Proportioning of reinforced concrete retaining wall elements such as stem, base slab, toe slab and heel slab is important to make the design economical. Proportioning involves calculation for the thickness and width of retaining wall components.
Proportioning of Reinforced Concrete Retaining Wall
Following are the preliminary dimensions of retaining wall:
a) Calculating Width of Base Slab:
Base slab of a retaining wall is so proportioned that the vertical soil reaction ‘R’ at the base with the front face of the stem. Base slab provides the stability to the retaining wall and helps to transfer loads from retained material to the ground by preventing overturning and sliding of the base.
The base slab consists of heel slab and toe slab. Proportioning of these components of base slab is important to provide safety and stability to retaining wall and prevent overturning and sliding.
Following figure shows the dimensioning of retaining wall.
Let h = height of the backfill from the soffit of the base slab
B = width of the base slab
Xh = width of heel slab
Xt = width of the toe slab
Ca = coefficient of active earth pressure
For economical design, the soil pressure resultant should pass line up with the front face of the stem. Assuming the soil pressure distribution as triangular with maximum pressure at the toe and zero at the heel, the resultant vertical pressure will pass through the middle third point.
Unnikrishna Pillai and Devdas Menon of IIT Madras have developed an expression for the minimum width of the heel slab as:
Width of Base = B = 1.5Xh and Xt = B/3.
b) Thickness of Base Slab and Stem:
The thickness of the base slab is assumed to be little more than the bottom thickness of the stem. The minimum thickness of the stem is considered to be 200mm and base slab to be 300mm for construction purposes.
The preliminary thickness of base slab is given by:
Tb = 0.08h but not less than 300mm.
Thickness of stem at bottom is assumed as
ts = tb
The stem thickness is gradually decreased towards top to a minimum value of 150 to 200mm for economical design. The front face of the retaining wall is maintained vertical.