Investigation Methods and Procedures - Geotechnical Engineering Investigation - page 113

Sampling Methods and Devices
The application, description, and penetration depths of the various devices and methods
are summarized in Table 2.17. The devices include the following:

Petersen dredge

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Harpoon-type gravity corer ( Figure 2.72)

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Free-fall gravity corer

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Piston gravity corer ( Figure 2.73)

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Piggot explosive coring tube

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Benthos Boomerang Corer ( Figure 2.74)

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Vibracore ( Figure 2.75) is the most practical method for obtaining long cores in
soft soils in deep water. It can also be used on land to sample remotely contam-
inated lagoons where a soft crust makes access hazardous to people.

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2.4.5 Rock Coring

Objectives
Rock coring is intended to obtain intact cores and a high percentage of core recovery.

Equipment
Rotary drilling machine, drill rods, a core barrel to receive the core, and a cutting bit are
needed.

Operations
The core barrel is rotated under pressure from the drill rig applied directly to it, while
water flows through the head, down the barrel, out through the waterways in the bit, and
up through the rock hole and casing (in soil) to return to the surface. When the rock is first
encountered in a borehole, the initial core runs are usually short because of the possibility
that the upper rock will be soft and fractured. As rock quality improves, longer core runs
are made.
The core barrel is generally rotated between 50 and 1750 r/min; rotation speed is a func-
tion of the bit diameter and rock quality. Slow speeds are used in soft or badly fractured
rocks and high speeds are used in sound hard rocks. If large vibrations and “chatter” of
the drill stem occurs, the speed should be reduced or core recovery and quality will be
severely affected.
Bit pressure is also modified to suit conditions. Low bit pressure is used in soft rocks
and high pressure is used in hard rocks. When vibrations and “chatter” occur, the pres-
sure, which is imposed hydraulically, should be reduced.
Fluid pressure should be the minimum required to return the cuttings adequately to the
surface to avoid erosion of borehole walls. If there is no fluid return, drilling should imme-
diately stop and the core barrel returned to the surface to avoid overheating the bit, which
would result in bit damage (loss of diamonds) and possible jamming in the hole.
Lack of fluid can result from:

Blockage of the core barrel, which occurs in clayey zones. Continued drilling
after a broken core has blocked entry into the core barrel results in core grinding.
Indications of blockage may be heavy rod vibrations, a marked decrease in pen-
etration rate accompanied by an increase in engine speed, return fluid more
heavily laden with cuttings than normal, and a rise in circulation fluid pressure.

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