Design and Construction of the Summit Senior Care Facility into the Side of an Unstable California Hillside-Paper

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The Summit Senior Care Facility in San Ramon, California, was built into a hillside affected by two landslides and nearby active faults. Geotechnical investigations identified a large, mostly dormant landslide extending beyond the property and a smaller, more recent active slide. The active slide was removed and replaced with engineered fill and subdrains. The remaining old landslide was stabilized with a tied-back retaining wall.<br /><br />The wall, up to 30 feet high, was designed to resist slope, landslide, and seismic forces. Analyses targeted factors of safety of at least 1.5 under static conditions and 1.1 under seismic conditions, with predicted earthquake displacement limited to six inches. Design used a peak ground acceleration of 0.85g. Drilled steel soldier piles extended at least 12 feet below the landslide plane, while permanent double-corrosion-protected tiebacks were anchored in stable ground beneath it. The wall was completed with shotcrete facing.<br /><br />Construction was complicated by the hillside setting, difficult soils, and delays in remediating the active slide. Large-diameter holes were drilled for the soldier piles, which were lowered into place by crane and encased in concrete. Tiebacks were installed using cased drilling after open-hole drilling proved unreliable; each was proof- or performance-tested. Drainage and reinforcing steel were added before applying the shotcrete facing.<br /><br />The project demonstrates how detailed historical and geotechnical investigations, specialized design and construction, and careful field oversight can make a hazardous hillside suitable for development. The facility is also supported by a deep reinforced-concrete pier foundation to address building loads and expansive soils.

Keywords

hillside stabilization

landslide remediation

tied-back retaining wall

geotechnical investigation

San Ramon California

active fault seismic design

soldier piles

permanent tiebacks

shotcrete facing

deep foundation