Prescriptive Specification Design: The Impact on Cost and Constructability

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The paper argues that prescriptive specification design (PSD)—which dictates conservative design parameters and methods—can make public transit projects unnecessarily costly and difficult to build. By restricting alternatives and discouraging innovation, PSD may increase material use, labor, and construction time. It can also foster an “overdesigned” mindset that weakens attention to construction controls and monitoring.<br /><br />The Launch Shaft 1 excavation for Toronto’s Eglinton Crosstown LRT illustrates these effects. The original design was replaced during construction with a contractor-led alternative developed using local geotechnical experience, finite-element analysis, and the Observational Method. The redesign simplified the headwall, eliminated jet grouting and 40 large drilled holes, reduced excavation bracing, and improved access for tunnelling. It saved more than 3% of construction costs and at least three months, enabling the shaft to receive two tunnel-boring machines on schedule. Measured movements were well below contractual review and alert limits. The authors suggest that a performance-based design from the outset could have produced further savings.<br /><br />The Observational Method uses monitoring to compare actual performance with an agreed baseline and modify the design when needed. The authors recommend embedding this approach in contracts through a geotechnical baseline report, contractor-submitted base and contingency designs, “base cost plus contingency” bids, agreed procedures and rates for changes, and monitoring thresholds established before construction. Effective implementation requires timely approvals and fair sharing of the risks and savings associated with design changes.<br /><br />The paper concludes that well-managed, performance-based specifications can improve safety, constructability, schedule, cost, and environmental outcomes by combining monitoring with contractors’ and consultants’ local expertise.

Keywords

prescriptive specification design

public transit projects

performance-based design

Observational Method

geotechnical baseline report

construction cost savings

Eglinton Crosstown LRT

Launch Shaft 1

construction monitoring

contractor-led redesign