PVD improvement combined with surcharge and vacuum preloading including simulations

This paper presents the study of PVD improved reconstituted specimen with and without vacuum preloading on large-scale consolidometer in the laboratory tests. Subsequently, the results of the laboratory tests were analyzed and simulated by 2D (axisymmetric) finite element method (FEM) to back-analyze and confirm the related design parameters which were used further in subsequent numerical experiments. The laboratory test results indicated that the increased hydraulic conductivity in the smear zone of PVD with vacuum preloading (Vacuum-PVD) resulted in the increase in the coefficient of horizontal consolidation (Ch) by 16% as well as the decrease in the ratio between the horizontal hydraulic conductivity of the undisturbed zone (Kh) to the horizontal hydraulic conductivity in the smear zone (Ks) or (Kh/Ks) of about 10%. The Vacuum-PVD and PVD only have the same settlement magnitudes with similar equivalent loads.     

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.     

Discharge capacity requirement of prefabricated vertical drains

Discharge capacity is one of the most important parameters affecting the performance of prefabricated vertical drains (PVD). An attempt has been made in this paper to develop an equation for the maximum discharge that can flow to a PVD. The required discharge capacity of the PVD was obtained based on the predicted maximum discharge in the PVD. The developed equation could be verified from the field data published in the literature.     

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。     

A simple solution for prefabricated vertical drain with surcharge preloading combined with vacuum consolidation

Prefabricated vertical drains (PVDs) with the surcharge preloading and vacuum consolidation has become considerably popular for ground improvement projects. A simple solution that incorporates the fundamental embankment features, such as the average degree of consolidation and excess pore pressure, are essential for the design of soft ground improvements by PVDs with vacuum preloading. However, most of the solutions for vertical drains with vacuum consolidation require numerical simulations, whose implementation tends to be laborious. In contrast, a simple solution for vacuum consolidation under time-dependent loading has not yet been proposed. In this study, a simple solution that can be easily incorporated into a conventional spreadsheet is derived for PVDs with vacuum preloading by applying the Laplace transform technique. The proposed solution accounts for several actual construction conditions, such as initial surcharge load, vacuum pump trial period, variations of radial permeability, and time-dependent loading. The results obtained from this proposed approach were validated with those from the finite element method and field data from the case study of the Cai Mep International Terminal project in southern Vietnam. The derived solutions, including the excess pore pressures and average degrees of consolidation, were in good agreement with the predicted and observed data.     

Preloading clayey deposit by vacuum pressure with cap-drain: Analyses versus performance

A method of improving soft clayey deposit by combining cap-drain (CPVD) with vacuum pressure is described. The method uses a surface or subsurface soil layer as a sealing layer with no need to place an air-tightening sheet on the ground surface. It is explained that the method has advantages for the following situations: (a) a higher air/water permeability layer at ground surface and (b) combining vacuum pressure with embankment load. A case history of consolidating a reclaimed clayey deposit by combining CPVD with vacuum pressure is analyzed and discussed. The site was in an under-consolidated state before starting the project. It is shown that the method is effective, and the method proposed by Chai et al. [Ground deformation induced by vacuum consolidation. Journal of Geotechnical and Geoenvironmental Engineering, ASCE 131(12), 1552–1561] for calculating the vacuum-pressure-induced ground deformation is useful for the design of the vacuum consolidation project. The back-calculation shows that for this under-consolidated deposit, vacuum pressure caused plane strain type isotropic deformation near the ground surface.     

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.     

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.     

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.     

真空联合堆载预压处理深厚软土效果分析

软土由吹填土和原状淤泥质土构成,平均深度为25m,且表层覆盖回填土层,采用真空联合堆载预压法进行处理。检测结果表明,该场地表层承载力达到100kPa以上,下部淤泥质土层承载力达到80kPa,平均沉降153.14~202.79cm,加固效果非常明显。实践证明,针对该区的深厚淤泥质软土地基,真空联合堆载法具有加固效果好、工期短和施工安全等优点,值得在该地区工程建设中推广应用。     

Effect of pressurization frequency and duration on the consolidation of a dredged soil using air booster vacuum preloading combined with prefabricated horizontal drains

The prefabricated horizontal drain (PHD) clogging problem often results in poor drainage and consolidation effects. To solve this problem, this paper adopts air booster vacuum preloading combined with PHDs to treat dredged soil. Laboratory model tests and microscopic tests are used to investigate the improvement effect of this method on dredged soil consolidation at different pressurization frequency and duration. The test results indicate that pressurization can effectively relieve clogging and improve the consolidation efficiency. The frequent pressurization is conducive to the consolidation at the same air-boost ratio, and the longer pressurization duration has an obvious promoting effect in the later stage of consolidation at the same pressurization frequency. Furthermore, pressurization can significantly alleviate the PHDs clogging problem has also been demonstrated from the micro perspective. These findings could provide an important reference for the treatment of dredged soil by air booster vacuum preloading combined with PHDs.     

Effect of FeCl3-conditioning on consolidation property of sewage sludge and vacuum preloading test with integrated PVDs at the Changan landfill,China

Extremely soft sewage sludge lagoon exists in many landfills of municipal solid wastes in China. A two-staged in-situ treatment method, vacuum preloading on the conditioned sludge combined with subsequent cement solidification treatment, was proposed to improve the sludge ground. In this study, vacuum filtration tests and oedometer tests were firstly performed to investigate the effect of FeCl₃-conditioning on the consolidation performance of sewage sludge. Then a pilot test was carried out in the field to examine the effectiveness of vacuum preloading on the FeCl₃-conditioned sludge by using an integrated type of PVD. The test results demonstrated that the coefficient of consolidation increased with more percentage of FeCl₃ added. For the sludge sample conditioned with 10% of FeCl₃ (dry basis), the coefficient of consolidation increased to 4 × 10⁻⁵–1.2 × 10⁻⁴ cm²/s, which was four-nine times greater than that of the non-conditioned sludge. 68 days' vacuum preloading on the 3.2 m thick sludge pit resulted in a magnitude of 70.9% in the average degree of consolidation and a reduction by 47.5% in the total volume of sludge. The water content of the sludge reduced from the initial 860% to 140%–450%. The undrained shear strength of the consolidated sludge with a water content of 140% was about 10 kPa, and decreased sharply with a further increase in water content. The integrated type of PVDs used in the pilot test showed a good performance with respect to the discharge capacity and the resistance against chemical corrosion. Comparative analyses show that the proposed two-staged in situ treatment method has advantages over the sole cement solidification method with respect to cost saving and environment protection.     

Laboratory tests of electro-osmotic consolidation combined with vacuum preloading on kaolinite using electrokinetic geosynthetics

Laboratory tests were conducted on kaolinite to investigate the effectiveness of electro-osmotic consolidation combined with vacuum preloading using electrokinetic geosynthetics (EKG). The results showed that the combined method could remove more water and induce larger surface ground settlements compared with the traditional vacuum preloading or electro-osmotic consolidation. Vacuum preloading was quite effective during the first 4?h, though the electro-osmotic consolidation took a main role in dewatering process after 9?h. The combined method could also hinder the development of cracks, induce higher negative pore water pressure and hence increase the efficiency of electro-osmotic consolidation. The results showed that deep electro-osmotic consolidation technique combined with vacuum preloading could result in significant water removal efficiency along with shorter electrode length. Furthermore, both electro-osmotic consolidation and the combined method could consolidate the soil efficiently with low energy consumption.     

Fully coupled analysis of consolidation by prefabricated vertical drains with applications of constant strain rate tests: Case studies and an open-source program

The paper proposes a new approach to use measured data of the constant strain rate test (CRST) for analysis of consolidation by prefabricated vertical drains (PVDs). Each PVD has an influence zone that idealised as a unit cell. Consolidation behaviour of a unit cell is studied with an axisymmetric finite element (FE) model based on Biot's theory. From a CRST data, ASTM-D4186 or the back-analysis method is used to obtain stress-dependent parameters for the model. An open-source FE software named CONAXIS was developed for these purposes. Data from two projects in Mekong Delta Vietnam were used in this study. In the first project, nine CRSTs with various depths from a borehole were conducted. Two tests were chosen to be simulated using the proposed approach implemented in CONAXIS and the soft soil model in PLAXIS for validation and comparison purposes. Comparing to the laboratory data, CONAXIS gave more accurate results than PLAXIS. Then CONAXIS was used to calculate the settlement of the ground surface during the construction process with different scenarios. For the second project, six CRSTs from three boreholes were used to set up the model in CONAXIS. Modelled results of both projects showed good agreements with field monitoring data.     

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.     

Architectural quality of the productive fa(c)ades integrating photovoltaic and vertical farming systems: Survey among experts in Singapore

Buildings could play a critical role in energy and food production while making high-density cities more resilient. Productive façades (PFs), as flexible and multi-functional systems integrating photovoltaic (PV) and vertical farming (VF) systems, could contribute to transforming buildings and communities from consumers to producers. This study analyses the architectural quality of the developed PF concept drawing on the findings of a web-survey conducted among experts – building professionals in Singapore. The developed design variants are compared with regards to key design aspects such as façade aesthetics, view from the inside, materialisation, ease of operation, functionality and overall architectural quality. The study also compares and discusses the results of the web-survey with the results of a previously conducted door-to-door survey among the potential users – residents of the Housing &; Development Board (HDB) blocks. The findings confirm an overall acceptance of the PF concept and reveal a need for synergetic collaboration between architects/designers and other building professionals. Based on the defined PF design framework and the results of the two surveys, a series of recommendations and improved PF prototypes are proposed for further assessment and implementation in order to foster their scalability from buildings into communities and cities.     

面向定制化装配式建筑的制造与装配设计策略研究 ——以莲花之居为例

针对建筑师缺乏关于定制化装配式建 筑制造与装配的技术认知与研究前置等问题, 文章旨在通过面向制造与装配的设计（DfMA） 理论探索,建构适用于建筑领域的DfMA策 略。策略研究分实施基础、面向制造的设计 （DfM）和面向装配的设计（DfA）三部分,并 基于莲花之居进行实践验证以形成实施框架, 从而为建筑师提供面向定制化装配式建筑的 DfMA策略指导与设计建造流程参考。     

约束及无约束泵送高性能与超高性能混凝土力学性能试验研究

本文系统地研究了一系列强度等级的无约束和箍筋约束泵送卵石和碎石高性能及超高性能混凝土的多种基本力学性能,包括立方体强度、轴压强度、弹性模量、峰值应变等。并与国内外大量的相关试验结果进行了比较。试验与分析表明:卵石与碎石高性能混凝土的基本力学性能基本相同,对于无约束高性能混凝土,轴压强度与立方体抗压强度的比值、峰值应变以及弹性模量均高于普通混凝土,峰值应变和弹性模量随着混凝土强度的提高而提高;高性能混凝土的应力-应变关系在峰值应力之前接近于直线,塑性变形很小;峰值应力后,高性能混凝土应力-应变关系的下降段很陡,表现出较大的脆性。对于箍筋约束混凝土,其强度和峰值应变均有较大的提高,分别提高20%～35%和32%～66%;其应力-应变曲线下降平缓,表现了较好的延性。与其他文献结果对比表明,本文高性能混凝土延性较好,弹性模量较低。