A factor of great importance to the strength of a soil is the range of particle sizes that are contained within it. We talk about a soil that has the same size particles as an ungraded soil, and a soil which has a wide range of particles within it as a well-graded soil. Now here we have a bed of model soil particles. They are all the same size, so this is a bed of ungraded soil particles. We want to measure the strength of this bed of soil, and we'll do that by loading a model foundation. There it goes, it's penetrating into the bed of uniform particles, and we will now measure how much it has penetrated with a simple ruler. And if we measure off to the baseline, we see that it is 22 millimeters. Now here we have a bed of well-graded soil, and you can see that the gaps between the large particles are filled with the smaller particles, and we will put one two batteries as we did before. And we can see it penetrating here. Now it's come to a standstill, so we'll measure how much it has penetrated. And we measure that it's penetrated 17 millimeters, whereas before in the ungraded material, the footing has penetrated 22 millimeters. So in general, other things being equal, a well-graded material is stronger than an ungraded material. You won't be surprised to learn that the shape of the particles are very important in determining the strength of the soil. We've looked at the effect of grading on circular particles. Here we have replaced them by angular particles, and for simplicity we've chosen triangular-shaped particles. We will load it with the two batteries and measure the penetration. So here we see it settling, and we will measure what the penetration is. But we see that the penetration is 12 millimeters, which is a lot less than for the circular particles. So the lesson from this demonstration is the more angular the soil particles, the stronger the soil.