The Civil Engineering Contractor June 2019 | Page 32

FEATURE
Shaun Nell, MD of geotechnical
contractor Terra Strata Construction. Dave Rossiter, non-executive chairman
of the GeoGroup. Trevor Green, geotechnical engineer
and head of the Geotechnical
Department at Jones & Wagner.
over the past 30 years. Recently,
the bridge over the Zambezi River
at Kazangula was investigated
successfully by Geomechanics using
a jack-up platform designed and built
by the company in its workshops in
Lanseria near Johannesburg.
“In this case, holes of larger
diameter than usual were drilled in
the river bed to establish the founding
conditions for the piers and abutments
of the bridge. These holes were drilled
with PQ core barrels which give a
core size of 84.7mm and Lefranc
permeability tests were conducted on
the shore boreholes in the alluvium.
The accuracy of the positioning of
these holes was of utmost importance
and a Trimble survey instrument was
used to position the barge and drill
rig within 20mm of the required
position,” says Rossiter.
Another recent investigation was
conducted successfully on lake Albert
in Uganda [see Infra Africa, page 10]
when Geomechanics modified the
same Jack-up platform to work in
11m of water and conducted CPTU
testing and NWD4 rotary core
drilling in the lake bed for a pipeline
to supply water to the oil company’s
central process facility. This facility
will prepare crude oil to be pumped
some 1 300km away to the sea at
Tanga in Tanzania for export.
The full geotechnical investigation
in Uganda and Tanzania has taken over
two years to complete and involved
drilling 15 holes in lake Albert,
seven holes on the Nile river where the crude oil pipeline crosses the
Nile river from the north, 60 holes
at the central process facility area,
numerous holes on the well pads,
as well as investigating the ground
conditions for the 1 300km pipeline
through both Uganda and Tanzania.
Dam sites and hydro power schemes
are generally located in mountainous
areas and often require specialised
equipment and methods to access the
sites like river barges or helicopters
and require specialised in situ testing
to provide the necessary design
information to the design engineers.
Geogroup also uses sonic drilling
where necessary. Geomechanics has
two sonic rigs both of which are
mounted on rubber tracks to access
rough terrain sites and to do so with
minimal damage to the environment.
Rossiter explains that this method
involves high frequency resonant
vibrations which are sent down the
drill string to the drill bit, with the
operator controlling the frequencies
to suit the soil/rock geology.
“The resonance magnifies the
amplitude of the drill bit, which
fluidises the soil particles at the
bit face, allowing for fast and easy
penetration through most geological
formations. An internal air spring
isolates these vibrational forces
from the rest of the rig, and by
providing the necessary rotational
and vibrational forces, the sonic rig
is able to core and case holes in any
overburden material, drilling where
most other rigs can't. The cores are held in the core barrel by friction
and/or by the use of core catchers
as required, and core samples are
gently extruded by vibration typically
into plastic sleeves. Compressed air
is available to assist core extraction if
necessary,” he says.
There are a number of benefits
to sonic drilling, though they are
extremely expensive. Rossiter describes
it as better suited to softer ground
formations such as dune sands where
mineral sands will be investigated or in
granular alluvial ground.
He lists the advantages as follows:
• Superior information.
o Continuous, relatively
undisturbed core sample
through any type of formation.
o Continuous core samples to
depths of more than 100m with
or without using any drilling
mud.
• It is two to three times faster.
• Superior well construction
causing minimal disturbance to
the surrounding borehole wall.
• Flexibility – it advances a
temporary outer casing as the
borehole is drilled, allowing you to
do more within a single borehole.
• Risk minimisation.
o Reduces the risk of project
failure due to unknown or
difficult subsurface conditions
o Recovery rate in excess of 95%
o Projects finish on time and
on budget
o Obtains the lowest total
project cost possible. nn
30 | CEC June 2019
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