Waterfront Toronto: Soil–Cement Mix Designs in Sands and Peats for Proposed Brownfields Redevelopment Project

Abstract

Fifteen soil–cement mix designs, cured for up to 270 days, were tested on sand and peaty sands from a Waterfront Toronto Brownfields site having a total petroleum hydrocarbon (TPHC) concentration of approximately 20,000 mg/kg. The peaty sand was created using fibrous peat and sands from the site blended at a 25:75 v/v ratio to reflect the lithology targeted for stabilization/solidification (S/S). Portland slag–cement blends (6–14 wt%) added to wet soil with 0.5 wt% bentonite applied to the sandy soil provided 28-day unconfined compressive strengths (UCSs) of the order of 400–2,500 kPa, which increased to approximately 700–4,300 kPa by 56 days. The performance of the peaty sands was lower: 8–25 wt% cement blends with 0.5 wt% bentonite had UCSs of approximately 140–1,725 kPa at 28 days and 175–3,400 kPa by 56 days. Brazilian tensile strengths of the S/S-treated soils were 10%–16% of UCSs. The minimum cement doses to pass the freeze/thaw test (12 cycles; ASTM D560) after 28 days were 10 and 25 wt% for the sandy and peaty sand soils, respectively. For similar reagent doses, the sandy soil mixes were typically 3–130 times less permeable than the peaty sand mixes after 28 days curing, with all mix designs except one having a hydraulic conductivity less than 10−6 cm/s. Total (n) and effective (neff) porosity testing conducted at 210 days showed that the sandy soil mixes had values of n and neff that were, respectively, 20% and 85%–90% less than the peaty sand mixes. The maximum values of neff of the S/S-treated sands and peaty sands were 9% and 14%, respectively. Computed tomography (CT) of the S/S-treated soils at 210 days confirmed the porosity trends and provided insight into the freeze/thaw failures of the peaty sands. TPHC leaching of the untreated soils and select 28-day cured mix designs as a function of the liquid/solid (L/S) ratio were similar, consistent with other studies where particle crushing of the S/S-treated materials occurred.