To solve seepage and slope stability problems at High Falls Dam, Wisconsin Public Service needed to construct a toe berm. Construction required drawdown of the reservoir, a popular recreation site. Knowing this would affect tourism, the utility worked with local businesses to create an alternative revenue opportunity and shared information with the public and the media about the construction and timeline.
By Virgil E. Schlorke, Dean S. Steines, and Todd M. Rudolph
Wisconsin Public Service Corporation’s 7-MW High Falls project, on the Peshtigo River in northeastern Wisconsin, began operating in 1910. From the time the dam was built, the project owner dealt with issues related to seepage. For example, historical documents show that excessive seepage occurred upon initial filling of the reservoir, and steps were taken to deal with the situation at that time. However, seepage continued to be a challenge at the dam, with areas of emerging seepage and minor slumping at the toe.
Wisconsin Public Service needed a way to address both continuing problems with seepage through the dam and concerns related to stability of the upstream and downstream slopes of the earth embankments on either side of the concrete dam. Thus, in 2004, Wisconsin Public Service began investigating options for performing remediation work at the dam. The plan chosen involved constructing a berm at the toe of the earth embankments that would provide stability for the slopes and contain drain fill to safely capture and convey seepage downstream. Because this work would require a reservoir drawdown and affect tourism at this popular recreation site, the utility worked with local businesses to create alternative revenue opportunities. The remediation work was completed in November 2005, and monitoring has shown that conditions have stabilized and the dam is performing adequately.
Analyzing the dam, discovering the problem
From east to west, High Falls Dam consists of a 2,200-foot-long earth embankment; a concrete dam with a central overflow spillway and a gated spillway; a concrete intake structure; and a 1,450-foot-long earth embankment. Both embankments are constructed of homogeneous silty sand. The maximum height of the east embankment is 34 feet, and the west embankment is 26 feet. Crest widths range from 6 to 8 feet. The west embankment has a concrete core wall founded on rock from the concrete dam to an outcrop where the embankment alignment changes sharply. Both embankments have a concrete facing on the upstream slope, except on the west embankment near the penstock intake where there is a core wall.
A drain system was installed in 1910 to collect seepage and convey it to a collection ditch that runs along the toe of the embankments in most areas. Seepage was historically measured at four locations, two at each embankment. Most seepage exits through the collection ditch. The drains were reportedly perforated, tongue-and-groove wood pipes buried in a free-draining sand fill.
In 1997, Wisconsin Public Service performed slope stability analyses for the east and west embankments to determine whether they met current stability criteria. Work performed included surveying embankment cross-sections and performing soil borings and direct shear tests to determine the soil shear strength. As a result of this analysis, Wisconsin Public Service concluded that the downstream slopes did not meet the Federal Energy Regulatory Commission (FERC) stability requirements. The upstream slope stability safety factors for sudden drawdown also were insufficient, but the utility determined that the drawdown rate could be controlled to prevent a slope failure.
To better categorize work needed to improve stability of the slopes, Wisconsin Public Service performed additional borings and installed 14 observation wells to monitor the phreatic surface in the embankments. Then, in 2003, the utility reevaluated the slopes. Again, results indicated that the downstream slopes did not meet FERC stability criteria.
In 2004, FERC approved a revised probable maximum flood (PMF) for High Falls Dam. Although the new PMF is lower, refined slope stability analyses completed in 2004 showed that the safety factors for the dam were still lower than FERC requires. In particular, critical safety factors were low at the east embankment and the areas of the west embankment that lack a core wall. Remediation measures were required for the east embankment and these areas of the west embankment to meet stability criteria for the new PMF.
As part of FERC’s dam inspection program, Wisconsin Public Service conducted a potential failure modes analysis (PFMA) of High Falls Dam in July 2004. This analysis included an extensive review of construction history. The review provided valuable information regarding the original embankment construction. Historical correspondence confirmed the presence of the core wall in the west embankment. However, the documents lacked definitive information on where the core wall terminated. Data from the observation wells indicates that the phreatic levels in the embankment are significantly lower where the core wall is present. Stability analyses indicate that portions of the embankment with the lower phreatic surface meet stability requirements, and no modifications are required in these areas.
Drain fill placed at the toe of the embankment dams impounding water for the 7-MW High Falls project provided a safe exit for seepage through the embankments and prevented a loss of fines.
The review of the historical documents showed that during initial filling of the reservoir in 1910, seepage increased to the point that it was considered dangerous. The reservoir was drawn down. The design engineer suggested that the seepage was occurring through the foundation subsoil, which consists of sand 40 to 50 feet thick. He recommended seepage control measures, including installation of an upstream seepage barrier and a downstream toe drain. The recommended upstream seepage barrier included a sheet pile cutoff wall and impermeable concrete blanket. The recommended toe drain was perforated wood conduit surrounded by gravel. However, the utility could not find detailed information to determine to what extent these measures were carried out.
In 2004, a consultant safety inspection report – including a field inspection, underwater inspection, and review of past reports and analyses – was completed. Field observations from the dam inspection and further research shed some light on the seepage control measures. Most of the upstream embankment slopes at High Falls Dam are lined with concrete. Photographs taken during a previous drawdown revealed what appeared to be the top of wood sheeting at the toe of the concrete blanket. The underwater inspection confirmed the presence of the wood sheeting. Toe drain outlets were present at the downstream ditches, as were finger drain channels, confirming that a toe drain was installed. Repairs to the toe drains in 1998 uncovered some of the wood drains, which extended about 6 feet into the embankment from the toe.
Developing a remediation plan
In 2004, Wisconsin Public Service began looking into options for solving the seepage and stability problems at High Falls Dam. The solution chosen was to provide downstream slope stability by constructing a stability berm at the embankment toe. A toe drain system incorporated into the berm would ensure that seepage from the toe was not impeded and would be filtered. This solution was chosen as the most economical and efficient way of solving the problem.
The berm consists of granular fill material placed at the toe of the existing embankments. The fill placed at the toe provides ballast to resist rotational failure of the downstream slope. The material consists of granular sand fill to allow free drainage. The berm is 10 feet wide to accommodate maintenance vehicle traffic. The toe berm elevation was determined based on additional seepage and stability analyses performed for the design. The berm runs the entire length of the embankments where there is no core wall.
Seepage issues were addressed by installing toe drains and repairing the upstream seepage barrier. Toe drains were installed to intercept emergent seepage and to provide a reverse filter that would prevent migration of soil particles from the embankment. The drain fill is located between the existing embankment and the toe berm fill, allowing the drain to intercept seepage at and above the existing toe and convey it to the downstream ditch.
The design also included repairs to the upstream concrete seepage barrier. The concrete had visible cracks near the normal reservoir surface elevation as a result of ice pressure and freeze-thaw cycles. A reservoir drawdown of 12 feet for three months during construction allowed for repairs to the cracked or deteriorated portions of the upstream concrete blanket.
As part of the design, Spaulding Consultants conducted a finite element seepage analysis to determine the expected phreatic level in the embankments under normal and flood conditions. Spaulding Consultants developed a model representing existing conditions to calibrate soil permeability conditions and to evaluate the effects of the upstream concrete blanket, sheet pile cutoff wall, and drain system on seepage. Because the sheet pile depth and toe drain composition were unknown, the ability to calibrate the model was somewhat limited.
The seepage model consistently predicted a higher phreatic level than was indicated using data from the observation wells. The utility accepted the results of the seepage model because it predicted higher phreatic levels and thus would result in a conservative design. Wisconsin Public Service used the results of the seepage analysis to determine the appropriate phreatic surface for slope stability analyses. The slope stability and seepage analyses were performed to determine the appropriate configuration for the toe berm.
Remediation work at the 7-MW High Falls project included repairs to the concrete seepage barrier on the east and west embankment dams. The contractor repaired the surface using ready-mix concrete with entrained air.
The reverse filter drain was designed specifically for the embankment and foundation soils at the site. The design gradations for filter drains were based on soil samples from the embankment and foundation near the embankment toe. The embankment and foundation are comprised of silty sand. Two filter layers with a riprap cover were required to provide an adequate filter and to protect the filter. The drain fill extended through the downstream seepage ditch to capture seepage exiting to the ditch.
Public relations issues during construction
Construction was scheduled to begin in September 2005. This date was chosen so that Wisconsin Public Service could delay reservoir drawdown until after Labor Day, when most of the tourism had subsided. Recreation and tourism provide significant revenue to the local economy, and a drawdown during the summer would create a significant hardship to local businesses.
Even an autumn drawdown would have a significant effect on the local economy. Business owners, sportsmen, residents, and vacationers were concerned about the effect of the reservoir drawdown on water-related recreational opportunities.
Wisconsin Public Service was sensitive to the effect the work would have on area businesses and residents. Stakeholders were made aware of the project about a year before the drawdown was to begin. In May 2005, Wisconsin Public Service began a comprehensive communication effort. The utility sent a news release to local media providing information on why the work was being done and a tentative timeline. Additional press releases were issued as the drawdown and construction proceeded. In August, Wisconsin Public Service issued a joint press release with the Wisconsin Department of Natural Resources (DNR) to emphasize safety and enforcement. The release explained restrictions on access to the reservoir bed (including all-terrain vehicle operation), use of metal detectors, collecting driftwood, and fishing regulations.
A month before the drawdown began, the utility placed posters at businesses, parks, and area boat launches. The posters served as public notice that the drawdown was to begin, provided a brief explanation of why it was necessary, and indicated when the reservoir would be refilled. The posters also highlighted safety messages and included Wisconsin DNR enforcement messages.
In addition, Wisconsin Public Service representatives attended Crivitz Recreational Association meetings to provide members with project updates. This association of local businesses recognized that the project would affect autumn tourism-related revenue associated with anglers and visitors viewing of the beautiful fall colors. Local businesses responded to this challenge by planning a family-oriented event in October, called “Wisconsin’s Biggest Beach Party.” The event featured food, music, drawdown T-shirts, children’s games, a sandcastle contest, and a horseshoe tournament. Wisconsin Public Service staffed an information booth at the event to answer questions about the drawdown and to display photographs of the work at the dam.
During the remediation work, Wisconsin Public Service established a web page with photographs of the site. Additional informational and public re- lations items included articles in the Marinette County Tourism Newsletter, a frequently asked questions document so that Wisconsin Public Service and Wisconsin DNR employees would be consistent in answers they provided, and informational letters to members of two local business associations.
Performing the remediation work
Construction began in September 2005 and needed to be completed by mid- November, when the construction season typically ends in northern Wisconsin because of freezing weather conditions. Initial construction activities included stripping topsoil and preparing for drain fill placement while testing blends of drain fill.
Relyco of Green Bay, the general contractor, found it difficult to produce drain fill materials that met the project specifications. Drain fill specified for reverse filters typically is not a readily available material and requires blending and sifting of existing materials. The material supplier produced several trial mixtures, and it took several weeks to produce a blend that conformed to specifications. Whenever possible, it was recommended to use existing material specifications from state departments of transportation for aggregates and sands. However, when this is not possible, it may be beneficial to contract separately with a material supplier to produce the drain fill before construction.
The stripping of the topsoil allowed for better visual inspection and location of existing drains. The drains consisted of wood conduits, some with clay tile pipe outlets, and polyvinyl chloride (PVC) pipe drains. The wood conduit drains that were uncovered generally were filled with sand and were ineffective. The contractor observed drains in an area where the downstream seepage ditch was located, a significant distance from the embankment toe. The construction drawings did not include drain fill at the outlet of these drains, as the seepage path and resulting gradients were deemed to be adequate to prevent piping. However, the extension of the drains into the embankment toe resulted in a decreased seepage path and potentially higher seepage gradients.
Based on these findings, Wisconsin Public Service decided that a trench drain should be installed in this area to intercept and filter seepage to these drains. The bottom of the trench drain extends to a depth equaling the bottom elevation of the downstream seepage collection ditch, or to bedrock if it is higher than the seepage ditch. Seepage was present, causing difficulties in keeping the trench open. No problems were encountered with loss of fines. Therefore, operations were modified to minimize the length of open trench and to initiate backfilling with the drain fill as soon as possible. This process worked well and allowed construction to proceed with little difficulty.
After the reservoir was drawn down, the upstream seepage cutoff measures were visible. The presence of wood sheeting was confirmed along the length of the embankment where there is no concrete cutoff wall. The sheeting extends well beyond the east end of the east embankment. The bottom of the upstream concrete seepage barrier is located at the top of the wood sheeting.
Concrete removal, surface preparation, and concrete replacement were completed for selected portions of the east and west embankments. After the reservoir drawdown, the contractor inspected the upstream seepage barrier and identified areas requiring repairs. The original design called for using shotcrete to repair the selected areas. However, in discussions with the contractor and Wisconsin Public Service, the decision was made to replace the shotcrete with ready-mix concrete with entrained air for improved durability.
Wisconsin Public Service increased monitoring of observation wells and seepage weirs, from monthly to daily, during the reservoir refilling. During reservoir refilling, the water levels in the observation wells increased to levels exceeding historic maximums. The refilling was suspended for the winter and resumed again in the spring of 2006. Daily monitoring continued during and after completion of the refill until the observation well levels stabilized and returned to normal levels.
The utility also developed a surveillance and monitoring program to monitor the changes in the phreatic level and seepage rates through the embankments and to observe the functioning of the drain system. Surveillance included observing the embankments for deformations and loss of fines in the seepage discharging from the drain system. Wisconsin Public Service also established threshold levels for the instrumentation, based on historic levels and the analyses completed for the design.
The remediation work addressed the primary concerns of slope stability for the PMF condition and seepage control. The stability berm provided additional mass that resists rotational failure of the downstream embankment slopes, resulting in acceptable factors of safety for the flood loading condition. The design also provides a safe exit for seepage at the downstream embankment toe and prevents the loss of fines from within the embankment.
As a result of the public relations effort, the local businesses and general public were well-informed and remained supportive of the work taking place. The informational booths at the beach party were instrumental in communicating to the public what the real issues were and how they were being addressed. The general public and local businesses were inquisitive but remained at a safe distance from the construction, resulting in a successful project.
Virgil Schlorke, P.E., supervisor regional generation with Wisconsin Public Service Corporation, provided construction management and coordination for the work at 7-MW High Falls. Dean Steines, P.E., formerly water resources engineer with Ayres Associates, is now senior plant engineer with Xcel Energy. He was the independent consultant for the potential failure modes analysis and Part 12 inspection, lead engineer for the remediation design, and quality control manager during construction. Todd Rudolph, P.E., water resources engineer with Ayres Associates, assisted with the design and provided technical assistance during construction.