2ft. Gravel Backfill Behind 15ft.High Retaining Wall 1

[cap4000]

I have a rock excavation that is 15 feet deep. I plan to use only gravel behind it. I have only 2 feet to backfill between the currently blasted 15 foot high rock face and retaining wall. There is NO soil pressure at all, only the new 2 foot of gravel behind it. My question is, what backfill pressure actually develops on the 15 foot high retaining wall?

 

[GeoPaveTraffic]

Very little, but it is difficult to quantify. I would draw the condition and place a Ko and Ka line extending from the base of the wall through the backfill and into the rock. Then looking at the lines and the relative amount of inplace rock versus backfill, make a judement call on the loading.

 

[StephenA]

Will you have weep holes to reduce the water table behind the retaining wall? Otherwise you could develop the full water head of 15'

 

[BigH]

You would probably use a toe drain pipe at the bottom. Unless your rock slope is going to show creep, as indicated you would have very little earth pressure and likely nominal steel would suffice. But, you can try to determine the pressures via "silo theory" - for that really is what it is - a silo - wall on one side and rock on the other. You should be able to confirm if there is any chance of creep over the long term.
As an aside - we put in an alternative bid a few years back for a co-generation plant. It was to be blasted into limestone rock. To cut down the costs, we suggested not even putting up a concrete facing wall - just let the rock act as the wall - a false wall would be used between columns with the open space behind - drains at the bottom with geomembrane placed down the rock face to control any seepages into the drain. Would have saved a bundle of moola. - relevance to post - no wall/no pressures!

 

[DRC1]

Shnable used to have a system where they would lean the sheeting back to reduce the earth pressure. They would compute the earth pressure by drawing the anticipated failure triangle if the wall were plumb. tThen they would draw the line of the laid back sheeting. The ratio of the original area to the reduced area was theratio of the pressrue of the vertical sheet to the laid back sheet. The same idea could be used for the sloping rock.This is detailed in a very back issue of ASCE civil engineering magizine. Another option is a graphical solution such as Cullmen's Diagram.

 

[PSlem]

I calc'ed a wall like this years ago. Look for notes on silo loading. Similar case. As I recall on the one I did, it was less than half the load of an unrestricted wall.

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