Abstract:
Constructed in 1949, the overcrossing ramp at Broadway provided access to the four‑lane US Highway 101 (US 101) for the City of Burlingame, California. Later, US 101 was widened to an eight-lane divided freeway, becoming a principal artery serving local and inter-regional traffic along the San Mateo Peninsula and for the greater Bay Area, and since that time, the 1949 structure has reached its capacity despite improvements in the late 1960s, 1970s and 1990s. In 2005, the San Mateo County Transportation Authority (SMCTA), formed in 1988 when voters approved a half-cent sales tax measure to fund transportation projects within San Mateo County, proposed reconstruction of the interchange at US101 and Broadway to improve traffic movement and to accommodate future increases in traffic volume within and adjacent to the interchange. A buttonhook/diamond interchange alternative was selected and the center piece of the selected interchange alternative was the overcrossing ramp structure at Broadway. To accommodate Caltrans’ freeway access ramp standards, approach embankment fills as high as 32 feet were required. Since the site is underlain by highly compressible, soft and low permeability clay, known locally as Bay Mud, long-term consolidation settlement was identified as the primary geotechnical concern at the site as this could result in downdrag loads on pile foundations, and excessive settlement of approach fills, retaining walls and existing transmission tower structure foundations. Instability of embankment slopes underlain by soft clays was of further concern, as well as potential liquefaction settlement and lateral spreading associated with a shallow groundwater table, artificial fills and loose native sands in this seismically active region. Because of limited right-of-way and the need to maintain traffic flow during construction, ground improvement methods to accelerate or eliminate consolidation settlement were evaluated, including surcharge loading and use of lightweight fill. Cement deep soil mixing (CDSM), a process whereby soil is improved by injecting cement through augers that mix the soil, forming in-place soil-cement columns, was ultimately selected since this ground improvement method would not only decrease compressibility and increase strength of the soft clays, but would also mitigate adverse effects of liquefaction and lateral spreading at the overcrossing structure. Unconfined compressive strength tests performed on CDSM column core samples collected during construction resulted in average strengths well above the specified 150 pounds per square inch. Field surcharge load tests and monitoring for areas treated to depths of 25 and 50 feet indicated essentially no post-construction consolidation settlement where the ground had been improved. This paper will review the project background and subsurface conditions encountered at site and present information pertaining to the design, load testing and construction of the CDSM ground improvement for this interchange reconstruction project.