Selection of Mining Method

The hard massive marlstone which forms
the overburden will require blasting before
removal can take place and this rules out the
use of a dragline or a bucket wheel excavator.
Rear-dump trucks and face shovels will have to
be used. Three alternative mining configurations
may be considered using this equipment:
(i) advance down the dip;
(ii) advance up the dip; or
(iii) advance along the strike.
The third method, also known as terrace mining, was
considered the most suitable with the particular
geological and geotechnical conditions
which exist at Soma.
Following this method a box-cut would be
excavated down the full dip of the deposit from
the outcrop until an economic stripping limit,
or practical mining depth limit, is reached.
This study concluded that a maximum mining
depth of 150 m was feasible, although constraints
were generally related to the geological
configuration and consideration of maintaining
footwall stability, rather than to stripping
ratio economics. Advance could then be in one

or both directions along the strike line, depending
on the box-cut location within the proposed
mining area. Waste disposal within the excavation
is practical (and usually desirable) as the
box-cut excavation is enlarged. Horizontal
benches would be formed in the overburden
along the advancing face and along the
highwall formed on the deep side of the excavation.
As much spoil as possible from the
advancing face would be removed along the
highwall benches for disposal to form an internal
spoil dump within the excavation and
behind the advancing face; the remainder having
to be dumped outside the pit. The front
of the spoil dump and the advancing face then
advance in unison as mining continues along
the strike of the deposit. The final
highwall is expected to have a maximum slope
angle of 45 degrees.
The application of this mining method will
bring several advantages:
(i) a minimum area of footwall clay will be uncovered at any time, thus reducing the risk of footwall failure; (ii) internal dumping of waste reduces transport costs, helps stabilize the footwall, and begins reclamation at an early stage of mining; and
(iii) the stripping ratio is more or less constant over the life of the mine, and hence the mining costs

are stabilized.