View Full Version : Hand shear relativity
02-12-2005, 09:07 PM
this is a good post i found on t-tips and i thought it might help out over here.
The hand shear is an excellent way of evaluating the bond of a relatively shallow layer to an underlying layer. A hand shear test is most easily done by digging the sides, bottom, and top of a block of snow (approx 12" by 12") using the handle end of a pole. Excavate down to and slightly through the suspected problem layer. Alternatively, in very soft snow, I may use just a gloved hand to create the block. I then spread my hands to support the back of the block relatively evenly and pull planar to the slope. So both Saturday and Sunday on slope angles of 27-35 degrees I guessed it took about 1 or maybe 1-1/2 pounds of pressure to fail the block at the weak layer (actually an interface in this case), an ice crust.
In the case of the hand shear, I am applying force planar to the slope angle. In a RB or CT test, I am applying force vertically, so only a certain portion of the force is directed along the slope plane according to the Sine trigonometric function. Looking up "trigonometric tables" on the internet, I can readily find these tables according to slope angle. The value of the sine function for a 25 degree slope angle is .42,
for a 30 degree angle, .50, and for a 40 degree angle .64. This can be easily understood from diagrams like this:
Therefore, when you apply pressure downward in a Rutschblock or Compression Test, only a portion (see numbers above) of the force is directed along the slope plane (which is the orientation of shear layers.
So, if I get a handshear of 1-1/2 lbs, when I am applying force along the shear plane (slope), then since a RB in round numbers is 5x6 or 30 square feet, it would take about 30X 1-1/2 lbs or 45 pounds of planar force in a RB to yield failure. Taking a 40 degree angle, 45 lbs/.64 =70 lbs ought to do it. But on a 25 degree slope, 45lbs/.46 = 107 pounds would be required. You can readily see the effect of slope angle.
Because of other factors, of course, (depth and structure) things aren't really this clean. But still, I can get the feeling that a 1-1/2 lb hand shear sure seems skier releaseable .
Whether a slab will release and propagate is another issue.
any thoughts or discussion?
02-12-2005, 10:17 PM
How does one estimate planar force without having used some sort of gauge to get a feel for it?
02-12-2005, 10:35 PM
good question. maybe take specified weights ( gallon of water at 8lbs, 5,3,&1lb dumbells) and put them on an incline (maybe plywood). at that point just hold your hand underneith them and get a feel for the force. change the incline to get a feel for steeper/mellower slopes.
i think the important thing is to get a feel for doing consistent tests that you can quantify to yourself regardeless of exact numbers.
02-14-2005, 12:51 PM
Can't remember what it is called, but there is a device that you can put on the back of the column when isolated. It is similar to a blood pressure cuff in that you fill it with air. A gauge reads the amount of air in the bladder and translates it to force . The column will fail once the force created by the air bladder is greater than the shear strength of the column. Once the column fails, you have your force required. It only measures planar force. It is certainly overkill for a nonprofessional bc traveler, but doing these types of tests over and over gives you a pretty good "feeling" for the amount of force required to have a slope fail.
02-17-2005, 11:06 AM
It seems that Tremper doesn't really like shear tests in general because they are so difficult to quantify. I can see that Gary is interested in hand shears as a quick test that can be done frequently but he's also an engineer and gets a lot more excited about applying trigonomic functions to avy questions than most people do about sex.
In Tremper's book he indicates that he hardly every uses shear tests any more and really like the shovel tap tests or compression tests more, of course this involves full on pit work. I can't remember if he really addresses a quantifiable way to address shear besides standard 1,2,3.
APD, do you feel that you've had success in getting a general read on snow pack and stability using hand shears? What other quick tests do you like? Stepping above the skin track and pushing down, hand shears, pulling on the snow with a pole basket, all interesting but unquantifiable.
BTW, what really constitutes a quick pit or hasty pit? Just digging down with your hands and doing some quick hardness tests or still pulling out the shovel but not spending a lot of time on other tests besides looking at layers?
02-17-2005, 01:11 PM
I rarely do shear tests. Most of the test's I do require very little time or effort. I generally get a good idea of the layers by pushing the grip side of my pole about every 100 vertical feet. I also use the baskets to pull up some of the crystals from down below. Not a great dea lof scientific eveidence gathering, but enough to give you an idea ofthe general make up of the snow pack.If I find something that raises a red flag, I'll pop the shovel off and dig around for a few seconds to see what's below. I like the compression tests as well, but they require experience to make the results tangible.
Ski cutting small slopes and cutting/stomping (when small) cornices give you a pretty good idea what the top of a slope will be like. I also look for small test slopes on the way up, like small little ravines or roll overs that are relatively safe to jump on (not anything that is under a slope capable of sliding or that is remotely close to a runout zone)
02-17-2005, 03:55 PM
i think the important thing is to get a feel for doing consistent tests that you can quantify to yourself regardeless of exact numbers.
02-17-2005, 04:53 PM
those hasty pits or pole pits as i usually call them are just a quick and simple way to get a look at the new snow layer(s). you can cut out a small block on the upside of your skin track by poking your pole in and isolating the block. once isolated you can do a quick shear test that should let you know how well bonded the new snow layer is to the old layer. after that i get a good look at the backwall of the pit and feel for any density inversions or faceted layers. you should aready have an idea if there are density inversions if you have been pole probing (like cololi mentioned) or breaking trail. stepping above the track, like you mentioned, is also a good way to tell if the snow is holding well. when digging a pit i usually do a 3 column 4-6' pit. it takes about 8-10min max if you are used to doing them. on the first column i do shovel shears. the second is a compression test and the third is used for any other test that i think may work best to identify the layers i am most worried about. i always feel the back wall of the pit for facets, density inversions and crusts. remember that your one pit is not always representative of the whole slope you are about to ski (spatial variability).
the bottom line is that i don't put too much faith in any one test. it is a combination of being out front and setting the skin track, feeling the snow, seeing how it is reacting and then lastly i use my test info. i try to confirm instability rather than the opposite. most of my information is already gathered from snotels before i leave the house. starting at the trailhead is where i gather my empirical data that should supplement what i already know. always start with a hypothesis and either prove or disprove it by the information you gather along the way.
btw, cololi mentioned the test slopes and cornice tests. they are both great and should not be passed up if they can be done safely. i don't mean go and drop cornices on every super weak slope you find but it is a great test before skiing a slope. just remember that a cornice may not always be big enough or hit the right spot to trigger the slide. check out the slope cut thread (http://tetongravity.com/forums/showthread.php?t=25195) too.
edit:trempers tests (http://www.couloirmag.com/articles/avy/tremper_series/part1.asp)
the thing that makes me really nervous about this hand shear test is, that it derives the quantity of shear forces a layer might support from the RB test, whereas the RB is a measure for the quality of a snowpack. so he is dealing with two completely different things. nevertheless he derives his theory from the one and applies it to the other. this might be quite some source of errors. multiplying those potential errors by 30 definitely doesn't make me feel more comfortable. in addition, I think the precise size of a RB is not that important (since it's a quality and not a quantity test). It has been simply chosen to have a comfortable surface to jump on.
besides this basic criticism, there are other grave sources of error.
1. maybe not so important but still bad enough: slight changes in sample size (as any other errors in the setup) are taken times 30.
2. wearing a heavy backpack or a light one, being tired or not, having climbed a lot, or just using your upward arm or your downward arm after a traverse is going to have a huge impact on subjective measurements of pullout forces. it's like measuring watertemperature with your hands. 10°celsius compared to 0° is quite comfortably warm, 10° versus 20° unpleasingly cold. those errors are again taken times 30.
to sum it up. i always do think it's a good idea to touch the snow (with a glove on) and to take a close look at what you are dealing with. but i honestly don't think one should try to derive qantifiable measurements out of this test or even in some sort regard ist as a quick substitute for a RB (since it's a quality and not a quantitytest)!
my kind of tardy thoughts, greg
I don't know, if anybody is still interested in this, but i asked an avyexpert on this and he confirmed me in my doubts. the problem is, that this test is highly deceiving because it is going to fail in those "hard to tell situations" that are at the same time very tempting for skiers. in those situations the result of the test might tell you, that an rb would hold maybe the double of the weight of skier whereas an actual rbtest would easily colapse..
so i really hope nobody is using this "test" including the author and he is not spreading it any further on the internet. maybe, somebody who knows this guy could actually inform him.
07-15-2005, 04:23 PM
greg, regardless of how your compare your test results to each other you should have a general idea of how a slope will rate in all tests just by looking at it. the tests are geared at finding out different info with some overlap but you can generally predict the results of the next test based on what you saw in the pit or from the first test. quantification of this stuff is extremely hard. i'd rather have a general idea than exact numbers because you run the risk of getting too rule based with numbers. there is a cross-relation between all tests but the formula the brain puts on it is too complex to break down. just rest assured that if your head tells you that something is wrong, then it is a no go.
i just don't see the point of creating a test which is most likely misleading in exactly those situations it was thought up for:
example: shallow layer, bonded and soft but at the same time brittle snow. relatively weak layer underneath. << i chose this example, because it is 1. what the standard skiers avalanche is made of and 2. it is what this test was thought up for: surface slabs of relatively shallow layers.
if you do an rbtest under these conditions, the forces that are exerted on the snowpack are never going to be spread over the whole surface of the rb, due to the brittleness combined with the shallowness of the slab. the force effects maybe only 50% or even one third of the rb surface, but it is still going to fail.
if you had a hard slab sitting on the weak layer or if the slab was simply thicker chances are good, that the exerted forces on the rb will be spread over the whole surface. in this case the rb might not fail.
this is a source of error of 200 to 300% to your disadvantage which in my oppinion is more than considerable. (there is obviously no need to even use trigonometry to eliminate the comparatively small error given by different slope angles)
i just don't get the point of creating a test, which is most likely going to fail in those situations it is made for (eg shallow surface slabs) and wich are at the same time tempting for skiers. it's just creating a false sense of security. that is all...
edit: the conditions in the example i discribed are probably comparable to the conditions during your failed ski cut. it's exactly these conditions which are most hazardous to skiers, and it is under exactly this kind of conditions under which this test is going to fail to the disadvantage of the skier's security...
07-16-2005, 04:03 PM
i'm not sure i understood your complaint before. it seemed like you didn't like the comparison of the two tests but now it looks like you have a problem with the rblock. i don't make these test up but use them when i need them. my choices to ski slopes are typically based on what i don't know about a slope rather than what i know. if you read bruce trempers report you will see the layering in the ski cut you speak of. had i even had a look at the layers in the pack w/o doing any tests i would never have ski cut the slope. all that aside, these tests (hand shear, rblock, compression, etc) are all designed to help give a piece of the pie from which to make decisions. i'm still not sure i see what you don't like here but i think any snow testing will go thru a critique process that can only make it better understood.
the danger i see in this test is, that your conclusion might be deceiving.
with a rbtest, you don't look at the shearforces which the snowpack might resist but at the whole chainreaction that leads to a slide. in order to do that you isolate a certain sample size and then you step or jump on it. due to factors like brittleness, layerthickness, impact, piercing of the layers.... the initial forces you exert on the rb are in many cases not going to affect the whole surface of approximately 3 squaremeters (eg see example explained in my last post), but just a small portion, which is going to fail and than as a chainreaction leads to the collapse of the rb as a whole.
what i'm basically critizising is that gary is using the whole size of a rb in his calculations which is misleading. he should use only a small portion of the surface of an rb to compare it with a sheartest and be on the safe side.
it's hard for me to explain in english. but what i'm basically saying is, if you do a test of shearforces on an rb you might get results telling you, that it is easily going to support a skier. then when you step on it, it will collapse if the skiers weight doesn't effect the whole rb. this is especially the case for shallow, brittle yet soft layers.
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