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Marine environments
Performing site investigations within the marine environment is critical both in terms of quality of data obtained and sensitivity to the operating environment (i.e. waves, currents, wind).
Typically, in exposed marine environments, weather down for conventional geotechnical plant and equipment (drilling platforms and barges and survey vessels) can be as much as 100-500% even under the management of experienced marine contractors.
Woolacombe Bay
Lankelma completed an intertidal marine site investigation for an outfall extension project, using a subsea ROV crawler unit combined with our Neptune CPT system within the surf zone at Woolacombe Bay.
Subsea CPT crawler unit
After initially considering performing the tests from the shore using conventional land CPT plant, Lankelma determined that the works could only realistically be performed using a subsea-crawler or a remotely operated vehicle (ROV). However, no such system existed with the necessary geotechnical capability.
Neptune CPT unit
Lankelma then selected to mount one of our seabed frame CPT units onto the ROV to enable underwater CPTs to be performed. The chosen system was the Neptune. This system, normally deployed from floating marine plant in water depths up to 2,000m is some 1.5 tonnes and uses a 2cm2 piezocone, with a depth capability of up to 10m below bed level.
The ROV CPT unit weighed some 4.5 tonnes (air), which was sufficient to provide the necessary reaction weight for the proposed 5m piezocone (CPTU) tests, yet light enough to overcome the hazard of potential aerated sands.
The Neptune was controlled via a strengthened deck umbilical linked to the ROV main umbilical.
System deployment
The system was deployed from a marine based landing craft after restrictions (due to tourist health and safety) were imposed by the landowner. Also, the distance between the access point and investigation area was too great for the available control umbilical.
During the first falling tide, the landing craft was moved onto a small section of the beach, next to the rocky foreshore. The craft then lowered the front plate and the crawler ROV was driven down onto the beach.
The landing craft was then moved out into deeper waters and the ROV positioned and made operational within the survey area.
Survey and positioning
Precise co-ordinates and levels were required both for the exploratory locations and the topographic survey. Therefore, a conventional shore based EDM total station was used, and an 8m mast, detail pole and single prism. All 19 CPTs and 55 topographic survey locations were determined to 0.1m horizontal and 0.01m vertical accuracy according to OS National Grid datum.
Single borehole
All CPTs were performed to depths up to 5m below bed level. Refusal was encountered within the underlying slate.
In order to determine the likely geotechnical properties of the underlying rock, a single borehole was drilled to 20.05m within the pumping station compound on top of the foreshore cliffs. The borehole was drilled by Geotechnical Engineering using a Pioneer rotary drilling rig in just over 2 days.
Ground conditions encountered
The ground conditions encountered consisted of topsoil underlain by very sandy very gravely silt to 1.10m in depth. The beach deposits consisted of very loose to medium dense sand. Beneath the overburden a series of very weak to progressively strong slate, with very closely to closely spaced inclined and sub vertical planar fractures.
The samples obtained from the hole were used to provide the best and worst case strengths and weathering grades of the underlying rock, should it be encountered by the outfall installation operations.
Marine conditions
Whilst the ROV was designed to operate within the North Sea environment in water depths up to 2,000m, the combined ROV and CPT system had never been used successfully before, particularly within a surf zone such as that at Woolacombe.
Throughout the project, the system was successfully deployed within 0.5-1m seas, operated in 2.0-2.5m seas and recovered in 1.5-2m seas (via on-board hiab).
Reporting & geotechni cal parameters
The following geotechnical parameter profiles were determined from the CPTs and reported to the client:
In addition to the factual report, a topographic survey plan was produced using the co-ordinate and level data.
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