San Francisco’s Central Subway: Part II – Underground Stations
The Union Square and Market Street Station (UMS) is located in a downtown area with abundant shopping and places to meet. As you can see in the section below, the platform will be deep underground and directly under Stockton Street between Geary and Market Street. The south lobby will connect to Market Street’s MUNI and BART lines.
Fortunately, the design of the station leaves much of the primary structural system exposed, providing us an opportunity to “read” it and learn its story which, as is true for much of civil engineering, is both fresh and familiar.
The drawing above is a view from the platform, where you can see most of the primary structural system. Most noticeable are the cylindrical steel struts, regularly spaced and spanning overhead. These struts brace the sand-blast-finish concrete station walls. A closer look at the walls reveals a palisade form made of separate long components, recalling the ancient fortifications of Greece and Rome. Interestingly, the Romans adapted this structural form and used it in ways that would eventually evolve into what we see at UMS.
When Roman bridge builders became adept at pile-driving, they took the palisade concept a step further to build cofferdams – box-like structures that create a dry, below grade workspace needed for making excavations for foundations in rivers and wet ground.
Roman palisade cofferdams worked well as long as piles of reasonable size were available and could be fabricated, transported, handled and driven using available technology. For much of human history, construction technology was muscle-powered and the piles were made from durable trees such as alder and oak. Large trees were not common so cofferdams were only used for important structures. Although replacing trees with a man-made substitute is advantageous, it is difficult, particularly for the larger sized piles. This is one reason why, today, we do not see palisade walls in every subway station of the world. To understand how and why the palisade wall evolved to make the version used in the UMS Station possible; we need to consider how an excavation for a subway station is made, and review how piling technology has changed since the time of the Romans.