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considered a“blind” system. Several tests have been carried out and it has been noted that the operator can easily handle the change of stratigraphy (i.e. stop the drilling or decide when to change the tool) within a rangę of 10-20 cm.

In order to increase the safety of the cones, there are two alternative ways to go:

1)    alteration of the length of the piezocone in front of the drillstring system and

2)    a safety system which allows for the “return” of the piezocone back into the core barrel when the maximum value of resistance has been reached.

CPTU, as described above but without the detection of the MWD parameters, can also be used. In that case, the data collector connected to the drill rig only detects the depth versus timc, in order to synchronise the CPTU data with the depth data, exactly like the “normal” CPTU tests in memory modę.

*    DOWN-HOLE CPTU: the whole drillstring is lifted 1.5 - 3 m; a special core barrel having the memocone protruding for the same length (i.e. 1.5-3 m) is lowered and hooked inside the drillstring; then it is pushed down by the drill-rig (without rotation) for the same length. This allows to check and calibrate the CPTWD having some “pure” (not affected by rotation and/or mud injection) CPTU data.

*    DOWN HOLE tests of other sort: at any depth it is possible to lift the core-barrel and perform any type of test (SPT, Permeability test, Dilatometric, Vane test, etc).

The different options exposed above can be selected according to the type of soil as well as the scope of the bore-hole. The drillstring actually remains the same and the tools related to the three options are interchangeable any time the operator decides to do so.

2.5 TECHNICAL DETAILS OF THE CPTWD

The generał scheme of the CPTWD prototype is shown below:

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The system is basically madę up of:

•    Casing tubes. Those are of the same type used in c.c. drilling. For the prototype, the extemal diameter was 127 mm, which allows them to collect 0 86 mm cores and 0 88.9 mm (standard) undisturbed samples. The bottom part of the casing is eąuipped with a specially designed drill- bit.

•    Hooking system. There are normally four hooks placed inside the core barrel, shaped in a way which allows them to get into (once they are unhooked from the overshot) the slot placed inside the casing, at a distance equal to the length of the core barrel.

•    Core barrel. In this case it is not quite appropriate to use the definition core barrel because it is not used to carry out c.c.drilling operations, but it is basically a wireline barrel used to carry out drillings in gravel, which