Case Study: Seismic Cone Testing

Proposed service harbour

This work was part of a project to provide sub sea-bottom information for a proposed service harbour in the south east of the country.  The ultimate client was the Venezuelan state oil company who are developing gas fields in the Caribbean Sea.

Jack-up barge

The seismic cone was operated in conjunction with Geohidra’s own CPT equipment.  Geohidra is currently the only Venezuelan company with the capability to carry out cone penetration testing.  A jack-up barge was used as the platform for the testing and the pushes were made with an onboard drill truck, the seismic cone being attached to Geohidra’s 45mm push rods.  Tip resistance, sleeve friction and inclination were recorded concurrently with the seismic data.

Shear waves

A single modular unit comprising of 3 geophones mounted in the 3 orthogonal planes (x, y and z) was used to detect shear waves down to a depth of 36.5m below mud line in a maximum of 15m of water.  The shear waves were created using a hammer provided by Gardline Lankelma and lowered on to the seabed from the barge. 

Data

The ground consisted of layers of soft clay, dense sand and gravel.  Processing of the data provided a range of travel speeds consistent with changes in density, these shear-wave velocities then provided the basis for calculation of dynamic shear modulus (Gmax).  The evaluation of Gmax is part of the geotechnical data required for projects in a region where the possibility of seismic activity has to be taken into consideration.