Abstract

Port Solent is a waterside development of a 36 hectare site at the north end of Portsmouth Harbour occupying what used to be the shallow, impounded Paulsgrove Lake and surrounding derelict land (SU635052). The existing lake level was lowered and approximately 310000 m3 of accumulated silt and other unsuitable material were removed. This exposed a formation of reworked (probably soliflucted) chalk overlying deeply frost-shattered, structureless chalk, becoming more structured with depth. Groundwater was artesian and it was only the cohesive fraction of this material which prevented instability.

A total of 2.2 km of in situ concrete retaining wall was constructed and approximately 600 000 m3 of imported fill placed to a maximum thickness of 5 m for land reclamation behind the wall. Chalk from the nearby Ports Down Hills was chosen as the fill material, from quarries at Warren Farm (SU603072), Downend (SU601066) and Paulsgrove (SU635066). An experimental excavation and embankment were used to formulate the method of working and supported by initial site trials a compaction specification was developed for 450 mm and 600 mm thick layers of chalk (Privett 1990). This ‘reduced effort’ compaction specification preserved the lump structure of the chalk and was designed to allow placing of chalk in adverse conditions where more severe compaction requirements would have caused problems of instability. The chalk was successfully placed on the relatively soft formation with artesian groundwater and throughout the winter season, sometimes in shallow standing water.

The integrity of the chalk fill was monitored by

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