Project Description

Sarafinchin Associates Ltd. carried out a soil study of marine clay deposits in the Greenboro and South Key areas in the southern portion of the former City of Ottawa.  The purpose of the Greenboro Area Soil Study (GASS) is to determine the cause of ground settlements, which were reported to intensify during the summer of 2001 with resultant cracking to residential houses and public roads.

Three drilling investigation programs were carried out comprising of deep, intermediate and shallow boreholes.  Soil and groundwater instrumentation devices were installed for long term monitoring observations.  Research of historical data was carried out plus a review and analysis of collected data.  Soil and groundwater assessments were performed to characterize subsurface conditions for the preconstruction and post development stages.

A total of 32 boreholes were advanced to depths down to 19m.  Field sampling and testing included standard split spoon samplers, thin walled Shelby tubes, Geonor field vanes, standpipe piezometers, pneumatic piezometers, sealed monitoring wells, and settlement monitoring devices. The shallow soil deposits comprise topsoil, peat and fill.  The subsurface soils have 1.0 to 1.5 m of sand, underlain by brown to grey, stiff to soft, silty clay.  The weather and dessicated crust of the marine clay varies from 0.5 to 2.0 m in thickness.  The silty clay is inter-layered with thin layers of clayey sand to silty sand.  The marine clay crust thickness varies from 2 m in the central area to 0.5 m in the eastern areas.  The marine clay is underlain by deposits of loose, sandy silt changing with depth into loose to compact sand.  The sand formations are followed by variable thicknesses of glacial till, approximately 1 to 2 m thick over dark grey shale bedrock of the Carlsbad Formation.  Based on a few deep boreholes, it is estimated that the bedrock surface is dipping from the east to the west in the study area.

A review was undertaken to establish whether vibration may have contributed to general foundation damages under normal heavy and light traffic loads including large buses and some trucks plus normal car traffic.
Based on a review of climatic records, in the year 2001 the precipitation was 117.6 mm below the annual average period.  The year 2001 was the sixth driest year since 1960.  Similarly, a review of temperatures indicated that year 2001 was the third hottest year since 1960 with average temperatures 1.7oC above average.

Based on a review of precipitation and temperature data, it may be concluded that 2001 is one of the hottest and driest years since 1960.  These drought conditions are a major contributor to a significant water deficit in the sensitive marine clay deposits.

A visual survey of selected houses was performed to observe the crack damages to the building and foundations and to categorize the severity of crack damages.

The contributing factors to soil shrinkage settlement were found to be climate change, urbanization and deep rooted vegetation.