Engineering challenges building on Grade D chalk in East Anglia

There is very little specific guidance on the design of foundations in Grade D chalk in published guidelines (Lord et al., 2002, CIRIA C574). A recent project in the East Anglia region has included shallow foundations on structureless medium/high density West Melbury Marly Chalk, New Pit and Holywell Nodular Chalk Formations at a site at 50 miles north east of Cambridge near Kings Lynn.

Grade Dc and Dm structureless chalk outcrops at the site. Following the recommendations of CIRIA C574, plate load tests were carried out to confirm the allowable bearing pressure for shallow foundations, expected to be of the order of 225 kPa as indicated in the CIRIA report. Unexpectedly the results of the tests at 0.8 m depth on the Grade D chalk indicated much worse settlement at the proposed bearing pressure than suggested in published data in CIRIA C574, requiring deeper excavation than originally planned. Without the use of plate load tests, it was not possible to identify the correct level for foundation formation based on visual descriptions from trial pits and laboratory testing. Furthermore, uncharted man-made buried obstructions were encountered. The paper discusses the results of the plate load tests and provide the strategy followed during the construction for selecting the correct founding level.