Retaining walls are structures used for supporting earth at different levels. There are different types of retaining walls such as cantilever retaining walls, gravity retaining walls, counterfort retaining wall, buttressed retaining walls, etc. Counterfort retaining walls have similarities with cantilever retaining walls, with the difference of having triangular web panels spaced at regular intervals at the back of the retaining wall.
These web panels are called counterforts, and they serve the purpose of tying the base slab and the wall (stem) together. By so doing, they reduce the internal stresses induced in the structure, and increase the weight of the structure for stability.
Counterfort retaining walls can be easily modelled on Staad Pro software, and loaded to obtain the internal forces and deformations due to the retained earth. We are going to demonstrate that using the video below. The data of the retaining wall is shown below;
Height of wall from base = 7 m c/c
Length of base = 4.5m
Projection of toe = 0.8 m c/c
Projection of heel = 3.7 m c/c
Thickness of stem wall = 0.3 m
Thickness of base = 0.5 m
Thickness of counterfort = 0.3 m
Spacing of counterfort = 2.5 m c/c
Unit weight of concrete = 25 kN/m3
Unit weight of retained earth = 19 kN/m3
Angle of internal friction φ = 30°
Surcharge pressure on retaining wall = 10 kN/m2
Modulus of subgrade reaction of supporting soil = 50000 kN/m2/m
We are going to neglect the effect of passive earth pressure on the retaining wall.
Coefficient of active earth pressure Ka = (1 – sinφ)/(1 + sinφ) = 0.333
Earth pressure at the back of the wall (triangularly distributed) = 0.333 x 19 kN/m3 x 7m = 44.289 kN/m2
Surcharge pressure at the back of the retaining wall = 0.333 x 10 = 3.33 kN/m2
Earth pressure on the base (heel) = (7m x 19 kN/m3) = 133 kN/m2
Surcharge pressure on the base (heel) = 10 kN/m2
Watch the video for analysis of counterfort retaining walls on Staad Pro below;