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。     

软土地基超载卸载再加荷的沉降研究

根据4 a多的沉降观测资料,分析了高速公路软土地基超载卸载再加荷的沉降变化规律。结果表明,采用塑料排水板超载预压处理高速公路软土地基,具有较好的效果,不仅使沉降大部分在填筑期和超载预压期间发生,且使卸载后再施工路面结构层时沉降速率仍然呈收敛的状态。超载卸载再加荷产生的沉降对于高速公路工后沉降标准而言不容忽视,其沉降也并非瞬时完成,不宜采用类似计算超固结土沉降时的回弹指数进行计算。对于试验段特定的条件,卸载后施工路面结构层的沉降计算,可采用分层总和法、利用压缩模量计算其主固结沉降,然后乘以大于1的修正系数。     

Improvement of ultra-soft soil using prefabricated vertical drains

A case study of using prefabricated vertical drains (PVDs) to accelerate the consolidation of an ultra-soft fine-grained soil with high moisture content for a land reclamation project is described in this paper. Large-scale laboratory model tests were carried out to assess the suitability of the selected PVD and the effectiveness of the PVD in the consolidation of the ultra-soft soil. The model tests indicate that the discharge capacity of the drain can decrease substantially after the drain has experienced large deformations. To overcome this problem, PVDs were installed in two rounds. The first round was before the application of surcharge, and the second round was after substantial settlements have taken place. Field instrumentations were utilized to monitor the performance of PVDs during consolidation. The monitored settlement and pore water pressure results are presented and discussed. The study shows that it is effective to use PVD for the consolidation of the ultra-soft soil if special care has been taken in selection and installation of PVD and in fill placement to overcome the difficulties involved in the consolidation of ultra-soft soil.     

Inflection point method for predicting settlement of PVD improved soft clay under embankments

Prefabricated vertical drains (PVDs) are being used to accelerate the consolidation of subsoil for construction of high embankments on soft ground. The construction is carried out in stages and the height of each stage construction depends on gain in strength of soft subsoil and target factor of safety. The inflection point method for estimating the degree of consolidation for vertical drainage has previously been published. The degree of consolidation was estimated to be 70% at the inflection point. After 70% consolidation, the rate of consolidation reduces to a minimum value and it is economical and technically feasible to allow the second-stage loading. Additional load at this stage would ensure increase in shear strength of the subsoil. In this paper, the authors have extended the procedure of inflection point method for vertical drainage to a three-dimensional drainage when PVDs are adopted for subsoil improvement. Theoretical graphs have been developed which can estimate the percent consolidation at inflection point (% U_i) for different subsoil thickness, drain spacing ratios and C_r/C_v ratios. Different factors viz. smear, drain spacing, depth of clay thickness and well resistance were also considered in the analysis. The degree of consolidation at inflection point is observed in the range of 61–78%. The inflection point method has been applied to a settlement data from a case history of two sites and the estimated percent consolidation at inflection point has been compared with the values predicted from developed theoretical graphs. The total primary settlement estimated from inflection point has been compared with that of Asaoka method, which is widely used in the field. It is concluded that inflection point method has the potential for field application and provides an alternate method for estimating of total settlement in field applications using PVDs and surcharge, and to determine the appropriate required waiting period for stage loading.     

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。     

双层地基超载预压固结理论及应用

针对未打穿竖井地基传统固结计算方法的不足 ,将深厚软土未打穿竖井地基合理转化为双层地基 ;分析了双层地基超载预压的固结、沉降机理 ,给出了多级等速加、卸载条件下双层地基一维固结孔压解析解。考虑时间因素的影响 ,提出了反映土结构性的固结沉降计算方法。最后 ,结合工程实例计算 ,发现采用超载预压法处理深厚软土地基 ,对于控制工后沉降是不利的 ;工后沉降主要是下卧层的压缩变形造成的 ,约占工后总沉降的 74%左右。     

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。     

竖向排水井地基粘弹性固结解析理论

本文基于Merchant模型分别推导了自由应变条件和等应变条件下打穿软粘土层的竖向排水井地基粘弹性固结问题解析解,同时考虑了井阻和涂抹作用的影响.并将两种形式的解编制成相应的计算程序,据此对粘弹性参数的变化进行了大量的计算,探讨了竖向排水井地基粘弹性固结的一般规律.最后利用本文的解对杭甬高速公路试验段塑料排水板软基沉降进行了计算,结果表明粘弹性固结理论较现有线弹性固结理论更加接近实际.     

地表超载对软、硬地层中既有盾构隧道影响的试验研究

为了探明地表超载对软、硬地层中既有盾构隧道的影响,通过隧道与地层相互作用的模型试验,对地表超载作用下隧道变形、土压力及土体沉降进行了量测。试验结果分析表明,相同的地表超载作用下,软土地层中的隧道横椭圆变形要大于硬土地层中的隧道横椭圆变形。当隧道穿越土层的土体压缩模量较小时,地表超载作用下隧道上覆土层表现为被动土拱土压力;当隧道穿越土层的土体压缩模量较大时则为主动土拱土压力。隧道竖向收敛变形与其穿越土层竖向压缩量之间的关系分析表明,隧道横断面变形刚度与穿越土层的土体压缩模量共同决定隧道上覆土层的沉降状态,从而决定了地表超载对既有盾构隧道的影响。研究成果定性地揭示了软土地区既有盾构隧道在地表超载作用下极易发生变形超限的机理。     

大面积深厚软土地基处理的实用方法

根据广州南沙大面积存在的软弱土层的工程特性,提出了采用搅拌桩和塑料插水板排水固结等方法处理的方案。通过计算确定桩长、堆载计划、地基处理分区和施工要求,既经济合理,又满足了不同的使用要求。     

Analysis of microstructure and spatially dependent permeability of soft soil during consolidation deformation

The fundamental understanding of the consolidation and deformation behaviour of soft soil is of critical importance in engineering geology. The purpose of this study is to develop a nonlinear consolidation model for investigating the deformation behaviour of the montmorillonite soft soil under different engineering loading conditions and the effectiveness of various drainage plate configurations for soil enhancement. The model considers the change of soil microstructure and spatially dependent permeability during the consolidation process. The model predictions were also validated by using collected engineering geology field-testing data and developing advanced imaging processing techniques. The results show that model predictions can reproduce the experimentally observed deformation behaviour of soft soil reasonably well. In addition, the permeability of the soft soil decreases with the decrease of the volume and diameter of pores with the discharge of water from pores during the consolidation process, and there is a negative exponential relationship between pore ratio and depth of the soft soil. Furthermore, it indicates that the vacuum drainage preloading could effectively accelerate the consolidation settlement of the soft soil through playing the role of vertical drainage and reducing the distance of the soil drainage, whereas the increase of the length of the drainage plate has no obvious effect on the settlement of the soil.     

真空-堆载联合预压法在软基处理中的应用

在软土层较厚时,采用真空预压不需要大量的人工堆载,节省工时,缩短预压时间。真空法所产生的负压使地基土的孔隙水加速排出,地下水位下降,可以增加施工期间的沉降量,减少路堤的工后沉降,能保证公路运营后路基的稳定性。     

太仓煤堆场软黏土地基堆载预压处理效果分析

采用塑料排水板堆载预压法对太仓煤堆场吹填土地基进行了加固处理,根据现场监测数据,分析了软土地基在堆载预压条件下的沉降和超静孔隙水压力的变化规律。研究结果表明:实测沉降—时间曲线与采用Logistic模型拟合得到的沉降—时间曲线较为吻合;塑料排水板插板深度范围内的各层地基土均发生了排水固结,而且地面以下占排水板总长度四分之三深度范围内的加固效果最佳。通过反演实测沉降曲线,得到了场地地基土径向固结系数平均值;最后对地基固结性状进行了分析,分析表明,本场地采用塑料排水板堆载预压法加固地基是有效的,研究成果可为后续工程地基处理设计提供可靠的设计依据。     

塑料排水板处理软土地基的离心模型试验研究

采用大型土工离心机对某海上机场工程近海软土地基上堤坝施工期及运行期进行了模拟。原型中采用塑料排水板固结法处理软土地基,模型中则根据固结过程相似的原理,换算成等效圆截面排水体,在模型制作中采用等效透水滤芯来模拟。试验中采用停机加载法模拟分级施工加载过程。根据激光位移传感器试验数据可以计算得出相应原型软土地基的沉降特性。利用试验得到的沉降曲线,采用"经验双曲线法"推算出了地基最终沉降,然后得出按沉降推算的分层地基平均固结度随时间的变化。对比沉降推算的运行5 a时软土地基固结度和理论计算结果,二者基本接近,表明文中塑料排水板的模拟方法用于离心模型试验是可行的。通过与理论计算结果比较,表明试验采用的塑料排水板模拟方法是可行的。     

沿海滩涂软土路基堆载预压沉降变形特征与卸载时机研究

沉降变形与卸载标准是铁路路基堆载预压的主要问题。针对处于沿海滩涂软土路基之上的铁路专用线,采用插塑板与堆载预压相结合的方案进行地基处理,开展了大量的表层沉降、孔隙水压力、分层沉降等观测项目。据实测资料,总结分析了路基表面的沉降规律,并考虑了路基分层沉降与孔隙水压力的变化规律,全面地揭示了整个路基地层的沉降变化特征,运用双曲线合理地预测了工后沉降,且推算出路基整体的固结度,采用工后沉降与地基固结度建立卸载时机判别标准。为堆载预压的卸载时机判别标准研究提供了一种新的思路。     

公路软基超载预压卸荷沉降速率标准研究

软土地基上的路基施工常采用沉降速率法确定超载卸荷时间。现有的沉降速率法主要依据地方经验沉降速率标准,无法与获得的工后沉降相联系。基于分级加荷条件下地基固结度计算的改进太沙基公式,推导出超载沉降速率与等载工后沉降间的关系式,据此建立按容许工后沉降控制的超载预压卸荷沉降速率标准。有限元数值模拟分析表明,相对于地方经验沉降速率标准过于严格,该沉降速率标准卸荷控制的工后沉降满足规范要求。通过应用于实际工程,给出了超载卸荷时间。     

双层砂井地基大变形固结方程差分解

基于砂井地基轴对称大变形固结控制方程,综合考虑双层砂井地基软土的压缩性与渗透性非线性幂函数变化、分级加荷、径向与竖向同时渗流排水、初始超静孔压几种分布模式等实际条件,采用交替方向隐式（ADI）差分法编程求解了建立的双层砂井地基大变形固结控制方程,通过算例分析验证了建立的方程与差分解法及编制程序的合理性。将本文方法应用于围海造陆工程吹填淤泥与海相沉积淤泥组成的双层砂井地基固结计算,预测的固结沉降过程较经典固结理论更接近于实测值,对双层砂井地基固结沉降过程预测进行了有益的探讨。     

公路软基超载预压卸荷时间确定的沉降速率法研究

推导出了超载沉降速率与等载工后沉降之间的关系式。该式表明,超载沉降速率控制标准不仅取决于允许的工后沉降,而且与土的固结性质、各级荷载增量的加荷速率和加荷起讫时间有关。基于分级加荷沉降预测理论,建立了超载卸荷时间快速预报方法。最后通过一工程实例,验证了方法的适用性。     

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.     

公路软基超载预压机理研究

通过不同超载比、不同作用时间的一维固结试验,研究软土在超载作用过程的变形机理。研究表明,超载作用时间过长会使地基的总变形量增加,相对于等载预压过程,超载的作用时间越长、超载比越大,超载卸除后的次压缩变形越小。以提早完成基准期次压缩变形量为目标,过大的超载比是没必要的。工程算例表明,在较长的超载时间情况下,较小的超载比也可以满足工程需要。