Civil Engineering Reference
In-Depth Information
generally be avoided if other means are available for making a boring. If the
large rig must be used, wire-line procedures will minimize the rig time devoted
to soil-boring operations.
Existing diver-operated equipment consists of fairly conventional drilling
equipment adapted to work on the sea floor with support from a surface vessel.
The diver-operated approach to drilling and sampling may become more com-
petitive with other traditional methods, but, diver training must also include
experience in soil sampling.
Several pieces of equipment have been developed to operate on the sea floor
by remote control from a surface vessel. To date, this equipment, known as a
remotely operated vehicle (ROV), does not have the capability to sample at
the depths required to investigate deep-pile foundations. As development con-
tinues, the use of such equipment may become more feasible.
Much attention has been given to use of small manned submersibles to per-
form various underwater tasks without exposing personnel to the pressures
associated with diving. These devices have been equipped with manipulators
of various kinds, and they have been used to perform in-situ tests at shallow
penetrations while resting on the ocean bottom. A logical extension of this tech-
nology would be to equip a submersible to drill and sample at significant depth;
no such equipment has yet been built, although a design has existed since 1996.
4.3 SOIL TESTS
The wet rotary process, which is commonly used with onshore soil boring, is
also used for offshore soil testing. In this respect, there is little difference
between onshore and offshore practice except for some details in the way the
objective is accomplished. However, the different offshore environmental con-
ditions have necessitated changes in soil-sampling procedures. An understand-
ing of onshore sampling techniques, tools and results will aid in understanding
the required alterations, the concessions made and the advantages of the off-
shore procedures.
Most onshore sampling is done by what is termed conventional means,
which are also used in offshore sampling. At the desired sampling depth, the
drill pipe is pulled from the hole and the drill bit is replaced by a soil sampler.
The sampler is run to the bottom on the drill pipe. After the sample has been
taken, the drill pipe is again pulled to retrieve the sample, then the soil sampler
is replaced by the bit and the drill pipe is run back into the hole to advance the
boring to the next sampling interval.
The cost of soil investigation offshore is very high, as is the case with pre-
liminary engineering. There are many sources of useful information and data
about soil characteristics as they relate to the geologic information for the plat-
form location, drilling records, acoustic data or weather and sea state.
The most common method of sampling cohesive soils (clay) is to push a
thin-walled tube into undisturbed material below the bottom of the drilled
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