I am doing large direct shear experiments (box plan are 300 mm x 300 mm , depth of sample 140 mm ) on dense granular soil. I am applying 40 mm to 60 mm displacement to study their behavior at large displacement. I am conducting the experiment at the normal pressure range of 15 kPa to 150 kPa using pneumatic systems.
My question is, while calculating shear strength, do I have to use the actual contact area between the top and bottom shear box, or should I go with full plan areas (300 mm x 300 mm).
You if are performing "reduced contact area direct shear tests" (ie soil-soil) you should apply area correction.
Yes. You should consider the contact area between upper and lower parts of the box.otherwise you will end up with a conservative failure envelope.
The main deficiency of direct shear test is the change of contact area during the test . For this problem, the researchers always use peak shear strength. However, If you want to calculate the shear stress during the test you have to consider the decrease of contact area during the test . Also the area reduction affect both normal stress and shear stress during the test.
Thank you all for your answers. I am interested in both peak and residual shear stress .
ASTM D3080-04(2001) does not recommend any area correction. Is there any standard or published results available for shear stress calculation using large direct shear box ?
It is true that ASTM D3080-04(2001) does not recommend the area correction but it refers to conventional (small scale) direct shear tests. See for instance ASTM D 5321 (2002) or EN ISO 12957-1 (2005)
I do feel that correction need to be applied when you are working on such large samples wherein changes in the area in context of the dept of the sample is changing, since this will result in changes in values of the normal and shear stresses. also since you have a large depth also the wall thickness of the box and the contact behaviour will also play an important role. however this is not mentioned clearly in any code as of what i know off.
will get back if i get something since we are working on the same size of box but for soft rocks
Dear Castorina , I don't have access to EN ISO 12957-1 (2005) . Can you please send me a copy . I will be really grateful.
Lalit,I am eagerly waiting for your next answer.Add your answer
Thank you every one.
If both boxes are 300x300 mm you should corect the shearing stress. In this case the lower box should have an extension for not allowing the material in the upper box to fall, otherwise you need to correct the normal stress, too. Some shearing boxes are made to avoid this kind of correction, meaning that the lower box is not a square, but a rectangle 300x600mm, so that the upper box travels over the same material allowing you to assess the post-failure behaviour (such as the residual one). In this case the trick is to fill the lower box to the top limit without letting any grain to be sheared between boxes edges. I recomand, if the equipment is new, to check the spacing all around the box using feeler gauges and if you have different spacing to polish it using a face ginder. Normally the spacing should be less than the minimum grain size of the material in the box. Another thing you will notice will be the cohesion you will obtain for cohesionless materials. This is normal, because the assumption according to which the soil grains are so small and stiff they do not break, no longer applies, so you will record structural cohesion caused by the large particles breaking. A good correction for this is by assessing the wearing resistence using Los Angeles test or micro Deval test.
Good luck with your tests!
The rectangular box mentioned by Manole-Stelian Serbulea should not be used. We have at University of Porto a direct shear device able to perform tests with rectangular or square lower box and we have been noticed that the soil blisters when the rectangular box is used. See "Soil/Geotextile Interface Behaviour in Direct Shear and Pullout Movements" by Lopes & Silvano
If you are using both block of same size, in that case some area correction should be apply because at every shear displacement contact area will be changed. In case, lower block is larger than upper one, no area correction will apply.
If you are just shearing cohesionless materials, the danger Ms. Vieira mentioned is irrelevant. The problem really occurs as described in the mentioned paper when you insert a geosynthetic material in the shearing surface because you cannot clean the extra material in front of the advancing upper box, which can be easily done with a brush if the soil in the lower box is exposed. The bulging of the material in front of the upper box is actually due to the dilatancy of the soil and not to a generalised failure (case in which the bulging would have been symmetrical in front and behind the box).
In conclusion: you may use whatever configuration of shearing box you have as long as you take into account all the effects and you correct the procedure as soon as you notice something wrong. As I mention in the first post is very important to ensure the clearance between the two boxes for avoiding grains to get between them and modify the friction condition.
dear sunil,
have been trying to locate something for the correction but the criteria given for soft rock shall not suffice with the case in soil, i am not very sure but there have been some studies on the contact behavior and frictional studies between some different sand sizes and different materials in contact, i do not have access to a very large amount of journals so sorry for not being able to help you any further.
It depends, how you are apply the normal stress, if it is simple uniformly distributed gravity load then area correction may not be necessary.
Can you show the schematic diagram of your shear device
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