Subsidence hazard in Berkshire in areas underlain by chalk karst

Abstract

Purpose of survey During the last ten years a number of ground subsidence events have occurred in the northwest part of Reading. Many of the subsidence events resulted in structural damage to existing properties (see Plates 1 and 2). On the basis of the properties inspected to date it appears that the local housing has been constructed mostly upon conventional strip footings bearing onto naturally occurring soils. The increasing number of recorded subsidence events is of concern to planners, developers and insurers. Consequently the aim of the survey was to identify the nature and extent of subsidence hazard in the local area. The site The northwest part of Reading, generally referred to as Caversham, is shown in Figure 1. It largely comprises a south to southeasterly dipping land surface, overlooking the River Thames. North of the Thames the land surface is dissected by a NNW-SSE trending valley feature known as Hemdean Bottom. This divides the westerly Caversham Heights area from the easterly Caversham Park and Emmer Green areas. The Thames lies at just below 40 m AOD and northwards the land rises to above 80 m AOD. The floor of Hemdean Bottom generally lies between 40 m and 50 m AOD. Geology The published geological map at 1:10 560 scale (British Geological Survey County Series Berkshire Sheet 29 SE) for this area shows the entire district to be underlain by Cretaceous Upper Chalk, overlain by a Tertiary Reading Beds outlier to the northeast side of Hemdean Bottom.