Everett Riverfront Cement Amendment Project

Everett, Washington

The City of Everett plans to redevelop the 66-acre Everett Landfill and adjoining properties along the western shores of the Snohomish River. In addition to compressible waste, this area is underlain by thick (up to 60 feet) deposits of soft, compressible alluvial soils (peat and silt). In 2007, Oliver McMillan (OM), a national developer, contracted with the City to purchase and develop the site that would be known as the Everett Riverfront. The agreement between the City and OM required the City complete specific site improvements in order to provide OM with a construction-ready site prior to taking possession. The City's obligations included: 1) mitigate the potential for future settlement across the site; and 2) provide the developer with an "all weather working surface".

The City's design team, which included Perteet Inc. (Perteet) and HWA Geosciences Inc. (HWA), worked with OM's team to address these issues. A phased site preloading program was agreed upon for mitigating the settlement potential over the project site. However, agreement was not easily reached on the measures necessary to provide an "all weather surface".

The developer's default approach was to import and compact a layer of structural fill soil (Gravel Borrow) on the site. HWA proposed to cement-treat existing soils at the site in order to provide the "all weather surface." This approach employed recycling/reuse of materials, and provided a less resource-intensive, sustainable, solution. Cement treatment of in-place soils is typically used to improve the strength of subgrade soils for road construction, although the finished product is typically too strong to excavate through (e.g., for utilities). HWA developed a soil/cement mix protocol to apply just enough cement to generate a finished product that simulated the all weather surface characteristics of compacted Gravel Borrow, but would allow future excavation.

HWA developed a laboratory and field testing program that allowed determination of the correct cement application rates, curing times, and compaction methods, to match the variable site soils conditions and produce the desired end product. Variations in on-site soils required fine-tuning the cement application rate in the field. HWA developed an innovative field testing protocol, and was given the authority to modify the cement application rate as soil changes were observed. Upon completion of cement treatment procedures, 5.5 million pounds of cement were spread and mixed across the site. OM was presented with a project site that had an "all weather surface" that remained excavatable.

HWA's innovative use of existing technology resulted in a cost savings of approximately $3.4 million to the City of Everett taxpayers.