In general, which is the most conservative, Coulomb’s or Rankine’s theory of lateral earth pressure and why? (05) Note: Your discussion should be correct in order to get any marks.
The Correct Answer and Explanation is:
Correct Answer: Coulomb’s theory is more conservative than Rankine’s theory of lateral earth pressure.
Explanation:
In geotechnical engineering, the lateral earth pressure is the pressure exerted by the soil mass against a retaining wall or structure. Two primary theories are used to determine this lateral pressure: Coulomb’s theory and Rankine’s theory.
Rankine’s theory, developed by William John Macquorn Rankine, is based on a simplified assumption that soil behaves in an idealized, frictionless manner. It assumes that the soil mass is homogeneous, dry, and that there is no cohesion between particles. Rankine’s theory focuses on the relationship between the horizontal stress in the soil and the angle of internal friction, neglecting the effects of wall friction and adhesion. It also assumes that the wall moves in a perfectly vertical direction with no displacement or rotation. As a result, the theory leads to a lower lateral earth pressure.
On the other hand, Coulomb’s theory, developed by Charles-Augustin de Coulomb, is more sophisticated and takes into account the effects of wall friction, cohesion, and the sloping of the backfill material. Coulomb’s theory accounts for both the cohesion and the internal friction between soil particles and the retaining structure. In this theory, the lateral earth pressure is affected by the angle of internal friction of the soil, the angle of wall friction, the angle of backfill slope, and the cohesion of the soil. Coulomb’s theory allows for more complex interactions between the soil and structure, which means that it typically predicts higher lateral earth pressures compared to Rankine’s theory.
Because Coulomb’s theory includes additional factors such as wall friction and soil cohesion, it generally results in more conservative (higher) estimates of lateral earth pressure. This is important in engineering practice, as a more conservative estimate ensures the design is safe and accounts for real-world complexities such as non-ideal soil conditions, wall friction, and variations in material strength. Therefore, Coulomb’s theory provides a more cautious approach, ensuring that structures are more likely to withstand the actual forces exerted by the soil.