Dams

Public Works and Government Services Canada owns and operates the Timiskaming Dam Complex which crosses the Ottawa River near the towns of Thorne, Ontario and Témiscaming, Québec.

Riggs was retained by Nor Eng Engineering and Construction Inc. to provide civil and structural engineering services for repairs to the Timiskaming Dam head works, piers and sluices, and new automated stop log and lift assemblies.

The basis of design was as follows:

• The design water levels were estimated using hydrologic frequency analysis of the difference in water levels between the upstream and downstream sides of the Timiskaming Dam Complex.
• The designs of steel and timber stop logs were based on both hydrostatic and hydrodynamic induced forces including seismic induced force.
• The design of steel end caps for both steel and timber stop logs was based on static and finite element analysis. Loads were considered for both pulling and compacting of the stop logs.
• The design of a timber protection wall to protect the workers during operation of the dam was based on static analysis.
• The design of an extension platform on the upstream side of the Ontario dam was based on static and finite element analysis.

Riggs was retained by the Upper Thames River Conservation Authority to study the drainage characteristics of the soil held by a concrete crib wall located in London, Ontario. As part of the investigation, Members of Riggs Associates Ltd. performed global stability analyses of the valley slope located behind the concrete crib.

2D finite element modeling was used to study the global stability of the slope behind the crib. Seepage analysis was used to quantify the groundwater table in the soil mass, which was later determined to have a great influence on the computed factors of safety of the valley slope. The results of the finite element analysis produced failure surfaces that were in line with field observations.

Riggs provided a number of different alternatives for improving the characteristics of the soil behind the crib and completed a cost assessment of the remedial options provided.

Riggs was engaged to perform an investigation of the baffle walls leakage at Wildwood Dam, located east of the town of St. Marys, Ontario. The integrity of the joints between the baffle walls and the north and south spillway walls are critical in providing the structural stability of the dam. Deterioration at the joint raised concerns regarding the overall safety of the dam.

Riggs examined the physical cracks in the baffle walls and researched the historical records for details of existing reinforcement. Analysis showed that the existing reinforcement provided was inadequate; however, the contractor at the time of construction utilized steel angles in the formwork. If the angles remained in the permanent work, they would strengthen the walls. Thus, finite element analysis was performed to assess the stress distribution in the baffle walls and found that if the angles were not in the permanent work the dowels would be overstressed and fortification would be required.

Riggs recommended and supervised an intrusive investigation which confirmed that the angles did indeed remain in the permanent work, thus avoiding costly emergency repairs.

Riggs was retained by Nor Eng Engineering and Construction Inc. to provide engineering services for the rehabilitation of the Bark Lake Dam. The dam is located in Jones Township on the Madawaska River in northeastern Ontario.

The project involved designing a floatation cofferdam system that minimized environmental impacts. Various concrete repairs, along with new access ladders and a handrail with fall-arrest device as well as vertical slide gates (replacing the original butterfly valves) were included in the project.