Twin-tunnelling-induced changes to clay stiffnesses

Tunnels used for transportation in urban environments are often constructed in pairs. Projects in which tunnels are constructed sequentially and within close proximity are referred to as ‘twin tunnelling’. Case studies and recent research indicate that the prediction of settlements for such schemes cannot be determined using existing simple methods derived from consideration of a single tunnel. To establish the reasons for the observed variation in settlements, a series of centrifuge tests was undertaken on various twin-tunnel arrangements in overconsolidated clay. The tests consisted of preformed cavities from which a specific quantity of supporting fluid could be drained, with precision, creating a predetermined magnitude of tunnelling volume loss. Data were obtained for surface and subsurface displacements, changes in pore-water pressure near the tunnels and the support pressure within the tunnels. The systematic use of cavity contraction models was found to be an informative method of explaining the observations. Use of an elastic–perfectly plastic cavity contraction model coupled with observations from the experiments enabled the shear stiffness of the clay around the tunnel to be described. Further analysis demonstrated a reduction in shear stiffness of the soil prior to and during the second tunnel excavation, explaining the increase in volume loss observed in that event

Explanation for twin tunnelling-induced surface settlements by changes in soil stiffness on account of stress history

In this article, a group of representative centrifuge tests were selected for numerical modelling to explain the surface settlements induced by sequential twin tunnelling. Both Modified Cam Clay model (MCC) and Three-Surface Kinematic Hardening model (3-SKH) were adopted in the simulation, which indicated the use of 3-SKH model conduced to mimicking more closely centrifuge model response. Via performing more contrastive numerical analyses with 3-SKH model, the influence of the first tunnel event on the stiffness of the soil around the second tunnel was quantitatively investigated, whereby the mechanism behind the observed surface settlements was finally made clear

Mechanisms for the disaggregation of soil cuttings in slurries

A series of tests has been undertaken on a variety of different soils to understand how soil cuttings disaggregate when pumped from the tunnel face to the separation plant during slurry tunnelling for pipe jacked tunnels. It is important to understand this process to ensure that the separation plant can be optimised to remove the maximum amount of soil from the slurry prior to the reuse of the liquid. In pipe-jacking operations this liquid is normally water, which is recirculated after the soil has been re-moved to minimise and if possible eliminate the production of liquid waste. The paper will present results from a series of “mixing tests”, devised by the authors to investigate disaggregation, and also from standard laboratory tests undertaken to establish basic soil properties such as soil strength. The “mixing tests” simulate the softening of the cuttings due to the presence of water and the shear forces applied to the slurry by the pumps and have al-ready allowed the effect of these factors to be quantified. This series of tests will demonstrate how the mechanism of disaggregation varies with strength resulting in different proportions of disaggregated soil particles in slurries subjected to the same mixing time and shear forces

Aphelenchoides sp. destroying mushroom mycelium

RESP-357

Test development for the investigation of soil disaggregation during slurry tunnelling

Slurry tunnelling uses a water based slurry to aid in tunnel face support and transportation of the excavated material. Unlike other tunnelling methods this requires expensive surface separation plant to remove the excavated ground from the slurry. Incorrect specification of this plant can lead to significant delays and added cost to a tunnel drive. Due to the tight budgets and space constraints this can cause contracts to become unprofitable, in particular where small diameter slurry tunnels are excavated by pipe jacking. For this reason accurate prediction of the size distribution of the soil particles and lumps in the disaggregating slurry is required. This research concerns the development of test procedures and methods of soil classification that will enable improved predictions of the degree to which soils/weak rocks will disaggregate during the slurry tunnelling process

Predictions of changes in pore-water pressure around tunnels in clay

Any underground construction causes changes to the stress state in the ground and this change generally causes the generation of excess pore-water pressures in saturated fine grained soils. Subsequent dissipation of these pressures can lead to settlements and potential damage and hence there is a need to understand and predict these changes in pore-water pressure. Simple plasticity and non-linear elastic solutions have been used to calculate pore-water pressure changes as a tunnel is constructed in clay. These are compared with previous centrifuge tests involving the simulation of tunnel excavation as well as new tests specifically designed to investigate the generation and subsequent dissipation behaviour of excess pore-water pressures. The paper reports on the new tests, presents the findings within the simple plasticity and non-linear elastic analysis framework

Service user suicides and coroner's inquests

This is an Accepted Manuscript of an article published by Taylor & Francis in Criminal Justice Matters on 22nd May 2013, available online: DOI:10.1080/09627251.2013.805375The expansion of victimology in the 1980s produced a more nuanced understanding of victims and victimisation. Yet responses of government, criminal justice agencies, media and general public to victims are predictably and predominantly focused on victims of ‘conventional crime’. We challenge this perspective, thus widening the victimological lens. We discuss the impact of self-inflicted deaths and subsequent coronial inquests on practitioners working on behalf of the state

Stem eelworm attacking carrots

RESP-355

A method for creating larger clay samples with permeability anisotropy for geotechnical centrifuge modelling

Long-term ground movements associated with geotechnical constructions are predominantly caused by the dissipation of excess pore-water pressures and are governed by the permeabilities of both the soil and the geotechnical structure. Natural soil has inherent anisotropy due to the layering and structure as a result of the natural deposition process. A significant factor that influences the rate of consolidation and seepage in natural soils is that the horizontal permeability can be orders of magnitude larger than the vertical permeability. This is often considered in numerical modelling during geotechnical design however, due to the lack of reliable field measurements available, validating these numerical models can be difficult. Geotechnical centrifuge techniques have successfully been used to investigate responses to complex construction events but are, generally, models created from reconstituted soil. This results in models with well-defined but homogeneous properties. There is a fundamental difference between centrifuge models and natural soil deposits. As a result, centrifuge models are better suited to simulating the short-term response of the soil to a construction event. The work presented outlines a procedure for creating large clay models suitable for geotechnical centrifuge testing with a sedimented structure. These models have anisotropy of the horizontal and vertical permeability allowing for more representative soil behaviour (in terms of dissipation of pore-water pressures) which can be used to investigate the long-term movements resulting from geotechnical construction events

Tests of varied sample preparation methods for centrifuge modelling

Centrifuge modelling is an established technique capable of investigating the ground’s response to complex geotechnical events. Centrifuge models are often created from reconstituted soil, with well-defined boundary conditions and known soil parameters. Clay soil models may be prepared by mixing clay powder with distilled water to form a slurry. This slurry is placed within a soil container and subjected to a vertical stress (usually in a consolidation press or consolidated inflight). This creates an isotropic model but there is a fundamental difference between this soil model and naturally occurring soil deposits. The structure and fabric present within a naturally occurring clay is not reproduced by this preparation process. It is well-established that structure and fabric in naturally deposited soils are as significant in their effect on soil behaviour as, for instance, the stress history. Inherent structure and fabric within clay soils creates anisotropy which can vary with depth, this is particularly apparent when considering the permeability. Creating a soil model for centrifuge modelling with representative permeability anisotropy would allow for a better representation of consolidation driven events and the ability to observe long-term behaviour of complex geotechnical events. Currently, there are limited methods of doing so, leading to a considerable gap in knowledge associated with the behaviour of layered ground. This paper describes the development of the equipment and experimental procedure for quantifying the structure developed by different sample preparation techniques for centrifuge modelling