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House arrest

Technical feature Ground stabilisation

A Georgian home perched above the River Avon has been threatening to slip into the water for nearly half a century.

Alan Sparks explains how movement is at last being halted.

Shortly after the Second World War a three storey, Grade II-listed Georgian house with a two storey Victorian extension started to slip perilously down a steep hill west of Bath.

The cause was put down to the failure of a 3m high, 38m long retaining wall which helped to stabilise the structure's foundations. Repairs were hastily planned and carried out.

Over 40 years later, the grand home resumed its progress towards the River Avon below.

Again the cause was identified as the hopelessly underdesigned retaining wall.

'It was amazing that the house managed to survive as long as it did, ' explains Richard Holmes, managing director of specialist geotechnical engineering contractor Van Elle. Its £220,000 contract covers designing and renewing drainage, landscaping and fascia works around the house.

But while 'the thrust of the scheme is to return the view from the nearby bridge exactly to how it was back in the 1800s, ' according to site foreman, Gordon Sumner, the backbone of the project remains the challenge of halting the movement of the ground around the house once and for all. For this Van Elle called for back up from retaining wall specialist Chris Raison Associates.

The solution chosen uses contiguous piles with a capping beam anchored into the limestone. Ground is interbedded with weak mudstone and stiff Lias Clay and overlain by a weak ancient landslip of clay fill to a depth of 3m to 4m. Piles of 300mm diameter, between 7.5m and 8m long were augered at 500mm centres with a minimum rock penetration of 3m.

Each pile is reinforced with six T32 bars to restrain the bending moment of 206kNm applied per metre run of the wall.

The high surcharge is due to the hefty nature of the Georgian construction. 'The stone walls are 600mm thick, reaching 12m in height, in addition to a heavy slate roof, ' adds Holmes.

Handling of the reinforcement cages was hindered by space and the sheer weight of steel. This was also true of the cages for the 38m long capping beam, which contained 10 T20 bars. Along its length 12 Ischebeck anchors which plunge a minimum of 7m into the rock to restrain pull-out forces of up to 200kN.

These anchors are preaugered through the fill and then screwed into the stone using a sacrificial drill bit on the end of hollow 50mm diameter bars.

Grout is then pumped through their centre. Plastic pipes cast into the capping beam cage create a perfect fit for the anchor connections, which are then grouted into position.

With little room for manoeuvre, the usual choice of 25t Hymac type excavation rig for installing the anchors was completely unrealistic, so modified mini-rigs were used instead.

Although only one and a half anchors can be installed each day compared with the four Van Elle reckons is normally possible, 'the modified system has worked very well with the minirigs working to their absolute limit - and then a little bit more, ' says Holmes.

Another challenge was concrete supply to the site. The only access is a road above, so concrete had to be pumped 80m from trucks 15m above the retaining wall. This operation carries its own problems, as the run of the grout can be faster than the pump - with air voids and seepage to be negotiated.

The mini-rig working on lower landscaping and septic tank excavation is 20m below the road with two walls to hurdle on route. The machine had to be lowered in using a cable winch.

How it will be retrieved at the end of the job remains one of the final challenges facing site workers.

Given the environment, it is also perhaps surprisingly that the client has continued to live in the house throughout the job.

This meant installing temporary drainage when switching between old and new septic tanks and pipes. A sheet piled walkway has also been constructed to provide a safer and more pleasant entrance to the house.

'The owners have been absolutely magic, ' says Sumner.

'The only restrictions placed on us have been to ensure that all deliveries are made after 8am and that the garage is clear overnight.'

To ensure that the work causes no sudden movements in the structure, all cracks within the house and the safety retaining wall a couple of metres below are continually monitored. Results so far show that unexpected bathing in the River Avon is becoming less and less likely in the future.

As a final touch the house will be bedecked in a new Bath Stone face. Mann Williams Consulting helped develop the concept design in association with local architect Peter Dlugiewicz. Stone will be laid by local specialist subcontractors using a traditional hydraulic lime-based mortar mix that includes Bath Stone dust and Bideford grit.

Pre-contract delays pushed the programme back by three months which means the stone face will now be laid in late April rather than February as planned.

This had its upside. 'It just never stopped raining and the subcontractor would not have even been able to get started, ' says Sumner. He is hoping that spring will be kinder, with higher temperatures to speed the traditional mortar's strength gain and allow the 22 week programme to finish in May.

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