location. Because of the complex geometry, Isherwood’s drafting team created a section for each of the 27 piles. Tiebacks employed 150 mm (6 in) diameter cased holes in soil and 115 mm (4.5 in) diameter in rock. With the 13 m (42.7 ft) high rock face assumed to be self-supporting, the design did not attempt to resist the release of the locked-in stresses. During rock excavation, prominent east-west vertical joints appeared. These were of concern at the south wall where the jointing was subparallel, creating thin vertical slabs with the potential to break loose. Isherwood decided to protect this face with a curtain of wire mesh and rock-bolts. The east and west walls were stable, as was the north rock face under 200 University. At the remainder of the north wall, between the existing building and University Avenue, Isherwood used the mesh as well. Under 200 University, rock anchors, installed in a grid pattern directly on the rock face, were designed to counteract the building’s weight. Layout included active anchors at the column lines and passive anchors elsewhere. FLAC Modeling Two-dimensional Fast Lagrangian Analysis of Continua (FLAC) modeling was completed for the proposed shoring and construction sequence at one location in the center of the east wall. For each stage, the model provided soil and structure behavior: soil stress and strain, structural displacements, axial, shear and moment forces. Soil parameters were derived from the geotechnical investigation conducted by Terraprobe Ltd. The geometry was based on a 7 m (23 ft) clearance between the excavation and the subway tunnel. The baseline analysis was conducted with a lateral rock stress of 3.8 MPa (40 tons/sf) and a rock bulk modulus of 3.8 MPa (40 tons/sf). Two parametric studies were also conducted, using rock bulk moduli of 2.9 MPa (30 tons/sf) and 7.7 MPa (80 tons/sf). The maximum move- ments predicted for the 2D displacement at the center of the excavated face ranged between 9 and 32 mm (0.35 and 1.26 in). Isherwood standard practice is to attach inclinometer casings to soldier piles at representative locations and targets near the top of each pile for survey monitoring by total station. At Shangri-La, the TTC subway and 200 University both required more comprehensive monitoring. At the east wall, the monitoring plan required for the TTC approval process comprised pile targets at the top and at each tieback elevation (as exposed during excavation) on every pile, as well as four inclinometers attached to shoring piles plus two in boreholes behind the shoring, extending 6 m (19.7 ft) below pile toes. These instruments had an accuracy of 2 mm (0.08 in) or better. In the subway tunnel, electrolevels were installed across 10 expansion joints to record relative displacement and tilt of the subway box sections in real time, with an accuracy of 0.1 mm (0.004 in). Precision survey targets at expansion joints, with an accuracy of 1 mm (0.04 in) or better, served as a back- up for electrolevel readings. Borehole extensometers installed at three locations monitored differential rock movement directly below the subway tunnel, recording elongation at sensors 5 m (16.4 ft) apart along the 30 m (98.4 ft) length. The inclinometer and pile target moni- toring indicated that overburden excavation down to the rock surface resulted in shoring wall movements in the expected 15 mm (0.59 in) range, and that excavation of the unsupported rock face below resulted in a further 10 to 13 mm (0.39 to 0.51 in), bring- ing the overburden and shoring with it. The three extensometers gave very similar results, indicating lateral movement of the rock just below the subway of 10 to 12 mm (0.39 to 0.47 in) at the excavation face and at the 5 m (16.4 ft) node, reducing to 7 to 8 mm (0.28 to 0.31 in) at the 10 m node (32.8 ft), 3 mm (0.12 in) at the 15.0 m node (49.2 ft), 2 mm (0.08 in) at the 20 m (65.6 ft) node, and less than 1 mm (0.04 in) at the remain- ing nodes. A comparison with the FLAC predictions indicates the displacement at the near edge of the subway of 7 to 9 mm (0.28 to 0.35 in) was significantly smaller than the prediction of 12 mm (0.47 in). However, the extensometers indicated the rock movement did not extend back to the far side of the subway, so differential dis- placement across the width of the subway structure of 7 to 9 mm (0.28 to 0.35 in) appeared to be larger than the FLAC prediction of 5 mm (0.20 in). Monitoring at TTC: A section of FLAC analysis and actual readings DEEP FOUNDATIONS • MAR/APR 2014 • 79
