Retaining Wall Issues: Inclined, Near Structure, in Limestone

[Mr. Skippy]

Attached is a picture showing the site conditions. The bag wall is not acceptable and needs to be replaced. The ground conditions consist of top soil over limestone. There appeared to be some leveling fill, but the corner of the house is resting on limestone rock mass.

My thoughts:
1. Global Stability of the rock mass supporting the corner of the house: The bag wall has no soil behind it and wasn't designed to take any structural loading. I would think that the engineer who designed the foundation of this house considered the subsurface exploration and took into consideration a safe offset distance. I will need to check into this further.

2. Lateral surcharge pressure: I have not done any rough calculations, and I have no testing on the limestone rock mass, and the slope of the cut between the house and the wall is about 45 degrees and about 15 feet way... Tentatively I am aiming to proceed with no surcharge affect from the house on the wall, due to the geometry and material.

3. Height of Wall: The height of the wall would need to be about 8 feet high at least. A Cantilever RC wall would need about 4 to 5 feet of foundation width. This would require, excavating limestone rock, cutting down trees, and cutting material out from around the base of the foundation of the house. This would cause a lot of difficulties and probably underpinning the house.

4. The incline of the R-wall Foundation: This would create complex loading and construction issues.


My plan:
Is to stair step the foundation of a modular block retaining wall, to its max height allowed of 4 feet (Assuming the design allows). Then above that retaining wall set back an appropriate distance construct another 4 foot modular block retaining wall. Using properly designed drainage and geogrid if necessary.

If the assumption that the geometry and material can handle the surcharge of the house, then the goal is really to control the erosion of that slope. In which case stacking modular block retaining walls seems like the way to go.

Any insight would be greatly appreciated.
Thanks

 

[PEinc]

Your attached photo is not very clear. If, as I think you said, the house is bearing on the limestone bedrock, there should not be a building surcharge on the proposed block wall. I would be surprised if the original, relatively thin, bag wall would be capable of supporting the full 8' height of retained soil, even if there is no building surcharge.
Terracing low block walls may be the way to go as long as at least the lower wall is sitting on the limestone. The question I have is, "Can the lower wall be excavated to sit on rock without failing the slope above?" Excavating a "top soil" slope of at least 45 degrees sounds problematic.

http://www.PeirceEngineering.com

 

[oldestguy]

Leaning trees may well mean you have a major overburden soil slippage situation. Anchorage into rock may be needed.

 

[BridgeSmith]

A stepped (terraced) wall may be the most economical solution, but a soldier pile wall, grouted into the rock, could be close to the same cost. It may be preferred by the owner for greater usability of the property. Might be worth looking into to present as an option.

 

[PEinc]

Drilling soldier beams could be safer than digging into a 45 degree slope to set block walls, but access for drilling a soldier beam wall might be a problem due to slope, trees, and close building. Can't really tell much from the photo.

http://www.PeirceEngineering.com

 

[Mr. Skippy]

Thanks for the replies. I wish I would have taken a better picture of the site. There isn't much top soil, the driveway is excavated into the limestone rock, so the lower retaining wall will be sitting on rock.

The leaning trees observation is good insight. The one nearest the bag wall is of some concern. Cutting the tree would remove stress applied to the slope, but it would also leave dead root voids. I need to take a closer look at that.

My biggest questions regarding a solider pile wall would be construction on that steep and narrow of a slope and any tieback grouted near and possibly under the foundation of the house.

 

[BridgeSmith]

I wasn't suggesting an anchored wall. If the limestone is fairly solid, you should be able to drill holes, drop steel piles or beams into the holes, secure the piles plumb, grout the holes, install lagging between the piles, and backfill.

 

[MTNClimber]

I'll never understand the appeal of tiered walls on any site that has proposed them, unless local codes require it. You will save space and provide more yard to the owner by using a non-tiered segmental retaining wall. Tiered walls could save money but you would have to set the upper tier back far enough so that it doesn't act on the lower tier, therefore losing a lot of yard. If the bedrock is in good condition then you could save money by using a MSE wall with the grid extended from the blocks to the bedrock.

On a project like this, drilled soldier pile and lagging is typically significantly more expensive.

 

[Mr. Skippy]

I was informed by the general contractor that R-walls taller than 4 feet had to be reinforced concrete to get permitted by the city. I looked up the codes for the city and this is not the case, they just have to be engineered. That was why I was thinking of doing tiered walls.

If one taller wall was used a tree would have to be removed, and geogrid would have to be connected to the bedrock at one end of the wall. How would the geogrid be connected the the bedrock? grouted bolts? Also in this case, I would really want a subsurface exploration program, but I just don't see any feasible way to do that.

 

[MTNClimber]

A tiered wall should take up more space than a single wall. I’m not understanding how you would need to remove a tree if you did a single wall versus tiered. How far did you plan on setting back the upper tier from the first 4’ wall? NCMA guides suggest if a tiered wall is placed within a horizontal distance less than twice the height of the underlying wall, a load will be applied to the lower wall. To have a non-engineered tiered system your walls should be set back appropriately. A normal MSE wall will have grid lengths of 0.6H to 1.0H. A non engineered tiered wall will be setback 2H.

For a MSE wall, the grid wouldn’t have to anchor into the bedrock. If the bedrock is stable then you essentially just need to reinforce the soil between the blocks and the bedrock. There are minimum grid length guidelines in these situations. If I remember correctly it’s 0.3xH but I don’t have that reference in front of me at the moment.

 

[Mr. Skippy]

Thanks MTNClimber. I will try to get a better picture of the situation this morning. I was going to set back the second wall 6 feet. The local building codes require 1.5H for tiered systems to not be counted as a single wall. Even at that, I am applying the second wall as surcharge felt by the lower wall and engineering the system. I am still in the process of calculating out proper embedment lengths for the geogrid by hand. The software I am using requires a minimum of 8 feet and my reference "designing with geosynthetics" by Robert M. Koerner suggest minimum of 8 feet as well. But I have found a Txdot publication suggesting .6H to .7H as typical and even .3H to .4H being used and working well. If at all possible one wall would simplify matters, going to go look at it to day and get some survey points to model the situation more accurately.

 

[BridgeSmith]

Per AASHTO, the 8 ft minimum only applies to walls that support a roadway or structure foundation. The 8 ft minimum may also be one of those 'unless it's engineered' kind of requirements.

I agree with MTNClimber; the geogrid would only need to go to the bedrock to stabilize the soil, not be attached to it. The reinforced zone farther up would have to extend far enough to keep it stable for overturning and sliding. You may need to check sliding at intermediate levels where the coefficient of friction is reduced at the interface of the backfill and geogrids. In lieu of product-specific values, the COF is reduced to 2/3 of the normal value (2/3 TAN of Phi vs. TAN Phi). Typical product-specific values are between 0.8 and 0.9.

 

[Mr. Skippy]

I made another site visit and attached a better photo. The corner of the house appears to be placed on fill, and the bedrocks location is uncertain.

1. I am in the process of getting the original geotechnical report for the house. The structural details show a waffle slab and a note says that if limestone is encountered that the beams depth can be terminated. I foremost need to establish what is going on with that corner of the house. The prior engineer and city inspectors signed off on it.

2. I am trying to get a hold of the city to understand what they require in this case in regard to a retaining wall in order to get the house permitted (it is being sold). The next door neighbor has a bare 25 foot vertical cut along their driveway.

Originally my understanding was that I was retained to replace the inadequate retaining wall for landscaping and erosion control. Under this pretense, my aim is to not disturb the house and replace the wall. But this will be very difficult to accomplish with out disturbing the fill the house is on.

Now it seems (and this is the first thing I will get clarified) that they need a way to ensure the stability of the corner of the house so the house can be sold. This is an entirely different analysis, that I will likely walk away from.

In the case, that they just want to replace the old retaining wall, and the stability of the house doesn't need an additional investigation, then my only concern is how to handle what is below the old bag wall. It looks like an infinite slope with seepage, as there is about one to two feet of topsoil sitting on a ledge of bedrock. Any excavation of the topsoil and removal of trees would disturb the roughly 45 degree slope of the fill. If it is left in place I am not sure that any retaining system would prevent the slope from sliding, and compromising the fill above it.

Could the top soil and trees below the old wall be removed with out disturbing the fill above? Any ideas, thoughts, or opinions would be greatly appreciated.

 

[BridgeSmith]

My suggestion in that case, I think, would be back to the soldier pile wall option, if you can get a drill rig in to auger the holes along the toe of the slope, you should be able to get the wall in place without disturbing the slope above.

Ground anchors embedded back into the rock is an option, but likely a much more expensive one because of the small size of the project. It requires specialized equipment, so the mobilization cost is typically very high.

 

[Mr. Skippy]

Thinking further about this, it seems like the edges of the fill pad for the house was just pushed to form its natural angle of repose overlapping the sloping topsoil and vegetation, as the main tree in the way is buried a couple feet.

The city okayed it, now the house is being sold and they are not okaying it. The interesting issue is that according to the cities codes there is no way to build a retaining wall that will pass, in this situation. The toe of the slope has to be a certain distance set back from the face of the wall. And, anything steeper that 3:1 is to steep of a slope. Even if a solider pile wall was built, the slope with the tree and topsoil is to steep {I am assuming I can't just bury the tree and loose topsoil) but any effort to remove it would compromise the fill above.

If that tree and top soil have to be removed, It seems to me the house has to be underpinned, but if I am not mistaken this would require digging under the house to do so.

In which case, I think it makes more sense to just build a MSE wall section by section as you go down the driveway. There seems to be enough room until you get to the tree. Once at the tree, it needs to be cut down, and by having the retaining wall completed behind allows for localized excavation to be less invasive. This seems less invasive then trying to underpin the house.

Ideally the city people need to relax, according to their standards you would have to retaining wall entire mountains in New Mexico for example. Really seems like all that is needed is erosion control and its less risk just letting it be.

 

[BridgeSmith]

As alluded to in an earlier post, the city code likely has an exception to many of its requirements if an engineered (stamped) design is completed. The prescribed limits are typically what someone is allowed to do without having a PE stamp on the design.

 

[MTNClimber]

The Rwall2 photo looks like shallow bedrock to me. Not sure what your options for drillers are but if you could get a tripod rig and probe the slopes for shallow refusal or get an excavator to dig some test pits. More information will help you decide on a course of action. There are a lot of unknowns here so it's hard to figure out what to do.

The bag wall can definitely be replaced with a MSE wall. I'm not sure what the proximity of the house to the proposed wall is. Underpinning may be required but the slope appears to be stable in the photos. It's possible a tiered non-engineered wall can work too if the bedrock is shallow along the entire slope.

For the lower slope you're looking at either a stamped letter that the slope is OK as-is (maybe cite the lack of damage in the driveway or other lack of evidence of slope movement), slope stabilization (I'm thinking nails), a top-down constructed wall, or ripping out the driveway and building an MSE wall.

 

[PEinc]

It does look like rock should be at about the driveway level. If so, is the driveway wide enough to drill in some cantilevered soldier beams as close to the bag wall as possible and then stack precast lagging high enough to fill in a flatter slope up toward the house? The soldier beams could continue down the driveway beyond the bag wall. If the driveway isn't wide enough to allow soldier beams, maybe the bag wall could be shotcreted and soil/rock nailed and soldier beams could still be installed where there is no bag wall.

http://www.PeirceEngineering.com

 

[Mr. Skippy]

Thanks for the great insights!

Meet with the city building inspector and the issue is larger than I was aware. (I was called a liar, and demand I tell him my PE number?!?!?) I had not walked around to the back side of the house. Basically a wall is needed around most the house. There are trees surrounding the house, some within two feet. There is no access. Its trees and brush sloping down at about a 45 degree angle. Since the slope is so steep, hopefully the bedrock is very shallow.

The city wants a 3H:1V slope for anything that supports a structure. So much of the driveway cut below the bags could be left as is if a retaining wall is continued around the other side of the house, since that slope will have no affect on the structure in that case.

Building MSE wall footings around the other sides of the house concerns me. The neighbors driveway has a large cut through the bedrock that I can see what to expect, very fractured and interbedded. Slippage and seepage could cause problems. Leveling with concrete seems like the only thing to do.

I like the appeal of Solider Beams in that they simplify subgrade preparation. I am going to call around and see if a company has any type of rig that could pull this off. Just to have something to compare to the MSE option.

Making use of that bag wall is an interesting idea. What if a portion of the driveway is removed (enough for a toe) and then shotcreted monolithicly to the bag wall? Then the toe of the wall resumes being apart of the driveway? There is no room to move the driveway.

 

[MTNClimber]

Best of luck with the City Inspector. Sounds like you're on thin ice for some reason.

Making use of that bag wall is an interesting idea. What if a portion of the driveway is removed (enough for a toe) and then shotcreted monolithicly to the bag wall? Then the toe of the wall resumes being apart of the driveway? There is no room to move the driveway.

I'm not sure if I'm understanding this. Are you saying the driveway is to act as part of the retention system? If so, I would advise against this. I'm not aware of any shotcrete walls that have toes to be honest. Plus making the 3 to 5 inch thick concrete drive as a structural part of a retaining wall is unreasonable. A thin, unreinforced concrete slab is not going to resist sliding, overturning, ect. You would need soil nails as PE, Inc. suggested. Just make sure you have enough room to keep the driveway wide enough. I have a sneaking suspicion that the driveway isn't very wide considering the slope below and the location of the bag wall.