The sequential excavation method (SEM, also known as NATM or New Austrian Tunneling Method) is a mined method that will primarily be used for the Chinatown Station. This method is used to control deformations and thereby mobilize and maximize the self supporting capacity of the surrounding rock or soil. The tunnel excavation is carried out in increments (headings or rounds) in numerical sequence, which are supported with sprayed concrete (shotcrete) and other supporting elements immediately after exposure, followed by installation of additional steel and shotcrete support elements until a safe stable opening is created. SEM provides a high degree of flexibility during construction and makes it possible to control virtually all kinds of ground conditions, thereby greatly reducing the risks of construction. This method of tunnel construction is used worldwide for small to large openings in a variety of ground types ranging from rock to soil.
After completion of the excavation and initial support, a waterproofing system is installed between the initial and the final lining. The final lining, which can be either reinforced cast-in-place concrete or reinforced shotcrete, is then installed.
The cut and cover method employing a “top-down” or “bottoms-up” approach will be used for the Union Square/Market Street Station, for the Yerba Buena/Moscone Station and for the Tunnel Portal. This method involves construction of a box frame structure within a trench excavation that is subsequently backfilled. In urban settings the cut-and-cover method requires utility relocation, traffic re-routing, and creates construction impacts in the form of noise, dust, and traffic, transit and business access disruption. “Top-down” refers to the sequence of final structures erected with the excavation, whereas with the “bottoms-up” approach the excavation utilizes temporary struts and bracings to the final level before erection of final structures from the bottom of excavation back up to surface.
Decking will be placed over the cut immediately following the first layer of excavation to reduce traffic disruption. The decking is removed and the surface restored at the end of construction. Temporary excavation support walls (or shoring) are installed before significant excavation commences. These walls must be supported with internal struts as the excavation is deepened to avoid instability and control settlement at the sides of the cut. The shoring used will either be cased secant piles or slurry walls.
Cased secant piles are non-driven piles that can be used for ground support in soft ground and hard ground. Secant pile walls are formed by constructing interlocked concrete piles reinforced with steel beams. Used extensively in dense population areas due to the minimal disturbance they cause to adjacent structures, secant pile walls are commonly used for shafts and stations in saturated soil conditions. The steel reinforcement in the form of wide-flange sections will be dropped or vibrated into place.
Slurry walls are reinforced concrete walls used in areas of soft earth close to open water or with high ground water table. A trench is excavated to create the diaphragm (water-blocking) foundation walls around the perimeter of the excavation. The trench is kept full of surry preventing collapse from the earth pressure and prevents water flow into the trench. The slurry wall uses steel reinforcement cages which are lowered into to the trench and filled with concrete to displace the slurry. The slurry wall serves as both the excavation of support and permanent wall structure of the station.
A rendering of the proposed tunnels under the existing Market Street subway lines