Retaining Walls

PROPORTIONING OF RCC RETAINING WALL

Proportioning of reinforced concrete retaining wall elements such as stem, base slab, toe slab and heel slab is important to make the design economical.
Following are the preliminary dimensions of retaining wall:
a) 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. 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
X_h = width of heel slab
X_t = width of the toe slab
C_a = 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 = 1.5X_h and X_t = B/3.
b) Thickness of Base Slab and Stem:
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
t_s = t_b
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.

TYPES OF REINFORCED CONCRETE RETAINING WALL

Retaining wall are generally used to retain earth or other material to maintain unequal levels on two faces. The material on the back face is called backfill.
Retaining walls are used in the construction of basement below ground level, wing walls of bridge and to retain slopes in hilly terrain roads.
Retaining wall can be constructed with masonry as well as reinforced concrete. In case of masonry retaining wall, the thickness of wall increases with height because masonry resists the lateral pressure by its weight. Thus it is also called gravity retaining wall. While the reinforced concrete retaining wall resists the lateral pressure by structural action such as bending and results in thinner section.
Following are the types of reinforced concrete retaining walls:

1. Cantilever Retaining Wall:

Cantilever retaining walls are most commonly and widely used type of retaining wall. The following figure shows the cantilever retaining wall.

Parts of a cantilever retaining wall and its actions:
1. Vertical stem:
Vertical stem in cantilever retaining wall resists earth pressure from backfill side and bends like a cantilever. The thickness of cantilever slab is larger at the base of stem and it decreases gradually upwards due to reduction of soil pressure with decrease in depth.
2. Base slab:
The base slab form the foundation of the retaining wall. It consists of a heel slab and the toe slab. The heel slab acts as a horizontal cantilever under the combined action of the weight of the retaining earth from the top and the soil pressure acting from the soffit. The toe slab also acts as a cantilever under the action of the soil pressure acting upward. The stability of the wall is maintained by the weight of the earth fill and on the heel slab together with the self-weight of the structural elements of the retaining wall. Cantilever type retaining walls are suitable upto 5m depth of backfill.

2. Counterfort Retaining Wall:

When the height of earth to be retaining exceeds 5m, the bending moment developed in the stem, heel and toe slabs are very large which results in large thickness of structural elements and becomes uneconomical. Thus, counterfort type retaining wall is adopted for larger heights.
Figure below shows counterfort retaining wall and its parts.

Counterfort retaining wall consists of a stem, toe slab and heel slab as in case of cantilever retaining wall. But it also consists of counterforts are regular interval which divides the stem. The stem with combination of counterfort behaves like a tee-beam with varying width.
The stem and heel slabs are effectively fixed to counterforts so that the stem bends horizontally between the counterforts due to lateral earth pressure. Thus the thickness of stem and the heel slab is considerably reduced due to the reduction of moment due to fixity of these slabs between counterforts.

CANTILEVER RETAINING WALL

• Cantilever wall are usually of reinforced concrete and work on the principles of leverage.
• Have much thinner stem, and utilize the weight of the backfill soil to provide most of the resistance to sliding and overturning.
• Most common type of earth- retaining structure.
• The cantilever retaining wall (“cantilever wall”) constructed of reinforced Portland-cement concrete (PCC) was the predominant type of rigid retaining wall used from about the 1920s to the 1970s
• Earth slopes and earth retaining structures are used to maintain two different ground surface elevations.

FUNCTION
To retain the soil at a slope that is greater than it would naturally assume, usually at a vertical or near vertical position.

Design Consideration
In order to calculate the pressure exerted at
any point on the wall, the following must be
taken in account:

• height of water table
• nature & type of soil
• subsoil water movements
• type of wall
• material used in the construction of wall

The effect of two forms of earth pressure need to be considered during the process of designing the retaining wall that is:
a)Active Earth Pressure
“ It is the pressure that at all times are tending to move or overturn the retaining wall”
b)Passive Earth Pressure
“It is reactionary pressures that will react in the form of a resistance to movement of the wall.

Two Basic Form of Cantilever Wall
1 ) A base with a large heel so that the mass of earth above can be added to the wall for design purposes.

Figure 1 :Typical reinforced concrete cantilever walls.

2 ) If form 1 is not practicable, a cantilever wall with a large toe must be used.

Figure 2 : Typical reinforced concrete cantilever retaining walls

From figure 1 and 2 :

• The drawing show typical section and pattern of reinforcement encountered with these basic forms of cantilever retaining walls.
• The main steel occurs on the tension face of the wall and nominal steel (0.15% of the cross-sectional area of the wall) is very often included in the opposite face to control the shrinkage which occurs in in-situ concrete work.
• Reinforcement requirements, bending, fabricating and placing are dealt with in the section on reinforced concrete.

Reinforcement of cantilever retaining wall.


Advantages and details about cantilever wall
Reinforced cantilever walls have an economic height range of 1.200 to 6.000 m; walls in excess of this height have been economically constructed using prestressing techniques. Any durable facing material may be applied to the surface to improve the appearance of the wall but it must be remembered that such finishes are decorative and add nothing to the structural strength of the wall.

Cantilever Wall Failure

• Effect of water: Ground water behind a retaining wall, whether static or percolating through a subsoil, can have adverse effects upon the design and stability.
• Slip circle failure: sometimes encountered wit cantilever wall in clay soils particularly if there is a heavy surcharge.
• Low quality of material that use in cantilever construction
• Low design reinforcement in cantilever wall.
• Mistake in calculate height of water table, nature & type of soil.
• Subsoil water movements.

Identifying Failure of Cantilever Wall

• Cantilever wall be in sloping position.
• Cantilever wall had curve on its surface/wall.
• Crack on wall structure.
• Cantilever wall awashed.