Ground water is a matter unique to geotechnical engineers. Whether or not having a good understanding of the ground water can be a demarcation line between geotechnical engineers and structural engineers. Similarly, the capability of easily and accurately simulating the behaviour of ground water can also be what sets professional geotechnical software apart from other generic use software. This blog post is dedicated to the basic principles of simulating the behaviour of ground water, in the context of soil mechanics.
Wet or dry
Soil is naturally a composite material consisting of solid (particles), water (i.e. pore water) and air. In typical geotechnical practice, it is normally assumed that the soil is:
If granular (i.e. permeable):
Fully dry (no water) above ground water table
Fully saturated (no air) below ground water table
If cohesive (i.e. non-permeable) – fully saturated regardless of ground water table. This is because of the capillary actions (suction effect)
Bear in mind that both ‘dry’ and ‘fully saturated’ are idealised conditions that are only assumed to simplify analysis. They never exist in real life.
Method of analysis
The mechanical analysis of soil separates water and solids in the soil:
For granular soil, water is always separated from the solids in the analysis. Mechanical properties of the solids are explicitly used in the analysis. Water has its own weight and lateral pressure coefficient, and does not contribute towards shear capacity of the soil. This is called ‘drained’ analysis, or ‘effective stress’ analysis.
For cohesive soil, in the ‘short term’, because water flows very slowly through the pore, water and solid is analysed in a combined manner, like a dough, which is called ‘undrained’ behaviour. Combined mechanical properties of both the solids and water are used in the analysis. The combined properties can either be defined directly, or calculated using separated properties of the soil skeleton and water. In the ‘long term’, water is still separated from the solid in the analysis, just like that for granular soil.
Some mechanical features of ground water
Most of the time ground water in geotechnical analysis does three things:
Generates (pore) pressure, which affects the effective stress conditions of the soil. Below a steady and consistent ground water table, the pore water pressure increases linearly with depth (can be called a ‘phreatic’ condition). In addition to the phreatic pressure, there is also excess pore pressure which is generated temporarily. A variation in ground water table over a distance close enough triggers ground water flow, and this moves the pore water profile away from a linear increase manner.
Modifies properties of the soil. Water has zero shear capacity. Therefore its coefficient of lateral earth pressure K is always 1, and there is no active or passive coefficient. Water can not be compressed (almost), and therefore the Poisson’s ratio of water is always 0.5. In undrained condition, properties for the combined water and soil is used. Under this condition, Poisson’s ratio is always 0.5, and the Young’s modulus is higher than drained condition.
Leads to consolidation over time, depending on the permeability of the soil. Consolidation is a process of dissipation of the excess pore pressure over time. It’s one of the reasons for foundation settlement over time, as an example.
Excess pore water pressure
Furthermore, for cohesive soil, ‘excess pore water pressure’ can be generated in the short term due to changes in the stress conditions of the soil. See this previous blog for more information. This basically means the water in the soil is being squeezed or stretched but has nowhere to go, at least not quickly enough, due to the low permeability of the soil.
When compression in the soil increases, positive excess pore water pressure is generated. This means the pore water is taking almost all the increase in compression
When compression in the soil decreases, negative excess pore water pressure is generated. This means the pore water is sucking and gluing the solids in the soil together and not letting go.
The maximum negative ground water that can be generated is around 100kPa, because this is the atmospheric pressure. The suction effect of ground water comes from the pressure of the atmosphere, and therefore can never go beyond this. Any suction effect that tries to go beyond this pressure will start to create vacuum – a void in space that has nothing inside.
When is a soil considered cohesive or granular?
For some cases the answer is clear – clay is definitely cohesive whereas coarse sand is definitely granular. But the boundary between the two classifications is quite fuzzy. Generally speaking, the boundary permeability is about 10-7 and 10-8 m3/s/m2. Below that is usually considered as cohesive, and above that granular.
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