 So the last thing I want to talk about before moving on from Claes is something called soil suction. Now soil suction happens above the water table in unsaturated pores where the space between soil particles is occupied by some water. Now the surface tension of that water creates menisci and a force that binds the soil particles together. And that creates an apparent cohesion that the soil has that goes beyond the attraction between solid particles. So the soil has an apparent cohesion and it's due to the negative pore water pressure that exists within these droplets of water. So the ability for soils to generate soil suction to generate negative pore water pressure is a function or directly proportional to the particle sizes within a soil. So clay sized particles will have a thousand times greater soil suction than a sand sized particle. So this is important when thinking about stresses within soils because we take the pore water pressure from the phreatic surface. So the phreatic surface is where the pore water pressure is zero and we calculate our pore water pressures as a function of depth below that. Now this can be complicated when we go into the field to try and measure where the phreatic surface is because the presence of negative pore water pressure above that creates conditions within the soil that can seem almost fully saturated. So even several meters above this phreatic surface. So where do we take the phreatic surface from when we're measuring high water contents above it? And this is even more trickier for soils with a fluctuating phreatic surface where we might have groundwater conditions that means that we don't have a constant phreatic surface. So there's this whole range of techniques that are applied to get over that, a range of pressure meters. And we'll talk about that later in the video series when we're talking about site investigation. But it's important to start thinking about soil suction and its influence on the phreatic surface now.