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.     

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.     

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。     

Consolidation of a composite foundation with soil–cement columns and prefabricated vertical drains

A new ground improvement method is proposed for embankment foundation on soft soils, involving the use of both soil–cement columns and prefabricated vertical drains (PVDs) to improve the shear strength and accelerate the pre-consolidation process. An analytical solution was derived for calculating the consolidation process of this composite foundation under time-dependent loading by considering the PVDs as cylindrical drain wells. The equivalent coefficient of permeability was acquired by matching the average degree of consolidation of a unit cell model. The analytical expression of consolidation was established according to the axi-symmetric analytical model, and its theoretical solution under time-dependent loading was achieved through the variable separation method. The analytical solution under ramp loading was verified by comparing the calculated results with the three-dimensional finite-element analysis. The influence of replacement ratio of the soil–cement column, column-soil modulus ratio, improvement depth and column-soil permeability ratio were explored. Field experiments on the Huai-Yan Highway indicated the calculated settlements agreed well with the field measurements.     

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.     

CAD on geocomposite vertical strip drains

In recent years, an increasing need has arisen for various types of construction on sites underlain by soft cohesive soils. In such cases, some methods of soil improvement are usually required to provide adequate bearing capacity and tolerable postconstruction total and differential settlements. These goals often may be achieved by precompression or preloading the site prior to construction. Precompression and preloading are frequently used in combination with vertical drains, especially in very thick soft deposits, otherwise the time required for consolidation may be unacceptably long or the instability of the foundation may be a serious concern. Therefore, the vertical geocomposite (jute and coir) drain may be used to accelerate the settlement of soil thereby making the site available for use in the shortest time. The computer program and the design chart for determining the spacing between geocomposite drains are presented. This technique is generally useful for embankment construction, tank foundation, underwater construction and landfill areas. Two projects (Haldia in West Bengal and Vashi Station Complex in Bombay) have utilized the application of geocomposite drains reported in this paper.     

Application of jute geotextiles as facilitator in drainage

For constructions on extremely soft foundation medium, the most common practice is to allow the soft soil to consolidate under the application of surcharge which generally consists of applying the necessary superimposed preload. However, because of low permeability of the in-situ soil, this often becomes a time consuming affair and also large quantities of material may have to be applied in the form of overburden. In some of the very fine grained soils encountered in practice, it may not be feasible to apply the surcharge without the danger of exceeding the bearing capacity of the existing formation soil. In such cases jute geotextiles may actually permit the construction to be carried out successfully and in a cost effective manner. A quantitative study on the efficacy of jute geotextile for consolidation purposes has been made and the outcome seems interesting. A design methodology involving selection and application of jute geotextile drains in weak foundation soil is suggested. Further, comparison of published standards with available properties of jute geotextile reveals that the fabric meets the criterion required for such purposes. Hence it may be judicious to explore the technically feasible, environmentally compatible and economically viable use of jute geotextile, as a suitable drainage medium for dealing with drainage problems encountered in the field.     

Geotextile reinforcement of embankments on peat

The effect of geotextile reinforcement on the stability and deformations of embankments constructed on peat, underlain by a firm base, is examined. Both end of construction and long-term conditions are considered. The effect of geotextile reinforcement is compared with the effect of alternative construction methods involving berms or light weight fill. The combined use of geotextiles with light weight fill is then considered. It is concluded that the use of either geotextile reinforcement or light weight fill may significantly improve embankment performance; however, the use of reinforcement in conjunction with light weight fill may be the most effective means of improving performance in regions where light weight fill is readily available.     

Land reclamation using the horizontal drainage enhanced geotextile sheet method

Increasingly waste materials or soft soil dredged from sea or river have to be used as fill materials for land reclamation. Although preloading using prefabricated vertical drains (PVDs) has been commonly used as the treatment method for soft soil, this method is time consuming as it can only be applied after all the fill materials have been placed. In this paper, a conceptual design for land reclamation using a horizontal drainage enhanced geotextile sheet (HDeGs) method combined with vacuum preloading is proposed. Large-scale model tests are carried out to verify the effectiveness of the HDeGs method. The proposed method is also compared with the existing prefabricated horizontal drain (PHD) method and the advantages and disadvantages of the HDeGs with vacuum preloading method are discussed. This study has demonstrated that the proposed HDeGs method is not only effective, but also more efficient compared with the PVD or PHD methods, as it can reduce substantially the construction time required for land reclamation.     

Effect of reinforcement form on the behaviour of coir geotextile reinforced sand beds

Coir geotextile, a natural geotextile manufactured out of coir fibres, has been recognized as a feasible alternative to geosynthetics for reinforcement applications, due to its longevity and excellent engineering properties. It is best suited for low-cost applications in developing countries due to its availability at low prices compared to its synthetic counterparts. This paper presents the results of plate load test on square model footing resting on sand beds reinforced with coir geotextile in geocell and planar forms. Keeping the characteristics and amount of geotextile the same, the performance of the geocell and planar forms were compared. The results indicate that bearing characteristics clearly depend on the form in which reinforcements are applied. For the same amount of material, coir geocell reinforcement provides better performance compared to planar forms. For a settlement of 15% of foundation width, the maximum improvement in bearing capacity for coir geocell was found to be 7.92 compared to 5.83 in the case of planar forms.     

A case study of geotextile-reinforced embankment on soft ground

Full-scale test embankments, with and without geotextile reinforcement, were constructed on soft Bangkok clay. The performances of these embankments are evaluated and compared with each other on the basis of field measurements and FEM analysis. The analyses of failure mechanisms and the investigations on the embankment stability using undrained conditions were also done to determine the critical embankment height and the corresponding geotextile strain. The high-strength geotextile can reduce the plastic deformation in the underlying foundation soil, increase the collapse height of the embankment on soft ground, and produce a two-step failure mechanism. In this case study, the critical strain in the geotextile corresponding to the primary failure of foundation soils may be taken as 2.5–3% irrespective of the geotextile reinforcement stiffness.     

软弱滩涂上堤基细砂垫层的适用性研究

 结合浙江省平湖市白沙湾—水口围涂一期工程实例及相应原位监测成果,从堤坝工程对砂垫层的渗透性能需求着手,探讨土工织物加筋和细砂垫层应用于软弱滩涂海堤工程的可行性。数值分析结果表明,基于对固结、变形和稳定的综合考虑,在实际选择砂源时,作为水平排水体的砂垫层渗透系数最好大于5×10－3 cm/s;土工加筋对堤基的沉降和孔压影响不大,对提高堤基的整体稳定性作用显著。当细砂垫层由于渗透系数不满足稳定要求时可采用土工织物加筋。实践结果表明,对于场地附近区域缺乏碎石料而砂源充足的软弱滩涂上的堤防工程,采用细砂作为水平排水垫层兼以土工织物加筋是可行的,所得结论和规律可为类似工程提供理论支持,并指导设计和施工。     

Construction of an offshore dike using slurry filled geotextile mats

A study on the use of clay slurry filled geotextile mats to construct dikes for land reclamation at Tianjin Port, China, is presented in this paper. The dike so formed was covered by a thin layer of grouted geotextile mattress for protection. Through laboratory tests, a type of low plasticity clay was chosen to be the fill for the mats. A simple method for estimating the required tensile strength for the geotextile mat and the height of the mat was proposed. A preliminary design for the dike was made. Numerical analysis and centrifuge model tests were conducted to verify the design and assess the stability of the dike before construction. A field trial was also carried out in which a 100 m long and 4.8 m high dike was constructed on soft seabed. The dike has been stable and the settlement has been within the expected limit since the construction was completed in September 2001.     

大型油罐软土地基设计

描述大型油罐软土地基设计方法。考虑建在海洋粉质黏土地基之上的油罐,位于加拿大安大略北部沿James Bay湖边的一个小镇Attawapiskat。2个油罐直径为29m,高12m,为Attawapiskat以西大约100km处的一个钻石矿提供燃料。每一个油罐具有7.5×106L的存储容量,并且发挥140kPa的最大油罐压力。考虑软土处理和席形基础的设计,针对潜在的整体地基剪切破坏和局部地基剪切破坏设计标准要求充分的安全系数,并达到严格的不均匀沉降限制。其他设计考虑包括冻结效应、有限的粗粒和堆石料、短施工时间和燃料溢出保护。稳定性分析表明,油罐边界局部基础剪切破坏模式比整体基础破坏模式更关键,同时,强调对于软土改进的需要。地基处理利用土工合成加筋材料加强油罐外围基础,利用预制竖排水管加速地基固结沉降,并结合预压软土降低工后沉降。设计研究确认在短期内建造大型油罐地基的可行性,但要结合上述的软基处理技术。     

A Simple Method for Calculating Settlement-Time Curves of PVD-Improved Deposits

Soil Mechanics and Foundation Engineering - A simple method has been proposed for calculating the settlement-time curves of deposits improved by prefabricated vertical drains (PVDs). This method...     

应用袋装砂井和土工布处理桥头软土路基

结合广珠东线软土路基施工实践，提出了用袋装砂井排水固结和土工布横向加筋的施工方案，并就其施工工艺作了阐述，取得了较满意的效果。     

Geotextile strain in a full scale reinforced test embankment

A geotextile reinforced test embankment was constructed on a soft organic clayey silt deposit at Sackville, New Brunswick, Canada in September/ October 1989. A relatively high-strength polyester woven geotextile (ultimate strength of 216 kNm⁻¹) was used as reinforcement. The reinforcement was instrumented with a number of electrical resistance, electromechanical and mechanical gauges. The details of this instrumentation and field performance of the geotextile reinforcement during the construction of this test embankment are described in this paper.

The field data indicated that the strain in the geotextile was comparatively small (typically less than about 0·7%) up to an embankment thickness of 3·4 m. The strain increased to a maximum of about 2% when the embankment thickness was increased above 4·1 m, suggesting the initiation of movement (or yielding) of the foundation soil. A large increase of strain was evident for thicknesses exceeding 5·7 m and the available evidence indicates that the soil approached failure at a fill thickness of about 5·7 m. The strain increased to over 8·5% when the embankment was first constructed to 8·2 m thickness and then failed as the soil continued to deform at constant fill thickness and the geotextile strain increased until failure (inferred tearing) of the geotextile occurred. After the embankment failed at a thickness of 8.2 m, the addition of more fill did not result in any     

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.     

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.