Soil Instruments Ltd has been involved in the supply and installation of geotechnical monitoring instrumentation to the Moroccan Government’s dam building program since 1990. The Direction des Aménagements Hydrauliques (La DAH), the Moroccan Government’s department implementing the program, has awarded a number of instrumentation contracts to Soil Instruments through their local agent, Sygeo of Casablanca. These include the prestigious hydropower project, Barrage Al Wahda, completed in 1997.

One of the most recently completed contracts was Barrage Ahmed El Hansali (formerly Barrage Dchar El Oued), on the river Oum Er Rbia at the northern foot of the Middle Atlas mountains near Khenifra.

Designed by INGEMA of Rabat and constructed by Casablanca-based main contractor SGTM between October 1998 and July 2001, Barrage Ahmed El Hansali has a height of 101m with a crest length of 342m. Barrage Ahmed El Hansali was the first concrete-faced, rock-fill dam to be completed in Morocco, designed for water supply, irrigation and hydropower.

The instrumentation supplied and installed falls into two categories; (1) Foundation and fill monitoring, and, (2) concrete joint monitoring. To monitor behaviour of the fill material, upstream-downstream profiles of long-based horizontal soil extensometers, with fill extension measured by 300mm range vibrating wire displacement transducers, were buried within the embankment at 3 levels. Neighbouring hydraulic overflow settlement cells, read manually via standpipe measuring units in downstream face gauge houses, provided fill settlement information.

At two sections in the adjoining earth fill saddle-dam, vibrating wire piezometers are installed at the interface of the rock foundation and the clay core, providing data on any uplift due to the passage of water along the contact. Further vibrating wire piezometers were installed in boreholes formed in the main dam’s upstream foundation, just below the grout curtain, to monitor the curtain’s effectiveness for preventing water flow.

During the pouring of concrete face slabs, selected plots were instrumented with strain gauges embedded in the concrete mass, in two axes parallel to the slab face. These will indicate stresses induced within the concrete either due to flexure of the face through further settlement and/or in reaction to the retained body of water post reservoir filling.

Movement at the vertical joints between selected adjoining face slabs and at the perimetric joint, where the inclined face slabs bear against a supporting upstream perimeter wall, is monitored by vibrating wire displacement transducers. Mounted externally on steel legs grouted into the slab surface and straddling the rubber joint between adjoining face slabs, ten uniaxial jointmeters with a 50mm range measure the opening or closure of the joints. Nine perimetric jointmeters comprising three 50 mm range displacement transducers are mounted orthogonally in a heavy tri-axial support frame and straddling the face slab / perimeter wall joint. Anchoring is again by grouting support legs into holes drilled in the concrete surfaces either side of the joint. Due to the vulnerability of a surface mounting both types of jointmeter are protected by external steel box covers designed to accommodate differential movement across the joints.

Additional manual tri-axial jointmeters monitor movement at joints between plots of the upstream crest wall.

The majority of vibrating wire instruments are cabled back to an instrument house where they are data logged by a PC controlled data acquisition system, designed and installed by Soil Instruments.