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Single bore multiple anchors are securing the quay wall at Rochester Riverside.

Damon Schünmann takes a look.

There are big plans for Rochester Riverside, a Thames Gateway peninsular in Kent.

Once the site of an old gasworks dating back to at least Victorian times, contractors are hard at work remediating the land for its future commercial and residential use (GE February 2007).

That work includes securing the 2.5km quay wall. Different sections are getting differing solutions depending upon the future use of that part of the riverbank and how close to the edge follow-on construction will be. The retaining schemes include dead men and a single bore multiple anchor system (SBMA) that retains the sheet piled walls.

Keller Ground Engineering has been busy installing anchors for the latter using a technique that allows tendon anchors to work through the full length of a borehole. This is in contrast to a standard tendon anchor that will only work, under normal load, in its upper region nearest the surface. The principle can be demonstrated by holding a rubber band tightly in a st, leaving an end sticking out. When one end is pulled the tension can only be felt where it emerges from the st. The entire length will only work at the point of failure, when the whole rubber band is pulled from the hand.

By using multiple anchors in the same borehole, with staggered bonded working lengths, each one is contributing to a collective strength all the way down the borehole. The bonded working length of each strand is called the unit anchor encapsulation length and this section transfers the load from the strand to the grout and into the surrounding ground.

Keller has installed 253 of the permanent SBMAs for the quay wall sheet pile retention. The high capacity anchors found in a combination of sand and gravel with underlying chalk. Anchor working loads are generally 800kN with applied proof loads of 1200kN.

Overall anchor lengths are normally from 26m to 30m and drilled at a 40º inclination below the horizontal. Site workers installed the anchors at 1.5m to 2.5m horizontal spacings along the walls, 2m below the coping.

The initial design of the multiple anchors for Riverside was based on the results of anchors installed in similar ground conditions in the opposite Medway river bank in Strood in 1998. There, trials demonstrated that upper anchors in each bore can found in the limited thickness of the Thames Ballast layer and the lower units in the weaker underlying chalk. Test loads in excess of 2500kN were accomplished without failure.

Multiple anchors can achieve higher capacities than those required at Riverside. The maximum number of anchors normally installed in a single grouted anchor borehole is six and the maximum unit anchor working capacity is typically 400kN meaning the maximum working load available on a single SBMA type soil anchor is 2400kN. Licensing company Single Bore Multiple Anchor director and the system's designer, Tony Barley, claims test loads up to 5000kN have been applied to a SBMA installed in a single grouted borehole in soil. Barley says SBMA capacity is probably about twice that of the highest load conventional anchor.

'You can use up to seven unit anchors in a single borehole but generally it's six, which gives a maximum 200t working capacity in soil. Even the best conventional anchors can only manage 100t, ' he says.

Keller won the £750,000 subcontract from Edmund Nuttall and began work last summer. It was due to install the last of the anchors at the end of February with tensioning continuing into the early part of this month.

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