Groundwater Induced Basement Wall Failures - Defining Hydrostatic Failure 1

[Forensic74]

Since it hasn't stopped raining for the past year on the east coast, I have been seeing a slew of CMU basement wall failures.

I see a lot of engineers chalk up these types of failures to "hydrostatic pressure" induced from "the pressure of the water column acting on the face of the wall". Makes sense on its surface, but one thing I have noticed is that the retained soil doesn't have to be even close to saturated to induce a failure. I've been to a few of these on the day of failure and while there was some moisture in the soil, its not like there was a water column of pressure behind the wall.

Got me thinking that maybe slight wetting of the retained soils is instead engaging active soil pressure by promoting the soil to displace. Maybe changing the internal friction angle. Not so much the water column explanation. Does anyone have any thoughts on this?

At the end of the day it might just be a case of semantics, but I like being as accurate as possible in describing whats going on.

 

[oldestguy]

Once it has failed the free water is gone. Maybe you are too late with these observations.

 

[Forensic74]

One other thing is that CMU walls are notoriously leaky. Cant imagine how these pressures are developing when the walls are usually already cracked and leak like sieves.

 

[Ron247]

When they "failed" did they just crack and lean, or did they literally collapse? If they just cracked and leaned inwards, you probably are not going to see the wet soil. It may be 2' outside the wall and not right against it. Also, the wetter soil will be deeper in the ground. It generally has to penetrate to the bottom, not find a way out and then start building upwards.

 

[Forensic74]

When I say failed, I mean the homeowner suddenly got a new walk-out basement. Red tagged structure, $60k+ to reconstruct. Side note, it is amazing how much bridging action there is in the first level wall framing when you see 40' of basement wall completely gone.

Sometimes, I can see when the soil is liquified and floods into the basement I can argue that there's pore pressure there. Many other times the soil is just damp and just crumbles in.

 

[Ron247]

I have only seen about 3 that totally collapsed. I have seen 100 or more that cracked and leaned inwards. Generally they were not built per the building code in the first place. No French Drain, wall not secured to wood floor with anchors and hollow block.

 

[dhengr]

Forensic74:
You’ve potentially got a bunch of things going on, take your pick. Potential soil swelling and expansion, heavier/wetter soil, lubricated soil in terms of its movement, etc. Where the conc. blk. walls well constructed, grouted, reinforced? What were the support conditions at the top and bot. of the wall? What was the wall cracking situation prior to failure, and what was the failure cracking pattern? Was the wall constantly wet or leaking, was it waterproofed, not just damp proofed, was there an ext. and/or int. drain tile system? Was the backfill done properly, with draining fill and then a topping layer to keep water out or to a min? What is the planting situation immediately around the bldg. such that it might retain water, what about gutters and down spouts directed away from the bldg? Is the area immediately around the bldg. graded to drain away from the bldg. properly?

Does the wall, as built, meet the old and/or the current code criteria? What was the quality of the construction? Nobody paid too much attention to these bsmt. walls until they actually failed, at which time the builder was long gone. That’s the way they’ve always been built, that’s the way my Grandpa showed me to build em, what could possibly go wrong. So what if the soil elev. is a couple feet higher along a long portion of the wall? The backfilling, proper waterproofing and drainage, and grading are often given short shrift. It’s really a wonder more of them don’t fail.

 

[Forensic74]

I think my question is pretty simple. Can the infiltration of groundwater into soil induce active soil pressure? I've never seen this in any references, and I think most just overgeneralize the term hydrostatic pressure.

I generally see about 6 full failures per year. Have seen more this year. The worst is when the septic tank slides into the basement.

 

[Ron247]

I would think the soil saturation at some point could affect the active pressure somewhat like a mudslide. I have never known anywhere someone researched the possible relationship.

 

[GeoPaveTraffic]

Forensic74,

Based on your question, I think you have some confusion about earth pressures.

Soil exerts pressure on a wall depending on how the wall moves.

If the wall doesn't move any, then you have at-rest pressures.

If the wall moves away from the soil, then the pressure starts to drop from at-rest towards active. If the wall moves far enough, which depends on the height of the wall and type of soil, then the pressure drops all the way to active.

If the wall moves toward the soil, then the pressure starts to increase towards passive. Again, depends on the wall height and soil type.

If the wall is at active and there is an increase in water pressure on the outside of the wall, the soil pressure doesn't change, i.e. there is no change in friction angle of the soil and no change in the active earth pressure coefficient. The only change in pressure on the wall is the water pressure. Note that if there is a 1/4" crack between the wall and the soil that is filled with water, the wall feels full hydrostatic pressure. Water pressure can also be transmitted by small cracks in the soil.

As for the full collapses that you described above, here is my best guess. Wall was somewhere between active and at-rest since the top of the wall was at least partially restrained. Wall was probably designed, if designed at all, for a pressure closer to active; so already highly stressed. Period of heavy rain caused the soil to saturate, displacing air and adding water causing the unit weight of the soil to increase thereby increasing the earth pressure. Wall displaced causing a crack either between the wall and the soil or cracks in the soil. Water filled these cracks placing full hydrystatic load on the wall causing the wall to further deflect and fail. The soil had enough apparent cohesion to mostly stand a short time, a few weeks or a month, without the wall.

Mike Lambert

 

[GeoPaveTraffic]

Ron247,

As for earth pressures due to a mud slide or a landslide. Yes pressures are higher, can even get to full passive pressure. However, the soil has to move a lot to reach full passive pressure and the structure has to be very stiff. I don't think either of these conditions apply to these basement walls.



Mike Lambert

 

[Forensic74]

My bad, yea you're right. "Can the infiltration of groundwater into soil induce at-rest soil pressure?"

Does anyone know offhand if the IRC tables are based on active pressures?

 

[Ron247]

I do not know what type of soil pressure they were based on but I am fairly sure they do not reflect lateral pressure increase from water.

 

[hokie66]

Another issue is the type of soil, whether it swells when wet. Many clays will swell, thus producing high pressure. But are these walls reinforced? If not, whether they stand or not is a lottery.

 

[Ron]

Agree with GPT but add one point.....water can cause the angle of repose to decrease (kind of like slump in concrete!), which would keep the soil at least at the at rest pressure during wall movement if the soil follows the wall as it moves. Active pressure mobilizes interparticle friction (inducing shear) in the soil that reduces the pressure against the wall. If water lubricates the soil enough to overcome this, the pressures will remain higher.