Creation of functional ground models in an urban area

Abstract

Rationale The ability to see the ground surface in an urban environment is limited by access and the amount of natural ground exposure. This is typically less than 10% by area. The assessment of the probable ground structure and the behaviour of critical ground elements can be achieved by comparison with non-urban environments exhibiting similar landforms. This model can be verified by a detailed study of historic maps and a review of existing local subsurface information. The site The City of Cardiff has largely been constructed in the last 150 years. The area studied relates to the whole city land area lying to the south of the A48T and between the west banks of the rivers Ely and Rhymney. Area model The area lies substantially on the Severn Levels, which comprise a recent tidal coastal margin zone of the Severn estuary. The area consists of a glaciated land surface comprising Mercia Mudstone and Dolomitic Conglomerate draped by a thin and partly discontinuous layer of glacial detritus that includes two identifiable layers of till and a consistent layer of fluvio-glacial sands and gravels which forms the dominant aquifer. These strata reflect Devensian and Flandrian glacial advance and recession. To the south of the historic natural shoreline a series of post-glacial foreshore alluvia are superimposed on the late-glacial surface and form a confining layer to the underlying fluvio-glacial sand and gravel aquifer. These alluvia represent a series of more recent marine transgressions modified by sea defence construction works of Bronze Age to late mediaeval antiquity.