Groundwater under building foundation 1

[nwitt]

The building in question is a manufacturing facility approx. 400'L x 200'W. The building was constructed on select fill over expansive clay; the select fill ranges from 7' to 14' thick. Even with the select fill thickness, the building has suffered substantial foundation damage due to swelling of the expansive clays. Poor landscaping irrigation and sewer line breaks below the slab were originally to blame for the excess water reaching the clays. However, these water sources have been removed (sewer line repairs completed in late 2007 and irrigation improvements completed in 2005). Unfortunately, the groundwater levels are not subsiding and the foundation is continuing to move. We are considering installing horizontal drain pipes below the foundation in hopes of removing/lowering the remaining groundwater above the clay layer. My question is this - what type technology exists to bore a drainage pipe 450'-500' under the building foundation? Whtt type pipe, sleeve, size, etc?

 

[hokie66]

Not an answer to your question, but I don't think it is clear that your footing movements are due to moisture change in the clay. If the groundwater level is constant, the clay may not be changing volume purely due to this reason. I would suspect consolidation of the clay due to the surcharge from the fill is at least partly responsible for the experienced movements. Installing drainage may exacerbate rather than improve the situation.

 

[nwitt]

Thanks for the response, but most of the movement has been from heaving and not settlement. The water levels were not constant until after the sewer line breaks were repaired. The water fluctuated pretty drastically before the breaks were repaired.

 

[cvg]

have you determined the source of your continuing groundwater? shouldn't that be done before simply trying to remove it? consider trying to remove it at the source, rather than installing an expensive drain.

 

[nwitt]

We are confident that the source of the water (maybe I shouldn't have called it groundwater?) was from the irrigation problems and the sewer line breaks. These sources have now been removed. I assume that after some time the water levels will eventually subside, but it may take years due to the properties of the expansive clay. These type soils don't allow water to escape easily. It's our thought that installing drains will expedite the water removal and help slow the swelling of the clay.

 

[escrowe]

Is the select fill freely draining?

The technology to install horizontal drains at the clay surface via directional drilling
is readily available.

Consider that drainage may trigger additional settlement of the fill...

BTW, I find the swelling clay theory rather unlikely,
though not impossible:
How much heave has occurred?
What is the strength of the clay soil?
What is the LL / PL of the clay soil?

For heave to occur, swell must be greater than consolidation (which may happen simultaneously),
and swell pressures must exceed the fill and structure weight.

For damage, there would generally have to occur
at least 1 inch of differential settlement,
depending on the structure type.

 

[fattdad]

Not sure it's ground water either, but I do wonder whether the "select fill" included any flyash, bottom ash, scrubber residue or slag. If so, I'll have alot to say regarding the potential for swell.

Regarding the expansivness of the site clays, can you provide natural moisture content and Atterberg limits?

f-d

¡papá gordo ain’t no madre flaca!

 

[aayjaber]

Why drainage was not installed in the first place? A retrofit drainage that can work and will not cause problems is too elaborate and might cost more than rebuilding the whole building again.

 

[nwitt]

To comment on and/or answer a few of the questions above:
The building is most definitely heaving; we have seen movements of 4-6 inches in some parts of the building. Other areas have experienced some settlement but nowhere near the magnitude of the heaving seen in other areas.
The expansive clay has average values as follows:
LL=63, PL=18, PI=45
No flyash, bottom ash, etc. was included in the select fill.
Drainage was not installed in the first place because there was no need for it at the time. I have seldom seen buildings in this area constructed with subsurface drainage.
We have been working with our preferred geotechnical consultant for several years now, and I have a lot of faith in his position that this movement is due to heaving/swelling in the expansive clays. We have completed 4 floor surveys, installed piezometers, completed borings, etc.
I am simply trying to get some input on how to remove the water from below the building.

 

[BigH]

vacuum dewatering?

 

[hokie66]

OK, accepted that the issue is that the clay has swelled, what will happen if you now drain below the building? As the clay dries out, you will have it reverting to the previous condition, with settlement resulting.

 

[cvg]

directional drilling can be used to install a pipe, without a casing or sleeve. The process uses drilling mud which will surround the pipe creating a filter cake and blocking out any water. But not sure how that will help with your drainage problem. To collect the groundwater, you need a perforated pipe encapsulated in free draining sand or gravel. This will almost certainly need to be installed by conventional cut and cover methods.

 

[nwitt]

Thanks cvg. You have stated the problem that I should have more clearly mentioned. How do you install a slotted drain pipe using directional drilling without clogging the slots/pores?

 

[escrowe]

Big H, vacuum dewatering sounds about right.

nwitt, yes, a dessicated PI 45 clay, expansion index about 130+,
even with a 1000psf surcharge might still swell 5 percent.

Have you considered a perimeter well-point system?
You would essentially surround the building footprint with a PVC manifold pipe
connected to numerous small diameter vertical screened pipes.

You would then pump the system until dewatering is complete. And likely watch the building settle...

This approach is typically for temporary dewatering,
but makes sense if the site drainage problems have been fixed.

http://www.eidw.ae/well_point_dewatering_system.html

 

[Friar6]

There is no indication of the depth to the groundwater in your original report. In addition, for an expansive clay, it is better to have the water content at least three to five percent over the optimum water content. If the distress in the footings is from swelling, then the fill may have been placed at a water content substantially the optimum water value. Have you considered installing a perimeter subddrain (a trench with perforated pipe and crushed rock in a filter fabric, etc.,) around the building? This alternative is economical depending on the depth to the source of the water! Good luck

 

[Delijosi]

Perimeter well point may not be possible in clay. I would suggest using electrokinetic technique. This technique is suitable for clays.

 

[oldestguy]

Years ago we had some drains installed in clay soil to stabilize a major slope failure. These were done from base of slope with casing, with auger inside it. A perforated pipe was installed in the casing and sand filter was blown in in the annular space, as casing was withdrawn. Resulting pipes were about 100 ft. long, and carried water "forever".

In your case, this would have to be done from trenches outside the building, probably way too expensive to consider.

Grouting contractors have installed grout pipes radially from large diameter man-holes. Could you do the same procedure by jacking well points radially from man-holes, then use sump pumps in the manholes?

I suspect drainage will not cure the problem however, due to the affinity that the clay has for water.

Has anyone looked at lime injection to reduce the clay affinity for water?