The Civil Engineering Contractor June 2019 | Page 31

FEATURE
Sonic drill rig in operation.
site, the geology present, the loads
that one is going to apply to the
ground and the type of structure to
be built, explains Dave Rossiter, non-
executive chairman of the GeoGroup.
Specialised equipment is used to
access mountains, lakes and rivers
and specialised drilling methods and
in situ testing is required for specific
types of structures. Generally, the
more weathered the ground, the
deeper one will have to investigate
until sold rock is found. “For instance,
the Fourways area is primarily
granitic; Centurion dolomitic; the
East Rand lies generally on Ecca series
shale; and the south predominantly
on andesite lava – and each rock type
has different characteristics.
“These different soil and rock types
will have different characteristics
which will determine how they
are investigated. For instance, for
lighter structures you can, in the
generally shallow weathered granites,
simply make use of a trailer mounted
Dynamic probe super heavy (DPSH)
rig to conduct a DPSH test to
understand the bearing capacity of
the soil. But in the dolomites, even
for lighter structures, you will need
information from greater depths
to understand if there are cavities
present due to the manner in which
the dolomites weather,” says Rossiter.
www.civilsonline.co.za
A different proposition is posed
by the lava rock in the south of
Johannesburg, which weathers
in spheroids, with large boulders
sometimes interspersed in soft clay,
‘hard and soft alternating layers
which can go on for about 30m,
which is also a troublesome rock to
build on because of its inconsistency,
especially for dynamic loads’.
“The dolomites are quite different
in that there’s no consistency. Below
a hard crust, there may be anything
from a wet muddy cavity full of
wad, to a fresh pinnacle of dolomite
with a strength of 200Mpa or into
a honeycomb mix of spoil and rock
which can extend for as much as 80m
below the surface,” he says.
This challenge would generally be
met with some geophysics testing
and backed up with the cheapest
form of drilling such as air drilling
or percussion drilling. This method
involves pumping air from a
compressor down a rod string and
into a ‘down the hole hammer’ which
hammers the rock and blows the
sample back up to the surface.
Rossiter explains that for this
investigation to be successful you
must have information on sample
return, air loss during drilling, water
colour and speed of drilling as well
as signs of sudden voids encountered
while drilling. This can be best
achieved by using the Jean Lutz (LT3)
data collection system which collects
a range of seven different parameters
electronically during drilling. Manual
recording of these parameters with
a stopwatch is often misleading and
confusing especially when cavities are
present.
During a recent piling project for
a bridge on the Berg River at Val de
Vie estate near Paarl, Geopile Africa
(part of the Geogroup), found the
underlying targeted Malmesbury
shale to be covered with about 7m
of large rounded quartzite boulders,
which without the combination of
the Atlas Copco Symmetrix drilling
and grouting system, they could not
have piloted stable holes through
the boulders to accommodate the
170mm diameter ductile iron piles.
“This is the same method that was
successfully used on the Gautrain
geotechnical investigation project, to
install PVC ground penetrating radar
tubes for Bombela on the dolomite
rock in Centurion,” he says.
“Geotechnical investigations for
large span bridges or water extraction
pipelines can often demand the use of
jack-up platforms on which to drill
from, over water. Geomechanics has
conducted numerous geotechnical
investigation in rivers and lakes
CEC June 2019 | 29