DEM simulation of effect of confining pressure on ballast behaviour

In this paper, an attempt has been made to investigate the influence of confining pressure on deformation and degradation behaviourof railway ballast using the Discrete Element Method (DEM). A novel approach has been employed to model the two dimensionalprojection of field size ballast particles as cluster of bonded particles. Bonded particles are held together by a bond, and debonding isconsidered as particle breakage. A series of cyclic loading simulations using DEM were carried out on an assembly of angular ballast particles at different confining pressures (10 kPa to 240 kPa). The results highlight that the development of axial strain during cyclicloading as a function of initial confining pressure and number of cycles. Very high axial strain and breakage of particles have been observed at low confining pressure (\u3c 30 kPa) owing to dilative volumetric strain behaviour. In terms of particle breakage, there existsan optimum range of confining pressures where breakage is minimal. In addition, the evolution of particle displacement vectors explains the breakage mechanism and associated deformations during cyclic loading

Consolidation Analysis of Soft Ground Improved by Stone Columns Incorporating Foundation Stiffness

© 2020 American Society of Civil Engineers. The consolidation of soft ground improved by stone columns is normally analyzed under equal strain or free strain conditions. In this study a new consolidation model for stone columns is proposed to capture actual field conditions that lie between these two hypotheses, where a permeable foundation layer on top of the unit cell is introduced. By considering the stiffness of this layer, a closed-form solution can be derived, which indicates a considerable difference between the equal strain and free strain conditions. The influence of foundations with varying values of stiffness is examined, and the results demonstrate that as the foundation layer becomes stiffer, the time needed to achieve a 90% degree of consolidation decreases and so does the differential settlement, but the steady stress concentration ratio increases. This is also confirmed by a parametric study carried out under varying dimensionless ratios with respect to soil modulus, column spacing, and permeability. A computational example is provided to show the implications of these results on actual design. Finally, a case study is presented to illustrate that the proposed model is able to provide more realistic predictions of settlement and stress concentrations on top of the unit cell

Biodegradable prefabricated vertical drains: From laboratory to field studies

© 2020 Southeast Asian Geotechnical Society. All rights reserved. Biodegradable prefabricated vertical drains (BPVDs) made from natural fibres have been in use for several decades to improve soft soil, especially in East and Southeast Asia despite the fact that this type of drain has still not been fully addressed and evaluated. This study presents a series of laboratory tests where a drain made from coconut cores wrapped in Indian jute sheath filters is compared to conventional synthetic prefabricated vertical drains (SPVDs). Discharge volume tests are carried out with and without soil clogging to understand how jute drains can resist soil clogging under increasing confining pressure. Along with these macro-hydraulic tests, the influence that the micro-characteristics of natural fibre drains can have on their hydraulic conductivity is also examined using micro-CT scanning and an optical microscopic to capture the micro-details of these drains. This study shows that the porous structure of BPVDs is much more complex than SPVDs, which causes them to have a lower discharge capacity. Unlike SPVDs, micro-properties also play an important role in the hydraulic properties of BPVDs. A pilot project in soft soil at Ballina, Australia, where BPVDs were installed in parallel to SPVDs, was used to evaluate their performance in assisting soil consolidation considering the biodegradation of natural fibres. The identical performance of these two types of PVDs added further evidence to prove how well BPVDs can facilitate soil consolidation

Influence of Salinity-Based Osmotic Suction on the Shear Strength of a Compacted Clay

As most previous studies have neglected the positive influence of salinity (osmotic suction) on most coastal soils in Australia, the design of transport infrastructure involving these soils has often been overly conservative. In this study, a laboratory approach based on direct shear testing was explained to determine the stress–strain behavior of compacted coastal silty clay (CL) at different levels of osmotic suction generated by various salinity (NaCl) concentrations. A broad data set for a total of 147 direct shear tests conducted on remolded and recompacted test specimens at seven different initial matric suction conditions was analyzed to develop a semiempirical model that captures the effect of osmotic suction on the soil shear strength. The results suggested that the greater the initial matric suction, the more pronounced the role of osmotic suction. The proposed semiempirical model was governed by an electrical conductivity relationship with the osmotic suction generated by soil salinity. A new parameter χ2 was introduced to quantify the role of soil salinity in the apparent soil shear strength corresponding to different levels of osmotic suction. When this novel relationship was coupled with the conventional matric suction theory, the overall unsaturated shear strength of a saline soil could be properly evaluated, as proven by the close proximity of the predictions to the measurements

Radial consolidation characteristics of soft undisturbed clay based on large specimens

In recent years, reconstituted small samples have often been used to assess the performance of radial consolidation due to prefabricated vertical drains (PVDs), but the permeability and compressibility of samples of undisturbed soil often differ from those of the remoulded ones. The problem seems more complex in marine environment due to the presence of random coarse particles including gravels, shells and natural partings. Performing small-scale laboratory experiment with reconstituted samples, especially in marine environment, cannot predict the exact soil behaviour in the field. This paper describes an experimental programme that measures radial consolidation using a conventional Rowe cell and a large-scale consolidometer, where the samples of undisturbed soil obtained from a site along the Pacific Highway (north of Sydney) were compared using measured settlements and excess pore pressures. Moreover, this paper highlights the implications of the smear effect and sample size influence, which are imperative in translating the laboratory testing practices to actual real-life behaviour. The effect of vacuum pressure on the coefficient of radial consolidation of a large-scale undisturbed test specimen is also discussed. The paper demonstrates that the extent of smear zone in the field can be very similar to the large-scale laboratory consolidation test using a scaled-down drain and mandrel, but considerably different from the data obtained for small laboratory specimens

松本歯科大学学会会則

© Springer Nature Singapore Pte Ltd 2020. Railway industries are facing greater technical and economic challenges to increase the train speeds and hauling capacity of the trains to cope up with the growing population and increasing traffic problems. However, increasing dynamic stresses from the passage of faster and heavy haul trains progressively degrades the ballast layer and other track substructure layers which inevitably leads to excessive settlement and track instability. Nowadays, heavier concrete sleepers used in most of the ballasted tracks are subjected to even greater stresses and faster deterioration. Under Sleeper Pads (USPs) are resilient pads installed at the bottom face of concrete sleepers to increase the degree of vertical elasticity in the track structure and also with the intention of enhancing sleeper–ballast interaction to minimize dynamic stresses and subsequent track deterioration. In this study, cyclic loads from fast and heavy haul trains have been simulated using a large-scale Process Simulation Prismoidal Triaxial Apparatus (PSPTA) to investigate the performance of ballast improved by the USPs. The laboratory results indicate that the inclusion of USP at the harder interface of concrete sleeper-ballast significantly curtails the stresses transmitted and minimizes the amount of plastic deformation and degradation of ballast