Concrete savings1 February 2002
A concrete arch fuse plug reduced risks associated with impounding the Masjed Solayman rockfill dam in Iran. In comparison with conventional steel plugs, this also led to to substantial savings in construction costs and time
In high rockfill dams, the first impoundment should proceed with maximum safety and flexibility. Using forecasting models this may be achieved after the flood season. Controlling the reservoir water below the elevation of the intake sill (ie morning glory sill) is generally done with steel plugs.
The Masjed Solayman dam is situated 40km northeast of Masjed Solayman town in Khozestan province, southwest Iran. At 178m high, this is the first highest rockfill dam to be built in Iran and has a reservoir capacity of 228M m3. The dam has a gated chute spillway on the right abutment with a PMF of 21,000m3/sec. With two 1000MW hydro power plants using pressurised intake tunnels, the dam's primary function is as a source of electricity generation.
A bottom outlet system was provided for the Masjed Solayman dam to enable the discharge of water downstream with the total shut down of the power plant, to draw down the reservoir for inspection and maintenance of underwater structures, and for any emergency situation. The outlet system comprised of an intake structure, a vertical shaft and outlet.
The elevation of the morning glory spillway sill was 300m asl, enabling the reservoir to be drawn down to 300m asl by operating the bottom outlet system. The Criteria and Guidelines for Evacuating Storage Reservoirs and Sizing Low Level Outlet Works, published by the US Bureau of Reclamation, provided guidelines for designing the bottom outlet elevation. It specified that generally low level outlet works in conjunction with other release facilities should be located and sized to draw down the reservoir, within a period of one to four months, to the lower of either a reservoir level commensurate with a storage capacity 10% of the initial reservoir level; or a reservoir level less than 50% of the hydraulic height of that at the initial reservoir level.
The discharge capacity of the bottom outlet provided in the tender design was approximately 500m3/sec at the reservoir level of 350m asl (spillway sill). When the outlet control gate is kept fully open, the reservoir can be drawn down from 350m asl to 300m asl in two weeks under the annual mean flow condition.
The reservoir storage below 300m asl is 25M m3. This is about 11% of the initial storage volume of 228M m3 for the maximum operation level (372m asl). The hydraulic height of the dam is 152m from the maximum operation level to riverbed (220m asl). Hence 53% of the initial hydraulic height remains without being drawn down. These percentages are slightly higher than those of the criteria (10% and 50%), but the difference is marginal.
The bottom outlet in the tender design practically satisfied the requirements of the USBR criteria. The filling rate at the initial impounding should have been such that it allows monitoring of the dam and reservoir performance. At Masjed Solayman dam, the reservoir water had risen from 260m asl to the bottom outlet intake level (300m asl) within three days after closure of the left diversion tunnel. After which the reservoir water level should have been kept at around 303m asl for three months until the left diversion steel fuse plug was ready for future impounding.
Up to reservoir elevation of 300m asl the water level had risen at a rather higher rate (13-15m/day), but this was not adversely affecting the stability of the dam itself. Continuous and extensive monitoring were carried out when the reservoir was rising and controlled at 303m asl. The second phase of reservoir impounding above 303m asl was scheduled only after confirmation of no unexpected situations.
It was extremely unlikely, taking into consideration state-of-the-art technology which had been used for embankment construction and foundation treatment, that the situation required evacuation of the reservoir during initial impounding to 300m asl. It was, however, prudent to have plans for evacuation of the reservoir below 300m asl to cope with any unexpected situation, providing such provision was insignificant to the initial investment.
Taking into consideration the foundation geology and the tender design of the foundation treatment, it was almost certain that the amount of seepage water would have been within a tolerable limit.
Even if the amount was more than this, seepage could be reduced by additional grouting from the grouting tunnels, which could be done without evacuating the reservoir. When the velocity is high enough to wash the grout away, usually treatment with chemical grout (a short gel time) may be effective to seal the seepage water flow path. Should the velocity be higher than the extent to which the chemical grouting is effective, the reservoir would be lowered to reduce the velocity (or hydraulic gradient along the under seepage path) for grouting to seal the path.
The bottom outlet system in the tender design was able to evacuate the reservoir down to 300m asl. Evacuating below 300m asl was seen as a prudent safety precaution.
One of the initial ideas studied was the installation of a steel lining in the plug concrete of the left diversion tunnel. A bulkhead cover is fixed to the end of the steel pipe liner with explosive bolts. When the need arises to evacuate the reservoir below 300m asl, the bulkhead cover would be removed by blasting. The wall structure closing the tunnel outlet will also be removed by blasting. Before removing the bulkhead cover, the reservoir should have been lowered to 300m asl by operating the bottom outlet.
The stoplogs installed at the intake should have also been removed but this was not possible: the intake structure did not possess sufficient strength against the unbalanced external water pressure; and shear failure mode was expected.
The steel lining and plug concrete should have been completed three months after closure of the left diversion tunnel in order to maintain power commissioning as scheduled. The additional investment cost of the provision of the emergency outlet (steel lining and bulkhead cover with additional plug concrete) was estimated at some US$250,000 in 1997.
Starting to impound
The designed shell thickness for the emergency steel plug liner was 25mm. An internal operating pressure of about 1.4 MPa and an external pressure of about 0.2 MPa was taken into consideration as design loads. A temperature rise of about seven degrees and a temperature drop of about 20 degrees were also considered. The seepage was to be controlled by contact grouting at the location of the contact between the rock and concrete, and steel rings in the contact part of steel and concrete. About 20 litres/sec seepage was estimated initially.
Reservoir impounding of the Masjed Solayman dam started on 19 December 2000 by installing the stoplog segments in the left diversion tunnel (DTL). On 21 December, water started to enter the bottom outlet facilities in the right diversion tunnel right (DTR) on which inlet sill is set at EL 300m asl.
DTL has an inside diameter of 9m and a length of 703m. The air supply was to be provided through the outlet of the DTL. Due to the time required for preparation of access to DTL, the water flow in the DTR was considered to be aerated about two days after first impoundment. On 23 December, it was revealed that the insufficient or delayed air supply had caused some technical problems in the DTR. Considering the emergency condition and based on the initial estimations, the fuse plug should have been constructed in a period of less than three weeks to one month. This time was not sufficient for construction of the steel fuse plug. Thus the concept of designing a concrete fuse plug was introduced.
Concrete fuse plug
A two staged concrete fuse plug was design for the DTL. The first stage was used up to an elevation of 360m asl (10m above the minimum spillway operation water level). The second stage was designed to carry the hydrostatic and hydrodynamic pressures (earthquake induced pressure). As the spillway was not ready for operation at the time of the first impoundment, overtopping had to be avoided. Warnings of forthcoming floods necessitated the shortest construction schedule possible, with the first construction stage scheduled for completion within two weeks.
The contractor successfully executed the first stage of the concrete arch fuse in 15 days. The emergency condition was consequently lifted and the Masjed Solayman dam was inaugurated in March 2001. The concrete plug led to 79% savings in construction cost and 74% savings in construction time.