success stories - geomechanics -cahora bassa · geomechanics cc 28 central road, sunrella, gauteng,...

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Geomechanics cc 28 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 Station Geotechnical Study and Assessment on Parameters for Civil Engineering Design of the Cahora Bassa Northern Bank Power Station - Mozambique OVERVIEW Geomechanics 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. SUCCESS STORY Page 1

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Page 1: Success Stories - Geomechanics -Cahora Bassa · Geomechanics cc 28 Central Road, Sunrella, Gauteng, South Africa PO Box 68063, Bryanston, South Africa, 2021 Tel: +27 11 451 7760 Fax:

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

Page 1

Page 2: Success Stories - Geomechanics -Cahora Bassa · Geomechanics cc 28 Central Road, Sunrella, Gauteng, South Africa PO Box 68063, Bryanston, South Africa, 2021 Tel: +27 11 451 7760 Fax:

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.

Page 3: Success Stories - Geomechanics -Cahora Bassa · Geomechanics cc 28 Central Road, Sunrella, Gauteng, South Africa PO Box 68063, Bryanston, South Africa, 2021 Tel: +27 11 451 7760 Fax:

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

Page 4: Success Stories - Geomechanics -Cahora Bassa · Geomechanics cc 28 Central Road, Sunrella, Gauteng, South Africa PO Box 68063, Bryanston, South Africa, 2021 Tel: +27 11 451 7760 Fax:

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.

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

Page 4

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.