Crib retaining wall Repair

[dcceecy]

I am working on a project about crib retaining wall repair.

The owner of the wall wants us to do a survey and provide some repair/replacement choices with budgets.

This is the first time I am working on such projects.
I walked through the walls: The retaining walls were constructed from precast concrete components and interlocked with timber beams to form open grids, which were then filled with soil. Fabrics were used between the timber beams to hold the soil behind the walls, as shown in the attached picture.

The biggest concern for me is that soil instead of gravel has been used to fill the open grids of retaining walls. (no construction drawings available). So the soil in the open grids will be saturated during the rain, and the lateral load on the retaining wall will be increased dramatically. Maybe that is the reason the corner of precast member were sheared off and timber beams slided out.

The property line is about only 5 ft away from the bottom of existing walls. I have some options in my mind:
1. Install soil nails between the timbers; leave the wall alone.
2. Build sheet pile walls (drive some posts into the groud and put FRP sheet between posts) just in front of the existing wall (leave the existing wall alone). maybe install some tie backs from the post if needed .
3. Demo the existing walls, and then build a modular block wall with geogrids.

for repair method #1 & 2, I am still worried about the drain of the soil behind the wall. Repair method #3 may cost too much.

Any suggestions and comments?

Thanks.

 

[msquared48]

I'm surprised no horizontal tension ties were placed at the corner intersection of the wood stringer cantilevers.

If the wall was improperly filled, I would remove the backfill and place the correct fill if possible with a free draining material as you suggested.

Are the downspouts in the area tightlined, or do they drain to the finish grade behind the wall, allowing additional water to collect behind the wall?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[dcceecy]

There are no tension tie, but only two toe nails for the connection. I think they thought it is a gravity wall, and the precast component will hold the stringers.

There are sidewalks, plants, hand rails at the top of the retaining walls as shown in the attached picture.
I did not see any downspout from the photos I took. I'll check that next time.

The retaining wall hold the soil between the building (has a basement wall) and retaining wall.

Thank you for your comments.

 

[SlideRuleEra]

dcceecy - IMHO, it is best not to do anything to the existing wall - it looks too far gone to salvage (structurally). I don't believe that soil nails (Option 1) would work for the long term.

Sheet piling (Option 2) has potential but I doubt if posts and FRP sheeting would be sufficient. Driven steel sheeting designed for cantilever should work, but would be expensive and not look very attractive.

Agree with you that total wall replacement (Option 3) is neither practical nor cost effective.

You did not mention how the height of the existing wall, but it looks to be 8 to 10 feet (compared to the height of the handrail in the photo).

Perhaps you could consider a true, unreinforced gravity wall constructed in front of and completely burying the existing wall. There are products to do this, see:

http://www.versa-lok.com/bronco/Default.htm

This would be expensive, too, but at least is has a high probability of success and would look good.

http://www.SlideRuleEra.net

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[dcceecy]

Thank you for your suggestions.
The wall in the 1st photo is about 10' high. The spacing of those stringers is about 12".

I was trying to develop some repair options in the report.

The next step is to estimate the cost for each option.
Where do you think I can find the unit cost for retaining walls (like 35$/S.F.) or I have to call contractors for quote?
I think a rough estimate is good for now.

Thanks.

 

[hokie66]

This appears to be a homemade attempt to build a cheap version of a concrete crib wall, and the attempt has failed. Based on your description that it only retains soil between the failed wall and a basement wall, so does not provide support for the structure behind, I would demolish it and replace with a properly engineered wall.

 

[SlideRuleEra]

dcceecy - Based on your photos, a project like this is very difficult to estimate accurately. This has little to do with the direct cost of repair / replacement options. The site looks confined, with limited access and nearby existing buildings, trees, etc. The unpredictable cost factor is the value a Contractor will place on what I'll call "protect existing facilities". Years ago, as a heavy construction Contractor, I bid on jobs with type restrictions; a difficult site can easily result in bids that are double (or more) the fair value of the actual work.

Suggest that you contact Contractors for help with estimates. When the time comes, include a specific bid item for "protection of existing facilities" (or whatever you choose to call it).

Also, for any option that involves removal of the existing retaining wall consider if the building's basement wall should be closely monitored for movement during this work (monitoring could likely be performed from inside the basement). If it does start moving the project could suddenly become more complicated and costly. This is one reason why I consider abandoning the existing wall in place (as-is) a good option.

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[dirtydude]

You could also consider a Redi-Rock wall if the final height is limited to 10 feet.

 

[dcceecy]

Thank you for your replies.

I have another question. As shown in the first picture, some corners of precast member, which suppose to hold the timber beams, was broken and no steel reinforcement was found at the corner.

Is it common that no steel reinforcement for the corner of such members? To me, the corner has a big shear load from the beam

Thanks.

 

[a2mfk]

I think Hokie is right, this is somebody's homemade crappy idea of a retaining wall, what garbage and an eyesore! Good thing they saved that money though, huh?

I agree with others, this would be a good site to consider a pre-cast, stackable, modular wall, maybe one of the types that you offset every course to follow the slope of the existing wall. They can backfill and compact in small lifts as they go up the wall. Maybe you can incorporate some type of weep hole details at the same time. I think this will give them flexibility also to do curves and bends, more so than a CIP wall with form work and rebar...

Maybe have them cover the existing wood that they can get at with an asphaltic coating or some other preservative to prolong the wood rot. But I would also explain this up front to the owner that over the long term they may have settlement issues, but this may be decades before the settlement at the top due to decaying wood is even noticed. And put some CYA notes on your drawings. It will become a a long-term serviceability issue, not one of safety.

 

[hokie66]

dcceecy,
To answer your latest question...the lugs at the ends of the tension tie members in a properly designed crib wall are not reinforced. However, they are a lot thicker and have radiused corners to prevent what has happened in this crude and cheap example.

 

[BAretired]

One further option occurs to me:

4. Demo the existing retaining structure and build a new deck full width between house and handrail. Could enclose space below to accommodate storage if desired.

BA