Retaining Wall without heel and toe 3

[philhelf]

I am considering a retaining wall without the heel and toe since it will be on solid limestone. Can the anchor steel be sufficient for overturning and can the rock be considered as an infinite footer? Wall height will be about 10'. Plan on using epoxy grouted anchor bars.

 

[jdmm]

hatrack

There will be a very large moment at the base of the wall that you have suggested controling with anchors. The traditional approach of a heel and toe are simple means of providing a lever arm to resist the moment. The other approach is to provide a wall that is wider at the base than the top. I suspect that a wall can designed and built without a heel by I would wonder why one would not use a heel?

 

[Focht3]

jdmm:

I think he's trying to avoid excavating the rock...

 

[geomo]

Sounds like your designing a cantilever wall that will behave like the stem of a wall w/ no footing. The reinforcement you design for the wall stem can be extended as anchors into the limestone to handle the shear and moment for the whole system. However, consider using a lot of reduncancy to keep anchor steel stresses low, limit deflections in wall, allow for some corrosion of steel at the concrete/rock interface. Also make sure the anchor bond length is sufficient for the quality of rock and bonding agent manufacturer's recommendations.

You might also consider socketing piles into the rock if the number of foundation anchor dowels gets to be high.

 

[BigH]

This is what I have been trying to tell the designers on my project - we have very soft ground at surface to 6m with lots of expected settlement. I told them to use piles at wide spacings on grade beam with a wall with no heel and no toe. How to handle the moments? I have suggested the use of two rows of deadmen anchors. - This is like reinforced earth (oops, Mechanically Stabilized Earth walls) - it is what is done in dock projects - why not work for you.

Best regards and

 

[Focht3]

hatrack:

What will be retained by the wall? Is an MSE feasible? Tie-backs and deadman anchors? You used the phrase, "Plan on using epoxy grouted anchor bars." Is the wall "retaining" limestone or decomposed limestone, with the anchor bars grouted into the "rock"?

BigH:
Too bad this Internet thing won't transmit/receive libations...the emoticons are great

 

[philhelf]

Thanks to all of your comments. The rock is already excavated, and is very competent. The wall will separate a driveway from the back yard, but to have a level yard, the wall needs to be about 10'. The backfill will only be about 6 feet wide against the excavated rock and the wall. The wall will have a railing on top as a precaution against falling. I am also considering tie backs into the limestone just to increase the safety factor.

The moment arm on the anchor steel is large. It is definitely a cantilever beam, but I was also worried about the anchorage steel development length. Calculations show it will work, but I like other opinions.

Thanks

 

[Focht3]

Use a crushed rock backfill - with proper drains, filters, etc. (No french drains, please!) That should significantly cut down on the lateral force.

In reality, you only have a 6 foot high retaining wall...the lower 4 feet of intact limestone won't put any significant force on the wall. Two levels of tie rods (say, 2-3 feet from top and bottom) should give you reasonable forces. Both rows should probably be battered - the lower row will probably have to be for installation purposes.

 

[ishvaaag]

Just add that

Maybe since only a prism of backfill acts one can design for lesser push if as narrow as the effect be accountable. Obviously this contradicts the misconfidence urging to use tiebacks to the then buried rock wall.

To reduce forces for vertical anchors taking moment, one can always widen the foundation (to its costlier expense).

 

[jimbeck]

Limestone can be a bit tricky as it may have voids. Be careful.

Suggest you consider post tensioned anchors. DSI (Dywidag) have an excellent system, and as the bars can be spliced easily, and protected from corrosion with their system, should be usable. You may have to have a wall that is thin at top, thicker at bottom. Do not use any section less than 6" thick.

You may have built his already, so my comments may be redundant.

Regards

Jim Beck, P. Eng.

 

[MSEMan]

Have you thought of using a timber crib wall? They're relatively inexpensive and fit in with a residential setting. Design procedure is much simpler than tie backs etc. since cribs are essentially semi-flexible gravity walls. If you use a high strength backfill, you should be able to make it work especially with the relatively narrow wedge of supported backfill.

 

[DBNodurf]

I hope my post is not too late, but many folks considering forces acting on segmental block retaining walls do not realize that the block facing is merely that: a facing. The actual wall is a gravity wall composed of the mass of reinforced soil. The global overturning and sliding forces are acting on the back of this mass, and therefore a cantilever footing is unnecessary; in fact, ANY footing is unnecessary. The "footing" used for the block is really a leveling pad. Mike Simac's footing bearing capacity procedure used for such walls, as published in the NCMA manual, is an exercise in futility as well as evidence of inane thinking.

Regards...

D. Bruce Nothdurft, MSCE, PE, PG, M.ASCE, etc, etc,...
Principal Engineer/Geologist
Atlantic Geoscience & Engineering
Charlotte, NC

 

[grits]

when dealing with volatile minerals (ie limestone) several environmental factors must be taken into consideration. Is the construction taking place in a cold climate area, is there many voids in the rock, is the rock close to a road where much salt would be used in the winter. Limestone, when dissolved in water can become highly corrosive. I simply recommend using "nick free" epoxy coated bars. For in my experience I have seen concrete removed from an epox bar with nothing but coating left. Good luck.

 

[DBNodurf]

Hey, we could all get together in Nashville at the ASCE conv and toss back a brewsky or two...

D. Bruce Nothdurft, MSCE, PE, PG, M.ASCE, etc, etc,...
Principal Engineer/Geologist
Atlantic Geoscience & Engineering
Charlotte, NC

 

[Focht3]

Bruce -

I didn't see your 9/9/2003 post until a minute ago; guess I closed that browser panel too quickly. I chuckled - glad someone else gets "fired up" from time to time!

BUT I'm going to conditionally disagree with the following statement,
Mike Simac's footing bearing capacity procedure used for such walls, as published in the NCMA manual, is an exercise in futility as well as evidence of inane thinking.

It is clearly wrong to think of the problem as a bearing capacity problem per se, but one does need to look at the overall (slope) stability of the arrangement. And let's remember that bearing capacity and slope stability have a lot in common.

(You clearly understand the "fine distinction" I'm making, but some readers might not -)


I completely agree with your "inane thinking" comment - looks like fuzzy thinking to me. Like the view one gets from a certain dark, smelly place...




Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[BigH]

Bruce is quite right in that the facing is really to prevent the soil from falling out. The original RE paneling was simply curved linear sheets: (___ rather than the more normal concrete panels today.

Focht3 and I have had our discussions on the calculations of "bearing capacity" of MSE walls - I really do believe it is a misnomer. However BS 8006 uses this as "bearing capacity" and most follow suit; but interestingly it is usually acceptable to use a factor of safety against the shear failure of 2 rather than 2.5 to 3. My view is that the slope stability concept is better suited to the safety of the structure. With "bearing capacity" - the one side is significantly higher than the other so the ultimate capacity doesn't really take in the 2pi x Su; but the 1pi x Su which is more geared to the slope problem. (Hope I got that right!).

By the way, I would love to get together with some at the Nashville Conference but, alas, I am back in the sub-continent.