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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 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 Ingrop 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

    Page 1

  • 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.

    S U C C E S S S TO RY Cahora Bassa Northern Bank Power Station

    Page 3

  • 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 o