Instrumentation and Performance of Tied-Back Shotcrete Shoring in Sand Adjacent to a Hospital Structure

Recent additions to the Brantford General Hospital expansion included construction of a new hospital wing, involving excavations of up to 11 metres (36 feet) depth, in loose to compact sand adjacent to an existing eight-storey hospital structure. The tendered contract called for interlocking caisson walls. An alternative method of temporary excavation support, tied-back shotcrete shoring, was proposed by HC Matcon Ltd. Due to a lack of familiarity with this method in Ontario, the uncertainty of attaining near-zero movements, and the proximity of adjacent ‘lifeline’ structures, the design-build team of HC Matcon and Isherwood Associates implemented a comprehensive program of quality control and assurance. The instrumentation for this program included inclinometers, standard and precision visual survey, electrolytic tilt-meters, and load cells. The inclinometers were generally placed directly behind the wall faces to ensure accurate monitoring of the shoring face and effects of installation procedures. Precision survey was used to monitor shoring and structural displacements. Electrolytic tilt-meters (electrolevels) were placed on the adjacent structures' foundation walls and floor beams to ensure an accurate differential movement history of the structure at critical points. Frequent data acquisition from the inclinometers and electrolevels provided timely feedback and permitted accurate assessment of the performance of the shoring system during installation. It allowed for rapid response by the design-build team to any unexpected movements of the shoring or adjacent structures. Movements of the shotcrete shoring face in the hospital wing phase of the project were limited to 3 millimetres or 0.03% of the shoring height - equivalent to that achieved by caisson wall in similar ground conditions. Of note, the adjacent hospital structures' movements were measured as less than 3 millimetres, better than expected from a caisson wall system due to ground loss problems often associated with large diameter vertical and horizontal drilling. The excellent performance of the shotcrete shoring in the hospital wing phase was attributed to shoring design features, good workmanship, and rigorous quality control efforts by the design-build team. The monitoring results allowed for ‘real time’ reaction.