Soil Instruments has installed monitoring equipment at the Al Wahda dam in Morocco. Suzanne Moxon spoke to Chris Spalton about the work the company carried out.

For nearly four years, north Africa was home to employees from soil-instruments, a UK-based geotechnical instrumentation company. In February 1993, the firm started work on Morocco’s largest embankment dam, the Barrage Al Wahda. The dam, which was completed late in 1997, is set to play a central role in developing the area’s water supply; providing flood protection and irrigating 250, 000ha of agricultural land.

Soil Instruments’ first association with Al Wahda was in 1992, when the company was awarded a contract for the work by the Ministère des Travaux Publics, the Moroccan authority responsible for administration of hydraulic facilities. French-speaking engineers from Soil Instruments spent 47 months on site, supplying and installing instrumentation for the dam. Much of their time was also dedicated to training the client’s personnel to use the new equipment and instrument monitoring.

Commenting on the nature of their work, Chris Spalton, senior engineer from Soil Instruments, said: ‘Our technical services included the preparation of installation method statements tailored to the site-specific conditions, technical support for all instrumentation matters including the design and supervision of the instruments’ cable scheme, together with planning the supply of the instruments in line with the progress of the works, as well as the provision of instrumentation and monitoring.’ Instrumentation at Al Wahda was designed by the consulting engineers (a consortium of France’s coyne-et-bellier; Morocco’s Hidrotecnica Maroc, CID Conseil and Ingema; Portugal’s Hidrotecnica Portugesa and Russia’s Hydroproject). The equipment was used to confirm design assumptions, check the stability of the dam during construction and provide long term safety monitoring.

The dam designer must ensure that detailed specifications state exactly what information is required about the dam, as the type of instrumentation used determines the type of data that is obtained. As Spalton said: ‘It must be remembered that the instruments are, after all, only tools that allow engineers to recover information; helping answer questions that they [the engineers] have about a structure.’ The types of instruments Soil Instruments proposed for the contract were mostly based on electronic transducers which employ the vibrating wire operating principal. ‘This,’ Spalton said, ‘is the type of sensor that is, by far, most suited to the environmental conditions and the need for long term monitoring.’ Vibrating wire instruments are reported to be very robust and capable of enduring the African environment, while the UK company favoured this equipment because it has proven to be reliable over long periods.

Work at the dam was not a trouble-free process. A number of problems were encountered, particularly in the early stages of construction. ‘The properties of the dam foundation, a highly over consolidated marl, were not clearly understood,’ Spalton explained. ‘Consequently, the forming and grouting of an 80m deep vertical multipoint rod extensometer borehole caused the local drilling contractor some problems.’ The drilling contractors suffered their fair share of problems as more technical difficulties were encountered when 50m vertical boreholes had to be created for the inverted pendulums.

Most difficulties experienced by Soil Instruments took place during construction. A number of instrument cables were damaged but, as these were detected almost immediately, they were repaired quickly. Cable failures during construction and the impounding period also led to the disappearance of a number of piezometers and pressure cells. Changes in the design of the instrumentation scheme in the early stages of the contract caused some logistical complications with regard to manufacture and supply of the equipment. However, as time went by the work became almost trouble-free. ‘On the whole,’ Spalton says, ‘we worked closely with the contractor, the client and the design engineers to make the scheme perform and provide the data required.’ When reflecting on the work carried out at the dam, Chris Spalton said the client should be pleased with the instrumentation and performance. ‘The instruments have provided useful information during the construction and impounding periods and will continue to provide long term performance data.’

Instrumentation used at Al Wahda

•Vibrating wire (VW) piezometers – 220 of these have been installed to monitor the build up, dissipation and changes in the pore water pressures in and under the body of the dam, as well as close to other associated structures.
•VW multipoint extensometers – these are used to determine small changes in distances within boreholes. At Al Wahda dam they have been used to measure the compression of the dam foundation during construction and to determine the effects of loading and movement on the existing material around concrete structures.
•Total pressure cells – 49 have been used to measure pressures generated by the fill at various elevations in the dam core and at the interface between the concrete structures and the core/foundation.
•Inclinometer/magnetic extensometer installations will measure the lateral and vertical movement in the dam body. The inclinometer generates an accurate profile of horizontal movement and the magnetic extensometer provides a settlement profile. Plain inclinometers have also been used in the cut slopes on the site to monitor their stability, while chain VW extensometers have been installed close to the dam crest to observe the horizontal movement of the fill material along a specific section of the dam.
•Two 50m deep inverted and two conventional pendulums were installed in the spillway gate structure to measure horizontal movement and linear inclination, with a single conventional pendulum installed in the intake tower.
•All electronic sensors are connected to one of three dataloggers on the site which are, in turn, connected to the central monitoring location where the data is gathered and processed. To reduce the risk of failures engineering staff from Soil Instruments conducted vigorous pre-installation checks on all transducers. Soil Instruments now regularly visits the site to help solve any problems that occur with the instrumentation.