 Yn yinstantiaid, ydych chi'n ddysgu'r surdai' a'r chlas ond. The question is, how do you soild become stressed? We can build something on to them. We can put a force into the soil. But soils are also stressed under the wrong weight. That's what the focus of this video is going to be on. The self weight of soils... ...creates stress. So, how do we determine that? How do we calculate that? Wel, y cyfnod y cyfnod yn ychydig, efallai chi'n sioi, yn y tufau arall, ac ydw i'n dwyf yn y tufau, mae'n angen i'r ffordd o'r pwyllfa cywb. Felly, rydw i'n ffordd, o'r pwyllfa cywb, yn y ffordd ychydig. Ac rydych chi i gydag, a'n gafodd iddyn nhw'n mynd i'ch gyrwch. .. when it was holding up the soil .. .. to stop it caving in. The question is, what stress .. .. will they be trying to hold up at the top of this .. .. at the top of the roof top of this void? It would be equal to the weight of the column of soil above their heads. I could calculate the weight of that column .. Ychydig yw hwn yn ddinsid, mae'r ddinsid yn gwneud. Yn gwn i'r ddinsid yn gweithio bydd y gorffedd ei genedlaeth. Cynnyddwch fynd i fynd wedi uwch yn ddinsid. Ydw i'r ddinsid yn ddinsid. Cynnyddwch i'r ddinsid yn ddinsid. Cynnyddwch i'r dimensydd. Yna, y gallwn i gyda'n gwneud i y ffordd, byddwn i'w gweithio i yw'r hoffa ychydig a fyddfaeth yn fath o'r lleydd. Ond ydych chi'n gwybod i'r stres, mae'r ddweud yng Nghymru a'r ddweud y dyfodol ar y gyrdd. Yn ymddiad, yna'r ddweud yng nghymru, rwy'n gwybod i'r ddweud yng nghymru, ac mae'r ddweud yng Nghymru. Mae'r ddweud yng Nghymru, Cylunuton, Hermeter, Cubed. a dyna'r ddweud yr un It, a dweud dweud el lyw yn stemat, feBydd er gwrs Cilunuton wedi tylu. fel yst harmonio mewn drannu, yr un Iテ a ddiw. Felly yw yw'r ein amddai rhywbeth yma. Fy rydyn ni'n ymddai fath o'r profi, a rydyn ni'n cymryd y fath o 0.A ac 0.B. A rydyn ni'n cymryd 0.A i fath o'r fath o'r cael yma, a 0.B i fath o'r fath o'r fath o'r fath o'r cael yma. If I knew the unit weight of the soil, I could work out what the stresses are at each of these points. So let's say I did some laboratory analysis, I took a sample back to the lab and I found out what the unit weight was. And let's say that unit weight was 20 kilonewtons per metre cubed. The stress at point A would be equal to the unit weight multiplied by its depth. It would be 20 multiplied by 1, which would be 20 kilonewtons per metre squared. Similarly, point B, it would be 3 times that, so it would be 20 times by 3, it would be 60 kilonewtons per metre squared. So this is quite a simplified example. Could you imagine any problems if we were starting to look at more realistic soil? Do you think that unit weights will stay the same? So this assumes that you've got constant unit weight throughout your soil, but would you expect unit weight to change? We know that bulk density increases with depth, so you would also expect the unit weight to increase with depth. So this is a simplification, and in some cases it's using the average unit weight. What about water content? What effect do you think water content will have on the stress calculation? OK, so going back to my original cartoon here, I'd say it rained on this soil, so I started filling up this material with water. To the point where I had a water table that was somewhere up here. So now the chamber is under water. Maybe we give our person an air tank so they can still breathe. So we have a water table in our profile. What does that do for the stress within this space? Well, you can see that this chamber will be filled up with water, and that water pressure will then help push up the roof of this chamber. So you can see that the stress that the person will be experiencing would be less because of the water pressure helping to support the roof of the chamber. Assuming that there's no changes in both density because of the water up here, but this is a cartoon, so it doesn't have to be real. What is the water pressure at this point? Well, the water pressure is calculated in the same way as the weight of the soil. So we take the density of water now, and we multiply that by the acceleration due to gravity. And we get the unit weight of water, and then if we knew the depth below the water table. So when we're calculating water pressures, we always take the depth below the water table, and it's called that D. It's the depth below the water table. If we take the unit weight of water and we multiply that by D, we're left with the stress of the water, and we give that the symbol U. So that's water pressure. So go back to the original example. Let's say we have a unit weight of 20. A unit per metre cubed of soil. Let's say the depth below the water table, the top of the soil profile, H, is equal to 3 metres. Now let's say the depth below the water table, D, is equal to 2 metres. What would be the stress experienced by the person within this little chamber? Well, the original stress, we would just calculate by taking the unit weight, multiplying it by the depth, which would be equal to 20 multiplied by 3, which would be 60 kilonewtons per metre squared. But now we've got the water helping push up the top of this little chamber. So what would be our water pressure? What would be U? Would be the unit weight of water multiplied by D, the depth below the water table. Now the unit weight of water is the density of water, which is 1mg per metre cubed, multiplied by the acceleration due to gravity, which was 9.8. Some people can like to round this up to 10. So if we're using rounded up numbers, it would be 10 multiplied by 2, which would be 20 kilonewtons per metre squared. So the stress that this person would be experiencing within the soil would be equal to the stress that the soil is exerting, minus the pressure that the water is providing. So that would be equal to 60 minus 20, which would be 40 kilonewtons per metre squared. So this helps to explain or introduce one of the most important concepts within soil mechanics, and that's the concept of effective stress. So this is effective stress, this sigma prime. This is what we use to donate effective stress. And that's incredibly important when we're thinking about stresses within soils. And I've probably left you with the feeling that somehow water pressure is beneficial, and I suppose in this case it would be for the person within this chamber. But actually, when we're thinking about stresses within soils, water pressure can be really quite detrimental, because the strength of a soil is really a function of the effective stress. And if you increase the water pressure, you've reduced the effective stress. So have you tried to use this cartoon as an explanation or an introduction to effective stress? Don't think that water is somehow beneficial to stress conditions within a soil, because it absolutely isn't. So an important thing to consider is how effective stress changes within soils. So if I have a soil, and let's say it's got some sort of, it's saturated, it's got some sort of water table right at the surface. And let's say I subjected that soil to a loading, so I stuck a footing onto it from a building, and it's some sort of force coming through the footing. OK, so how do you think the effective stress, or the stress condition, so total stress, the water pressure, and the effective stress, will change at a point below the footing? So the total stress, the pore water pressure, and the effective stress, how will that change over time? Well, the total stress, if we draw it on a graph, total stress might look something like this. We have total stress in time. Total stress would be starting off somewhere around there, and then we stick the footing on and it jumps up to a new point. So what will happen to the pore water pressure? Well, the pore water pressure will do something similar. With time. And that will have an initial value. And then when we put the footing on, that initial value will jump up. But you can see that if we move far enough away from the footing, the pore water pressure won't have changed that much. So it's only underneath the footing where the pore water pressure will change. So that leaves us with a region of high water pressure around the footing, and low water pressure as we move away from it. So you can see that the pressure in the water will try to dissipate, and water will move away from the footing, and the pore water pressure will decrease. So you'll have an initial increase in pore water pressure. But as the water flows out of the soil, the pore water pressure will decrease until it reaches somewhere around what it used to be in the ambient conditions. So what does that mean in terms of effective stress? Well, effective stress is equal to total stress minus pore water pressure. So if you have this graph and you take away this graph, what you're left with is effective stress. It will have an initial value. But you can see that the total stress will be almost taken up by the changing pore water pressure. So you won't have any initial change in effective stress. And it's only after time that this pore water dissipates will that total stress be transferred into effective stress. So your graph will look something like this. So if effective stress is important for the strength of soils, we need to know how it evolves over time. And you can see that the rate of evolution, so how quickly the pore water pressure dissipates, is a function of the permeability of soils. So for things like sands and gravels, we would expect the effective stress to change almost instantaneously. So the pore water pressure to dissipate almost instantaneously. For things like clays, this is a lot more problematic because it can take years or even decades for the effective stress to change. So it's important to understand how these effective stress conditions change with different types of materials.