Soil Mechanics : What Is The Difference Between Total And Effective Stress Analysis?
what is the difference between the total and effective stress analysis, where do we use them?
I hav this assignment problem I’ve been working on for quite a while: 2 meters of sand on top and clay below it to a depth of 10 meters. the water table is 1 meter below the surface. the soil is excavated to the depth of 2 meters (1 meter below the water table), which type of analysis should we use? total or effective? and why?
Tags: Analysis, Between, Difference, Effective, Mechanics, Soil, Stress, Total, What
February 6th, 2010 at 9:06 am
Imagine your soil profile divided up in to a series of isolated layers – this is the free body diagram. The total stress is the stress applied to the outside of each free body – it must be in equilibrium with the sum of the external forces applied to the free body and the self-weight of the body itself. Total stress is the stress applied to the _outside_ of an imaginary block or layer of soil.
Effective stress and pore water pressure are measures of how this externally applied stress is carried _inside_ the soil inside your block or layer, the free body. The pore pressure is the component carried by the pore water. The effective stress is the component carried by the contact forces between the soil grains.
To answer your second part, you’ll need to know the unit weight of the soils, whether they are dry or saturated, and what you are trying to achieve in your analysis. To get the stress profiles, the usual first step, you first work out the total stress variation with depth using vertical equilibrium, then work out the pore pressure variation with depth by knowing where the water table is and the saturation profile, then after that you calculate the effective stress profile by subtracting the pore pressures from the total stresses. All you can say for now is that in the initial case the sand above the water table can be assumed to be dry, so the pore water profile is u=0kPa at the surface where the sand is dry, u=0kPa at the water table (z=1m from the surface), u=1kPa at the base of the sand (z=2), and u=11kPa at the base of the clay (z=12m). In the second case, you need to know whether you are trying to get the stress profile immediately after excavation or once equilibrium has been reached. In the first case, the total stresses will have changed because some of the external load (i.e. the sand) on the clay has been removed but the effective stresses will be unchanged because the soil particles haven’t had a chance to move around yet and change their contact forces. Over time, as consolidation occurs the particles will move around and the pore pressure will go back to being in equilibrium with the regional water table – in this case, that means you would expect a 1m deep puddle and the pore water profile would be as it was before. At the moment, I don’t know what you are after. To get the effective stress profile, you’ll need to work out the total stress profile from equilibrium, and for that you need the unit weights.
All of this assumes that air pressure effects can be ignored. Unsaturated soil mechanics is more complicated, and is usually an advanced or postgraduate course. At introductory level, most courses deal with stresses in soils that are either saturated or dry i.e. the extreme cases. Fortunately, this gives reasonable results most of the time.
The concept of effective stress is probably the single most important concept in soil mechanics. It underpins most of the discipline. It is worth spending a lot of time with a lot of textbooks to get comfortable with the concept.