Riyadh’s new metro Line 3 is the longest of the large six-line metro system in Saudi Arabia’s capital with a total length of 41.6 km and 22 stations. Eight stations are underground. The US$ 25.3 billion metro program includes 180 km of underground and at-grade LRT, along 75 stations. It started in 2013 and is expected to be operational by 2019.
The lead design firm of Line 3 – IDOM Engineering – retained Pedelta for the design of two underground stations named 3E6 and 3E1. The scope of services comprised all permanent concrete and steel underground structures including foundations, slabs, walls, stairs, accesses, canopies and temporary works.
Both station 3E6 and 3E1 were built by using a bottom-up construction method, with the temporary support of excavation consisting of a secant wall. The stations have a rectangular box of 148 m in length with a variable width between 19.9 and 28.3 m, and extend 33 m below street level. In addition, there are entrances and ventilation shafts. The stations have a middle-covered atrium that visually connects lower and upper levels. The atrium allows the light to reach the platform level and provides an appealing open space. Stairs and escalators provide vertical circulation.
The architecture of the stations governs the structural form and layout within the stations. The reinforced concrete permanent walls of the main box have a variable thickness of between 0.8 and 1.4 m, which is proportioned to the bending caused by the lateral earth pressure. The base slab is curved with a maximum depth of 2 m. The inverted arch slab is a cost-effective structural system to reduce the bending due to the underground water pressure. Expansion joints at walls and inverted slabs are spaced every 30 to 40 m to reduce tensile stresses on the walls due to concrete shrinkage and temperature variations. The stations have a concrete roof slab and four intermediate levels. The concrete floors consist of prefabricated pre-stressed concrete girders with a reinforced concrete slab. The connections between walls and the floor system along with the construction sequence have been carefully assessed so that the slabs provide permanent lateral support to the walls.
Structural Finite Element models were carried out to determine the load effects by considering the soil-structure interaction. In addition, a soil-structure analysis using two-dimensional Plaxis © models was conducted to determine lateral earth pressures and to calibrate soil-spring stiffness used in the structural models. This design strategy allowed the rapid evaluation of changing soil conditions and the construction sequencing prior to the detailed design.
Even though these stations are located in an arid region, in the KSA, the specific site conditions, with a high water table, imposed challenging conditions of buoyancy. Lateral, inverted shear keys were designed at the sides of the bottom slab to anchor the structure vertically to the rock. Additionally, transverse shear keys were designed to minimize the longitudinal superimposed deformation and optimize the design of longitudinal reinforcement in the walls and slabs.
Deep foundations with secant-pile walls, High water table