In areas where soil cover is thin, the location
and testing of bedrock mineralisation is made
relatively straightforward by the examination
and sampling of outcrops. However in locations
of thick cover such testing may involve
a deep sampling program by pitting, trenching,
or drilling. Pitting to depths of up to 30 m is
feasible and, with trenching, forms the simplest
and least expensive method of deep sampling
but is much more costly below the water table.
For safety purposes, all pits and trenches are
filled in when evaluation work is completed.
Drilling penetrates to greater depth but is more
expensive and requires specialized equipment
and expertise that may be supplied by a contractor.
Despite their relatively shallow depth,
pits and trenches have some distinct advantages
over drilling in that detailed geological
logging can be carried out, and large and, if
necessary, undisturbed samples collected.1) Pitting :-
In areas where the ground is wet, or labor is
expensive, pits are best dug with a mechanical
excavator. Pits dug to depths of 3–4 m are common
and with large equipment excavation to
6 m can be achieved. In wet, soft ground any pit
deeper than 1 m is dangerous and boarding
must be used. Diggers excavate rapidly and pits
3–4 m deep can be dug, logged, sampled, and
re-filled within an hour. In tropical regions,
thick lateritic soil forms ideal conditions for
pitting and, provided the soil is dry, vertical
pits to 30 m depth can be safely excavated. Two
laborers are used and with a 1 m square pit,
using simple local equipment, advances of up
to 2 m per day down to 10 m depth are possible,
with half that rate for depths from 10 to 20 m,
and half again to 30 m depth.2) Trenchimg :-
Trenching is usually completed at right angles
to the general strike to test and sample over
long lengths, as across a mineralized zone.
Excavation can be either by hand, mechanical
digger, or by bulldozer on sloping ground.
Excavated depths of up to 4 m are common.
pliz talk about drilling also
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