INTRODUCTION:-

Retaining walls are used to retain earth of other materials which have the tendency to slide and repose at a particular inclination.They provide lateral support to the earth fill,embankment or other materials in order to hold them in a vertical position.Retaining walls also have applications in buildings and bridges such as basement ,foundation wall,bridge abutment etc.

TYPES OF RETAINING WALLS:-

The different types of free standing retaining walls used to retain earth or other materials.The materials retained exert a  push on the retaining wall which tends to overturn or slide.The stability of the wall against overturning and sliding is maintained by the weight of the retaining wall and weight of the earth on the base of the retaining wall.These walls are subjected to vertical load in addition to lateral thrust from the retained earth.

          

The different types of retaining walls are described below:-

1.Gravity Retaining wall:-

               A gravity wall is made of plain concrete or brick masonry.The stability of the wall is maintained by its weight.It isgenerally made up to a height of 3 m of wall.

2.Cantilever Retaining wall:-

               It consists of vertical wall,heal slab and toe slab which act as cantilever beams.Its stability is maintained by he weight of the retaining wall and the weight of the earth on the base of the reaining wall.It is generally made when the height of wall is in range of 3 m to 8 m.

3.Counterfort Retaining wall:-

               When the height of retaining wall is more than 6-8,it shall be economical to tie the vertical wall with the heel slab by counterforts at some spacing.The counterfort acts as a tension member to support the vertical wall and reduces bending moment in it.It also provides support to the heel slab and reduces bending moment in it. Generally, the counterforts are spaced at approximately one third of the height of the wall.The stability of the retaining wall is maintained by the weights of the earth on the base and its self weight.

FORCES ON RETAINING WALLS:-

The forces acting on retaining wall may be grouped as follows.

  1.Self weight of retaining wall

  2.Weight of soil above the foundation base

  3. Earth pressure on retaining wall

  4.surcharge,i.e.forces due to loads on the earth surface

  5.soil reactions on the footing

  6.Frictional forces on the footing due to sliding

   The main force that acts on the retaining wall is the lateral pressure induced by the retained earth.It exists in three different states,i.e. at rest, int the active state and in the passive state.The lateral pressure exerted by soil on an immovable and rigid wall is known as the pressure at rest.Usually such a state of earth pressure does not exist and the wall tilts and moves slightly under the action of earth pressure.Even a small movement of the top of the wall away from the earth fill by about 0.1 to 0.5 per cent of the height of the wall reduces earth pressure appreciably from that  occurring at rest state.this is know as active earth pressure.

STABILITY OF RETAINING WALL:-

In retaining walls it is not sufficient if we design the sections of various parts for moments and shear.It is necessary to check the retaining wall for the following two types of complete failures.
        1)Overturning about toe
        2)Sliding

 Overturning:-

           The earth pressure on the stem causes overturning moment about the toe.The weight of backfill earth,surcharge,self weight of retaining wall cause stabilizing moment about the toe.Hence factor of safety against overturning is given by
                           
                                      F1=Ms/Mo
         
Where Mo is overturning moment about toe and Ms is stabilizing moment about toe.A minimum factor safety of 1.4 is to be used.As per IS 456-2000,only 0.9 times the characteristic dead load shall be taken into consideration.
                     
                                F1=(0.9*Ms)/Mo

Sliding:- 

          The horizontal pressure H on the stem tries to slide the retaining wall away from the back fill.This is resisted by fractional force between the soil and heel and toe slab.If μ is the coefficient of friction  and Σw is the total downward load, the maximum resisting frictional force is
                         
                                F= μ*Σw
If PH is total horizontal pressure,then factor of safety against slidingis given by
                             
                               F2= (μ*Σw)/ PH

As per IS 456-2000 in this case also a minimum factor of safety of 1.4 is to be provided and only 0.9 times characteristic dead load is to be considered.

                                       
                                          F2= (0.9*μ*Σw)/ PH