Napier Gateway, Luton

Napier Gateway is a large mixed use development close to Luton Parkway Airport that will eventually deliver 685 residential apartments, a hotel, retail and leisure outlets and a large new landscaped public park.

The £300 million scheme is being built on a 6.9 acre site formerly occupied by Vauxhall Motors. It is being built in four phases, with the new apartments being built first.

Phase one includes three residential towers up to 11storeys high sitting above ground floor retail units, along with a multi storey car park, communal gardens and the central park. Offices, commercial space and the hotel will be built in later phases.

Phase one contractor JJ Rhatigan appointed Central Piling to install the bored piled foundations for the three residential towers, Blocks A, B and C, which are nine, 10 and 11 storeys high respectively. In addition, Central Piling has installed a contiguous piled retaining wall to create the podium level. The piling works were carefully coordinated with those of the main contractor following behind in order to achieve the project deadline.

Ground Conditions

The variable ground consists of made ground above a thin layer of clay that contains flint, with an underlying chalk bedrock.

Solution

A detailed geo-environmental assessment of the site indicated that, due to the natural weak soils, a traditional shallow foundation would not be feasible for the high-rise nature of the new structures, and a contiguous piled foundation solution was recommended.

Central Piling carried out the pile design for both the residential tower foundations and the 250m long retaining wall, which was designed to facilitate the construction of the podium and ground level on what was a naturally sloping site.

The pile loads ranged from 50kN to 1700kN, with all piles designed to embed into the underlying chalk as competent bearing strata. The maximum pile length was 21.5m.

There is an underlying aquifer on the site, which could have become contaminated if traditional intrusive piling techniques were used. Instead, Central Piling opted for continuous flight augered (CFA) piling, as it generates minimal disturbance to the soil.

CFA is a fast and reliable method of pile installation that also has reduced impact on the environment in terms of noise and vibration.

Central Piling carried out the £500,000 piling contract in a single two month visit between July and September 2019. The first priority was construction of the embedded retaining wall, which was formed of 177, 450mm diameter CFA piles. This was followed by the installation of the 687, 350mm diameter CFA bored bearing piles for the residential tower foundations.

All the piling was carried out using a Soilmec SR-75 CFA rig – the largest rig in Central Piling’s fleet.

Challenges

The main challenge was piling on a site that had different ground levels to compensate for the natural slope. This required a well-organised sequence of works, making use of ramps and temporary berms.

The piling works also had to be carefully coordinated with those of the main contractor, which had to follow immediately behind in order to meet the project deadline.

Using its most powerful rig on the contract enabled Central Piling to meet – and even exceed – the daily targets, despite the variable ground conditions.