As an alternative to push the blade using a penetrometer, the blade can be advanced using a drill rig, by the torpedo(en/ita) method. Drill rigs (typically 2-4 tons) are much lighter than penetrometers. Hence, unless the soil is soft, the drill rig can only push a few meters.The insertion of the torpedo has some analogy with SPT, as the torpedo is lowered at the bottom of a hole before testing. The casing is almost always necessary, unless one can be sure that the hole sides will not cave in, damaging the cable.

The torpedo(en/ita)  is composed by an upper adaptor, 3 ms of rods, a lower adaptor, a DMT blade. The upper adaptor has a slot permitting the lateral exit of the cable. The torpedo executes 3 m of sounding starting from the bottom of a borehole. If the soil is easily penetrable the torpedo can be 4 ms or more, otherwise say 2 ms or less. The torpedo is generally advanced by pushing the top of the drill rods (typically OD 76 mm, unlikely to buckle). If the soil is hard the penetration will be less than 3 ms, but it is desirable to have at least 0.6 m or better 1 m of penetration, in order to reach soil not softened by the hole.

The torpedo is pre-assembled before starting the test, and is connected at the bottom of to the drill rods each time it has to be lowered downhole. The cable does not run all the way inside the rods up to the surface,  a time consuming operations, but exits laterally at the top of the torpedo. Above this point the cable is taped to the outside of the rods at 1-1.5 m intervals up to the surface. Since the unprotected cable is vulnerable, the adaptor on top of the torpedo has a collar (see figures). The collar has a vertical channel for the cable and has a diameter larger than the upper rods so as to insure a free space between the upper rods and the casing. The operator should not allow the slotted adaptor and the exposed cable to penetrate the bottom of the hole, to avoid damaging the cable.

The torpedo method takes longer than using a penetrometer, but has the advantage of permitting to overcome (destroying) obstacles. The torpedo method permits the execution of very deep DMT soundings (max depth: 135 m in L’Aquila 2009).

Fig. 1. Upper slotted adaptor for the lateral exit of the cable and with a collar to protect the cable above the adaptor.

 

Sequence of operations of Torpedo Method

  • Lower the torpedo down to the bottom of the hole..
  • Push the torpedo and do DMT for 3 m (stopping before the exit of cable reaches the bottom of the hole).
  • Extract the torpedo.
  • Remove the soil that was penetrated and tested by the blade, thereby advancing the borehole down to the max depth previously reached by the blade.
  • Lower the torpedo down to the bottom of the hole and resume testing..

The diameter of the casing of the borehole should be as small as possible, to minimize the risk of buckling (possibly 100 mm ID). The penetration must always occur in “fresh” (not previously penetrated) soil.

It is generally convenient to use a 4 m long extendable pneumatic-electric cable, connected to the blade and egressing from the upper slotted adaptor. Then normal length pneumatic-electric cable will be attached to the 4 m cable.

More details on the Torpedo Method

Illustrative page [Torpedo method]

TC16 Report – Drill Rig

TC16 Report -Rod Adaptors 

Fig. 2. Torpedo about to be inserted in a Borehole.

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