Project Overview
The Oakland County Water Resources Commissioner retained NTH for the geotechnical exploration, evaluation and analysis of the existing Pontiac Lake Dam. Pontiac Lake is an impoundment of the Huron River in central Oakland County. The lake is formed by earthen embankments (designated as Dike Nos. 1 through 6 and 8), the largest of which (Dike No. 1) is located on the South side of the lake, along the North side of Pontiac Lake Road. Based on available information, the lake was constructed prior to 1930 (estimated to be circa 1926–1927) by a private developer. A combined concrete spillway and control structure is located on the Dike No. 1 embankment. Discharge of the Huron River flows over the concrete spillway and under Pontiac Lake Road, through a concrete box culvert. The facility has been experiencing seepage issues for some time and the owner is contemplating on reconstructing major portions of the dam including a new spillway, and other measures to improve the dam stability and resolve the seepage issues. The owner wanted to collect adequate data in order to seek proposals for the future revamping of the dam.
Project Scope
The Owner retained NTH to provide geotechnical engineering services to evaluate seepage and stability of the dam and provide recommendations and options to resolve the issue. The scope of work included drilling of several test borings on the downstream face of the earthern embankment and installation of Vibrating Wire Piezometers at different locations and elevations to monitor pore water pressure within and below the embankments. The scope also included geotechnical laboratory testing of the soils to obtain information on the composition and density of the embankment fill materials and foundation soils.
NTH monitored the piezometers for a period of time to collect data on the static piezometric head elevations and used the data to analyze the stability of the embankment slopes. NTH trained the owner on the use of the piezometer readout unit to allow the owner to do their own monitoring in the future. Once the data was collected, NTH developed a two-dimensional seepage and stability model that represented the subsurface conditions of the affected embankment. The seepage analysis was performed using the finite element analysis module within SLIDE2 software. The results of the study along with the collected data were published in an engineering report. Several options were provided to the client including installation of cutoff walls or placement of an earthen buttress along with advantages and disadvantage of each option.
One of the major advantages to using an earthen buttress is that the addition of fill will raise the downstream face above the piezometric surface to mitigate wet slope conditions, as well as provide an opportunity to re-establish a downstream ditch with proper slopes. The client intends to use the study to allow replacement of the spillway and repair of the dam within the next 10 years.
Client Benefit
NTH provided various recommendations for resolving the current seepage issues, which included the installation of cutoff walls or earthen buttress. Now, the Oakland County Water Resources Commissioner has a study that they can rely on for advancing any concepts for the replacement and repairs moving forward.
