Subsurface Conditions Mission Bay

San Francisco, California

Presented by Hadi J. Yap, PhD, PE, GE

DTX Project Meeting San Francisco County Transportation Authority

11 October 2011

Mission Bay Site Location

United States Coast Survey Map - February 1852

1982 Aerial Photograph with 1857 Coast Survey Overlay

T&R Projects in the area

N3A

42

28

Typical Stratigraphic Layers

• Fill

• Bay Mud

• Colma Sand

• Old Bay Clay

• Colluvium/Alluvium

• Franciscan Complex Bedrock

Typical Stratigraphic Layers

• Fill

– Non-engineered, generally loose to medium dense

– Heterogeneous material consisting of sand, clay, silt, gravel, bedrock fragments, abandoned structures, construction debris, and organics

Typical Stratigraphic Layers

• Bay Mud

– Soft clay and silt with occasional sand lenses and organic material

– Low strength and high compressibility

Typical Stratigraphic Layers

• Colma Formation– Dense to very dense sand with occasional

clay lenses

– Generally contains up to 20% fines (silt and clay)

– High strength and low compressibility

Typical Stratigraphic Layers

• Old Bay Clay

– Stiff to hard clay and silt

– Occasionally contains sand and gravel lenses

– Moderate strength and moderate compressibility

Typical Stratigraphic Layers

• Colluvium

– Dense sands and gravels and stiff to hard clays

– High strength and low compressibility

Typical Stratigraphic Layers

• Franciscan Complex bedrock– Deformed bedding planes and shear zones

due to seismic activity

– Highly variable in hardness & strength

– Deeply to moderately weathered

– High strength and low compressibility

N3A

42

28

Idealized subsurface Conditions – Block 42