Retaining Wall and Water Proofing

Retaining Wall:

A retaining wall is a structure designed and constructed to resist the lateral pressure of soil. Retaining walls are built in order to hold back ground. Lateral earth pressures are zero at the top of the wall & a maximum value at the lowest depth. Earth pressures will push the wall forward or overturn it if not properly addressed. The basement wall is thus one form of retaining wall. Most often used to refer to a cantilever retaining wall which is a freestanding structure without lateral support at its top.

Function of retaining wall

Strength stability & durability
Resistance to overturn
Resistance to horizontal slide
Resistance to overstress in the materials of construction
Resistance to overstress in the soil on which the wall rest

Design Consideration:

Retaining wall must be ensure that;

Overturning does not occur
Sliding does not occur
The soil beneath the wall is not overloaded
The materials in the wall are not overstressed

Forces acting on Retaining walls

The effect of 2 forms of earth pressure need to be considered during the process of designing the retaining wall that is:

1. Active Earth Pressure

“ It is the pressure that at all times are tending to move or overturn the retaining wall”

2. Passive Earth Pressure

“It is reactionary pressures that will react in the form of a resistance to movement of the wall"

Angle of repose

It is the natural slope taken up by any soil; It is given in terms of the angle to the horizontal base line. It varies from 45-0̊ for wet soil but for most soil this angle of repose is 30̊

Wedge of soil

This is the soil resting on the upper plane of the angle of repose

Surcharge

This is the additional mass of soil above the top surface of wall

Factors affecting strength, stability & durability

Nature and type of soil
Height of water table
Sub-soil water movements
Types of wall
Materials used in the wall

Types of retaining walls

Gravity or mass retaining wall
Cantilever or L-shaped retaining wall
Sheet piling retaining wall
Anchored retaining wall

1. Gravity or mass retaining wall

Gravity walls depend on the weight of their mass (stone, concrete or other heavy material) to resist pressures from behind. With of the base is usually H/4 to H/2 ,where H is the height of wall. For efficiency the wall is sloped in front face. Reinforced to avoid cracking. Height is usually limited to 1.8 to 2 m.

2. Cantilever or L-shaped retaining wall

Cantilevered retaining walls are made from an internal stem of steel-reinforced. These wall have much thinner stem and utilize the weight of the backfill soil to provide most of the resistance to sliding and overturning, less expensive than mass gravity walls, most common type of earth retaining structure.

3. Sheet piling retaining wall

Retaining properties and water proofing of basements

Damp Proofing Course (DPC) for basement
Provided on outside surface of wall and underside of floor of basement
DPC must withstand the water pressure from underside
Basement must have sufficient dimension
Base concrete (PCC) of sufficient thickness to be provided with minimum projection of 15cm beyond outer wall as a protective before DPC
RCC wall and slab be provided after DPC course
Asphalt layer is best DPC in basement and it should be continuous
There must be proper lapping of DPC in joints and cracks

Methods of water proofing

1. Membrane water proofing -rater repelling substance like bituminous felt, asphalt, silicon etc.

2. Integral waterproofing -adding certain waterproofing compounds into the concrete mix.

3. Surface treatment -Filling up the pores of the surface subjected to dampness-paint, cement slurry etc.

4. Pressure grouting

5. Cavity of construction

Civil Engineering Nepal