Repair for Segmental Retaining Wall Undermined at Base Course

[gte447f]

I am assessing a situation involving an existing segmental retaining wall (SRW) that is undermined at the base course due to inadequate embedment of the base course(s) below grade. The wall is approximately 150 feet long
and approximately 16 feet high (maximum). The adjacent grade at the toe of the wall slopes up gradually from one end of the wall to the other, so the base course of blocks steps up periodically as the grade slopes up. Due to inadequate embedment at the base of the wall and the stepped base course, there are two locations along the wall where the base course of blocks is above the adjacent grade (i.e., zero or negative embedment). At each of these locations the face blocks have been partially undermined due to loss of aggregate base from underneath the base course of blocks where there is negative embedment (see photos and sketch). The wall has been in service for approximately 20 years and appears to be in serviceable condition other than these two locations where the face blocks are undermined. Each of these locations is approximately 6 feet long.

I am looking for options to repair this condition to prevent further undermining and loss of bearing support under the base course of blocks at these two locations. Any suggestions on how to underpin the wall in these locations?

 

[gte447f]

Maybe something like this?

The intent of the underpinning concept shown in the sketch would be primarily to act as a bulkhead to prevent further undermining by loss of aggregate base from underneath the base course of blocks.

Thoughts?

 

[MTSOE]

I take it that this is in a private owner's backyard and thus aesthetics are important and loss of usable space would be preferably avoided. I wonder if you could do something with coir fibre logs in front of the wall to build up the grade a little. If you wanted to assure yourself that these would stay put, perhaps you could anchor them in place with some duck-bill anchors or a similar product. By all means, do underpin the areas which have already been undermined, but this may be a simpler, and less labour intensive solution than a continuous concrete bulkhead. I'm just spit-balling here.

 

[SlideRuleEra]

gte447f - I would consider your suggestion, but with a few changes:

1) Don't dig immediately adjacent to the existing wall. Move back, say, 1 foot, to excavate.

2) Slope top of concrete to ensure that rain water falling on the block is directed away from the wall.

3) Use reinforcing steel in the concrete.

 

[TigerGuy]

If you construct a big block of concrete as shown in the above diagrams, where do you drain the water to from behind the wall?

The wall should have been constructed to a couple feet below finished grade. You may need to do some localized grading to bury the first couple rows of block, again being mindful of drainage.

 

[gte447f]

SlideRuleEra, I was already concerned about potentially excavating directly adjacent to the front of the wall, so I like your suggested modifications to my original concept sketch. I am leaning toward recommending something like your sketch.

MTSOE, this wall is located behind a row of townhouses. The back yards are already quite small/narrow, so preserving space is a consideration, but ensuring the integrity of the wall is most important. Regarding your suggestion to use coir logs to build up the grade in front of the wall, do you mean at these problem areas where the wall has already been partially undermined, or at the other areas along the wall with insufficient embedment but no undermining? You suggestion seems to be similar to TigerGuy's to build up the grade in front of the wall to bury the bottom course or two of blocks. I think the suggestion has some merit at areas along the wall that have not already been undermined, but I would think something more permanent than coir logs might be needed. Won't they biodegrade eventually?

TigerGuy, I appreciate your concern about possible drainage issues. The wall has existing weep holes periodically, although probably not enough. I think there are 3 weep holes along the entire 150 ft length of wall. There are no weep holes at either of the two 6 feet sections where the wall has been undermined. My gut tells me that bulkheading two 6 ft sections of the wall would not have a significant effect on the wall drainage, but I will give it some more thought. I am not opposed to adding soil in front of the wall to bury the bottom course or two of blocks, but I think a concrete bulkhead will function better at the two locations where the wall has already been undermined. I might consider recommending adding soil along the rest of the wall where it is clear that there is also insufficient embedment but the wall has not been undermined. However, adding 1-2 feet of soil at the toe of the wall would require more space to transition to existing grade than would adding the concrete bulkhead, unless some sort of planter wall was added a couple of feet away from the existing wall. What do you think?

 

[MTSOE]

gte447f - you are correct about the degradation of the coir logs. I should have educated myself more about their lifespan before I responded. I initially suggested them because I was thinking of something that could act as a filter to prevent loss of your wall base material (something that wasn't necessarily structural). I agree that some sort of structural repair is needed at the areas where the base material is lost. However, I thought a continuous concrete bulkhead was excessive. Perhaps I misunderstood you and it was your intention to build this bulkhead only opposite the areas needing immediate repair. I thought the issue of lack of wall embedment existed across the entire wall but loss of base had only occurred at two locations thus far.

 

[dik]

I don't know if it's the camera perspective, but the wall appears to be leaning. This may be unusual with the stepping of the precast units shown in the detail. The step is common with segmental retaining walls. Does the wall lean? The photographs don't show the step.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dik]

That should be taken care of by the drained backfill. The affected area of the top of the retaining wall is quite small. One of the first parkades I designed (over 50 years ago), I sloped the spandrel beam tops to drain to the outside. This caused a bunch of random drip lines (including the raindrop 'dirt') from rainfall to be visible on the outward face of the spandrels and has/had a terrible visual effect.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[gte447f]

Dik, the wall is not leaning, that is just an optical illusion in the photo. The wall is batttered as usual, i.e., each course of block is setback 1" from the course below. The stepping I am referring to is different. I mean the bottom elevation of the wall steps up periodically as you move along the wall from one end to the other. This is evident in the first photo where you can see the gap below the bottom course of blocks at one of these step-ups.

Regarding SlideRuleEra's comment about sloping the top of the concrete to ensure that rain water falling on the block is directed away from the wall, SlideRuleEra was referring to the concrete underpinning/bulkhead shown in the sketches above. SlideRuleEra is correct that the concrete needs to be sloped away from the wall.

 

[dik]

Thanks...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[geotechguy1]

What does the original design say about requirements for burial of blocks? I thought usually even the manufacturers want a few courses of these things buried.

 

[gte447f]

geotechguy1, I have no idea what the original design documents say about embedment because this thing is 20+ years old and there are no records of the original design documents, but we all know that proper design and construction of segmental retaining walls requires the base blocks to be embedded some distance. I am simply trying to come up with a decent repair to prevent further undermining of the wall at locations where insufficient embedment has resulted in loss of bearing.

 

[Jrit]

Are those base blocks trimmed at the bottom? Are they set on an aggregate base, or possibly concrete? To me, easiest fix is to replace the leveling course and embed the wall 6 or 8 inches by adding compacted soil in front. Monitor over time and replace soil as needed. You could armor the added soil with some rip rap if needed maybe just vegetation would suffice.

 

[AKSherpa]

The referenced "undermined" blocks appear to be deteriorated or damaged? Have you noted any quality issues with the other blocks?
Where is the wall located?

 

[gte447f]

Jrit, the blocks are not set on concrete. They appear to be set on an aggregate base, but I do not know the width or thickness because I did not do any excavation. It does not seem to have the dimensions of a typical aggregate base leveling pad, since the bottom of the blocks is at or above grade and there is turf growing right up to the face of the wall.

I don't think just dry packing aggregate base into the space under the undermined blocks will be able to achieve proper compaction to re-establish bearing support under the undermined blocks, and I don't think I trust 6 or 8 inches of soil to be able to develop adequate passive pressure to confine the newly placed aggregate base, so I am still leaning toward a concrete bulkhead/underpinning.

 

[gte447f]

AKSherpa, the rest of the wall is in relatively good shape. There is one section that is stained from water overtopping and running down the face, but the wall doesn't show any other signs of significant damage or deterioration. The bottom edge of the undermined blocks does look somewhat ragged and damaged. I don't know what caused that. Possibly freeze-thaw damage, but this is in Georgia where freeze thaw isn't a huge concern, but we do get occasional freezing. One thing of note is that the bottom course of blocks are 8 inches tall where these step ups occur, but the rest of the typical blocks are 12 inches tall. Could the 8 inch blocks have been cut down from standard 12 inch block, resulting in damage along the bottom edge?