 The last thing I want to do before I move on is give you an example of calculating stress in a slightly more complex soil profile. This is maybe a typical exam question that you might get. I'd say we have a soil profile that looks like this. We have a ground surface and underneath it we have four metres of sand, four metres of clay and some gravel. Let's say we know the unit weight of each of these layers. So we have a water table that's one metre below the ground and the sand above that water table has a unit weight of 19 kilonewtons per metre cubed. Let's say the unit weight below that water table is 20 kilonewtons per metre cubed. The clay has a unit weight of 22 and the gravel 25 kilonewtons per metre cubed. Now we might be interested in trying to work out what the stress is within this gravel and let's say we're trying to work out the stress two metres below the top of this gravel. So what is the stress conditions here? Well let's first of all start with the total stress. Now the total stress we can calculate by taking the unit weight of the soil and multiplying it by its depth. But how do we deal with that if we've got different unit weights? Well what we can do is just sum the individual contributions of each of these layers. So we can find the contribution of stress from this layer, add it to this layer and add it to this layer and we'll be able to find what the stress conditions are at this point. So for the total stress we can take the dry sand above the water table and treat that as a single layer. So we take the unit weight of that sand and we multiply it by its thickness. So 19 multiplied by 1. And then the sand below the water table we treat as a separate layer and we have 20 multiplied by its thickness which is 3. So we can then take the clay layer and it has a unit weight of 22 and we multiply that by its thickness which is 4. And then finally if we're going two metres into the gravel we take the unit weight and we multiply that by 2. And if we, out of all of those together we'll find the total stress at this point down here which comes out to be 217 kNm2. Okay, so we found the total stress. What about the water pressure at this point here? We want to know what the pore water pressure is here. How do we work that one out? Well all we have to do is take the unit weight of water and multiply that by the depth to which our point is below the water table. So our water table is here, our point of interest is here and we just multiply that depth by the unit weight of water. So our pore water pressure is equal to 10 multiplied by this point's depth below the water table which is 3 plus 4 plus 2. And that comes out to be 90 kN per metre squared. Okay, so the last thing we need to work out is the effect of stress. So the effect of stress is equal to the total stress minus the pore water pressure which equals 217 minus 90 which equals 128 kNm per metre squared. And while that sort of initially might have looked like quite a complex problem it really is not too difficult to solve if you follow these sequential steps.