Merrill Gardens at Hillsboro

Scope of Work:

Merrill Gardens at Hillsboro is a proposed 8-story mixed use building. It will incorporate assisted-living spaces, a medical clinic, and retail opportunities into one structure in a central urban location, also representing the tallest building ever planned in downtown Hillsboro.

Central Geotech’s subsurface investigation began in September of 2018, with CPT testing and solid-stem auger boreholes. These tests yielded penetration testing data, as well as both disturbed and in-situ soil samples for analysis. Seismic response analysis was also performed on the site, using both our collected data and historical data from the region. Utility locating, drilling exploration, and laboratory testing were all performed by COBID-certified firms. This investigation allowed us to provide a Geotechnical Report dated 11/20/19.

Deep soil improvement columns proved cost-prohibitive at this site. Central Geotech and KPFF collaborated to implement a hybrid wall and backfill-supported “bathtub” slab design, to address concerns of excavating below the observed groundwater table. This design was implemented to ensure the structure’s ability to withstand earthquake loading, adequately spread axial loads, and withstand full hydrostatic resistance as expected from the observed condition of site soils and groundwater. Two piezometer-based monitoring wells were installed on-site per our recommendation for continuous monitoring of site groundwater conditions.

After the expected soft soils and groundwater were encountered as excavation began, an extensive dewatering program was put into place following CGS’ recommendations. A massive pump system was installed and connected to upwards of 50 drilled wells around the site perimeter, draining continuously into a Baker tank while construction operations were paused for weeks to allow sufficient time for dewatering.

Throughout the design and construction phases, Central Geotech issued six technical memorandums as necessary to support the evolving design process. Once groundwater levels had been lowered sufficiently to allow for further earthwork, a continuous cut-and-cover style excavation was performed to install compacted structural fill into zones of otherwise unacceptably soft soils. After excavation and backfill was complete, the structure’s slab pour marked a significant milestone for the project; representing a triumph over the design concerns that had until then dominated project discussions. Settlement monitoring markers were installed on structural columns of the building, to ensure that hydrostatic uplift remains a non-factor as construction progresses.

Central Geotech continued to provide subgrade analysis and backfill compaction testing for project elements such as electrical vaults, utility trenches, exterior construction scaffolding, and the structure’s underground entryway ramp. As the bulk of the project’s earthwork and subsurface improvements have been completed, Central Geotech will remain attached to the project for isolated backfill testing and settlement monitoring until project completion.

                                                                                                                                 

Features:

  • Challenging site with undesirable groundwater properties
  • Ongoing design and recommendations for site dewatering operations
  • Soft soils encountered with over-excavation required
  • Monolithic ‘bathtub’-style slab design
  • Analysis of provided settlement monitoring data                                                                                                                                           

Results:

Central Geotech was able to help our clients and their contractors navigate a tricky geological environment throughout every phase of the project. Understanding the hydraulic forces in the site was crucial for the design team to inform their specifications, and Central’s presence on-site during construction helped the contractors navigate construction of a solid and structural subgrade upon which the monolithic slab could be supported.