Following the previous blog on soil-structure interaction (http://www.si-eng.org/post/soil-structure-interaction-1-what-interaction), this blog continues on the thread to talk about lateral earth pressure – why it is the critical link in soil-structure interaction design, and how lateral earth pressure is affected by the structural design.
The retaining structure and the retained ground are essentially two materials that work in composite to each other to maintain equilibrium – the ground has certain capacity to stand up by itself to certain extent; the retaining wall is pushing back on the ground to prevent it from collapsing. In a composite action with compatible strain, the material with higher stiffness attracts more stress. When the structure is relatively stiff, it moves less and attracts more loading from the soil. When the structure is relatively soft, it moves more but relieves stress from itself. This critical link maintaining the equilibrium between ground and structure is the lateral earth pressure. It is an output from the geotechnical analysis and an input from the structural analysis.
Lateral earth pressure has three different states, each representing a different equilibrium – the active state, the passive state and the at-rest state. In the at-rest state, the wall is nearly infinitely stiff, thus it is working on its own to retain the ground. The ground isn’t making any contribution. In the active state, the utilisation of the soil’s own structural capacity is fully utilised, thus relieving the contribution of the structure to a minimum possible. In the passive state, however, the structure is pushing against the ground, making the structural capacity of the ground working against the ground. The ground has the overcome both the soil’s pressure and its negative contribution of structural capacity, thus taking on even more load than the at-rest state. These are three theoretical states and represent idealised extremities of the spectrum. Real situations are mostly somewhere in between the extremities.
Now, a key question is – under what circumstances does the pressure qualify as active and passive?
This is actually a very tricky question to answer. Perhaps nobody knows for sure.
The active state of lateral earth pressure is much ‘easier’ to mobilise than the passive state, in terms of the amount of lateral movement required to achieve full mobilisation of each state. Both states are easier to be mobilised in soil with densely packed particles than loosely packed particles – i.e. the amount of lateral movement required is smaller.
· Full active pressure can be mobilised with 1/1000 to 1/500 wall deflection (away from the retained soil) for densely packed soil and about 1/200 wall deflection for loosely packed soil.
· Full passive pressure can be mobilised with 1/10 to 1/20 wall deflection (towards the retained soil) for densely packed soil and about 1/15 to 1/5 for loosely packed soil.
Some other factors that has influence over lateral earth pressure:
1. Surface profile of retained earth. Having an upwards sloped surface profile for the retained soil results in an increase in both active and passive pressure. The higher the angle of the slope is, the more the increase is, topping at the internal friction angle of the retained soil. This is because the internal friction angle is the maximum angle the retained soil is able to stay stable. Any steeper angle will result in a collapse theoretically.
2. Friction between retaining structure and the ground. The friction between the retaining structure and the soil reduces active pressure or increases passive pressure. As the wall-soil friction goes up, the reduction to active pressure also goes up, topping at about 15% reduction, where the friction is as strong as the internal friction between the soil particles. Any stronger wall-soil friction than the internal friction is useless, because the internal shear failure of soil particles will happen before the shear failure at the interface. As for the increase to passive pressure, the effect of wall-soil friction increases as the internal friction of soil increases. In other words, the stronger the internal friction of the soil is, the stronger the effect of wall-soil friction is.
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