Ground Improvement at US 101/Broadway Interchange Reconstruction Project-Paper

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The US 101/Broadway Interchange reconstruction in Burlingame, California, required approach embankments up to 32 feet high on soft, compressible Bay Mud, with loose, potentially liquefiable sands also present. The site’s low-permeability clays created risks of substantial long-term settlement, pile downdrag, embankment instability, and damage to nearby utilities and structures. Conventional surcharging with drains would have required years of consolidation and was incompatible with maintaining traffic and construction schedules.<br /><br />After evaluating surcharge, lightweight expanded-polystyrene fill, and ground treatment, the project selected cement deep soil mixing (CDSM). The method mixes cement into the ground to form soil-cement columns, improving strength and reducing compressibility. Columns were arranged in treatment zones beneath approach fills and around abutments; overlapping panels provided additional resistance to slope movement. Treatment also reduced liquefaction-related settlement and lateral-spreading risk.<br /><br />A contractor pre-production program established an effective water-cement ratio and cement dosage. Core testing showed that more than 95% of samples exceeded the specified 160-psi strength requirement. In a field test, a treated area was subjected to a 3,000-psf surcharge for 60 days. Settlement remained below one-tenth of an inch and became negligible after about two weeks.<br /><br />Monitoring and testing indicated that CDSM would keep post-construction settlement within project limits while providing sufficient strength for embankment stability. The treatment eliminated the need for surcharge and a prolonged waiting period, enabling construction to proceed more efficiently while meeting the project’s geotechnical objectives.

Keywords

US 101/Broadway Interchange

Burlingame California

cement deep soil mixing

Bay Mud

approach embankments

soil-cement columns

ground improvement

liquefaction mitigation

settlement control

embankment stability