Turkey’s Yusufeli Dam is one of history’s most iconic structures

Situated on the Coruh River in the Artvin Province of the Black Sea region of Turkey, Yusufeli Dam is a 275 m high double-curvature concrete arch structure and has a developed crest length of 540 m and a section thickness of 8 m at the crest and 90 m at the base on the crown cantilever.

A view of the foundation excavation looking in the direction of stream flow. Various foundation stabilisation and reinforcement measures were required. The dam foundation excavation was completed in 2018, with a total of 3.9 million m3 of rock being removed

The highest in Turkey, the dam was constructed with conventionally vibrated mass concrete and the structure contains 4 million m3 of concrete, impounding 2.1 billion m3 of water at full supply level.

Excavation for Yusufeli Dam commenced in 2014 and the final concrete lift in the dam body was placed in 2021. Impoundment was initiated at the beginning of 2023 and the water depth had reached 256 m by early September.

The dam has a controlled crest spillway, with three gates, which is supplemented by two controlled spillway tunnels on the right flank discharging into the river approximately 1 km downstream of the dam. The dam has a system of four mid-level outlets, which discharge into the spillway plunge pool immediately downstream.

The Yusufeli hydropower project includes an underground power station located beneath the right abutment immediately downstream of the dam, with an installed generation capacity of 558 MW and an average annual energy production of 1.8 billion kWh. It is predicted that the dam will contribute an annual amount of 1.65 billion Turkish Lira to the Turkish gross domestic product.

Owner and project team

The DSi (Turkish State Hydraulic Works) is the owner of the project, which was constructed by Limak Construction S.A. For the detailed and construction design, Limak appointed IC Consulenten of Austria for all geotechnical investigations and design, and a team comprising ARQ of South Africa and Su Yapi of Turkey for the dam design. Su Yapi was also contracted for the power station design.

The ARQ-Su Yapi team for the dam design was led by Dr Quentin Shaw of ARQ. ARQ was responsible for all dam design, which included numerous complex and detailed studies to model all conditions and modes that could be experienced during construction and operation.

During investigation, design and construction of this super-project, many challenging geotechnical, topographical and logistical conditions were overcome through significant innovations and advances in engineering technology, and the project’s success was in a large part due to an effective cooperative relationship between owner, contractor and designer.

Typography and excavation

The Yusufeli Dam site is located in a remote mountainous area, with relatively extreme weather conditions and challenging site access. The local topography comprises a river with steep canyon slopes of complex rock mass formations showing signs of surface relaxation. The steep riverbank slopes create a topography suitable for an arch dam, while the igneous rock mass forming the V-shape valley demonstrated some challenges that required particular management. Excavation of the rock material was achieved by a combination of controlled blasting and mechanical excavation. As the excavation progressed downwards, the canyon cut slopes were stabilised with grout-injected post-tension prestress rock anchors. In general, 250 tonne, 80 m long anchors were used.

During excavation for the dam foundation, a cable crane and multiple batch plants were erected in preparation for casting of the concrete arch structure. The 3-line cable crane was suspended from concrete platforms on the upper banks of the excavation and each crane had a 28 tonne capacity.

The primary concrete batch plant that fed the cable cranes was located on the upper left abutment and this was supported with one supplementary plant located on the downstream coffer dam and another located upstream of the dam, both feeding the placement by conveyor. The batch plants collectively had a concrete

The Yusufeli Dam project won the award for Best International Project at the CESA AON Engineering Excellence Awards held in Midrand, South Africa in August 2023. From left are David Leukes, CESA vice-president; Ryan Cassells, structural dams engineer, ARQ Consulting Engineers; Henry-John Wright, director, ARQ Consulting Engineers; and CESA board member, Jabulile Msiza production capacity of 740 m3/hour and were operated 24 hours a day to meet the ambitious project production requirements.

The majority of the aggregate materials were sourced from site excavations, although it was necessary to open a supplementary quarry to complete the works. The aggregate crusher was located downstream of the dam site and indicated a capacity of 2 000 tonne/hour. Crushed aggregates were transported almost 2 km by conveyor, with a capacity of 200 m3/hour, to the batching plant at the dam site.

Concrete placement and cooling

The placing of concrete in the dam body commenced in late 2018 and continued without interruption for 30 months until mid-2021, during which time 4 million m3 of concrete were cast. Concrete was placed into forms by a combination of a 9 m3 bucket suspended from the cable crane, and temporary conveyor belts leading directly to the forms. Concrete was spread in the box by rubber-tracked bulldozers and compacted using gang-mounted immersion vibrators attached to an excavator with low-pressure tracks.

Extensive temperature control measures were implemented at Yusufeli Dam, both in terms of pre-cooling and post-cooling of the mass concrete. Pre-cooling was implemented using a cooling plant to chill the mixing water and more importantly the coarse aggregates prior to batching in order to meet the maximum allowable placement temperature specification of 18 °C. Post-cooling was achieved by circulating chilled water through 32 mm cooling pipe loops (2.5 m c/c) installed in the bottom of each lift cast.

As the concrete construction progressed towards the dam crest, the thickness decreased and the crest length increased, meaning more monoliths were being cast concurrently. As is conventional practice for the construction of a conventionally vibrated concrete dam, a leaderfollower block approach was applied, whereby the monolith casting heights are staggered between neighbouring monoliths.

The final stages of concrete placement occurred in May and June of 2021, when the spillway peers and bridges were formed and cast. These components of the dam comprise reinforced concrete to account for concentrated loading of the structural steel spillway gates to be installed.

Filling the reservoir

Impoundment of the dam reservoir commenced in November 2022 and is currently still progressing. The rate of water reservoir rise during impoundment is controlled by release from the dam outlet works, as well as the release from upstream dams in the Coruh River Development Scheme.

The reservoir impoundment speed has been limited to a daily rate of rise not exceeding 1.5 m, pending the availability of sufficient river flow volume. At the current stage of impoundment, the reservoir elevation level has reached a height of 256 m above lowest foundation level.

Finite Element (FE) structural modelling

The unique engineering challenges addressed during the dam design included the development of systems to transfer load from high strength, high deformation modulus concrete into a rock mass with a lower and variable deformation modulus. Verification processes developed to ensure that the design foundation characteristics were achieved were complemented with real-time instrumentation monitoring. With impoundment completed in six stages, the structural behaviour measured on the prototype dam was verified against the respective predictions of the calibrated FE structural modelling at each stage.

FE modelling and analysis was used extensively throughout the design development and verification studies and the modelling capabilities of the latest versions of the MIDAS FEA NX software proved extremely valuable in enabling a very accurate representation of the actual dam and foundation rock mass characteristics. The capacities of MIDAS software were exploited to develop possibly the most complex foundation rock mass model yet for a dam design analysis.

The advanced design analyses undertaken by ARQ included linear elastic, thermal and seismic/dynamic analyses. Non-linearities were built into the FE model to accurately replicate the structural behaviour under both static and thermal loads, while non-linear seismic time-history analyses and non-linear modelling, including some plasticity in the dam/foundation interface, were applied to replicate as accurately as possible the actual effects and conditions anticipated.

A structural calibration study of the dam FE model was conducted whereby the predicted behaviour of the dam, as output by the FE analysis, was validated against the observed instrumented behaviour of the dam during impoundment. The FE model proved to be marginally conservative, overpredicting displacements of the dam by 5-10 %.

While the average concrete placement rate of the order of 140 000 m3 per month achieved at Yusufeli Dam represents a world record for conventional mass concrete, as much of a challenge as the actual concrete placement was safe concrete post-cooling and joint grouting to provide lateral support to the cantilevers sufficiently rapidly so as not to slow concrete placement. The realisation of this requirement involved real-time instrument monitoring and a reactive management of postcooling processes.

Additional aspects for which ARQ was responsible included foundation rock mass stability analyses on both flanks, assistance and advice on concrete mixes, assistance with the spillway model tests and design, design of the plunge pool reinforced concrete lining, design of foundation stabilisation infills, design of the concrete post-cooling systems and stability design of the inlet for the twin tunnel spillways on the steep upper right abutment. In total ARQ provided more than 4 000 person-days, or 200 person-months of input on the project.

A celebration of excellence

As the central component of the Coruh River hydropower development, Yusufeli Dam is a landmark achievement for Turkey and a significant contributor in the country’s progress towards carbon-neutrality. From an engineering perspective, the project is significant in the increased accuracy of the FE modelling achieved with state-of-the-art software, allowing a substantially better representation of actual conditions, and the consequential development in the understanding of superhigh arch dam behaviour.

In recognition, the Yusufeli Dam project recently won the award for Best International Project at the CESA AON Engineering Excellence Awards held in Midrand, South Africa in August 2023. The dam is also currently the subject of an article published in the USA’s Engineering News-Record (ENR).