真空预压法中塑料排水板弯曲对固结的影响

 工程实践表明,真空预压法处理超软弱地基时因过大的地基压缩量而使塑料排水板弯曲,导致其纵向通水量减少,进而影响深层土体的加固效果。为降低这种影响,提出二次插板方案,即先对浅层土体进行处理,待其达到一定强度后,再插设较长的塑料排水板对软基进行整体加固。结合浙江省温州丁山垦区围垦造陆的真空预压工程,对不同弯曲率的塑料排水板进行纵向通水量的测试以确定其纵向渗透系数在加固过程中的变化,并基于ADINA有限元软件,开发邓肯–张本构模型,在三维有限元数值模拟中考虑排水板纵向渗透系数的变化和超软弱土中未消散的初始超孔压。分析比较数值计算结果与监测数据,结果表明,二次插板方案所产生的沉降与孔压消散值均比一次插板方案的要大,计算时考虑排水板弯曲对固结的影响比未考虑这种影响的计算值更接近实际值,所得结论可为类似工程的设计和施工提供理论支持。     

Description of partial sandy layers of dredged clay deposit using penetration resistance in installation of prefabricated vertical drains

The dredged soil dumped into a reclamation facility is generally heterogeneous. If the reclamation is executed using hydraulic transportation through pipes, large particles will be deposited around their outlets, and fine particles will be deposited apart from those outlets, resulting in significant grain size segregation. Therefore, ground improvement by applying a preload or vacuum to the dredged soil deposit with prefabricated vertical drains (PVDs) may result in an unexpected differential settlement. In the present study, partial sandy layers in a dredged soil deposit were identified as three-dimensional information using the penetration resistance of the mandrel in the PVD installation, which was recorded as dense information for a wide region. It was clarified that the depth profile of the penetration resistance of the mandrel in the PVD installation was useful for investigating the soil stratigraphy, because it is closely related to the depth profile of the tip resistance in cone penetration tests (CPTU). The relative penetration resistance, defined as the penetration resistance eliminating the data trend that reflects the effects of the overburden stress, shear strength, sleeve friction and buoyance, is useful for identifying the partial sandy layers in a dredged soil deposit. A classification equation was proposed for identifying the partial sandy layers. Firstly, the depth profile without the sandy layer was approximated, and then the threshold value of 1.0 MN/m² was used to identify the partial sandy layer. To verify the availability of this proposed method, the depth profiles were compared with the results of CPTU tests. In addition, the predicted settlement, calculated on the basis of the stratigraphy obtained using the penetration resistance of the PVDs, was compared with the ground surface profile leveled after vacuum consolidation.     

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.     

Preloading using fill surcharge and prefabricated vertical drains for an airport

This paper presents the field measurements and analysis of a preloading project with the installation of prefabricated vertical drains (PVDs) in Wenzhou, China. At the site, PVDs were installed to a depth of 22?m from the ground surface with a spacing of 1.5?m in a triangular pattern. The preloading fill thickness was 6?m with a unit weight of approximately 18 kN/m³. After a total elapsed time of 310 days, approximately 3?m thick fill was removed. The measured preloading settlement was approximately 1.5?m. The measurements and analytical results indicated that the soil layer with PVD improvement reached almost 100% primary consolidation when part of the fill was removed. After partial unloading, the PVD-improved zone was in an over-consolidated state. After the runway was opened for traffic, a settlement increment of approximately 7?mm was monitored over a period of 11 months. Analysis indicated that the settlement was mainly due to the consolidation of soil layers below the PVD-improved zone and post-surcharge secondary consolidation of the PVD-improved zone. The values of the parameters related to PVD improvement were back-estimated from the field measurements. These findings can be used to guide the design of PVDs improvement along the east coast of China.     

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.     

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.     

Probabilistic design of ground improvement by vertical drains for soil of spatially variable coefficient of consolidation

The design of soil consolidation via prefabricated vertical drains (PVDs) has been traditionally carried out deterministically and thus can be misleading due to the ignorance of the uncertainty associated with the inherent variability of soil properties. To treat such uncertainty in the course of design of soil improvement by PVDs, more rational probabilistic methods are necessary. In this paper, a simplified probabilistic method is proposed in which the inherent variability of the coefficient of consolidation, which is the most significant uncertain soil parameter that affects the consolidation process, is considered. An easy-to-use design procedure and charts are provided for routine use by practitioners.     

Deformation characteristics of soil between prefabricated vertical drains under vacuum preloading

The deformation characteristics of soil among prefabricated vertical drains (PVDs) subjected to vacuum pressure are investigated using a model test conducted on dredged slurry. Red iron particles are used to indirectly indicate the lateral displacement of soil under vacuum preloading. Test results showed that, in addition to the settlement of soil between two PVDs, there was also lateral displacement that varied with consolidation time and lateral distance from the PVD because of lateral vacuum suction. The lateral displacement arose successively with the increasing lateral distance. And it increased from zero on the PVD surface and dropped back to zero again at the midpoint between the two PVDs. There should have been a maximum value of the lateral displacement at a point near the PVD. The combined vertical and lateral displacement formed a soil pile around the PVD and showed a ‘V’ shaped soil surface.     

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.     

Prefabricated and electrical vertical drains for consolidation of soft clay

The use of prefabricated vertical drains to consolidate soft clay is a common ground improvement method. In large projects laboratory testing of PVDs for selection and quality assurance is considered important. This paper presents a review of PVD laboratory testing. The need to provide simulated site conditions in the test is emphasized. In addition instrumented PVDs show that installation stresses in deep soft clay deposits could cause filter rupture under tensile failure. It is also shown that the maximum required discharge capacity of a PVD is obtained by equating the flow rate of the PVD under the installation and consolidation states to the maximum rate of volume reduction of the influential clay cylinder of the PVD. Consolidation can be enhanced much faster in clay soils if vertical drains manufactured with conducting polymer are used. Some laboratory tests, field tests and field applications of such electric vertical drains (EVD) are presented. A minimum current density at appropriate applied voltage is required to benefit from the electric osmosis (EO) application. EVD in dewatering clay soils, extracting heavy metals in clay soils and few other geotechnical applications are also presented.     

基于现场监测的软土地基联合处理机理研究

通过现场原型监测对地基处理的机理进行探索,是常用的地基处理研究方法。本文在一高速公路软土地基处理试验段现场监测的基础上,通过对监测结果的分析,研究了水泥土搅拌桩与塑排板预压固结法联合处理的机理。研究结果表明:联合处理方法由于短桩的向下刺入,使桩土应力分担变得均匀,桩土沉降相对协调;附加应力通过短桩的传递和塑排板的存在,可加速下部软土层的排水固结。     

砂土地基中静压桩沉桩及其承载特性室内试验研究

静压沉桩过程中沉桩阻力计算是桩机选型的关键,而目前对沉桩阻力计算方法尚未统一,且沉桩过程中引起的桩周侧压应力分布规律尚不明确。为此,通过室内模型试验模拟砂土地基中静压沉桩,研究沉桩速率和桩长对静压沉桩过程中沉桩阻力、沉桩端阻力、桩周侧压应力和桩体极限承载力的影响。结果表明：沉桩阻力、沉桩端阻力、桩周侧压应力和桩体极限承载力主要与桩长有关,沉桩端阻力与沉桩阻力之比随着沉桩深度增加而减小,但在沉桩深度为12倍桩径位置的沉桩端阻力占沉桩阻力的比例仍高达75%;桩周侧压应力随着埋深增加逐渐趋近于被动土压力,其主要与沉桩对土体挤密作用有关,且增加桩长和降低沉桩速率均可改善沉桩挤密作用;桩周侧压应力存在显著的“退化”现象,并与沉桩过程和桩体回弹有关。此外,基于太沙基极限承载力理论建立了沉桩阻力拟合计算式,并结合朗肯被动土压力理论,提出了桩周侧压应力计算的朗肯被动土压力修正计算公式,可用于静压桩沉桩完成后的桩周侧压应力计算。     

Experimental investigation on resistance and response of jacked model piles in sand

静压沉桩过程中沉桩阻力计算是桩机选型的关键，而目前对沉桩阻力计算方法尚未统一，且沉桩过程中引起的桩周侧压应力分布规律尚不明确。为此，通过室内模型试验模拟砂土地基中静压沉桩，研究沉桩速率和桩长对静压沉桩过程中沉桩阻力、沉桩端阻力、桩周侧压应力和桩体极限承载力的影响。结果表明：沉桩阻力、沉桩端阻力、桩周侧压应力和桩体极限承载力主要与桩长有关，沉桩端阻力与沉桩阻力之比随着沉桩深度增加而减小，但在沉桩深度为12倍桩径位置的沉桩端阻力占沉桩阻力的比例仍高达75%；桩周侧压应力随着埋深增加逐渐趋近于被动土压力，其主要与沉桩对土体挤密作用有关，且增加桩长和降低沉桩速率均可改善沉桩挤密作用；桩周侧压应力存在显著的“退化”现象，并与沉桩过程和桩体回弹有关。此外，基于太沙基极限承载力理论建立了沉桩阻力拟合计算式，并结合朗肯被动土压力理论，提出了桩周侧压应力计算的朗肯被动土压力修正计算公式，可用于静压桩沉桩完成后的桩周侧压应力计算。     

Effectiveness of a Geocomposite-PVD system in preventing subgrade instability and fluidisation under cyclic loading

Instability of subgrade soil sometimes associated with soil fluidisation can lead to uncontrollable deformation and failure at a critical number of loading cycles for a given cyclic deviator stress and frequency. Although numerous laboratory experiments on the performance of Prefabricated Vertical Drains (PVDs) and geocomposites have already been carried out in the past, how effectively this combination can mitigate the potential for subgrade fluidisation under repeated (cyclic) loading is still not properly understood. The primary objective of this paper is to evaluate the integrated role of PVDs and geocomposite in preventing subgrade fluidisation using Dynamic Filtration Apparatus (DFA). Laboratory experiments show that the continuous dissipation of EPWP and the substantial reduction in drainage path lengths by PVDs can prevent subgrade fluidisation at shallow depths, while geocomposite can provide adequate surficial drainage and effective confinement at the ballast/subgrade interface. By measuring the Excess Pore Pressure Gradients (EPPGs) during cyclic loading, the test results convincingly reveal the promising performance of PVD-geocomposite combination under different loading conditions.     

连云港港区深厚淤泥地基筑堤数值分析

连云港徐圩港区淤泥层深厚,土质条件差,拟在淤泥地基上修筑斜坡堤。准确开展筑堤后地基固结预测分析,可优化设计,并为港区后期建设提供技术支撑。采用能较好反映超软淤泥性质的修正剑桥模型,模拟斜坡堤分级施工过程,采用不同涂抹效应参数进行计算,通过与实测结果对比以确定海上排水板施工的合理涂抹参数。计算结果表明,当涂抹区井径比s=5,渗透系数降低比k_h/k_s=5时,原位监测结果与数值模拟结果吻合较好。要满足油气管廊基础对工后沉降的要求,应保障排水板施工质量,同时堤身预压时间不低于13个月。     

Creep analysis of an earth embankment on soft soil deposit with and without PVD improvement

In this paper, an anisotropic creep constitutive model, namely Creep-SCLAY1S is employed to study the installation effects of prefabricated vertical drains (PVDs) on the behavior of a full scale test embankment, namely Haarajoki embankment in Finland. The embankment was constructed on a natural soft soil with PVD installed to improve the drainage under one half of it. The Creep constitutive model used in this study, incorporates the effects of fabric anisotropy, structure and time within a critical state based framework. For comparison, the isotropic modified Cam clay (MCC) model and the rate-independent anisotropic S-CLAY1S model are also used for the analyses. The numerical predictions are compared with field measurements and the results indicate that the creep model provides an improved approximation of field settlements, and excess pore pressure build-up and dissipations. In addition, the application of two commonly used permeability matching techniques for two dimensional (2D) plane-strain analysis of the PVD problem is studied and the results are discussed highlighting their limitations and advantages.     

静力压桩临界深度和最小厚度探讨

根据土塑性力学基本原理 ,用空穴球形扩张理论推得了沉桩时沉桩阻力沿深度变化的理论计算方法 ,对有软弱下卧层的桩基持力层、桩尖离下卧层顶面应留的最小厚度 ,无软弱下卧层的桩基持力层、桩尖进入持力层的临界深度 ,以及最小厚度或临界深度在不同土层或土层组合时的差别等三个问题进行了讨论。计算结果与工程实测结果对比 ,二者比较接近。还就现行规范中桩基进入持力层深度或离软弱下卧层距离大小的规定进行了讨论。     

The effect of geotextile reinforcement and prefabricated vertical drains on the stability and settlement of embankments

The construction of four dikes on deep strata of very soft clay has required the application of several measures to improve the performance of the foundation, such as very wide berms, basal geotextile reinforcement and prefabricated vertical drains (PVDs). In order to control the rate of construction, the foundation and the dikes have been monitored with settlement plates, topographic stakes, inclinometers and piezometers. The use of back-analysis has allowed finding the adequate material model, the smearing of drains and the coefficient of secondary compression necessary to attain a good agreement between the measurements supplied by the instrumentation and the calculated values obtained with an elastic-viscoplastic (EVP) finite element (FE) program. Both the geotextile reinforcement and the PVDs produce an important increase in the safety factor (SF). The PVDs produce a significant acceleration in settlements, but the influence of the geotextile in the settlements is negligible. The combined use of the geosynthetic reinforcement and PVDs enhances embankment performance substantially more than the use of either method of soil improvement alone. The importance of flow in the results has been established.     

考虑砂土渗透性变化的吸力锚沉贯及土塞特性研究

吸力锚负压沉贯过程中锚内部常常产生土塞现象,这将阻碍锚体的进一步沉贯,影响吸力锚在海床土体中的安装深度,最终导致吸力锚承载力的降低,本研究通过1g条件下室内模型试验研究土塞的形成机理。通过试验模拟吸力锚水下砂土中吸力沉贯过程,借助高分辨率相机记录锚体整个沉贯过程,同时利用微型孔压传感器测量锚内外负压值随沉贯深度变化情况。试验过程中发现吸力锚内部土柱在向上渗流作用下发生细颗粒迁移现象,且随着沉贯深度的增大,土塞隆起的高度逐渐增加。提出土塞的形成是由于吸力锚内部土体膨胀引起的,进而引起渗透系数的增加。本研究分析了渗透系数在锚体沉贯过程中的变化情况,并对Houlsby和Byrne提出的吸力锚砂土中沉贯吸力计算模型进行了改进,从而获得更加准确的沉贯吸力随沉贯深度变化关系理论计算方法,为吸力锚的工程设计和施工提供理论依据。