状态变量理论提供了牢靠的试验依据。     

水位下降过程中气相对土坡稳定性的影响

基于饱和-非饱和水-气二相流模型,对土坡在水位下降过程中的孔隙水压力、孔隙气压力、毛细压力及水相饱和度的变化过程进行模拟;基于模拟得到的不同时刻的渗流状态,采用Bishop简化方法推导出考虑孔隙气压力及坡外水压力的土坡安全系数计算公式,分析了水位下降过程中不同渗透性土坡安全系数的变化规律,及基质吸力和气相的存在对边坡稳定性的影响。结果表明:水位下降过程中土坡的安全系数逐渐减小,水位下降后土坡的安全系数略有增大;考虑气相影响后土坡的安全系数降低,考虑基质吸力影响后土坡的安全系数增大;岸坡土体的渗透率越大,水位下降过程中安全系数较大,而水位下降后安全系数较小。     

An Active Control System for Matric Suction Measurement

Matric suction is an important stress state parameter in unsaturated soil mechanics. Many studies have been carried out in the past to determine the matric suction through direct and indirect methods. Direct measurement of matric suction has been proven possible with high-suction tensiometer; however high-suction tensiometers are still susceptible to cavitation. The axis translation technique developed by Hilf (1956) has been employed in many laboratory tests for unsaturated soils to avoid problem of cavitation in the water pressure measurement system. However in laboratory testing of unsaturated soils, air and water pressures are usually independently controlled and there is no need for a feedback control. The matric suction of soil can be measured using a modified pressure plate apparatus by actively changing the air pressure to maintain the water pressure to be close to zero thus imposing negligible water content change in the soil. A major setback of the existing practice is the need to manually adjust the air pressure of the modified pressure plate in response to the changes in the water pressure. This paper presents an active control system for the modified pressure plate apparatus for matric suction measurement. The experimental results obtained from modified pressure plate apparatus with active control system show good performance as compared to the high suction tensiometer.     

非饱和土强度随含水量的变化

吸力的量测和计算比较困难,而含水量的分布在工程中却容易确定,在非饱和土强度的研究中,选择含水量代替吸力,研究非饱和土强度随含水量的变化,具有重要的实用价值.首先,在改进的普通三轴仪上进行非饱和土的强度试验.试验中控制含水量不变,同时保证气压消散,以模拟工程施工期间气压消散快、水压消散慢的工程实际特点.其次,根据试验结果分析含水量对非饱和土强度的影响,建立非饱和土的实用强度公式.试验结果表明：随着含水量的增大,强度明显减小,表明含水量对强度的影响较大;且在一定含水量范围内,强度指标随含水量的增大线性减小,并在此基础上建立了引入含水量的非饱和土实用总应力强度公式.公式运用时由土层含水量的分布确定强度,避免吸力的量测和计算的困难,简单实用.     

Experimental Study on the Behavior of Unsaturated Compacted Silt Under Triaxial Compression

Most of the experimental investigations conducted on unsaturated soil have been performed under a constant air pressure. Changes in air pressure during deformation are in some cases important in practice. For example, in order to explain the stability problems of embankments during earthquakes and seepage flow, and grounds containing gas associated with the dissociation of methane hydrates, it is necessary to consider the interaction between the soil and the pore fluids. In the present study, we carried out fully undrained tests as well as drained tests, namely, constant water and constant air shearing tests. We performed the fully undrained tests using an air-controlled valve to measure the pore air pressure. For the stress variables of the unsaturated soil, skeleton stress values were used to describe the experimental results. From triaxial compression tests on silty soil, we found that the initial suction, the confining pressure, and the strain rate of unsaturated soil strongly influence the stress-strain behavior of unsaturated silt.