RETAINING WALLS

THE BASIC FUNCTION OF A RETAINING WALL IS TO RETAIN SOIL AT A SLOPE WHICH IS GREATER THAN IT WOULD NORMALLY ASSUME

THE NATURAL SLOPE TAKEN UP BY ANY SOIL IS CALLED ITS ANGLE OF REPOSE AND IS MEASURED IN RELATION SHIP TO THE HORIZONTAL

IT IS THE WEDGE OF SOIL RESTING ON THIS UPPER PLANE OF THE ANGLE OF REPOSE WHICH A RETAINING WALL HAS TO SUPPORT

THE DESIGN OF RETAINING WALL IS BASICALLY CONCERNED WITH THE LATERAL PRESSURES OF THE RETAINED SOIL AND ANY SUBSOIL WATERGREATER THE ANGLE OF REPOSE OF A

MATERIAL, THE LESS IS THE PRESSURE EXERTED

INCREASED PRESSURES MUST BE ALLOWED FOR WHEN THERE IS A SURCHARGE OR WHEN THERE ARE BUILDINGS OR TRAFFIC CARRYING ROADS NEAR THE TOP OF THE WALL

FACTORS AND CONSIDERATIONS IN THE DESIGN OF RETAINING WALLS COULD BE

1.NATURE & TYPE OF SOIL

2.HEIGHT OF WATER TABLE

3.SUBSOIL WATER MOVEMENTS

4.TYPE OF WALL

5.MATERIAL USED IN THE CONSTRUCTION OF THE WALL

TYPES OF WALLS COULD BE

1.MASS RETAINING WALL

2.CANTILEVER RETAINING WALL

3.COUNTERFORT RETAINING WALL

4.PRECAST CONCRETE RETAINING WALL

5.PRECAST CONCRETE CRIB RETAINING WALL

RETAINING WALL TERMINOLOGY

Counterfort

Gravity RWT-Shaped RW

L-Shaped RW

BackfillBackfill

Counterfort RW

Buttress

Backfill

Buttress RW

Batter

Drainage Hole

Toe

Cantilever Retaining wall with shear key

Gravity Poured Concrete Retaining Walls

Gravity retaining walls depend on their own weight and any soil resting on the concrete in resisting lateral earth forces. They are generally economical up to 10 feet in height for cast concrete structures. Usually are sufficiently massive to be unreinforced. Monolithic cast walls are generally formed on site.

Semi-Gravity Retaining Walls

A specialized form of gravity walls is a semi-gravity retaining wall. These have some tension reinforcing steel included so as to minimize the thickness of the wall without requiring extensive reinforcement. They are a blend of the gravity wall and the cantilever wall designs

Cantilever Retaining Walls Cantilever retaining walls are constructed of reinforced concrete. They consist of a relatively thin stem and a base slab. The base is also divided into two parts, the heel and toe. The heel is the part of the base under the backfill. The toe is the other part of the base

Use much less concrete than monolithic gravity walls, but require more design and careful construction. Generally economical up to about 25 ft. in height. Can be precast in a factory or formed on site.

Counterfort Retaining Walls

Counterfort retaining walls are similar to cantilever walls except they have thin vertical concrete webs at regular intervals along the backside of the wall. These webs are known as counterforts.

Counterfort retaining walls:

The counterforts tie the slab and base together, and the purpose of them is to reduce the shear forces and bending moments imposed on the wall by the soil. A secondary effect is to increase the weight of the wall from the added concrete. Can be precast or formed on site. Counterfort retaining walls are more economical than cantilever walls for heights above 25 ft.

Provisions for Joints in the Construction of Walls

Cast concrete retaining walls may be constructed with any or all of the following joints:

Construction Joints:

These are vertical or horizontal joints that are used between two successive pours of concrete. Keys are used to increase the shear resistance at the joint. If keys are not used, the surface of the first pour is cleaned and roughened before the next placement of concrete. Keys are almost always formed in the base to give the stem added sliding resistance. The base is formed first, and the stem constructed afterwards

Contraction Joints:

These are vertical joints or grooves formed or cut into the wall that allows the concrete to shrink without noticeable harm. Contraction joints are usually about 0.25 inches wide and about ½ to ¾ inch deep, and are provided at intervals of not exceeding 30 feet.

Expansion Joints:

Vertical expansion joints are incorporated into the wall to account for expansion due to temperature changes. These joints may be filled with flexible joint fillers. Greased steel dowels are often cast horizontally into the wall to tie adjacent sections together. Expansion joints should be located at intervals up to 90 feet.

Backfill Drainage of Retaining Walls One area that can be commonly overlooked, or at least underestimated, is the necessity to drain the backfill of rainwater and/or groundwater. Hydrostatic pressure can cause or induce retaining wall failure, or at least damage.     Drainage of water as a result of rainfall or other wet conditions is very important to the stability of a retaining wall. Without proper drainage the backfill can become saturated, which has the dual impact of increasing the pressure on the wall and lessening the resistance of the backfill material to sliding. Granular backfill material offers the benefits of good drainage, easy compaction, and increased sliding resistance.

Drainage systems usually utilize weep holes and drainage lines.

  Weep holes actually penetrate the retaining wall and drain the area immediately behind the wall. Weep holes should have a minimum diameter so as to permit free drainage; for large walls, 4 inch weep holes are common. Adequate spacing between weep holes allows uniform drainage from behind the wall. Weep holes should always have some kind of filter material between the wall and the backfill to prevent fines migration, weep hole clogging, and loss of backfill and caving.

Drainage lines are often perforated and wrapped in geo textile or buried in a granular filter bed, and serve to carry water to the weep holes from areas deeper within the backfill.

FORCES ACTING ON A RETAINING WALLTHE DESIGNER IS MAINLY CONCERNED WITH THE EFFECT OF TWO FORMS OF EARTH PRESSURE- ACTIVE & PASSIVE

STABILITY OF RETAINING WALLSTHE OVERALL STABILITY OF A RETAINING WALL IS GOVERNED BY THE ACTION AND REACTION OF A NUMBER OF LOADS

ACTIVE PRESSURE IS EXERTED BY THE RETAINED MATERIAL & WATER PRESSURE ON THE BACK OF THE WALLPASSIVE PRESSURES ARE THE INDUCED LOADS AT THE TOE AND THE FRICTION BETWEEN THE UNDERSIDE OF THE BASE AND THE SOILGROUND WATER BEHIND A RETAINING WALL CAN HAVE ADVERSE EFFECTS UPON THE DESIGN AND STABILITY OF THE RETAINING WALL

THE DESIGN OF RETAINING WALL MUST ENSURE THERE IS NO

1.FAILURE DUE TO OVERTURNING

2.FAILURE DUE TO SLIDING

3.FAILURE DUE TO BENDING

THE RESULTANT THRUST ON THE SOIL SHOULD BE IN THE

MIDDLE THIRD OF THE BASE

MASS RETAINING WALL

RCC CANTILEVER RETAINING WALL

COUNTERFORT RETAINING WALL

COUNTERFORT RETAINING WALL

-M

-M

TOE

COUNTERFORT

+M

+M

STEM

HEEL SLAB

PRECAST CONCRETE RETAINING WALL

PRE CAST CONCRETE CRIB RETAINING WALL

BASEMENT WALLS

REVETMENTSGABION

SRIP RAP

          

Active System with the mesh anchored on the rock facing.

 Passive System with simple 

drapery system.