11_GOGA_FM201W_Geo_04072022_Add1_0001.pdf

7 April 2022

Barbara Clement, AIA, NCARB

Associate Partner

FFA Architecture and Interiors, Inc.

520 SW Yamhill Street, Suite 900

Portland, Oregon 97204

Subject:

Addendum #1

Geotechnical Investigation

Building 201 – West Wing

Fort Mason

San Francisco, California

Langan Project No.: 731677402

Dear Ms. Clement:

This addendum presents supplemental foundation and underpinning recommendations for the proposed strengthening of the West Wing of Building 201 at Fort Mason in San Francisco, California. We provided our conclusions and recommendations regarding the geotechnical for the project in a letter dated 4 May 2021.

SUPPLEMENTAL FOOTING RECOMMENDATIONS

Per our 2021 geotechnical investigation letter, we recommended new footings that are five feet wide be designed using an allowable soil bearing pressure of 3,000 pounds per square foot (psf) for dead plus live loads (factor of safety of 2), and embedded at least 3 feet below the building slab or below lowest adjacent outside grade, whichever is deeper.

We understand that the design team is considering supporting some of the planned improvements on new footings with a width of 4 feet. New footings with a width of 4 feet may also be designed using an allowable soil bearing pressure of 3,000 pounds psf for dead plus live loads. This value can be increased by one-third for total loads, including wind and/or seismic forces. For footings embedded as described above, static settlement should be about 1 inch.

UNDERPINNING RECOMMENDATIONS

We understand that at locations where the existing concrete walls will be strengthened with shotcrete, underpinning piers will be installed beneath existing building foundations. These piers will need to be designed for any surcharge from any adjacent building foundations. The size and elevation of the bottom of the building footings to be underpinned should be confirmed by others prior to excavation and underpinning.

We anticipate underpinning to consist of conventional hand-excavated piers that extend at least two feet below the bottom of the existing or new footing adjacent excavation. Hand-dug underpinning piers are usually on the order of 30- by 48-inches in plan dimension and soil is

Building 201 – West Wing

Fort Mason

San Francisco, California

Langan Project No.: 731677402 retained between piers using pressure-treated timber lagging. The piers are reinforced with steel and filled with concrete, the pier is jacked against the building to engage the bearing of the pier, and the top of the pier is dry-packed to fit tightly with the base of the underpinned foundation.

Underpinning piers should be designed to resist the vertical building loads and lateral earth pressures. The underpinning piers should be designed for at-rest lateral earth pressures above and below the design groundwater level, provided in Table 1. In addition, the underpinning should be designed for the surcharge from foundations within a line drawn up from the bottom of the underpinning element at a 2H:1V. Lateral forces may be resisted by passive earth pressures against the embedded vertical faces of the piers and calculated using an equivalent fluid weight of 240 pcf.

TABLE 1

Equivalent Fluid Weights for Underpinning Piers

Retained Material

Restrained

At-Rest Condition

(pcf)

Soil Above the

Groundwater Level2, Drained

Undrained or Soil

Below the

Groundwater Level

Notes:

1. The more critical condition of either at-rest pressure (static condition) or active pressure should be checked.

2. 10 feet below the ground surface, is the recommended design groundwater level.

3. Structural engineer to determine appropriate load combinations for design of below-grade walls.

Underpinning piers should bear on medium dense to dense sand and may be designed using a maximum allowable soil bearing pressure of 3,000 psf for dead plus live loads. To reduce movement and provide adequate foundation support during installation of the underpinning piers, adjacent piers should not be excavated concurrently. If intermittent piers are used, the excavation face between the underpinning piers should be retained using lagging. We should observe the underpinning pier excavations to check that the exposed soil can support the design bearing pressures and that they are properly cleaned. Piers may need to be deepened and either backfilled with lean concrete or overpoured with structural concrete if the bearing soil is not encountered at the planned depth. We recommend underpinning piers be preloaded prior to dry packing to reduce settlement as the foundation load is transferred to the piers.

Building 201 – West Wing

Fort Mason

San Francisco, California

Langan Project No.: 731677402

We trust that this provide you with the information needed at this time.

If you have any questions regarding our findings or conclusions, please contact us.

Sincerely, Langan Engineering and Environmental Services

Ron E. Noche, PE Maria Flessas

Project Engineer Principal/ Vice President

731677402.02 Addendum1_Building 201 Fort Mason West Wing