Arching in backfills
Arching in backfills
(OP)
I was asked to look at the potential for some rough (100mm down) backfill material to arch against a revetment and a quarry face (I've even drawn you a little diagram).
http://img4.imageshack.us/img4/8317/arching.jpg
Do you think this is plausible depending on the height of the fill and if so can anyone point me in the direction of some kind of document that might tell me how much arching there might be?
Cheers guys.
http://img4.imageshack.us/img4/8317/arching.jpg
Do you think this is plausible depending on the height of the fill and if so can anyone point me in the direction of some kind of document that might tell me how much arching there might be?
Cheers guys.





RE: Arching in backfills
f-d
¡papá gordo ain't no madre flaca!
RE: Arching in backfills
RE: Arching in backfills
Is there anything else you can tell me that might help me out? I don't think the mid term disolution of the clay over the boundary should be too much of an issue but it will be something I raise.
Cheers.
RE: Arching in backfills
f-d
¡papá gordo ain't no madre flaca!
RE: Arching in backfills
Do you have Winterkorn and Fang? In the 1975 edition, it's in Chapter 5 by Arpad Kezdi. On page 216, see "Pressure of Sand Located Between Two Rough Vertical Walls."
Also, Frydman and Keissar, "Earth Pressures on Retaining Walls Near Rock Faces," ASCE JGE June 1987, vol 113, no. 6, pp. 586-599.
My impression is that if the clay can compress a little and/or steel revetment can "give" a little between the tiebacks (which I suspect is the case), the horizontal earth pressure would be much less than the classic Coulomb or Rankine active pressure because the backfill is so thin relative to the assumptions that go into those models.
However, if they can't give at all, there is potential for arching to cause very high lateral pressure and very low vertical pressure in the back fill. This is a big deal for cutoff trenches for earth dams. If there is arching across a trench that is too narrow, the vertical pressure can be less than height*gamma, and when the reservoir is filled, the water pressure can be high enough to pry cracks open, admitting water easily, which can lead to erosion and really bad things happening.
Regards,
DRG
RE: Arching in backfills
Cheers for the references on the literature front. I don't have those books to hand but I'm going to try and track the information down.
I'm interested that you mentioned that the fact that the thin layer of backfill in relation to the Rankine/Coulomb model will mean that the active earth pressures are much lower. I was wondering if you knew how much difference there would be? (Order of magnitude, no accuracy required)
They've actually started testing this revetment and backfill combination and have had some problems with the stability of the steel revetment frame which is something I haven't even begun to cover yet. It's a horrible case of someone trying to use an old design for a new purpose and not really thinking it through.
Cheers for the reply.
RE: Arching in backfills
BTW - no cement in the clay liner, as was suggested on the other thread! Keep it as ductile/plastic as you can for avoiding cracks. That cvg fellow generally gives advice that should be heeded.
DRG
RE: Arching in backfills
Can you expand on this a bit? Most of the dam cutoff trenches I have seen are filled with clay or concrete or geotextile / HDPE liner or possibly even sand to capture seepage - but not usually with rock backfill. Would clay arch? I guess I don't know what "stoney backfill" means in regards to the OP. Is that soil with a few rocks or rocks with a little soil?
RE: Arching in backfills
Nobody knows exactly how erosion initiated, but one of the credible explanations is that the relatively narrow, vertical-sided trench in fissured volcanic bedrock allowed the fill to arch. This meant that the weight of overlying fill was carried by the trench walls, rather than as vertical stress in the core backfill. When the reservoir filled, this could have introduced 150(?) feet of pressure head in an area of the cutoff trench where there was little vertical stress in the fill, much less than 150*62.4. (Finite element analysis for the failure investigation indicated that Sigma-v was pretty low.) Water seeping in could have hydrofractured (wedged open horizontal fractures), which would allow the water to flow through, cause erosion, etc., eventually causing 11 fatalities. There was no filter on the downstream side, and the discontinuities in bedrock were not sealed by grout or concrete. There was a thread about this a few months back that had some pictures to go with it. http://www.eng-tips.com/viewthread.cfm?qid=253131 I see that you were part of that. The pictures might be worth 1000 words.
The portion of the cutoff in alluvium had flatter slopes, and probably would never have been a problem.
Not sure about clay, but clayey sand can arch, as can low-plasticity silt. Clay might be able to if not placed wet of optimum.
Concrete and soil-cement are, of course, not so easily erodible, but they aren't always economically viable. Sand, by itself, would not make a good cutoff, but modern practice often includes a sand filter to back up the cutoff.
Did I answer the question?
RE: Arching in backfills
RE: Arching in backfills
RE: Arching in backfills
RE: Arching in backfills
One of Marston's ideas was called the Imperfect Trench and I have used it on occasion when it was necessary to place added load over existing buried conduits that could not be taken out of service,such as sewage force mains. No doubt there is arching and Marston took advantage of that.
Here is only a little of a Google search.
http://eng.lacity.org/techdocs/str-man/h-300.pdf
http://cedb.asce.org/cgi/WWWdisplay.cgi?0701176
RE: Arching in backfills
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: Arching in backfills
I admit to not being a geotech guy, just a practical construction engineer.
I'd really like to know how you expect to place a uniform 0.5m thickness of clay against the steel and then get the stoney backfill material into place without intermixing them.
The detail looks fine on paper - how is it accomplished in the field?
Ralph
Structures Consulting
Northeast USA
RE: Arching in backfills
RE: Arching in backfills
msucog I understand that a close approximation of the design intent can be accomplished - I'm just curious as to how it's done, and how close the final result really is.
Ralph
Structures Consulting
Northeast USA