Dear analogue modellers, 

I have been using wetted sand (just connate water) to demonstrated the formation of open tension fractures, with dry sand this is not possible.
Using just connate water the cohesion equals the surface tension of the water, but the internal friction angle does not change.
Less cohesion can be made with alcohol, or with water alcohol mixtures. The cohesion is proportional to the surface tension of the fluid (mix) that is applied.

I apologise if this message was sent twice, I needed to sign up for Jiscmail and I am now not sure if my first attempt was sent or not.

Have fun,   Dirk

Dirk Nieuwland
4e Binnenvestgracht 13
2311NT Leiden
URL: www.newtec.nl
E: [log in to unmask]
M: +31 (0)621547949

> On 02 Nov 2015, at 19:05, Janos Urai <[log in to unmask]> wrote:
> 
> Fully agree. But, if you let the models be just a little bit cohesive, you can scale them for faulting in the failure mode transition domain, for example at Mid Ocean Ridges, or cemented carbonates at relatively shallow depth.
> 
> kind regards,
> 
> Janos
> 
> 
> 
> 
> 
> On 02 Nov 2015, at 1844:, Mark Brandon <[log in to unmask]> wrote:
> 
> Janos,
> Good point. I was thinking that the questions were about sand to use for classroom demonstrations. A fully submerged experiment is probably not warranted for this case. And, as you emphasis, the cohesion term (in this case, apparent cohesion) was still be large relative to the friction-angle term.
> Mark 
> 
> 
>> On Nov 2, 2015, at 11:25 AM, Janos Urai <[log in to unmask]> wrote:
>> 
>> Sand only becomes cohesive when it is partly saturated: if you do the experiments under water  the is still an apparent cohesion but this is only because the friction angle increases rapidly towards zero normal stress.
>> 
>> 
>> 
>> 
>> 
>> On 02 Nov 2015, at 1628:, Mark Brandon <[log in to unmask]> wrote:
>> 
>> Note that wetting of sand increases its cohesive strength. Given the small length scales associated with most "sandbox" experiments, the yield strength of the sand will be dominated by the cohesion term of the Coulomb friction law. In other words, the mean stress will be low, so the contribution of the cohesion term will be large. As a result, the material will behave like a perfect plastic, with a single yield stress, rather than like a Coulomb plastic, with a pressure-sensitive yield stress. It will also tend to fail in a brittle fashion (localized shear), rather than a ductile fashion (distributed shear). Is this what you mean by "much more reliable behavior"?
>> Best,
>> Mark Brandon
>> 
>>> On Nov 2, 2015, at 4:16 AM, Potts, Graham <[log in to unmask]> wrote:
>>> 
>>> Hi
>>> 
>>> I think that this response highlights a key point
>>> 
>>> well sorted sands produce much more reliable behavior particularly when wet; presumably the cohesion caused by the surface tension of the water is more homogeneous (suggests that intermediate "strengths" and "behaviors" might be achieved by mixing grain sizes!)
>>> 
>>> our supplies came from a garden in an area of sand dunes to the north of Liverpool
>>> 
>>> it might have been the high degree of sorting or maybe it was because it came from Juan Watterson's garden
>>> 
>>> Graham Potts
>>> From: Tectonics & structural geology discussion list [[log in to unmask]] on behalf of Urai, Janos [[log in to unmask]]
>>> Sent: 29 October 2015 16:14
>>> To: [log in to unmask]
>>> Subject: Re: Where to purchase sand for analog modeling - USA
>>> 
>>> Phil,
>>> 
>>> for my classes in Oman I use sand form desert dunes - perhaps you can find a way to get this in the USA.
>>> 
>>> for the ductile material in teaching I have used different kinds of hair gel that you can buy in any drug store. Just go for the ones without perfume. 
>>> 
>>> A great addition also is a good strong lamp to illuminate the models in low angle light.
>>> 
>>> Finally, perhaps you want to consider using gypsum powder which you can harden and excavate and can fail in both modes. This is very easy to do. We described this in
>>> 
>>> Changes in structural style of normal faults due to failure mode transition: First results from excavated scale models
>>> M Kettermann, JL Urai
>>> Journal of Structural Geology 74, 105-116
>>> 
>>> Also, there are some ink types which you can use to colour the sand easily.
>>> 
>>> kind regards,
>>> 
>>> Janos
>>> 
>>> Prof. Dr. Janos L. Urai
>>> Structural Geology, Tectonics and Geomechanics
>>> RWTH Aachen University, Lochnerstrasse 4-20
>>> D-52056 Aachen, Germany
>>> T: +49 241 809 5723  M: +49 151 140 42552
>>> e-mail: [log in to unmask]
>>> www.ged.rwth-aachen.de
>>> 
>>> 
>>> 
>>> 
>>> 
>>> 
>>> On 29 Oct 2015, at 1552:, Resor, Phillip <[log in to unmask]> wrote:
>>> 
>>> Hello All,
>>> 
>>> I am adding sandbox modeling experiments to my structural geology course this year and am struggling with where to find appropriate sand (well sorted, fine-grained). I have seen “play sand” suggested online, but everything that passes as play sand around here is quite poorly sorted. I think perhaps the only requirement is that it is washed.
>>> 
>>> Thanks for any suggestions you may have.
>>> 
>>> Best,
>>> 
>>> Phil Resor
>>> Wesleyan University