Flow cone–new CPTU add-on module trialled in Halden silt

Abstract

Hydraulic conductivity is an important soil parameter for design of shallow foundation concepts for offshore wind, and there is a need for a new tool that can measure this parameter in a reliable way. A new module has been developed that can be mounted behind a standard piezocone test (CPTU) probe. Water can be injected in a controlled manner into the CPTU equipment and flow out into the surrounding soil through a filter an offset behind the friction sleeve, while water pressure (uf) is measured by a transducer mounted in the filter itself. During penetration of the CPTU probe, water flows out at a constant rate, while uf is measured in addition to CPTU parameters qc, fs and u2. At desired depth intervals penetration can be stopped and either a dissipation test or constant flow rate test can be carried out to determine hydraulic conductivity. Hydraulic fracture tests can be performed in low permeability soils, where water flow is used to induce a vertical crack in the soil. The vertical crack is then allowed to close while pore pressure is monitored, from which the closing pressure can be determined and subsequently used to determine the in-situ K0-condition. This paper describes a series of hydraulic conductivity tests and a hydraulic fracture test carried out at one of Norway’s recently established geotechnical test sites, a silt dominated site in Halden. Several tests were successfully carried out and the results were benchmarked against hydraulic conductivity as measured by falling head tests in standpipes and laboratory tests. In general, the results compared well to credible benchmarking tests, showing a promising potential for this tool.