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This photo from July 2013 shows the rim of the Berkeley Pit were a slough deposited surface material into the Pit lake in Feb. 2013. Photo by Fritz Daily.

Study details slope stability

The rate of rise of water levels in the Berkeley Pit and connected monitoring points is affected by many factors, including rain and snowfall and occasional ‘sloughs’ or ‘slumps’ of material from the Pit’s sidewall slopes.

The most recent slough occurred on February 8, 2013. An estimated 820,000 tons of material from the southeast wall collapsed into the Pit. Montana Bureau of Mines and Geology (MBMG) monitoring showed that the water rose about 0.6 feet as a result. For comparison, over the past several years the water has risen about 0.65 feet per month.

This photo from July 2013 shows the rim of the Berkeley Pit were a slough deposited surface material into the Pit lake in Feb. 2013. Photo by Fritz Daily.
This photo from July 2013 shows the rim of the Berkeley Pit were a slough deposited surface material into the Pit lake in Feb. 2013. Photo by Fritz Daily.

Sloughs or landslides are relatively common in open pit mines and can potentially raise water levels. To address the potential effects of future sloughs on the Pit’s rate of rise, EPA required the Potentially Responsible Parties (PRPs) for the site, Montana Resources and the Atlantic Richfield Company (AR), to study the stability of the slopes around the rim of the Berkeley. Publication of the final report on that study is expected later in 2015, and it will be published here on the PitWatch website.

EPA and the PRPs have stated that preliminary results indicate that the rising Pit water level will continue to increase the potential for slope failure, especially in the southeastern part of the Pit. Future sloughs are expected to occur in the absence of any stabilization or mitigation measures, but, based on past sloughs, are not expected to significantly affect the Pit management timeline.

Two much smaller landslides, which had no noticeable impact on the water level, occurred in August and November 2012. A larger landslide occurred in 1998. The November 2012 slide damaged the Montana Resources pontoon boat used for water quality sampling in the Pit. Following the 2013 slide, those sampling activities were suspended for the safety of the MBMG scientists who conduct the sampling.

The area of the slough that occurred in Feb. 2013 can be seen in approximately the center of this photo of the Berkeley Pit, taken in July 2013 by Fritz Daily.

Berkeley Pit slough

On February 8, 2013 material from the southeast wall of the Berkeley Pit collapsed into the Pit water in what is known as a rotational slump or slough. Such sloughs are relatively common in open pit mines. For example, a similar slough occurred at the Berkeley Pit in 1998.

This photo from July 2013 shows the rim of the Berkeley Pit were a slough deposited surface material into the Pit lake in Feb. 2013. Photo by Fritz Daily.
This photo from July 2013 shows the rim of the Berkeley Pit were a slough deposited surface material into the Pit lake in Feb. 2013. Photo by Fritz Daily.

The recent slough was about 550 feet wide and caused an estimated 820,000 tons of material to collapse into the Pit. Montana Bureau of Mines & Geology (MBMG) monitoring showed that the water level in the Pit lake rose about 0.6 feet as a result of the slough. For comparison, over the past several years the water in the Berkeley Pit has risen about 0.65 feet per month.

Current projections still estimate that water levels at one of the surrounding monitoring compliance points for the Berkeley Pit system will reach the Critical Level (5,410 feet) around 2023.

Pumping and treating of Berkeley Pit water will be required when water levels at any of these compliance points reach the Critical Water Level. Currently, the highest water level is in the Pilot Butte shaft to the north of the Pit. As of June 2013, the Berkeley Pit water level was 5,310.89 feet above sea level, and the water level in the Pilot Butte shaft was 5,335.72 feet above sea level, or about 75 feet below the critical level.

Click here for more information about the Critical Water Level.

This aerial photo taken in 2001 shows the location of the Continental fault east of Butte, Montana. It has been monitored closely for 25 years and has not shown enough activity to prompt earthquake concerns.

What if an earthquake were to strike?

This aerial photo taken in 2001 shows the location of the Continental fault east of Butte, Montana. It has been monitored closely for 25 years and has not shown enough activity to prompt earthquake concerns.
This aerial photo taken in 2001 shows the location of the Continental fault east of Butte, Montana. It has been monitored closely for 25 years and has not shown enough activity to prompt earthquake concerns.

There are several reasons why we don’t need to be overly concerned about the Pit in the event of an earthquake, including the fact that there has been no significant seismic activity in nearby faults during the 28 years that the Earthquake Studies Office has been monitoring the area.

In the mining region, the recorded seismic activity is mainly caused by a few mining blasts per week, however, two landslides in the pit have been recorded in the last 28 years and a few very small, non-mining related earthquakes are recorded within Butte Silver-Bow county annually. Most seismic activity in Montana occurs outside of Silver-Bow County, and in the worst-case scenario, a large earthquake could cause landslides or sloughing in the Pit, but would not cause the Pit to overflow. Such an earthquake would cause considerably more damage to buildings and structures in the Uptown area than to the landscape or Pit.

After the initial PitWatch article on earthquakes in the Spring of 2005, there was a 5.6 magnitude earthquake centered near Dillon on July 25, 2005. This earthquake was felt in Butte, but there was no evidence of any sloughing or rise in water level in the Pit.

According to the Earthquake Studies Office, there have been approximately 20 very small earthquakes within 25 km of Butte in the past year, ranging in magnitude from -.2 to 1.9. Nineteen of these were less than magnitude 1.0, and only 4 were located within Silver-Bow County. Three of these events were non-mining related events in Butte and were not large enough to be felt.

There have been no reports, past or present, of any earthquake damage to the Pit and the last earthquake greater than magnitude 1.0 in the area was a 2.8 on October 9, 2005, located 3.3 kilometers west-southwest of Butte along Silver Bow Creek.

This topic was covered in the 2005 Spring and Fall issues of PitWatch.

Have there been any major rock slides along the Pit walls?

Yes. Wet weather may have also played a role in the 1998 Pit wall slough that sent about 3 million tons of rock and dirt into the water. Rising groundwater saturated and gradually weakened that section of the southeast wall, eventually causing it to break away.

Montana Resources, Inc. (MR) is taking steps to stabilize the piles of waste rock that form sections of the Pit walls. Two options are available: The first is to remove material from the tops of the dumps (the crests) to relieve pressure, and the second is to add material to the bottoms of the dumps (the toes) to bolster their foundations.

MR is employing both strategies to minimize future problems. Using a bulldozer, crews shaved the crest of what is called the “Bird Watch Dump” along the Pit’s south wall. And to shore up the underwater toe of another dump in the Pit’s southeast corner, crews pushed in material from the Bird Watch Dump, plus additional waste rock and dirt from the active Continental Pit.

The Berkeley Pit, Continental Fault, and the two wells that showed water level changes after a July 2005 earthquake.

Earthquakes did not affect Pit

The Berkeley Pit, Continental Fault, and the two wells that showed water level changes after a July 2005 earthquake.
The Berkeley Pit, Continental Fault, and the two wells that showed water level changes after a July 2005 earthquake.

A 5.6 magnitude earthquake centered near Dillon on July 25, 2005 did not affect the Berkeley Pit. There was no Pit wall sloughing or change in the water levels in the Berkeley Pit, the underground mine shafts, the alluvial aquifer wells, or the majority of the bedrock monitoring wells.

However, two bedrock monitoring wells (A&B) showed changes. Well A showed an initial water level decline of about one (1) foot after the earthquake, and the level stayed lower for a number of days before rising again. Well B, which is located in an area that wasn’t dewatered as extensively by historic mining activities as other portions of the bedrock aquifer had a 9-foot drop in water levels in the month following the earthquake. Recently, the water elevation in Well B is rising again.

One possible explanation for the lower water level in these wells is that the earthquake opened up existing fractures in the bedrock surrounding the wells. Water then flowed into these fractures until the bedrock adjacent to them became saturated. When that happened, the water levels began to rise again.

Since the July earthquake, there have been two additional quakes in the region, one of which was centered in the Butte Basin. Both of these other quakes were considerably smaller in magnitude, and no effects were noted in the Berkeley Pit or bedrock monitoring wells.

This aerial photo taken in 2001 shows the location of the Continental fault east of Butte, Montana. It has been monitored closely for 25 years and has not shown enough activity to prompt earthquake concerns.

Can the Pit Withstand an Earthquake?


PitWatch Issue Volume 10, Number 1

Tsunamis, volcanoes and earthquakes in recent months have created an increased interest in seismic activity. Many readers have written, called, or stopped by questioning what will happen to the Berkeley Pit if an earthquake occurs in Butte, Montana. To help answer these questions, local experts were asked to explain the likelihood of an earthquake and what effect it would have on the Berkeley Pit.

Probability of an Earthquake in Butte

This aerial photo taken in 2001 shows the location of the Continental fault east of Butte, Montana. It has been monitored closely for 25 years and has not shown enough activity to prompt earthquake concerns.
This aerial photo taken in 2001 shows the location of the Continental fault east of Butte, Montana. It has been monitored closely for 25 years and has not shown enough activity to prompt earthquake concerns.

Mike Stickney, Director of the Earthquake Studies Office at the Montana Bureau of Mines and Geology says that Butte is not likely to suffer a severe earthquake anytime soon. Large earthquakes are certainly possible in western Montana as demonstrated by the 1959 Hebgen Lake earthquake (magnitude 7.3), but are most likely to occur in the more seismically active regions located to the north, east, or southeast of Butte. The state of Montana is unlikely to experience earthquakes larger than the 1959 earthquake because the faults are not large enough to produce earthquakes greater than magnitude 7.5.

Stickney also explained that Butte has been monitored closely for seismic activity over the past 25 years. There has never been any significant seismic activity recorded that suggests the nearby faults to be active enough to cause a large earthquake. Most seismic activity that registers in the Butte area is caused by blasting at the open mine site, and very minor underground subsidence, especially near the old block caving zones under the Kelley Mine.

Effects of an Earthquake

Even assuming a worst-case earthquake scenario, the Berkeley Pit would not overflow. Experts suggest that there would be far more damage to buildings and other structures in Uptown Butte than would be caused by adverse impacts from the waters in the Berkeley Pit.

Studies show that the Yankee Doodle Tailings Pond dam would withstand at least a 6.5 magnitude quake. It can also be assumed from similar studies that such a quake could cause some sloughing on the pit walls, but the resulting movements would not discharge enough rock and materials to cause the water in the pit to overflow.

Sloughing and Landslides

Although earthquakes are not likely to be a problem, landslides and sloughing of the Pit could occur. The majority of the Berkeley Pit walls are made of “solid” bedrock. However, the southeast wall is composed of “loose”silts, sands and gravels, and this is the area where sloughing is most likely to occur, with or without a major earthquake.

In September 1998, about 1.3 million cubic yards of “loose” alluvium on the southeast wall sloughed into the Pit. This event caused a 3-foot rise in the water level and surface waves greater than 20 feet.

The water rise associated with any pit wall sloughing would ultimately depend on the volume of material that breaks free and displaces the water. But it should be noted there is enough space for more significant events. For example, there is more than 150 feet between the current Pit water level (5,252′ above sea level) and the Critical Water Level (5,410′), and there is another 100′ feet up to the rim of the Pit.

Summary

If an earthquake were to occur, the effects of seismic activity at the Berkeley Pit would be the least of Butte’s worries. Since a large earthquake is not likely anytime soon, and because landslides are relatively manageable, the public should not be overly concerned. There will probably continue to be some sloughing on the benches and old roads, but not enough to cause the Pit water to rise more than a few feet.