PUZZLING SOFT ZONES IN A REGIONAL AQUITARD LOCATED IN CENTRAL ONTARIO, CANADA

Anomalous zones of soft material have been documented in the Newmarket Till, a thick sandy silt diamict that is highly overconsolidated, at a field site near Pickering, Ontario. Where they are observed, these ‘soft zones’ have increased permeability and water content, decreased strength, and are normally consolidated. These anomalies appear to vary in size, from centimeters to meters, and are often seen in conjunction with secondary structures such as slickensides and fractures. Resulting from their increased permeability, soft zones may act as preferential flow paths for groundwater, providing connections between post-depositional features such as fractures, and syn-depositional features such as sand and gravel lenses. Although the Newmarket Till is reported to have a matrix vertical hydraulic conductivity of 1x10^-11 to 1x10^-10 m/s as derived from laboratory tests, the bulk vertical hydraulic conductivity is 1x10^-9 m/s (Gerber et al., 2001).

In addition to those found in Ontario, soft zones are documented in Saskatchewan and Alberta where they exhibit similar characteristics, and similarly appear in conjunction with fractures and slickensides. To date, soft zones have been found in deposits with low primary permeability and that have been overconsolidated by a past stress such as glaciation. Through detailed characterization of the material (geotechnical index properties, Atterburg limits, density, mineralogy, grain orientation, fabric etc.) we are looking to identify the mechanisms by which these soft zones were formed. Currently it is our belief that these soft zones formed as a result of shearing during the last glaciation, followed by an influx of glacial melt water through fractures or other features that enable the rapid movement of groundwater in short time periods.