Retaining Wall Design

Concrete Retaining Wall Design

Firstly, we need to check the stability of the retaining wall. Overturning and sliding shall be checked before proceed with the design. For more details about above checks click here.

In addition, we need to check the bearing under the foundation. If the wall is supported on soil, bearing pressure under the foundation of wall shall be less than the allowable bearing capacity.

First of all, we need to analyze the retaining wall and find the bending moments and shear forces. This can be done with a analysis software like SAP2000 or PROKON or any other software or by manual calculations. If the there are variations such as steps in the retaining wall and we are more concentrated on stress variation of the wall, we may use software like SAP2000 or ANSIS.

Manual calculations can only be done for basic retaining walls that do not have many  complications. For example, cantilever retaining wall can be analyse by simplifying its behavior. We can assume the boundary condition "fixed" at base to find the bending moments and shear forces of the wall. After finding wall forces, we can find the forces of the base of the wall. The easiest method is to use a software at least to find the bending moments and shear forces.

Design Example

Consider a wall of height 3 m and width of base 2.3 m. Assume friction angle of 30 degrees and water table is at 1 m above the base of the foundation. Wall thicknesses are considered as 0.3 m. This analysis is done from prokon software. Input details and analysis results are as follows.

As per the above analysis, wall and base have maximum bending moments of 39.81 kNm and 46.85kNm respectively. Design of each element is carried out as per the guide lines given in the BS 8110 Part 01 1997.

Design of wall

Cover to the reinforcement             = 40 mm
Assume main bar diameter             = 12 mm
Horizontal reinforcement dia          = 10 mm
Thickness of the wall                      = 300 mm
Characteristic strength of concrete = 25
Characteristic strength of steel        = 460 N/mm²

Effective depth                               = 300 - 40-10-12/2
                                                       = 244 mm

Bending Moment                           = 39.81 kNm
  
M/ bd²                                            = 0.669 N/mm²                                                      

100As/bd                                        = 0.158

As                                                   = 385 mm²

Provide T10 at 200 mm centre to centre spacing. It should also be noted that this design does not cover the reinforcement requirements for cracking of concrete due to thermal effects.

Design of Base 

Cover to the reinforcement             = 40 mm
Assume main bar diameter             = 12 mm
Horizontal reinforcement dia          = 10 mm
Thickness of the wall                      = 300 mm
Characteristic strength of concrete = 25
Characteristic strength of steel        = 460 N/mm²

Effective depth                               = 300 - 40-12/2
                                                       = 254 mm

Bending Moment                           = 46.85 kNm
 
M/ bd²                                            = 0.726 N/mm²                                                      

100As/bd                                        = 0.172

As                                                   = 437 mm²

Provide T10 at 175 mm centre to centre spacing. It should also be noted that this design does not cover the reinforcement requirements for cracking of concrete due to thermal effects.