不同真空加载条件下吹填土固结特征

在吹填土加压固结过程中,吹填土的固结过程与真空加载过程密切相关。为了分析吹填土固结强度增长过程与真空荷载条件的关系,本文在室内开展了真空加压砂井下排水方式的吹填土固结试验。试验结果表明:吹填土固结与渗透路径的长短、荷载大小、真空泵的容量、荷载持续时间密切相关;在加固初期,由于蒸发等环境因素的影响,各参数表现出两端小、中间大的特征;随着真空荷载作用时间延长,距离竖向排水体一定距离的土体各工程地质参数的变化情况与深度不相关;距离排水体较近的土体孔隙水压力变化明显,施加荷载孔隙水压力按照一定幅度下降,持续施加荷载一定时间,孔隙水压力消散幅度不断减小。在加固时应做好加荷过程的控制,以期在较短时间内达到预期的加固效果。     

预排水动力固结法加固吹填粉土地基试验研究

针对吹填土含水量高、孔隙比大、强度低,多呈流塑状态,在动荷载作用下极易产生沉降和液化的特点,提出了将井点降水和强夯法相结合的预排水动力固结法,即首先利用井点降水降低地下水位和土体含水量,并采取振动碾压的辅助降水措施,让施工设备能够进场作业,然后采用小能量强夯对吹填土地基进行加固,利用插设的井点来加速强夯后超孔隙水压力的消散和土体固结。结合工程实践对上部为吹填粉土、下部为饱和软土的地基进行了现场试验,对加固过程中的降水辅助措施、土体变形、孔隙水压力变化、工艺参数、加固效果等进行了分析和讨论。预排水动力固结法进一步拓展了井点降水和强夯法的应用范围,对吹填土地基加固提供了一种新途径。     

电渗–真空联合作用下富水黄土的排水固结特性分析

以富水黄土隧道开挖前围岩的快速排水为目的,提出电渗联合真空的方法,并对其排水固结过程进行了理论分析和试验研究。基于多孔介质理论和非饱和土力学原理,建立非饱和土体在电渗–真空作用下电场–孔压场–位移场相互耦合的数学模型,利用有限单元法数值分析土体在电渗–真空作用下的排水固结规律,并结合室内模型试验进一步验证该方法在富水黄土快速排水中的应用效果。研究结果表明：电渗–真空联合作用可以有效的提高土体的排水效率,但当初始渗透率较大时,联合排水后期会出现“孔压反转”现象,这种现象会导致联合排水后期排水效率低下,揭示联合排水后期增大电压并不能有效提高排水效率的机制;电极间距主要对电渗–真空联合排水后期孔隙水压力的变化有较大影响,且阳极电势一定时,孔隙水压力不随电极间距成单调变化。     

混合电源输出方式下的一维电渗试验研究

利用改进的一维电渗固结试验装置对饱和软黏土进行电渗试验,通过监测电渗过程中的电流、电压、电势、排水量、电渗前后的含水率和抗剪强度,利用电渗透系数、电迁移系数、能耗系数来研究不同电源输出方式下的电渗排水状况。试验结果表明:相比于稳压输出方式,压流模式下的累积排水量更小且排水速率更慢,有效电势利用率变化幅度略小,电渗透系数和电迁移系数衰减得更快,含水率的总下降值和归一化抗剪强度略大;相比于稳流输出方式,流压模式下的累积排水量更大且排水速率更快,有效电势利用率变化幅度更小,电渗透系数和电迁移系数衰减得更慢,含水率总下降值和归一化抗剪强度更大。通过对不同电源输出方式下能耗系数递增速度的对比,得出流压模式下电渗工况更经济合理。     

线性堆载下软黏土一维电渗固结理论与试验分析

 考虑线性堆载下水流和电流的相互作用以及孔隙水压力的消散情况,通过将单向渗流固结过程的普遍方程推广到电渗领域,建立线性堆载下软黏土一维电渗固结方程,并给出阴极排水、阳极不排水和阴阳极都排水2种情况下孔隙水压力和平均固结度的解析解。通过参数和试验分析,研究最大堆载和加载速率对孔隙水压力消散的影响,以及电压对理论固结度和试验固结度的影响,绘制不同最大堆载和加载速率时,单面排水和双面排水下孔隙水压力消散曲线、不同电压下理论固结度和试验固结度变化曲线。结果表明：最大堆载和加载速率越大,孔隙水压力消散得越快,同时最大堆载和加载速率对双面排水的影响要比对单面排水的更大;电源电压越大,理论固结度和试验固结度的增长速度都越快,在相同时间段内试验固结度的变化幅度小于理论固结度的变化幅度。     

外荷载随时间变化下考虑起始电势梯度的一维电渗固结解析解

将起始电势梯度的概念引入电渗固结理论，基于相关假定建立了外荷载随时间变化下考虑有效电势衰减的一维电渗固结控制方程，利用变量代换和分离变量法获得了电渗固结通解，并给出了常见加荷形式下解析解的表达式。将退化解与已有解析解进行比较，并将所提解与有限差分解展开对比，验证了所提解答正确性。基于所提解析解，分析了相关参数对软土地基电渗固结特性的影响。结果表明：起始电势梯度的存在降低了超孔隙水压力绝对值和沉降量；水力渗透系数的减小有利电渗排水固结过程的进行，且电渗透系数与水力渗透系数比值越大，沉降量越大、电渗排水固结效果越好；当采用电渗联合堆载预压法加固时，电渗作用可降低因外荷载引起的正超孔隙水压力的最大值，提高固结排水时土体的稳定性。     

天津市滨海新区吹填土工程处理现状及技术改进试验研究——吹填土水平辐射真空排水固结技术初探

本文在对天津滨海新区吹填土处理现状及存在问题分析基础上,针对吹填土的物理特性,应用水平辐射井排水原理,对吹填土地基处理技术进行了改进,提出了吹填土水平辐射真空排水固结技术,改变了吹填土排水固结路径和排水条件,由上排水改为下排水,实现了边吹填边进行吹填土的固结排水,大大缩短了工期;另外,改塑料排水板为波纹塑料排水管,提高了排水效果,而且排水井排水过程压力可有效控制;吹填土工程结束后,将排水系统进行回填密封处理,阻断竖向排水通道,从而减小了吹填土处理后的工后地面沉降量。该技术在应用试验中取得了较好的工程效果。     

湖相淤泥排水速率及残余含水率的影响因素分析

为模拟实际工程中不同土体组成及工况对湖相淤泥排水固结的影响，开展了真空预压法、真空预压联合电渗法的室内试验，获取了在充分排水情况下土体的排水量、含水率等参数的时程变化规律。讨论了淤泥中不同掺砂比、不同真空度、以及不同电压等因素对湖湘淤泥排水结果的影响并进行回归分析。结果表明：提高真空度、掺砂比、电压均可在不同程度上提高湖相吹填淤泥的前期排水速率，降低后期试样的残余含水率；真空预压试验中，残余含水率与掺砂比在大范围变化区间内存在显著的线性关系；联合处理试验中，残余含水率在不同掺砂比情况下，与电压存在显著的线性关系，且在不同电压下，与掺砂比之间满足一次傅里叶函数关系；本文的研究成果，在较小参数变化区间范围内与其他学者的研究结果较为符合。     

吹填土次固结特性试验研究分析

对滨海地区真空预压处理后和未处理吹填土进行一系列一维压缩次固结试验研究,获得应变、孔隙比与时间的关系,对比分析其次固结特性,为今后解决围海造陆工后沉降问题提供试验依据。结果表明:吹填土的次固结变形与土样高度及排水条件有关。双面排水条件下土中弱结合水容易排出,最终变形高于单面排水;加高后6cm试样最终变形明显低于2cm试样,相同排水条件下二者相差2.2倍;经过真空预压加固处理的土体结构性受到破坏,排水过程结束后土体直接进入次固结阶段,其次固结变形量低于未经处理吹填土。     

电渗法加固软土地基试验研究

 在自制的电渗固结装置上,进行轴对称工况软黏土不同初始条件及通电条件下的电渗试验。通过监测电渗过程中的电流、电势、排水量、抗剪强度、含水率和沉降,对比分析不同条件下的土体电渗固结性状,并得出以下结论：(1) 电渗法对软黏土地基处理效果良好,可达到减少土体含水率、提高土体抗剪强度、减小土体工后沉降等目的;(2) 高电压及高含水率土体的电渗效果好于低电压及低含水率的土体;(3) 含水率及电压相同条件下,间歇通电时的土体电能利用率较高,且电渗后土体的整体差异性较小;(4) 电渗也会造成一定的电极腐蚀,低电压及间歇通电条件下,电极的腐蚀相对较小。     

真空–电渗联合加固技术的固结试验研究

针对真空预压与电渗联合加固时存在水流方向相互干扰的问题,在室内试验中开展真空预压与电渗不同联合方式对淤泥脱水效果的研究。试验结果表明:真空预压与电渗异步加固时能够获得比其他联合方式更好的加固效果,而真空预压与电渗交替加固的时间对加固效果也有显著的影响。若交替时间太短,真空预压与电渗将不能充分发挥排水作用,若交替时间太长,真空预压与电渗所提供的能量将不能被充分利用。在现场实际应用中,应根据实测排水量或排水速率的变化不断调整真空预压与电渗交替加固时间。     

基于大变形的未打穿砂井固结理论研究

 吹填土往往具有含水率高、初始孔隙比大、压缩性大等特点,加固过程中渗透系数、压缩系数及渗流路径都会发生较大变化,加固过程中砂井以下土体在砂井排水通道和真空压力下也会发生固结。基于大变形固结理论,在流动柱体坐标系下,通过建立渗流连续方程,进而推导了能够同时考虑砂井加固层及下卧土层的未打穿砂井大变形固结控制方程,并采用有限差分方法对建立的方程进行了求解。结合黏土渗透压缩模型,利用研究成果对一吹填地基浅层加固工程进行了分析,计算结果与实测结果吻合较好,证明了计算方法的可行性。     

堤防工程中吹填土的固结分析

结合某堤防吹填工程实例,考虑外荷载随时间的变化过程,采用太沙基单向固结理论估计堤身及地基孔隙水应力消散,对堤防的固结进行相关分析。计算结果表明固结过程与吹填材料的渗透性能密切相关。     

Non-mechanical behaviors of soft clay in two-dimensional electro-osmotic consolidation

To investigate the soil behaviors in a direct current field on both spatial and temporal scales, a 1: 5 scale model test was conducted in laboratory to simulate the two-dimensional (2D) electro-osmotic consolidation of soft clay foundation. Volume of drainage, intensity, voltage, water content and pH value of water collected in the cathodes were monitored. The pH values of soil and the mass of anodes were measured before and after the test. The test results indicate that the unsaturated state, resultant from fissures induced by the differences in water contents, is favorable to dynamic compaction of soil during electro-osmotic drainage. The results also demonstrate that water content, degree of saturation and electric potential distributions can be used to deduce the electro-osmotic drainage process. Water content of soil decreases first near electrodes, while keeps nearly constant in the center of the model. The area with constant water content is larger than half of the sample surface. Moving anodes towards cathodes by about one third of the electrode spacing is effective to improve the treatment effect after electro-osmosis stops due to the large resistance. Moreover, it is observed that during electro-osmosis, the corrosion rate of anodes becomes smaller, while the variation in pH values of soil near anodes becomes larger.     

孔隙水运移特性及对尾矿细观结构作用机制试验研究

 利用自行研制的尾矿细微观力学与形变观测试验装置,研究尾矿充水过程、荷载作用和排水过程中孔隙水的运移特征及其对尾矿细观结构作用机制。结果显示：尾矿水位线实际达到高度大于计算获得的水位线高度;受荷载作用,充水条件下各层尾矿颗粒沉降位移显著大于未充水条件下相同位置颗粒的沉降位移;充水条件下尾矿颗粒沉降位移随荷载的增加可分为线性增长阶段、显著增长阶段和稳定阶段,显著增长阶段出现在200～400 kPa荷载下;受荷载与孔隙水的作用,尾矿细观结构会形成曲折复杂供水运移的通道,该通道的形成过程发生在尾矿颗粒沉降位移的显著增长阶段;孔隙水夹带尾矿小颗粒运移,对尾矿颗粒分布有一定的影响;尾矿颗粒沉降位移与细观结构受排水影响较小,其变化主要在排水初期。最后,提出水的毛细管作用下尾矿坝实际浸润线的概念与计算方法,研究孔隙水与荷载综合作用下尾矿细观结构变形破坏机制,为深入探索水引起的尾矿坝灾变机制及稳定性评价等具有重要的实际意义。     

Laboratory model study of newly deposited dredger fills using improved multiple-vacuum preloading technique

Problems continue to be encountered concerning the traditional vacuum preloading method in field during the treatment of newly deposited dredger fills.In this paper,an improved multiple-vacuum preloading method was developed to consolidate newly dredger fills that are hydraulically placed in seawater for land reclamation in Lingang Industrial Zone of Tianjin City,China.With this multiplevacuum preloading method,the newly deposited dredger fills could be treated effectively by adopting a novel moisture separator and a rapid improvement technique without sand cushion.A series of model tests was conducted in the laboratory for comparing the results from the multiple-vacuum preloading method and the traditional one.Ten piezometers and settlement plates were installed to measure the variations in excess pore water pressures and moisture content,and vane shear strength was measured at different positions.The testing results indicate that water dischargeetime curves obtained by the traditional vacuum preloading method can be divided into three phases:rapid growth phase,slow growth phase,and steady phase.According to the process of fluid flow concentrated along tiny ripples and building of larger channels inside soils during the whole vacuum loading process,the fluctuations of pore water pressure during each loading step are divided into three phases:steady phase,rapid dissipation phase,and slow dissipation phase.An optimal loading pattern which could have a best treatment effect was proposed for calculating the water discharge and pore water pressure of soil using the improved multiple-vacuum preloading method.For the newly deposited dredger fills at Lingang Industrial Zone of Tianjin City,the best loading step was 20 kPa and the loading of 40-50 k Pa produced the highest drainage consolidation.The measured moisture content and vane shear strength were discussed in terms of the effect of reinforcement,both of which indicate that the multiple-vacuum preloading method has a better treatment effect not only in decreasing the moisture content and increasing the bearing capacity,but also in increasing the process uniformity at different depths of foundation.     

Laboratory Investigation on Electro-Osmosis Properties of Singapore Marine Clay

In order to be able to design the electro-osmosis scheme, two major properties of clay, electro-osmosis permeability and coefficient of consolidation, need to be known. These properties can be measured in the laboratory by running the electro-osmosis consolidation test. This type of test is carried out with the help of modified triaxial apparatus in which the soil sample has an electric current applied, and its volumetric change accurately measured. It was found that the electro-osmosis permeability (k_e) of Singapore marine clay ranges between 10^–8 and 10^–9 m/v-s, whereas the electro-osmosis coefficient of consolidation (c_ve) varied between 0.17 and 2.81 m²/yr for tests under an electric potential gradient between 2 to 12 volts; c_ve increased with voltage applied. Physical and compressibility properties changes were noted after the electro-osmosis process. Gain in effective stress due to electro-osmosis was significant and magnitude of effective stress gain was directly proportional to the voltage applied. However vertical displacement was found to be smaller and significant magnitude of volume change was contributed from lateral deformation. This paper describes the test apparatus, and procedures and discusses the results from electro-osmosis tests, carried out on Singapore Marine Clay.