The project comprised the infilling of a former chalk stone pit for future residential development of over 700 units. At its deepest, the chalk stone pit was up to 52m deep and had direct access (via a tunnel) to an adjacent chalk pit, which is now the site of Bluewater Shopping Centre.
We joined the project team after partial infilling had already taken place. We were required to review the existing filling strategy, compile and propose alternative suggestions for backfilling the pit. Acknowledging the significant depth of the pit we proposed an observational approach with surcharging to accelerate the settlement development.
Acting on behalf of our Client we proactively discussed this approach with the NHBC. Initially, we constructed a trial embankment to provide proof of concept. We installed magnetic extensometers and vibrating wire piezometers beneath the embankment to monitor settlement and pore water pressure response. The results were favourable and indicated settlement was typically complete within 12 weeks and pore water pressure dissipated far quicker. Accordingly, the NHBC approved this methodology.
We monitored the infilling of the pit long term, over a period of 10 years, and the results agreed with those which we encountered during the trial. Following this, our team then undertook ground modelling to predict future long-term creep settlement beneath dwellings.
The alternative methodology of surcharging the infilled material and validating through the use of extensometers allowed conventional earthworks testing to be considerably reduced. This transpired into a significant cost saving for the Client, of circa £150k.
We took a proactive approach of obtaining NHBC agreement in principle at an early stage followed by full warranty approval upon completion of the development platform. Consequently, the value of the land considerably increased at the point of sale.