Field Instrumentation for an Innovative Design-Build Excavation Adjacent to Heritage Structures

This case study highlights how design, construction and field monitoring can be combined to satisfy low movement criteria in the presence of difficult site constraints. The R.C. Harris Filtration Plant is located in south-eastern Toronto, overlooking Lake Ontario, and provides forty-five per cent of Torontonians with potable water. The existing structures are designated as both historically and architecturally valuable under the Ontario Heritage Act. A proposed expansion to accommodate a residue management facility required that construction activities would not interfere with the plant's operations. In order to preserve the historical and architectural presence of the site, the new facility will be housed in two cut-and-cover stepped excavations, 12m and 10m deep, between two highly-valued and sensitive structures. Concerns regarding potential movements at an existing, critical expansion joint within the potable water tanks influenced the Owners’ designers to limit the allowable lateral deflection of one excavation support wall to 4 mm. A comprehensive monitoring plan was implemented to include: inclinometers, electrolevels, extensometers, precision and pile target monitoring, load cells, SMART cables, and thermistors with baseline readings obtained over several weeks to determine if mandatory accuracy limits were attainable. The design build team awarded the contract, crafted an alternative design that resulted in substantial savings over the original concept. Extensive FLAC (Finite Layer Analysis of Consolidation) analyses were performed to predict movements at each stage of construction. Monitoring of the movements and comparison with the FLAC predictions allowed the team to assess the need for design or methodology modifications during construction. Based on the contractor’s willingness to commit to an extensive monitoring program to justify the optimized design, the contractor was able to satisfy the owner’s objectives with a cost effective solution. Extensometers, installed within the potable water tanks, were imperative tell-tales for joint movements at the critical junction between the Filter Building south face footing and the slab beyond. Over the subsequent 12 months, real-time monitoring of the extensometers showed negligible movements across the joint at the Filter Building’s south face footing while the accompanying instrumentation proved total constructionrelated horizontal movements of 6 mm to 10 mm at the same location proving that the