Direct Shear (small shear box test) help 1

[TechnicalUK]

Hi,

I'm trying to learn about shear box testing using BS 1377-7:1990. I have 3 sets of data for a clay sample but the more I read the standard and other resources the more it confuses me like I'm missing something.

First I don't understand the terminology so first I will describe the tests which were carried out:

- vertical loads were 0kg, 10kg and 20kg.
- Porous bottom and top plates were used (does this make it drained?)
- It wasn't carried out in water
- Results were taken up around 20 minutes

Firstly I read in the standard that undrainaed is when the pore water pressure isn't given time to change, so in this case its clay and the load was applied fairly quickly, does this mean its undrained? Then the standard says the test should only be carried out if its drained (or is it only referring to sand?) Then I read in a book that undrained can be used but its known as the quick test.

My second main thing is I understand I need to plot horizontal displacement (X axis) against shear stress (Y axis) to find the maximum shear stress but the data I have doesn't plot anything like what is suggested in books etc. Should the horizontal displacement be the cumulative one or just the interval? I'm either thinking the data is duff or I'm doing this wrong. I can post some data if it helps.

As you can see Im very confused, I hope someone can shed some light on this for me.

Thanks

 

[dgillette]

If direct-shear tests on clay were done in 20 minutes, with or without flooding of the sample, you probably have a partially drained test, which is nearly impossible to interpret, for either effective stress or undrained strength.

Direct shear is RARELY used for clays. When I've seen that done, it was to try to measure residual friction angle on a presheared or precut specimen.

Hate to say it, but it sounds like you haven't got the right test. Who's telling you to do direct shear tests on clay, and why? Have you had a class in soil mechanics?

Good luck!
DRG

 

[TechnicalUK]

Thanks alot for the reply. Yes I am doing a module at University on Geotechnics. I was told although shear box tests are not commonly used especially for clays they were carried out because it was quick and easy as apposed to carrying out a number of triaxial tests. They showed us and explained the triaxial setup they have but we don't have any test data.
The sample was dry and water was added to it until it was reasonably moist.

I think our aim is to determine maximum shear stress and then from that determine the maximum angle (can't remember the terminology).
I was expecting to get a smooth graph with a peak which indicated maximum shear stress but instead I have 3 graphs of a slope upto a bunch of spikes.

It says on the test results that the tests were undrained and I know the tests were not flooded but were moist. Undrained indicates that water doesn't have time to escape right? Which would make sense in a clay?

I talked with others and I think the data may be duff, probably because the test wasn't carried out appropriatly as you said.

 

[dgillette]

Afraid you're right about the data being duff, if that means what I think it means in UK slang.

20 minutes is not enough time for excess pore pressure to drain off completely from a clay specimen. It is too much time for you to consider the test truly undrained, and if the soil is not totally saturated, you may have effects from capillarity or from pore fluid that is compressible and therefore doesn't cause the same excess pore pressure you would get from shearing soil that is below the water table and fully saturated. Therefore, you can't know what the effective stress is, and you have no way to verify that the measured strength is anything like the true drained strength. In short, I'm afraid your data are hosed, and I expect you know what that means in US slang.

By the way, you will find that people here get a bit grouchy when students post school problems on what is intended to be a forum for active practioners. Therefore, after this discussion is complete, you should hold off until you are in practice and have on-the-job issues.

 

[TechnicalUK]

Thanks for the great explanation, it makes sense which is great because I thought it was just me. In the British standard I'm sure it mentions that the test should be drained only so you can control the conditions and thus get reliable results (baring in mind that it's probably talking about sand and not clay).

Point taken, although I do work in the industry (for a Civil engineering consultancy) and go to University part time. My area of expertise is highways however, not geotechnics/soil mechanics/ groundworks unfortunately. Thanks for the warning anyway and thanks for the advice.