Canadian Consulting Engineer

Feature

Award of Excellence: Library of Parliament Excavation

The Library of Parliament in Ottawa is the only remaining part of the original Centre Block and the most prestigious structure on Parliament Hill. Completed in 1876, the building was acclaimed both in Canada and abroad for its beauty and grandeur.





The Library of Parliament in Ottawa is the only remaining part of the original Centre Block and the most prestigious structure on Parliament Hill. Completed in 1876, the building was acclaimed both in Canada and abroad for its beauty and grandeur.

Planning studies for a comprehensive conservation, rehabilitation and upgrade program for the building were launched in 1995 and a three-year construction phase began in 2002. Renovations were required all the way from the weathervane on the pinnacle of the roof, down to the heating, ventilating and air-conditioning (HVAC) system in the sub-basement.

To provide proper indoor environmental conditions, a much larger space was required for the mechanical systems and ducts. A new, visible, above-ground expansion of the Library building was unacceptable, since any new structure would affect existing aesthetics and the national heritage environment of Parliament Hill.

Excavation risks

Substantial technical risk was involved in undertaking excavations close to and under the rings of heavily loaded, masonry walls. Any work had to avoid damaging the building’s dome and roof, its leaded windows and its masonry support walls and flying buttresses.

Golder Associates, as geotechnical subconsultant to the architects, had to tackle the geotechnical risks of creating the required space underground in a cost-effective manner, while maintaining the integrity of the heritage building.

The company developed innovative rock engineering solutions and used a staged excavation sequence to allow the rock to be safely removed while preserving and improving the strength and integrity of the fissured, bedded sedimentary limestone bedrock.

The level of technical risk associated with the rock excavations beneath the Library is comparable with other world class heritage rehabilitation projects, such as the underpinning of the mediaeval towers of York Minster or the recent remediation of the Leaning Tower of Pisa.

Design approach

Golder set stringent design requirements: (a) that rock mass excavation would be by mechanical methods only, (b) that vibrations would be maintained below critical threshold peak particle vibration (PPV) and frequency limits, (c) close perimeter line drilling would be used, and (d) an elaborate array of pre-support and rock reinforcement, plus grouting of prominent fissures would be specified.

Control of the rock excavation sequencing and near real-time monitoring of the movements of both the rock and the foundation zones were critical. Various analytical and two- and three-dimensional numerical modelling methods were used to optimize the excavation geometries and formulate rock support layouts. Similarly, models were developed to time the pre-support installation and to develop appropriate sequencing for the staged excavation.

Based on the modelling, discrete areas of rock mass overstress or building impact were identified either for concentrated rock reinforcement, or to rearrange the excavation layouts to mitigate any stability problems.

Areas identified as requiring additional support were the abutment zones to the intersection of the Link Tunnel with the Main Mechanical Room, and also its intersection with other discrete excavations, such as the Elevator Shaft. In these areas the rock mass could have lost confinement due to the effects of stress interaction. Also, because of increased degrees of freedom created by re-entrant (indent) excavation geometries, wedges or blocks might be freed to slide or rotate on pre-existing discontinuities and along newly created tensile fractures.

For rock mass zones where general de-stressing had the potential to extend into the crowns or side walls of the excavations, the engineers developed patterned reinforcement layouts to keep the total movements below levels that might affect the overlying masonry structure.

Because access to the heritage structure was limited, the use of typical large-scale civil engineering equipment was constrained. Golder decided that mining approaches, more usually used for ventilation and ore pass construction, would work better for the complex HVAC system duct work requirements.

Golder proposed that it would be best not to use the existing historic openings through the masonry walls, but instead to create an external access shaft and tunnel under the existing foundations, thereby allowing more rapid excavation progress.

The project used a mix of conventional construction methods such as hoe-ramming to remove rock and percussion drilling for the anchor installations, along with advanced techniques such as:

sawing and precision line drilling;

continuous perimeter controlled coring for very tight tolerance areas;

specialized installation of extensive steel and fibreglass pre-support;

application of hydraulic rock splitting technology;

use of a raise bore drill for construction of vent ducts;

installation of needle beams and concrete packing for underpinning the masonry walls.

During the excavation, arrays of high-precision instruments were used to monitor displacements of the rock mass (dilation, tilt, settlement, shear), and of the masonry ring walls (deflection, tilt, settlement), as well as potential groundwater level changes. The state-of-the-art geotechnical instrumentation included multi-point and sonic probe borehole extensometers, joint meters, piezometers, tilt beams, strain gauges, as well as convergence and inclinometer arrays.

The rock engineering aspects of the Library project met the approved budgets and schedule for the design phase. All major rock excavation and associated monitoring was completed on schedule by December 2003. The project was completed without damaging the heritage structure, and without disrupting the operations of Parliament or public events such as the Changing of the Guard ceremony.

The Parliamentary Precinct, including the Library of Parliament, is a National Historic Site, designated as one of only three heritage cultural landscapes in Canada, and a major tourist attraction. The protection of the building’s heritage status and the guaranteed operation of essential building systems is key to preserving this heritage treasure as a national trust for future generations.

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Name of project: Excavation for Renovation of Library of Parliament, Ottawa

Award-winning firm: Golder Associates, Mississauga and Ottawa (Dr. Trevor G. Carter, P.Eng., Gerry S. Webb, P.Eng., Philip de Graaf, P.Eng., Mark J. Telesnicki, P.Eng., Daryl Miller)

Owner: Public Works and Government Services Canada

Client: Ogilvie and Hogg, Desnoyers Mercure & associs, Spencer R. Higgins and Lundholm Associates, architects in joint venture

Other key players: Adjeleian Allen Rubeli (structural engineering); Pageau, Morel & associs (mechanical & electrical engineering); Thomas Fuller Construction (general contractor); Dibco

Underground (excavation); J.S. Redpath (raise bores); GeoFoundations (grouting); Fondex (instrumentation)

Suppliers: Marathon Drilling (drilling); Multicrete Systems (Shotcrete); Roctest (instrumentation)