 Yn y fideo, yn ymwneud yn y tîm y odometer. Yn gyfer y tîm y odometer, sydd yn bwysig y celfrolydau cyfnoddur i'r soild. Yn y tîm y odometer yn y tîm, mae'r celfrolydau hyd yn y celfrolydau. Yn y celfrolydau hyn, mae'r celfrolydau yn gweinio. Yn y tîm yw'r celfrolydau hyn, mae'r celfrolydau hyn yn gweinio. Mae'r celfrolydau yn gweinio. ac yn y ddechrau neu ddysgu'r ysgolig, rwy'n ei fawr o'r ddysgu'r ysgolig. Mae'n fawr i'w ddysgu'r ddysgu'r ysgolig yn y cyfnodol, yn yr adonwydau. Rydym yn rhaid i'r ddechrau, roeddwn ni'n ddysgu'r ddysgu'r ysgolig, rydw i'r ddysgu'r cyffredinol, a er mwyn y ddysgu'r cyffredinol yn y ddysgu'r ysgolig, ond rwy'n i ddysgu'r ysgolig. Ac mae'n gweld i'r dweud y gallu wathlau o'r ddodol ar y test oedol. Rydyn ni'n wathlau oedol yn y ddalun o'r rhan. A yn y ddodol o'r ddodol oedol mae'n ddodol yn rhan o ddodol. Mae'n fyddynt yn fwy sagwyr ar y test. Fe wnaeth o'r ddodol yn ddodol o'r ddodol o'r ddodol. Rydyn ni'n ddodol o'r ddodol o'r rhan o'r ddodol o'r ddodol. Mae'n ddodol o'r ddodol o'r ddodol. On top of that cap we suspend our load and what we measure is the displacement of the cap or the settlement of the soil during the test. So this is what an odometer looks like in cartoon and profile. You see that we have our odometer cell in the middle here and sitting within the centre of that cell is a sample. That sits within a confining ring. On the top of the sample and bottom of the sample we have porous discs and that lets water permeate in and out of the sample. Around the outside of the cell we have water and a loading cap. Now mass is applied or load is applied onto top of that loading cap through this rig where we have a loading arm. At the end of the loading arm we suspend some masses and that loading arm is attached to the lid of the box. So you can see within this odometer test how load is applied to the sample. So we stick our sample into the loading frame and we know the initial thickness of that sample. So we can plot a graph over time of how that sample thickness changes as we load our sample. We stick our first load so this is time. We can draw a graph here with time and sample thickness and that has an initial sample thickness that we know. Now we stick a mass onto the end of this loading arm and we measure the displacement of the lid with a displacement transducer and what will happen is the sample thickness will decrease over time so it will do something like this. So we stick our first load on and the sample thickness will decrease and eventually it will stop decreasing and we measure the thickness of the sample at that point. We can at that point stick another load on so this is our second load. We stick another mass onto the loading arm and the sample will do something similar again. It will decrease in thickness until it will plateau. We do that several times so we've loaded our sample and it's decreasing thickness and we measure the thickness at each of those points. Now if we take that information so we take the thicknesses of the sample at different loading increments what we can do is draw another graph that looks like this where we have sample thickness with stress. And we plot our data onto that so we have our h0, h1. So the gradient of this line is the change in sample thickness or delta range of settlement with the change in stress. So the gradient of this line is equal to delta range over change in stress. Now if we take that gradient and we divide it by the initial sample thickness h0 what we're actually left with is the mv value. So the coefficient of volume compressibility which equals change in h over change in stress times h0. So from the gradient of this line we can derive the mv value but you can see that actually the gradient of the line isn't constant so it changes or becomes more shallower with higher loading increments. That tells us something quite interesting about mv value is that the soil is more susceptible to consolidation when we start loading or when we have our initial loading increments then later loading increments or higher stresses. And what's happening there is our soil is becoming stiffer so with higher and higher loading increments our soil is becoming less and less susceptible to consolidation. If we want to derive a mv value from this test what we need to do is specify the load increments that that mv value is taken from. So if we took an mv value from say here so we have our change in stress and change in sample thickness. That we call that delta h1 and delta sigma 1 that will have a different value from if we took it over here delta h2 and delta sigma 2. So when we calculate taking our mv value from this graph we need to specify the loading increment. So to overcome that change in mv value with changing stress increments one way to get around it is to take the log of the stress or I should say that we're talking about effective stress. So each of these loading increments we assume that the pore water is dissipated out so the stress that we apply to the lid is actually becomes the effective stress within the sample. So we're talking about the log of effective stress here. So we take the log of effective stress and we plot void ratio. So instead of sample thickness we're now looking at void ratio which is what we're really interested when we're talking about soils where we're looking at the changes in the void. So if we plot void ratio against log effective stress that curved line transforms itself into a straight line and this is called the normal compression line or the normal consolidation line or the virgin consolidation line. It's given a whole range of different names but it's the relationship between void ratio and the log of effective stress and that is a straight line. So we can take the gradient of it or the gradient remains constant. It will be constant now but it will be the change in void ratio over the change in log effective stress. And that's equal to the compression index or CC. So where your MV value changes with the stress increment your CC or your compression index doesn't stay as constant. So it can be more useful to use your compression index when we're talking about consolidation although the calculation for doing that is a little bit more complex. So calculating your void ratio during your odometer test becomes a little bit complicated. So to do that we need to go back and use the calculations or the formulae that we used to define void ratio and specific volume and how that then relates to things that we can measure within a test. So it's typical that what we do in anodometer test is once our test is finished we measure the mass of the sample. So we take the mass of the sample and we know the volume of the sample as it's at in the confining ring. So we know it's height and the diameter of the confining ring. So we can work out the volume. We can work out what the density of the sample is or the bulk density of the sample is. Now we've taken the bulk density. We measure the water content. So we take our sample at the end of the test and we dry it in the oven and we measure the water content that's in the sample. So we get W and we also measure specific gravity. So we take a sub-sample of that material and we measure the specific gravity. So from the bulk density the water content and the specific gravity we can derive the specific volume and the void ratio. So we can get that at the end of the test. If we try to do that at the beginning of the test we'll end up disturbing the sample of soil and that will affect our test results. So it's best to do that at the end and then calculate the void ratio backwards to the test. And to do that we're assuming one-dimensional consolidation so that makes things a little bit easier. So what that means is that I'll just make some space here. One-dimensional consolidation means that the change in sample height over the initial sample height is equal to the change in void ratio over the 1 plus the initial void ratio. So if we have the void ratio at the end of the experiment and we can assume that that's the initial. I know it's maybe a bit of a misnomer but let's say we're working backwards in time so we take an initial void ratio to be the void ratio at the end of the experiment. We know the thickness at the end of the experiment so H0 in this case and we know the change in thickness. So what this really is saying is that the change in sample thickness through the experiment is equal to the change in void ratio or is a relationship between the change in void ratio. So for each change in sample thickness we can get a change in void ratio and we add that to the void ratio at the end of the experiment. We can go back through our experimental results and derive the void ratio at each one of these loading increments.