 Mae'n ffordd o'r bod rattling i gyn processed o'r materiodau llnoedd, pan dwi'n cei'r materiodau llio'r llnod. Maewn dwi'n cei'r llio'r materiodau llio'r parteidol, a'r materiodau llio'r partidol yn gynradd. Mae'r plenifio ar ddefnyddio'r materiodau llio'r partidol. a we try to share this material. You can see that if we share, where we share the material and on the shear plane, if we try to share these top particles away from the bottom particles, you can see that the top particles in initially dense material will want to try and ride up over the particles underneath all the particles below. So you get the particles moving from an initially dense packing to what they are up here, to something like the situation below. So the material would increase in volume, and this is what we call dilation. So the soils dilate when they're initially dense, and because of that it takes a little bit extra shear force to get the particles over the underlying particles, the particles on top of the underlying particles. That's why we get a peak shear strength. However, if the situation was initially reversed and we had an initially loose material, we would start off with a situation with loose packing, and if we shared the top particles over the bottom particles, you'd see that we'd force a number of these particles, or a lot of these particles, into the spaces. We'd force the particles to fill the spaces between the particles below. So we'd go from an initially loose material to a more compacted particle distribution. This is called compaction, and it's what would happen to initially loose materials. So there are two graphs that we can plot during a shear box test to describe this behaviour. The first is if we take volumetric strain that we calculated throughout the test and plot that against shear strain that we calculated throughout the test. The sample would look like this. So as we're increasing the shear strain and initially dense material, we'll do something that looks a bit like this. So this is dilation, so this is an initially dense material. So it's important to point out now why we put volumetric strain on an inverted y axis. The reason for doing that is like compressive stresses within soils taken to be positive, we also take compressive strains also to be positive. So material that might be compacting is taken as a positive volumetric strain. So it's a little bit counterintuitive, but it's not too difficult to figure out. The reason why we flipped the axis is we kind of like the idea of putting dilation as a sort of increasing positive feature on the diagram. So it's just a convention. We could flip it around and it should be exactly the same, but this is just the convention. So this is dilation for an initially dense material. For an initially loose material, it might look something like this where we have compacting. So another graph we can plot is a specific volume against shear strain. We can measure the specific volume at the beginning of the test and then calculate how it changes through time with volumetric strain. But if we plot these graphs, we can see that an initially dense material will increase its specific volume. So it will dilate and it will dilate until it reaches a certain level. What's interesting is that for the same material, that's initially loose, we'd get a compaction, or a decrease in specific volume, but that should theoretically decrease to the same point. And that point is what we would call the critical state, or the critical specific volume. Now we'll come back to critical states when we talk about that in later videos, but it's quite interesting to note now that whether it's initially dense or initially loose, it will change in specific volume or change until it reaches the critical specific volume. It's quite interesting to think that the pores within the other voids within the material will reach a situation whether it's initially dense or initially loose. That's pretty much the same.