Rock Wall Evaluation 2

[GATV]

Hi. I am working on the evaluation of a rock gravity wall and run into this forum. The wall in question was built fairly steep (about 80 deg), and a section failed after a period of heavy rain. I've heard about the ARC design manual and am in the process of obtaining a copy. In the meantime, I was thinking that proper evaluation of the rock wall would entail checking the wall overturning and sliding capacity at each rock layer, taking in consideration any reduced contact area. Because of the "open" nature of these walls, hydrostatic forces would typically not be considered. Wouldn't such walls be able to withstand a large through-flow of storm water? Any guidelines with respect to potential wash-out of filler material? Thanks!

 

[kmerl]

Well I think the largest problem with these walls in storm events relates to several things. The tractive forces removing rocks from overtopping, hydrostatic forces on each rock unit, and the global.

So to get started you need to determine if there was a reason for failure, was a small rock uplift and heaved. Was water frozen behind oir on the wall and prevented proper drainage. Although there is a drainage of the rock large rain events could overload the drainage system. Are the soils behind the wall clayey and caused excessive lateral loading due to saturation?

How high was the wall? An intially high wall with a 20% batter could have been unstable intially and been pushed over the edge, so to speak, during the event due to overtopping load.

Please elaborate on the conditions as known....



Keithe J. Merl

 

[kmerl]

Also refer to thread:
thread255-82018

This has some very useful information...



Keithe J. Merl

 

[GATV]

Thanks for responding! The wall ranges in height from 8 to 15 ft, and backfill soils are mostly medium sand. No freezing, just runoff accumulation during a storm-event. Haven't started on the wall stability evaluation itself, wanted to check if anybody knew about a more proper way to do that than by using regular gravity wall design procedures, since this wall would not really act as a coherent unit, and also if anybody knew what storm-event, magnitudewise, could a properly built wall withstand...'tis my first time looking at one of these.

 

[kmerl]

Well look at the local standards, did you check the other thread there, I think there is some national codes refered there. Fifteen feet high is rather large for a dry rock wall. Remember that these walls to rely on gravity to determine the stability. You act as though the wall is one conherent structure for global, however if you determining the casue of failure it needs to be specific to the wall situation.

I really feel that the failure, becasue of the rainstorm events mentioned, is related to hydrostatic loading. If you think of each rock layer as a lift jiont you can caluculate hydrostatic pressures assuming the water filled up behind the wall and uplifted the blocks. With this force you can determine the minimum size rock that should have been on the wall at each layer and survey the remainder of wall for the failure source.

I think sources of information on forsenic failures of these walls may be lean, but I'll refer you to the other thread for more info on standards.



Keithe J. Merl

 

[kmerl]

Here is a source form the army corps of engineers for engineering with large stones, it may help:

http://www.usace.army.mil/publications/eng-manuals/em1110-2-2302/entire.pdf

Keithe J. Merl

 

[Robert168]

Why that section? Any difference in drainage and soil properties between the failed section and other sections unaffected? A comparison may give you some hints.

 

[g7mann]

g7mann [geotechnical]

Dear geotechgal - since I was the primaryu author of the ARC Guidelines perhaps I can help. I often engineer rock walls for seafront bulkheads wher the hydraulic pressure can be truly huge. Fortunately this is not often an issue.

The rock wall cannot be accurately "designed", at least not like a reinforced concrete wall. Ordinarily I use a wedge analysis to determine the requisite mass of rock in-place to resist lateral sliding and overturning. Sliding is virtually always the big issue. Once you determine the "correct" amount of rock to resist movement you will need to bury the toe, typically two to three feet, so that you can generate a small amount of passive restraint along the toe. [Neglect the upper one foot in your analysis.]

Once you have detemined the appropriate size of the structure, and we are presuming that you are building against a reasonably competent "native" cut soil face, you will need to provide a drainage layer between the rockery rocks and the cut face. Seafront walls typically have between about two and three feet of "drain rock" installed. Obviously, the thickness of this drain rock layer will also vary with the shapes and sizes of the wall rocks. Use a two to four inch sized crushed quarry rock for this purpose. If you are in a situation where the water level will fluctuate regularly you may omit any basal drain pipe. If not, install a six inch diameter, perforated, smooth-walled plastic drain pipe along the back edge of the keyway excavation, where it should be at the lowest point behind the wall. This drain pipe must be extended to a controlled discharge beyond the ends of the wall, and preferably into a permanent drain system. You must also remember that the larger vopid spaces between the rocks, typically those of about six inches and above, must be chinked from behind to help prevent seepage [or drainage flow] from dislodging and removing either soil fines or drain rock. Do NOT chink the wall from the front.

Where the protected [and retained] soil is granular and susceptible to erosion or dislodgement when subject to seepage or dranage flow, you should also consider using a layer of geotextile. The geotextile should be hung over the full height of the cut face and spread over the base of the keyway. Set the basal row of rocks on the front edge of the geotextile to "fix" it in-place. Then carefully construct the rock wall making sure that the drain rock backfill is firmly tamped-in-place so that it "forces" the geotexctile firmly against the soil face. This should help reduce the rick of long term clogging of the fabric.

There are a few other issues to be aware of, such as preventing the wall contractor from stacking the rocks one atop the other like a stack of shoe boxes. If this occurs, regardless of the results of your "engineering" the wall will fail, particularly if it's more than about eight feet tall.

It's also important that the second row of rocks, and opf cource the succeeding rows, be offset on the row below so that each rock is, wherever possible, supported by two underlying rocks. Avoid point contact wherever possible -it tends to devlop high stresses that can, though not often, lead to rock crushing.

When building the wall make sure the contractor is setting the rocks so that the face is inclined back at about 1H:6V and that the individual rocks are flush with each other horizontally.

Most rockey contractors will price a wall on a square footage of wall face basis. To make money they will then maximize the indiviedual rock exposure - placing the largest dimension outwards. Do NOT under any circumstance let the contractor place any rocks with the largest dimension parrallel to the face of the wall. The largest dimension should always be set back into the wall - after all, the engineering determines the thickness of the wall required to resist sliding and overturning. If you do not have sufficient mass-in-place, the wall will probably fail.

Perhaps most important of all is to make sure that the contractor is capable of building the wall the way you engineer it. He must have [at least access to] appropriately sized equipment to lift the larger rocks. Has he built any similar walls in the past and, if so, where are they. Go and look! You'd be surprised at the stories you'll be told - verify the contractors' capabilities always.

There, now you can go pout and build your wall safely. good luck.

 

[jdmm]

If I understood correctly, you are evaluating a wall that failed in heavy rain. The question is how can this occur if the facing is essentially open allowing the flow of water out of the wall.

When it comes to groundwater we tend to talk about hydrostatic conditions. In fact, most instances involve hydrodynamic conditions. Although the pore pressures at the face of the wall may be zero, there may be pressure head as a result of drainage of the groundwater to the face of the wall. Lamb & Whitman and NAVFAC DM7 both discuss this issue with respect to retaining walls.

It is very dangerous to assume that there is no increase in pressure on the wall from pore pressures just because the front face is drained. For retaining wall design (in the pacific northwest rain forest) I usually use a drainage blanket along the base of the wall and a drain at the heel of the wall in order to try to lower the phreatic surface well back of the face of the wall.

You are correct to assume that for 'dry stacked rock walls' that it is necessary to check stability at each change in geometry.

For your situation, if the backfill behind the rock facing was not free draining (such as till fill...just a guess) then the development of pore pressures in the backfill and consequent increase in earth pressure on the wall is likely the problem.

 

[cphi]

g7mann:
What are the ARC Guidelines, and how and where can I purchace them?
Thanks

 

[BigH]

Might want to check out UK's TRL (now TRRL, I believe) Overseas Research Note 16 "Principles of low cost road engineering in mountainous regions." They have some basic information on dry masonry walls, mortared masonry walls.