EFFECTIVE STRESSES VS. TOTAL STRESSES

Soil typically consists of 3 parts: solid, water, and air. Therefore, it might confuse to analyze soil stresses using classical theories of mechanics. Karl Terzaghi () has introduced the Effective Stresses concept to simplify stress analysis in soil particles. Effective stress can be defined as

According to Terzaghi, by assuming that the summation of the area of particle contact is small as compared to the total area of the soil mass

When we apply total stress to soil mass with water inside the void space, so-called pore water. If the soil is saturated (100% of void space is filled with water), the applied stress will be carried only by pore pressure so that effective stress is zero. Here we have two scenarios:

- For high permeability soils like gravel, sand

Pore water can dissipate fast enough that effective stress quickly equals total stress. Therefore, pore pressure vanishes due to quick drainage. This is the so-called drained condition.

- For low permeability soils like clay and silt

Pore water can't dissipate fast enough that the transfer of stress from pore pressure to soil inter-particle is slow or requires a very long time to complete, i.e., consolidation.

Example

The above example shows the different methods to compute vertical stresses at points A and B. In the total stress method, the stress from water and soil weights is lumped into total stress. While in the effective stress method, separating soil inter-particle stress from pore pressure is necessary. The only reason to know effective stress is when we use drained shear strength from CU or CD tests.

Total stress vs Effective Stress Analysis

1. Due to different water permeability, it is common to use effective stress analysis (drained condition) for granular soils (short-term and long-term) or fine grain soil in long-term loading conditions. The shear strength obtained from the triaxial test (CU, CD) is necessary for effective stress analysis.

2. Total stress analysis (undrained condition) can analyze fine-grained soil in short-term loading conditions. Total stress analysis usually deals only with cohesive soil (phi = 0). The total stress analysis uses undrained shear strength of soil (obtained from UU or UC tests) for design purposes.

Which analysis type is safer?

It is not always true that undrained analysis is always safer than drained analysis. For example, in slope stability problems, groundwater above the failure surface can lead to lower FS compared with undrained analysis. In drained conditions, cohesion is lower, while shear strength primarily relies on friction at the failure interface. Assuming c = 0 for all soil types is always safe for drained conditions.

Any calculation difference in case of submerged soil?

- In effective stress analysis, the submerged unit weight of soil is used for soil below the water level, and the hydrostatic water pressure is then added up to the lateral pressure.

- In total stress analysis, the saturated unit weight of soils is used for soil below the water level so that we don't separate water from the soil as in effective stress analysis.

Ref.

DP Coduto, MR Yeung and WA Kitch, Geotechnical Engineering: Principles & Practices, Pearson, 2nd Ed., 2018