Mohale Dam, Situated in the Lesotho Highlands near the town of Mohale, is a 145m High Rock-Fill Face-Slab (RFFS) structure with a 620m long crest.

The Dam is situated in the Senqunyane River and construction started in March 1998.

In contrast to Katse's high-tech structure, Mohale works on a simple principle - a mass of rock compacted across a valley, add a concrete face slab to make it relatively waterproof. Mohale dam is considered to be large by world standards and consists of 7, 8 million m3 of compacted rock, 600m thick at the base, and 10m thick at the crest.

Dam capacity is large and active water storage volume is 947 million m³.

During construction an earth fault on the right flank was discovered and an observation tunnel needed to be constructed for close observation of this earth-fault. The inspection tunnel houses various Vibrating Wire Rod Extensometers for monitoring the fault movement.

Mohale Dam Instrumentation consists of the following:

• Convergence points in the Outlet-Tunnel measuring expansion/contraction.
• Vibrating Wire Strain Gauges in and around the Outlet-Tunnel measuring concrete and re-enforcing strain.
• Vibrating Wire Load Cells and Vibrating Wire Pressure cells measuring total load and pressure.
• Vibrating Wire Piezometers measuring pore pressure.
• Five sets of Vibrating Wire Perimetric Joint Meters are installed on crucial structural points on the plinth and the concrete face slab, measuring three-dimensional movement between the plinth and the starter slab.
• Eighteen Wire Embedment Jointmeters are installed in the top parts of the various vertical joints of the concrete face slab measuring movement between the joints.
• Two Horizontal Inclinometer casings on the face slab measuring deflection of the face slab as such.
• Vibrating Wire Settlement Cells in the rock-fill for measuring settlement.
• Standpipe Piezometers for measuring water table.
• Vibrating Wire Transducer measuring seepage.
• Vibrating Wire Rod Extensometers measuring zone movements in rock.
• Mechanical Rod Extensometers measuring zone movement in rock.

Instruments installed in and around the Dam Embankment are intended to monitor:

• Settlement of the Embankment;
• Pore pressure within the Embankment and foundations;
• Opening of the concrete face slab joints;
• Stresses and deflection in the face slab concrete and fault line;
• Rock zone movement;
• Measurement of the starter slab and the plinth;
• Total pressure and Load;
• Strain within the face - slab.

Installed Instruments Specification:

Seepage (Dam, Conduit and Spillway)

• Geokon Vibrating Wire Transducers Vented Model 4580

Piezometers (Clay Core against Retaining Wall)

• Geokon Vibrating Wire Model 4500S

Joint Meter (Concrete face and plinth)

• Geokon Vibrating Wire Model 4400

Strain Gages (Face and Conduit)

• Geokon Vibrating Wire Model 4210/4200/4202

Pressure Cells (Conduit and Main Embankment)

• Geokon Vibrating Wire Model 4800

Settlement Cells (Fill)

• Geokon Vibrating Wire Model 4650

Convergence Meters (Conduit)

• Geokon Model 1610 Tape Extensometer

Data Acquisition System CR800/1000 (Valve House)

• Geokon Micro-1000 Data logger Model 8021

Standpipes (Manual measuring holes downstream from dam)

• Geotechnical Instrumentation Casagrande Standpipes

Rod Extensometers (Spillway)

• ITMsoil Vibrating Wire Transducers Model E7-1.1-6

Crack-Meter (Movement)

• Geokon Vibrating Wire Model 4420

Designers: N/A

Consulting Engineers: Mohale Consultants Group(MCG) comprising SMEC (Snowy Mountains Engineering Corp of Australia), BKS, Melis & Du Plessis and Steward Scott Incorporated (SSI) of South Africa, Harza Engineering of the USA and Nippon Koei Co. of Japan

Contractors: The Contractors comprised a joint venture of Hochtief (germany) contract leader Impregilo (Italy) and Concor Roads & Earthworks (South Africa)