Client: Landmark Trust
Lundy is a SSSI and the surrounding seas are a Marine Nature Reserve. JGP has worked on Lundy since 1987 engineering improvements to the island’s infrastructure. In 2010, the works achieved Winner of The Institution of Civil Engineers South West Region Civil Engineering Project of the Year Award: Minor Projects.
Projects engineered by JGP on Lundy have included:
The Jetty, Ancillary Buildings, Infrastructure and Adjacent Road:
JGP provided civil, structural and rock slope engineering consultancy services for the design and supervision of the construction of the jetty, adjacent access road, harbour infrastructure and ancillary buildings. The new jetty and road allowed access to the island at all states of tide and weather. Lundy Island has a tidal range of 9.2m.
The 90m long open structure jetty was engineered to sustain forces generated by the high energy wave climate; the design wave height was calculated to be 8m. The structure jetty comprises a pre-cast concrete deck, cross tensioned and braced by cross head beams. It is supported on 1200 diameter tubular steel piles, extended 5m into competent rock. The deck is vented, and its soffit profiled to rapidly vent air and water, which would otherwise by compressed by extreme storm wave energy.
The adjacent access road was formed by both excavation into the sea cliff and construction of sea walls across the foreshore using rock anchored reinforced sprayed concrete. The shore building, erected on reclaimed land, comprised a steel framed timber clad building. Works were completed in 2000. Project value: £2.2M.
The Landing Beach Slipway, Adjacent Access Road and Coastal Retaining Walls:
In 2008, JGP assessed the stability of the retaining walls north-east of the Landing Beach. These wall supporting the only access road along the island. The original stone retaining wall was built in 1836 and was cracked and distorted. The old concrete foreshore defences had deteriorated due to marine erosion to the extent that the road was considered to be vulnerable to collapse. The road surface exhibited evidence of compatible settlement. The stabilisation works involved construction of an anchored reinforced sprayed concrete retaining wall, which acted compositely with the existing wall and underlying rock mass.
The foreshore defences were also upgraded and have been designed to guard against the effects of wave slamming, which is a significant feature of this high energy marine environment.
In 2009, the Landing Beach slipway and adjacent 20m length of road were reconstructed to an elevated profile using rock anchored reinforced sprayed concrete walls. Project value: £1.1M.
The Historic Boat Cave:
The historic Boat Cave is located to the rear of the slipway on the Landing Beach. It was thought to be originally used as a cold store for fish. The rock slope above and to the north of the cave has been retrogressively slipping over the last ten years. An actively receding rock mass wedge failure was identified. It was predicted that unless stabilised, the boat cave would be lost. Furthermore, the 12th Century Marisco Castle at the top of the slope would be adversely affected.
In 2009, JGP designed the stabilisation works for the Boat Cave. These included rock bolting and reinforced sprayed concrete stabilisation within the cave and slope drainage. The rock bolted, reinforced, sprayed concrete was designed to form a stiff tension skin integrated with the rock face. The slope stabilisation works were designed to limit the potential for future landslips and halt the active wedge regression.
The Access Road Along the Coastal Slope:
The only access road traverses a marginally stable coastal slope. The slope comprises bouldery ground underlain by a glacially disrupted rock mass fabric. The access road across the coastal slope is essential as it is the only route along which supplies for the island can pass. If the road were to close due to slope failure, there would be no means of access for the 20,000+ annual visitors to the island.
A 75m long section of the road and existing stone retaining walls exhibited significant down-slope movement. JGP designed a piled anchored reinforced sprayed concrete solution formed over sections of the existing deteriorating walls and road. It was designed to resist down-slope forces and withstand predicted shallowly rooted landslip impact without sustaining more than readily repairable damage.
Careful programming was required as the road was still in-use for access whilst the work was progressed. The sensitive SSSI environment and unique flora on the slopes (the Lundy Cabbage is a floral species unique to Lundy) meant that works were designed to have minimal impact on the environment. Works were carried out by specialist contractors and completed for £0.5M.
The south-eastern cliffs of Lundy Island comprise highly fractured meta-sedimentary Morte Slates with numerous doleritic and trachyte intrusions. Marine erosion has exploited the discontinuities within the highly fractured rock mass, resulting in pneumatic quarrying and the formation of caves. Many of these caves often extend many metres inland and have often compromised the stability of the overlying access road.
JGP has provided a number of coastal defences for various sea caves that have developed below the access road. Bespoke remediation solutions have been designed for each cave. These typically comprise robust reinforced sprayed concrete facing walls. These incorporate drainage and ventilation systems to allow air to escape and prevent wave-induced pneumatic pressure build-up.