Arching in backfills 1

[NJonesUK]

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.

 

[fattdad]

Are you asking whether the horizontal earth pressure will be reduced owing to the limited spacing between the exposed face and the rock face? If so, then yes, the horizontal earth pressure will be reduced. I'm not sure that "arching" is the correct soil mechanic, however. If you are asking whether the vertical stress at the base of the soil will be less than gamma*z, then I'd doubt it.

f-d

¡papá gordo ain’t no madre flaca!

 

[ishvaaag]

Of this detail I would be more concerned by the midterm disolution of clay at the rock backfill-clayey material interface upon iterated washing. This could produce some movement at the surface upon migration of fine material to the voids in the backfill. Whilst the non-coherent status of the right part of the backfill is preserved, you may calculate the push of that part by some references; old Spain's code MV-101 I think to remember had the case. Combined with the presence of the clayey material that except when of some particular use or by some kind of mandate must be used, the lateral pressure of the pair of vertical layers is more difficult to determine.

 

[NJonesUK]

I suppose this 'arching' I'm concerned about is sort of like the effect produced in silos or the backfill of trenches over pipes where you get relief of pressure due to the specific settlement in soils.

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.

 

[fattdad]

Can you please let us know if you are referring to horizontal forces or vertical forces?

f-d

¡papá gordo ain’t no madre flaca!

 

[dgillette]

I'm having a sense of deja vu all over again, vaguely recalling an almost identical sketch I saw recently. Is this a liner for a MSW landfill being constructed in an old quarry? Revetment has tiebacks into rock? Danged if I can find it on eng-tips, however. Anybody else recognize it?

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

 

[NJonesUK]

DRG, you're bang on the money. It was my diagram last time though I'm asking a slightly different question this time. I must admit, geotechnics is not my strong point but that seems to be the only work available in the office at the minute so I'm trying to get my head around this stuff.

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.

 

[dgillette]

How much lower? Sorry, but I wouldn't dream of putting out numbers without doing some analysis.

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

 

[cvg]

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...

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?

 

[dgillette]

Teton Dam is the classic example. The core material was low-plas silt placed dry of optimum, which is not inherently problematic if one designs for it. (Not exactly what you'd prefer, but you have to build with the materials that are available.)

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?

 

[cvg]

I'm glad you reminded me of that. The main key trench was very wide with sloping sides. However, I went back and looked closer at the Teton slides and see the failure was at the very bottom of the key trench in a a very narrow, vertical trench. There was a grout cap, but the entire bottom of the key trench (and probably up the sides also) should have been slush grouted. A granular seepage collection system might have helped also. I am curious also why additional grouting and exploration wouldn't have been done when they found the extinct fumaroles in the abutment.

 

[dgillette]

Politics, budget, and a schedule that wouldn't wait for trivial little details like that. The USBR culture at that time was to go-go-go-build-build-build, and to minimize interference from the designers. You think I'm joking about that last part? Wish I was.

 

[cvg]

it wasn't just the USBR but others who shall remain nameless that were continuing that philosophy even after Teton failed. I've been looking at a dam which was finished in 1980 and it exhibited significant seepage in the abutment through untreated cracks 70 feet above the outlet works on the first filling. Since it is a flood control dam, it has drained and remained dry pretty much since then. The solution now is to put in an inclined sand blanket, toe drain and massive soil buttress at very significant cost when a little (more) grout could have prevented the leak in the first place...

 

[oldestguy]

Here is something to consider. Professor Marston at Iowa State (USA) did a significant study of conduits in trenches way back, possibly in the 1920's followed by a professor Spangler(now maybe dead also).

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/

http://WWWdisplay.cgi?0701176

 

[dicksewerrat]

In the Plastic Pipe Insitute website, there is a formula for caculationg the "arching" coef. It is using in some circles to determine the vert. load on buried pipe.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[RHTPE]

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

 

[msucog]

rhtpe...thin lifts well-compacted (yes "stone" too) with seperation fabric and a little bit of care/effort on the contractor's part. it's typically not all that difficult to get something reasonably close to the intention of the design...it's usually more an issue of the contractor doing what they're supposed to do from my experience. i've had to deal with similar situations as both a geotech on the testing/inspection side and contractor performing the work.

 

[RHTPE]

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