San Francisco’s Central Subway: Part II – Underground Stations
Contiguous Pile Walls
As we mentioned earlier fabricating and installing a large pile, reliably, is a major challenge. Up until the years approaching World War II, the basics of pile installation hadn’t changed much since Roman times. Although the process was mechanized, piles similar to trees were still largely pounded into the ground and pounding a very large solid pile, say 4-ft in diameter was not practical. For that matter, it was not practical to pound down a 4-ft diameter metal cylinder (In fact it still isn’t for most on-shore projects). Since I.K. Brunel’s day, builders had become more comfortable with the large iron, and then steel cylinders. However, these were not pounded into the ground. Typically, these were essentially “sunk”, or lowered into place since they were large enough to be advanced by excavating the soil at the bottom and letting gravity do much of the work. As you can surmise, the soil conditions suitable to this method are limited.
But wait a minute, aren’t the UMS walls concrete? Yes they are, owing to a machine and a method devised by a French company, BENOTO, just after WWII. The method installs a steel casing that is to be used as a mold to cast-in-place a large reinforced concrete pile. Instead of driving the casing in the ground, the machine would work it in, via rotation and oscillation. Since steel was (and remains) expensive, the design was tweaked so that casing could be reused. Moreover, to make things more manageable for the machine, the casing length was divided into segments. These segments would interlock to create a continuous cylinder, a sealed mold for the concrete pour. The continuous cylindrical casing would also act as a vertical “tunneling shield” of sorts. With the casing keeping the hole open, boring equipment, such as the clam bucket, auger, or rock cutters, etc. could be lowered inside to advance the excavation.
After completing the excavation the casing would then act as a guide to install a steel reinforcing-cage or a steel “H” beam. Finally, with the beam or cage secure in place, the casing mold is slowly withdrawn, via oscillation and rotation, as the concrete is cast. As a result of its heritage and its reliance on the casing, the method became known as the BENOTO or “all casing” method. Using this method, two types of contiguous pile walls, shone below, can be created: a tangent pile wall and a secant pile wall. Both of these wall types will be used at UMS. Each resembles the ancient palisade wall; however, the former can be considered truer to form since the piles abut one another, while in the latter application the piles overlap a bit.
Although BENOTO is no longer in business its pioneering method lives on and remains one of several effective ways to build contiguous pile walls which fulfill two roles: providing temporary and permanent excavation support and reducing the steps in the cut-and-cover process. At UMS, “all casing” was the method of choice because the underground station space is wider than Stockton Street above. “All casing” makes it possible to construct an inclined wall to create the wider underground space as well as to avoid employing extensive underpinning of the buildings that flank the station. It’s conceivable that building reconstruction would have been needed if “all casing” technology did not exist. Fortunately, UMS will not be a first time application of this solution. Both the Munich U-Bahn (c. 1960s) and the Frankfurt U-Bahn (c.1970s) transit systems in Germany successfully constructed this type of inclined wall in similarly tight circumstances. As shown in the following figures, creating the wall, a pile at a time, enables builders to work in a tight footprint, making it possible to maintain street life. The reliability and accuracy of “all casing” also makes it possible to install these contiguous pile walls after the tunnels have been completed.
The drawings shown below show how the All Casing method can be used to create inclined contiguous pile walls while maintaining street life.
“All casing” has another advantage as well. It can be employed effectively in the less intrusive variation of the cut- and-cover method selected by the UMS design team; it is known as the Top-down or the Milan method. In Part III we’ll find out more about Top-down construction and its Milanese origins.
Since Top-down was first applied using a cousin of the pile wall, the so-called Slurry Wall, we will take a slight detour next post and begin with the Moscone Station for which a slurry wall is planned.
COMING SOON: Part III – Underground Stations: Top-Down Method and the Slurry Wall