Strabag won the tender for construction of the Wadi Minqal asphalt road including wadi Minqal gorge crossing.

The tender solution of the wadi Minqal crossing consisted of an embankment over a large culvert. This solution was not considered feasible due to difficult foundation conditions resulting in a variation changing this arch culvert to an arch bridge over the gorge.

Strabag has commissioned Renardet SA & Partners to analyse and design this bridge.

The bridge consists of twin steel arches supporting a composite steel-concrete deck and braced in the transversal direction by cross beams and diagonal bracings. The arch has a span of 115m and the deck has a total length of 160m. Two 22m high steel piers support the two approach spans and share the foundation with the arch. All the elements of the steel structures were partially assembled at the factory to confirm dimensional accuracy before being transported to site. Once the sections arrived on site erection could proceed rapidly reducing the probability of damage or disruption during the construction period due to flash floods in the wadi.

Spherical PTFE bearings were used at the abutments and piers to allow deformation and rotation of the structure, especially for temperature induced variations in dimensions.

The anti corrosion system adopted follows the ISO 12944-5 code requirements, resulting in a minimum period of 15 years to first maintenance. The coatings applied to ensure this level of resistance comprise a zinc rich epoxy prime coat followed by 2 coats of Micaceous Iron Oxide and a polyurethane final finish coating. Before painting operations the steel received grit blasting to SA 2.5. The Wadi Minqal Bridge site is located across a steeply incised wadi valley and consequently the steep side slopes present natural abutments to an arch crossing the wadi. The wadi slopes show a superficial appearance of fractured rock and consequently unweathered rock suitable for founding at relatively shallow depth was expected.

However, geotechnical investigations including boreholes; seismic tomography and electrical resistance measurements revealed weathering, cavities and open fractures to considerable depth at the location of the arch and main pier foundations.

Traditional piles were not considered appropriate due to the nature of the topography and the steeply inclined nature of the combined resultant reaction of the arch and pier. Micropiles were considered and a possible solution developed in sufficient detail to obtain a firm construction price. In parallel with this solution we considered a grouting solution to inject cement grout into a regularly spaced borehole pattern to fill the joints and cavities with grout thereby strengthening the entire rock mass and limiting the expected movement.

Considering the analysis of the seismic results along with economical factors, shallow foundation with ground improvement would provide the best founding solution.

Ground improvement by Pressure grouting was performed by Swissboring LLC under contract to M/s Strabag Oman LLC acting on behalf of Ministry of Transport and Communication.

The whole work consisted of 83 and 78 grout holes (comprising Primary, Secondary and Tertiary holes by upstage and downstage techniques) at Pier 1 and Pier 2 foundations respectively, to a maximum depth of 25m below the foundation levels. Water pressure results of Lugeon tests were used as parameters to evaluate the ground improvement and define the successful completion of the work. The ground improvement was completed on 28 March 2016 with the submission of Project Completion Report.

The settlements of the foundation were monitored over the whole duration of the bridge construction. The maximum settlement at completion of the bridge construction resulted in 4-5 mm total settlement, lower than expected, showing the successful achievement of the ground improvement.

Settlement pegs are still installed on the pier foundations and can be used to monitor the bridge settlements over its lifetime.