Geomechanics cc28 Central Road, Sunrella, Gauteng, South Africa PO Box 68063, Bryanston, South Africa, 2021
Tel: +27 11 451 7760 Fax: 086 663 3896
THE PROJECT
Cahora Bassa Northern Bank Power StationGeotechnical Study and Assessment on Parameters for Civil Engineering Design of the Cahora Bassa Northern Bank Power Station - Mozambique
OVERVIEWGeomechanics is contracted by Ingérop Africa to perform a geotechnical study and
assessment on parameters for the Civil Engineering Design. The objective is to further
investigate the Northern bank for the Cahora Bassa North Bank peaking Hydroelectric
Power Plant located in the Zambezi River of the Tete Province, in the north west of
Mozambique.
The geotechnical investigation including drilling, water pressure tests and parts of the
in situ testing on the steep slopes of the North Bank began in April 2011 and is due to be
completed in August 2011.
About 240 km below its confluence with the Luangwa River, the Zambezi passes through a major gorge incised
into large intrusive rock masses. The eastern end of the gorge is dammed to form the Cahora Bassa Reservoir
which regulates the Zambezi flow regime for hydropower production. The total generating capacity of Cahora
Bassa hydropower station is 2075 MW through 5 hydraulic turbines.
About 240 km below its confluence with the Luangwa River, the
Zambezi passes through a major gorge incised into large
intrusive rock masses. The eastern end of the gorge is dammed to
form the Cahora Bassa Reservoir which regulates the Zambezi
flow regime for hydropower production. The total generating
capacity of Cahora Bassa hydropower station is 2075 MW
through 5 hydraulic turbines.
This project was initiated to increase the hydro power production
from the Cahora Bassa hydropower station by 1,245 MW.
A total of thirteen vertical boreholes will be drilled to a depth
between 60 and 260m. Seven of these will be drilled with the
conventional core barrel, hole diameter 75.7mm and core
diameter 60.3mm, and the remaining six holes will be drilled with wire line core barrel, hole diameter 75.7mm and
core diameter 50.6mm. In addition there is one borehole that will be drilled horizontally from a 50m deep, adit
situated on the left abutment of the dam wall.
Some borehole positions are situated on steep rock faces, others downstream of the
Cahora Bassa dam wall alongside the raging Zambezi River. Three borehole positions
had not yet been marked at the start of the project because of the difficulty of accessing
them.
An indication of the severity of the environment is the fact that a Bell 407 helicopter is
required to place the drilling equipment at the borehole locations.
S U C C E S S S TO RY
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Geomechanics cc28 Central Road, Sunrella, Gauteng, South Africa PO Box 68063, Bryanston, South Africa, 2021
Tel: +27 11 451 7760 Fax: 086 663 3896 Page 2
S U C C E S S S TO RY Cahora Bassa Northern Bank Power Station
MEETING THE CHALLENGES
Geomechanics knows the local conditions and the challenges presented very well as a result of work carried out in
the North Bank during a 1999-2001 investigation into 3 potential dam sites on the Zambezi River. Despite flood
conditions on the Zambezi in two successive seasons, Geomechanics completed the works timeously and within
budget.The challenges are not necessarily as a result of the
type of drilling required but as a result of the range of
unusual tests required in some of the boreholes and
the fact that the boreholes are all of varying depths
and scattered around a mountain side. Some of the
borehole positions are on the edge of cliffs with a
150 -300 m drop either onto a rocky out crop or
straight into a very turbulent Zambezi River.
The only way to ensure that equipment and personnel arrive safely at drill
positions is to use Bell 407 helicopter which brings with it, its own set of
challenges.
THE PROJECT
Testing
Water pressure testing is performed using a single packer system as the drilling proceeds on each borehole and
dilatometer tests are performed at the end of each drilling phase.
A total of 100 water pressure tests and 30 dilatometer tests with a probe allowing for assessment of the anisotropy
will be carried out. 20 of the dilatometer tests will be carried out at a depth range of 0 – 100m, with the remaining 10
tests being carried out in the 100-200m depth range.
In addition, core sections are tested on site with the point load test equipment and core samples are taken to be
laboratory tested. 70 point load tests will be performed on core sections to evaluate the homogeneity of the intact
rock strength and the zones of weathering and weakness. Sclerometer tests will be performed at the location of the
point load tests to develop a correlation between the parameters measured by the two tests.
Additional tests are required on the horizontal borehole. Continuous and oriented coring will be placed in boxes for
logging. Four over coring tests will be performed at distances between 20 and 50m from the wall of the adit for
measuring in situ stresses. Stress relief will also be measured while over coring.
Geomechanics cc28 Central Road, Sunrella, Gauteng, South Africa PO Box 68063, Bryanston, South Africa, 2021
Tel: +27 11 451 7760 Fax: 086 663 3896
MEETING THE CHALLENGES
Preparing the borehole sitesMany of the drill locations require preparatory bush clearing and the building of sturdy drill platforms before the drill
rigs can be lowered via helicopter to their positions.
The platforms have to be built either into or onto rock faces and need to be large enough to accommodate a
machine the size of a LY44 with its tripod and sturdy enough to withstand the pressures of a borehole 260m deep.
Cement platforms 2.4m x 1.6m x 300mm deep are being built.
Moving equipment and personnel to borehole locationsMoving the drill rigs to these platforms is challenge in itself. Across the dam, at the Geomechanics Laydown area
on the Southern Bank, a team of technicians, supervisors and mechanics prepares the rigs for their flight across
the waterway to their respective platforms. Each machine has to be disassembled into parcels not exceeding
750kg, the maximum weight the helicopter is permitted to lift. Each parcel is safely rigged and then inspected and
approved by the pilot for lifting. Only once the platforms are completed and all the parcels of machinery are
approved is the team of supervisors able to start dispatching the helicopter. This process has to be repeated as
much as three times per machine. The horizontal borehole situated at the base of the dam wall, in a 50 meter
deep adit presented its own set of challenges.
The original plan to utilize a 50ton crane at full extension to lower the
generator and equipment to a 30 meter square platform at the entrance to
the adit failed. The only option was to get the
helicopter to sling the generator and equipment
into place but this was not a simple task. The
location issurrounded by obstacles; a 150m
vertical rock face on the left bank; sluice gates
releasing 1000 cubic meters of water per
second on the right bank, and conditions that
resembled a tropical thunder storm.
Some positions are situated in small crevasses so
an entire structure has to be built to support the
cement base for the drill platform.
Local Mozambique casuals, now qualified stone
masons, clear the bush and prepare the locations for
the drill platforms using only what Mother Nature
provided. Once the platforms are prepared bags of
aggregate for the concrete bases are carried up the
steep slopes to the various positions. Two RB35
Bean pumps push water from the edge of the dam up
the steep slope some 300m high to a 12000lt
Aquatan dam situated high above the various
platforms. A web of PVC piping makes its way down
to each platform and with gravity to assist provides each platform with water.
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Geomechanics cc28 Central Road, Sunrella, Gauteng, South Africa PO Box 68063, Bryanston, South Africa, 2021
Tel: +27 11 451 7760 Fax: 086 663 3896
THE OUTCOME
MEETING THE CHALLENGES
According to
co-ordinate such a technical operation while at the same time trying to minimise helicopter flight hours to
control costs. Team work is the factor that defines our success in this project.”
Brett Mannix, Geomechanics Site Agent, “It takes a very skilled team of people to plan and
Drilling and testing is continuing on this project.
About GeomechanicsGeomechanics and Geomech Africa provide a drilling and testing service to the minerals exploration market as well as the Geotechnical and Civils industries and have over 20 years experience in Southern Africa.
Today, through a continued policy of upgrading of staff and equipment through training, replacement and maintenance, Geomechanics is one of the leaders in its field.Geomechanics cc is the founding company of the GeoGroup. The GeoGroup is a group of companies comprising Geomechanics, GeoCrush, GeoStone, GeoPlant, Geomech Africa, GeoPower Africa, Terra Power Solutions, GeoSolar Energy, GeoSolar and Green-a-Planet.These companies provide a range of services from geotechnical and exploration drilling, renewable energy and emergency power supply systems.
The essence of the GeoGroup is entrepreneurial initiative, integrity and honesty.
Health and Safety Health and Safety in such a hostile environment are key.
Information and safety boards throughout the site remind
personnel of the dangers of falling, snake bites, malaria and other hazards.
Crews have built pathways and installed life-lines between the various borehole positions and rules have been
instituted which prevent people from walking between borehole positions alone.
All crews and supervisors have hand held radios and are in contact with each other, the helicopter and the base
camp.
Malaria is another concern so crews are issued with a weekly dose of prophylactics to try and prevent it. Malaria
test kits and treatment medication is held at base camp in the event that a crew member succumbs to malaria.
In addition to the natural hazards, there are hazards associated with working in such close proximity to helicopters.
Relevant personnel are briefed on these additional safety issues by the pilot every morning.
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Once the equipment was lowered onto the platform at the
entrance to the adit, the blocs of the adit wall and the
entrance door had to be disassembled to allow the machine
access to the borehole position.
Only when platforms and machinery are in place, does the
daily task of getting equipment, consumables, personnel
and full core trays efficiently and safely to and from the sites
begin.