MicroPiles in Expansive Clay

[emmgjld]

I have been confronted with using MicroPiles beneath residential foundations in highly expansive clays (CH) and Elastic Silts (MH). I am not a real fan of deep foundations in an arid or semi arid environment, in that the definition of moisture penetration during development of residential subdivisions can be vexing, to say the least.

I need some real information regarding:
Any studies for Pullout resistance in the lower 1/2 to 1/3.
Issues regarding the connection between the MicroPile and the gradebeam.

I am always interested in the discussions which can also develop regarding deep foundations for lightly loaded residential applications.

 

[jdonville]

emmgjld,

I'm no specialist, but can only imagine that the micropile contractors will strongly recommend against using micropiles in expansive soils as the load-resisting layer. I would think that they would want to install the micropiles into more stable soil/rock and include updrag/downdrag as part of the load to be resisted by the element.

I suggest that you contact a micropile contractor for definitve info (Nicholson, Hayward-Baker, etc.)

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

The views or opinions expressed by me are my own and do not necessarily reflect the views or opinions of my employer.

 

[oldestguy]

Check with the Chance anchor folks.

They are always pushing their method as suitable for many purposes. Maybe they have a guarantee also?

 

[emmgjld]

Thankyou for your responses.

jdonville,

The expansive strata of CL, CH & MH (Mancos Formation) is up to 40' thick, underlain by more expansive shales and claystones with interbedded argillaceous siltstones and sandstones(Dakota Formation). Some Sandstones in the Dakota are non-expansive. Deep fracturing is present. Other sites have more than 300' and up to 1000' of expansive clays.

Defining the depth of 'Seasonal Water Change' is problematic, considering the site is dry and developoment (to include landscape irrigation) is just beginning. The deep fracturing of the formations, combined with the formational bedding (includes very thin siltstone and sandstones) and the presence of significant secondary gypsum deposits in the fractures and some bedding planes results in water infiltration and movement being unpredictable.

I see the same potential design problems as drilled piers, except the diameters are smaller, which MIGHT be an advantage. Hence my question regarding pullout resistance in the lower portion, assuming the upper portion has been isolated (little or no side friction in upper part) or the element has been instrumented so as to allow computation of side friction throughout the MicroPile length.

oldestguy,

These clays are very stiff (SPT >50) and relatively dry. Under similar conditions, We have not been able to accomplish penetrations of more than 10' and sometimes less than 5' without 'twisting off', even using a 4" diameter flight. Most of our Helical Applications in such ares have required predrilling.

I am not seeing any real reason to go with these relatively expensive MicroPiles when a narrow (8" to 10" dia.) drilled pier can be placed at 1/2 the cost. The drilling is a little slow but, godd contractors can be had, usually from out of town. I have found NO documented case which shows MicroPiles to be a better alternative. I have seen cases which indicates MicroPiles to be probably as good as normal drilled piers.

The slow response to this thread indicates that I am again on the Frontiers of Science. What a Joy. Try something new and learn a lot (be a white rat and possibly meet some new lawyers) or stay with the familiar.

I tend to want to stay with either a shallow foundation on a thick structural fill or a matte foundation, possibly Post Tensioned. As I mentioned in the origional post, I am not a real fan of deep foundations in an arid environment. Too many failures, very few were ours.

 

[oldestguy]

Stay with it.

Our lake clays are in Wisconsin, not that severe.

Yours sounds to me like a place I would not want to work in, unless what is built is capable of withstanding the movements. My experince is with CH zones that dry down maybe to 15 feet. Later they take on water and swell.

We have stablized some structures by just adding water in the summertime and removing fast growing trees. No piles were used. I really am amazed how well adding water works, even to correct some significantly distorted structures.

In your case, I was wondering about establishing a stable reinforced slab that moves, via shallow piles, together with buried utilities. I'd think the greatest problem would be differential movement of those with respect to the buildings.

One ought to be able to make the building relatively inflexible, but not so with the utilities.

If you are able to create a non-moving structure, then what will happen to moving attachments to the building?