Tieback monitoring at the Bisha condominium project, Toronto.

The BISHA Condominium project is an exclusive new private residence and hotel property in Toronto. Rising to 41 storeys and designed by award-winning Wallman Architects and Munge Leung Design Associates, Bisha will be a coveted new address in the heart of Toronto’s Entertainment District. A deep excavation is being dug for several levels of underground parking. On one side of the excavation is a very large building at close proximity. Reinforcement with a concrete caisson is mandatory and monitoring for movement must be conducted during the entire construction phase.

The d-Cable

The d-Cable is based on a miniature inductive strain-gauge small enough to be recessed into a surrogate tubular king-wire. The gauges are configured with base-lengths of 20cm to 2m. The d-Cable will measure how a certain length of the cable bolt is stretching in response to load. Either a single strain-gauge or an array of strain gauges may be specified. These can be concentrated at locations where it is predicted that the cable may intersect a known geotechnical feature.

During manufacturing the entire strain-gauge array is independently manufactured and calibrated and then recessed into the king-wire and swaged in place. Since the technology involves strain gauges distributed within the cable rather than displacement sensors housed in an instrumentation head, the “signal processing head” of the d-Cable is reduced to a diameter equal to that of the cable. This has significant advantages in applications that require plating or pre-tensioning.

In many applications cables require posttensioning using a plate and grips. Since the readout head of the d-Cable in the same as that of the cable itself, the grips and tensioning jack can simply be placed over the end of the instrumented cable.


Installation

Tiebacks are made of cable bundles of 9 units of 15.9mm section. One YieldPoint d-Cable is used as one of the 7 cables in the bundle. This d-Cable is 14.5m in length and reads loads in Tons at 5 different locations along the length. 12cm boreholes 14m in length are being drilled into the rock.

Drilling a single borehole can take an hour in easy rock and up to 7 hours in heavy clay. This jig is used to bundle together the 9 cable units. Cables are then tied together using duct tape.

The boreholes were pre-filled with cement and yet the cables were very easy to push in. The entire bundles went down without effort and the instrumented cable among them.

The boreholes were pre-filled with cement and yet the cables were very easy to push in. The entire bundles went down without effort and the instrumented cable among them. The stainless steel tubing used to house the d-CABLE’s lead wires is clearly visible. It is a lot more resistant than a standard plastic tube. The lead wires run back all the length of the borehole because the instrument’s head is located at the toe.

Results

A d1Logger set at 1minute monitoring interval was used to record the response of the tieback during the jacking process. The results have been analyses using Minescope-Cable. The anchor configuration of the ties back is shown on the LHS below.


The response for the one tiebacks is shown on the RHS above. During the jacking process it appears that a constant load is applied with a jack for approximately 10mins and is then released so that the tension is maintained at the faceplate. A 2-3ton relaxation occurs at the end of the jacking period which only affected gauge 5.

The detail of how the tension is transferred along the tie-back during the application of tension is consistent with expectations(see below). During the jacking period (constant load) a gradual increase in load is observed down the tieback for the Gauges 4 and 3 (at approx. 4.0 and 6.0m depth) whereas at Gauge 5 the load decays slightly and adjustments appear to be made by the operator to maintain a constant value. This is exactly as would be expected as the load transfers down the tieback in a time dependent manner. .

The results indicate that the deepest anchor points register very little load. For 130265002 the deepest anchor registers only 200N during tensioning.

A complete report on the project is presented here.

YieldPoint Inc. provides mine operators worldwide with tools, technology and service. YieldPoint's Digital (direct-to-digital) sensors combine accuracy and durability while keeping costs low.

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