This project was part of the $1 billion San Francisco Clean Water Program that was mandated and funded by the Federal Government. San Francisco’s sewer system is combined with its storm water system as in the name “combined flow”. This is a typical situation in old cities throughout the world. The obvious drawback with this is the massive amount of water that must be treated as sewer water when it rains. Many times when there are large rainstorms the sewage/rainwater overflows into the bay or ocean untreated. Mariposa facilities is a storage basin which acts as a sewage/rainwater buffer in order to minimize overflows into the San Francisco Bay. Along with this underground storage basin is a pump station and many associated pipelines

The sewage box is 400’ long and 30’ deep and encompasses 5,000 cubic yards of cast-in-place concrete. Because it was built in the middle of a busy street, the excavation had to be decked to allow for traffic flow. Much of the work occurred under the deck and the deck was only removed when it was absolutely necessary. The shoring and dewatering was tricky due to the fact that the structure was only 150’ from the bay. However, Nationwide/Shimmick installed a soldier pile and lagging shoring system instead of the more typical and expensive shoring for this situation, sheet piles. This was an educated gamble that paid off. Nationwide/Shimmick was able to dewater this area next to the bay because the bay mud in this area is very impermeable, therefore, very little water entered the excavation even though it was 25’ below sea level. The pump station forced sewage through 5000’ of new pipeline towards the nearby treatment plant. Nationwide/Shimmick was considerable under the next closest bidder, but still made a healthy profit on this project due to their clever construction method, brought on by experience and unique insight.