Construction dewatering in Chalk

Abstract

Construction dewatering is often required to control groundwater when excavations are made in Chalk, which is a relatively soft fissured limestone. The rate of groundwater inflow and any associated instability in an excavation are controlled by the hydrogeological setting and the degree of weathering of the chalk. Structureless Chalk or highly weathered structured Chalk may exhibit groundwater-related instability comparable to coarse geotechnical soils such as sands and gravels. Conversely, less weathered structured Chalk may act like massive fissured rock, where large water inflows can occur along pre-existing bedding planes, fissures and other geological features such as sheet flints. The applicability of construction dewatering techniques is strongly influenced by permeability, which can be related indirectly to weathering grade of Chalk. This paper includes data on 49 case histories, from the literature and the authors’ experience, of construction dewatering in Chalk and presents guidance on dewatering methods to be used in different grades of Chalk; this is an update of previous guidance provided by the authors in 1990. Recommendations are given for appropriate ground investigation techniques relevant to construction dewatering, including well pumping tests and dewatering trials.