Complex Design and Build Earthworks Rolls On...

Works are progressing ahead of programme at a former quarry site on the outskirts of Exeter in preparation for a 225 home residential development.

VertaseFLI was engaged in late April 2013 by Harrow Estates to undertake extensive enabling works of the former Aggregate Industries quarry at Bishops Court. The client had chosen to let the contract on a ‘Design and Build’ basis, in essence passing the design of the works and the associated risks for any changes onto the contractor. The primary reason for this was the complicated and extensive earthworks required prior to construction could commence.

Following completion of sand extraction at the quarry in the early 1990’s, 300,000m3 of unconsolidated sand fill from a nearby development had been deposited on site at heights of up to 20m above the quarry floor. In order for the development to progress, the sand required levelling and improving in order to provide a development platform suitable to support ground bearing foundations.

Initially the client had suggested undertaking traditional drop weight dynamic compaction following on after the remodelling. Whilst this would have left a suitable, uniform platform, the process is expensive and leaves uncertainties as to the suitability of fill below its 6m influence. There were also doubts about the suitability of the technique in poor weather due to the 10-15% silt content within the sand.

Working with the clients engineer, Peter Brett Associates, and our own geotechnical consultant, Wardell Armstrong, VertaseFLI proposed a different approach to the works that would provide both programme and cost benefits and reduce the overall risk profile of the works.
Instead of attempting to consolidate the fill following completion of re-profiling works, we proposed additional site investigation, followed on by targeted treatment of delineated soft areas through rolling dynamic compaction or replacement will engineered fill. The re-profiling works would then follow on utilising engineered fill verified in real time through the use of a system of nuclear density moisture gauge, traditional geotechnical testing, CPT and skip load tests.

Despite encountering significantly larger soft areas requiring treatment than initially expected, re-profiling works are still progressing ahead of programme, moving approximately 5,000t of sand per day, and are due for completion at the end of July.