Prefabricated Vertical Drains Research Articles

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.

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.

FIELD PERFORMANCE OF TRANSITION RIGID PILED EMBANKMENT WITH SURCHARGED VERTICAL DRAIN OVER SOFT GROUND

Commonly, thick filling works over soft clay is aided with piled embankment at bridge approach to reduce differential settlement between the piled abutment and embankment over soft ground. However, long stretched thick filling works over soft ground requires substantial resources and time to construct rigid piled embankment. Generally for such long stretched and thick filling development over soft ground, other ground treatment such as prefabricated vertical drain (PVD) with surcharge is introduce to reduce the cost of construction. However, excessive total and differential settlement at the surcharged prefabricated vertical drain (SPVD) and rigid piled embankment intersection during post construction period may leads to rectification works over the time. This paper presents the field performance of an alternative design approach for long stretched thick filled embankment over soft clay at structure approach to control the differential settlement between the two conventional ground treatment approaches. With the introduction of the alternative ground treatment approach at the intersection of the two conventional ground treatment, post construction total settlement and differential settlement is reduced significantly.

Impacts of organic content and pH on consolidation of clayey dredger fill by vacuum preloading method

The vacuum preloading method combining prefabricated vertical drains (PVDs) and surcharge preloading from vacuum pressure is widely used in the improvement of dredger fill foundation. However, a clogging effect arises within the soils around the PVDs during the vacuum preloading, which significantly affects the soil consolidation. Previous studies indicated that the organic matter and soil pH circumstance influence the soil-water interaction and soil microstructure. In order to investigate the influences of organic matter and soil pH on the consolidation of clayey dredger fill, a series of small scale vacuum preloading tests was conducted on soils with different organic contents and soil pH values. Subsequently, Scanning Electron Microscopy observation test and vane shear test were carried out to evaluate the effect of vacuum preloading consolidation. The results showed that a low organic content or a high soil pH is beneficial for soil consolidation by vacuum preloading method, resulting in a more compact microstructure and a greater vane strength. There are two actions on the organic matter during the vacuum preloading consolidation, namely, material migration and organic matter dissolution. The organic matter dissolution effect is predominate at soil pH > 9.1, so that the consolidation effect is improved. However, the material migration effect is predominate at pH = 8.1, clay particles and organic matter can be transported with the water flow and redeposit near the drain pipes, resulting in a clogging effect. Moreover, a higher pH can induce the change of microstructure from flocculated association to deflocculated and dispersed association of clay particles, which is positive to soil consolidation. The results in this paper may offer helpful references for engineering practice.

Analytical solutions to the axisymmetric consolidation of a multi-layer soil system under surcharge combined with vacuum preloading

Surcharge combined with vacuum preloading is a common technique for accelerating the consolidation process in ground improvement. A unit cell model for the axisymmetric consolidation of a soft soil using a prefabricated vertical drain (PVD) under a surcharge, combined with vacuum preloading, is investigated in this study. Based on this model, analytical solutions for a multi-layer soil system are put forward and the explicit expressions for two-layer and one-layer systems are presented. The accuracy of the proposed solution is verified using an analytical solution available in the literature. In the parametric study, the influencing factors on the consolidation process, such as, the smear zone, the PVD spacing, the hydraulic conductivity in the radial direction, the coefficient of vacuum decrease, are taken into account. The water flow in the radial direction plays an important role in the consolidation process while the impact of the vertical flow mainly develops around the interfaces between two adjacent layers. In addition, the proposed analytical solution is applied in a case history with three different layers and the results are reasonable.

Equivalent ‘smear’ effect due to non-uniform consolidation surrounding a PVD

This paper investigated the effect of non-uniform consolidation within a prefabricated vertical drain (PVD) unit cell on the average degree of consolidation through laboratory model tests and finite-element analysis. By considering the permeability (k) changing with the void ratio (e), the results of the finite-element analysis showed that the effect of non-uniform consolidation has a considerable effect on the average degree of consolidation of a PVD unit cell. Further laboratory model tests of a PVD unit cell were conducted, and analyses of the model test results confirmed the effect of non-uniform consolidation. Finally, a method of considering the effect of non-uniform consolidation for the analysis of PVD-induced consolidation was established with an equivalent ‘smear’ effect concept.

ANALISA PENURUNAN TANAH LUNAK DENGAN BEBERAPA METODE KONSOLIDASI PADA PROYEK JALAN TOL PALINDRA

Based on Daily Traffic Average of Palembang - Inderalaya in 2007, traffic volume of 2 ways of Palembang City for traffic capacity had been reach at 19900 vehicles/2 ways/day. Therefore, in order to decrease traffics jamthen government has built Palembang – Inderalaya Highway (Palindra). According to topography of Palembang City, it has lying on lowland areas with soft soils type which has high compressibility. The objectives of this study are to estimate soft soils settlement with various method of consolidation which are oedometer test, pre-fabricated vertical drain (PVD), and vacuum consolidation method (VCM). The results of this study showed the numbers of settlement of soft soils 1,479 m for 17,24 years in oedometer test, 1,342 m for 105 days in PVD with square pattern, and 1,354 m for 90 days in PVD with triangular pattern. Meanwhile, the numbers of settlement with VCM method have 2,250 m for 105 days with PVD square pattern, and 2,251 m for 90 days with PVD triangular pattern. Based on those results, it can be concluded that VCM triangular pattern more effective than others.

Centrifuge modeling of mitigation-soil-foundation-structure interaction on liquefiable ground

Significant progress has been made in recent years toward a better understanding of the liquefaction phenomena. Yet, the combined effects of excess pore pressure generation, permanent soil deformation, and ground shaking, with and without mitigation, on the performance of the soil-foundation-structure system remain poorly understood. Moreover, there is a lack of physical model studies incorporating these important effects for a range of conditions to validate numerical models. This paper presents an experimental study of the performance of 3-story structures with shallow foundations on a saturated soil profile including a thin liquefiable layer. The influence of three different mitigation techniques was evaluated: 1) ground densification; 2) enhanced drainage with prefabricated vertical drains (PVDs); and 3) reinforcement with in-ground structural walls. Densification was observed to slightly reduce excess pore pressures and permanent foundation settlement and tilt, but amplified the demand transferred to the superstructure. Use of PVDs reduced permanent foundation settlement and rotation by reducing the duration of large excess pore pressures, but amplified roof accelerations and flexural drift. The performance of the stiff structural wall depended on the properties of the earthquake motion. During more intense, longer-duration motions, confining the soil and inhibiting flow inside the structural wall led to liquefaction, larger settlements, and larger translational and rotational accelerations on the foundation. In this case, the dissipation of seismic energy through additional foundation movements reduced the moment-rotation demand on the columns. These experimental results emphasize the importance of evaluating the potential tradeoffs of liquefaction mitigation, which may reduce settlement and sometimes tilt, but result in larger transient drifts and damage to the superstructure.

Analytical Modeling of Soft Soil Stabilized by Deep Mixed Columns Combined with Vertical Drains

A combined method involving the use of deep mixed (DM) columns and prefabricated vertical drains (PVDs) was proposed to reinforce the soft soils. To obtain analytical solution of the composite foundation with DM columns and PVDs, the drainage system was converted into a cylindrical drain wall with the equivalent properties, and only the radial drainage was considered. An axi-symmetric unit cell was established and the solution of average degree of consolidation was derived. Based on the proposed solution a design chart was developed and was applied to a case history of an embankment on soft soil with short DM columns and long PVDs. A good agreement was found between the predicted average degree of consolidation and the field data.

Combination of vacuum preloading and lime treatment for improvement of dredged fill

Numerous land reclamation projects that utilise dredged fill are currently being executed in the eastern regions of China to cater to the demands of economic development and address the land shortage problem. However, the dredged fill has high water content, low shear strength, and mainly consists of fine-grained particles. A long period is thus normally required for the treatment of dredge fill by vacuum preloading, and clogging always occurs around the prefabricated vertical drains (PVDs), resulting in insufficient consolidation of the soil. To address this issue, combination of vacuum preloading and lime treatment is proposed in this paper. In this method, a certain percentage of hydrated lime (Ca(OH)2) is added into the dredge fill slurry before vacuum preloading, to enhance the engineering properties of the fill, such as the shear strength and permeability. The added hydrated lime would induce cation exchange on the clay surface and flocculation of the fine soil particles. As a result, this method would significantly mitigate the risk of clogging around the PVDs with the enhanced soil permeability, thereby improving the consolidation efficiency. In this study, dredged slurry with a water content of approximately 187% was utilised. Lime modification optimum (LMO) was first determined using the vacuum preloading method, and a comparison test was then conducted to verify the effectiveness of the proposed method. It was found that, compared to the conventional vacuum preloading method, the proposed method significantly increased the vane shear strength of all the soil layers, especially the deep soil layers, and afforded a higher consolidation rate.

Laboratory determination of smear and transition zones due to prefabricated vertical drain installation

ABSTRACTThe smear zone, which develops during the placement of the prefabricated vertical drain (PVD) in the ground using a steel mandrel, is a significant factor that influences the performance of PVDs. The determination of the width and hydraulic conductivity of smear zone is an important consideration in designing ground improvement by preloading with PVDs. Thus, the extent and hydraulic conductivity of the PVD smear zone have received significant attention; however, there is still uncertainty and the topic remains discrepant among investigators. There is limited or no smear zone hydraulic conductivity data that is directly produced by laboratory or field tests. In this study, a laboratory smear zone model experiment that was developed as a performance test for determining the extent of the smear zone and measuring directly its hydraulic conductivity was used. Based on the findings obtained from two different materials (Craney Island dredgings and Hydrite R kaolinite soil), it is indicated that the disturbed zone is made up of two areas, namely smear zone and transition zone, and that the hydraulic conductivity in these areas is lower than the undisturbed soil. The ratio of the hydraulic conductivity of the smear and transition zones to that of the undisturbed soil was found to vary between 0.32 and 0.50 and between 0.57 and 0.82, respectively in different soils. The diameter of the smear zone was found to be 2.3–3.3 times higher than the equivalent diameter of the mandrel depending on the soil. The diameter of the transition zone extended 5.2–7.3 times the equivalent diameter of mandrel again depending on the soil. These directly measured values, in general, are supportive of the published values based on indirect means and the test device can be used to study the impact of mandrels of different size and shape and other factors by minor modification.

Improved Vacuum Preloading Method for Consolidation of Dredged Clay-Slurry Fill

AbstractAn improved vacuum preloading method was developed to consolidate clay-slurry fill that is hydraulically placed in seawater for land reclamation. This method adopted both a vacuum and an air-pressurizing system. In the vacuum system, a new connector to connect each prefabricated vertical drain (PVD) directly to a horizontal vacuum pipe is used in lieu of a horizontal sand blanket. In the air-pressurizing system, compressed air is injected into the soil to accelerate consolidation. Field pilot tests using both the improved and the existing vacuum preloading systems were carried out at the same site. The testing data showed that better consolidation results were achieved using the improved vacuum preloading method.

Fractal properties of filter membrane for silt clogging evaluation on PVD improved soft clays

Soft clays are characterized by having large fine particle content, and prefabricated vertical drains (PVDs) are used to accelerate the consolidation of soft clays to improve foundation strength. During consolidation, the fine particles move and accumulate around the polyester filter membrane, which causes silt clogging of the PVDs, thereby reducing consolidation efficiency. The microstructure images of the polyester filter membrane before and after clogging were collected by applying electron microscope technology. The computer program processes the micro images, including preprocessing, binary threshold segmentation, pore distribution, and fractal properties analysis. Results show that the new fiber lines are clear, with no junk in the space and a certain sense of hierarchy. The clogged filter membrane is blurred, with black shadow lumps or sand-sized dots. The fractal dimension of clogged filter membrane is increased, particularly in the sand cushion. Thus, analyzing the fractal properties of the filter membrane is proposed as a new quantitative method for silt clogging on PVD-improved soft clays.

Influence of biodegradable natural fibre drains on the radial consolidation of soft soil

Natural prefabricated vertical drains (NPVDs) produced from biodegradable materials such as jute and coir have some distinct advantages over conventional polymeric prefabricated vertical drains (CPVDs). For instance, NPVDs are not only able to discharge excess pore pressure effectively but are also beneficial to the environment thanks to their biodegradability. However, due to the biodegradability of the natural fibres, NPVDs can sometimes deteriorate too quickly in an adverse environment such as in highly acidic clay, which hampers the dissipation of excess pore pressure. In this paper, an analytical solution for radial consolidation that considers the time-dependent decay of drain discharge capacity is therefore proposed. The solution is applied to an exponential form of reduction of the drain discharge capacity, and then verified with the experimental results obtained in previous studies. The effect of drain degradation on soil consolidation is simulated using a finite element method (ABAQUS) and these numerical results are then compared to those obtained from the analytical approach. In addition, a new and flexible method using a matching factor to convert the parameters from axisymmetric to plane strain models is introduced and applied to radial consolidation. The modelling outcomes indicate a significant retardation of excess pore pressure dissipation due to drain degradation and this suggests the need to exercise caution when using biodegradable NPVDs.

Model Test and Theoretical Analysis for Soft Soil Foundations Improved by Prefabricated Vertical Drains

AbstractModel testing for soft soil foundations with and without a prefabricated vertical drain (PVD) was performed to assess the consolidation and long-term performance of PVDs. Surface settlements and excess pore-water pressures at different depths were monitored throughout the applied multistage loading. Classical methods with improved solutions that considered variations in the discharge capacity and non-Darcian flow were applied via a back-analysis of the consolidation results. Parameters related to the smear effect and well resistance were discussed in these solutions. It was found that the foundation improved with PVD had an approximately 20% increase in the total degree of consolidation compared with untreated soft soil foundations; the well resistance of the PVD system not only existed but it also changed over time; the average discharge capacity in the model test under multistage loading condition was far less than the reported value; and the advanced solutions, especially the solution that cons...

Experimental study on the improvement of marine clay slurry by electroosmosis-vacuum preloading

Laboratory tests were conducted on marine clay slurry to investigate the feasibility and effectiveness of a combined electroosmosis-vacuum preloading method in tideland reclamation. This method consists of two main systems: the vacuum preloading system and the electroosmosis technique. In the vacuum preloading system, integrated prefabricated vertical drains (PVDs) are connected to the vacuum pipes directly through a new airtight cap to minimize vacuum loss. In this combined method, the electroosmosis technique is applied towards only the final stage of vacuum preloading, where both the dissipation of excess pore water pressure and the development of settlement have become slow. The test results show that the overall effect of this treatment on marine clay was enhanced significantly when electroosmosis was applied at the final stage of vacuum preloading, and the energy consumption in electroosmosis was only approximately 8% of the energy consumption in vacuum preloading.

Numerical simulation of high-speed railway foundation improved by PVD-DCM method and compared with field measurements

A PVD-DCM method combined with prefabricated vertical drains (PVDs) and deep cement mixing (DCM) piles is presented in this paper based on the field case of Beijing–Shanghai high-speed railway in China. The field performance of soft soil foundation improved by this method is comprehensively analysed, and shows that the PVD-DCM method takes full advantages of PVDs and DCM piles, in which the DCM piles increase the bearing capacity and reduce the total settlement of the soft foundation, while PVDs accelerate the consolidation of the soil by shortening the drainage paths. A plane strain numerical simulation on the field case is then conducted, which is converted by a three-dimensional (3D) problem with equivalent properties and dimensions. The numerical simulation is found to be an effective method to predict the true behaviour of the foundation improved by the PVD-DCM method, such as the load transfer and settlement, with reasonable accuracy. The PVD-DCM method is finally assessed both by PVD method and DCM method in terms of the settlement, implying that the PVD-DCM method can provide an effectiveness of foundation improvement and create a more economical solution.

연직배수재가 부분 관입된 점토지반의 피압에 따른 압밀 거동에 관한 연구

Many researchers reported that artesian pressure exists in thick soft ground of Busan Nakdong river estuary. Artesian pressure in soft ground could affect rate of consolidation, settlement and drainage capasity of prefabricated vertical drain(PVD). This paper investigated consolidation behaviors of soft ground with partially penetrated PVD subjected to artesian pressure. Laboratory tests with 1-dimensional large column equipment and their numerical analyses were carried out. Test results showed that the consolidation settlement of clay ground with artesian pressure was higher than that without artesian pressure. Due to artesian pressure, the dissipation rate of excess pore water pressure was reduced in soft ground with artesian pressure, especially at bottom part of clay ground. Numerical results were in good agreement with experimental test results.

Laboratory Model Test for Heavy Metal-Contaminated Soil Remediation Using PVDs

Laboratory remediation experiments were carried out through the use of Prefabricated Vertical Drains (PVDs) to remediate soil contaminated with heavy metals. In the ground with lower permeability, the extraction of contaminants and the transmission of fluids are difficult; thus, to improve the injection or extraction of surfactants and to overcome difficulties due to micro pores in the soil, PVDs were used. For band-type drains, experiments comparing the effectiveness of filters coated with reactive material with the effectiveness of uncoated filters were conducted, and for cylindrical-type drains, experiments comparing the effects of the injection of surfactants were carried out. The heavy metal contaminants used in the experiments were Cu, Pb, and Cd, and concentrations over time were measured to assess the remedial efficiency. In the experiments, the initial concentrations were determined in consideration of the Korean Soil Contamination Warning Standards. The results from the experiments showed that the removal rates for Cd and Cu were 40% while that for Pb was 35% in the case of extraction alone. In the case of extraction with the application of reactants, the removal rates for Cu and Cd were 60% while that for Pb was 55%. In the case using both the reactant coating and cleansing agent injection, the removal rates were 74%, 65%, and 58% for Cd, Cu, and Pb, respectively.

Laboratory investigation of axisymmetric single vacuum well point

Vacuum well point is a new but faint soft ground treatment method. This work focuses on the consolidation behavior of a reconstituted soft clayey specimen under vacuum well point combined with surcharge loading. The laboratory test was conducted through a vacuum-surcharge consolidation apparatus, and the vacuum loading scheme was adopted for vacuum pressure application to investigate the vacuum effect on soil consolidation. In the testing process, some key parameters such as vacuum pressure, pore water pressure and settlement deformation were timely recorded. Furthermore, the water content, void ratio and permeability coefficient of samples collected after loading were measured to reflect the consolidation characteristics. By comparing with the membrane system and membraneless system, something different was found for the vacuum well point method. The results indicate that the consolidation behavior of an axisymmetric single vacuum well point is almost identical to the behavior of vacuum preloading combined with prefabricated vertical drain (PVD), except for the distribution of the vacuum pressure along the well drain due to the structure of the vacuum well point. And the vacuum well point method may be useful for the improvement of soft clayey deposit in a certain depth.