真空-堆载预压法在夹砂淤泥质软基中的应用

本文从加固机理出发,根据工程现场的监测数据对预压期间各土层的沉降、侧向位移、孔隙水压力进行分析,前期真空预压时沉降主要发生在淤泥层,后期加上堆载后填土层沉降较大,淤泥层发生蠕变现象。判断其与竖向荷载大小、层间土弹性模量比及软弱层的厚度有关。另外,由于水压消散值的测定受真空和堆载联合作用而随时间变化较复杂,推荐用沉降量推算固结度。     

真空联合堆载预压加固覆盖层深厚软土地基研究

根据现场监测资料和检测数据,研究真空联合堆载预压处理大面积覆盖层下深厚淤泥质软土地基的表面沉降、超静孔压、深层沉降、侧向位移等固结变形特性和加固效果,结果表明:由于覆盖层的应力扩散和"顶托"作用,地表沉降曲线比较顺滑,高填土堆载过程中无明显"坡降"段;地基土体压缩变形主要发生在覆盖层下部的深厚淤泥质软土层,覆盖层较厚时不利于消除下卧层土体压缩量。孔隙水压的增长和消散受真空负压和填土堆载共同作用,与覆盖层厚度有一定的关系,覆盖层较薄时,孔隙水压响应较快,堆载引起的超静孔隙水压值较大;覆盖层较厚时,孔隙水压响应较滞后,孔隙水压曲线较平缓,堆载引起的超静孔隙水压值较小。厚覆盖层的存在不利于软土的排水加固,方案设计和施工时应予以重视。     

真空堆载预压加载速率对预压期沉降量的影响分析与探讨

以排水固结原理为基础的真空联合堆载预压法,作为加固深厚淤泥层的处理方法,在我国沿海软土地基加固施工中已得到广泛的应用,并取得了良好的工程效果。依据真空堆载预压施工现场监测的数据,从加载速率、孔隙水压力、分层沉降、深层土体水平位移监测几个方面,分析了加载速率对预压期沉降量以及工后沉降的影响。     

真空-堆载联合预压加固高等级公路软基的工程实例分析

根据浃里陈大桥桥头试验段的现场监测成果,对真空-堆载联合预压加固软基试验段中加固区的孔隙水压力、地表沉降随时间变化的规律、影响区的地表沉降特征及其对周围建筑物的影响、真空能量与地表沉降的关系以及停泵后的地表回弹量等进行了分析。分析表明,真空-堆载联合预压的联合方式实质上为孔隙水压力差的叠加,真空-堆载联合预压能大幅消除沉降量,减少工后沉降量,达到防止“桥头跳车”的作用。     

深厚软土地层真空联合堆载预压地基处理对周边建筑物变形影响研究

为深入探讨真空联合堆载预压法对周边建筑物变形的影响,以某深厚软土地层为例,开展了真空联合堆载预压现场试验研究,分析了孔隙水压力、真空度、沉降、分层沉降、地基土侧向变形等随着时间的变化规律,并阐明了产生该种变化规律的具体原因,同时分析了真空联合堆载预压处理深厚软土地基对周边建筑、周边管线的影响,得到如下结论：(1)真空预压和堆载共同作用使得土体孔隙水压力消散加快、土体的有效应力增加,最终加速了土体的固结和沉降,有效改善了土体的物理性质;(2)在真空预压阶段,土体的竖向沉降随着时间的增加逐渐增加,此时土体几乎没有明显的侧向变形,经堆载预压处理后,土体的竖向沉降和侧向位移逐渐增大,本工程中真空预压预压对土体的影响深度最大可达25 m。(3)真空联合堆载预压施工对周边建筑影响较小但对周边管线的影响较大,会导致周边管线发生不同程度的沉降。(4)真空联合堆载预压施工会造成地表处土体出现侧向变形,对于地基沉降及侧向变形十分敏感的建筑,需对其沉降量进行实时观测,避免危险发生。研究成果为真空联合堆载预压法处理深厚软土地层提供了设计参数和理论指导。     

真空堆载联合预压加固软基机理的试验研究

通过对浃里陈大桥桥头试验段的真空度、地下水位、流量、孔隙水压力、地表沉降等现场监测结果的分析 ;阐述了真空堆载联合预压的联合机理 ,真空预压与堆载预压的联合方式不是真空荷载与堆载荷载的叠加 ,而是真空预压引起的孔压差与堆载压力引起的孔压差的叠加。试验结果表明 ,用真空堆载联合预压法处理软基是极为有效的。     

真空联合堆载预压法处理软基

结合某深厚淤泥质软土地基处理工程,介绍了真空联合堆载预压设计方案,以及现场监测方案,结合现场沉降观测数据对场地最终沉降以及工后沉降进行预测,计算结果表明真空联合堆载预压能够有效完成预定的地基加固效果,满足工程建设需要。     

预压作用下软土固结蠕变及微观结构试验研究

为了揭示预压荷载作用下软土路基工后沉降的机理,开展了室内模拟真空、堆载以及真空—堆载联合预压加固软土的固结蠕变三轴试验及微观结构试验。分析了在三种不同的预压作用下土体固结蠕变性状的不同和微观结构的变化机理。结果表明:在三种不同的预压荷载作用下,固结过程中,是真空—堆载联合预压条件下土体的沉降量最大,其次是堆载预压,最小的是真空预压。蠕变过程的沉降量是堆载预压的最大,其次是联合预压,最小的是真空预压。预压荷载作用下土体的固结和蠕变具有耦合效应;微观结构参数的变化受预压加载方式的影响。微观结构参数的变化率与宏观物性参数的变化率有相似的性质。     

深港西部通道大面积填海地基处理工程及其主固结沉降计算与讨论

深港西部通道位于深圳与香港之间的深圳湾浅海域,面积1.5平方公里,水下均沉积了一层6~24米厚度的高含水量、高塑性、高压缩性的滨海淤泥。自2002年开展了填海造陆工程,其软基处理面积已达到一百五十万平方米。滨海淤泥的加固采用价廉简便的堆载预压排水固结法,但是并不是预压时间越长,淤泥中孔隙水排出的越多,当淤泥含水量降低到一定程度后,(达到—个"极限含水量"),延长预压时间对淤泥固结不起应有作用,研究表明这个"极限含水量"和淤泥的"液限含水量"相当。本文最后探讨了堆载预压排水固结处理软土地基的主固结沉降量,极限主固结沉降量等新的计算方法。     

吹填淤泥在真空联合堆载预压过程中的工程性质变化特征

本文以深圳前湾A区真空联合堆载预压处理吹填淤泥的工程为例,采取室内试验和原位测试分析,详细地研究了吹填淤泥在砂垫层施工阶段、插塑料排水板后加载前阶段、联合堆载前纯真空预压阶段、真空联合堆载预压阶段的工程性质变化特征,随着施工推进,吹填淤泥的含水量和孔隙比逐渐降低,湿密度和干密度逐步增加,吹填淤泥土性质不断得到改善,强度持续增加。最后,对真空联合堆载预压达到设计卸载标准后的吹填淤泥加固效果进行了检测和分析,结果表明加固效果达到了设计要求,为吹填淤泥的处理提供了借鉴作用。     

真空预压处理软基效果分析

采用真空预压法处理某高速公路软基,处理淤泥层的平均厚度达到10m以上,软基上施加真空压力80kPa,150d后地基表面沉降量达到1m以上,地基平均固结度超过90%,满足设计要求。处理后的软土地基物理力学性质变化明显,表明采用真空预压法处理本工程软基是可行的,同时对真空预压引起的土体水平位移进行分析。     

A pilot test on a membraneless vacuum preloading method

A membraneless vacuum preloading method is proposed in this paper for soft soil improvement. The method offers several advantages over the conventional vacuum preloading in which membrane is used to create the airtight condition and sand blanket layer to distribute vacuum. To assess the effectiveness of the proposed method, a pilot test was conducted at a land reclamation site in Tianjin, China. The ground settlement and the pore water pressure (PWP) at different elevations in soil were measured. After vacuum preloading, the average water content of the soft soils reduced by approximately 12% and the undrained shear strength increased twofold. The average degree of consolidation at the end of the vacuum preloading achieved 85.1% based on the settlement data and 84.5% based on the PWP data. The pilot test data have shown that the proposed method exhibits similar efficiencies to the conventional vacuum preloading method.     

Experimental study on a dredged fill ground improved by a two-stage vacuum preloading method

The vacuum preloading method has been wisely chosen among many ground-improvement methods considering the time limit of many projects and the characteristics of reclaimed soil. However, the loss in vacuum with soil depth, the clogging around prefabricated vertical drains (PVDs), and the deteriorative consolidation of the deep soil layer, among other factors, create a large challenge to vacuum preloading for dredged marine clay fill. Thus, this study proposes a two-stage vacuum preloading method and focuses on its feasibility and effectiveness. Contrasting laboratory tests are performed in two identical experimental tanks with dredged marine clay fill from the Wenzhou land reclamation site in China. In one tank, the one-stage vacuum preloading method is used to serve as a baseline for this study. In the other tank, use of the two-stage vacuum preloading method is proposed for consolidation; it comprises two stages. In the first stage, the dredged marine clay fill is conditioned by vacuum preloading using half of the PVDs, where the dissipation of the excess pore water pressure tends to be steady. In the second stage, vacuum preloading is activated using all the PVDs. The results show that a better consolidation effect is achieved with the proposed method in terms of the settlement, vacuum pressure, pore water pressure, water content, vane shear strength, and soil particle microstructure after soil consolidation.     

Improvement of silty clay by vacuum preloading incorporated with electroosmotic method

A laboratory test was performed to assess the effectiveness of vacuum preloading incorporated with electroosmotic (EOM) treatment on silty clay (combined method) for reclamation projects like new disposal ponds, where the horizontal electrode configurations beneath the soil layer were possible and the drainage pipes and the prefabricated vertical drains (PVDs) system could be easily installed in advance before the sludge dragged from sea bed or river bed was filled into the site. Three groups of tests were conducted on the silty clay from Qinhuai River in Nanjing, China. The model is able to apply vacuum pressure at the bottom of the soil layer and a direct current electric field simultaneously. It is also possible to measure the pore pressures at different depths of soil column, and the changes in settlement and volume with the elapsed time. In this study, the vacuum preloading method, vacuum preloading applied at the bottom (VAB method), was applied and the cathodes were installed beneath the soil layer. The results obtained indicate substantial reduction in water content, and increases in dry density and undrained shear strength in comparison with those obtained by the vacuum preloading only, particularly at the positions close to the anode. The combined method utilizes the vertical drainage flow created by the electroosmosis integrating the horizontal drainage flow created mostly by the vacuum pressure. The total drainage flow can be calculated as a result of the vertical drainage flow by electroosmosis only and the horizontal drainage flow by the vacuum preloading only. The way of placement of the cathode and the anode in the combined method also overcomes the disadvantage of EOM method itself, i.e. the appearance of cracks between the anode and the surrounding soil. Moreover, it is observed that the vacuum preloading plays a primary role in earlier stage in deduction of free pore water; meanwhile, the electroosmotic method is more efficient in later stage for absorbing water in the diffused double layers o     

堆载预压和真空堆载联合预压的比较

堆载预压和真空联合堆载预压都是工程中处理软土地基的常用方法。通过舟山富通电缆项目地基处理工程项目中堆载预压和真空联合堆载预压施工的比较研究,得出真空联合堆载预压比堆载预压具有更高的施工成效效率,更好的沉降稳定性和沉降性,以及水平位移量更小,使周围软弱边界的变形处于较为稳定的状态。     

Pilot tests on vacuum preloading method combined with short and long PVDs

Dredged marine clay has been widely used as a filling material for land reclamation in China. The difficulty of using the vacuum preloading method to improve the dredged marine clay together with the bottom sediment clay is the different spacing requirement of the PVDs. To solve this problem, the Vacuum Preloading method combined with the Short and Long PVDs (VPSL) is proposed in this paper. The short PVDs are installed only into the dredged marine clay layer in-between the long PVDs which are installed through the whole clay layer. Pilot tests are also conducted at a land reclamation site in Tianjin, China, to investigate the performance of the proposed method. The ground settlement, the applied vacuum pressure and the pore water pressure in the soil are monitored during the pilot tests. The average degrees of consolidation are calculated based on the monitored settlement and pore pressure data. It is found that the proposed VPSL method is more effective for improving top dredged clay together with the bottom sediment clay than the conventional vacuum preloading method. The vane shear strength profiles of soil layers after ground improvement also show that the VPSL method is more effective to achieve a uniform soil strength profile.     

真空预压联合搅拌桩在有轨电车软基加固中的应用

有轨电车以其绿色环保的运营方式正在引领中国交通行业向可持续方向发展。针对南京河西地区软土地基的结构特点和工程特性,提出了真空预压联合搅拌桩软基加固方法,结合施工期表层沉降、分层沉降、水平位移和孔压及运营期累计沉降监测结果,分析了该软基处理方法的加固效果。研究结果表明:在抽真空过程中,孔隙水压力随着深度的增加而逐渐减小;水平位移和分层沉降变化率均逐渐减小;表层总沉降量在抽真空2个月后趋于稳定,表明软土层接近完成固结;施工期的表层累计沉降量为440 mm;运营后半年累计沉降为4.2 mm,预测运营10 a后累计沉降约为9.8 mm,远低于设计警戒值。说明真空预压联合搅拌桩的方法可以有效加固有轨电车软土地基。     

Experimental study of PVD-improved dredged soil with vacuum preloading and air pressure

The PVD-vacuum method is the main method used to treat dredged soil, but sometimes it cannot completely meet engineering needs due to the limitations of vacuum pressure. Therefore, based on the traditional PVD-vacuum method, a method of air pressure combined with vacuum preloading is proposed in this paper. The air pressure is transmitted into the dredged soil by using an impermeable and flexible membrane airbag embedded in the dredged soil. A comparison of the traditional PVD-vacuum method and the combination of air pressure and vacuum preloading is investigated by the model test. The data of the settlement and water discharge during the test are monitored. After the tests, the water content and shear strength at different positions are measured. Finally, a method for calculating the volume change of dredged soil is proposed, and the results of the test are used to prove the correctness of the proposed method.     

真空堆载联合预压作用下软土地基位移和孔压变化规律研究

根据实测资料分析了真空堆载联合预压作用下加固区地表沉降、土体侧向位移、孔压、超孔压变化规律,并对孔压消散原因进行微观解释,结果表明:真空预压形成的负压与堆载预压形成的正压二者在加固软土地基的效果上是一致的,场地中心处的沉降要比四周大,用固结度对数配合法推定最终沉降,停泵时的平均固结度达到97%;真空预压阶段土体侧向位移方向是指向加固区内的,而堆载预压阶段是指向加固区外的;开始抽真空后浅层土体孔压消散非常明显,深层土体孔压消散较为缓慢,后期较深土层中孔压才有较大变化;超孔压在抽真空后变为负值,在真空预压阶段绝对值越来越大。