Malcolm Drilling, Inc. and Dan Brown and Associates, LLC were the design-build team for a hybrid ground improvement system consisting of vibro-stone columns (VSC) and cutter soil mixing (CSM) panel grids to mitigate liquefaction and achieve target settlement performance criteria for a mixed-use development in Santa Cruz, California.

The development is being done by the Lincoln Property Company and consists of three separate 7-story buildings including 175 apartments and ground floor retail. The separate buildings provide public access from the street to the San Lorenzo River multiuse path. The project is an anchor for the City of Santa Cruz’s plan for revitalizing downtown with enhanced access to the river and increased density of affordable housing.

VSC were used on the interior of the site to densify granular soils while the CSM grids were designed to limit shear demand around the perimeter of the site where existing sensitive buildings and a USACE-controlled levee along the San Lorenzo River prevented the use of vibratory methods. The final project included 125 CSM panels for ground improvement and 558 VSC.

The hybrid ground improvement system supported a mat foundation beneath a single-level basement that spanned the entire site, atop which three individual seven-story mixed-use structures were constructed. The grade-level diaphragm resulted in combined seismic participation from the three structures which led to significant mat bearing pressures beneath shear walls and at the site perimeter.

In true design-build fashion, final geotechnical information was not available until after construction began and existing buildings on the site were demolished. This allowed access to install trial patterns of VSC with pre- and post-installation testing as well as additional exploratory CPT to identify the depth transition for the Purisma Formation bearing layer from about 60 ft in the northern portion of the site to about 20 ft in the southern portion of the site. VSC and CSM panel depths through the transition zone finalized while construction of the first production CSM panels was underway at the southern site edge.

Along with the Owner’s geotechnical engineer, Rockridge Geotechnical, DBA and Malcolm implemented multiple quality assurance and control measures. The CSM design and QA/QC program included a bench-scale study using soil samples from the geotechnical investigation, test panel installation using three different binder mixes with variable cement content to investigate soil mix compressive strengths, daily wet-grab sampling of production columns for strength testing, and coring of select columns for visual observation of mix continuity and sampling for strength testing of cured in-situ soil mix. Ground improvement through VSC was verified using post-installation CPT which were analyzed and evaluated on an individual basis against project performance criteria.

The RiverRow project demonstrates the successful use of the design-build method to optimize site-specific ground improvement technologies to meet multiple project needs while safeguarding the interest of a wide variety of surrounding stakeholders, including sensitive structures and a USACE-controlled levee.