Design of all concrete structures, Kárahnjúkar Dam

The project's objective

The main objective of this project was to detail design all concrete structures of the Kárahnjúkar Dam and issue associated construction drawings.

The Kárahnjúkar Dam is one of three dams creating the Hálslón reservoir, the Kárahnjúkar hydroelectric power plant main reservoir. The dam is Europe's highest concrete-faced rockfill dam (CFRD), 700 m long and almost 200 m high where it is highest.

Concrete and filler

Approximately 9 million cubic meters of different embankment materials were used to construct the Kárahnjúkar Dam, in addition to great quantities of concrete. The embankment is divided into several zones, where each zone represents a fill material with specific criteria. The lowest part of the dam crosses the Hafrahvammar Canyon. 

The canyon is approximately 50 m deep and 60 m wide beneath the upstream side of the dam where a massive concrete wall fills the canyon, the so-called Toe Wall. The upstream surface of the dam is a concrete slab that varies in thickness from 600 mm on top of the toe wall to 300 mm at the top of the dam. On both sides of the canyon, a concrete plinth supports the concrete slab. 

At the top of the dam, the concrete slab connects to a concrete parapet wall that rests on the dam's crest. The plinth is aligned to the rock surface on both sides of the canyon and connects to the parapet wall at each end of the dam. 

The concrete slab is divided into 48 vertical sections, each 15 m wide. Upstream of the left end of the dam is the Hálslón Reservoir spillway. On the dam crest is an asphalt-paved road that is confined upstream by the parapet wall and downstream by a crest wall, a low concrete guardrail wall. 

Bridge lies over the spillway shute. The shute conveys the spillway overflow into the canyon downstream from the left bank. The dam downstream surface is rip-rap from the canyon bottom to the crest wall.

Geological consideration

The longitudinal axis of the dam has two break points to avoid geological unsound substratum that otherwise would have been in its footprint. The dam substratum is crossed by few tectonic faults as well as several other types. One of the main tectonic faults crosses the substratum underneath the toe wall in the bottom of the canyon. 

The dam design considers the derived consequences of the faults, the possible leakages, movements and the substratum settlements. As consequences of these and the natural settlement of the dam, the face slab was divided into the 15 m wide vertical elements, as previously described, that in some places were divided also by horizontal joints. 

A contraction joint is located at the perimeter of the face slab, i.e. its connection to the toe wall, the plinth and the parapet wall. The contraction joints between the face slab sections, as well as the various structures, were designed watertight and to allow foreseeable movements and settlements. Where the plinth and the toe wall crossed tectonic faults, special measures were implemented.

Major challenges were faced

This is by far the largest dam that has been designed and constructed in Iceland. Consequences of dam breach would be severe and endanger inhabitants downstream. Therefore, the design criteria had to be thoroughly established.

• The load on individual concrete elements is much greater than for conventional structures.
• The long-term settlement of the dam fill constituted some challenges. Firstly, the dam crosses the almost vertical 50 m deep river canyon where differential settlement as well as significant movements of the face slab occurred. Secondly, the two break points in the dam axis caused differential movement of the face slab elements resulting in closing of the movement joints and creating very high in-plane forces in the slab.
• The biggest challenge in the design of the dam was the tectonic fault in the canyon, directly beneath the Toe Wall. The wall had to be designed to withstand potential movements of the fault and increased load if the fault would open up. With 200 m deep water at the bottom of the canyon, the Toe Wall became very extensive, resulting in various other challenges such as thermal cracking during construction.

EFLA's role

• Toe Wall
• Dam plinth
• Dam face slab
• Retaining walls
• Parapet wall
• Intake and diversion tunnel plugs
• Bottom outlet

Detailed design of the concrete structures such as

• Construction assistance
• Review and approval of shop drawings
• As-built drawings

The project's long term benefits

Kárahnjúkar Dam is by far the largest of the three dams that form Hálslón Reservoir, the main reservoir for the 690 MW Fljótsdalur HPP. Creation of the Hálslón Reservoir is a prerequisite for such a large HPP.

Kárahnjúkar Dam is a part of Landsvirkjun's most extensive project. Valuable knowledge was acquired during the planning and design of the project by the employer, the consultants, and various stakeholders.