Rockfall Question

[TrogdortheBurninator]

I feel incredibly bad for the friends and families of those involved in the accident, and don't feel it is respectful to include a question in that thread. Instead I will pose it separately.

 

Rockfall is one of those things that always scares the crap out of me in the mountains. While party inflicted rockfall can be minimized by experience, natural rockfall will occur regardless. I recently watched a program on discovery on waste water induced rockfall in yosemite, where it specified that hydrostatic pressure from water was the major cause of natural large scale rockfall. Obviously melt freeze is a major concern for rockfall in the mountains, but how much consideration do the more experienced mountaineers here give to precipitation when assessing objective rockfall risk. What might be tell tale signs that this is a concern? I am not speculating what may have caused the rockfall in the accident. I am merely looking for suggestions so that I may make more informed decisions in the future. I really hope this doesn't appear insensitive.

[Ireneo_Funes]

I'd guess that rising temperatures have more to do with increased risk of rockall in the mountains. I think it'd be pretty hard to calculate increased risk of rockfall from precipition, at least for the mountaineer who's in an area for a short time only... but what do I know?

[Dru]

 

You need a lot of rainfall and a lot of rock. mUch more likely to affect rockfall in the valleys than rockfall in the mountains. And if its raining in the mtns that bad you are probably pulling plastic in the gym.

[TrogdortheBurninator]

Now that I think about it, I guess I read this in a magazine. Anyhow, I thought that the rockfall was occuring after heavy rain, not during. This would be more akin to going to bed in a downpour and awaking to a beautiful morning, and being forced with a decision as to whether this type of rockfall would be a concern.

[Bill_Simpkins]

These are some personal guidelines, probably left some out.

I avoid resting in areas with loose rock above.

I move fast in places with loose rock above.

If you are in a talus field, how fresh does the talus look?

The rock chunks got there somehow.

Look at the base of the face you are on, is there lots of fresh rockfall?

Does the snow below the rock have rock on it or signs of recent rockfall?

Is it warming up fast? Be extra careful.

What is your gut feeling? Speed up? Get out of here?Follow it, within reason.

Depending on place and rock, rockfall is a bit worse early season.

What is your plan if you see rocks hurling toward you? Run back? Lay down? Is there a safe spot very close?

Are there other climbers around? Could you hit them, can they hit you?

Will your rope bring lose rock with it when you pull it?

If you the rock that you are holding onto or standing on pulls will you fall? Do you have something else on something that may not pull with it?

Do you hear other rockfall around you?

 

I like travelling alone around loose rock or not talking so I can hear whats going on around me. I get a little nervous when people are blabbing away and theres choss city above us.

 

Biggest rule, move fast and careful.

[Dru]

Yeah you read it in Climbing or Rock and Ice an issue ago. Basically this type of rockfall is not really an issue above treeline because freeze/thaw and snowmelt induced rockfall are so much more common that they strip off rock so fast (relatively speaking) that the big masses never get the chance to fail.

 

For instance, look at the "Perkins Pillar fell over" thread in the BC forum. That fell over because the glacier that lies under it, undermined its support. Not because of rainfall.

[Szyjakowski]

go with your gut feeling. it usually is correct...when in doubt, go to the bar. rockfall can also be triggered by seismic or volcanic activity not just weather...the first two generally are not predictable...course neither is the weather in the cascades. so, seriously, when in doubt, go to the bar...if you are underage...you are S.o.L

[Dru]

Signs that large rockfalls may be about to occur include -

 

widening cracks, IE, topo says 2" and crack is 6"

small rockfalls from the base of flakes and pillars

creaking, groaning noises

grating sounds

dust drifting out of cracks

new cracks forming on what used to be blank faces or clean slabs

 

if you see any of these signs, run away

 

some or all of these were reported immediately before the rockfall on Trango tower (see Alpinist 11) and the 1990's rockfall on the Dru in Chamonix.

 

neither of the above mentioned rockfalls had anything to do with rainfall.

[Szyjakowski]

Is gravity the culprit?!?!?!?!

[Dru]

Does a change in the factor of safety result from gravity

[TrogdortheBurninator]

if pressure = density * gravity * depth, then maybe

[Dru]

Most bedrock cliffs are metastable anyways. But the release of strain (or is that stress, ) takes 10,000 to 20,000 years due to the elastic modulus of granite being low, so...

[TrogdortheBurninator]

probably release of stress, and probably a result of the ratio of the storage and loss modulii, not just the elastic modulus which would assume a hookean type solid incapable of stress relaxation

[TrogdortheBurninator]

Although even 10,000 to 20,000 years seems like a surprisingly short time scale for these type of phenomena

[Dru]

yeah but the glaciers only melted 10,000 years ago

[TrogdortheBurninator]

good point, although it seems a more likely mechanism would have to do with some critical flaw size and flaw growth. However, I know nothing about geology and this is pure speculation.

[Dru]

all i'm parrotting for you is a rock mechanics course i took about 9 years ago. however, that's just slightly better than knowing nothing. it DOES say in my standard disclaimer that Assessment of the potential for bedrock failure is beyond the scope of this report

[TrogdortheBurninator]

I am just a nerd who has crapped his own thread for the sake of interesting science speculation.

[iain]

in structural geology:

stress = force applied to a material

strain = magnitude of displacement of material

[TrogdortheBurninator]

so they are not normalized as in force per area and elongation per unit length?

[Szyjakowski]

strain is what you see in rocks... like stretched pebbles, dikes/sills or minerals.

what quanity is visible depends on the strength of the rock body (composition, previous fractures, etc.) and the amount of force (stress) good ole baby jesus inflicted on it.

but, don't listen to me I am only a field geologist with a big mouth for

[iain]

so they are not normalized as in force per area and elongation per unit length?

 

actually they are. stress is force per unit area, usually measured in pascals

1 Pa = 1x10^-5 kgm^-1s^-2

[Szyjakowski]

[iain]

you know you love it

[Szyjakowski]

i can't belive you beat me to the punch....i guess I just had more to say.