properties of schists 1

[39dd]

We are currently investigating ground conditions for bridge piers in the west of ireland. The investigation has shown competant boulder clay N50 for between 5-8m overlying a dark peletic schist. We have been unable to obtain any decent core from the schist as the material is highly micaceous, and would appear to wear away using a water/polymer flush. The design engineer is thinking of loads of the order of 500Kpa, and while the boulder clay theoretically should sustain these loads there would be a greater degree of comfort if we could assess the strength of the underlying schist. Anybody any ideas as the expected strength of this schist, and how would it behave under light or moderate loadings given the seemingly high mica content.

 

[Focht3]

Micaceous materials can be troublesome; if you can't get "good" samples, how about a test pier? One added benefit: you might achieve a capacity significantly higher than 500 kPa (10.4 ksf.)

 

[phillyfrank]

I assume you are diamond core drilling. You are probably in a weathered zone of the schist initially, which will be very soil-like. Have your driller use less water and control the feed mre carefully. Try contacting Boart Longyear and getting a triple tube core barrel. They will core through dog feces and get 100% recovery.

http://www.boartlongyear.com/

The trouble with schists is that they are highly anisotropic and shear more readily along the schistocity planes. You must determine the strike and dip of the schistocity (unless it is very folded) and consider this in the design loadings. Bearing strength is less important that shear strength along the schistocity.

 

[pigdog]

39dd,
With slower feed, less water, slower rotation and pay on basis of recovery, we used to get 100% recovery in red clay saprolite using standard equip and longyear drillers.

Micaceous schist core will probably yield very low unconfined compressive strength. In the ground though, unweathered schist is confined and should be able to hold up the world. The test pier should give you a feeling of confidence.

In response to scour questions, we typically core to a depth that not only yields bearing capacity but also resistance to overturning. Scour is sort of a black art, but the idea is that turbulent flow around your pier, (from the 500year flood) may wash the soil away from the pier and let your bridge tip over.
pigdog

 

[kmerl]

The denison core barrel will receive a semi-undsitrubed sample in the upper zones of the weathered schist. These samples can be used to complete similar testing to determine phi angle and confined bearing tests. Highly micaceous schists can have large zones of weathering but are very stable and can generally support larger loads than materials from other types of rock.

Also if the loads are on the order to which mentioned above then it is a good idea to attempt a trial loading, the foliations in schist can change dramatically in a short distance thereby reducing the effective maximum loading available to the substratum.

Another method to determine subsurface in-situ maximum bearing is using CPT, depending on how dense the schist is this may be a cost effective option to determine the strength and compressability of the schist at depth.