Ground engineering round up

Crossrail keeps stent on its toes

Although it is not yet built, London’s Crossrail tunnel has proved to be a problem during piling work for the Central Saint Giles mixed-use development near central London’s busy Tottenham Court Road.

Stent Foundations, part of Balfour Beatty Ground Engineering, completed the piling contract on the former site of a medium-rise 1950s office building for Bovis Lend Lease and Stanhope. The development required the installation of nearly 500 load-bearing piles and 500 secant wall piles over a 5,000m2 site. The load-bearing piles were a variety of sizes up to 1.5m in diameter and 44m long, while the secant wall piles were 0.6m, 0.75m and 1.2m in diameter.

Concerns that the construction of Crossrail would overload the piles led to the installation of a sequence of large-diameter, reinforced concrete piles with permanent slip-lined steel casings. The unusual depth of these piles meant a bentonite support fluid was used during construction and the bases of the piles were grouted.

Meanwhile, a temporary continuous flight auger wall was installed along the site’s southern elevation to enable the excavation of a two-storey basement. This had to be installed to very tight tolerances due to external services that run next to the site boundary. Subsequently, a permanent reinforced concrete basement wall was also built.

When completed in 2009, the Renzo Piano Building Workshop-designed development will include 109 homes, 40,000m2 of office space, retail outlets and an open air plaza.

RB gets to the root of it

When a 400-year-old farmhouse in Gillingham, Kent, began to subside, a system of cantilevered concrete foundation beams allowed repairs to continue without the owners having to vacate the property.

The cause of subsidence, at one corner of the property, was traced to a large horse chestnut tree in the garden, the roots of which had drawn moisture from the clay-heavy soil. ‘The fact that the house was built using lime mortar, which allows the structure to move a little more sympathetically with the ground, helped limit the damage,’ says Dave Clarke, contracts supervisor for underpinning specialist Roger Bullivant.

The system of 12 cantilevered beams, installed by Roger Bullivant at the front and along one side of the property, was chosen in preference to constructing an internal piled raft which would have required the owners to leave the premises.

Each beam is supported by a pair of foundation piles: one next to the property, which takes the load, and another anchoring the beam into the ground. The beams are designed to ‘float’ in the ground above a 150mm-deep void, which accommodates any clay heave should the tree die and release moisture back into the ground.

The voids were created by placing sacrificial cardboard supports wrapped in polythene on a firm strata of ground before casting the beams on top. Once the beams were cured, the polythene wrapping was pierced to allow water to enter, causing the sacrificial supports to erode. Backfill was then compacted around the beams.

Piling work began with the installation of 24, 250mm-diameter augered piles to depths of up to 15m. Each foundation pile was created by first installing a leading 1m section of flighted auger into the ground, with a further 14 sections of flight connected one to another above.

After reaching the full depth, concrete was pumped in through the central hollow stem as the augers were removed. A three-bar reinforcing cage was inserted into the top of each pile before a single rebar was inserted to full depth.

Trenches were then excavated in front and to the side of the property and a series of temporary jacks placed beneath the perimeter walls. A ring beam of concrete reinforcement installed beneath the walls transferred the load to each cantilevered beam. The temporary jacks were then removed to complete the work.

Slope situation solved

An extremely unstable slope, underground tunnels and the possibility of an emergency evacuation were among the problems that faced piling and foundation specialist All Foundations during enabling works for a housing development at Rosary Road in Norwich.

After Suffolk-based developer Hopkins Homes bought the site, which was previously home to a book warehouse, investigations revealed potentially deep tunnels and chasms created by the historic hand mining of flint.

The instability of the slope to the rear of the site is well documented. A collapse in the 1940s had forced the owner at the time, Norwich City Football Club, to re-locate to its current Carrow Road stadium and the slope’s factor of safety – the ratio of the available shear strength to that required to keep the slope stable – is now considered at best an ‘unsafe’ 1.0.

To stabilise the slope, the enabling phase involved installing a row of 600mm-diameter, heavily-reinforced piles at close centres at the base, together with a heavily reinforced concrete capping beam. A hybrid of pile types, including non-anomaly and anomaly designs, was considered the most suitable.

The delicate condition of the slope meant the pile and capping beam installation and the cutting out of the existing concrete apron – which was providing some resistance to the base of the slope – had to be executed in a very strict sequence. Slope stability was continually monitored during the work, with an emergency evacuation procedure in place for both the site and the occupants of houses at the top of the slope, in case any any movement occurred.

‘Roughly 250 piles were installed on the site during the first visit, requiring almost 1,300m3 of ready-mixed concrete and nearly 24 tonnes of pile reinforcement,’ says Michael Lloyd, contract manager at All Foundations. ‘The first phase of the project was completed in three weeks during July, ahead of schedule and without any movement occurring. The second phase should begin towards the end of the year.’

Persimmon’s stable investment

With greenfield sites becoming increasingly scarce, housebuilders are left with no choice but to look at alternative land sources. For Persimmon Homes that meant taking over a 20 ha disused chalk quarry near Rochester in Kent, where it is building 400 homes, as well as offices and shops.

Before its Medway Gate Development could get under way, however, a £2m programme of groundworks was required to bolster the A228 Cuxton Road that runs on top of the quarry’s eastern boundary, providing access to the site from the nearby M2 motorway. In preparation for the extra traffic generated by the development, consultant Peter Brett Associates designed a 170m-long, 25m-high, 45o sloping buttress wall to run against the quarry face.

The surface of the reinforced earth structure features more than 2,000m2 of Maccaferri’s green Terramesh wire gabions, which form 2m-long, 600mm-high building blocks, each filled with compacted backfill taken from the site. The gabions work in conjunction with 23,000m2 of Maccaferri’s Enkagrid-Pro polyester geogrid reinforcement to form a steep, very strong grass-faced reinforced earth wall.

To stabilise the friable (small particles can become dislodged and enter human lungs) chalk near the site entrance, subcontractor CAN Geotechnical installed more than 3,000m2 of Maccaferri’s Armater cellular soil containment matting to transform the rough rocky slope into a grass embankment. The concertina-like matting, which comprises interlocking 500mm-diameter hexagons, creates a grid of 100mm-deep pockets to hold topsoil. When seeded, root growth binds the soil layer to the underlying materials, preventing it from slumping down the slope.

Finally, realising that some rock face erosion was inevitable, main contractor CA Blackwell installed a woven-steel mesh drapery system to completely enclose some areas of the near-vertical quarry face. The drapery catches falling debris, protecting users of the development.