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justen
Nov 5, 2004, 8:38 PM
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Ok,
I have seen a lot of verbage about rocks being more prone to break when wet a lot on this site, including comments about not climbing until the rock dries.
I am a hydrogeologist and working on my masters in geology-
I can think of no mechanistic reason to support sandstone (or any other rock) being prone to breakage when wet.
If you increase the pore pressure in rocks you can "hydrofrac" (in drillers terms) a rock unit lowering the youngs modulus of the rock. These are usually very high pressures though, and surface exposures have a pore pressure of virtually nil.
I think this is just an old climbers tale...
unless it is simply the rock being wet and people falling more often, jerking on holds.
I would love to hear any plausible explanations.
Justen
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j_ung
Nov 5, 2004, 8:40 PM
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Edit: what a tencnologically challenged person I am.
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j_ung
Nov 5, 2004, 8:42 PM
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I'm not knowledgeable enough about geology/hydrology to argue this intelligently, but I've heard plenty of anecdotal evidence to dispute your assertion. I suppose it's possible that, yeah, it's an old climbers' tale, so to speak.
Thankfully, I live on the EC where the sandstone is almost always bullet hard, regardless of its water content. :D
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killclimbz
Nov 5, 2004, 8:45 PM
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Go climbing at Garden of the Gods after a good soaking, holds become more friable at places with soft rock, and tend to blow. You'll notice it. There is plenty of bulletproof sandstone that it's not a worry, such as Eldorado. I've climbed on that rock plenty, right after a soaking.
Remember we are not talking about the overall structure of the rock, just the skin so to speak. Holds can weaken on certain types of rock when they get wet. If you don't believe it, you haven't been climbing much.
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taualum23
Nov 5, 2004, 8:46 PM
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I also lack the hydro/geo backround to argue scientific-like, but ask the old hands in red rocks. I REALLY doubt it's an old climbers' tale.
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petsfed
Nov 5, 2004, 8:46 PM
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Okay, then soak a piece of sandstone in water, then try to break it with your bare hands. Then compare with the amount of force to break a dry piece of sandstone. You will see the difference.
Just because you can't think of a reason why it is so, doesn't make it not so. In other words, when the theory doesn't fit the data, don't refute the data.
For the longest time I couldn't understand or explain how TVs work, but they still worked.
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imnotclever
Nov 5, 2004, 8:51 PM
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My guess would be that as you get pore water pressure you get greater lateral pressures on the rock. Especially in sandstone where you could get continuous column of water in the pores and have massive lateral pressures develope.
This explanation is better thought of as an analogy to retaining walls. Be they retaining sand or clay, having a water table present increases the lateral pressure (62.4 * height of water) more so than with the just the soil and much more than the break you get from the reduction in the soil lateral pressures from buoyancy.
Things to think about. Boiling in retaining walls and upward seepage forces cause the reduction in laterial pressures. I'm thinking that the running of water from top to bottom in the rock is at a slow pace and it is this that causes a complete water column and therefore the great lateral pressures.
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pancaketom
Nov 5, 2004, 8:51 PM
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It depends a lot on what is cementing the sandstone together. If it is something that is easily weakened by water (like clay or salt), then the rock will be weakened by water. This is probably especially true for things like the desert mud towers.
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pk
Nov 5, 2004, 8:53 PM
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Climb in Utah after 2 days straight of rain then tell me rock does not absorb water and become a little more brittle.
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wlderdude
Nov 5, 2004, 9:05 PM
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Many materials that are porous loose strenth when they absorb moisture.
Go get a piece of paper.
Get it wet.
Notice how much weaker it is.
Fiberglass and other composite materials weaken substantially when wet, but of course to a lesser extent.
I learned some of the mechanisms for this kind of weakeneing in a class in college, but I can't remember them and don't have my text book any more. I do remember that the math for % mass change (ie water absorption) works out very much like the decrease in strenght you get from temperature changes in composite materials.
Mineral based materials are a bit of a mystery to me, so I can't comment on the properties of sandstone. Perhaps phenomena from composite materials don't correspond to those of sandstone, but perhaps they do.
However, Young's modulus is a property that is measured under very idealized conditions and is only usefull if you understand the assumptions and limitiations of your information.
Whatever you do, don't be stupid and assume that if you can't understand the mechanisms for the wekening, that the material doesn't weaken. All material science really does is try to explain tha physical phennomina OBSERVED in materials and find udefall ways of describing them. Do some observing before you run to your books to reach conclussions.
From first hand experiece I know bricks are weaker when they are wet, dirt is easier to dig when it is wet and I have no reason to doubt sandstone is weaker when wet. So when people tell me from their first hand experience it is, I believe them, regardless of my deficient understanding of water's effect on Young's modulus of sand based materials.
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travelin_light
Nov 5, 2004, 9:06 PM
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there might be some vulnerability to the part of the rock we consider to be a "hold" to become affected by water infiltration. for example for a feature to develop on a rock over time to create a "hold" there has to usually be some type of parting or a "flaw" in the rock to become succeptable to the elements in the first place. water may be simply finding its was way back into the rock each time along this "flaw" after a significant rain event. cementation and/or overall inherent rock strength may become more succeptable to breakage from this. there may be many forces at work here. but overall... water defintely can't make the rock stronger...right!
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pbjosh
Nov 5, 2004, 9:08 PM
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In reply to: I can think of no mechanistic reason to support sandstone (or any other rock) being prone to breakage when wet.
As others have said, a failure of reasoning / theory does not change the facts.
If you climb on some types of rock (notably some sandstones) when they are good and wet after a good dosing you will break holds.
There are sufficient cases of this to go beyond anecdote and be statistically valid at Red Rocks - particularly on sport routes that people refuse to let dry after a good rain. Holds break under all conditions, yes, but the frequency with which they break, relative to the amount they're climbed on, goes up tremendously the wetter the rock is.
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camhead
Nov 5, 2004, 9:10 PM
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on a similar note, you can often retrieve stuck cams from soft rock like wingate sandstone by squirting water onto the rock around the lobes.
FYI.
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imnotclever
Nov 5, 2004, 9:23 PM
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In reply to: on a similar note, you can often retrieve stuck cams from soft rock like wingate sandstone by squirting water onto the rock around the lobes. FYI.
That might be more along the lines of how water reduces the friction. There might be more to it than that as well.
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tico
Nov 5, 2004, 9:25 PM
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A little knowledge is a dangerous thing, huh? The original poster is applying specific theories to a problem, finding an answer which is totally out of whack with experimental data, then declaring the (massive amount of anecdotal) experimental data to be false. Sounds like a dude fresh out of college to me.
Sedimentary rocks can become more friable when wet because water dissolves the cement that is holding the grains of rock together, this cement can be calium carbonate or silica.
The sandstone in ol' kentuck isn't as prone to this problem, but i've broken holds in the red and the new while climbing after several days of rain.
And don't let me get started on all the "rain on pitch 9 or 10 or 11 in Zion" pants-filling religion-getting semi-epics I've endured.
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curt
Nov 5, 2004, 9:28 PM
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In reply to: Ok, I have seen a lot of verbage about rocks being more prone to break when wet a lot on this site, including comments about not climbing until the rock dries. I am a hydrogeologist and working on my masters in geology- I can think of no mechanistic reason to support sandstone (or any other rock) being prone to breakage when wet. If you increase the pore pressure in rocks you can "hydrofrac" (in drillers terms) a rock unit lowering the youngs modulus of the rock. These are usually very high pressures though, and surface exposures have a pore pressure of virtually nil. I think this is just an old climbers tale... unless it is simply the rock being wet and people falling more often, jerking on holds. I would love to hear any plausible explanations. Justen
I'm afraid I can't provide you with the scientific mechanism, but the statement is true. And, it is true for many types of rock including Hueco Tanks syenite and Oak Flat dacite--as well as sandstone.
Curt
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crankenstein
Nov 5, 2004, 9:45 PM
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I agree that it's the cement that largely defines the phenomenon. More soluble cements = rock that is more weakened by water.
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miklaw
Nov 5, 2004, 9:59 PM
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Even Nylon ropes are weaker when wet.
And my ankle is too! I broke it soloing on wet rock (pulled a good jug off).
In Dresden /Ebelsandstein you're not even alowed to climb after rain fo a bit
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md3
Nov 5, 2004, 10:07 PM
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I bow to science, but please scientist man, don't break my favorite soft sandstone climbs.
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davidji
Nov 5, 2004, 10:23 PM
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In reply to: I am a hydrogeologist and working on my masters in geology- I can think of no mechanistic reason to support sandstone (or any other rock) being prone to breakage when wet. |
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knoxville
Nov 5, 2004, 10:29 PM
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For a hydrogeologist, you're missing the big picture. As many people have posted earlier, any sedimentary rock with calcareous cement is subject to dissolution from contact with acid, such as meteoric water. Also on the granular scale, higher water content decreases the friction between individual grains.
I'm not sure what cliffs you've climbed on that prompted your post, but the reasons lots of sandstone cliffs are climbable is large failures at bedding planes creating steep angle faces. What's a bedding plane if not a large preferential flow path for water to move through a rock unit? Maybe we shouldn't be talking about failures on the outcrop level instead of on a lab scale (Young's modulus, etc.).
I'm sure there are lots of articles out there on triaxial tests of rocks when saturated. Check georef or something similar.
If anyone's interested, there's a pretty basic but interesting article on the geology of West Virginia climbing areas in Mountain State Geology (1987). Not totally relevant to this thread but a good discussion of sedimentary geology relative to climbing.
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apolobamba
Nov 5, 2004, 10:36 PM
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..................
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cloudbreak
Nov 5, 2004, 10:38 PM
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What your saying is true, BUT, not all rock is the same. Nor is all sandstone the same.
Take into consideration poorly indurated, or cemented, rock. The marine sourced sandstone around Santa Barbara is of this characteristic, and therefore is extremely friable, or chossy in climbing terms, especially when wet. Hell, when it's dry you can carve your intitials in it with your finger nail. Now imagine what it's like when wet!
How or why you were correlating subsurface well fracing to that of holds breaking off on the rock we climb on is beyond me.
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iclimbtoo
Nov 5, 2004, 10:40 PM
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Yeah, what knoxville said.
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t-dog
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Nov 5, 2004, 10:58 PM
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yeah sure, but out of personal experience, holds DO break after a rain, it's not a myth if you're climbing on sandstone with little flakes :shock:
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