burst buried water pipe lifted slab on grade - remedial procedure

[ajk1]

A burst water pipe lifted an existing 30 year old 5" thick slab on grade about a foot. The water was shut down about 75% about an hour after the flooding was noticed, but due to a faulty valve was not totally shut down for another 10 days and the water continued to run under the slab for that time, although it was also being pumped out from a pit.

The portion of the slab that lifted has now been cut out. Under the slab is about 6" of gravel and a poly sheet and then clay material under that for as far as we dug ...at least 4 feet and probably much deeper. The slab performed well in the past so that is not an issue. The questions I have are:

a) The clay is now very wet. Can we just place a 6 mil polyethylene sheet and pour a new slab, or is there any possibility that if the clay slowly dries out, it will shrink over the months and years ahead, and the slab will settle? Is there any other concern about placing a new slab over the wet clay (there is 6" of gravel under the slab before the clay starts).

b) There are buried sheet metal ducts for the supply and exhaust ventilation. Currently there is water in them but it is being pumped out and the ducts will then be vacuum cleaned. Is there any possibility that the water in the clay will slowly leach into these ducts, or can we count on the water in the clay to be retained in the clay and not go anywhere?

c) grout or urethane injection can possibly be used under the portion of the existing slab that has not to-date been cut out, to fill any possible voids under the slab, but would it be effective in correcting any soft spots in the clay material under the gravel?

d) if grout injection is used, might its pressure collapse the buried sheet metal duct that runs in the area where the buried water line broke and the area that would be injected? At what pressure is the grouting done?

e) might the grout find its way into the buried air ducts?

 

[a2mfk]

You need to call a geotech and get them involved on this project, if you don't already. I would recommend a minimum of 5-10' deep hand augers and see what type of soil you have for sure. You mentioned clay, but not all clays are plastic. Some hand cone penetrometers would give you some idea of what kind of density you have also. Holes can be cut/drilled in the existing slab to perform the testing.

 

[ajk1]

LOL. As an experienced structural engineer, I fully understand that a geotech is needed and have had some preliminary contacts about this job, with a couple of geotechnical engineers with whom I have worked over the last 5 decades, but there is some opposition to retaining one till now, although I do expect authorization to do so soon. In the meantime I was hoping that someone on here was a geotechnical engineer and could tell me whether my questions were soundly based or whether I was worrying too much. I guess there are only structural engineers on this system.

 

[TigerGuy]

As far as the clay goes, your best bet is to moisture condition it to near optimum and recompact. Easier said than done, I know.

You state that the water was on for 10 days. The slab raised 12 inches. If the clay soaked up that much water in that amount of time, I would be concerned about what the extent of damage is. Additionally, it generated enough swell pressure to lift a 5 inch slab.

Good luck

 

[csd72]

ajk1,

a) I would be concerned that the clay has sand lenses that are now filled with water as well as the gravel beneath the slab. These will take months to be absorbed by the clay and will therefore produce swell during that period.
b)if it has leaked in before it can again. Dont forget that most of these buried items will be surrounded by sand backfill.
c)toe) you should discuss these with the grouting contractor.

 

[cvg]

sheet metal vents buried in ground under a slab? is that even allowed by code? yes, water will probably get in the vents and the grount will also

grout might fill the voids but it will not correct "soft spots", it will also saturate the entire gravel layer. that will take a lot of grout. and yes, if the pressure is too high it could damage a duct, unless your "duct" is really a pipe that has some strength to resist the pressure. you can deterine the allowable pressure on the duct so that it doesnt collapse.

you are expecting the grout to find and fill voids in the clay, if those voids are in the duct work, then it will find its way in.

 

[Ron]

TigerGuy...I doubt that the slab was lifted by swell pressure from the clay...moreso by hydrostatic uplift from the water....slab was less resistive than underlying soil.

As for just putting a vapor barrier on it and re-placing the slab, I would not recommend that because the clay will shrink as it dries.

You might consider a pre-load wick, using clean sand as both the pre-load and wick. Monitor settlement and moisture content. Check the moisture content in the clay away from the area of concern and try to equilibrate. It will take a little while (maybe a couple of weeks) but it should fix the issue.

 

[a2mfk]

Glad I could make you laugh.... You posed a question about a slab and soil problem, but you don't have all the facts, ie, geotechnical data, or at least didn't share them with us. If this were my job I would want soil testing prior to jumping into the mitigation phase of the project. It may end up answering most of your questions. But that's just me and my 1.3 decades of experience.

 

[ajk1]

Thank you everyone for responding.

Ron's comments about the pre-load wick are particularly interesting.

Also cvg's response about why grouting may not be a good way to go is interesting and exactly what I had been thinking. Thanks cvg.

The rise of the slab was from the water pressure during the one hour that it was on full force; not due to clay swelling. After that time the City shut the gate valve at the street, but they could not shut it totally, likely due to corrosion of the valve, and it continued to run for 10 days till we dug down and shut the water off.

Yes the ducts are sheet metal, not pipes. I assume it was permitted in 1965 when it was built because it was done by an establsihed architectural firm and went through the building permit appplication process. Has never been a problem except now that the water pipe burst.

We now have approval to retain the geotachnical engineer that I recommended, after a week of my urging them to do so. Will be interesting to see what he says.

 

[ajk1]

They have now cut out the slab and there is a 4 foot deep washout hole in the soil at the break in the water pipe. We also found the buried air distribution ducts...I erred in saying they were sheet metal, they are actually transite pipe and the investiagtion so far indicates that they are in good condition, although water got into them and we have now pumped it all out.