Direct measurement of pressure behind a basement wall

[DPAJR]

I am working on a report dealing with a bulging, leaky basement wall where a contractor used perf pipe instead of solid pipe to divert flow from roof downspouts. As a result the soil under the garage floor is so saturated that a second contractor drilled a hole in the floor and was able to push a nine foot probe all the way in with one hand. This was witnessed by the homeowners insurance agent.

I intend to calculate the pressures both dry and wet to illustrate the severity of the mistake but since we may be dealing with a jury it would be helpful to have direct pressure measurements at various heights along the wall. I worry that jurors eyes may glaze over when math comes into play. Especially here in my state where they are discussing dropping the math part of the achievement test for high school students because very few of them are passing it.

Can anyone recommend an arrangement of pipe and pressure gages to directly measure the hydrostatic pressure behind the wall? There seems to be enough water back there to activate a pressure gage.

P.S. The contractor who made the mistake tried to claim he had never done any work at that address. Invoices and cancelled checks prove he was lying so I have no sympathy.

Thanks

 

[VAD]

DPAJR:

I would suggest that you investigate the situation properly before writing your report. very often what seems obvious at first review may not be the cause. Just a few questions:

Is this a new home - Likely not

Was the first Contractor the new building Contractor or was he called in to provide a fix for a pre-existing problem.

The garage slab is a slab on grade. Water presumably travelled via the slab on grade to the basement wall. Was this bulging as a result of frost heaving. Was there leakage into the basement.

The above questions are based on a presumed arrangement of the garage and basement.

Anyhow be careful since the first Contractor will also engage his engineer as well.

These problems though seemingly simple are very difficult to pinpoint blame unless you know the history of the site re construction etc.

It may cost you more engineering time that you may have bargained for.

Anyhow it is your observation. Just some opinions from my past experience.

 

[oldestguy]

I'd suggest checking out porous tip piezometers, those fitted with a plastic tube. Then with an ohmeter and co-axial cable measure the depth to that water surface in the tube. Then a little math to compute pressure.

I'll bet local geotech firms even have a stock of these on hand.

You could shove these to various depths in soft stuff or hand auger holes, then properly backfill with sealing material (bentonite).

 

[fattdad]

this is a case of bad compaction. You may want to blame the perforated pipe, but I'd bet it's not the real culprit. If you can push a bar 9 ft into any soil there is no strength. Water alone doesn't reduce soil strength, otherwise embankment dams would fail all the time!

What you have is saturated mud. Use a dilatometer to find soil modulus, and correlations to soil strength and behavior. I'd also stick a piezometer 20 ft into the ground and measure the depth to the static water table. If you don't have water table conditions above the basement grade how would the hydrostatic pressures actually build up on the basement wall?

Watch out for your assumptions. Some may differ with my post; however. . .

f-d

f-d

¡papá gordo ain’t no madre flaca!

 

[rowingengineer]

I agree with the above, I would question should the wall have been designed for the saturated clay? Is there not sufficient drainage behind the wall? There are so many more questions. I would also make sure you don't rely on what the insurance or 2'nd contractor have advised, you will end up getting drilled on it in court.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[fannanxx]

I agree with f-d, I am working on a site compacting fill for a garage, the compaction was done with small lifts of 4" made of spoils of crushed soft rock and top soild (no organics). The site was a water trap this winter since I dammed the flow of water from adjacent properties and installed filtration fabric to avoid sediments escaping the site. We had a heavy winter this year and at times I had 3 to 4 inches of water stagntating over the compacted fill area. Now the site is dry and I hardly cam shave off enough materials to level the place where the grade beams will belaid. I think the proble is in the soild under the garage and the way it was compacted.

 

[DPAJR]

Thanks to all of you for the responses.

VAD, there is a lot more history than what my question would indicate. I am just looking for a way to present the data in the way a jury of unknown makeup and background can understand.

Fat Dad, Good point about the earthen dams. However, it would be impossible to push a rod 9 feet into any dry soil with one hand no matter how poorly compacted. The soil under the garage floor is no longer soil but soup. Borings in the garage floor and the use of a bore scope show about a two inch air gap between the soupy soil and the bottom of the garage floor slab. The water build up appears to be due to the fact that the downspouts from the roof are being routed into the soil by the perf pipe which was buried just outside the garage door. Prior to the installation of the perf pipe the downspouts were routed into the ground into gravel beds which then seeped into the adjacent soil.

The contractor who made the mistakes in question was called in to fix an existing condition. However he made it worse. He may not have created the problem but at the very least he should refund the homeowners money. The contractor I am dealing with now was called in to investigate. He has about $150,000 worth of instruments to aid in the investigation including ground penetrating radar, expensive rebar locators not the hardware store variety, thermal imaging electronic bore scopes etc.

Thanks,
dpajr

 

[smb4050]

How is GPR going to help with this? Have you check if the perforated pipe has negative of positive slope? What is the slope? What rate does the perforated pipe drain under no or very low head pressure? What's the soil's infiltration rate? How much rain has this local had during this period?
Next you'll be claiming this homeowner is so badly damaged that the floor and wall all need to be removed and "properly" rebuilt! Maintain your objectivity as the owner's expert engineer, or you'll get your head taken off by lawyer who gets enjoyment from it and has had a lot of practice at it. The lawyers will get paid, the homeowner won’t spend the money from the settlement, and you end up looking like a whore.
I started a thread a few years ago asking for ideas on what could cause the webs of a exterior CMU to split causing the wall to bow above an adjacent roof, and similar to what you are doing asked for ideas. In my case an architect, using a cheap probe looked inside the wall and saw the webs were broken, and came to the conclusion the webs separated due to severe moisture infiltration and the wall was in eminent danger of collapsing. That made absolutely no sense to me, but the architect was obstinate about what he saw. I finally got my client, who was a cheapskate lowlife, to open up the wall to do a visual inspection. Guess what we found? The broken webs seen were due to the CMU being laid around a steel support beam, and the wall was built with a bow in it and the masons just left that way.
Be careful that you know the difference between what you think you know and what you know. Engineers get paid for being smart, not for being brave.

 

[fattdad]

Yes, dry mud has greater strength than wet mud (for some reason, I'm reliving my childhood). That said, in either form it's not compacted. The water releases the dry strength of poorly compacted soil, but it was poorly compacted when it was dry also (after all, we are talking about dry density).

You need to demo the slab, you need to pull out the bad dirt, you need to replace the bad dirt with compacted fill. The drain pipe can be fixed concurrently, but your problem is not the drain pipe, it's the soil compaction.

f-d

¡papá gordo ain’t no madre flaca!

 

[PEinc]

One small question: No mention was made that the wall was settling, only that the wall was bulging and leaking. If the soil beneath the slab is so badly flooded and uncompacted for at least 9 feet deep, why isn't the wall footing also settling? It seems to me to be unlikely that the slab is on such unstable soil ("soup") yet the immediately adjacent wall footing isn't.

http://www.PeirceEngineering.com

 

[fattdad]

PEinc: I visualize this problem as the foundation soil and the backfill are both saturated, but the foundation soil was undisturbed and firm at the time of construction and the backfill was not. They've both been subjected to saturation moisture contents, but the distinction between dry strength and saturation strength was much greater for the improperly compacted wall backfill.

f-d

¡papá gordo ain’t no madre flaca!

 

[PEinc]

Well then, how do you explain the 9 feet of "soup" under the slab? It is unlikely that the slab was over-excavated by 9 feet but the wall footing was not. Therefore, the footing is probably on bad fill also or the slab was not over-excavated. It seems to me that were are just guessing due to a lack of information. At least one soil boring or test hole just outside the wall would shed some light on the problem.

To me, the first step is to fix the downspouts and site grading to get the surface water away from the building - especially if the wall is on the north side of the house.

http://www.PeirceEngineering.com

 

[rowingengineer]

I think we need a sketch before we can visualise.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[fattdad]

O.K. I'm back on board. What's going on below the slab?

A sketch would help for sure!

f-d

¡papá gordo ain’t no madre flaca!

 

[VAD]

A Structural Engineer once said we pay very little attention to a subject - houses which some many millions are being spent on daily basis and within which we also spend most of our lives.

The responses given so far are enlightening as it demonstrates how complex a commonly presumed unsophisticated engineeering issue - a house - can get.

There is absolutely no need for all the sophisticated techniques to answer this problem. Observation, past history - talk to the owner, previous owner, neighours, look at past development drawings if available, aerials,examine the basement properly on the inside etc are fewof the basic things that need to be done. In other words try to understand what could lead to the issue at hand. If excess hydrostatic pressure has resulted in the wall bulging, is the weeping tile system of the foundation working or was non installed. This could be possible or it may now be defunct.

Core the basement wall if possible from the inside. Water is the culprit then concentrate on determinimg how this has resulted.

Was the building performing well in the past.

There are a number of steps one can take or else youone may end up facing what smb4050 has indicated.

This is one area that many of us are not good at. Step back and give it some thought before plunging. Lawyers and your fellow engineer love these issues. having lawyers involved can get nasty as they will always find a buddy engineer who will take pride in providing the ammunition for him to shred you.