Mitagation of High Water Table
Mitagation of High Water Table
(OP)
Any ideas and comments would be greatly appreciated. An accountant friend of mine wants to construct a small, 2000 sf footprint, two story office building (I was not involved in design). Helical pier foundation with crawlspace. After completion of foundation excavation approx. 2 feet of water collected in the hole over a 6 week period. On my recommendation the hole was pumped out and approx. 2" of water collected in one week. Soil profile is alternating layers of sandstone and hardpan (moderate to high swell potential). The geotech's tech (not the engineer) told the owner she'd be crazy to build there. The owner is concerned about mold in the crawlspace, sick building syndrome, etc. Site is located in Colorado's front range, semi-rural, no water features nearby (large or small), average annual precip is about 15". My opinion is they're dealing with perched groundwater and that sinking a well adjacent to the building with the well head 20' to 30' below crawlspace level would lower local water table enough to permit safe usage. Provide proper foundation drains, vapor barrier and powered crawlspace vents and no worries, mate. They could even put a slab on top of the vapor barrier. The owner is going ape because if I'm wrong she can't build on half of her property. What do you guys think?






RE: Mitagation of High Water Table
2' is an awful lot of water. If this is suface runoff collecting in the excavation, then maybe this is not so bad. If this is truly groundwater traveling along sandstone lenses, then I would be concerned.
Your water mitigation methods sound OK to me. I would talk to the geotechnical engineer and ask where the water table is in relation to their their boring log(s). They may want to over-excavate down to the suspected water lense and intercept it before it enters the building footprint. The effectiveness of the sump pit would be a function of the soil permiability. Helical piers implies expansive (and impermiable) clay overburden on the Front Range.
RE: Mitagation of High Water Table
Agree with sundale about using hot dip galvanized products, but don't be too concerned about damage to the coating. Unlike painted and epoxy coated steel, the zinc bonds to the metal and will self-protect the damaged area. See "Abrasion Resistance / Resistance to Mechanical Damage" at this link
http://
www.SlideRuleEra.net
RE: Mitagation of High Water Table
The geotechnical engineer should have typical details for drainage systems that could be used, as well as provide information on vapor retarders and ventilation (I know my company does). I do not think a well type system as you describe would work for the soils you mention, but there are several good alternatives. Also keep in mind that if you are sinking a well deep and pumping, you may have to deal with water rights issues.
RE: Mitagation of High Water Table
RE: Mitagation of High Water Table
RE: Mitagation of High Water Table
BTW, the tech that made the comment should be fired (or at least sternly reprimanded). If the site was that bad, then the Geotech should have had the guts to say it from the start, otherwise, the tech is contradicting the PE that stamped the report that gave recommendations indicating the site was constructible. That is not very good client development.
RE: Mitagation of High Water Table
RE: Mitagation of High Water Table
JAE....The original EOR has been notified of my involvment. The owner has lost faith in the entire design team and they really don't seem to be working too hard to restore it. My feeling is the owner went with the cheapest proposal she could find and got what she paid for. Before anybody starts yelling I agree that is no excuse for a lack of professionalism. But in my experience that's what happens, more often than not, when the owner goes that route.
RE: Mitagation of High Water Table
As for tieing down your structure, unless your surface rock is more like residual soil, I, too, have a hard time fathoming how helical piers can be installed (unless they are known to work locally). Short stubby drilled caissons - or small diameter auger holes may work to tie down the foundation - or to support the structure by going deeper than the potential swelling - Else, you could use a suspended crawl space separated from the walls - so that even if the rock swells, it will swell to an open space. Three are a number of engineers on the site with a lot more experience of swelling soils than I so hopefully they can shed some light on the subject.
RE: Mitagation of High Water Table
The other posters' comments about predrilling are right on. Most of the helical pier contractors that I worked with used a single 8" diameter helix for expansive soils. This will NOT typically penetrate any but the most weathered of bedrock. Most of the helical pier designs I did were for a foundation underpinning repair which had an expensive lift head weldment. These cost roughly $1400 each installed. New construction requires a much simpler "T" head at the top of the pier shaft, which would be a maybe $400 less (guessing here). Even with the cheaper T-head, I seriously doubt that a steel helical pier is the best solution given the added cost of pre-drilling.
If one needs to pre-drill the hole, why not just drill a bigger hole and drop a rebar cage and fill it with concrete? If I recall, drilled piers ("caissons" in old school talk) cost about $300-$500 a hole depending on the diameter and bedrock penetration. The rebar and high slump concrete are a minimal added cost.
If the geotechnical engineer only recommended helical piers, did not mention any pre-drilling issues, and apparently muffed such a groundwater problem, I would consider telling your friend/client to seriously consider obtaining another soils report.
RE: Mitagation of High Water Table
RE: Mitagation of High Water Table
RE: Mitagation of High Water Table
It seem to me the issue with piers leaves more questions that need to be answered. What do the core samples reveal? Will the drilled shaft require casing? Is a concrete pump truck required to deliver concrete on a saturated site? will piers require belling or is a friction pier ok.
As to the water issue if a suspended concrete deck is placed and foundation is properly vented (power vents with humidistat) mold is not an issue you just have a wet crawl space. And i was wondering why you couldn't soil cement the pad site and would this help dry that out.