CONDITION ASSESSMENT OF EXCAVATION WALLS AT DECOMMISSION PIT AT 160 COCHRANE DRIVE, MARKHAM, ONTARIO.


The project includes a Contamination Pit on the interior of 160 Cochrane Drive which occupies an area of approximately 120 x 70 feet.  The site subject site was occupied with storage tank and other sources of contamination detected as being the primary cause(s) of contamination at the subject site.

As part of remediation work it is planned to excavate a pit to the impacted thought to be about 9-10m (25-30 feet) below the existing Floor Slab Level (FSL). The contaminated soil is then excavated, recycled and/or replaced with clean material to the floor level. The approximately 200mm floor slab is then replaced.  The groundwater level at the subject site is believed to be about 4m below FSL. 

During the excavation of the pit local instability of the excavation walls where noted. Groundwater seepages were also noted locally at the base of excavation.

Several columns are located within the centre of pit (a total of four) presumably surrounded by contaminated soils. These columns (amongst others) are supported by approximately 2.5m (8 feet) long individual caissons. 

Temporary truss structures are put in place to support the loading of the roof while these columns are dismantled during the clean-up period. The steel columns will then be founded upon sonic piles which we believe are to be founded upon the native bearing soil.

Several boreholes were drilled at the FSL elevation indicating the site is generally underlain by sandy silty clay/clayey sandy silt. Some fill material is also evident just below the floor slab which is approximately 200mm thick

Report recommendations include excavations to be carried out in accordance with the most recent Occupational Health and Safety Act (OHSA).  The sides of excavations in the clayey silt/silty clay above groundwater table can be expected to be temporarily stable at relatively steep side slopes for short periods of time but they should be cut back at slopes no steeper than 1:1 to comply with the safety regulations.  Any excavation in water bearing sandy silt to silty sand will require positive dewatering, otherwise it will result in an unstable base and flowing sides. Upon advancing excavation, this water bearing stratum has the potential to become unstable, hence destabilizing the upper crest of cohesive silty clay/clayey silt. General shear failure and/or local slip may occur consequently. Excavation in to this material must be undertaken with extreme care and under controlled conditions.  Monitoring of the existing structures such as cladding, building, columns and roof is recommended during the entire operation. A baseline reading should be established prior to commencement of any work. An alert system must be established should displacements and/or cracks are exhibited above the threshold limits.

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