Direct Shear or Triaxial Tests on Boulder SIzed Rockfill 3

[BigH]

G'day all,

Does anyone have personal or corporate experience and/or references on the laboratory strength testing of boulder sized rockfill, particularly direct shear but traixial would help. The rockfill is typically a maximum of about 500 mm [20 inches](rounded to subrounded boulders) and without significant elongation. Sand content and finer is typically 20 to 25%. Most papers I see on rockfill have maximum size of 100 to perhaps 150 mm [ 4 to 6 inches]. Another material type that will be used is 200 mm [8 inch] maximum size - but presently, interested in the boulder sized material.

Secondly, what might your personal thoughts be on scalping the rockfill to produce a "parallel" gradation but with, of course, a smaller, say 100 mm to 150 mm [4 to 6 inches] max grain size.

Your experience would be appreciated.

 

[GeoPaveTraffic]

My college thesis was related to triaxial testing of large size materials. Specifically course coal refuse. Mind you this was almost 34 years ago, but at that time there was very little in the literature about triaxial testing of materials over about 8 inches in diameter. The common practice was to strip out the oversize material, run the largest test you could, and then say that the result were conservative because the larger (and assumed stronger) materials had been stripped out.

For my thesis, I ran 1.5-inch-dia, 3-inch-dia. and 6-inch-dia. triax tests on the same material, by stripping out the oversize stones. The results showed about a +3/-3 degree variation, 6-inch-dia to 3-inch-dia, in results. The variation was related to the durability of the material, lots of shale in the refuse. Coefficient of uniformity and slake index were pretty good predictors of the delta in phi angle.

Let me know if any of this is of interest to you. If you can't find it elsewhere, I can dig out my copy. Title: Effects of Particle Durability on Triaxial Strength of Coarse Coal Refuse, Michael Lambert, M. Eng. Theis, 1988 University of Louisville, KY.


Mike Lambert

 

[GeoEnvGuy]

The lab run by Marsal claims to have handled 113 cm specimen rockfill.

Table from Kurt Douglas thesis on the shear strength of rock masses. The book Geotechnical Engineering of Dams references his thesis and also indicates the following regarding shear strength of rockfill based on size.

Most accept that shear strength
increases with particle size (Chiu,
Douglas), but some claim no effect
(Charles et al.), or the opposite effect
(Anagnosti and Popovic)

 

[BigH]

Thanks GeoPaveTraffic. I recently found a paper by Gupta (Effrect of particle size and confining pressure on breakage factor of rockfill mateerials using medium triaxial test (2016). On thing he showed was that, for his samples, particle breakage was significant at confining pressures of about 1.2 MPa. In our case, it is quite probable that the confining pressures will be up to 4.5 to 5 MPa depending on the condition (i.e, during placement and then after impounmding). Some want direct shear tests done and on a QC (50k m3) basis - and of course with 70M m3+, Hmmm - I find that this isn't reasonable for the big size and scalping 600 mm minus to, say 100 mm minus will have its problems. Also, the confining pressures needed - ranging from, say 0.2 MPa to 4 MPa would be difficult to carry out.

I'd be interested in discussing this with you.