Retrofitting weep holes after the fact 1

[beej67]

I'm seeing more and more instances recently of poured concrete retaining walls without weep holes, which are coming under hydrostatic load from rising groundwater tables, and then leaking through cracks to damage floor level condos and the like. I understand that normally you'd install a weep hole to tie in to porous backfill behind the retaining wall that collects water and helps convey it to the weep hole without pulling sediment / backfill from behind the wall. In cases where a weep hole does not exist at all, and a maintenance contractor needs to drill one to relieve hydrostatic pressure behind the wall and dry the backfill out, what's the best procedure? I worry that if I tell a maintenance contractor to simply drill a hole through the wall, that the weeping might advect sediment through the hole and create a problem. Is there a standard practice used for retrofitting weep holes that handles this? Any advice on weep hole spacing?

Thanks in advance.



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[oldestguy]

Assuming you don't get any advice from those that have done it, consider what can be done to filter the water coming out to retain soil behind the wall. Off hand I'd look to holes about 1" diameter spaced about 3 feet apart. If 1" diam. is not practical, of course larger. If you have ever cleaned a shogun barrel after a hunt and run patches up the barrel, something similar might work. I am thinking cut small patches of geotech filter fabric and, with a rod, shove them back to the end of the hole. For a small diameter hole, maybe only 1 or two needed. However, too many and there may be plugging of the weep. For a weep hole of 2 inches diam. perhaps some support to keep the fabric there, such as a few inches of gravel.

 

[gbam]

To build on oldestguy's recommendation, One could cut slots or drill holes for about 6" to 1' from the end of PVC Pipe, smaller in diameter than your drill hole. Cover the the end just drilled with geo-filter and insert. Use grout/sealer to seal the exposed end of the PVC pipe and wall. Hope all works out.

 

[cvg]

unless you have previously backfilled the wall with free draining granular material or installed geocomposite wall drain, the 1 inch weepholes will do very little to reduce overall hydrostatic pressure on the wall

 

[beej67]

Any other ideas CVG? I'm looking at something right now where there's literally zero way to excavate back behind the wall, and it's got intermittent groundwater staging up behind it. I've got a laundry list of things the owner may be able to do to keep water from accumulating back there, but no good way to drain it if it does accumulate.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

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[oldestguy]

Even though there may be questions abut how well to drain the backfill, if you don't try some simple, cheap method, you won't know much better than guessing. Installing some observation wells behind the wall (cheap) will help to check on any treatment results.

 

[cvg]

well, the first thing I would do is determine if the problem is with surface water or ground water. If it is surface water, there are a lot of things you can do to keep the water on the surface and prevent it from soaking into the ground next to the basement. If it is groundwater rising up than an active dewatering system such as a weeping tile draining to a sump pump is recommended, along with waterproofing the back of the wall. Weep holes without a collection system only work where the backfill has a high hydraulic conductivity. If you cant excavate the wall down to the footing, than your options are really limited. OG's idea of drilling a few holes (through the wall or vertical observation wells behind the wall) and observing is a cheap experiment that will allow you to sample the wall backfill and also observe any water flow.

 

[beej67]

Originally it was a broken water lateral, now surface water is exacerbating the problem. Split basement condominium building 2 stories at the street, 3 stories on the back side, and the foundation wall between the front and back units is the culprit on the lower side. The foundation wall daylights and then becomes an exterior wall outside the lower unit that had the water problem.

I've attached a photo of the wall on the exterior of the unit. Here it serves as a retaining wall, then it extends back under the building, behind the point of view of the photo, and becomes more of a foundation wall.

I can't put a monitoring well on the back side of the wall because there's a horizontal slab there serving as the sidewalk for the ground level unit. You can see staining from water leaking through the seam between the upper horizontal slab and the foundation wall. I've given the owner a laundry list of means to prevent surface water from accumulating behind the wall, but I'm not convinced the back side of the wall has dried out fully, and need a way to promote that.




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[oldestguy]

Explain your "laundry list" of handling the surface water. Are you using any bentonite?

 

[cvg]

does water drain toward or away from the wall?
seal the slab joints and especially the joint between the slab and wall
I dont see where you could use bentonite if the ground is already covered with concrete slab

 

[emmgjld]

Lots of discussion, mostly dealing with little to no information as to ability to actually drain the wall backfill. At this point, Simple experimentation seems to be in order. It would be a simple matter to try oldestguy's first suggestion and see what happens. I will quote:

Off hand I'd look to holes about 1" diameter spaced about 3 feet apart. If 1" diam. is not practical, of course larger. If you have ever cleaned a shogun barrel after a hunt and run patches up the barrel, something similar might work. I am thinking cut small patches of geotech filter fabric and, with a rod, shove them back to the end of the hole. For a small diameter hole, maybe only 1 or two needed. However, too many and there may be plugging of the weep. For a weep hole of 2 inches diam. perhaps some support to keep the fabric there, such as a few inches of gravel.

 

[beej67]

Explain your "laundry list" of handling the surface water. Are you using any bentonite?

Lots of small problems. Roof drain leaders were put underground by a prior contractor. They enter the ground as black corrugated HDPE and daylight in the curb as PVC, so who knows how they were spliced together. Also the PVC is at an adverse slope, so the thing actually catches water from the curb instead of draining freely, which could be a big deal if the connection between the HDPE and PVC isn't water tight. Some slabs are cracked above the wall. Some gutters drain to the surface at the adjacent building and then drain through landscaping to the back side of the wall at an area not pictured, which traps the water up against the wall, and needs to be regraded. The slab outside the problem unit is flat, so needs a slot drain or trench drain. Some other stuff. Our soils are silty clay here, so I'm not that concerned with soil addenda.

I suspect the list of site improvements I gave will ameliorate the problem, but it'd be nice to provide relief otherwise, as well as have an easy way to test if the soil behind the wall is wet. I included oldestguy's suggestion in the writeup as part of an overall plan, so we'll see how it goes.

It's possible the wall was originally built with weep holes and appropriate backfill, but that the holes were covered over by some kind of surface treatment. Look closely at the image, you can see where it looked like someone had resurfaced the wall a while back.





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[oldestguy]

Is there weeping of the wall inside the building?

As to an observation well or two, coring of slabs is no big job and later filling can be barely visible. The observation well might even be as small as slotted electrical conduit, set in an augered hole. A piece of 2 conductor wire and an ohmeter is the measuring device if you can't read it with a tape measure.

If slabs have to be removed to change grades, think about a layer below them of a mix of 10% bentonite, at the most. Otherwise heaving can occur. That layer need only be 4 to 6 inches thick, and can be done with a garden rototiller.

I've cleaned out joints in slabs and have dribbled in bentonite. I'm leary of contractors that will "waterproof" walls by injecting bentonite. It can be vicious stuff if in a thick layer.

 

[beej67]

The event that provoked the investigation was weeping through a crack in the wall behind the kitchen cabinets of the lower unit. Wrecked the cabinets and did some damage to the floor, ate up some drywall, etc. This was after a water lateral break. While the cabinets were torn out to fix the crack, additional weeping was noted by the owner during storms. The crack repair was very well done by that contractor, one of these hydrophilic grout injection deals. Photo attached.

In the photo, you can see two important clues. The vertical crack itself is where the bulk of the water intrusion occurred during the lateral break, but you can also see staining of the wall from light water intrusion from above. That's from the seam between the foundation wall and the horizontal slab, which is hidden behind the stud wall in this photo, but visible in the prior photo further up the thread. My hope is that weep holes or similar, in conjunction with my "laundry list" of site improvements, will keep the groundwater below this upper seam.





Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

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[oldestguy]

My interpretation of what you present is this. Weep holes in the exterior wall may do some good,out there, but not inside unless backfill is quite porous. Aside from that interior crack, and possible weeping at top, the interior wall appears intact, not leaning in, etc.
I'd concentrate on fixing up surface drainage and surface infiltration first. Handing the owner a list of work items may well not do it if the contractor is not well experienced or closely supervised by an experienced person. Blending of bentonite with on-site soil can be done wrong, especially if granulated bentonite is used vs powdered. Using laborers to do the mixing needs someone to say "OK that's good", etc. or a half a$$ job results. Will settling of new slabs result?

Buildings appear rather modern. Was there a building code requiring a footing drainage system? It's quite common in my area (Wisconsin).