3800 RIVERFRONT BLVD Geotech Report 2023-05-04

March 26, 2019 HWA Project No. 2015-061-21 Task 600-7 4.3 ANTICIPATED CONSOLIDATION SETTLEMENT The refuse,peat and organic silt underlying the proposed Riverfront Boulevard alignment are highly compressible. Although the site has been previously surcharged, additional settlement is expected to occur over the design life of the proposed roadway. These settlements will occur as a result of several mechanisms including long term secondary compression and biodegradation. A more detailed description of each of these mechanisms is provided below. 4.3.1 Secondary and Biodegradation Settlement Secondary compression is associated with structural deformation within the soil as the result of applied load. In mineral soils the secondary compression is relatively small in comparison to the primary consolidation. However, secondary compression in organic soil and refuse is much larger than in mineral soil, due to the potential for cellular deformation within the organic material. Secondary compression continues for many years after the application of load. Biodegradation occurs as organic materials within the refuse decompose. The rate and magnitude of biodegradation is independent of applied load and depends on several factors including groundwater level and the nature and amount of organic material present. For the landfill refuse, the magnitude of biodegradation is expected to vary widely and be dependent on the composition of the waste stream as it was placed. As both secondary compression and biodegradation occur slowly over time, it is generally difficult or impossible to distinguish the individual components of each mechanism. Modified secondary compression indices for the refuse and alluvial soils were determined through back calculations of observed settlements at settlement plates and settlement instruments across the landfill. As observed long-term settlement readings across the landfill were a result of a combination of both secondary settlement and biodegradation, HWA assumes that the calculated modified secondary settlement index accounts for both mechanisms. Therefore, specific biodegradation settlement calculations of the refuse and alluvium were not performed. The secondary settlement can be estimated using the following equation: dH—Cae*H*log(dt) • AH is the long-term or secondary settlement. • Cote is the modified secondary compression index. • At is the time over which the long-term settlement occurs in years (the design life of the facility). • H is the initial thickness of the compressible layer. Geotechnical Report Revision 3 26 2019.doc 12 HWA GEOSCIENCES INC. 111