Measuring Foundation Hydrostatic Pressure

[mmbridges]

I am looking to measure the hydrostatic pressure at the outside of my homes basement walls. The basement has poured concrete walls with the north side of the basement completely below ground level but with the south basement wall above ground level with a walk-out basement door and no stairs.

I would like to drop a 1" pvc pipe 10ft down along the outside north basement wall. Cap the top with an 1/8" fitting so I can attach my Dwyer Mark II manaometer.

Would such a makeshift piezometer get me a decent reading of the hydrostatic pressure at my basement walls?

Thanks!

 

[oldestguy]

How about a tape measure dropped down an open pipe, to footing level only, and open there, not plugged? Top of water table should be enough. Once you get that, then what?

Many basement water problems are due to surface water getting there, perhaps with temporary saturation conditions, not necessarily with much of any pressure, as with a sponge. A piezometer may not say much about that, yet dampness can show up inside.

You can't hurt by what you say, but it may not be sufficient for knowing what to do.

 

[mmbridges]

That sounds like a much simpler and elegant solution!

I suspect a portion of my exterior drain tile is clogged but I was trying to get a feel for how much a working drain tile system would limit the hydrostatic pressure. I have a small leak in the corner of the north basement wall that only appears after a hard rain. I regraded the ground away from the foundation at the proper slope and installed a plastic building skirt over the first 5 feet from the foundation.

The leak appears at a vertical hairline crack in the corner about 6 ft from the basement floor.

Would that suggest the water table around the foundation has risen to that level and furthermore would the tape measure dropped down the dip tube indicate that level?

 

[oldestguy]

Could be that high a saturation and temporary water table exist for you. Somewhere on this WEB site there is a thread where I explained my method for waterproofing the ground to help on that score. You probably did that with the "roof" installed. If interested, perhaps a search using key words such as "bentonte", "ground waterproofing". Maybe try using my name for that, but I have done a lot of comments.

If your footing drain is clogged, that may be the case. On that score, search for my comments about "french drains" where I explain how so many of those drains done by well meaning folks go bad. The main fault usually is the backfill to them is not filtered to keep mud out of the pipes. Gravel backfill is crummy for filtering out mud. In those types of comments by me you also may find the ground waterproofing comments.

 

[mmbridges]

My sump pit has two 4" black corrugated drain pipes entering it which I suspect form the exterior perimeter drain tile system. During heavy rains only one pipe drains water into the sump pit. I have never seen anything draining from the other pipe. I suspect it is this pipe that is supposed to drain the corner with the leaking crack but is clogged.

In your experience, with a properly functioning exterior drain tile, what maximum water height should I see in my dip tube next to the outside basement wall?

Do I have the physics right? Assuming my ground "roof" is working and all of my down spouts are discharged sufficiently far from the foundation, 1.) will the ground water start collecting at the bottom of the foundation footer/clogged drain tile pipe and slowly start rising up the outside basement foundations wall until it hits the 6ft point of the crack? Or 2.) is the surface water getting under my ground "roof" and getting to the foundation wall in such a way as to start saturating the ground near my foundation from the top down?

I am trying to visualize the soil saturation gradient around my foundation wall. It would seem if the ground water is getting to the 6ft level of the crack from the top down, before it reaches the drain tile, then it is not clear a properly functioning drain tile would help. Is this the low hydrostatic pressure temporary saturation scenario you hinted at in your first response?

I appreciate your thoughts!

 

[BigH]

As a first step rather, why not try to seal the hairline crack. Polyethelene injection can be tried . . . if that doesn't work, then more extensive measures that others have suggested would be warranted if this really bothers you. My dad installed a "fake" baseboard moulding at the bottom of his basement - hollow on the inside and then the water would run to a inside the moulding to a sump; though, that would probably be better if the water was coming up along the base of the wall to the floor . . .

 

[mmbridges]

Thanks BigH. I actually might try that when I get to the solution stage. However, I'm the type of guy that likes to get a detailed understanding of the actual problem first hence my questions about how water might build up around my basement wall and how the use of a dip tube down to the footer might help in my understanding. Based on your experience, with a properly functioning exterior drain tile, what maximum water height should I see in my dip tube next to the outside basement wall?

 

[oldestguy]

Interesting question about what actually happens. Usually, if there is a wet basement, walls or floor, a contractor is hired to re-do the system. No attempts (by me or my guys) to measure anything. Just get it done.

BigH: Not so sure about sealing anything. I have seen walls cave in, due to pressure outside, aggravated by water there. If walls are strong enough, maybe OK, but I would not seal any place where there might be pressure from water.

Down spout dump should be beyond any backfill. Reason is that backfilling usually is done by shoving in from the side and layers develop sloping towards the wall, which direct water that way, not just down.

While we are at it, any replacement should be with filtered systems, not GRAVEL. Gravel has large voids that do not filter out mud.

Chances are the footing drains are slotted pipe with sock. However, mud coming there will plug the sock over those slots. So any backfill above and near the pipe should be a filter. The best all-around filter is fine aggregate sand for concrete. ASTM-C-33 fine aggregate. Same cost as gravel.

 

[mmbridges]

I understand when your fixing a problem on the job you don't have time to conduct a science experiment (time=money). As a homeowner, I don't have such constraints. Can I ask you to speculate? what does the spoil/groundwater science say on the matter? If the drain tile is clogged does the water build up along the outside of the foundation from the footer or if you look at the wall does it saturate from the top down?

I also do understand your point about the interface between the backfill and undisturbed soil sloping towards the foundation. But if this happened, and I had a working drain tile system, would that prevent the saturated water level in the soil from rising to the 6ft crack level?

I saw some advertisement about the ability of different coatings placed on the outside of the foundation wall being able to handle different levels of hydrostatic pressure. In particular, one company claimed their coating could withstand 3 ft of hydrostatic head while typical coatings can only handle 1 ft. Does this suggest my dip tube water level measurement should never even get over 1ft from the bottom of the footer if I had a working drain tile system?

I'm a little confused as to whether the water level measured in my dip tube = hydrostatic pressure at the foundation wall.

Hope you or others can shed some light.

Thanks!

 

[oldestguy]

From my standpoint of looking at job situations, usually the trouble is not an even on going thing. Usually it is spring of the year and sometimes after a rainy few days. Then, in excavating we see saturated ground, several feet high in the area. Actual water table, usually called "perched" may be to the top of that saturated ground. From experience, if we left the hole open, the water may stabilize in a few hours, or in more cohesive soils it may take days.

From a home owner's perspective (I am one), any water is no good and I don't want it. So, I then take efforts to prevent that happening again. That has been done on two houses I owned and also has occurred where clients have the problem. No one wants to wait it out.

If you want to get scientific, I would have to takeseveral test borings, leave observation wells in several places, run soil gradation tests and permeability tests on representative samples in the lab. Then I would prepare a flow net diagram to explain the water tables and the flow of that "ground" water, even if it is perched. Such a study could be done, but I have only done it for examining the flow situation of water seepage at sanitary landfills and earth dams. The expense for such as study would be many times the cost of digging out the failed system and doing it correctly. We can make a judgement on site by looking at what is available and be close enough. However, then what do we do? Usually I recommend replacing a failed system. Also, I have added the ground surface waterproofing thing where that may appear useful. Sometimes that is all that is needed. On one of my houses (and several jobs) only the ground surface waterproofing was needed. If you have not searched for that in previous threads, I can go through that here. I probablyshould keep a copy on file for these occasions.

 

[mmbridges]

Your introduction of the term "perched" had me heading to Google. So my search introduced me to the wonderful world of hydrology and partial differential equations. Fascinating stuff! Being an EE, I especially appreciated the analogy between Darcy's Law and Ohm's Law. So I do want to get a bit scientific but not to the point of doing the study you mentioned.

I wonder if you or someone else here could answer the following hypothetical which I think will improve my understanding of things:

Suppose my drain tile is clogged and it has been raining for a while such that the water table just outside my wall is 3 ft above the footer. Further, suppose the soil directly above the water table up to the ground underneath my "ground roof" is 100% saturated. I believe this would be called the capillary fringe. Since it is above the water table, would the hydrostatic pressure at my wall at the 6ft crack level be below atmospheric pressure? If so would that mean it would be impossible to get a leak at that point in the crack under these conditions?

Is it realistic to have a capillary fringe zone underneath my "ground roof" near the wall that is this thick (i.e. ~7ft = 10ft wall/ground height above footer - 3ft water table above footer)?

Thanks!

 

[oldestguy]

I assume your water table is found by a well of some sort open to any water in the zone between footing and ground surface, as a perforated pipe. Per your question, the pressure on the wall is the product of distance below the water surface times water density, using suitable units. In the capillary fringe there is no pressure from the water there, since it is held by capillary tension. However. if the pores in the wall are so fine that the capillary attraction (suction) there is greater than for the soils outside, then water will enter the wall's pores. However, very fine pores are darn near waterproof.

For your earlier statements about various contractor applied sealants on the outside, well I'd not believe any claim. Many are available and most apparently have very low hydraulic conductivity (permeability). I'd be very careful about injecting bentonite, since it can swell to many many times its original volume. Asphalt is a common sealant for initial construction.

Personally I'd never use a contractor to seal up things. Damp proofing is all you need. I'd not go for waterproofing as with sealing cracks. If the pressure builds up to the extent that the wall caves in, you won't be a happy guy, maybe a camper until it is fixed.

Have you tried getting a snake up the drain that carries no water to see what goes on? If that goes, maybe a garden hose will to jet things out.

 

[mmbridges]

So just to clarify, you are saying that if the crack at 6ft is "fine" enough to create capillary attraction (i.e. suction) greater than that of the soil then water can leak into the basement through the crack even though the water table is well below the 6ft crack level. Is that correct?

If so which do you think is more likely, 1) a leak from the capillary fringe zone through crack at 6ft or 2) the water table rising higher than the 6ft crack level causing the leak?

I have not snaked the line but that is on my list of potential courses of action along with possibly using an inspection camera.

Regarding water table measurement at the wall. At the top of this thread you suggested instead of using a manometer at top if a 1" pvc dip tube I could just leave the top open and drop a measuring tape down to see where it hit the water.

If my PVC pipe is not perforated along the entire length and just open at top and bottom, how is my water table measurement accuracy degraded?

With a properly functioning drain tile system should I be trying to attain a water table at the wall that never rises higher than say 12" above the footer/drain tile pipe?

Thanks for your patience.

 

[oldestguy]

Lot's of questions.

So just to clarify, you are saying that if the crack at 6ft is "fine" enough to create capillary attraction (i.e. suction) greater than that of the soil then water can leak into the basement through the crack even though the water table is well below the 6ft crack level. Is that correct?

Answer "NO". While a crack may take on water due to capillary action, there is no "leakage" resulting. Think of capillary action as a vacuum action. That's maybe why British use the word "Suction" and we use the term "Tension". This property can be measured with centrifuges overcoming the"tension" with an increased gravity.

If so which do you think is more likely, 1) a leak from the capillary fringe zone through crack at 6ft or 2) the water table rising higher than the 6ft crack level causing the leak?

Answer: Only a rise of water table to that point, but also a few possibilities of free water dribbling on its way down the wall after rain dumping water against the wall up there..

Regarding water table measurement at the wall. At the top of this thread you suggested instead of using a manometer at top if a 1" pvc dip tube I could just leave the top open and drop a measuring tape down to see where it hit the water.

A fully perforated pipe is best, but very likely for most cases, an open pipe at bottom will work. Add some perforations and rapid changes in water conditions can be checked more accurately.

With a properly functioning drain tile system should I be trying to attain a water table at the wall that never rises higher than say 12" above the footer/drain tile pipe?

With a properly functioning perimeter drain system, the water table will never rise above the top of the pipes. Ideally, most of the time it will be below the invert of the pipe. Now there is another word for you, "invert".

Hey: Bite the bullet and replace the noon-functioning system with a properly filtered backfill. For details on filtering water from earth do a Google search. This one has some ideas.

http://web.mst.edu/~rogersda/umrcou...ures/subsurface_drainage/GE441-Lecture5-9.pdf

 

[mmbridges]

Please forgive the wall of text. This is really me kind of thinking out loud as I try to understand what might be going on outside my basement wall in the context of subject matter that is new to me. I appreciate folks pointing out any errors they may see in my thinking.

Oldestguy, thanks again for the thorough replies as they clear up quite a bit. However before I bite the bullet and make the considerable expense to "replace the noon-functioning system with a properly filtered backfill" I still am not clear whether it is the drain tile system that has failed or the ground roof.

You have ruled out the scenario 1) where the capillary fringe is at the 6ft crack level and causing the leak because the fringe zone still is under negative pressure right?

I have ruled out the scenario 2) where water dribbles down wall after wall has been splashed by a heavy rain because I have observed the wall during such a heavy rain event and it is fairly dry. The eaves stick out enough and the ground roof butts up again the wall and has enough slope such that I do not believe any water is seeping down at the "ground roof" to wall seam/interface.

You have ruled as most likely scenario 3) where the ground water rises above the 6 ft crack level and qualifies as one definite way for the crack to leak into the basement. Am I right in thinking this is because in such a scenario the groundwater pressure head at the crack is >0 whereas the basement is at 0 so pressure diff causes water flows into the basement?

So I am still unclear as to the possibility of scenario 4) where water infiltrates down through the soil (not dribbling down the wall) to the 6ft crack level and leaks in. The only way for this to work is for the saturated soil above the "wetting front" to have positive pressure head so once the wetting front moves down through the soil and falls below the 6ft crack level the leak begins.

Reading up on the hydrology science I found a notional set of pressure head vs depth plots in a time sequence that showed the effects of rain water infiltration. The y axis showed height (z) above mean sea level and the x-axis showed pressure head (psi).

What I got from the time sequence plots of height (z) vs soil pressure head (psi) is that, as the wetting front migrates downwards, the soil pressure head above the wetting front stays zero! If one looks further below the wetting front it becomes more negative but eventually goes back to zero as it passes through the capillary zone and hits the water table. This was unintuitive to me.

So it would seem the column of soil outside my basement wall during a heavy rain has two capillary zones. One between the ground surface and the wetting front and another between the water table and the unsaturated zone. Both of these cap zones have zero head pressure. As rain water infiltrates it increases the size of cap zone 2 by moving the wetting front boundary down. Cap zone 1 remains constant thickness. As the wetting front moves down, the unsaturated zone thickness decreases until the two cap zones meet. At this point there is one large saturated cap zone with zero head pressure. It would seem prior to this point in time no leak could occur at the 6ft crack level.


So the question is what happens after this point? Could additional infiltration cause an increase in head pressure near the 6ft crack causing a leak BEFORE the ground water rose to the 6 ft crack level? Or does the new single unified cap zone maintain a zero head pressure with all the additional infiltration flowing down to the water table causing it to rise without any increase in head pressure above it?

The latter would eliminate scenario 4.

I made a text drawing to help me visualize what I wrote. Thanks again for everyone's patients and comments!

[pre]
wall

|
|
outside air |------------------- <- basement ceiling
|
-------------------| <- ground surface
|
cap zone 2 |
|
-------------------| <- wetting front = interface between capillary fringe zone 2 and unsaturated zone
|
o <- basement crack 6ft above slab where leak is seen
|
unsaturated zone |
|
-------------------| <- interface between capillary fringe zone 1 and water table
|
|
cap zone 1 |
|
-------------------| <- water table
|
|
|
---------------------- <- basement floor/slab
drain | |
tile -> ( ) | | <- footer
[/pre]

 

[mmbridges]

Oops! found an error in my diagram. The second interface should actually read:

<- interface between capillary fringe zone 1 and unsaturated zone

[pre]
unsaturated zone |
|
-------------------| <- interface between capillary fringe zone 1 and unsaturated zone
|
|
cap zone 1 |
|
-------------------| <- water table
|
|
|
---------------------- <- basement floor/slab
drain | |
tile -> ( ) | | <- footer

[/pre]