Gneiss Rock

[EireChch]

Hi all,

Has anyone used Gneiss as an aggregate for structural fill?

If so, is there any good FHWA literature on the material, which discusses its typical aggregate values like LA fragmentation, crushed and rounded particles etc. I have seen online that LA for Gneiss can range from 30-60.

The request stems from a client of ours who is looking to supply this aggregate to their client. But their client has aN LA fragmentation limit of 30, which I feel is too stringent. The Design Manual for Roads and Bridges (DMRB) allows an LA value of up to 50.

If anyone knows of any other standard which has less stringent LA values or specifically information on Gneiss I would love to hear it.


TIA.

 

[Mad Mike]

So gneiss is effectively a granite which has undergone enormous compression under ductile conditions (deep in the earth a long time ago), such that it's mineral constituents are now strongly aligned, or "foliated". The foliation is usually produced by mica.

The term "gneiss" is very generic, and covers a whole family of layered, granite-like rocks.

In my area some of the gneiss is horrible material- there can be such enormous variation between the layers that it becomes impossible or impractical to separate and control during construction. On the other hand, I've seen gneiss which behaves effectively as a granite and is reasonably consistent.

Watch out for large amounts of mica in the gneiss, which makes it very friable and susceptible to flagging, among other things.

Doesn't answer your question, but some geological context to consider.

All the best,
Mike

 

[EireChch]

Thank Mike.

The Gneiss I will be working it is in your neck of the woods so good to know. We will have site specific testing so will get a good handle on the quality of the rock.

 

[Mad Mike]

EireChch- which neck of my woods? I'm based in Durban on South Africa's east coast.

 

[BigH]

Mad Mike - he's close if compared to going to the moon!

 

[Mad Mike]

BigH- this is what happens when you chaps insist on clinging to the old Imperial measurement system!

How many cubits away is he?

 

[fattdad]

gneiss is a reference to degree of metamorphism. Shale is a sedimentary rock. When metamorphosed it goes by degree to slate, phyllite, and gneisse. As the degree of metamorphism increases (higher pressures and temperatures), there is opportunity for mineral grains to grow. Such tendency (and pressures) align bands of minerals. Typically, this separates the mafic and alkaline components - i.e., biotite, hornblend and feldspars.

I would agree that if you metamorphosed granite, you'd also get some banding - I mean it's possible. I would not agree that the genesis for gneiss is metamorphosed granite. Well, in the felsic bands, we do often get a granite-like texture.

There can be a lot of biotite in gneiss. Such layers can be friable. So, I'd rather get LA abrasion data and base my conclusions on the performance of the actual rock supply.

f-d

ípapß gordo ainÆt no madre flaca!

 

[EireChch]

I myself am not close but the job is. Well close as in on the same continent as you, within 2.08e^-4% of the distance to the moon...do the math

But thanks for the info again.

And thanks f-d too, you mentioned a lot of geological processes, minerals and lingo that I should really know more about!

But yes all assessments will be based on actual site specific lab data. Just wanted some general background info.