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©2012 Civil-Comp Ltd |
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D.A. Gunn, H.J. Reeves, J.E. Chambers, P.B. Wilkinson, C.J. Munro, P.I. Meldrum, G.A. Williams, M.J. Raines, E. Haslam, S.J. Holyoake and J. Wragg
British Geological Survey, Nottingham, United Kingdom
Keywords: geophysical, geotechnical, ground model, embankment, earthworks, surface waves, cone penetration test, property distribution, stiffness, resistivity, moisture movement.
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This paper discusses the development of fit-for-purpose property distributions or ground models for rail network planning and system modelling.
Case studies include the effect of engineering interfaces on the propagation velocity of surface waves and the movement of moisture fronts through an aged, Victorian end-tipped embankment.
Simple, half-space or even layered models do not readily accommodate the heterogeneity of superficial geology, artificial and aged engineered ground. Three-dimensional ground models that capture the heterogeneity with resolutions that are fit-for purpose are required, especially to improve the rigour of route corridor assessment and process modelling.
Poor ground conditions can be identified during route planning using network scale ground models based on former geological mapping and site investigation. Base ground models are constructed in platforms such as GSI-3D [1] and capture engineering geological characteristics within a composite digital terrain model. Increasing heterogeneity can be accommodated within the model with additional incorporation of subcrop information, such as from boreholes. Databases that store related borehole and core test property data with locational information enable spatial attribution of geotechnical and geophysical properties to the engineering geological units within a three-dimensional ground model. Synthetic engineering geological sections can be created along any route corridor digitised across the land surface.
The risk of amplification of ground displacements generated by heavy, high speed vehicles is likely where Rayleigh waves propagate from firm to soft soils. In the absence of direct Rayleigh wave velocity measurement, density-effective stress controlled algorithms, specific to each of the engineering geological units, can be applied to model route corridor stiffness, body shear wave and Rayleigh wave velocity-depth sections. Interfaces that are likely to be associated with ground deformation, pumping of fines and differential compaction along the rail route can be identified during the route assessment using Rayleigh wave velocity sections derived from the ground models.
Process modelling and monitoring would benefit from a ground model that best captures the true site heterogeneity of the ground. This can be achieved by using property distributions based upon integrated geotechnical and geophysical ground investigation data as input matrices. Three-dimensional models of small strain stiffness can be constructed from surveys employing continuous surface wave or multi-channel analysis of surface wave methods to support an interpretation of the materials, variability and overall condition along the embankment.
By installing permanent, in situ electrode arrays, groundwater movement within the subsurface can be holistically visualised using a series of three-dimensional time lapse, interpreted resistivity-difference images. These remote systems can monitor the impact of ground water movement on soil moisture, related geotechnical properties (consistency) and surface movement. Recent innovations in time lapse, differential resistivity image processing also support movement tracking of the individual sensors within the monitoring network. Such tools provide far greater insight into the cause and effect relationship between subsurface processes and surface movements.
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- 1
- H. Reeves, H. Kessler, K. Freeborough, M. Lelliot, D.A. Gunn, L.M. Nelder, "Subgrade beneath railways in Manchester", Proc. 8th Int. Conf. Railway Engineering, London, 2005.
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