1,720,966 research outputs found
The slip surface in the D Zone of the Barton Clay
No full textThe most dominant of the preferred bedding plane surfaces of shearing in the Eocene Barton Clay coastal cliffs of the Hampshire Basin is that near the base of the D Zone. This is utilized as the basal surface of compound landslides virtually throughout its 2.3 km outcrop from near the cliff top to beach level, an elevation change of nearly 30 m. The shear zone is located along a thin seam of dark chocolate brown clay, which has been investigated by X-ray powder diffraction mineralogical analysis, X-ray fluorescence chemical analysis, scanning electron microscope study of the microfabric and ring shear tests. The dark seam is slightly more clay rich and has a slightly lower value of residual shear strength than the ambient D Zone clay. The reason for its preference during landsliding is discussed. The available evidence suggests that although some previous shear displacement by flexural slip during folding may have occurred, the main displacement results from the lateral rebound response to coastal recession, involving a reorientation of any previous clay particle alignment. Lateral rebound initiates progressive failure, which leads to the compound landsliding of the in situ clay slopes
Back-analysis of an embedded retaining wall with a stabilizing base slab
No full text--------------------------------------------------------------------------------A series of finite element analyses has been carried out as part of an investigation into the long-term behaviour of an embedded retaining wall in London Clay supported by a stabilizing base slab. Three different soil models were used: the Brick model, the Schofield model, and an elastic/Mohr–Coulomb plastic model. In the case of the Schofield and elastic/Mohr–Coulomb plastic models, two different soil stiffness profiles were investigated. The wall movements calculated in the finite element analyses depended more on the soil stiffness than on the soil model. With the temporary prop in place, the lateral stresses and wall bending moments calculated depended on both the soil model and the soil stiffness; however, in the long term following placement of the stabilizing base slab, the calculated bending moments depended more on the soil model than on the stiffness. The calculated ground movements depended on both the soil model and the soil stiffness.<br/
Long term performance of an embedded retaining wall with a stabilizing base slab
No full textIn 1993, a field study was commenced to investigate the in-service performance of a bored pile retaining wall constructed with a stabilizing base slab below the carriageway in front of the wall. The wall was founded in London Clay and constructed at South Woodford in London in 1976. Instruments were installed in front of and behind the wall as well as in a free field location to provide an assessment of both the lateral stresses acting on the wall faces and the in situ lateral stress existing prior to construction. Total lateral stresses were measured using push-in spade pressure cells, a Marchetti dilatometer, a self-boring expansion pressuremeter and a self-boring load cell pressuremeter. Pore water pressures were recorded using piezometers incorporated in the spade cells. In this paper, the field measurements are presented and discussed in the light of limit-equilibrium-based calculations typically used in design
Critical train speeds and associated track movements – a case study
No full textAs train speeds are increased, ballasted railway tracks that have previously performed acceptably may experience large movements as a result of what are commonly termed critical velocity effects. These occur when the train speed approaches the speed of surface (Rayleigh) waves in the underlying ground, and can lead to increased rates of track geometry degradation, poor ride quality and in-creased maintenance costs. Critical velocity effects are also a potential concern for new high-speed lines. An improved understanding of the causes of the ground and track movements, through field instrumentation and modelling, will help to identify potentially problematic locations and to develop more cost-effective remediation methods. This paper presents the results of a study of the ability of a semi-analytical model (TGV) to calculate realistic ground movements at train speeds approaching the critical velocity. Several ground geometries and loading cases are considered, and a site on the classic railway network is used as a case study for validation purposes. The track at the study site experienced large displacements following an increase in line speed from 160 km/h to 200 km/h. Geotechnical investigations showed that the site is underlain by a horizon of peat of low stiffness. By refining the parameters used in the model, assuming that the peat horizon was the primary cause of the large track movements at high speed, it was possible to obtain reasonably close agreement between site measurements and the model in both quasi-static and dynamic analyses
The influence of structural response on ballast performance on a high speed railway
No full textTrack for modern ballasted high speed railway lines typically uses continuous welded rail with continuous ballast. Continuous ballast is often specified over features involving discontinuity of track support including structural movement or expansion joints found in long railway bridges. Accelerated degradation of track geometry has been observed at these types of location, resulting in unplanned maintenance. A location was identified on a viaduct of an operational high speed railway, where a reoccurring track defect develops just following a structure expansion joint which was designed for continuous ballast. Trackside monitoring techniques have been used to capture the response of the track in the vicinity of the defect and of the bridge spans on either side of the structure expansion joint under normal operational conditions and to evaluate a typical maintenance process. This gave insight into the performance of the track, demonstrating that the defect was due to voiding and recurring as maintenance was ineffective at filling these voids. Monitoring also provided evidence of bridge behaviour which could have an adverse effect on the ballast over the joint and may be responsible for the original formation of the defect. Evidence from this monitoring has given new insights into the reasons for defect occurrence and recurrence allowing for a more informed approach to specifying maintenance, given the knowledge that there is little that can be done to alter the behaviour of the viaduct structure without major intervention
Modelling and measurements of critical train speed effects and associated track movements
No full textAs train speeds are increased, there is a greater likelihood of encountering critical velocity effects at sites with soft soil. These can lead to increased rates of track geometry degradation, poor ride quality and increased maintenance costs. These phenomena occur when the train speed approaches that of the Rayleigh waves of the underlying ground. A study of critical velocity effects is presented, based on a semi-analytical model of the vibration of the vehicle/track/ground system. Results from this model are compared with measured track deflections obtained at a site on a classic UK mainline at which large track displacements were observed following an increase in line speed from 160 km/h to 200 km/h. Geotechnical investigations have shown that the site is underlain by a layer of peat of low stiffness. By refining the parameters used in the model, assuming that the peat was the primary cause of the large track movements at high speed, it was possible to obtain reasonably close agreement between site measurements and the results from the model in terms of maximum deflections and frequency spectra of rail displacements. As the water table has been observed to be quite high at the site, an additional investigation is presented into the effect of the water level on the critical speed effects. Three levels of saturation are considered for the peat layer, ranging from fully saturated to relatively dry. The results suggest that a reduction in the water levels at the site is unlikely to lead to a reduction in the critical velocity effects
The behaviour of railway level crossings: insights through field monitoring
No full textThe development of reliable methods for measuring deflections as trains pass has enabled valuable insights into railway track behaviour to be gained. This is especially useful for problem areas such as transitions from normal ground onto hard substructures and complex track geometries such as switches and crossings.To date, much of the research on transition zone behaviour has focussed on transitions associated with underbridges and other substructures. Switches and crossings have received some attention and level crossings generally very little. This paper describes and discusses the behaviour of a transition onto a level crossing in the south of England, UK. Measurements are presented from both trackside and on-train instruments. It is found that at this crossing, maintenance constraints have resulted in a group of unsupported or hanging sleepers on the approach to the crossing; and that this fault is not effectively rectified by tamping. Comparisons are also made between the way the fault shows up in measurements from trains of the loaded track profile and data from trackside measurements.<br/
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