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Borth coastal protection ground investigation
Tuesday, April 9th, 2013
A popular tourist destination, the village of Borth is built on a shingle bank above a sandy beach and underlying soft clays. The village has a history of coastal erosion and flooding, due to overtopping of coastal defences built in the 1960s and 1970s and breaching of the shingle bank. Work is now underway to replace the timber breastworks and groynes with new rock beach control structures, a multi-purpose reef and beach nourishment.
Lankelma, working for Ceredigion County Council and supervised by Royal HaskoningDHV (RHDHV), carried out cone penetration testing (CPT) on the beach to investigate nearshore geology using one of its 4×4 track trucks. Working either side of low tide, Lankelma carried out nine piezocone CPT to a maximum depth of 25m through the underlying sand and clay.
Targeting the deformation characteristics of the highly compressible soft clays, 24 excess pore pressure dissipation tests were undertaken during four inter-tidal day shifts to acquire data and derive the in-situ measured coefficient of consolidation.
With such a short working window between the tides, the dynamic positioning of the CPT truck was critical. Lankelma used Leica Viva GS08+ land survey equipment, to a field accuracy of +/-50mm, which allowed the truck to track onto position, carry out the testing and get clear of the advancing tide in the shortest amount of time. An onboard engineering geologist provided real-time interpretation of soil conditions and assisted RHDHV in the selection of suitable locations for dissipation tests within the underlying zones of compressible material.
Carlton Hall, Engineering Director at Lankelma, said: “We are very pleased with how smoothly the investigation for the latest phase of the Borth coastal protection scheme went. The value of abundant data in such a laterally variable beach environment is high and CPT is the ideal technique. Its selection enabled us to demonstrate the speed, versatility and accuracy of the method when gathering geotechnical data in such a challenging environment.”
George Marjane, Geotechnical Engineer at Royal Haskoning, added: “Undertaking ground investigation work in intertidal areas can be very challenging. Lankelma managed the significant health and safety challenges by selecting appropriate plant and by implementing a well thought-out action plan in case the rig became stuck on the beach on a rising tide.
“The site team were well trained and very flexible which allowed us to complete the work safely and within the programme. Finally, the factual report produced by Lankelma was in a simple and easily digestible format, making our assessment of the site ground conditions more efficient.”