# Direct Shear test

When we want to test the shear strength of soil vs. how much we press on it vertically, we can use a mechanism like below, which is called direct shear test. It is the most basic shear test method for soil. In it’s simplest from it can be shown as:

Direct shear test is a very simple but common test. In this test, as you can see, the failure plane is fixed as horizontal. So we can not draw a Mohr’s circle here, because as you have seen before in our previous post for Mohr’s Circle, that circle represented different failure planes. (That post is under structural section but it makes no difference, principles for drawing Mohr’s circle are the same. Therefore, not controlling failure plane is the major limitation of direct shear test.

And then, for various values of N and T, we can obtain a simple graph as follows:

This angle here, is the angle of internal friction of this sand, depending on its initial density. This graph is for dense sand. The reason we drew two lines here is because, for the same force N, the force T to shear the sample varies between beginning and final stages, after large amount of sliding. For dense sand, at first, we spend a lot of effort, to start pushing, and that is why, the Ø in the beginning is Øpeak but in later stages, pushing is easier, once it starts to move. That angle then is called Øresidual or Øultimate. This is similar to trying to push a heavy block on the floor. At first you spend more effort, but once it starts moving, then you can continue pushing with less effort.

This means for example, when you analyze a soil slope stability problem, if you see that the soil was dense but it deformed greatly, you must use the residual, lower value of Ø. If you use the initial higher value, your design will not be safe. If however no deformation or very little deformation has occurred, it is okay to use the higher angle value, which results in more strength value for soil.