Advance Design of RC Structure

Lecture 11

Retaining wallsRetaining walls

Dr. Ali TayehDr. Ali Tayeh

Retaining wallsRetaining walls

A retaining wall is a structure built for the purpose of holding back or retaining or providing one-sided lateral confinement of soil or other loose material. The loose material being retained pushes against the wall, tending to overturn and slide it. Retaining walls are used in many design situations where there are abrupt changes in the ground slope. Such situations occurred when the width of an excavation, cut, or embankment is restricted by conditions of ownership, use of the structure, or economy.

These structures are commonly used in construction projects and may be classified generally as:

1. Gravity retaining walls. 3. Cantilever retaining walls.

4. Counter fort retaining walls.

Types of retaining walls:Types of retaining walls:

1. Gravity Retaining Walls1. Gravity Retaining Walls

Gravity retaining walls are constructed with plane concrete or stone masonry. They depend on their own weight and any soil resting on the masonry for stability. This type of construction is not economical for high walls.

In many cases, a small amount of steel may be used for the construction of gravity walls, thereby minimizing the size of the wall sections. Such walls are generally referred to as semi-gravity walls.

2. Cantilever Retaining Walls2. Cantilever Retaining WallsAre made of reinforced concrete that consists of a thin stem and base slab. This type of wall is economical to a height of about 25ft (8m).

33 . .Counter fort Retaining WallsCounter fort Retaining Walls:: Are similar to cantilever walls. At regular intervals, however, they have thin vertical concrete slabs known as counter forts that tie the wall and the base slab together. The purpose of counter forts is to reduce the shear and the bending moments. To design retaining wall properly, an engineer must know the basic soil parameters, which are the unit weight, angle of friction, and cohesion.

Retaining wallsRetaining walls

Types of retaining Types of retaining walls:walls:Gravity retaining wall depends entirely on its own weight to provide the necessary stability. Semi gravity retaining wall use in it very light reinforcement. Cantilever retaining wall it is a reinforced concrete. Counterfort retaining wall.

In buildings, retaining walls are usually temporary cantilevers, which are later propped by the adjoining floors, so the analysis of the wall is simply support because the construction of floor is before the backfill of the soil.

(a) Gravity, (b) Semi gravity, (c) Cantilever, (d) Counterfort.

Loads on retaining walls:Loads on retaining walls:

Retaining wallsRetaining walls

Downward load due to the own weight of the wall and base, and the weight of the soil resting directly on the base (considered as dead load and subjected to load factor1,2).

In addition to any surcharge that may exist, (considered as live load and subjected to load factor1.6).

Lateral pressure produced by the soil retained behind the wall and that in front of the toe and the key, (considered as live load and subjected to load factor1.6).

Upward pressure developed below the base as a result of the applied downward and lateral load, (they are subjected to load factor1.6).

Horizontal frictional force developed on the plane of contact between the base and the soil below as the retaining wall tends to slide ferward.

Lateral pressure on retaining wall: Lateral pressure on retaining wall:

Retaining wallsRetaining walls

The actual pressures that occur behind retaining walls are quite difficult to estimate because of the large number of variables. On the assumption that soil is dry and granular (noncohesive), and its surface is horizontal and free from any surcharge, the lateral pressure exerted by the soil on retaining wall is termed active earth pressure (Pa)

Retaining wallsRetaining wallsLateral pressure on retaining wall: Lateral pressure on retaining wall:

whCp aa

The pressure developed by the soil in front of the wall and the key, if provided, as the wall leans against the soil is termed passive earth pressure.

'whCp pp

Where Ca and Cp are the approximate coefficients of active and passive pressure is the unit weight of the soil, h is the distance from the surface to the point of question.

If the backfill be horizontal, the value of Ca and Cp are:

sin1sin1

aC

sin1sin1

pC

Where: is the angle of internal friction of soil .

Retaining wallsRetaining walls

The total active horizontal pressure is given by:

Lateral pressure on retaining wall: Lateral pressure on retaining wall:

2

2whCH a

a

If there is surcharge pressure, the total horizontal surcharge pressure acting to the wall is given by:

WhCH as

This force acts through the centroid of the uniform pressure distribution diagram, i.e. at h/2 above the bottom of the base.Note: In building the wall may be restrained, so (at rest) is used.

oC

Retaining wallsRetaining walls

Stability requirement Stability requirement

To check stability of retaining wall, the following steps are necessary:Check for overturning about its toe.

•Check for sliding failure along its base.

To have a good sliding resistance, the key be used below the base of retaining wall.•Check for bearing capacity of soil.

•Check for over all instability (slope instability).

0.2.

O

Rgoverturnin M

MSF

5.1.

d

Rsliding F

FSF

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