Upgrading Hebgen Dam to Withstand a Potential Second Earthquake

This aerial view of Hebgen Dam and its intake during the rehabilitation work shows the circular sheet pile cofferdam cells in front of the intake (at top left).

Work is nearly complete on a major upgrade project at Hebgen Dam designed to bring the intake up to modern seismic design standards and prevent failure should another major earthquake occur.

By Benjamin J. Cope

Benjamin Cope, P.E., is senior hydro engineer with PPL Montana and lead on the Hebgen Dam intake rehabilitation project.

Hebgen Dam has already survived a major earthquake … barely. On Aug. 17, 1959, a fault located only a few hundred yards from the right abutment of the earth embankment dam ruptured during a 7.5 magnitude earthquake. The Madison River downstream of the dam was completely blocked by a massive rockslide, which also killed 28 people. The dam held but was overtopped by a series of waves, and the dam and flow-control structures experienced severe damage.

The dam was repaired and operated well for the next few decades. Then, in 2008, several stoplogs in the intake structure failed, bringing the need to rehabilitate the intake to the top of the priority list for PPL Montana. The rehabilitation since undertaken, now nearly complete, should allow for reliable and safe operation of the intake for decades to come.

Background on the situation

Hebgen Dam is located near West Yellowstone, Mt. At over 6,500 feet in elevation and tucked among the mountains, conditions at the dam are harsh. Weather is among the coldest and snowiest of any location in the lower 48 states; winter temperatures have recently been below -30 degrees Fahrenheit. The nearest city of any size is 100 miles away. Montana Reservoir and Irrigation Co., a predecessor of Montana Power Co., originally build the dam, from 1910 to 1914. PPL Montana, a subsidiary of PPL Corporation, has owned Hebgen Dam and 11 other hydroelectric developments in Montana since 1999. (PPL Montana has announced an agreement to sell its hydroelectric projects to NorthWestern Energy, pending regulatory approvals.)

The 700-foot-long earth embankment dam was constructed with a concrete core wall. A concrete-lined and gated spillway was constructed near the right abutment. A small powerhouse was constructed to provide local power service, but by the 1950s it was no longer operating. The low-level outlet for the facility is the intake structure, a 75-foot-tall freestanding concrete tower at the upstream toe of the embankment on the left side. Flow through the intake tower was controlled with stoplogs in four bays for the full height of the structure.

The reservoir was significantly lowered in 1960 to allow repairs after the earthquake. The core wall was cracked in several areas and required grouting. The downstream embankment was cracked, displaced and eroded, and it required repairs. The spillway and its chute were damaged beyond repair and required complete replacement. The intake structure experienced some damage but was repaired and upgraded. Two vertical slide gates were installed near the bottom of the intake in two bays, while stoplogs were left in the other two bays. The outlet from the intake to the river downstream is a 12-foot-diameter woodstave pipe through the dam and core wall downstream to the tailrace.

In the 100 years since the reservoir was created, Hebgen Lake upstream of the dam and the Madison River below have developed into pristine fishing and recreational waterways. Hebgen Lake bolsters a healthy population of rainbow trout in its cool, clear waters, and lakeside residences and resorts near Yellowstone National Park make the reservoir its own destination. The steady cool flows released from Hebgen Dam make the river downstream a world-class fly fishing stream, which generates millions of dollars of tourism and business revenue. As a flow-control and storage reservoir, the 386,000-acre-foot volume of Hebgen Lake provides a benefit to the 11 downstream hydroelectric plants on the Madison and Missouri rivers in Montana. The recreation and environmental implications of Hebgen Dam, which are now a regular consideration of operating the facility, were something the original designers did not foresee, complicating maintenance and repairs to the facility.

Basis for the design

Over the course of independent consultant reviews required by the Federal Energy Regulatory Commission (FERC) as a component of the part 12D safety inspections, a seismic deficiency with the intake structure was identified. At first the vulnerability of the intake was not deemed critical because it was not integral with the dam and the spillway provided redundancy in flow control.

Further reviews attributed more importance to the intake however because it was the only low-level outlet and was needed for dewatering the lake should damage to Hebgen Dam itself occur from an earthquake. Plans were undertaken in 2008 to both better assess the condition of the intake and start analyzing the seismology to determine the necessary design loading.

Acceleration by near-disaster

Labor Day weekend of 2008, as design of a seismic rehabilitation of the intake was beginning, several stoplogs in the bottom of Bay 4 of the intake failed. For a month, PPL Montana personnel worked feverishly to stop the uncontrolled release. More than 3,000 cubic feet per second (cfs) of water was being discharged at a time of year when only 1,000 cfs is normal. The main concern was for the integrity of the aged wood outlet pipe and subsequently the dam itself. After several ideas were attempted without success, banded packets of stoplogs were jacked into the opening 50 feet below the water level, and flow was mostly stopped. The need to rehabilitate the intake and reduce the possibility of another failure quickly accelerated to a top priority for PPL Montana.

URS Washington was retained for the intake rehabilitation design, and Pacific Pile and Marine in Seattle was hired for the design-build contract for a cofferdam to isolate the intake. The intake rehabilitation design was a unique undertaking, worthy of its own substantial narrative. The seismicity was a complicated process, involving multiple experts and discourse with FERC, and a design event of a magnitude 7.3 earthquake at a distance of 100 meters, resulting in horizontal and vertical peak ground accelerations of 1.5 g, was selected. Substantial permanent anchorage and post-tensioning of the structure was immediately identified as a necessary feature to combat the multiple load directions and reversals during an earthquake. The finite element analysis URS performed was an intricate symphony of elements that took into consideration the multiple existing and new materials, the planned anchorage, foundation materials, and partial submergence in the reservoir, among many other factors.

Many factors were incorporated in the design just to allow access for construction and stability until the permanent features were in place. For example, an array of temporary grouted rock anchors were specified for the back wall of the intake to stabilize the structure during dewatering until the permanent post-tensioned anchorage and new concrete structure were complete.

The cofferdam to isolate the intake structure was designed as a free-standing sheet pile structure with three cells containing fill material. A primary goal of the design was that the cofferdam not rely on the intake for stability and not transmit significant load to the intake. The center cell was designed with a single vertical slide gate. During the winter months, Hebgen Reservoir is drafted below the crest of the spillway to allow flood storage volume for spring runoff. This fact necessitated the use of the cofferdam for a low-level outlet during the winter to ensure minimum river flows are maintained and flood storage volume is adequate for spring. The obvious downside is that every winter, construction on the intake has to be suspended and the entire river flow diverted through the area under construction. This unique aspect of the project has impacted the project schedule, sequence and budget.

This view of the intake after dewatering and removal of the existing stoplogs and gates shows the completed rock anchors and post-tensioned multi-strand anchors on the back wall of the intake.

Completing the work

Construction on the intake rehabilitation project began as the cofferdam was being completed. Sletten Construction of Great Falls, Mt., was awarded the contract for the rehab. Very early on, the first major difficulty was revealed. The hydrostatic pressure on the back wall of the intake was unbalanced when the intake was dewatered by the cofferdam. Two test wells were drilled behind the intake to quantify the number and capacity of dewatering wells needed to relieve the hydrostatic pressure on the back wall. The test wells revealed that the permeability of the highly fractured quartzite behind and beneath the intake was 100 times greater than what was estimated by the geotechnical report. With limited power service available at the site and limited area behind the intake, installation and operation of the required number and capacity of dewatering wells would be difficult. The hydraulic storage coefficient revealed by the test wells was also low, which meant that a failure of the dewatering system could introduce catastrophic unbalanced loading quickly.

The project team had to develop a solution to counteract the unbalanced loading. The cofferdam contractor, intake contractor, intake design engineer and PPL Montana all contributed to the brainstorming of possible solutions. It was determined that the best solution would be to brace the front of the intake until the temporary rock anchors were installed and the intake was stabilized. The challenge was to design a bracing solution that did not introduce detrimental or unbalanced loading to the intake and cofferdam but still fit within the limited interstitial space and could be removed when no longer needed.

A plan was developed for the placement of cementitious controlled low-strength material (CLSM) in the interstitial space between the cofferdam and intake. CLSM is frequently used in utility construction for trench backfill and at road crossings. It flows readily, is quick to place, is quick to develop strength, and is more economical than standard concrete. However, it is generally placed in dry conditions. The challenge for the Hebgen project was to create a CLSM mix that would not wash out when placed by tremie application in areas with flowing water, would self-level for even distribution, and would be of the target strength. The material would have to cure and not transmit significant lateral loads to the structures like a loose fill material, but would also need to provide a target compressive strength of 500 to 1,000 psi so it could be excavated. Mix designs were developed and tested, and Sletten began the placement of CLSM in front of the intake in the fall of 2010.

The front face of the intake required extensive work by Sletten’s diving subcontractor, Harbor Offshore Inc., to reduce the more than 100 cfs of leakage through the stoplogs and minimize loss of CLSM during placement. CLSM was placed in 3-foot lifts to limit hydrostatic pressure from the uncured material. The material cured enough overnight to allow placement of subsequent lifts daily until the full 40 feet of material, up to the level of the cofferdam gate, was in place. A concrete armoring slab was constructed on top of the CLSM to limit erosion over winter.

 

In this photo, placement of the stage 1 interior concrete and demolition of the existing interior walls of the Hebgen Dam intake is complete.

Installation of the 84 temporary rock anchors in the back wall and invert of the intake began before placement of CLSM was completed. The drilling contractor worked with a custom drill rig inside each of the four intake bays from platforms suspended from cables; access and safety was a daily concern. Even though an extensive grouting program was completed before the drilling began, the nearly horizontal holes proved difficult to complete. Installation of the anchors was completed in late 2011.

The intake tower for Hebgen Dam is shown under construction in 1911.

At that time, the contractor began work on the nine permanent 39-strand post-tensioned multi-strand anchors also through the back wall of the intake. Similar to the rock anchors but amplified because of the larger holes needed for the 39 strands of 7-wire 0.6-inch-diameter steel (as opposed to the single threaded bars of the rock anchors), these anchors were also difficult to complete. One anchor failed during testing and had to be removed and replaced.

In the summer of 2012, two years after starting construction on the intake, it was completely dewatered for the first time. Starting in 2011 and finishing in 2012, Sletten painstakingly removed the CLSM bucket by bucket using a mini-excavator, and the stoplogs and gates were demolished in the front face of the intake. Immediately after CLSM removal was completed, another major problem materialized. Stresses in the cofferdam had been monitored with a series of strain gauges since it was completed two years prior, with results within expected ranges. Stresses were increasing incrementally as the CLSM was removed, a behavior that was expected because the CLSM had served to limit deflection of the sheets. It was not expected when the gauges started indicating stress in the south cofferdam rising to within 5 ksi (5,000 pounds per square inch) of the 60 ksi yield strength of the steel sheet piles.

Work was immediately suspended in the intake and the project team again came together to develop a solution. While the exact cause of the high stress readings in the south cofferdam still has not been identified, it was decided that the most effective and fastest course of action would be to dewater the south cofferdam to remove the element of differential loading from hydrostatic pressure. PPL Montana installed three dewatering wells to draw down the water within the cofferdam cell fill, and stresses quickly abated. Other ideas for permanent solutions for reducing stress on the cofferdam were explored, but the dewatering system proved most effective and robust. Site power was upgraded and extended, an automatic transfer and standby generator system was installed, and instrumentation and reporting in the cofferdam was upgraded to real-time data logging with on-site and remote alarms. The dewatering pump system has performed well since installed, and stresses have remained within tolerable ranges.

The fall of 2012 saw the start of construction on new concrete for the rehabilitated intake. Construction of the north and south end walls started in the fall of 2012 and was completed in the summer of 2013. After the end walls and temporary internal struts were completed, the interior diaphragm walls were demolished using diamond-wire saws. In the fall and winter of 2013, second-stage concrete for the invert and front face of the intake was started from the bottom moving upward. From January 2014 through early summer, work focused on installation of the 20 post-tensioned multi-strand anchors located behind the intake. These 57-strand anchors are located at varying dip angles and positions directly behind the intake and bear on mass concrete, delivering additional post-tensioned horizontal and vertical pinning action to the intake and mass concrete.

The rehabilitated intake, when completed in 2015 after an estimated total cost of almost $30 million, will have multiple gates with redundancy, protections, and automated instrumentation and controls to allow reliable and safe operation long into the future. The project has been wrought with unique challenges, delays and difficulties since before construction began. The fortitude, innovation and collaboration of PPL Montana and its project partners will allow the project to be a success, and the energy consumers and recreationalists of Montana will realize the benefits.

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