Experimental study on the clogging effect of dredged fill surrounding the PVD under vacuum preloading

Clogging effect surrounding prefabricated vertical drains (PVDs) is a typical problem when vacuum preloading is applied to a dredged fill foundation. A large-scale model test was designed to clarify the cause and mechanism of the clogging effect, and the basic physical and mechanical parameters of the soil in the clogging zone were tracked during the test. The results demonstrated that a clogging zone was formed around the PVD in the early stage of improvement with conventional vacuum preloading, and the boundary of the clogging zone was approximately 0.2–0.4 of the boundary radius. The clogging zone surrounding the PVD was formed because of the overall movement of the soil toward the PVD under the high vacuum pressure gradient, rather than fine particle migration. The soil in the clogging zone exhibited permeability anisotropy and equivalent ‘smear’ effect. The permeability ratio (k_h/k_v) was less than 1, and the ratio of horizontal permeability coefficients at the test distances of 45 cm and 10 cm were 9.6 at a depth of 20 cm and 8.9 at a depth of 80 cm. An analysis of the microstructure of the soil in the clogging zone demonstrated that the clay particles tended to be vertically oriented. The re-orientation of the clay particles reduced the horizontal permeability coefficient and led to the permeability anisotropy of the soil in the clogging zone. Thus, decrease in the horizontal permeability coefficient and equivalent ‘smear’ effect of the soil in the clogging zone affect the consolidation of dredged fill, which leads to the clogging effect. The permeability anisotropy also slightly affects consolidation.     

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.     

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.     

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.     

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.     

Analysis of field performance of embankments on soft clay deposit with and without PVD-improvement

This paper presents a case history of the performance of two full-scale test embankments constructed on soft clay deposit in the eastern coastal region of China. One embankment was constructed on natural subsoil and the other was constructed on prefabricated vertical drain (PVD) improved subsoil. The thickness of the soft clay deposit without PVD-improvement was 19 m and with PVD-improved case was 23 m. The PVDs were installed to a depth of 19 m with spacing of 1.5 m in a triangular pattern. Field performance of the two embankments was analyzed using the finite element method. The following influential factors: (i) hydraulic conductivity of subsoil in field and (ii) discharge capacity of PVDs were investigated numerically. The back-analyzed results for the embankment on natural subsoil showed that the hydraulic conductivity ratio (C_f) of field to laboratory values is about 6. To analyse the PVD-improved subsoil, a simple approach using the equivalent vertical hydraulic conductivity of PVD-improved subsoil was employed. The analytical results show that PVDs increased the bulk vertical hydraulic conductivity of soft subsoil by about 30 times compared to the original non-treated subsoil. The discharge capacity of PVDs in this field case is 79–100 m³/a, which is consistent with the findings from laboratory tests and other reported values in literature.     

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.     

Effects of temperature circulation on dredged sludge improved by vacuum preloading

Vacuum preloading is an effective and common method used for clay soil improvement. However, the smear zone generated by the installation of prefabricated vertical drain (PVD) hinders additional efficiency improvements. PVD combined with heat is applied to overcome this problem. This study presents a series of model tests conducted on clayey soil improved by vacuum preloading with different rectangular-wave temperature circulation modes to investigate the effects of cyclic temperature on vacuum consolidation. During the test, the settlement, pore-water pressure, and drainage were monitored. The degree of consolidation was analyzed to evaluate the effectiveness of this method, and the coefficient of energy consumption was used to quantify the energy consumption at different cyclic temperature modes. The results indicated that the rectangular-wave temperature circulation mode of 30–75–30 °C was the most effective. The results of this study contribute substantially to the state of knowledge regarding the cyclic temperature effects on dredged slurry performance subjected to vacuum preloading. Concurrently, a novel approach is introduced for the determination of the optimal soil consolidation.     

Field Performance of PVD Combined with Reinforced Embankment on Peaty Ground

This paper describes the field performance of prefabricated vertical drains (PVD) used in combination with reinforced embankments on peaty ground. A large-scale field test of PVD adopted in combination with an embankment reinforced using galvanized steel grid was conducted on peaty ground in Hokkaido, Japan. Although the site was characterized by extremely soft peaty ground, a stable, high embankment 11.8 m in thickness was successfully constructed through the combination method. A consolidation-accelerating improvement from the PVD and a reinforcement effect from the steel grid were clearly observed, and settlement of the PVD-improved peat layer corresponded roughly with Barron's solution with consideration to well resistance. However, it was necessary to make the coefficient of consolidation nine times as large as that seen in the results of oedometer testing. The surcharge embankment allowed a reduction of the coefficient of secondary consolidation for the overconsolidated peat layer (overconsolidation ratio: approx. 1.13) to approximately 60% that of the normal consolidated layer.     

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。     

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.     

Back-analyses of flow parameters of PVD improved soft Bangkok clay with and without vacuum preloading from settlement data and numerical simulations

Prefabricated vertical drains (PVDs) with embankment preloading (conventional PVDs) and with embankment combined with vacuum preloading (Vacuum-PVDs) are examined using the field data obtained from the site of the Suvarnabhumi Airport, Thailand. The flow parameters were back-analyzed by comparison of measured and predicted or simulated data. The flow parameters were illustrated in terms of the horizontal coefficient of consolidation (C_h) and the ratio between the horizontal hydraulic conductivity in undisturbed zone (k_h) and the horizontal hydraulic conductivity in smear zone (k_s) or (k_h/k_s). Numerical simulations using one-dimensional FEM PVDCON software with equivalent vertical permeability, k_ev, to determine the appropriate C_h and k_h/k_s of PVDs with conventional embankment preloading and with embankment combined with vacuum preloading schemes were made. Furthermore, numerical simulations using axisymmetric FEM by ABAQUS software, incorporating horizontal (k_h) and vertical (k_v) permeabilities, to determine the appropriate k_h/k_s based on back-calculated C_h of conventional PVD and Vacuum-PVD schemes were also done. The Vacuum-PVD scheme indicated faster rate of settlement than conventional PVD scheme by about 1.7–1.8 times with slight reduction of the k_h/k_s ratios. For conventional PVD, it was demonstrated that the increase in k_h/k_s ratios reduced the simulated rate of settlement.     

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。     

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.     

New methods for measuring the installation depth of prefabricated vertical drains

The installation depth of prefabricated vertical drain (PVD) is one of the key factors affecting the outcome of soil improvement when PVDs are used to accelerate the consolidation of soft soil. As PVDs are installed underground, it is difficult to verify the installation depth of PVDs for quality control purpose.     

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.     

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.     

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。     

Experimental investigation on electro-osmotic treatment combined with vacuum preloading for marine clay

An electro-osmotic consolidation (EO) combined with vacuum preloading (VP) was investigated on marine clay using laboratory tests. To improve consolidation efficiency and reduce the settlement difference, a new prefabricated device was designed to combine EO and VP for the tests. The results indicated that the vacuum preloading with intermittent electro-osmotic consolidation (VP–I-ECM) provided more water discharge with higher discharge rate and produced larger soil settlement compared to traditional vacuum preloading and electro-osmotic consolidation. For the combined method, the VP effectively removed water from the soil for the first 12 h, and its efficiency decreased with the time. After 12 h, the intermittent EO was used to further consolidate the soil and maintain a high level of drainage rate. Test results also showed that the combined method of VP-I-ECM significantly improved the shear strength and bearing capacity of the marine clay to satisfy the construction requirements with a significant reduction in the anode erosion and the energy consumption. This research study provides useful information for the design guide and practical application of the combined technique for improving marine clay.