Pad Failures Due To Recent Rains

mmoncrie

Has anyone experienced any soil failures as a result of the recent Texas rains?

We built a pad on top of an existing dry creek, by means of overexcavating and replacing material per GeoTech recommendations. After the historical rains in Texas (25" in 1.5 months), we began to experience slab on grade failures. Upon exploratory investigation by borings, we found that the water table was 4' below our finished floor, and the subgrade's moisture content underneath our pad is around 15% above optimum moisture (it should be 3%). This water was not encountered during drilling / placement of piers, which went down over 30'.

We are now required to remove over 50,000 SF of our in place slab on grade and possibly "repair" the pad. Has anyone had a similar experience recently?

pwrbildr

Am I understanding you correctly that the pad passed inspection by the geo tech inspectors at the time of placement? Is there field test reports to back that up? If so I would think this might fall under builders risk?

What type of repairs are you going to have to make? Water table is water table and it can't be stopped. If you simply need to bring down the moisture content of the pad I'd suggest digging some sump pits around the perimeter and if necessary maybe a couple in the middle of the slab. Perhaps a perimeter drain that drains to a storm structure or a low point of the grade around the pad. But even with that being said, what are you expected to do, keep the moisture content below a certain point for the life of the building?

K2

The engineer should have a risk policy and the building should have a risk policy. I would be looking to the insurer on the building first. "Sudden and Severe" is the term I've heard used. I would consider 1.5 months sudden to the life of a building.

K2

I remember redoing a city owned swimming pool with the specs written by a city employee who was out there inspecting the work at least 4 hours every day. 2 months later when the work failed they came demanding this and that. That lasted about 15 minutes.

mmoncrie

We had a full time geotech onboard for the duration of the job. We currently have 7 borings inside the building monitoring the water levels. The water has actually risen to 2' BFF, after not experiencing rain for over 3 weeks.

We have tossed around the following ideas:
1) Remove / Replace damage slab on grade and replace with a 15 Mil Stego vapor reducer
2) Install perimeter french drain system tied into storm sewer
3) Install subsurface drainage system over the entire building pad tied into the storm drain.

We are going to remove portions of the slab on grade beginning Monday, and will have our geotech provide further investigations of the in place subgrade. I found a company who has a computer program that can actually locate all underground water by using probes and electric currents. At this point, it looks like we will have to provide remedial work to significant portions of the building subgrade, as the water table has caused yielding of the soils.

As for the life of the slab on grade concerning moisture content, there is a product manufacturer by Koester, that significantly reduces moisture contents in slab on grades. My company has used this product numerous times in Florida and have not experienced any failures of finished flooring. I believe the product is Koester VP-1, and is essentially an epoxy compound spread over the slab on grade. Only downside...it runs $3 / SF installed.

pwrbildr

Is this all taking place at your cost or do you have an open ended T&M change order in the works?

Unless I'm missing something here (fine print in the contract documents) how could you be held responsible for a pad/slab failure after you installed it per spec.? Time to get a lawyer involved before you end up undercutting half way to China.

mmoncrie

Fortunately this is not on our dime! No lawyers required. We ended up solving the issue today.

Turns out the water wasn't coming from below the pad. The water collected in our 4" capillary break between the vapor barrier and lime stabilized cap. The water had nowhere to go, as we had not excavated our tilt panel ditches, due to developer delays and owner directed delays. The water eventually weakened the subgrade, which was confirmed today via nuclear density testing and test pits, and when a load was exerted on the slab, the damages occured.

According to our geotech, capillary breaks underneath SOG are useless, especially on a 4" unreinforced pad. They offer no additional benefit to the performance of the slab on grade.