Derelict Salford Quays are being transformed into a new arts centre.
The Lowry Centre, north west England's new £97M arts development, includes two theatres and three galleries.
Many of its construction challenges result from its location on the former Pier 8 of the Salford Quays complex next to the Manchester Ship Canal.
Initial desk study work showed that the roughly triangular centre straddled the former channel of the River Irwell, which was infilled when the Ship Canal was constructed, explains Tom Skailes, associate with consultant Buro Happold's geotechnical engineering division.
Storage warehouses had formerly occupied the site and the mass concrete foundations had been left in place when they were demolished.
Ground investigation revealed that the site was capped by about 3m of made ground (up to 8m in the infilled river channel), underlain by soft alluvial deposits. These overlaid glacial tills resting on the Sherwood Sandstone which dipped steeply across the site from about 12m to 20m depth. Groundwater levels were consistent with the canal and dock water level.
The relatively heavy structural loads were carried on contiguous flight auger piles and socketed into the sandstone while the 6m to 8m deep basement was built within a permanent watertight diaphragm wall.
Foundation contractor Kvaerner Cementation Foundations worked closely with management contractor Bovis from early in the design process. Kvaerner says this enabled implementation of the most appropriate and cost effective foundation solutions.
For example each pile was sunk from the pier's original ground level and its upper 4m length later exposed as the basement area was excavated around it. Pile tops were then cut off and pile caps formed at basement level to support the various structures above.
The old warehouse foundations presented a significant obstruction to the groundwork, which was overcome by stitch drilling and localised breakout for the diaphragm wall and repositioning of the pile layout as necessary. The remnants of the foundations were removed during the general excavation work.
Skailes says searches of the Manchester Ship Canal Company archives had uncovered the original as built documents for the quay walls. There were two different constructions: a mass concrete gravity wall variously founded on the sandstone or the glacial till and an arched structure that did not actually act as a retaining wall. The latter type was considered adequately stable, but the main quay gravity wall showed evidence of displacement.
Analysis suggested a factor of safety only just in excess of one where the wall was founded on glacial till. The till frequently contained varve laminations and these were perceived as a potential failure slip surface. The wall was stabilised by piling through it to enhance the factor of safety against sliding.