Dispersive soils were tested at the job site daily and crack testing was performed at the dam crest for each phased dam section completed.
Portal Lake (Dam Site WP-6) Geotechnical Engineering, Permitting, and Construction Phase Services
Background
Dam site WP-6 was identified in the Papillion Creek Watershed Partnership Plan to provide regional detention in the West Branch Papillion Creek Watershed in conjunction with dam site WP-6. This sub-watershed is one of the most rapidly developing watersheds in the metropolitan area, and these sites were selected at the time the plan was developed to maximize flood control, given what open ground remained in the area. These two sites were at the top of the list for the Papio-Missouri River Natural Resources District’s (P-MRNRD) re-prioritization study due to the pressure of impending development adjacent to the dam sites.
The Project
FYRA Engineering (now part of Houston Engineering, Inc. [HEI]) conducted the preliminary design through construction phases for the project. The geotechnical investigation and design is being highlighted for this project description.
The geotechnical design proved to be both interesting and challenging. Remote pockets of material affected a sizable geotechnical investigation with widely varying laboratory testing results affecting consolidation times and strength parameters. The design was comprised of a phased embankment construction with a principal spillway pipe that had a significant vertical and horizontal curve incorporated to meet conduit joint extensibility.
Due to the steep valley and varying lab results, the concern for differential settlement was taken into account in the development of the plan documents. A redundant plan consisting of settlement plate monitoring, phased construction, a vertical chimney drain, and embankment crack testing were all employed during construction.
The site proved to be one of the most geotechnicaly unique sites in the watershed, which has nearly 20 other larger dams already constructed.
To add to all other challenges, dispersive soils existed throughout the site, challenging borrow source and placement coordination within the dam embankment.
Project Geotechnical Highlights
- Navigated extremely unique geology in this project.
- Uncovered unreliable, varying laboratory sub-surface sampling results.
- Phased, redundant design provided to address the potential for differential settlement.
Project Details
Client: Papio-Missouri River NRD
Location: Sarpy County, Nebraska
Project Highlights
Project Geotechnical Highlights
- Navigated extremely unique geology in this project.
- Uncovered unreliable, varying laboratory sub-surface sampling results.
- Phased, redundant design provided to address the potential for differential settlement.
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