What are claimed to be some of the longest ground anchors installed in the UK are helping improve flood protection in Great Yarmouth.
Great Yarmouth, on the east coast of England, has seen its fair share of devastating flooding – notably the Great Storm in 1953, when hundreds died during a storm surge. Since then, flood defences have been improved on a number of occasions.
The latest of these schemes is a £28.6M Environment Agency project that aims to protect more than 2,000 homes by upgrading sea defences in Great Yarmouth harbour, including refurbishing and raising quay walls.
As part of this project, Keller Geotechnique, working on behalf of main contractor Interserve, installed ground anchors to strengthen two sheet pile walls at Portland Quay on the west bank of the River Yare. Keller Geotechnique operations manager John Graham says these ground anchors, which are up to 62m long, are some of the longest ever installed in the UK.
Ground anchors were chosen instead of installing a “dead man wall” (a secondary sheet pile wall which is placed behind and tied to the front wall) as there was no space to do so, with a number of buildings on the quayside, Graham explains. “And,on quay walls in soft ground, such as in Great Yarmouth, installing a second row of sheet piling to improve stability is often uneconomic. Ground anchors are a good alternative.”
Ground conditions on site are made ground, over soft silty clays and peaty clays, with sand below. The sheet piles, which are approximately 22m long, are founded in the sand. “Suitable founding material for the anchors – the medium dense to dense sand – is at considerable depth, more than 15m to 20m,” Graham says. “It is also of marginal quality for the high anchor loads proposed: 754kN axial safe working load (SWL).”
Keller proposed using its single bore multiple anchor (SBMA) system to meet the high load requirements, although as Graham points out: “This is a high load, even for the SBMA system, where numerous fixed lengths work together to achieve the required capacity.”
The SBMA system involves installing multiple unit anchors in a single borehole. Unit anchors are encapsulated at staggered depths in the borehole with the load transferred from each unit anchor to a discrete length of the borehole.
“Installation was carried out from specially-designed platforms cantilevered out over the River Yare”
“The system ensures a uniform mode of load transfer to the ground over the entire fixed length and increases the efficiency in the way it mobilises the ground strength,” Graham explains. “The system offers an almost unlimited fixed length over which the load can be transferred.”
Graham adds that SBMA anchors can increase the ultimate load capacity by 200% to 300%. “When the starting point is marginal soils, this increase can become crucial. While it is suitable for most ground conditions, SBMA offers particular advantages in clays, mixed cohesive granular soils and weak rock, where conventional anchors only achieve a limited capacity.”
Achieving the design load at Great Yarmouth was critical, so in September 2014, Keller installed four trial anchors to depths of between 26m and 31m, using a 133/76 rotary-percussive water flush drilling system.
Trial anchors (each comprising five SBMA units of two 18mm Dyform prestressing strands) were grouted using a grout of neat CEM1 (52.5N) in a 0.45 w/c ratio. These were tested to ultimate tendon capacity of 3,040kN.
“Cyclic trial testing to BS 8081 mobilised the ultimate grout/ground bond stresses for most of the SBMA units. This allowed us to back-calculate the SBMA unit working capacities and lengths to suit the 754kN design load,” Graham explains.
A total of 94 permanent SBMA anchors were installed at an average spacing of 1.45m. Two lengths of anchors were used: 58m, installed at an angle of 32.5°; and 64m, installed at 27.5°. Each comprises six fixed length units, each of a single 15.2mm Dyform strand, giving a total fixed length of 21m.
“The anchor bores were considered to be at the limit of standard practice for a fully cased and end-of-casing pressure grouted drilling system, so the trial also involved drilling and grouting two holes, without installing anchors, to prove the depth could be reached,” says Graham.
The permanent anchors were installed through the sheet pile walls over the River Yare, using two specially-adapted CAT315 excavators and drill masts. The modifications allowed the machines to drill beneath themselves, with the rig on the quayside and drilling carried out from a cantilevered platform over the water, designed and supplied by Interserve.
“This approach meant much of the work could be carried out on ‘terra firma’ rather than having to overcome the challenge of tidal working or working off jack-up barges,” Graham says. “Clearly, working on land is much safer so this approach significantly reduced the risk to operatives.”
Working anchors were cyclically tested to 150% axial SWL before being locked-off at a calculated prestress value that will allow for future dredging and loading conditions.
All anchors were successfully installed by the end of December 2014.