真空预压联合堆载预压下软土地基工程特性研究

通过真空预压联合堆载预压的现场试验研究了预压过程中地基土的工程性能,监测了地表沉降和孔隙水压力消散规律,依据平均固结度理论,计算了预压完成时场地的平均固结度,采用基于应变形式和应力形式定义的场地平均固结度均在90%以上,表明真空预压联合堆载预压下地基土已完成了大部分的沉降,明显地减少了工后沉降,取得了满意的处理效果。     

机场场道软土地基处理工程的监测分析

根据宁波栎社国际机场三期扩建工程机场场道软土地基处理现场监测及检测结果,对机场场道堆载预压加固施工全过程中地基土体的孔压变化、地表和分层沉降、地表和深层土体水平位移进行了分析。结果表明堆载预压能较好促进场道地基孔隙水压力的消散,提升土体固结度,并能提前大幅消除软基沉降量,减少了工后沉降的风险。研究有效地验证了堆载预压技术的处理效果,能较好地指导软土地基施工,并为同类工程提供参考经验。     

剩余沉降及卸载时间的推算研究

在堆截预压地基处理工程中,有多种推算最终沉降量的方法,却没有理论公式可直接计算在给定的卸载沉降速率时所对应的剩余主固结沉降量和卸载时间。而工后沉降(包括剩余主固结沉降和次固结沉降)和沉降速率是卸载的重要标准,次固结沉降可估算在一定范围之内,工后沉降满足要求与否则主要由剩余主固结沉降所控制。在Asaoka法中,可推算出堆载预压任意时间时的沉降速率。运用沉降速率和剩余主固结沉降的关系,可推算出任意时间的主固结剩余沉降及卸载时间的理论公式,称之为“速率法”。沉降速率的对数和堆载预压时间又成线形关系,从而由图解法也可确定卸载时间。此方法在深港西部通道填海及地基处理工程中得到很好的应用,具有较高的推广和应用价值。     

某机场场道堆载预压与真空预压地基处理对比分析

某机场跑道选用堆载预压,停机坪选用真空预压地基处理。文章通过堆载预压与真空预压处理效果的对比,得出两种地基处理方式均能达到深层软基处理的设计要求,但堆载预压效果略优于真空预压。堆载预压地基处理后的固结度、地基反应模量等指标均略高于真空预压;采用堆载预压地基处理后的区域,工后沉降相对比真空预压地基处理的区域更小,更快趋于稳定。     

潮汕机场软土地基处理试验研究及沉降规律分析

为加速地基沉降,减小工后沉降,结合潮汕机场软土地基处理,在试验区不同位置分别采用塑料排水板堆载预压法、袋装砂井堆载预压法和砂井堆载预压法3种不同的地基处理方法进行研究。同时对软土表面沉降、分层沉降、孔隙水压力等进行监测。监测结果表明,在潮汕机场软土地基处理采用堆载排水固结法明显改善了软土层的排水固结过程,技术可靠。并通过堆载预压试验和计算分析对比,对工后沉降进行了预测分析,提出了适用于潮汕机场软土地基处理沉降估算经验公式。     

真空-堆载联合预压加固软基的特点和应用

介绍了真空-堆载联合预压法的发展概况及特点。结合现场监测数据,对真空-堆载联合预压加固区的地表沉降、分层沉降、孔隙水压力随时间的变化规律进行分析。结果表明：真空堆载联合预压能加快地基沉降的速率,大幅消除地基沉降,减少工后沉降量,是一种行之有效的软土地基处理方法。联合堆载预压阶段,真空预压引起的负的超静水压力与堆载引起的正的孔隙水压力不会抵消,也不是两者简单的叠加。     

两种软路基处理工艺的应用比较

介绍了打设塑料排水板＋真空联合（30 kPa）堆载预压排水固结法和振冲碎石桩复合地基在青岛四方欢乐滨海城软路基处理中的加固原理和设计方案,并通过比较2种处理方案的沉降速率、工后沉降和固结度等数据,得出了预压期和预压荷载对于软路基处理效果至关重要的结论。     

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。     

对工后沉降法确定卸载时机的研究

针对经常将剩余沉降用于卸载时机确定的现象,在分析剩余沉降与工后沉降区别的基础上,提出两者之间的换算公式。工后沉降法中最终沉降应是设计荷载——等载预压下所对应的最终沉降,对于超载预压、欠载预压,应根据监测资料确定沉降与填土高度的关系,从而根据超载预压、欠载预压监测资料推算得到等载预压对应的最终沉降量。对多土层地基,目前预测工后沉降的方法易导致提前卸载和实际工后沉降偏大,因此应分层确定工后沉降,然后累加得到总的工后沉降。     

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。     

真空覆水预压处理房建软基效果评价

针对深厚软基的房建项目在后期使用过程中因仅对主体建筑进行地基处理而引起的差异沉降、甚至主体结构开裂倾斜等病害;采用真空联合覆水预压法对某房建项目之深厚软基进行前期处理,并从理论计算、现场施工、施工监测、工后检测等方面对该工法进行分析,认为真空覆水预压法处理房建项目深厚软基可以消除后期项目运行过程中的差异沉降等问题。该工法处理房建项目深厚软基可满足设计要求、施工方便、工期较短、且可消除以往直接采用桩基处理等方法产生的病害、切实可行、值得推广应用。     

长板–短桩工法处理高速公路软土地基的数值分析

长板–短桩工法综合了排水固结法和水泥土搅拌桩法在加固软土地基工程中的优点,适用于处理高速公路深厚软土地基。以江苏省淮安–盐城高速公路盐城试验段为原型和依据,利用Plaxis软件,针对长板–短桩工法、排水固结法和水泥土搅拌桩法三种工况下的路堤和地基性状进行了二维有限元计算分析。从数值分析的角度研究了长板–短桩复合地基的加固机理和特性,进一步认识了总沉降、分层沉降、侧向位移、潜在滑动面及超孔隙水压力等指标的特征,得到了一些有益的结论。     

Combined effect of PVDs and reinforcement on embankments over rate-sensitive soils

A numerical study of the behavior of geosynthetic-reinforced embankments constructed on soft rate-sensitive soil with and without prefabricated vertical drains (PVDs) is described. The time-dependent stress–strain-strength characteristic of rate-sensitive soil is taken into account using an elasto-viscoplastic constitutive model. The effects of reinforcement stiffness, construction rate, soil viscosity as well as PVD spacing are examined both during and following construction. A sensitivity analysis shows the effect of construction rate and PVD spacing on the short-term and long-term stability of reinforced embankments and the mobilized reinforcement strain. For rate-sensitive soils, the critical period with respect to the stability of the embankment occurs after the end of the construction due to a delayed, creep-induced, build-up of excess pore pressure in the viscous foundation soil. PVDs substantially reduce the effect of creep-induced excess pore pressure, and hence not only allow a faster rate of consolidation but also improve the long-term stability of the reinforced embankment. Furthermore, PVDs work together with geosynthetic reinforcement to minimize the differential settlement and lateral deformation of the foundation. The combined use of the geosynthetic reinforcement and PVDs enhances embankment performance substantially more than the use of either method of soil improvement alone.     

Instability of a geogrid reinforced soil wall on thick soft Shanghai clay with prefabricated vertical drains: A case study

A 7.6 m high geogrid reinforced soil retaining wall (RSW) was constructed at the end of an embankment on very thick, soft Shanghai clay with 12 m deep prefabricated vertical drains (PVDs). The settlement of the ground, the wall movement and pore water pressure were monitored during the construction. From day 118, halfway through the construction, unexpected pore water pressure increment was recorded from the pore water pressure meters installed in the PVD drained zone indicating a possible malfunction of the PVDs due to large deformation in the ground. After the last loading stage, on day 190, a sudden horizontal movement at the toe was observed, followed by an arc shaped crack on the embankment surface at the end of the reinforced backfill zones. The wall was analyzed with a coupled mechanical and hydraulic finite element (FE) model. The analysis considered two scenarios: one with PVDs fully functional, and the second one with PVD failure after day 118 by manually deactivating the PVDs in the FE model. The comparison between the measured and simulated ground settlement, toe movement, and pore water pressure supported the assumption on the malfunction of the PVDs. It is believed that the general sliding failure in the wall was caused by the increase of pore water pressure in the foundation soil and soils in front of the toe. It is suggested that possible failure of PVDs should be considered in the design of such structures, and the discharge rate of the PVDs and the pore water pressure should be closely monitored during the construction of high soil walls on soft soils to update the stability of the structures, especially for grounds where large deformations are expected which may cause the failure of the PVDs.     

深圳机场中心变电站地基处理沉降监测分析

深圳机场110/10KV中心变电站建在填海场地之内,该场区淤泥平均层厚9.5m,具有高含水量、高压缩性、极低强度、欠固结等特点。采用排水板+堆载预压的方法对该软土地基进行处理,对整个施工和预压过程进行了全程监测,监测数据显示地基土性质明显改善,工后沉降满足设计要求。     

真空堆载联合预压加固沿海滩涂软基的效果分析

结合工程实例,对采用真空堆载联合预压法处理大面积沿海滩涂软基的加固效果进行分析和评价。通过表层沉降监测、孔隙水压力监测以及地基施工完成后的十字板剪切试验和浅层平板载荷检测等,得出处理后的软土层沉降量接近1．5m、软土地基物理力学性质变化明显、地基承载力得到大幅提高,表明采用真空堆载联合预压法处理沿海滩涂软基是可行的,具有良好的推广前景。     

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。     

Full-Scale Embankment Consolidation Test using Prefabricated Vertical Thermal Drains

In this study the field feasibility of an innovative thermal technique to improve the performance of prefabricated vertical drains (PVD) used in conjunction with the preloading ground improvement method is investigated. For this purpose, two identical 6.0 m high full-scale test embankments for preloading were constructed over the soft Bangkok clay where a conventional PVD system was installed underneath one embankment and a novel prefabricated vertical thermo-drain (PVTD) system was utilized for the other. The PVTD unit consists of a U-tube made of cross-linked polyethylene plastic (PEX) that is attached to a conventional PVD unit. Preheated water at about 90°C is circulated through the attached U-tube to raise the soil temperature underneath the PVTD embankment. The behavior of the two test embankments were compared in terms of excess pore water pressure and consolidation results. The comparison shows the advantage of a PVTD system over a conventional PVD system. The rate of consolidation increases significantly in the PVTD system due to the temperature effect on the hydraulic conductivity. Moreover, the embankment with the PVTD system generates more settlement due to the thermally induced irreversible contraction of saturated normally consolidated soft Bangkok clay.     

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.