Retaining structure wall

Retaining Structure Wall

When the earth is cut or will be filled to approximate a vertical condition. 

Some type of retaining structure wall is needed. 

We can ignore to use retaining structure wall if the type of soil is rock which will 

remain itself quite well on a vertical face in any case. 

An earth cut will attempt to reestablish its "comfortable” slope , and a retaining

 wall is necessary to keep it in position.

Type of Retaining Structure Wall

Retaining structure wall are generally of 2 types: gravity structures and cantilever 

structures.

Within these 2 general types many variants are possible. A gravity structure 

employs a crib system , a depth of stacked rock, or soil anchorage to produce a 

block of material that is stable in its own right, sufficient to resist the lateral forces 

of the soil behind it without overturning or sliding.

Cantilever structures are usually reinforced concrete and employ a foundation 

that is securely supported by the ground and vertical element, which is 

structurally capable of resisting the horizontal force of the soil. These are

probably the most of common types of retaining structures walls in use. and 

concrete cantilever retaining wall are frequently seen. 

There are also hybrid retaining structures that could be compound cantilever

structures, such as cantilever walls with tiebacks to " dead men" or anchor piers 

further back from the front of the walls. Anchored bulkhead typically are sheet 

piling with an anchored tieback.

The below sketches some types of structure retaining walls...   

General Design Principles

-Earth Pressures

The term earth pressure refers to the horizontal forces on a retaining structure 

generated by the retention of the earth.

this pressure depending on the type of soil material, the height of the soil 

material, drainage condition and the rigidity of the wall itself.

Horizontal earth pressure are typically assumed to be a triangular-shaped 

loading, which means that the horizontal pressures near the bottom of the back 

of the wall are much greater than those near to the top.

The soil is an equivalent fluid ranging from a unit weight of about 30 pcf to over 

100 pcf. The heavier the unit weight and the greater the height, the higher the 

lateral pressure.

For triangular loading distribution, it is assumed that the horizontal forces are 

totaled and concentrated at a point one-third of the distance up from the base of 

the wall , and this loading concentration is used by the designer to determine the 

overturning resistance of the wall as a whole and to design the elements of the 

wall structurally to avoid being damaged by the forces. 

- Bearing and Sliding Resistance

In addition to the horizontal earth pressures applied to a wall, the resistance to 

bearing failure under the wall footing and the resistance to sliding of the entire 

structure along the surface of the soil must be evaluated by the designer. In the 

case of a cantilever retaining wall , the horizontal footing is being pushed away 

from the upper soil tending to cause overturning , and the soil pressure is 

increased at the front toe of the wall footing compared  to the rear or heel. A 

check should be made to be sure that the front soil pressure do not exceed the 

safe bearing pressures of the soil at that location.

- How we can estimate sliding resistance?

Sliding resistance is estimated based on the friction factor between the base of 

the wall and the ground, for both a gravity wall and cantilever concrete wall, to 

restrain the full force of the horizontal pressures. This resistance is 

usually expressed in terms related to the vertical load or normal (vertical) 

pressure on the footing multiplied by a friction factor. The friction factor came 

from the Geo-technical engineer and the normal forces can be calculated by the 

structure engineer.

Clay sub grades commonly are considered to provide cohesion not dependent on 

normal force. some designs utilize a resistance at the front of the footing, which 

is buried in the soil a few feet to aid the sliding resistance. This is not always a 

good idea since the front edge may be removed by excavation or erosion and 

may not be present when needed. 

- Active and at-rest pressures

A bridge abutment, a braced basement wall ,or split-level structure wall will be 

virtually rigid when subjected to horizontal forces. In these cases, the horizontal 

pressure values used for design will be greater than if the wall were permitted a 

slight amount of movement because if the soil is permitted to move a little bit, the 

small strains in the soil mass increase resistance to further movement, and 

reduces the amount that has to be added by the resistance of the wall. If the 

small amount of movement is not permitted because the wall is rigid, the 

pressures will be higher. If the wall type permits a small amount of movement of 

a friction of a percent of the wall height, this would be analyzed as an active 

pressure case. At rest pressures applicable to rigid walls can nearly double the 

horizontal pressures on a rigid retaining structure and must be considered in the 

design.

- Reinforcement steel in concrete walls

Reinforcement steel in cantilever steel concrete wall is subject to high tensile 

forces, which are maximized at the base of the wall adjacent to earth face. 

Therefore, the steel must be embedded in the concrete footing and spliced 

lapped with the wall steel as necessary to provide a continuity of steel in this area.

Also, reinforcement steel concrete wall should have: -

- Cover of at least 3 inches of concrete at earth side to protect steel reinforcement from corrosion.

- Vertical steel reinforcement along with the earth side and horizontal steel to guard against shrinkage or thermal cracking.

- Steel should be clean from rust, dirt, or grease to bond with concrete.

- Concrete retaining walls should have a full vertical expansion joint with slip steel dowels at 100 feet spacing to allow for thermal changes. the horizontal steel reinforcement should not continue through this type of joint.

- Vertical shrinkage control joint should be provided at 20 feet intervals; the horizontal steel reinforcement can continue through this type of joint.  

- Water and retaining structure 

The common reason for retaining wall failure is by water. If retaining wall does 

not have a well-drained system behind it, will found water can build up behind the 

wall. and this will add 62.4 pounds per cubic foot of unit weight, which is all 

translated into horizontal pressure added to the soil design pressure. Thus, built 

up of water behind a retaining wall will double or more than double the horizontal 

forces and this led to loss of the factor of safety and the failure of walls.

To avoid such as failure, designer can provide:

- Drainage system behind the wall or through the wall to carry any water build up away.

- Designer can provide a clean granular drainage material behind wall, usually with geotextile separator to avoid the loss of sand or clogging by silt.

- Cantilever concrete walls may be have drain system through the wall by fix weep holes at wall near the base which typically 4 to 6 feet on centers and about 6 inches above the  lower grade and 1 to 2 inches in diameter. This should penetrate the wall and contact a drainage material behind the wall, such as a clean sand or washed gravel. To relieve any water that build up behind the wall.

- Designer can provide a continuous drain behind the wall using a granular drainage material with a perforated pipe sloped to run out at some point to daylight. This method used if the wall is facing an occupied area which water is not desirable. such as a sidewalk.

The pipe should have a minimum of 6 inches below the floor slab.