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Innovative Shoring and Excavation Solutions Keep Water Table Challenges at Bay

About the project:

500 Folsom St. is a 42-story, 728,814-square-foot residential tower project with six levels of below-grade parking. There are 545 residential apartments in 546,401 gross square feet; 288 parking stalls in 177,504 gross square feet; and 4,909 gross square feet of retail area. A portion of the above-grade residential project includes a nine-level “podium” structure, which is larger in footprint than the tower. The project also includes the following amenities: community room for entertaining; fitness room; spin room; yoga room; dog wash room; public and private plaza areas at grade; outdoor terraces which include a spa/Jacuzzi, multiple bar-b-ques, seating areas, and green landscaping.

Above-Ground Site Conditions

The 500 Folsom St. site conditions—both above and below ground—create extreme logistical challenges for construction, staging, and equipment that required the project team to formulate innovative solutions during the excavation and shoring processes. The site encompasses 30,000 square feet, which extends to property lines (back of sidewalk) on all sides, making it a zero-lot line project. In addition, it is constrained by heavy city traffic on First Street to the East, Folsom Street to the South, and Clementina Street to the North. A CalTrans off-ramp from Interstate 80 is situated adjacent to the north of Clementina Street. An existing residential building abuts the property directly to the North-West.  

To provide some context for just how much traffic surrounds the site, First Street is an access street for San Francisco commuters to the I-80 Oakland Bay Bridge and is consistently listed as one of the busiest urban streets in the U.S. between 3:00 and 7:00 pm. Folsom Street is an access street for San Francisco commuter buses to the Oakland Bay Bridge. The Interstate I-80 off-ramp is one of the arterial access ramps for commuter traffic coming into the San Francisco financial district from the Oakland Bay Bridge.

Below Ground Site Conditions 

All of these visible conditions affected �۽�ֱ��’s approach to the excavation, but so, too, did those beneath the surface. The site was encumbered with contaminated fill from the 1906 Great San Francisco earthquake to a level of approximately 10’ below the surface. Due to the possible existence of buried artifacts, an archeologist was required to be on-site full-time during the excavation. Additionally, the site was once home to an elevated CalTrans freeway, which was demolished after the 1989 Loma Prieta earthquake. Foundations from the elevated freeway were left on-site below grade.

The building is founded in dense Colma sand to the East and in rock to the West. The Colma sand, which is below the water table, has a phenomenon called liquefaction whereby saturated or partially saturated soils substantially lose strength and stiffness in response to applied stress, usually earthquake shaking, causing it to behave like a liquid.

Excavation Challenges

The basement footprint is approximately 30,000 gross square feet, and the excavation is 75’ deep from the highest grade to the lowest point of the mat foundation. The site’s water table is located 40’ below the surface, which is 35’ above the bottom of the excavation. A portion of the project called the “podium” is located nine levels above grade and has a larger footprint than the tower. As such, the weight of the “podium” is not heavy enough to overcome the hydrostatic head pressure of the below-grade water.

Because of these complex site conditions, the project team needed to solve the following challenges during the excavation process:

1. With the longest building length at 250’, a 15% grade ramp for trucks to access the excavation only allows for feasible truck access to the excavation via a ramp up to 25’ in depth. A solution to remove soil from below 25’ needed to be found.
2. Excavation below the water table and mitigating water intrusion into the excavation.
3. Soil enhancements to mitigate the liquefaction of the dense Colma sand formation needed to be found. These soil enhancements are necessary to mitigate potential settlement and/or differential settlement of the building, which could be caused by the liquefaction of the Colma sand formation.
4. Access to the excavation due to the site’s zero lot line design.
5. Tie-down anchor installation sequence needed to be resolved with other operations.

Excavation Solutions

Initially, �۽�ֱ�� interviewed five shoring subcontractors and six excavation subcontractors, with the intent to facilitate a “marriage” between the two. Additional subcontractor proposals were received for soil enhancements and dewatering. Each of the bidding companies offered different means and methods for their respective scopes of work. Ultimately, the project team elected to mitigate risk by awarding the entire responsibility to “get to the bottom of the hole” to one subcontractor, Malcolm Drilling. Malcolm’s subcontract, teamed with Evans Brothers for excavation, includes shoring, excavation, removal of existing CalTrans foundations, contaminated soil removal, temporary dewatering wells, soil enhancements, and tie-down anchors.

Because the logistical constraints of the site did not allow adequate room to stage and operate a mobile crane, the team elected to build a trestle supported by pin pile columns over the excavation. This trestle allows a 200-ton mobile crane to be situated over the excavation, which will also be used to hoist reinforcing steel and form materials for the mat foundation. Once the mat is poured, the team can successfully erect a tower crane.

The trestle also enabled conveyor machinery to be set to extract soil from the deep excavation. Since the excavation extended too deep for conventional truck ramps, a conveyor system was installed once the excavation extended below 25’. Equipment was hoisted into the excavation with the mobile crane to “feed” the conveyor hopper.

Unique Shoring System 

When builders excavate below a site’s water table, they run the risk of incurring water intrusion. If this happens, it renders excavation virtually impossible, as the hole remains continually flooded.

Malcolm’s unique approach to this challenge was to create a substantially watertight shoring system, which also provided support for the adjacent building. Malcolm installed a three-meter thick Cement Soil Mix (CSM) cut-off wall with wide flange steel soldier piles in addition to three rows of side pocket tie-backs. The fourth row of tie-backs was located in the water, so a center-pocket tie-back with a “plug” was utilized.

At 36” above the top of the mat, another row of support was required; due to hydrostatic head pressure, however, the team elected to use a cross-lot internally braced steel support system to mitigate any issues with “blow-out” of tie-backs, which would have caused flooding of the excavation and potential soil erosion of adjacent properties. This CSM shoring wall creates a water barrier wall to enable excavation without the risk of water intrusion that would normally occur with a more traditional and typical beam and lagging shoring system.

Soil Solutions

Contaminated soil existed on this site to a depth of approximately 10’ from the 1906 Great San Francisco Earthquake. Industrial hygienists were on-site full-time during contaminated soil removal. All contaminated soils were hauled to and disposed of at the appropriate facility based on the classification of contaminates. 

Since the building is partially founded in dense Colma sand, the team needed to strengthen this formation to mitigate liquefaction during a seismic event. A grid of jet grout columns across the site was utilized to achieve the necessary strengthening of the soils. Jet grouting is a highly specialized technique used to create an in situ mass of improved soil also called soil-cement. A drill rig advances a relatively small diameter (4” – 8”) jet grout tool to the bottom of the proposed treatment zone and horizontally injects fluid grout at high velocity. The drill tool is withdrawn and rotated at a constant rate as the grout erodes and mixes the surrounding soil to create a soil-cement column from the bottom elevation upwards.

Water Table Solutions

The first nine levels of the project contain a podium that is larger than the tower that extends up to 42 stories. To mitigate uplift due to the hydrostatic head pressure of the water table, 15 hold-down anchors were installed at areas outside the tower area. These hold-down anchors were load-tested and approved by the Geotechnical Engineer.

While the CSM shoring wall acts as a barrier to mitigate water from migrating horizontally into the excavation, there was still a need to prevent the migration of water from below. At the mid-point of the excavation, the presence of groundwater required interior dewatering. The team installed nine temporary dewatering wells to draw down the water table. These wells will remain active until the tower structure reaches a height of 24 floors and before the building is heavy enough to withstand the hydrostatic uplift pressure. To monitor any future movement or settlement, the project team will survey the top elevation of the mat foundation every month until the structure and curtain wall are complete.

To mitigate the risk of below-grade water migrating into the basement once construction is complete, Grace 160R vertical waterproofing will be utilized at poured-in-place concrete perimeter walls, which occur from the top of the mat up to Level B1, well above the water table. Grace SCS vertical waterproofing will be utilized at shotcrete perimeter walls, which occur from Level B1 to grade. Grace 300R below the mat foundation horizontal waterproofing will be placed over a three-inch mud slab. This membrane is covered by a four-inch fiber-reinforced protection slab. Cetco Voltex (volley) horizontal waterproofing will be placed over the protection slab as a secondary means of waterproof barrier. Grace injection tubes are installed at critical cold joint areas as directed by the waterproofing consultant.

Global Collaboration

The 500 Folsom St. construction team is collaborating with other teams across the global �۽�ֱ�� enterprise in regard to their experiences on this project, including systems approach, procedures, and lessons learned. Teams facing similarly challenging sites will benefit from leveraging and applying these innovative solutions to their projects.

Authored by Chet Brians, SVP, Oakland, CA �۽�ֱ�� office
Rendering and Plan courtesy of SOM, LLP
Photos courtesy of Malcolm Drilling and Jobsite