Mohale Dam

Type: Rock-Fill, Face Slab Dam
Construction Date: May 1995 - October 2001
Location: Mohale, Lesotho
S 29.457535° E 28.097062°
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)

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