Groundwater control during building of a new residential development in Greenwich is being achieved by construction of a grout plug. Claire Symes reports.
Constructing a deep basement to house a gym complete with swimming pool below a new residential development may not sound that unusual these days, especially in London. But locate the basement close to a major river, in water-bearing strata while guaranteeing minimal groundwater pumping, and the work certainly couldn’t be described as run of the mill.
Spie Fondations has recently completed ground improvement work at a site in Greenwich to allow developer Hadley Mace to construct a basement in just these conditions. To achieve this, Spie has constructed a grout plug to significantly reduce the permeability of the Upper Chalk and prevent the swimming pool from being a self-filling feature.
“Grout plugs are not a commonly used technique in the UK, but Spie has experience gained from using it on other projects around the world “
Adrian Mercer, Spie
The Greenwich Square development is part of a £250M regeneration project designed by architect Make to create 645 homes, leisure facilities and community services on the site of the former Greenwich District Hospital, which closed in 2001. The phased development will be spread over several blocks - the first of which will be completed at the end of this year. It will be centred around a new public square with the whole site will be completed over the next three years.
Although the Greenwich Square site extends over more than 3ha, Spie’s focus is on a much smaller area. “We are working on a 40m by 80m section of the site,” says Spie business development manager Adrian Mercer. “The main piling work has been carried out by Miller Piling, but the work to form the grout plug was sublet to us.”
Miller completed the secant piles for the basement’s retaining walls in August. These are formed from soldier piles to 18m and grout only female piles to 12m, tension piles to tie down the final basement slab and bearing piles. This cleared the way for Spie to move onto site in November last year.
The site lies on the Kempton Park Gravel Formation which overlies the Upper Chalk with the bottom of the two- level, 6.5m deep basement lying at the boundary of the two geological formations. With the site investigation predicting groundwater at 5m below ground, groundwater issues were flagged up a serious concern for the construction process at an early stage.
“It was impossible to dewater because the capacity of the sewers in the area wouldn’t have coped”
Adrian Mercer, Spie
The grout plug designed by Spie and consultant Meinhard is designed to reduce the hydraulic conductivity of the Chalk to between 10m/s and 6m/s to reduce inflow rates to around 10litres/s. “Inflows would be 10 to 100 times worse without the grout plug,” says Mercer.
“It was impossible to dewater because the capacity of the sewers in the area wouldn’t have coped and discharges when the site is complete are limited to 30litres/s. Handley Mace considered piping the water to the Thames or constructing a deeper cut off wall but in the end the grouted plug was a more cost effective solution.”
According to Mercer, the grout plug technique differs from the more commonly-known jet grouting method in that the grout plug uses permeation through natural fissures, whereas jet grouting is a destructive technique.
“Grout plugs are not a commonly used technique in the UK, but Spie has experience gained from using it on other projects around the world and we brought over experienced technicians from France to ensure the project went smoothly.”
Nonetheless, some new ideas were introduced on the Greenwich job. The first involved materials used and the other was the use of three different tubes to inject the grout.
The system used by Spie at Greenwich involved using a combination of two materials - a Holcim cement grout, supplied by Sika, and sodium silicate called Crystal 75 supplied by PQ Corp - to form a two level grouted plug. Working from within the partially excavated basement, Spie used a Casagrande C8 rig and a DCH rig to drill down to 10m below Ordnance Datum (14m below original ground level) using a tricone and cement bentonite to maintain stability in the open hole.
The grout plug is being formed 4m below the bottom level of the final basement floor slab and the plug is 1.5m thick.
“The outlet at the top of the plug was injected with cement grout first to form a capping layer at the top of the plug”
Adrian Mercer, Spie
“Once the hole has reached full depth, we placed a three tube system that was developed specifically for this job into the hole,” says Mercer. “There are two small tubes - one with an outlet at the bottom of the plug formation level and the other with its outlet at the top of the plug.
“The outlet at the top of the plug was injected with cement grout first to form a capping layer at the top of the plug and the process was stopped once the volume reached 340litres or the pressure reached 30bar.
“The sodium silicate was injected later through the bottom outlet and the process was only stopped once the volume reached 680litres or the pressure reached 30bar. The tubes are sacrificial but we could wash them out if reinjection was needed.”
Spie had a central, computer-controlled compound located outside the basement itself for mixing the grouts. Six pumps and 12 lines to took the material on to the site.
In total, Spie installed 1,050 boreholes for the grouting process. There were generally laid out on a 1.9m grid spacing.
As Spie was finishing its work on the grout plug, WJ Groundwater started work on dewatering from five boreholes located outside the basement. The process will lower the water table to 500mm below the slab construction level and maintain it at that level for two years during the construction process.
Miller installed tension piles in the area where the swimming pool wall will be located because there will be no columns in this area, but the design relies on the building’s weight to keep the slab in place.
Now the grout plug is in place, groundworks contractor O’Keefe has started work on excavating the basement. Mercer is confident that the work will proceed without any issues with groundwater. Issues related to rainfall may be another matter altogether though, but that is something out of Spie’s control.
Greenwich Square architect Make has said that the redevelopment of the former hospital site will provide much-needed homes for families and create a welcoming new community focal point in an area blighted by “unsympathetic ad-hoc development”.
According to Make, the scheme is “designed to mesh with the surrounding urban fabric and extend existing routes through the site. The buildings are arranged to create secure and welcoming residential streets and communal squares that echo traditional street patterns, while the accommodation offered ranges from apartments and maisonettes with balconies and rooftop gardens, to family-focused terraced housing with gardens.”