Challenge

Earmarked as one of the government’s strategic infrastructure projects, construction is already proving its worth through the promotion of economic activity, community development, service delivery and job creation at both a regional and provincial level. President Ramaphosa visited the site in September 2021, stating that the project would generate between 21,000 and 28,000 indirect jobs during construction and 8,000 direct full-time jobs once completed.

SMEC was appointed as the lead partner in the HVA Joint Venture to oversee design and supervise construction.

Solution

One of the key aspects of the bridge is its two identical, inverted Y-shaped, reinforced concrete pylons. Each of the 127 m tall pylons comprises two inclined legs, straddling the roadway below, and a tall cylindrical spire. The spires have a diameter of 6 m at the bifurcation, where the two legs meet, tapering to 4.5 m at the top. Anchored into each pylon are 17 pairs of main stay cables (connected to the deck) and 17 pairs of back stay cables (anchored 130 m behind the structure).

Grouting trials for anchor blocks

Pre-stressing grout trials were also a vital part of the construction verification process. Full-scale grouting trials were conducted on site to ensure high quality bonded tendons and the durability of structure. The longest tendon was simulated with the maximum curvature and steepest incline to test the equipment, methodology and operators.

The anchor blocks are 49 metres long, 10 metres wide and 17 metres deep, containing over 5,980 m3 of concrete, weighing approximately 15,500 tonnes. Each group of back-span cables are anchored in a buried concrete gravity anchor. U-shaped prestressing tendons extend the full depth of the anchor blocks to provide structural capacity to activate the full deadload of the anchor blocks. The anchor blocks unique shape provides maximum interlock with the in-situ rock mass, ensuring maximum efficiency in lateral bearing capacity to resist tension from the stay cables.

Composite deck

The composite deck, spanning 530 m and supporting a dual 2-lane carriageway, comprise of two box girders connected by transverse truss like cross frames. The deck is simultaneously constructed from both banks, each side comprising of 17 deck segments typically 15 m long. The composite steel and concrete deck of the main bridge is 22.8m wide and includes foot walks to the outer edges of the deck.

The composite deck is highly flexible and structurally indeterminate. An analytical model was created with more than 300 stages to determine the erection sequencing and methodology. The erection engineering included a stay cable tuning exercise to optimise the force effects in the structure as well as the monitoring of displacement in each construction stage to verify the final deck profile.

Wind testing

The bridge design was subject to full wind tunnel testing to ensure the aerodynamic stability during the in-service condition. The Contractor was also required to undertake wind tunnel testing of the deck at various stages during construction.

Concrete thermal modelling and monitoring

Large volume concrete elements (Anchor Blocks, Pylon Bases, and Ladder Decks) were modelled to determine the maximum peak temperatures and peak differential temperature range needed to avoid any thermal cracking. Thermocouples were then installed into the concrete to monitor temperatures during the hydration process. SMEC closely monitored the concrete hydration process to ensure tensile stress remained within the range of the approved temperature gradients.