Method of Construction

• Frequently Asked Questions

Portland limestone was used for groyne construction, and was brought to site by road.  Each rock for the head section of groyne weighs three to six tonnes, and for the section from the head back to the promenade one to three tonnes.

At the beach, sand is first excavated from the area where the groyne will be located.  The area is then backfilled with a bedding layer of stone to prevent subsidence of the groyne structure.  Using this method of construction the groyne is extended seawards with bedding material and rock, building up to a level that will permit plant to run on it. 

As construction progresses to full groyne length manhole rings are used to create voids to take the piles for the walkway that allow public access to the groyne.

Walkways are independent structures consisting of concrete beams spanning piles driven along the length of the groyne.  Walkway sections and pile caps are made at the beach from reinforced precast concrete and lifted into position.

Construction proceeds with the head of the groyne being built to its full height.  Piles are then driven into position through the voids by a vibrating rig (and air hammer if required).  Working shoreward pile heads and walkway sections are put into position at the same time as rock is built up to the finished level.

The whole operation is then able to retreat off the groyne leaving a near completed structure; just the in situ joints and infill between the walkways and pile heads are completed later.

Access ramps are situated between the back of the groynes and the promenade, made from concrete poured in-situ and tied back into the existing stepped sea wall.  First the wall sections of the ramps are poured, then the ramp is filled with beach sand and the slab section is cast on top.

Groyne construction methods have changed little since groynes were first built at Sandbanks in 1996 and again in 2001 (photos above are from those previous projects).

Frequently Asked Questions

What are the large trenches/ramps for?  The large excavations at the back of the beach are for construction of ramps.  Ramps allow access, for instance, across the beach for the beach rake and tractor.  Ramp foundations are taken to below the level of sand before beach replenishment (2006/07).  The size of the excavation is due to the working area required to provide the long 1:20 slope that complies with Disability Discrimination Act (DDA) regulations.

Where will all the sand go?  Sand from the excavation is placed seaward to give protection to the workforce and the working area.  Once construction work is complete the sand will be replaced in the excavation trench.  Current beach levels will means that the majority of the ramps will then be covered by sand; the extent of the ramps will ensure they can be utilised if/when beach levels drop.

What other access improvements are provided by ramps?  Ramps provide visitor access from the promenade to the beach and the groyne walkways.  They also allow easier access for the less able and those using mobility scooter or wheelchairs, buggies and prams.

David Robson, Senior Engineer

Coastal & Drainage Team, Borough of Poole

Email: d.robson@poole.gov.uk

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