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Se San River, Vietnam: The Ialy hydropower scheme, scheduled to come on line at the end of 1999, will increase Vietnam's generating capacity by 720 MW.

Ialy Dam was completed in late 1998. Designed by Vietnamese engineers and built by Vietnamese construction companies, the clay-core, rock-fill dam is 1,200 meters long and 75 meters high.

This was the first project in Vietnam to employ modern geotechnical instrumentation. The various construction companies involved were not familiar with procedures for handling and installing instruments, so the installation coordinators, Geotech International of USA and CE Instruments of Malaysia, were kept very busy with training and supervision.

Dam instrumentation included 60 VW piezometers and total pressure cells , 24 strainmeters , and five inclinometers , which included magnet extensometer targets. A number of portable readouts were used during construction, and an automated CR10 data logging system will monitor the piezometers, total pressure cells, and strainmeters later on.

Despite the inexperience of the work force and despite many incidents when signal cables were cut by construction machinery, only 7 out of 146 instruments were lost during construction, an impressive success rate of 95%. Tests performed by Slope Indicator technicians showed that most failures were due to faulty cable splices.

Ialy Power House and other underground structures were designed by Hydroprojekt of Russia. The designers specified instrumentation to monitor the heavy concrete lining and the stability of stone columns between adjacent chambers, which could be damaged during blasting operations. Eighty-four VW strain gauges were installed to monitor strain in the reinforcing bars in the lining. The strain gauges, which have built-in temperature sensors, were also used to monitor the heat of concrete after casting. Fifteen VW piezometers were installed to monitor water pressure and seepage, and twenty load cells were installed to monitor the pre-stressed cable tiebacks that were installed to stabilize the stone columns. Despite drilling blasting, welding, and frequent cutting of signal cables, only five gauges failed after installation. The success rate was again about 95%, due in part to continuous remedial action by Geotech International.

During construction of the powerhouse, Hydroprojekt engineers analyzed sensor readings and found that the strain values were slightly higher than the designed values, but still satisfactory. The temperature readings from the strain gauges provided useful information for analyzing some cracking that occured in the concrete liner, and strain readings and water-pressure readings provided information for the design of remedial measures. The load cell data were the only means of evaluating the performance of tiebacks after the installation. In addition, readings obtained during stressing of tendons prevented serious accidents in several cases, when the hydraulic pressure gauge failed in the operation.

The Ialy hydropower project will serve as a good example for the design, selection, installation and monitoring of modern instrumentation systems, and the experience gained by all the companies involved will certainly assist future hydropower projects in Vietnam.

Thanks to Marshall Silver and Nghiem Phuc Hai of Geotech International, Ltd for supplying this story. Geotech International, in addition to providing geotechnical consulting services, distributes Slope Indicator equipment in Vietnam. Their office in Hanoi can be reached at tel +844-825-3029 or fax +844-825-3704.

The authors wish to thank the Vietnamese Power Company No.3, the Ialy Project Management Board, and the Institute of Geological Sciences for their cooperation in producing this article. Other companies involved were Hydtroprojekt of Moscow, Vietnamese Power Design and Investigation Company No.1, Ukrhydroprojekt of Kharkov, Ukraine, and Song Da Corporation, the largest government-owned contractor in Vietnam.

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