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padge
Nov 5, 2004, 11:16 PM
Post #26 of 33
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Registered: Feb 18, 2004
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Surface adsorbtion can have big impacts on the pore pressure of the water within the sandstone. Even small changes in moisture within the air can affect the energy of the chemical bonding within the Sandstone.
If you apply the ideal gas law for sandstone in a vacuum vs. sandstone in the presence of saturated air (at standard temp and pressure). The difference in the surface energy between the two states is 300 mJ/m^2. This also increases the ratio of pore pressure from 0 in a vacuum to 0.8 in saturated air. Summarized from "Physics of Rocks" by Gueguen and Palciauskas.
Many factors can affect the strength of rocks when wet. Michalske and Freiman (1982) suggest adsorbtion of water molecules leads to hydrolysis of Si-O bonds, which are replaced by weak O-H bonds.
Another factor may be smectites (swelling clays) within the matrix of the sandstone. The introduction of water within the T-O-T (tetrahedron octahedron tetrahedron) sheets produces expansion of several times their original size. This can be seen anywhere adobe clay is present, by large cracks in the soil in the winter and heaving foundations in the summer. When performing earthwork with adobe we recommend that the clay is moisture conditioned to 4% above the optimum moisture content, and compacted to 88 to 92% relative compaction.
This expansion accompanied by increase in pore pressure may lead to the weakness we experience in wet sandstone. I too have broken holds on wet days, but that might just be my fat ass or lack of technique.
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padge
Nov 6, 2004, 5:50 AM
Post #27 of 33
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Registered: Feb 18, 2004
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Edit. Dessication cracks in the summer and heaved foundations in the winter. Whoops.
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anykineclimb
Nov 6, 2004, 6:04 AM
Post #28 of 33
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Registered: Mar 30, 2003
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In reply to: 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. |
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coldclimb
Nov 6, 2004, 6:04 AM
Post #29 of 33
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Registered: Jan 14, 2002
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Science might say water won't soften sandstone, but I've also been told that it's proven that slacklining can't help your climbing. ;) All those people can't all be wrong.
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karlbaba
Nov 6, 2004, 6:24 AM
Post #30 of 33
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Registered: Jul 10, 2002
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I believe in science way more than Bush does, but sometimes it fails to see the big picture.
For example, a while back I read a study that claimed to prove that the friction of climbing shoes on rock was not affected by the rock being wet. They had the friction coefficient to prove it.
Things that seem complicated are often simple. and Simple things are usually way more complicated than they seems. Too many factors weigh in that theory fails to consider
Peace
karl
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skinnyjim
Nov 6, 2004, 9:10 AM
Post #31 of 33
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Registered: Dec 1, 2003
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As an AP Chem student one thing comes to mind. Solubility constants. Everything is soluble to some degree and in some types of rock this small degree of errosion due to the solubility of said rock and its composing elements maybe be enough to make a noticeable change to us climbers.
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rrrADAM
Nov 6, 2004, 12:58 PM
Post #32 of 33
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Registered: Dec 19, 1999
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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.
For an expert, you need to sample more rock brutha... Go check out the rock at Garden Of The Gods in Colorado, or most of the Navajo Sandstone out in Southern Utah, Northern New Mexico, and Northeastern Arizone. Don't know how much you travel, but not all sanstone is as solid as that in the South. Hell, even Stoney Point in Southern California is little more than super hard dirtclods.
Look for more research/data on those types of rock, or better yet, "touch it" and see if you still hold the same opinion. :wink:
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grigri
Nov 6, 2004, 2:32 PM
Post #33 of 33
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Registered: May 23, 2004
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Its no myth. I live in South Afica where most of the rock is very hard quartzitic sandstone. After rain the more granular or rough textured rock at the base of the boulders in some areas becomes extremely friable, the smoother more polished looking rock (more compact?) on top of the boulders and in other areas seems unaffected however. I have heard that the locals at Fontainbleau (also sandstone) prefer it if you dont climb immediately after the rain to avoid damage to the rocks.
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