Soft ground improvement by vacuum-assisted preloading

This paper describes the behaviour of soft soil foundation stabilized with vacuum-assisted preloading at the New Bangkok International Airport, Thailand. An analytical solution considering the variation of soil permeability and compressibility and a finite element analysis based on an equivalent plane strain model developed by the authors are employed to investigate the performance of the test embankment. The converted equivalent plane strain parameters are incorporated in the finite element code ABAQUS. The associated settlement, excess pore pressure and lateral movement are predicted and compared with the available field measurement. The data indicate that the efficiency of the prefabricated vertical drains depends on the magnitude and distribution of vacuum pressure as well as on the extent of air leak protection provided in practice. The height of sand surcharge and consolidation time are significantly reduced in comparison with the conventional method of surcharge alone. The effectiveness of this method, its economies and its merit potential are also discussed

Mathematical Modeling and Field Evaluation of Embankment Stabilized with Vertical Drains Incorporating Vacuum Preloading

This study presents the analytical modeling of vertical drains incorporating vacuum preloading in both axisymmetric and plane strain conditions. The effectiveness of vacuum pressure (i.e. both constant vacuum pressure and varied vacuum pressure) applied along the drain is considered. A multidrain plane strain model is employed to analyse an embankment at the site of Second Bangkok International Airport (SBIA) stabilised with prefabricated vertical drains. At this site, a significantly reduced height of sand surcharge was applied by reducing the pore pressures through vacuum preloading. The results of FEM analysis confirm the efficiency of vacuum preloading in comparison with the conventional method of surcharge alone

Performance Appraisal of Ballasted Rail Track Stabilised by Geosynthetic Reinforcement and Shock Mats

Rail tracks serve the principal mode of transportation for bulk freight and passengers in Australia. Ballast is an essential constituent governing the overall stability and performance of rail tracks. However, large repetitive loads from heavy haul and passenger trains often lead to excessive deformation and degradation of the ballast layer, which necessitate frequent and expensive track maintenance works. In Australia, the high cost of track maintenance is often associated with ballast degradation, fouling (e.g. coal and subgrade soil) and associated poor drainage, differential settlement of track, pumping of subgrade soils, and track misalignment due to excessive lateral movements. With increased train speeds, the track capacity is often found to be inadequate unless more resilient tracks are designed to withstand the substantially increased vibration and repeated loads. A field trial was conducted on a section of track in Bulli, New South Wales, and findings indicated that the moderately-graded recycled ballast when used with a geocomposite resulted in smaller deformations in both vertical and lateral directions in comparison to uniformly-graded fresh ballast. Installing resilient (shock) mats in the track substructure led to significant attenuation of high impact forces and thereby mitigated ballast degradation. In addition, a series of full-scale field experiment was undertaken on track sections near Singleton, New South Wales to investigate the effects of geosynthetics on the performance of the track built on subgrade soils with varying stiffness. The finding suggested that geogrids can decrease vertical strains of the ballast layer and a few selected types of geogrids can be used more effectively with soft subgrade soils. This state of the art & practice (SOAP) paper describes the results of two unique full-scale field trials, series of large-scale laboratory tests and numerical models to assess the improved performance of ballasted rail tracks using synthetic grids and shock mats

Analytical and numerical solutions for a single vertical drain including the effects of vacuum preloading

A system of vertical drains combined with vacuum preloading is an effective method to accelerate soil consolidation by promoting radial flow. This study presents the analytical modeling of vertical drains incorporating vacuum preloading in both axisymmetric and plane strain conditions. The effectiveness of the applied vacuum pressure along the drain length is considered. The exact solutions applied on the basis of the unit cell theory are supported by finite element analysis using ABAQUS software. Subsequently, the details of an appropriate matching procedure by transforming permeability and vacuum pressure between axisymmetric and equivalent plane strain conditions is described through analytical and numerical schemes. The effects of the magnitude and distribution of vacuum pressure on soft clay consolidation are examined through average excess pore pressure, consolidation settlement and time analyses. Finally, the practical implications of this study are discussed

An analytical model of PVD-assisted soft ground consolidation

Prefabricated vertical drains (PVDs) are widely used to accelerate the dissipation of excess pore pressure in soft estuarine deposits in coastal areas under fill surcharge or vacuum preloading. Vacuum preloading can also control any lateral outward movement of the embankment toe, although excessive inward movement must be avoided. An equivalent 2D numerical modelling is proposed as a predictive tool for multi-drain conditions, but unlike a 3D simulation it has a greatly reduced computation burden. A unit cell model incorporating key factors such as the smear effect, vacuum distribution, nonlinear compressibility and permeability, soil disturbance and large-strain geometry, has also been developed. This paper presents selected work at the University of Wollongong on analytical solution for PVD-assisted ground improvement. The model is applied to a case history at Tianjin Port in China and then compared with the numerical result and field measurements

Evaluation of smear zone extent surrounding mandrel driven vertical drains using the cavity expansion theory

In this study, an attempt is made to analyse the extent of the smear zone caused by mandrel driven vertical drains, employing the cavity expansion theory for soft clay obeying the modified Cam-clay model. The predictions are verified by large-scale laboratory tests, where the extent of the smear zone was estimated based on the indications such as the pore pressure generated during mandrel driving, change in lateral permeability and the water content reduction. This study reveals that the radius of smear zone is about 4-6 times the equivalent vertical drain radius, and the lateral permeability (inside the smear zone) is 61-92% of that of the outer undisturbed zone. Finally, the predicted size of the smear zone using the undrained cavity expansion solution is incorporated in the finite element code PLAXIS to study the performance of a test embankment selected from the Sunshine Motorway, Queensland, Australia. A good agreement between the predicted values and field measurements was found

Analysis of the behaviour of stone column stabilized soft ground supporting transport infrastructure

This paper presents an analytical and numerical study on the behavior of stone column stabilized soft ground supporting transport infrastructure. Analysis has been carried out on the response of reinforced soft ground under static and cyclic loadings relevant to transport corridors

Characterization of smear zone caused by mandrel action

In this study, the smear zone due to vertical drain installation is studied using a large, in situ sample to capture the realistic characteristics of the smear zone in relation to the in situ soil structure. The smear zone extent for Bulli clay (New South Wales, Australia) is quantified on the basis of normalised permeability and the reduction in the water content prior to consolidation. The permeability and compressibility of the soil are used to determine the extent to which the soil surrounding the PVD had become disturbed. In laboratory testing, the soil consolidation behavior due to a prefabricated vertical drain (PVD) is studied using a large scale consolidometer apparatus

Use of geosynthetics in railways including geocomposites and vertical drains

Australia relies heavily on rail for the transportation of bulk commodities and passenger services, and has introduced faster and heavier trains in recent years due to a growing demand. Large cyclic loading from heavy haul and passenger trains often leads to progressive deterioration of the track. The excessive deformations and degradations of the ballast layer and unacceptable differential settlement or pumping of underlying soft and compressible subgrade soils necessitate frequent costly track maintenance works. A proper understanding of load transfer mechanisms and their effects on track deformations are essential prerequisites for minimising maintenance costs. The reinforcement of the track by means of geosynthetics leads to significant reduction in the downward propagation of stresses and assures more resilient longterm performance. The geocomposite serves the functions of reinforcement, drainage and separation, thereby reducing the vertical and lateral deformations. Stabilization of soft subgrade soils by using prefabricated vertical drains (PVDs) is also essential for improving the overall stability of track and to reduce the differential settlement during the operation of trains. The effectiveness of using geocomposite geosynthetic and PVDs has been observed through field measurements and finite element analyses. These have been the first fully instrumented, comprehensive field trials carried out in Australian Railways, and it was very encouraging to see the field observations matching the numerical predictions

Improved Performance of Ballasted Tracks under Impact Loading by Recycled Rubber Mats

Ballasted tracks at transition locations such as approaches to bridges and road crossings experience increasing degradation and deformation due to dynamic and high impact forces, a key factor that decreases the stability and longevity of railroads. One solution to minimise ballast degradation at the transition zones is using rubber energy absorbing drainage sheets (READS) manufactured from recycled tyres. When placed beneath the ballast layer, READS distributes the load over wider area and attenuate of the load over a longer duration thus decreasing maximum stress, apart from reducing the energy transferred to the ballast and other substructure components. Subsequently, the track substructure experiences less plastic deformation and degradation. These mats also provide an environmentally friendly and cost-effective alternative. In this study, a series of large-scale drop hammer impact tests was carried out to investigate how effectively the READS could attenuate impact loads and help mitigate ballast deformation and degradation. Soft and stiff subgrade were used to investigate the load-deformation response of ballast (with and without READS), subjected to impact loads from a hammer dropped from various heights (hd =100 - 250 mm). Laboratory test results show that the inclusion of READS helps to reduce the dynamic impact load transferred to the ballast layer resulting in significantly less permanent deformation and degradation of ballast, apart from significant attenuation of load magnitude and vibration to the underlying subgrade layers