Geotechnical Engineering (DOC download)

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					       Design of retaining walls

The main function of any retaining structure is to act as earth retaining structure for the whole or
part of its height on one face, the other being exposed to the elements. Liquids are also retained.
For example, in rivers, dams, reservoirs, etc.

7.1 Types of retaining walls/structures

       (a) gravity wall
       (b) reinforced concrete wall
       (c) sheet piling
       (d) basement
       (e) bridge abutment, etc.

7.2 Design of retaining structures

According to the principles of limit state design, an earth retaining structure must not
   (a) collapse or suffer major damage
   (b) be subject to unacceptable deformations in relation to its location and function
   (c) suffer major damage which would necessitate excessive maintenance, render it unsightly
       or reduce its anticipated life

Design of these structures is generally based on ultimate limit states with the incorporation of a
suitable factor of safety with respect to passive resistance to satisfy the requirements of
serviceability limit states. Serviceability limit states are those involving excessive deformation,
leading to damage or loss of function.

7.3 Modes of failure of gravity retaining walls
7.4 Limit states for gravity retaining walls:

   (i)     OVERTURNING

               Fot = ∑(Moments about the toe of W, q, etc.)/∑(Moments about the toe of Pa,
               etc) ≥ 2.0

   (ii)    SLIDING

                       Rs  Pp
                Fs               2.0
                         Pa

The sliding resistance Rs = Vtanδ or Rs = CwB


   (iii)   TENSION FAILURE AT JOINTS (OR STRUCTURAL FAILURE)

               MIDDLE THIRD RULE must apply

   (iv)    ROTATIONAL SLIP FAILURE
   (v)     BEARING ON THE GROUND

The combination of the downward-acting weight of the wall and overturning moment will set up
a bearing pressure between the base of the wall and the ground upon which it rests. Failure will
occur if the applied pressure exceeds the ULTIMATE BEARING CAPACITY of the soil
causing the soil at this point to be disturbed. In practice the applied pressure is not allowed to
exceed the ALLOWABLE BEARING CAPACITY of the soil, which is related to the shear
strength and compressibility characteristics of the soil and incorporates a factor of safety which
should be at least 3.0.

Ground bearing pressure consists of a direct stress component and bending stress component.
The ground bearing pressures (stresses) at A and B are obtained from the algebraic addition of
the direct and bending stresses i.e.
       V Ve
qA     
       A z


       V Ve
qB     
       A z



                              1* B 2
Putting area, A = B and z =          we get
                                6

       V 6Ve
qA     
       B B2


       V 6Ve
qB     
       B B2

where
V = resultant ground reaction
e = eccentricity of V about the centreline of the base
A = area of the base = 1*B (per unit length of wall)
z = section modulus of the base about its centreline = 1*B2/6 (per unit length of wall)

Factor of safety against failure of foundation is

F = Ultimate bearing capacity of the soil/Maximum ground bearing pressure ≥ 3.0
7.5 Determination of the position of the ground reaction, V

				
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Description: Design of retaining walls