A new test, called the seismic cone penetration test (SCPT) is described. A small rugged velocity seismometer has been incorporated into an electronic cone penetrometer. The combination of the seismic downhole method and the CPT logging provides and extremely rapid, reliable and economic means of determining stratigraohic strength and modulus information in one sounding. Results using the seismic cone penetration onshore in-situ techniques. Results are also presented from seismic cone testing performed offshore in the shallow Canadian Beaufort Sea.
In the offshore environment, the dynamics behaviour of soil can be of great importance. Rapid development has taken place in the past decade on new analytical and dynamic testing techniques.
For onshore investigations, the crosshole and downhole methods have become the standard techniques for dynamic testing to determine the in-situ shear wave velocity. A polarized shear wave is generated in one borehole (or at the surface), and the time is measured for the shear wave to travel a known distance to the geophone in the borehole. Elastic theory related the shear modulus, G, soil density, p. and shear wave velocity, Vs' as follows:
Hence, the shear modulus can be determined using in-situ seismic methods for the determination of the shear wave velocity. The shear modulus is largest at low strains and decreases with increasing shear wave strain (Seed and Idris, 1970). The shear strain amplitude in in-situ seismic methods for the determination of the shear wave velocity. The shear modulus is largest at low strains and decreases with increasing shear stain (See and Idris, 1970). The shear strain amplitude in in-situ seismic tests is usually low and of the order of 10-4%. Thus, the very low strain level dynamic shear modulus, G max' is usually obtained. The cost of such a test is usually high because of the requirement to have one or more boreholes. This has generally made the technique difficult or impossible for offshore use.
A new test, called the seismic cone penetration test (SCPT), which can dramatically reduce the cost associated with the in-situ determination of shear wave velocity, will be described in this chapter.
The cone penetration test (CPT) is already used extensively offshore for geotechnical investigations. A cone of 10cm2 base are with an apex angle of 60°is generally accepted as standard, and has been specified in the European and America n Standards. A friction sleeve, located above the conical tip, has a standard are of 150 cm 2. A pore-pressure transducer has recently been added to measure the dynamic pore pressure during penetration. The cone penetrometer is pushed at the standard rate of 2.0 cm/s. A small slope sensor is usually incorporated to monitor sounding verticality.
Full details of the design of an electronic cone are given by Campanella and Robertson (1981) and Schaap and Zuidberg (1982). The piezometer-CPT is regarded as the premier test for the continuous logging of soil stratigraphy and shear strength. An example of the extensive data obtained from a piezometer-CPT is shown in Fig.1.
