Prefabricated Vertical Drain Installation Research Articles

Analytical solutions for consolidation of soft soil improved by air‐boosted vacuum preloading considering clogging effect

AbstractAir‐boosted vacuum preloading is a newly emerging soft soil‐improving technology and increasingly draws attention of researchers and engineers. However, few analytical theories are available in the literature to facilitate the design and calculation of it in practice. To fill this gap, an analytical model for the consolidation of soft ground improved by air‐boosted vacuum preloading is proposed based on the assumption of equal volumetric strain and Darcy's law. Moreover, a time‐dependent clogging effect is taken into consideration to describe the gradual reduction in the drainage capacity of prefabricated vertical drains (PVDs) during the consolidation process. In addition, the smear effect is also considered due to the installation of PVDs. An air booster pipe and the surrounding PVDs are introduced into the unit cell for analysis based on the principle of equal cross‐sectional area, and analytical solutions under two typical time‐dependent horizontal loading are then developed. A comparison is made with the existing analytical model of conventional vacuum consolidation, which indicates that air‐boosted vacuum preloading can promote the consolidation of soft soil more effectively than conventional vacuum preloading. Furthermore, the obtained solution under instantaneously loading is used to investigate the influence of several parameters on consolidation behavior. Finally, to verify the accuracy of the analytical model with or without clogging effect, a comparative study is carried out with a field test.

A large-strain radial consolidation model incorporating soil destructuration and isotache concept

Prefabricated vertical drains (PVDs) are widely applied for soft soil improvement to accelerate the consolidation process. The soil destructuration caused by mandrel-driven PVD installation, the varying strain-rates with time and the occurrence of large-strains in soft soils are commonly encountered, however, there is currently a lack of analytical model that can capture the combined effects of these issues. This study thus presents a semi-analytical solution for large-strain radial consolidation model that incorporates the effects of destructuration and strain-rate based on the isotache concept. The proposed method is compared with previous radial consolidation models through an appropriate case study (i.e., Ballina Bypass trial embankment), where the destructuration and isotache parameters could be determined experimentally. The results show that the current method adopting the isotache concept can predict the consolidation behaviour with improved accuracy, especially in large-strain situations where soil destructuration has been ignored. Furthermore, a parametric study is carried out using the proposed model, which helps clarify when the effects of large strain and varying strain-rate must be considered in practical designs.

ALTERNATIVE IMPROVEMENT OF SUBGRADE AND REINFORCEMENT OF EMBANKMENT ON PELABUHAN KUALA TANJUNG TOLL ROAD, INDERAPURA - KUALA TANJUNG SECTION IN NORTH SUMATRA (STA 0+000 S/D STA 3+500)

Planning the Kuala Tanjung Ruas Port of Inderapura - Kuala Tanjung Toll Road is one of the efforts to support economic growth in North Sumatra. The toll road will be built on a pile with a height of 2 ms / 9 m at STA 0 + 000 to STA 3 + 500. Due to limited land, the toll road section must be built on the poor subgrade. Based on the N-SPT test results, it is known that the subsoil consists of a clay layer with a thickness reaching 36 meters and a layer of sand at the top and bottom with a low N-SPT value. If the soft subgrade is burdened with a high heap, it is estimated that the subgrade will experience landslides, so it is necessary to plan the improvement of the subgrade and reinforcement of the pile.The soil improvement method chosen is the pre-loading method with a combination of the prefabricated vertical drain (PVD). Pre-loading method aims to spend the compression that occurs on the subgrade so that there is no compression during the service life of the road. The combination with PVD is intended to speed up the compression time in thick clay layers. The stability of the embankment will be calculated using a computer-assisted program, and geotextile reinforcement planning will be carried out if the value of the safety factor does not meet the requirements.Based on the analysis of compression computation will be divided into five zones. The amount of compression obtained under various piles of 2 ms / d 9 m was 2.25 m, 1.49 m, 1.75 m, 2.57 m, 3.65 m, 4.3 m, 4.69 m, and 5.06 m with an initial heap height of 5.02 m, 5.26 m, 6.51 m, 8.34 m, 10.41 m, 12.07 m, 13.46 m, and 14.83 m. Compression time required for subgrade is relatively long with a period of more than five years so that the planned acceleration with PVD. To accelerate the compression time to 5-6 weeks, a triangular PVD installation pattern is used with a spacing between PVD of 1.5 m and a depth of PVD installation as thick as a soft soil layer. From the results of the pile stability control, it is predicted that the pile will experience slides so that the reinforcement is installed with geotextile.Rp1.598,213,522,582.08.

Full scale consolidation test on ultra-soft soil improved by prefabricated vertical drains in MAE MOH mine, Thailand

The reclamation of slurry pond with ultra-soft soil deposit using the prefabricated vertical drains (PVDs) with preloading technique in the Mae Moh mine, Lampang, Thailand is a challenging work and is illustrated in this paper. Geotextile reinforcement was used to strengthen bearing capacity of the soil foundation prior to the installation of sand platform. The delay of excess pore water pressure dissipation at the early loading stage occurred despite the occurrence of large settlements was a distinct behavior of the ultra-soft soil. Within the delayed time, the calculated average degree of consolidation based on measured settlements, Us increased while the calculated average degree of consolidation based on measured excess pore water pressures, Ue and the undrained shear strength, Su remained unchanged. Beyond the delayed time, both Ue and Su increased significantly with time and when Us > 90%, the difference between Ue and Us was observed to be small. It was suggested to use Us for approximation of Su when Us > 90% based on the SHANSEP's method. The successful installation and application of PVD to improve ultra-soft soil in this research is applicable for the ground improvement of similar problematic ultra-soft soils in international land reclamation projects.

The Effectiveness of Vacuum Consolidation to Soft Soil Settlement

The main issue of building constructions on soft soils is not a uniform settlement of soft ground, which has a slow process that effected construction failures and rising maintenance expenses. For many decades, to the improvement of ground soil stability, then applied using prefabricated vertical drain (PVD). Along the development of science and technologies, which followed by improvement of the soil stability method, that is called as Vacuum Consolidation Method (VCM). The objectives of this study are to determine how the effectiveness of vacuum consolidation with various patterns to soft soil settlement of constructions, and to find the right spacing of PVD installation due to un-uniform settlement of soft soils. The analysis of this study compared the numbers and rates of settlements in preloading, PVD, and VCM with various patterns, and to get a uniform settlement by two scenarios analysis, which are various spacing with single suction pressure (scenario 1) and single spacing with various suction pressures. Based on the result of analysis which showed the settlement of VCM in both patterns are 2.247 m in 105 days (square pattern) and 2.252 m in 90 days (triangular pattern). It means with a triangular pattern has 70.2 more effective than others in the rate of the settlement period. It also showed the VCM has larger than others in volume settlements. The results of 2 scenarios analysis showed that the implementation of scenario one was difficult due to various spacing, while scenario two can be implemented because numbers of settlements depend on suction pressure of the vacuum.

Analysis of settlement prediction due to preloading and vertical drain applications on runway construction

Consolidation settlement is a general geotechnical problem particularly found in the area where is composed of soft soil. It is caused by the discharge of pore water pressure induced by the increase of stress in the soil mass. Construction of runway above soft soil requires analysis for stability related to the reduction of consolidation settlement and the recovery. This study aims to analyze the settlement comprehensively using empirical methods of Prefabricated Vertical Drains (PVD) and preloading installation. Preloading is a technique by which consolidation of soil can be achieved to a substantial amount before the imposition of actual construction load. According to soil investigation, the characteristic of the soil layer is clay soil, which has the potential to consolidation settlement. The result of the settlement analysis of the taxiway in the research area is from 33 cm to 214 cm. It takes ten years for primary consolidation to reach a 90% degree of consolidation. However, in the Hansbo method of Prefabricated Vertical Drains (PVD) and preloading are applied, with triangular configurations in depth of 11 meters and duration for variation embankment spacing of 1 m is 79 days, 1.5 m is 202 days and 2 m is 390 days. The conclusion of efficient distance of PVD installation and preloading is spacing of 1 m with 79 days for primary consolidation.

Vertical compression of soft clay within PVD-improved zone under vacuum loading: Theoretical and practical study

Reliably estimating soil vertical compression under vacuum loading is still challenging for geotechnical engineers. In this paper, the soil vertical compression within the prefabricated vertical drain (PVD)-improved zone under vacuum pressure was investigated based on the theoretical analysis, reported laboratory tests and the case histories. A theoretical equation was developed to evaluate the effect of lateral displacement on the soil vertical compression. The observed small overall lateral-to-vertical strain ratio at the perimeter of the PVD-improved zone soil showed that the vertical compression of PVD-improved zone was close to one-dimensional (1-D) state, but 1-D compression method yielded smaller predictions than the measurements from the case histories. The back-analyzed soil modulus of the natural soft clay in Shanghai Disneyland Resort project showed considerable degradation as compared to the corresponding constrained modulus from the laboratory test. Such degradation could be due to the soil disturbance caused by PVD installation. The 1-D compression method was modified by introducing two factors to consider the effect of soil lateral displacement and to correct the constrained modulus due to the soil disturbance caused by PVD installation, respectively. The recommendations were provided after applying the proposed method to a case history.

An analytical solution for consolidation of PVD-installed deposit considering nonlinear distribution of hydraulic conductivity and compressibility

PurposeIt is well known that the prefabricated vertical drain (PVD) installation process generates a significant soil disturbance around PVD. This disturbed zone significantly affects the rate of settlement and excess pore pressure dissipation. However, the characteristics of these zones were still uncertain and difficult to quantify; there remains large discrepancy among researchers. This study aims to develop a simple analytical solution for radial consolidation analysis of PVD-installed deposit considering mandrel-induced disturbance.Design/methodology/approachThe proposed solution takes into account the nonlinear distributions of both horizontal hydraulic conductivity and compressibility toward the drain. The proposed solution was applied to analyze field behavior of test embankment in New South Wales, Australia.FindingsBoth effects significantly increased the time required to achieve a certain degree of consolidation. The effect of hydraulic conductivity on the consolidation rate was more significant than the effect of compressibility variation. And, the increased compressibility in the soil-disturbed zone due to mandrel installation significantly increased vertical strain of the PVD-improved soil deposit. The predicted results using the proposed analytical solution were in good agreement with the field measurements.Practical implicationsA geotechnical engineer could use the proposed analytical solution to predict consolidation behavior of drainage-installed ground.Originality/valueConsolidation behavior of PVD-installed ground could be reasonably predicted by using the proposed solution with considering variations of both hydraulic conductivity and compressibility due to PVD installation.

Radial consolidation of PVD-Installed normally consolidated soil with discharge capacity reduction using large-strain theory

The radial consolidation rate of prefabricated vertical drain (PVD)-installed soft deposits is known to be closely related to the PVD discharge capacity, which usually decreases during consolidation. Conventional solutions for radial consolidation of PVD-installed deposits have been developed to consider discharge capacity reduction using small-strain theory, in which the volume compressibility coefficient and soil permeability were assumed to be constant. This paper formulates a general expression for discharge capacity reduction with time in numerical analysis based on large-strain theory. Soil disturbance effects caused by PVD installation, such as a nonlinear distribution for radial hydraulic conductivity, are captured in the proposed solution. The proposed solution was applied to field data from a test embankment at Saga Airport. The proposed solution provides a good result which is close to the measured data.

Finite element modelling of prefabricated vertical drains using 1D drainage elements with attached smear zones

Prefabricated vertical drains (PVDs) are frequently used to accelerate the dissipation of excess pore pressures in low permeability soils subjected to load from overbearing structures, such as embankments. However, it is well known that the installation of PVDs leads to the formation of a smear zone in the surrounding soil, which will have a reduced radial permeability that hinders the flow of pore water towards the PVDs. It is essential that the smear effect be taken into account for estimating the rate of consolidation, but setting up finite element models which capture the geometry of many individual PVDs and smear zones involves building complicated meshes. In this paper, a new simulation tool is introduced which involves the attachment of a smear zone to drainage elements, allowing automatic reduction of the radial permeability at mesh integration points that fall within the smear zone, together with consideration of non-uniform hydraulic properties. This saves the analyst having to set up multiple separate regions within the mesh to capture smear effects, at no expense of accuracy.

1-D finite strain consolidation analysis based on isotach plasticity: Class A and Class C predictions of the Ballina embankment

Trial embankments with prefabricated vertical drains (PVDs) or Jute drains were constructed on the National Soft Soil Field Testing Facility (NFTF) at Ballina, NSW, Australia. The results of extensive laboratory and in situ testing and sophisticated field instrumentation of the soils at the site offer an excellent opportunity to investigate the mechanical behaviour of natural estuarine soft clays. An advanced hierarchical constitutive model based on isotach plasticity, viz., the Hunter Clay (HC) model, was used to characterise key features of this soft clay, including the effects of inter-particle cementation, fabric anisotropy and strain rate dependency of the soil behaviour. Incorporation of the HC model into finite strain consolidation theory leads to a fully coupled hydro-mechanical analysis of natural soft clays. The finite difference method, incorporating an explicit solution scheme, was used to solve numerically the highly non-linear partial differential equation governing this initial value problem. A Class A prediction of the settlement of a PVD-treated embankment was carried out. Comparison of this Class A prediction with field measurements of settlement and excess pore water pressure indicated the need for more accurate characterisation of the effects of PVD installation, particularly in terms of the value of the equivalent permeability that was adopted in a simple one-dimensional consolidation analysis of the embankment problem. A Class C numerical prediction was subsequently conducted, showing the capacity to capture accurately the evolution of settlements with time as well as the dissipation of excess pore pressures at various depths in the clay profile. It was found that the Hunter Clay model was able to successfully characterise the mechanical behaviour of the natural soft clay at Ballina, demonstrating its potential for implementation in numerical methods to solve boundary and initial value problems in geotechnical engineering.

Briefing: Effect of drain installation patterns on rate of consolidation

Prefabricated vertical drains (PVDs) are employed to accelerate consolidation by decreasing the drainage path length. In the present study, using analytical solutions, an attempt was made to evaluate and quantify the effectiveness of two non-conventional PVD installation patterns, involving a parallel drain wall compared with a circular drain ring pattern, in contrast to conventional PVD installation (square or triangular patterns). The governing equations are based on the equal strain theory including the smear effect, and they provide a relative comparison between the two newly proposed installation patterns and the conventional square PVD grid, in terms of both the consolidation time and the equivalent drain spacing. The comparisons between the new and conventional installation patterns are made based on a single drain analysis and the density of PVDs per unit area.

Quality management of prefabricated vertical drain materials in mega land reclamation projects: A case study

The quality management of Prefabricated Vertical Drain (PVD) material installation in ground improvement works in land reclamation projects is a critical task for designers, contractors and clients alike. Only if a good quality management system is established, can the expected performance of the PVD improvement works in the field be ensured. A case study of quality management of PVD materials in the mega Changi East land reclamation Project in the Republic of Singapore is presented in this technical report. The quality management of PVD works consisted of several processes, starting from selection of the type of PVD, properties of the PVD materials and ultimate performance verification in the field. This paper describes selection of PVDs against the comprehensive specification adopted in this project. The paper also describes the selection of the PVD installation rig and accessories based on the in-situ ground conditions. Test results from quality control test laboratories such as tension, permeability and discharge capacity are also presented and discussed. This paper seeks to set a benchmark for material quality management of PVD works in large-scale PVD ground improvement projects.

Investigation of field instrumentation in a preloading project

Preloading with prefabricated vertical drains (PVDs) is one of the most successful methods for improvement of soft soils settlement. A preloading project is described in the southwest of Iran near the Persian Gulf, in which well-planned instrumentation was used. The purpose of the project was placement of an 8 m embankment with installation of PVDs to accelerate the consolidation of subsoil for construction of a concrete water tank, 100 m × 150 m with total volume of 100 000 m3. The thickness of soft soil was about 15 m. This paper presents results of soil instrumentation to measure dissipation of excess pore pressure, differentiating settlement in different soil layers and a comparison of the measurements with predicted settlements and consolidation–time variations according to radial consolidation theory, the results of Settle3D analysis and the Asaoka method. In addition, smear zone effects caused by PVD installation are studied and suitable parameters of soil permeability and smear zone are proposed for future projects. The results clarify that if consolidation parameters are selected adequately based on laboratory tests, the maximum settlement and consolidation–time relations are estimated properly both by conventional theoretical solutions and by more detailed solutions such as Settle3D.

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 1.0MN/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.

Optimum PVD installation depth for two-way drainage deposit

For a two-way drainage deposit under a surcharge load, it is possible to leave a layer adjacent to the bottom drainage boundary without prefabricated vertical drain (PVD) improvement and achieve approximately the same degree of consolidation as a fully penetrated case. This depth is designated as an optimum PVD installation depth. Further, for a two-way drainage deposit under vacuum pressure, if the PVDs are fully penetrated through the deposit, the vacuum pressure will leak through the bottom drainage boundary. In this case, the PVDs have to be partially penetrated, and there is an optimum installation depth. The equations for calculating these optimum installation depths are presented, and the usefulness of the equations is studied by using finite element analysis as well as laboratory model test results.

Detection of smear zone using micro-cone and electrical resistance probe

The installation of a prefabricated vertical drain (PVD) using a mandrel results in soil disturbance adjacent to the PVD, which is known as the smear zone. The smear zone is investigated by a series of radial penetration tests including a micro-cone penetrometer and an electrical resistance probe in the clay specimen prepared in a large-scale chamber. The 5 mm diameter micro-cone penetrometer and the 2.1 mm diameter electrical resistance probe produce continuous soil strength and electrical resistivity profiles around the PVD. Almost-constant values of the cone tip resistance and electrical resistivity are measured within the undisturbed zone. The outer boundary of the smear zone is clearly defined by the locations where the deviations of the measured cone tip resistance and electrical resistivity from the constant values are observed. Significant changes in both measurements are observed in the smear zone with remarkably consistent patterns. The smear zone induced by the PVD installation using a rectangular mandrel has an elliptical shape and extends to approximately 3.2∼3.4 times the half-length of the mandrel and 5.3∼5.4 times the half-width of the mandrel.

Design of prefabricated vertical drains considering soil disturbance

In the present study, the effect of soil disturbance on prefabricated vertical drain (PVD)-enhanced soft soil consolidation was studied using finite element (FE) analysis. Both square and triangular PVD installation arrangements were considered. The actual shapes of the PVD, disturbed zone and unit cell were preserved in the FE analysis. A parametric study for a wide range of possible hydraulic conductivity profiles with a sigmoidal shape indicated that the degree of disturbance in the immediate vicinity of the PVD and the extent of the disturbed zone are the two factors that significantly affect the consolidation rate. Proper evaluation of these factors is therefore necessary for satisfactory PVD design. A comparison was made between the degree of consolidation calculated using an analytical solution assuming a bilinear hydraulic conductivity profile and FE analysis assuming a sigmoidal hydraulic conductivity profile in the disturbed zone surrounding the PVD. The curves of degree of consolidation plotted against time factor that were obtained from these solutions were in reasonably good agreement. Based on the analytical solution, design charts for a variety of conditions and hydraulic conductivity profiles were developed. They can be easily used in practice to determine the required PVD spacing for the desired degree of consolidation and specified time. The design charts can also be used for conditions in which overlapping of disturbed zones occurs.

Experimental Investigation on Effectiveness of a Vertical Drain under Cyclic Loads

This study examines the effectiveness of a prefabricated vertical drain (PVD) installed in reconstituted soft clay, where a large cyclic triaxial device was used to simulate repeated loading representing a rail track environment. The experimental data demonstrate that PVD successfully prevents excessive buildup of pore water pressure during cyclic loading, and that dissipation continues to occur after the load is removed. The findings of this study have direct relevance to rail track environments, e.g., in coastal Australia, where PVD installation beneath rail tracks constructed on low-lying estuarine soils has been recently introduced.

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.