Site name  Salisbury Road, Hoddesden

Description  Residential Development

Details  Piling Issues/Ground Instability

Role  Geotechnical Consultants

The proposed development comprises the construction of two, multi-storey apartment blocks with basements and associated areas of hardstandings and landscaped areas.  Due to high structural loads associated with the proposed development an augured piled foundation solution was adopted with piles extending into the underlying Upper Chalk strata.  At the time of the supplementary investigation the site was a construction site which had been cleared and demolished.  A piling mat had been constructed and piling operations had commenced with a total of nine piles being installed.

The first piles installed to approximately 20m depth suffered significant concrete loss during pumping of the concrete to the point where it was not possible to fill the pile cavity.  The concrete level dropped by typically 3m to 4m and in one pile dropped out of sight with the cages becoming exposed.  Test piles to 8m did not record any concrete loss.  A further test pile was extended to 15m and some concrete loss was recorded.

On review of the available information ASL considered the issue of concrete losses was associated with the deeper ground conditions at the site.  ASL subsequently commissioned to investigate the reasons for the loss of concrete during piling.  A series of rotary cored boreholes were constructed across the site to the depth of the

The rotary boreholes were positioned in the vicinity of existing pile locations to establish the underlying ground conditions and to determine possible reasons for the concrete loss observed during pile installation.  In addition a probing exercise was completed across the site to check for the presence of solution features that may have contributed to the concrete losses.

Concrete was encountered in some of the boreholes at the interface of the chalk and overlying granular drift deposits.  Significant coarse flint gravel inclusions and possible bands of flint gravel and cobbles together with sandy and chalky material were encountered with the concrete.  Based on the findings of the intrusive investigation and results of the test piles, it is considered that the concrete loss recorded may be associated with a band of flint gravel and cobbles present at the interface of the Upper Chalk and overlying granular materials at depth.

The bands of flint were found to be extensive across the site and as a result it was recommended to complete a grouting exercise at the proposed pile locations using a ‘weak’ grout mix and then pile through the grout to avoid further significant concrete losses during piling.

The pile arrangement for the development was to be reviewed to avoid contact with the concrete from failed piles which had formed obstructions.