As part of the Stacy Witbeck/Herzog team, BAI performed geotechnical investigations and geologic hazard evaluations along the length (approximately 41 miles) of the Initial Operating System One (IOS-1). This stretch included investigations for bridge abutments at 10 bridge replacements, new track, and dozens of grade crossings.
Our work consisted of researching available pertinent reports and existing data for all 14 station sites and 10 bridge sites, conducting subsurface geotechnical investigations at station locations, replacement bridge sites, and at over 100 key locations throughout the rail alignment. Our investigations targeted geotechnical factors that may influence design and construction of fills, bridges, and station elements. Project challenges included developing efficient, effective approaches for addressing weak, compressible or expansive surface soils, liquefaction potential in response to earthquake shaking, settlement of bay mud under fill, pile driving through bay mud into hard Franciscan rock, and shallow groundwater.
Specific challenges included the following:
- Weak deposits, such as young bay mud, are prone to compression and settlement when loaded, and consequently are not suitable for support of fills or structural foundations.
- Potentially expansive surface soils that underlie much of the SMART alignment in Sonoma County.
- Groundwater at depths less than 10 feet deep, and the associated potential for caving; necessity for dewatering of excavations extending below these depths.
- Seismic considerations, including the effects of earthquake shaking on foundations, embankments and retaining walls, and the loss of shear strength and potential soil volume reduction in loose, saturated sandy/silty soils below the groundwater table via liquefaction.
- BAI provided construction observations and testing including pipe pile installations and cut and fill slopes and retaining structures.