Keeping cliff face stable during construction process?

[Redacted]

Hi there, I am working on stabilizing a cliff face with a retaining structure. I am mainly thinking of using gabion baskets but I will size an RC retaining wall as well to compare the pricing.

I am trying to visualize how this can be done from a construction point of view. The green line is showing the proposed regrade of the cliff face. Regrading at 30 degrees from the toe of the cliff won’t work because it would undermine a building on the top (so disregard the first diagram).

To make an RC retaining wall or gabion basket structure work, I would need to cut away some of the cliff face (red line) to create space for the construction of the RC retaining wall heel or the gabion baskets.

My question is how can you keep the rest of the cliff face stable during the construction process? What are potential temporary stabilization methods?

The graph is showing a grid in meters.

 

[STrctPono]

There are so many options. This also has a lot to do with the type of soil that you are dealing with. Based on your profile, the slope face is standing pretty vertical as is. Are you dealing with rock or mud rock?

If there is room, they can cut it back to a 1.5H to 1V slope. This would be a stable cut in most conditions. If they can't get that, they can bench it with 5ft max vertical bench heights (OSHA would regulate this requirement). If it's really bad and there is no room on the backside, they could sheet pile or put in a secant pile wall but those would both be expensive.

I have never designed a gabion basket wall but I assume it's basically like a gravity wall. 8ft seems reasonable for something like this but since I have never designed one I am not too familiar with all the details. Do you line the backside with a geotextile to keep fines from migrating through the rock?

 

[Redacted]

Hi STrctPono thanks for your reply.

The cliff face that am dealing with is soft rock that appears to have large cavities of sand pockets (which has caused concern for a building below, which is the reason for building the retaining structure).

The first diagram in the attached photo actually shows what a 30 degree slope would look like, which is similar to what a 1.5H to 1V would be. It can't be cut back to that degree because it would undermine the building above. I'm trying to visualise the benching that you recommend, each of the squares in the attached photo are 2mx2m (~6.5'x6.5') and if using those as a rough guide for the benching it doesn't seem like you can do it without scaling a good portion of the entire cliff face off, from the bottom to the top of the cliff (or maybe my visualisation is off?)

Sheet piling is an option but like you said, may end up getting quite costly. The heel of a rc retaining wall this size would perhaps be around 3m (one and a half square in the grid).

Yes, that is correct a gabion basket wall is a gravity retaining wall. It is not 8 feet (units are in m) it is actually almost 8m (~26') tall (which may be pushing the limits for a gabion wall). Yes, the back of the gabion wall would normally have a filter fabric.

This would be the tallest retaining wall I have designed and I may need to go the RC counterfort route.

 

[PEinc]

In the United States, it would be an OSHA violation to work in front of the existing steep slope unless it were a steep, stable, rock slope. Therefore, you would have to backfill against the slope to flatten it and stabilize it. Then, you could build a wall (a soldier beam or SSP wall) using top down construction or install temporary sheeting to build some other type of wall.
If the slope is rock, then you might be able to excavate a fairly steep slope (1:1 or steeper) and then build a wall.

http://www.PeirceEngineering.com

 

[Redacted]

Thanks PEinc that information is helpful.

I'm not sure what you mean by 'In the United States, it would be an OSHA violation to work in front of the existing steep slope unless it were a steep, stable, rock slope. Therefore, you would have to backfill against the slope to flatten it and stabilize it' because before you backfill to flatten the slope I would have thought that you would need to work in front of the existing slope regardless to construct something either permanent or temporary to hold up the backfill/existing cliff?

It seems in this case that due to the space constraint it would require temporary sheeting. Or as you mention, something that can hold it up temporarily and/or then be used as part of the permanent works like an ssp wall (at this height would most prob need tiebacks). Although that's getting into the realm of deep foundations, which I was trying to avoid.

The slope is more like sandstone in the sand pockets and most likely has an angle of repose more than my assumed 30 degrees. However, where I am located currently, there aren't any geotech's, so I am trying to make the most conservative assumptions that I can with slope.

 

[STrctPono]

RStars. This may help shed some light on US practice.

https://www.osha.gov/laws-regs/regulations/standardnumber/1926/1926SubpartPAppB

Are you working with the Contractor at all or are you just curious how they would do it? The reason I ask is because as long as it is buildable (which I feel fairly confident that it is based on what you've presented) the responsibility for figuring out how to do it is on the Contractor and (possibly) the Engineer they hire.

If it's soft rock, the Contractor might risk it and cut the slope back at a much steeper angle if they are comfortable doing so. As has been mentioned, there is always the option for soldier piles. Sheet piles may not be the best choice in rock.

 

[PEinc]

RStars, a contractor can use a loader, dozer, or excavator backhoe to safely push or place soil against the unsafe slope without endangering the equipment operator. This temporary stabilizing embankment soil would not be compacted except for what little compaction the equipment can provide. Once the stabilizing berm has been placed, a temporary sheeting wall would be needed in order to build the permanent wall. Instead of a temporary sheeting wall, perhaps a permanent, non-gravity, retaining wall could be installed from the stabilized berm. The stabilizing berm would need to be big and flat enough to allow equipment access to the temporary or permanent wall location. Unfortunately, most people don't want to spend the money to install a stabilizing berm. They would rather work under an unsafe slope and hope they don't get caught and fined and hope no one gets hurt or killed. The main problem arises when contractors bulk excavate the site and then leave an unsafe slope where the wall needs to be built.

http://www.PeirceEngineering.com

 

[Redacted]

@STrctPono thanks for that document it is helpful! I'm not working with the contractor but I wanted to make sure that if I spend the time to design something that it can be built safely. I like to have a construction methodology in mind for all designs that I do (even though the temporary shoring etc will be the contractor's responsibility).

@PEinc thanks for the clarification

 

[dhengr]

RStars:
Looking at your cliff profile on the right, I’d built a gabion wall from elev. 22m to 26m. I would step each course back into the hill, and further, lean the whole wall plane back into the hill. Now you are approx. at the natural shelf at elev. 26m (your red line and a potential working surface), where you can step a second upper wall back 3 or 4m at its base. This second wall tops out at about 32m, another natural bench and potential working surface (your blue line). Some of the cliff clean-up material ends up as gabion fill and some ends up as potential fill behind the drain and geotextile plane behind the gabions. This can all go up in unison with each gabion course. I would dress (clean up loose material, etc.) the cliff face to try to match the backfill and gabion rock fill. And, I would leave the red bench at about 26m and the blue bench at abt. elev. 32-33m as level catchments and drainages. I would not fill to the green slope, below and to the right of the blue bench.

Whatever you do is going to be expensive, that’s why the lot was (or should have been) so cheap in the first place. I would not tackle this project without considerable GeoTech help, and I don’t mean just some guy who has a long handled shovel. You might do a bit of a cut-n-fill study to approx. match your cut/good useable fill/waste materials. This will also indicate that you might move the walls back into the hill another meter to cause a material balance. This approach probably has the potential of min. final cost, when you consider labor rates, forming and materials transport, site vibrations, etc. for other approaches.

 

[Colostruct]

I've done tall walls that had similar height. Maybe a dozen walls that exceeded 25 feet in my career. These were all very difficult designs that needed to consider slope stability and soil retaining pressures. Sure, you can use some guesses on geotech properties of the soils. But, when you get to walls this height you really need a geotech involved. I know you mentioned they are not available in your area. But, you can easily eat up a mobilization fee for a geotech by guessing too high on soil pressures. Not to mention the liability of a difficult construction.

That said. I have a few questions...
1. What is the length of the wall?
2. What are the end conditions of the wall...ie do you need to extend side walls. Often these designs are three dimensional and sometimes the side conditions can be as big of an issue.
3. What will be built below the wall? A house, parking...
4. What are the bearing conditions at the base of the wall?
5. Where is the property line of the upper property?

I've done retaining walls at the base of sandstone cliffs with geometry not all that different from yours. We needed to do rockfall mitigation on the upper slope to prevent a danger to workers and then did soil nailing into the cliff face. It was a lot less money than the huge footing that would have been required. I also did a building in front of one of these that was a bit lower and we decided (the geotech did) that retaining pressures were limited to the soils in the backfill behind the wall and our wall was pretty minimal. It's a good thing too because the soil was not that great below the wall as the natural cliff face kept going and a cantilever footing would have been huge.

I've also done boulder faced walls on sandstone. But, not at that height. So, if the formation can step back you could possibly do a stepped wall as suggested above. But, remember that a stepped wall requires you to consider the zone of influence of the bearing surface below the wall and the lower walls need to be outside this zone if designed as gravity walls (not an issue if formation). Gravity wall sliding is a BIG concern. So, if you go with a gravity wall be very careful to consider this.

As you can see. There is SO much to consider here. A good geotechnical engineer is a must.

 

[Redacted]

Thanks dhengr and Colostruct

I agree; I did have a discussion with my colleague/boss earlier this week and pretty much emphasized that due to the uncertainties in the ground conditions we will need to bring in a geotech to do a ground investigation, which he agreed is the way forward.

Otherwise as you said Colostruct the perhaps overly conservative assumptions of soil properties may likely cost more than just flying in a geotech to do a study and assist with the design. It of course reduces the risk as well (which is quite high, especially considering the height of this).

To answer your questions Colostruct
1. ~50'
2. The end conditions were something that I saw potentially being an issue as well. They will most likely be wing walls at the ends.
3. Nothing will be built below the wall. There is an existing school about 3m away from the fence at the bottom.
4. Without a geotech it's all a guess, I was going to check what the sizing would be for 150kPa.
5. There is no property line, the entire area is owned by the client. The start of the upper property is shown as the green square in the left hand attached image.

Going forward, the slope will need to be analysed by a geotech to make sure there are no potential issues like circular failures of the earth etc and to also provide the soil properties and discuss alternative solutions.

 

[Colostruct]

Thanks for the response.

Since the upper building is part of the property that may allow you to push the limits of the excavation and layback. It also allows your geotechnical engineer to review the bearing of the upper building.

The wall 3 meters away from an existing building might be, surprisingly, difficult to accomplish with a large footing. This is making a great case for some sort of soil nailing solution...if it available.

Construction a Gabion wall so close to an existing building might be tough!