KARIBA DAM

ZAMBEZI RIVER

GEOTECHNICAL INVESTIGATIONS

INTRODUCTION

Zambezi River Authority appointed consultants Coyne et Bellier (Tractebel Engineering France) to provide the necessary engineering studies tothis project on Kariba Dam. In the framework of these studies and future works, the need of further geotechnical investigations was identified.

Prinsloo Drilling Namibia was appointed as the geotechnical drilling contractor, due to their extensive experience on large dam geotechnicalinvestigations.

These investigations are intended to verify the geomechanical characteristics of the rock mass, in terms of fracturing, weathering, alteration,hardness, abrasiveness, strength, deformability and permeability.

Below describes in brief the technical specifications the investigations.

1 SITE AND ACCESS

1.1 Site location

Kariba dam is located on the Zambezi River, at the boarder between Zambia and Zimbabwe. It is found at 130 km SSE of Lusaka (Zambia),and 280 km NW of Harare (Zimbabwe). The international road M15 crosses the border on the dam crest.

1.2 Access

Both banks have existing access roads. On south bank, this road goes to the dam toe. On north bank it only goes to the top of power houseoutlets.

Information about site constraints are shown in the following photographs.

Figure 2. Kariba dam – North and South accesses and areas of work.

1.2.1 South bank

A ramp already exists to access the bottom of south bank.

Figure 3. South bank access and approximate location of boreholes (options A).

1.2.2 North bank

The access to the North bank and its general view is shown in the following Figure 4

Figure 1. General view showing access to north andsouth banks of the dam wall

Figure 2. General view showing access to the southbank of the dam wall

Figure 3. General view showing access to north bankof the dam wall

2 SCOPE OF WORKS

2.1 General

The purpose of the work being undertaken is to determine lithology, weathering and alteration, fracturing and structures, geomechanical andhydraulic characteristics of the rock mass. This is to be accomplished by means of rotary drilling, in-situ testing, collection of undisturbedsamples of rock and laboratory testing and includes inclined boreholes up to 150m metres deep core drilled from North and South banks.

2.2 Investigation report

The detailed and complete general investigation report will include:

- a historical description of the campaign,

- a description of the equipment used: rigs, casings, rods, core barrels, samplers, packers, water test instruments, pumps, CCTV etc.,

- the operating methods used with regards to drilling, boring, coring, water testing, measuring bearing and dip of inclined boreholes,undisturbed sampling, drilling and ground water levels (daily and before –after pressure test),

- a detailed geological and geomechanical log of each borehole as described hereafter,

- the results of the Lugeon water pressure tests (table and figures),

- colour photos of the core boxes presented in albums, with one (1) numerical set.

3 METHOD OF WORKS

3.1 Boreholes

Boreholes are core drilled through a gneiss rock mass, which is supposed to be in a good geomechanical quality, locally fair due to spot highfracturing and biotite-rich friable horizons.

All drilling operations are performed on field in a manner to comply with high quality standards and accepted practice by skilled technical staff,acquainted with such specific investigation works in order to meet all the following requirements:

- minimum 76 mm core diameter in soft material,- minimum 48 mm diameter full coring of rock.

Drilling in rock is carried out with the aim of provide 100 % of core recovery. For this purpose, triple tube core barrels are used. Coring diameteris NQ3-size (76mm external).

No drilling lubricants are used in the borehole other than normal drilling water or air. All water used is clean and free of any suspended matter.

3.2 Field water pressure test in boreholes (Lugeon tests)

Single packer rock permeability (Lugeon) tests are carried out in downstage sections of the drill hole as the hole is core drilled. By this method,the bottom section of the hole (5m in length) is isolated and the rate of water loss from this section under different pressures is measured.Double packer equipment is also used, depending on the rock conditions.

3.2.1 Equipment

Water Pumps

Water used, is from the Plunge Pool which is clean and free of suspended matter and sediment.

A Clemac/ Orbit PP5402 centrifugal pump able to deliver at least 500 litres per minutes against a pressure of 0.8 MPa at constant waterpressure is used.

Electronic Data Logging including Electronic Flowmeter and Pressure Transducer

Automated data acquisition systems capable of measuring, displaying and recording Lugeon test and grouting data in real time have becomeavailable over the last few years. This equipment measures flow rate and pressure at regular intervals of time and displays the information onan LCD display and is downloadable onto a PC for analysis and graphics production.Since this equipment is able to measure both pressure and flow rate in real time it is possible to monitor the behaviour of the Lugeon value asthe test proceeds. In order to take advantage of this possibility, analysis of the Lugeon test results is possible using the flow loss vs. pressurespace, with flow loss defined as the flow rate divided by the length of the test interval (q/L).For this reason a DAT Instruments DSP 100 data logging system with an electronic flowmeter and pressure transducer was chosen for theLugeon Testing with its ability to show real time monitoring and analysis software using a wide range of parameter settings and graphicsprinting ability. No operational problems with the DSP 100 equipment have been experienced on site to date.Water pressure is measured using an electrical pressure transducer located at the borehole head elevation. A digital display of the electricaloutput of the pressure transducer, in terms of kilopascals (accuracy + or – 0.1 kPa) is provided at the ground surface.The electronic flowwater meter is coupled directly in a straight section of pipe between pump and drill rods. The water line is free of angularbends as far as possible.

Manual Flow and Pressure Check Equipment - (Used as a check for the Electronic Logging system)

The manually read water meter is capable of accurately reading to 0.1 litres and capable of handling flows up to 500 litres per minute. A secondspare meter is available as a check; both meters have been recently calibrated. The calibration of these water meters is checked prior to theiruse in the field.

Manually read Water pressure gauges are provided as a back up and check for the pressure transducer. The pressure gauges are precisiongauges of durable construction and of standard test quality. Pressure gauges are selected such that they vary with pressure range. Suitablegauges are:

- 0-500 kPa graduated in 20 kPa increments

- 0-1000 kPa graduated in 50 kPa increments (for inflating packer)

Packer EquipmentThe test section is sealed by an approved Bimbar wireline pneumatic single or double packer. For other than a packer coupled to “NQ” rods orfitted with a downhole electrical pressure transducer the head loss within the rods and packer is measured for various rod lengths.

All measuring equipment is frequently calibrated. Friction loss through the water system is particularly important. Packers are individuallymanufactured and each has different frictional characteristics to water flow. The friction loss through each packer and water line is establishedby connecting the packer and a section of water line to the pressure gauge, water meter and pump. Hydraulic packers are calibrated in the“open” position. Water is pumped through the packer and line flow rates recorded for several readings and backpressure. The addiction of asecond section of water line provides an incremental increase in friction loss for each length of line. Calibration sheets identify the packer, rodtype, length and date.

3.2.2 Test Procedure

Before testing, the static groundwater level is measured at the beginning of the shift. Immediately after drilling the test section, the hole isflushed with water pumped through the drill rods until the water return is clear. The packer is then be lowered into position and inflated. Theexact position of the packer and the distance from the collar of the hole to the pressure gauge is measured.

Water is pumped into the drillhole for each test section of drillhole in five (5) steps following three (3) increasing pressure steps, followed by two(2) decreasing steps.

Every pressure step is held for 15 minutes after reaching a steady water absorption rate. During the step, water absorption rate is checkedevery 2 minutes and yield variation must not exceed 0.1 litres per second.

Pressure steps depend on rock quality, depth of the section tested and can vary according to specific investigations purposes.

Generally, pressure steps are linked to rock quality conditions as follow:

Rock quality Pressure steps (MPa)Weak rock conditions 0.1 - 0.2 - 0.3 - 0.2 - 0.1Average rock conditions 0.1 - 0.3 - 0.5 - 0.3 - 0.1Intact or hard rock 0.2 - 0.4 - 0.8 - 0.4 - 0.2

During the testing, the details of time, gauge pressure, water meter readings, water loss, leakage rate, sealing properties of the test and anyother relevant information are recorded on the Testing Report.

On completion the water pressure test, and removal of the test equipment from the hole, the time of completion is entered on the log, the waterlevel in the hole measured and recorded, and the Test Report signed by the Drilling Supervisor.

If the test section cannot be sealed satisfactorily and the leakage occurs between the packer and the hole wall, the packer is re-set at lowerdepths until an adequate seal is obtained.

The results of this test indicate the water loss in relation to the effective pressure (gauge pressure plus water column pressure less friction loss)which is the interpreted Lugeon value. Results shall be tabulated and plotted graphically to illustrate the pressure/water loss relationship. Thebest-fit straight line (not necessarily through the origin) indicates the incremental floor/pressure relationship. The parallel line to this best-fit line,passing through the origin, represents adjustments for water column pressure and is then extrapolated to 1000 kPa (10 bars) to guide theinterpreted Lugeon value.

3.2.3 Test report

The test report related to the Lugeon test includes the following information: General data (site name, date, operator, etc.),

Drilling method and hole diameter,

Water table recorded before testing,

Length of the section tested and its depth (mid point between packers),

Rock quality description at test depth,

Technical features of testing equipment,

Hole geometry and test procedures,

Data sheet of field recording (time, yield, pressure gauge reading),

Interpretation of the results and calculation of the Lugeon Value for the test.

Figure 5. Lugeon Testing with DAT Instruments DataLogger DSP100 and Electronic Flowmeter andPressure Transducer – South Bank BH 4

Figure 6. Bimbar Wireline Packer System with Nitrogeninflation arrangement

Figure 7. Lugeon Testing with DAT Instruments DataLogger DSP100 and Electronic Flowmeter andPressure Transducer – North Bank BH 3

Figure 8. Lugeon Testing with DAT Instruments DataLogger DSP100 and Electronic Flowmeter andPressure Transducer – North Bank BH 3

Figure 9. Lugeon Testing with DAT Instruments DataLogger DSP100 and Electronic Flowmeter andPressure TransducerNote : The manual flowmeter and pressure guagechecks including a UPS power supply and earthingwire.

Figure 10. Lugeon Testing with DAT Instruments DataLogger DSP100 and Electronic Flowmeter andPressure TransducerNote : The manual flowmeter and pressure guagechecks including a UPS power supply and earthingwire.