Down hill bridge launching

Project Overview
The Mgeni Viaduct is located in Durban, South Africa and bridges the Mgeni river to link residential and industrial zones in the greater Durban area, as well as providing a link road to the new King Shaka Airport. The project started in July 2007 and is due to be completed in March 2009, with a project value of R187 million.

The Mgeni Viaduct was designed by BCP Engineers (who were later bought out by SSI www.ssi-dhv.com ) and constructed by Rumdel Constitution Cape JV (www.rumdelcape.co.za ), with the temporary works design being done by Jeffares & Green ( www.jgi.co.za ) The project was undertaken for the Kwazulu Natal Department of Transport ( www.kzntransport.gov.za ). The viaduct was incrementally launched at a 9% gradient from the east abutment, with the launching system being supplied by Enerpac ( www.enerpac.com ).

The Mgeni viaduct consists of two independent, pre-stressed concrete single cell box girders with a total length of 410m, and has 9 spans of 34m, 36m, 6 x 50m and 40m. Each deck has a width of 14,4m and a depth of 3,7m and is designed to carry three lanes, of 3.5m, in each direction The decks are separated by a 3m wide raised median, and contained on either side by precast New Jersey barriers.

The deck weights 25 tons per meter and follows a constant sagged vertical curve with a radius of 3,7 km, the east abutment being 8.74m higher than the west abutment. The piers are hollow slid form structures founded on solid rock, varying in height from 50m to 15.5m with the highest 4 piers being stayed for the duration of the launch. The deck structure is lead by a 30m long 56 ton steel nose, and the deck is cast in 18 segments varying between 11 and 25m.

Launching System
Launching a deck on a 9% gradient offered many challenges as the structure was unstable and inclined to move under gravity. The launching system is required to restrain the deck at 1,3 times the down slope force of the deck, assuming zero friction. The system also needed to be able to pull the deck back in the event of a bearing miss-feed with a maximum force 10.500 kN.

This was achieved by using 3 x 350 ton hollow plunger cylinders coupled to the deck via retaining sticks installed through the box section and connected to the cylinders via 75 mm Macalloy bars. The cylinders were set up behind the left abutment with the macalloy bars running through the abutment. The 350 ton retaining cylinders act as the “anchor” with the pressure release valve being set to only allow the cylinder to collapse when a force of 1,3 times the down slope force of the deck is applied to the cylinder. Thus the weight of the deck on its own will not collapse the cylinder so the deck does not move. These are the largest cylinders made by Enerpac to date.

In order to launch the deck it is then pulled forward by three 150 ton pulling cylinders, coupled to the deck via pulling sticks and 50 mm Maccalloy bars. When the pulling cylinders are extended the force of the pulling cylinders, plus the down slope weight of the deck is enough to collapse the 350 ton retaining cylinders and overcome the friction, and the deck moves forward. The deck is thus moved in a safe and controlled manner and the launch can be safely stopped at any stage by stopping the extension of the pulling cylinder. This safety feature is critical as in the event of a loss of power or pressure the deck will not move on its own as the “anchor” remains in place.

Each movement advances the deck by 750 mm before the cylinders are reset for the next sequence. The load on the retaining cylinders is transferred to the transfer cylinders at the end of each cycle allowing the retaining cylinders to be extended. The deck is then locked off on the retaining cylinders again, the transfer cylinders are collapsed and the deck is launched again.

The launching system is controlled from a central computerized control panel where pressure parameters for the retaining and pulling cylinders may be set. This allows the launch controller to set the system to shut down if the anticipated launch force required for launching of that segment is exceeded. This may occur if a bearing is fed in upside down or something is obstructing the deck. The system may be operated in manual or automatic, with the automatic cycle making use of electronic sensors in the hydraulic cylinders indicated to the control panel when each movement in the launch cycle is complete. The release pressure for the retaining cylinder is set with a manual pressure release valve and the cylinder pressures can be read of dial gauges on the valve block or off the control panel.

The launching system also includes emergency switches on all the piers that are linked into clinometers installed on the 4 highest piers. The clinometers are equipped with electronic sensors which automatically shut the system down if the pier deflects beyond the stipulated tolerance.

Modifications to facilitate the launch

Due to the high launch loads imposed on the box section through the pull sticks the deck and soffit were required to be modified. The area around the pulling and retaining stick holes was thickened and additional steel reinforcing installed to cater for the launch loads. The thickening gave the pull/retaining sticks a larger bearing area on the soffit. The steel arrangement was then re-detailed to include the original design steel to facilitate easier and faster construction. As the deck grew longer and the launch loads increased the steel was increased to cater for the increasing loads. These holes were then filled in after the segment had been launched.

The pull/retaining cylinders kick against the left abutment and sleeves were left in the abutment to allow the Maccalloy bars to pass through. During the construction of the left abutment foundation it was found that a fault line ran through the centre of the abutment. The possible slip was overcome by installing additional ground anchors at different angles 16m into the rock face. Additional sleeves were left in the abutment in case any of the ground anchors failed. Fortunately none of the additional sleeves were required.

Construction
The casting area is situated in a cutting and the rest of the establishment for the construction of the deck on the adjacent fill which made for a very congested site. The limited storage area made the scheduling of stock delivery and control critical as not much could be stored on site.

The decks were launched simultaneously with segments being constructed in a cycle time of 12 to 14 days. The concrete is batched on site using a karoo batcher and is placed using 2 cubic meter buckets. The box sections are cast in three sections, first placing the soffit, then the webs and finally the deck. Due to the high cement content and summer temperatures in Durban in excess of 32 degrees the concrete is required to be placed below 30 degrees centigrade. This is achieved by cooling the water to 1 degrees and keeping the aggregates under insulated roofing. The 19 mm stone was kept wet using sprinklers to reduce its temperature and ice is on hand in a freezer container if required.

A strength of 35 MPa was achieved in 36 hours before the concentric pre stressing was done. Cubes are match cured on the deck under insulated boxes to get an indication of the actual strength of the element, while an onsite lab cures cubes in curing tanks for quality control purposes. The steel is prefixed outside the shutter form and rolled into place once the previous segment has been launched. The casting bay was constructed on the radius of the vertical curve with a dip of 25mm over the 25m segment. The skid beam was set up to with in a 1mm tolerance and the formwork is raised and lowered on 100 ton jacks. No settlement was experienced as the casting bay foundations were constructed on the bedrock in the cutting.

Completion We are currently busy with the final segment and the viaduct is due to be complete in March 2010. The deck will be pulled into its final position from the right abutment. The pulling cylinders will be relocated to the right abutment shortly and holes have been left in the second segment so that the pulling sticks may be installed.

The diaphragm walls will then be cast over the piers and the drape cables stressed before the New Jersey barriers are installed. The New Jersey barriers have been precast at the precast yard at the right abutment and are ready for installation. Piers 3, 4 and 5 have fixed bearings and the retaining sticks and cylinders on the left abutment will remain in place until these have been installed. The temporary bearings will be replaced with the permanent bearings by jacking the deck using 4 x 500 ton pancake jacks supplied by Enerpac. The temporary bearing will then be broken out and lowered to the valley floor. The permanent bearings are already located on the piers and will be moved into place and grouted. This is due to start early in the new year.

Although the project offered many challenges these have been overcome by the close working relationship between the various design consultants, the contractor and Enerpac.