Tannur dam: Paving the way for RCC in Jordan

Tannur dam, located 150km south of Amman in Jordan, forms part of the Southern Ghors project for water supply and irrigation in the Jordan valley. Tannur, Mujib and Wala, the three dams incorporated in the project, are the first RCC dams in Jordan and are being constructed for the Jordan Valley Authority. Construction of the Tannur dam commenced in January 1998 and completion is expected on time in May 2001, significantly ahead of the other two dams.

Tannur is 60m high, 250m long and impounds 18M m3 of runoff from Wadi Hasa, one of the major wadis (watercourses) discharging to the southern Dead Sea area. The elevation at the dam site is approximately 400m compared to the elevation of the Dead Sea at el 370m, some 30km downstream.

Construction of the Jordanian Dinars 21.7M (US$16M) dam is being undertaken by a French joint venture between Bec Frères SA and Camperon Bernard SGE. Supervision of construction is by another joint venture comprising UK consultant Mott MacDonald, Australian consultant Gutteridge Haskins and Davey, along with local Jordanian company Consulting Engineering Center (CEC). The consultants were also responsible for the initial review of the tender design and the resulting design modifications.

The dam site is located in a narrow V-shaped valley with abutment sides rising at about 30° for up to 100m above wadi bed level. The rock foundation for half of the dam height comprises interbedded limestone and marl, highly fractured in parts, and with the presence of thin gypsum seams and clay (weathered mudstone) layers. Above mid dam height, the foundation is a better quality limestone but with a relatively high permeability.

The dip of the bedding is approximately 20° in a direction downstream. The area of the dam site is one of high seismicity, due to the influences of the Dead Sea and El Hasa faults, and consequently design ground accelerations have been 0.2g for the design basis earthquake (DBE) and 0.5g for the maximum credible earthquake (MCE).

Due to the dry conditions, the natural water table at the site only rises very gradually from river bed level on each abutment. However the Wadi Hasa gorge is over 500m deep which ensures an inflow situation. Nevertheless it would be expected that at each filling of the reservoir there will be a significant absorption of water into each abutment, which will return into reservoir storage with drawdown.

Climate and hydrology

Tannur dam is located in the Jordanian desert where the climate is arid, with rainfall and runoff only occurring in the winter months between October and April. Annual rainfall is low, generally varying between 63-113mm. Temperatures can vary from up to 43°C in summer to less than 5°C, and occasionally freezing, in the winter.

The catchment area of Wadi Has a up to the dam site is 2160km2 and the spillway has been designed for a probable maximum flood of 3080m3/sec. The 1 in 20-year return period flood for diversion purposes is 350m3/sec. However for most of the year there is minimal flow in the wadi. The narrow site, reasonably good bedrock conditions at shallow depth, and the high spillway design flood clearly favoured the choice of a concrete gravity dam. The availability of pozzolan in Jordan led at the tender design to the obvious selection of RCC.

The spillway, which is integral with the dam wall, has a crest length of 180m and the downstream face is stepped to partially dissipate the energy. The flow is collected into a 50m wide stilling basin by a cascade structure on each abutment.

The dam at the tender design stage had an upstream face of 0.3:1 and downstream face of 0.5:1 but, following further stability studies, the dam section was changed to a vertical upstream face (with a 10m high 1:1 batter at the heel) and a downstream face increased to 0.8:1. The axis of the dam was also marginally changed to exploit rock levels on the upper left abutment. Dynamic analyses using the spectral response approach were undertaken for the revised dam section under earthquake loading. These indicated minimal potential cracking under the DBE case, and over less than one-sixth of the dam footprint width under the MCE.

Founding depths have been generally about 6-12m below ground level. Originally, the maximum dam height was 69m but, following excavation of the dam footprint, the extent of alluvium in the wadi bed was found to be significantly less and it was possible to raise the foundation level here by 9m.

With the bedding in a rather unfavourable direction and zones of weak rock, a particular emphasis in the design review was the sliding stability. This was confirmed, but as an additional safeguard the dam and downstream cascade were cast against the downstream rock face where the significant excavation depths enabled considerable passive resistance; effectively the dam was ‘locked’ into the abutments.

During excavation of the dam footprint on the right side, clayey silt seams from the complete weathering of the adjacent mudstone were exposed. These were found to have the expansive clay mineral, Smectite, resulting in high swelling pressures. Excavation to found below the seams would have resulted in a major change in the dam design on this abutment, with costs and delays to the construction contract. Instead it was decided to found above the seams, but introduce two 3.5m high adits; the first to provide a cutoff through the seams along the line of the grout curtain and the second, a drainage adit 10m downstream, to depressurise the foundation in this location.

The drawoff and bottom outlet works are located in a twin 4m high culvert located at the bottom of the left abutment. The 10m high intake structure, incorporating the drawoff controls and the 1.5 x 2m bottom outlet gate, was at an early stage of construction relocated from within the dam wall to directly upstream at the request of the contractor to enable uninterrupted RCC placing.

placing RCC

The RCC mix is a ‘high paste’ type with a required characteristic compressive strength at 90 days of 20 MPa. At the start of RCC placing, the cementitious content of the mix was 200kg/m3, comprising 100kg of OPC and 100kg of the naturally occurring pozzolan which was processed by the cement manufacturer. In terms of price, the cost of cement and pozzolan were similar and, as the contractor’s quality control improved, the mix was changed gradually to 125kg cement and 50kg of pozzolan.

It is understood that the pozzolan has been omitted at Mujib and Wala dams, but at Tannur it was found to provide benefits both in terms of increased workability and a 20% increase in strength. The aggregate was quarried from a limestone source within the reservoir area. The contractor found it difficult to obtain the required gradings in the finer fractions and a natural fine sand was imported from 20km away to rectify this. It was found that the addition of 7% of the fine sand produced a much improved aggregate grading, which in turn ensured good workability of the RCC without the risk of segregation.

The specified maximum placing temperature for the RCC was 26°C. Within the specification this restricted RCC placing to the winter period from September to April. In fact the contractor only commenced the RCC placing for the dam wall in January 2000, but was allowed to continue into the summer months, so long as the maximum placing temperature was not exceeded. The contractor achieved this in the summer months by the introduction of suitable cooling measures such as placing at night, evaporative cooling of the aggregates and cooling the mix water, plus the use of a retarder. In addition to the placing temperature, there was also a requirement that the final temperature of the RCC following hydration should not exceed 40°C. This temperature was established by a finite element analysis relating the concrete characteristics, the placing programme and ambient temperatures throughout the year.

The maximum spacing between transverse joints was 15m.

Slope layer method

The total volume of RCC in the dam was 215,000m3 and in addition a further 4000m3 was placed in the stilling basin. The contractor managed to place around 80% of this volume by June 2000 before average daily ambient temperatures became too high at 25-30°C. The RCC was stopped at this stage and recommenced in October and completed in December 2000. The final temperature of the RCC in the dam wall did not exceed 40°C at any time.

With the adoption of a vertical upstream face, the placing of the upstream facing of the dam and the stepped downstream face was changed (following negotiations with the contractor) from a slip-formed facing to a grout enriched RCC (GERCC). The specified slip-forming was replaced by conventional formwork and RCC placed directly up to each face, using light vibrating rollers in the vicinity of the formwork.

Correspondingly at the transverse joints, the stainless steel (carpi) joint shown in the tender design was replaced by a conventional waterstop.

The GERCC was formed by the application of a 1:1 grout mix to the placed RCC over a 400mm width at each face, which was then vibrated using conventional pokers. This was a new method for the contractor and there was an initial training period for both senior staff and the workforce. Following completion of the RCC and the stripping of all forms, there is every indication that a sound dense facing has been formed on both the vertical upstream face and stepped downstream face. This change has resulted in a saving on the contract of approximately £600,000 (US$900,000).

A further innovation introduced by the contractor following discussion with the consultants was the placing of RCC using the slope layer method. This system involved the placing and compaction of the RCC in 300mm layers that were sloped along the dam axis at approximately 7%. A 1.2m high lift height was adopted to correspond with the downstream steps and within each lift there were four 300mm layers.

The advantages of the system were a reduced clean-up that was confined to the ‘cold joints’ at the base of the lift, a corresponding reduction in bedding mortar to be applied to the lift joints, and laying rates that were considerably better than for the conventional horizontal layer method of placing.

The grouting works, which are still ongoing, comprised a two-line curtain to a depth of up to 55m, supplemented by contact grouting at the upstream heel to a depth of 10m over the 60m length of the maximum dam height at the wadi bed. The contact grouting was added at a late stage to improve the bedrock at the critical heel location and also lengthen the seepage path in the foundation. Consolidation grouting elsewhere in the dam foundation was not needed.
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