Rock and Boulder Fall, and Snow Avalanche Mitigation

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AVALANCHE AND ROCKFALL

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Natural Hazard Control / Mitigation is essential. Any time of the year Rockfall can be a serious concern. In the winter snowslides are of prime concern.  Springtime and early summer is when we see most of the mud, and rock avalanches, although the can occur any time of year.

The above is Cover photo from a piece I did for a Rocscience Newsletter, a publication put out by a computer program development organization in Toronto, ON, Canada.  It is one of several computer programs I use when evaluating rockfall hazards. Computer simulation gives me thousands of variables, so I can plan, and determine more accurately what the “risk exposure” is?

The above 200 ton +/- some, a boulder that was position in such way it made sense to use our sequencing system to bring it down under controlled circumstances. Although, in some cases, I still use of explosives. When I do I use them I do so in a very controlled and careful way. Things like ignition rate, placement, and timing delays on multiple charges, make a big difference in the resultant.

Proactive Mitigation prevents accidents, saves lives, and money. Closing a major highway costs $$$$ not to mention the potential damage and threat to life.”

The above boulder we called “Big Bertha”  it weighs about 1,250 tons, After doing and OHAR (Objective Hazard Assessment Report) we determine the best plan of action, in this case, stabilizing this did not make good sense, so we chose to eliminate.   Using our sequencing system, we were able to “Kick it” so it had topspin and it rolled completely over the highway.  Afterward, UDOT cleared the debris and traffic began to flow as per normal.

Natural Hazard Control is a tricky game. It is pretty hard to guesstimate exactly how mother nature will end up doing things. On top of that we must deal with physics and engineering principles. The first step is we do a hazard assessment and determine the Risk Exposure for the area of concern. It may be low enough nothing else needs to be done.

Big Bertha about 1,000 tons, is no longer above State Highway 24

 

This was staged.   The Park Service  was working with a person who was producing a safety video about natural rockfall for visitors to watch before they hiked the trail to the cave.  Over the many prior decades the NPS prevented many potential accidents, but there have been a couple of injuries and one death due to natural rock fall.  Understanding the principles associated with natural rock fall and safety is a good idea.  Things like: don’t make rest stops in gullies, notice the trail, and trees etc.  are they damaged by previous rock fall?  If so, move through the area quickly.  Listen, rockfall makes a distinct sound.  If you hear it, take cover close to the cliff wall, or quickly move out of gullies, etc.  Places that have lots of trees, that show no signs of rock fall damage are better places to make rest stops. and…
This staged picture was nixed, it was considered a little too overboard for the safety film.

Shortly after returning from active duty in the military I began working at Timpanogos Cave National Monument as a National Park Ranger in Visitor Protection and Resource Management. Timpanogos Cave has a mile and a half trail (2.5 kilometers) that leads to the cave entrance and another short section that leads from the exit, back to the main trail.

The trail is located on a north-facing slope which means it has less natural rockfall than south-facing slope would. South-facing slopes are less steep (generally) because they experience many more freeze-thaw cycles than do north facing slopes.

Freeze-thaw cycles, or “Frost Wedging” break rocks loose. It is like having water in steel pipes, that freezes and breaks the pipe, except, in this case, the sun melts the snow and ice, then free water fills the cracks and freezes at night, breaking the rock to pieces.

With computer simulations, we can add another degree of understanding and safety to our projects.

As the sun hits them, or it warms up, these rocks are influenced by gravity and fall down to the canyon floor. If people happen to be in the way, well, it can be serious . . . Over the years, there have been a couple of serious incidents on the trail and one death.   The NPS is quite proactive and designed a rockfall mitigation drape that was retractable, to go in the main chute above the cave trail, and a roof to go over the exit to protect visitors.

This is the Rockfall drape in its retracted position.

Back then I was fairly green, but I was a pretty fair rock climber and OK at the technical rescue.  Since I was a climber I was asked to help the contractor install the Drape.   My grandfather, an engineer for US Steel Corporation, taught me many engineering principles, but rockfall mitigation was not my specialty, I learned quite a bit.  It was my first major project of this size.

So I went to work for the contractor, doing what I loved to do. Although, training the contractor’s personnel and trying to watch over things to prevent accidents was nerve-racking.

Each fall the rockfall drape is retracted, and the gates opened. Then in the spring, all of the rocks that were not cleaned out by snow avalanches, are cleaned out manually. Then the gates are closed and the drape is pulled across the chute for the summer visitor season. It works well.

Anyway, that is where my natural hazard skills started to grow. I took these pictures years later after I had become an engineer and specialist in Rockfall and Avalanche Elimination and Mitigation. I took these pictures as our team installed the new rockfall fence at the exit, and I made sure the above system was working ok.

One of our lead professionals rappelling above the exit to Timpanogos Cave to drill holes for a rockfall barrier we installed. You can see the paved exit trail below.  We are upgrading work we did about 20 years back.

Depending on the situation, we sometimes bolt the boulders, and flakes to the mountainside, thus locking them in place. With time they get old, and they do need inspection. We prefer to eliminate them, but it depends on the location, and some other considerations.
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I n some cases, it is pretty straight forward, like this basic drape we installed near Eureka, on U.S. #6, but in most cases, it is a mixed approach. Over the years I have developed a system to establish the risk exposure for various areas of concern (AOC). A good example of the work we have done with the State of Utah. They have many AOC.

This is one of the rockfall drapes we have installed for the State of Utah. In some case, a simple drape will work fine, in other cases we use erosion control along with various drape configurations. That way the rockfall hazard and erosion problem gets better
This is another drape we installed for the BOR. You can see 3 of or team members working on this one. Two underneath and on the outside.

Quite a few years ago, we helped install the below special retractable hanging drape for the Department of the Interior, NPS.

One of our mitigation projects

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Snow Avalanches change each year.

Paths are fairly common but the severity and frequency changes each year.

One of my favorite pictures, a small spring snow avalanche at Timpanogos Cave National Monument, American Fork Canyon. Park Personnel are known for their “Can Do” attitude. Notice the shovel. He’s ready to go to work.

It was one of these spring snow avalanches that first made me think I was going to die for sure! We were hiking up the cave trail (early spring and the trail was closed to visitors) when we stopped for a break at a place called “dead dog” (I don’t know the full story, but a dog is supposed to be buried there.)   While we were resting we heard a low rumble.  It got louder and louder.  I thought is was a jet was flying over, although I could not see it. It seemed really close to us.

Then the ground seemed to shake. I did not know what to think. I looked up above us and a monster avalanche hit the big rock buttress above us. I did not stop to watch I began to run for cover.  As I was running I thought to myself: ” I always figured I would die, but not this way.”  I did not watch the other two Park Rangers, as I ran and took cover behind the downside of a big boulder on the ridge we were resting on.

When things settled down I noticed the avalanche had actually split into two, when it hit the rock buttress above us. It buried the trail on the east side of us, and continued down the chute on the west side of us! Wow! Here’s a good habit for anyone who spends time in the mountain. Be conscious of “Objective Hazards- (things we do not have direct control over.)   Stop in sheltered places. It has saved my life a number of times.

One of the Rangers had transferred to Timp Cave and was new to these mountains. He was my boss, Chief Ranger over our department, “Visitor Protection and Resource Management. He had transferred for White Sands National Park. Those type of experiences can be overwhelming. I think it was almost a month into summertime before he went back up the trail.  With time we get used to it, but that was an abrupt introduction for him.

I had another interesting avalanche experience about a year later. We hunting deer on the face of 12,000 foot high Mt. Timpanogos. It had been snowing a fair amount (about 80% of all natural avalanches happen during or shortly after a snowstorm.)  When we were coming back we found an avalanche had come down the North Face of Timp and had covered the dirt road. There we a couple of other trucks that were trapped on the west side. They were waiting for the forest service to come up and clear the road. They did not know that backcountry roads don’t usually have those kinds of services.

So we all grabbed our shovels and made a notch in the avalanche path, where the roadway was. I put some chains on my 4 x 4 and got a run at it. I hit the avalanche and the truck climbed up on top of it and I was partway across.  Just as I hit the middle I started to sink. Wow! My heart sunk! “That’s all I needed to do is have my truck stuck in the middle of an avalanche path!” During a snowstorm, it is rather common to have slides run the same path, just a short time after previous ones.

Luck was with me, just as it seemed it was going to become hopelessly stuck, it grabbed something, or I got some divine help, either way, I did not argue. It continued to go up and over the avalanche debris to the other side.  The others quickly followed.

This is a picture I took a number of years ago. It is Provo Canyon Highway 189, the old route.  This came out of Slide Canyon which is on the south side of the top of Sundance Ski Resort… This slide was about 1/4 mile wide, and over 30 feet deep. Some years we do not have many avalanches, others we have monster avalanches.

Below are some Avalanche Stabilizing Fences which we installed.  They were the first installed in the USA,)  Ee installed these at the Alta Ski Resort.  They protect homes and the highway.
 They do not “stop” avalanches, rather they prevent them from starting by holding the snow in place until it melts in the spring.

Each of the fences holds 100’s of tons of snow weight. After we do our pre-job evaluations and studies, we determine the size, type, and where to place these so they will be in starting zones. Basically speaking, there is a starting zone and the runout path. If we were to place them in a runout path, the force of the avalanche would tear them to pieces.
Notice the center right two of our avalanche stabilizing fences can be seen in the open slope just below the cliff with the big black mark on it. These fences are just above the highway in Little Cottonwood Canyon. With time the natural trees grow back (clear cut while lumbering in the early days) and as can be seen, even in such a short time 10 years, the trees are already hiding the fences, and the trees help naturally stabilize the slope.
 Using an inclinometer to measure slope angle. The prime angle is about 38°, 25° to 60° is fairly frequent. Steeper they sluff off before getting too big, and less than 25° they tend to stabilize and melt before sliding.
Avalanche crossing highway U-210 Little Cottonwood Canyon, Utah Notice the mountain has a story to tell. The trees are missing, broken, or bent over. A tell-tale sign of frequent avalanche activity and “a poor place to have lunch or set up camp.”

A major avalanche at Bridal Veil Falls (a very rare 50-100 year cycle)

This was a unique year. We had a midseason storm dump in the bowls on the north-north-east side of Cascade mountain, and then they released! We lost a great activity. Although, if it was rebuilt with the current understanding we have now, this would not have happened.  Here are the remains of the tram building (center). Notice the Railroad Box Car (Middle left) white top, laying in the Provo River. The Avalanche knocked it off the tracks and into the river. Notice the destroyed trees, and all of the scrub oak has been pulverized. Each year brings different hazards. Understanding them can save lives and prevent property damage.
This is the old bridge across the Provo River (buried), below Bridal Veil Falls Notice the pulverized trees on the surfaces. The area has been scoured to the ground. This was a mid season avalanche. Unlike many spring avalanches, this may have bee going over 100 mph and the avalanche blast was. . . !!!!!###%%%^^^
Bridal Veil Falls Tram Building remains. Notice the Train Box Car (center) laying in the Provo River. The pulverized branches and trees laying all around.

Bottom line.

These hazards can be identified and evaluated so that we can  eliminate them, or mitigate (lessen the potential for release and minimize the effects associated with the hazard. )

Unfortunately, this home was destroyed during a snow season cycle that occurs maybe once every 50 or 100 years. Had they hired us to do a pre-construction evaluation we would have been able to warn them, and suggest ways they could have still built their home their.
Of course, “I do not know it all, but experience and study has taught me more than I used to know.

I, along with a great team of professionals, are looking forward to saving lives, preventing property damage, and much more. Please do feel free to contact us with any questions or concerns you might have.