KCI was contracted to perform engineering evaluation and design services for the Columbia Gateway Dam as part of an On-Call Stormwater and Watershed Management Evaluation/Design/Build Contract.
Our team’s responsibilities included performing dam inspection services and preliminary geotechnical exploration studies to assess the dam, identifying problem(s), and developing, recommendations and cost estimates for remediation.
The project involved an existing stormwater management (SWM) pond. The dam, situated at the eastern end of the pond, is approximately 30 feet high with a ten-foot reinforced concrete barrel pipe spillway outfall. It is classified as one of the county’s high hazard dams. The spillway has a riser and two antiseep collars. The length of the pipe is approximately 120 feet and includes 17 joints. The pipe begins at an intake structure on the upstream end and daylights at the downstream end. The pipe also traverses an 18-inch sanitary sewer pipe enclosed in a concrete casing towards the downstream toe of the dam. The downstream end has a headwall and two wingwall/retaining walls.
As Engineer-In-Charge, we developed the remedial design including preparation of design drawings, specifications, construction sequence and cost estimates, permits and performed the construction QA/QC.
KCI engineers conducted geotechnical explorations and studies including dam inspections to assess the existing dam embankment, spillways, drainage features and appurtenant structures. Our team identified issues and provided recommendations in a report. Observations included:
- the presence of significant reddish-brown staining due to potential groundwater seepage and/or soil inflows at several joints inside the reinforced concrete spillway pipe
- wet soil conditions at the wingwall outfall areas, with continuous water seepage over the wingwalls
- significant reddish-brown staining on the face of the headwall and wingwalls
Based on the preliminary studies and recommendations, each of the existing 17 pipe joints and the interface between the pipe and headwall needed to be grouted to ensure proper joint sealing against groundwater and soil inflows into the pipe. Recommendations also included drainage and seepage control measures be implemented behind the wingwalls to minimize hydrostatic water buildup and water seepage over the walls. The county then requested that KCI develop design documents for repairs, including
- Design of a wingwall drainage system – Our team provided evaluation of construction excavation and dewatering adjacent to existing structures
- Design of remedial grouting repairs to concrete pipe spillway – This task included preparation of design package to solicit Design/Build proposals from specialty contractors, review and evaluation of proposals, provide recommendations to the county, and coordinate with selected specialty contractor
- Preparation and submission of maintenance and repair permit for the remedial work on the dam
- Construction monitoring and oversight during construction