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Multiple choice The first use of ground anchors on the Norfolk Broads has provided an economical and environmentally sound solution to the problem of preserving the river banks of this sensitive area.

The Norfolk Broads is a popular tourist destination all year round, with boatloads of holidaymakers gently chugging up and down the scenic waterways enjoying the area's rich and varied wildlife.

So the thought of piling rigs and drilling crews shattering the tranquil atmosphere, regardless of the crucial nature of any work, must have given great concern to residents, tourists and conservationists.

'The environmental aspect of our work here is one of the major concerns,' confirms Keller Ground Engineering director of engineering Tony Barley. The company is in the middle of a major ground anchoring contract to bolster flood defences west of Great Yarmouth.

Careful planning to protect the fragile environment and the use of environmentally friendly methods and materials were crucial to obtain Environment Agency approval for the work. This was especially important as Keller, working as subcontractor to main contractor Tilbury Douglas and engineer Halcrow, proposed a design change for the structural element of the flood protection work.

Originally, tie bars were to be used to hold back the new sheet pile walls being installed along the banks of the River Yare, but Keller suggested halving the number of ties by using the more environmentally friendly and ultimately cheaper alternative of its patented Single Bore Multiple Anchor system.

Broadland is the flat, low-lying area surrounding the tidal reaches of the River Yare and its two major tributaries, the Waveney and the Bure. The region is one of the UK's most important wetland marshes, containing numerous sites of special scientific interest and since 1986 has been designated as an environmentally sensitive area.

Much of the area is susceptible to flooding either from high freshwater river flows or, more frequently, high sea levels. At the moment, flood protection is provided by flood walls through Great Yarmouth and earth embankments along the Broadland rivers.

While defences in Great Yarmouth have recently been improved and upgraded, apart from routine maintenance and emergency repairs, little has been done on the Broadland defences as a whole since a major flood in 1976, when defences at North Breydon were breached.

Since then, embankments have settled and their condition has deteriorated to the extent that the risk of overtopping and breaching is now 'very serious', according to the Environment Agency.

The agency's Broadland Strategy (see box) aims to reduce this risk, by taking care of the existing defences and protecting and enhancing the local environment. Most of the 240km of river banks in Broadland need extensive strengthening and renewal of toe protection. In addition, some banks need to be raised by up to 375mm to take account of settlement and sea level rise to prevent breaching.

Keller's work forms part of the first 10 year phase of this rolling programme, covering a stretch of the River Yare between Reedham and Berney Arms Reach.

Here, crumbling timber pile walls are to be replaced by new steel sheet piles. Original design called for tie rods placed at 2.4m centres to hold back the new walls, fixed to a second row of sheet piles placed in a trench excavated behind. But Keller suggested replacing this tie back solution with a ground anchor alternative.

A Single Bore Multiple Anchor (SBMA) is made up of a number of unit anchors each having a 2.5m to 4m fixed unit length, placed at staggered depths within a single borehole. Typically, three with 3.3m fixed lengths will give a total length of nearly 10m, rather than a single 10m fixed length. This gives double the capacity, because using short fixed lengths reduces or eliminates progressive debonding, which is common in long fixed length anchors. Barley says that installation, testing and use of around 35,000 unit anchors has given a host of data that has allowed anchor capacities in most soils in the 2,000kN to 4,000kN range.

Installing normal anchors at the same spacing as the tie bars would have been uneconomic, Barley says. However, using the SBMA system, these high load anchors can be placed in weak soils at twice the proposed tie rod spacing (ie 4.8m). He explains that by halving the number of retention points, albeit with a nominally heavier waling, cost savings could be made and the need for any trenching or cutting of existing river banks was removed, further reducing environmental impact.

He says these were the main reasons that this first use of ground anchors in Broadland's weak soils, was accepted by the Environment Agency - with the backing of the Ministry of Agriculture, Fisheries & Food which provides 75% of the funding for the agency's flood defence works.

As work has to be carried out with minimal impact on the existing bank, all plant and materials have to be brought to site by boat, with the sheet piling installed down to 15m from a crane barge using a silent piler. Drilling and anchor installation takes place from a specially designed cantilever platform developed by Keller and Tilbury Douglas to allow the drilling mast to be kept at a constant level while not loading the bank.

Boreholes are drilled using the Duplex system with a Klemm rotary percussive head. Water flush is used as the casing and drill rod are advanced to the full depth through the 15m thick layer of very weak alluvial material and into a 4.5m thick fine sand layer, where the anchors are fixed. This suits SBMA as the angle of the anchors means the layer is very thin for the high loads needed.

As the rods are pulled back, water is kept at a constant head to prevent piping up the borehole. The strands are then inserted to the base of the hole and the Ordinary Portland Cement grout tremied in under 5 bar to 10 bar of pressure, with the casing pulled out in stages. Contamination of the river with drilling spoil or excess grout is eliminated by returning drill flush to the 'settling tank', basically the gap between the old and new sheet pile walls.

Each strand is 15mm or 18mm diameter and, in the free length, is covered in a plastic coating, which is surrounded by another layer of plastic. Within the 65mm diameter fixed unit length, each unit anchor strand also has a double protected, pre-grouted encapsulation. Each strand can carry a working load of up to 150kN (15mm diameter) or 190kN (18mm diameter) and on this contract the greatest anchor working load is 640kN. After the grout has cured, the anchors are tested and then locked off at 10% above working load, with caps to protect them against the corrosion effects of the brackish water.

In all, 277 anchors have been installed on both sides of the river. At Berney Arms Reach, eight 450kN working load anchors were installed at 40degrees and splayed to avoid the windmill foundations, with 109, 600kN working load anchors installed at 25degrees along the rest of the bank.

For the Ravenhall contract 26, 600kN working load anchors were placed in an area requiring emergency work. Some 134, 600kN working load anchors were installed for the main wall. All production anchors were load tested to around 900kN and at Berney Arms Reach two were tested to 1,200kN, which Barley says is a very high load to achieve in the thin sand layer.

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