Ground engineering round up

Holy piles

Sinking slowly into soft estuary deposits beside the river Humber, the grade II-listed St Giles’ Church in the village of Marfleet near Hull needed a unique, and virtually vibration-free, piling solution if it was to remain earthbound.

Rising to the challenge, foundation specialist Roger Bullivant took on the £450,000 contract, supported by a restoration grant from the English Heritage Lottery, to install a series of jack down and stitch pile foundations.

The perimeter walls were first secured by tying them into a new reinforced concrete floor slab supported by 56 jack down piles, and needle beams were inserted into the walls. Extra support was provided outside by installing several stitch piles driven beneath the buttresses.

The process of work was as follows. A total of 30, 400mm-deep square pockets were drilled into the perimeter walls using hand-held jack hammers. Each pocket received a needle beam of reinforcing steel, which tied into ring beam reinforcement running round the inside of the walls. Two layers of reinforcement were then installed throughout the interior of the church to provide added strength to a new 500mm-thick concrete floor.

Pile positions were marked with sacrificial polystyrene cones secured to the reinforcement. After a concrete pour, the cones were removed to leave sockets into which the piles were installed.

A two metre-long leading section of 140mm-diameter steel tube was inserted into the void and attached to the jack, whose twin hydraulic rams forced the pile section down through the soft ground. Further sections were connected to the previous lengths of steel tube with a spigot joint.

Each pile was designed to take a load of between 100kN and 185kN and reach down to 15m, at which point layers of silt and sandy clay give way to a firm clay and chalk strata. Ready mixed concrete was then pumped into each pile position to complete the foundation and form a secure connection with the reinforced concrete floor slab.

‘This form of piling is virtually vibration-free and almost silent,’ explains Bullivant site supervisor Kevin McCabe. ‘We couldn’t afford to drive the piles into the ground in a conventional manner because of the risk of causing structural damage.’

A diamond core drill was used to make holes in the exterior of the church at 1m intervals to receive stitch piles, each of which was driven to a recognised ‘set’, then concreted to provide added support to the perimeter walls. Work was completed in January.

Aarsleff’s new power base

The winds of change are literally bringing in new work for Aarsleff Piling.

To date it has installed foundation piles for more than 50 wind turbine bases and expects to complete 44 more this year.

Wind turbine bases require chunky precast concrete piles – up to 400mm square sections – to deal with the high cyclical loadings. So Aarsleff has invested in a Juntann PM26 LC piling rig with a 9-tonne accelerated hammer which can deal with big piles and hard driving.

Pictured here is the new rig in action on its first windfarm project at Ransonmoor, near March, Cambridgeshire. It is being used to install 350mm square section 26m-long piles for contractor R G Carter Civil Engineering.

You can see how hard the driving was by the smoke that is coming from the timber packers between pile top and hammer.

R G Carter is constructing the foundation bases and associated access roads for a pair of 2MW wind turbines under a £790,000 design-and-build contract for client Fenpower. The foundation package is worth £85,000.

Bachy forms square

The Cube in Birmingham is under way. At the end of last year Bachy Soletanche completed piling works for the Ken Shuttleworth-designed development, which will include up-market apartments, retail outlets and a roof-top restaurant. The 17-storey building will have a five-floor basement to make the best use of the 60m x 70m space.

Contractor BuildAbility awarded Bachy the the £1.7m contract to design and deliver a retaining wall. Working with the developer, Bachy chose a contiguous piled retaining wall. Using the large diameter auger piling technique, BSL installed a total of 226, 900mm-diameter piles.

However, when piling works started, unexpected groundwater meant that Bachy had to change its method of pouring concrete into the bored piles to use a tremie pipe. This is inserted to the toe of the pile and then gradually lifted out as the concrete fills the pile.

‘By filling the borehole from the base up, the groundwater is displaced to the top of the cast concrete,’ explains Bachy contracts manager Steve Mallinson.

Bachy also installed mini piles in two areas of the site and 30 king post piles to provide foundation for a ramp, which is required for future building works.

The site has now been excavated by between 14m and 19m to create a new ground level.

Bachy returned to the site after the excavation to construct 12 temporary anchors to hold back the retaining piles on the east side of the site.

End of the stone age

Two ancient stone arch culverts presented a bit of a headache for Tameside Metropolitan Borough Council when it wanted to rebuild a retaining wall in a valley in Mossley, Manchester. Heavy fill material above the culverts, which carry a stream, would have caused them to collapse.

Main contractor A&E Yates had to replace a 5m-high, 70m-long section of dry stone retaining wall that had reached the end of its working life with a reinforced concrete L-shaped cantilever.

Yates used traditional fill in areas away from the stone culvert, but on a 20m section of the wall over the culvert the council specified expanded polystyrene, a stepped 1.8m layer of Vencel Resil’s Fillmaster high-strength FM500 grade, with an additional layer of fill placed over the top. This reduces the pressure on the culverts.