When tenders were invited, two routes had been selected for tunnel no.
4, the longest tunnel.
One would be straight and more or less on the original location, but with the eastern portal relocated further away from the N1 national road.
The other would be curved to pass through shale material, that would make the use of a tunnel boring machine an economical proposition.
Tenderers were invited to quote for circular or horseshoe profiles and concrete or shotcrete linings for each of the two profiles and for each of the proposed routes.
After the engineering, geological and economic factors had been analysed, the straight route with a horseshoe profile and concrete lining was finally selected.

The tunnel was constructed by Compagnie Interafricaine De Travaux (Comiat), a division of Spie-Batignolles in Paris, France.

The contract for tunnel no.
4 was awarded on 13 August 1980 at a tender price of R26,770,082 and with the completion date four years later on 12 August 1984.
The contractual completion date was later extended to 25 February 1986.
Construction commenced in September 1980, with tunnel excavation commencing in January 1981.
As a result of unforeseen adverse sub-surface conditions  that were encountered during the execution of the contract, however, the tunnel was only completed in November 1988.

Tunnels no.
2 and 3 are similar in construction to the long tunnel, but were completed under a separate contract at a cost of R9 million.
Both of them had been partly excavated when work was suspended in 1949, the 1.1 kilometres (0.68 miles) tunnel no.
2 to a distance of 583 metres (1,913 feet) of which most was concrete lined, and the 1.2 kilometres (0.75 miles) tunnel no.
3 to a distance of 467 metres (1,532 feet), but only lined in areas of poor ground.
The contract called for both to be widened to new design standards to allow for overhead electrification and broader loading gauge clearances.
When tenders were invited, two routes had been selected for tunnel no. 4, the longest tunnel. One would be straight and more or less on the original location, but with the eastern portal relocated further away from the N1 national road. The other would be curved to pass through shale material, that would make the use of a tunnel boring machine an economical proposition. Tenderers were invited to quote for circular or horseshoe profiles and concrete or shotcrete linings for each of the two profiles and for each of the proposed routes. After the engineering, geological and economic factors had been analysed, the straight route with a horseshoe profile and concrete lining was finally selected.
The tunnel was constructed by Compagnie Interafricaine De Travaux (Comiat), a division of Spie-Batignolles in Paris, France.
The contract for tunnel no. 4 was awarded on 13 August 1980 at a tender price of R26,770,082 and with the completion date four years later on 12 August 1984. The contractual completion date was later extended to 25 February 1986. Construction commenced in September 1980, with tunnel excavation commencing in January 1981. As a result of unforeseen adverse sub-surface conditions  that were encountered during the execution of the contract, however, the tunnel was only completed in November 1988.
Tunnels no. 2 and 3 are similar in construction to the long tunnel, but were completed under a separate contract at a cost of R9 million. Both of them had been partly excavated when work was suspended in 1949, the 1.1 kilometres (0.68 miles) tunnel no. 2 to a distance of 583 metres (1,913 feet) of which most was concrete lined, and the 1.2 kilometres (0.75 miles) tunnel no. 3 to a distance of 467 metres (1,532 feet), but only lined in areas of poor ground. The contract called for both to be widened to new design standards to allow for overhead electrification and broader loading gauge clearances.
Hex River Tunnels