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。     

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。     

真空联合堆载预压处理软土地基测试与分析

结合山东某港口大面积吹填软土地基处理实践,阐述了真空联合堆载预压施工相关监测和检测内容及其基本操作方法;分析了真空联合堆载预压施工过程中土体表面沉降、分层沉降、深层水平位移以及孔隙水压力的发展变化过程和规律。表明真空联合堆载预压法加固软土地基具有真空预压和堆载预压的双重加固特征,堆载能加速软土地基的固结变形,但在堆荷较小情况下,被加固土体总体仍表现为土体向加固区中心位移的收缩固结变形特征。从工后现场原位测试及试样室内土工试验成果可以看出,淤泥土层加固后含水量、孔隙比均有大幅减小,土体物理力学性质得到较大改善,土体强度增长幅度可达100%～300%。说明真空联合堆载预压应用于处理大面积吹填软土地基是成功可行的,其加固效果明显,有效影响范围可达排水板深度。     

真空联合堆载预压法处理深厚超软淤泥时卸载标准的研究

从真空联合堆载预压加固原理和影响土体变形、固结的主要因素出发,针对目前真空预压(或真空联合堆载预压)卸载标准的局限性,提出卸载宜按固结度和沉降速率双重控制,且沉降速率应根据使用荷载、施工荷载、土体性质和土层厚度等因素进行修正.经将修正方法运用到工程实例中,证明修正后的卸载标准是合理的、符合工程实际的.     

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。     

真空预压和堆载预压模型试验研究

采用室内模型槽试验,研究了真空预压法和堆载预压法加固软土的固结机理和特性,得出两种加载情况下土样的含水率都随深度的增加而增加,十字板剪切强度随深度的增加而减小的结论。在相同压力作用下,真空作用下土样的含水率比堆载作用下的土样偏小。而且真空作用下土体前期固结沉降的速率远大于堆载作用下土体固结沉降速率。     

含承压水软基真空联合堆载预压加固试验研究

运用真空联合堆载预压法加固含承压水软基,简要介绍了防止承压水贯通的施工工艺。对港湾变电所软基加固试验中的地表沉降、分层沉降、土体深层水平位移和出水量等现场测试结果进行了分析。试验结果表明:真空联合堆载能有效消除含承压水层软基沉降,地基平均固结度为83%,工后沉降较小,能满足变电所软基工后沉降要求。对地基土体的加固效果进行了室内和现场试验。结果表明:真空联合堆载预压后土体的抗剪强度和承载力显著提高,加固效果十分明显。从单泵出水量和承压水层的压缩量来看,真空联合堆载预压过程中并未贯通承压水地层。加固后软基底部含水率比中部小,表明底部软土层向圆砾层排水加速软基固结。试验的成功进一步拓展了该法的应用领域,对类似地质条件下的真空联合堆载预压设计、施工和科研具有借鉴意义。     

真空预压与堆载预压加固软基的微观效果评价

本文以某高速公路路基软土为研究对象,在地质条件相近的、不同试验路段的软基加固方法（堆载预压、真空预压、真空．堆载联合预压）前后（加固时间达到3个月）,工程单位分别进行了原位试验并采取软土样,开展了室内物理力学试验。结合工程单位所取的软土样进行了软土的微观结构试验,在试验的基础上,获得了软土在天然状态以及3种不同工况下的微结构图像,通过对图像处理得到加固前后的微结构特征参数。研究得出：在工程地质条件相近以及加固时间相同的情况下,无论是从宏观、微观结构,还是从固结过程和强度的增长上来看,3种工况的加固效果依次为：堆载预压→真空预压→真空．堆载联合预压。     

软土地基填海机场堆载预压沉降预测分析方法

软土地基填海机场的沉降预测是机场建设所需重点关注的问题。基于厦门某在建机场,通过研究对比实测沉降曲线拟合法、有限元分析法和一维固结理论解析法三种计算软土地基堆载预压期的沉降量及平均固结度,并对目前机场地基固结度进行评价。结果表明：三种预测方法计算得到的沉降量及固结度差异不大,均能够很好预测机场地基堆载预压效果;三种方法的计算结果与实测沉降曲线较为一致,其结果可以相互比较,以用于研究软土地区沉降发展规律;机场在建区域在最后一级堆载预压荷载作用下的平均固结度超过90%,主固结沉降已基本完成。     

排水板真空堆载预压法在大厚度软基处理中的应用

真空-堆载预压法在处理大厚度淤泥质软基上的公路差异沉降和稳定性问题中应用广泛,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、孔隙水压力进行了分析,认为前期真空预压时沉降主要发生在淤泥层,堆载加压后填土层沉降较大;堆载作用下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。     

真空预压地基卸荷变形特性试验研究

为认识真空预压地基在真空压力卸载中的变形特性,对原状软黏土土样进行了常规三轴仪等向压缩与膨胀试验以及CKC三轴仪压缩膨胀试验,以模拟真空预压地基中各种应力状态下的土体单元在卸除真空压力这种球形压力后的变形特征。研究结果表明,真空压力卸载过程中地基土体既产生弹性的体积膨胀,因偏应力的存在,土体还产生剪切变形,在真空联合堆载预压中,堆载产生的较大偏应力将导致真空压力卸载时地基回弹很小,甚至地基继续沉降。真空压力卸载时,地基的沉降效果是土体膨胀变形和土体的剪切变形的叠加。     

含竖向排水体的软土路基固结分析

对含竖向排水体地基的固结理论进行了分析,研究了加载模式、竖向排水体间距及直径对软土路基固结的影响。结果表明,通过超载预压或者真空堆载联合预压可以较快地使软土固结,有利于减小工后沉降。     

考虑板土相互作用的排水板通水特性试验研究

为研究排水板在实际工况下的通水特性,研制排水板纵向通水量测试新仪器,采用室内真空预压模型试验、堆载预压模型试验和直接充灌淤泥等方法来制作板土单元体(试样),并开展板土单元体(试样)通水能力测试。试验结果表明:无论是直接充灌淤泥法、堆载预压法还是真空预压法,高性能排水板通水量均大于现行规程试验结果,而普通排水板通水量均小于现行规程试验结果,现行规程方法高估了真空预压后普通排水板的通水能力。对比结果表明:高性能排水板在堆载预压后通水能力与真空预压后通水能力接近,而普通排水板在堆载预压后通水能力明显高于真空预压后通水能力。因此,对于变形大且固结时间长的新近吹填淤泥地基加固工程应优先选用高性能排水板。     

Comparative analysis on performance of vertical drain improved clay deposit under vacuum or surcharge loading

This paper presents two well-instrumented large-scale field tests of PVD-improved soft soil with vacuum and surcharge preloading, respectively. The two large-scale field tests were conducted adjacent to each other with the same preload. A comparative analysis was performed to investigate the performance of subsoil (i.e., the ground settlement, the layered settlement, the lateral displacement of subsoil and pore water pressure) under vacuum preloading and equivalent surcharge preloading. Some design methods were verified based on the field data. Cone Penetration Tests (CPT) and Vane Shear Tests (VST) were conducted to assess the improvement effects on subsoil after preloading. The results showed that as compared with surcharge preloading, vacuum preloading mitigated the differential settlement of the ground. The vacuum pressure transmitted into the soil with a minor loss through the PVD length. From a practical point of view, the improvement effects by vacuum preloading and surcharge preloading were similar in terms of influence depth and soil strength based on the in-situ tests.     

Effect of vacuum removal on consolidation settlement under a combined vacuum and surcharge preloading

The combined vacuum and surcharge preloading technique is extensively used to accelerate the consolidation process of subsoils. The effect of vacuum pressure is often considered as a loading/unloading cycle of mean effective stress, such that elastic rebound occurs after vacuum removal, which cannot explain the observed postconstruction settlement in the field. In this study, the stress state of subsoils subject to vacuum and surcharge preloading is analyzed and decomposed into two components: (a) geostatic consolidation at a different depth, and (b) loading/unloading in the minor principal stress direction. A series of consolidated drained triaxial tests is conducted to simulate the soil behaviour after vacuum removal. Results show that the contribution of unloading in the minor principal stress direction outweighs the magnitude of elastic rebound after vacuum removal, and hence continued settlement dominates. A field case for highways is provided to further demonstrate the proposed mechanism.     

Performance of marine clay stabilised with vacuum pressure: Based on Queensland experience

Stabilising soft marine clay and estuarine soils via vacuum preloading has become very popular in Australasia over the past decades because it is a cost-effective and time-efficient approach. In recent times, new land on areas outside but adjacent to existing port amenities, the Fisherman Islands at the Port of Brisbane(POB), was reclaimed to cater for an increase in trade activities. A vacuum preloading method combined with surcharge to stabilise the deep layers of soil was used to enhance the application of prefabricated vertical drains(PVDs). This paper describes the performance of this combined surcharge fill and vacuum system under the embankment and also compares it with a surcharge loading system to demonstrate the benefits of vacuum pressure over conventional fill. The performance of this embankment is also presented in terms of field monitoring data, and the relative performance of the vacuum together with non-vacuum systems is evaluated. An analytical solution to radial consolidation with time-dependent surcharge loading and vacuum pressure is also presented in order to predict the settlement and associated excess pore water pressure(EPWP) of deposits of thick soft clay.     

真空联合堆载预压处理软土地基沉降试验研究

武汉市新区梅子路广泛分布湖积相软土,采用真空联合堆载预压法处理地基。试验段长300 m,分三个试验区共设6个测试断面观测地面和地基深层的变形。结合试验成果,建议真空联合堆载预压地基按堆载条件下附加应力和真空预压条件下负压渗流场分别考虑,然后叠加计算沉降。     

排水板真空度损耗的排水固结解析解

无砂真空预压吹填软土在国内造地工程中广泛应用,但实践表明传统固结解析解已不足以预测其加固性状和加固效果。从固结方程出发,在原有的等应变假设基础上,在定解条件中增加了吹填土特有的高度欠固结特性和无砂真空预压特别明显的真空度损耗,重新推导得到径向和竖向完全协调的新的等应变解析解——JJJ解答,以解决无砂真空预压吹填软土的排水固结设计理论问题。JJJ法作为固结方程的原生解答,兼容以往所有该类解析解的同时,增加了针对高度欠固结土类和排水板真空度损耗严重的排水固结问题的解决能力。经现场和试验验证,JJJ解析解可相当精确预测无砂真空预压吹填软土的复杂固结过程和加固土体超孔压分布和孔压消散分布,该解答同时也适用于堆载和真空预压的工况。