Mitagation of High Water Table 3

[Engrman]

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?

 

[sundale]

I would first recommend either a serious hot dip galvanized coating on the piers (that is compromised/scratched upon install) or perhaps even a cathodic protection system. Drilled concrete piers would not be a bad idea here.

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.

 

[SlideRuleEra]

In the coastal, southeastern US there are no problems with swelling soil or frost, but a high water table (sometimes essentially at ground level, year round) is an "everyday" situation. Houses on slabs (with vapor barrier) and crawlspaces (with lots of foundations vents) perform nicely. Powered vents are only needed in the worst cases and can be added later, if needed. The best solution (here) is to place 6" to 12" of compacted fill under the footprint of the house before construction begins. This brings the footprint area higher than the surroundings (for surface drainage) - that's normally all it takes.

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.galvanizeit.org/showContent,288,332.cfm#resist

http://www.SlideRuleEra.net

 

[TDAA]

There is 2 feet in a crawlspace excavation, and it was not encountered in the Soils and Foundation investigation? It sounds like your friend may have hired one of the cheaper firms in the area although it may have not been enough to notice in the holes. With that much water, it should have likely been noticed in follow up water measurements taken at least 24 hours after drilling. Did they do them? Based on the apparent decrease in the water entering the hole, I would have to assume this is perched water.

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.

 

[PEinc]

"Soil profile is alternating layers of sandstone and hardpan." Can helical piers be screwed through sandstone and hardpan? I would not bet on it. Perhaps it would be good to talk to a local helical pier contractor about the ground conditions.

 

[Engrman]

Thx for the responses. All helpful as I've come to expect from this forum. TDAA....The geotech is one I've had prior experience with and I'm not impressed. PEinc...their helical pier recommendation kind of baffles me, too. But as I said, I wasn't involved in the design. I don't think water rights would be an issue but I was rethinking the well anyway. I think a properly designed fdn and underfloor drain tied into a sump would work fine.

 

[TDAA]

The helical may be a strange recommendation, depending on the actual soil profile. I am assuming they recommended them due to the swell potential of the soils. They will be relatively useless, in that regard, if adequate penetration cannot be achieved. If they are bearing within the seasonal moisture variation, they will likely move. Did they even indicate in the report that there are other options, such as drilled piers, or sub-excavation? There are so many options here in this area! For information, what city is this near?

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.

 

[JAE]

Engrman - you keep saying, "I wasn't involved in the design". This is a little off-topic, but if there is an original Engineer of record, it is appropriate for you to notify them of your involvement. I would think that if there is an EOR out there, they would certainly want to be involved in any decision pertaining to a change in design.

 

[Engrman]

TDAA....helical piers was the ONLY recommendation contained in the soils report. I'm not yet aware of any converstaions between the consultants, contractor and geotech regarding other options. The site is near Colorado Springs. I agree about the tech opening his mouth. If he worked for me it would be the last time he would have talked directly to the owner about a topic that serious. The soils report did indicate water in the drill hole but well below the crawlspace level. I'm guessing the foundation excavation gave some nearby perched a place to run to.

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.

 

[BigH]

If you have water coming into a shallow crawl space excavation - it is most like seepage from weather fractured rock and/or surface water. I would suggest that you consider a fully surrounded french drain taken a couple of feet below the base of your footings. Tie good slab underdrainage as suggested by SlideRuleEra to it. Get the french drain to go to a low spot and use a sump - or if you are lucky, you can tie into the city/town sewers. Main thing is to try to keep the area free of water that could cause the potentially swelling rock to swell.
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.

 

[sundale]

On the Colorado Front Range, in general, a drilled concrete pier is a much more cost effective deep foundation element than a steel helical pier.

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.

 

[PEinc]

Right on, sundale! Good post.

 

[JStephen]

I grew up on the Texas coast, and similar to what SRE described, none of the houses there have basements. Then moved to Fort Collins, and it's kind of the expected thing there. That basement isn't just empty space, you use it, and miss it if it's not there.

 

[cassion]

Helical piers may work well as would drilled and poured concrete piers. Both have advantes and dissadvantges.
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.