Abstract Description: Wet areas have been observed along the downstream toe of the Clear Fork Reservoir Dam since at least 1979. The reservoir and dam are operated and maintained by the City of Mansfield, Ohio for water supply. To supplement the water supply, the City also relies on a network of groundwater wells, including one well screened in the native alluvial soils directly below the dam embankment. The City of Mansfield worked with Stantec to design a modification addressing the observed wet areas. Initial assessments included review of historical data, performing subsurface exploration, conducting seepage and slope stability analyses, and evaluating 15 identified potential failure modes (PFMs) associated with seepage. Stantec developed a seepage model to evaluate exit gradients and uplift pressures at the dam toe. Erratic piezometer (PZ) readings were observed, indicating up to five feet of fluctuation over periods as short as 15 minutes. The piezometer readings were compared to available groundwater well pumping records, which indicated a direct correlation. Stantec worked with the City to discontinue pumping from the groundwater wells nearest to the project area for approximately one month. Stantec monitored the piezometers to evaluate the pressure increases as conditions stabilized without well pumping. The design seepage model was validated by comparing predicted pressures with the stabilized field piezometer readings. Analyses indicated exit gradient factors of safety were below standard criteria at the dam toe in the areas of observed seepage. To address the risk-driving PFMs and the deficiencies identified through the engineering analyses, a graded filter toe berm was designed. The toe berm design considered the seepage PFMs and the impact of the groundwater well network. Construction of the modification was completed in October 2023. The nearest groundwater supply well was operated continuously during construction to lower the groundwater table and aid in constructability. The project is an example of how risk assessment of dams can be scaled down from a full PFM Analysis to focus on specific, targeted issues that have been observed. It also shows the importance of considering nearby infrastructure (groundwater wells) to design and construct an appropriate dam safety solution.
Learning Objectives:
Learn about targeted development of potential failure modes related to seepage.
Demonstrate the importance of reviewing and understanding instrumentation readings as related to design.
Develop a seepage model representative of known field conditions.
