The construction of the Chinatown Station is underway following the Sequential Excavation Method (SEM). The caverns are being constructed in the bedrock of the Franciscan Complex (sandstone, shale and melange) and the Colma Formation (dense sand with interbedded stiff clay).
The next milestones were reached in October 2017 with the completion of the Platform Caverns, North and South. The Platform Cavern has a width of 55 feet (17 m) and a height of 46 feet (14 m) resulting in a cross section of approx. 2000 sqft (204 m2). The Platform Caverns were constructed to the North and South of the Cross Cut Cavern (completed in 2016), beneath Stockton Street.
The length of Platform Cavern South is 180 feet (55 m) and this part of the station was predominantly constructed in the Franciscan Formation (sandstone, shale and melange). This unit is highly variable in composition, hardness, and strength, ranging from friable to moderately strong. The observed fracture spacing varies from very close to close.
The Platform Cavern North has a length of 120 feet (36 m) and was predominantly constructed in the Colma Formation. The Colma Formation is composed of a complex, interbedded sequence of estuarine and nearshore sediments. It generally consists of well- bedded dense to very stiff sand with interbedded stiff to very stiff clay. The overburden varies from 50 to 60 feet and both the Platform Caverns, North and South, were constructed following SEM Methods in partial drifts with grouted pipe pre-support and SEM Toolbox items.
In addition, an emergency egress shaft and two tunnels were completed in 2016 and early 2017.
Currently the construction of the Crossover Cavern is underway following SEM Methods in partial drift excavation. The completed Crossover Cavern will have a width of 57 feet (17 m), a height of 38 feet (11 m) with a cross section of approx. 1900 sqft (180 m2) with a total length of 300 feet (90 m).
Dr. Sauer & Partners delivered the final design of all SEM structures for the Chinatown Station. The design of Chinatown Station includes an array of risk mitigation measures including dewatering, partial drift excavation, grouted pipe pre-support, face support anchors, compensation grouting and SEM toolbox items. Construction of the SEM structures and Design Representation on site started in May 2016.