The concept of a marine Free-fall CPT systems revisited
Abstract
Cone Penetration Tests (CPT) are a widely used method for the geotechnical in situ characterisation of sediments in onshore settings. In the course of enhanced use of the continental shelf and slope by humans impact, the need for time- and cost-efficient solutions in the marine realm is emerging. In order to avoid the challenges of lowering heavy gear to the seafloor for the penetration tests, free drop devices have been developed since the 1970s, however, the use of the majority of them was discontinued owing to technical difficulties in measurement and data interpretation caused by the impact. Based on those experiences, two different free-fall CPT lances were designed for the in situ measurement of strength (tip resistance, sleeve friction), pore pressure and temperature. In addition, deceleration and tilt are monitored for vertical profiling of the penetrated sediment column. Both CPT systems rely on an industry 15 cm2 piezocone with the sensors at the tip and a pressure housing containing a microprocessor at the top. The lightweight (40-100 kg), shallow water (200 m depth) lance works completely autonomous with a volatile memory and battery package, and can be deployed from any platform, even without a winch. The sturdier, deeper water (2500 m depth) system uses both power and telemetry for data transmission from the research vessel. The length and weight of either system can be varied according to the sediment stiffness and hence allow variable penetration depth (usually less than 5 m). Initial use of the CPT systems attests their efficiency and reliability in the measurement of sediment physical properties. While most of the data are collected within the first seconds of the CPT experiments, long term (pore pressure dissipation tests have also been successfully carried out for durations in excess of 6 hours. In a variety of estuarine and marine settings, our studies served objectives such as the assessment of slope stability, navigability of harbours/estuaries, consultancy for cable laying and trenching, or ground-truthing of geophysical data.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2005
- Bibcode:
- 2005AGUFMOS21A1508K
- Keywords:
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- 0450 Hydrothermal systems (1034;
- 3017;
- 3616;
- 4832;
- 8135;
- 8424);
- 0468 Natural hazards