Retaining walls are built in order to retain the soil masses located at suddenly abruptly changing elevation and provide lateral confinement.
The retain mass on another side of the wall exerts lateral force and overturning moment to the constructed wall, and thus the wall is needed to design to encounter such forces and moment and safely transfer it to ground strata.
The restoring forces and moment are either provided by self-weight (in case of gravity retaining wall) or by flexural strength of the wall ( cantilever and Counterfort retaining wall)
Gravity walls restrain such forces only by their self-weight, so they are extremely massive in size. Thus, for space efficiency cantilever walls are constructed, in which such restrain is done by flexural strength and partial self-weight of the wall.
Thus, such a wall has comparatively smaller dimensions (width) along with the provisions of the reinforcement bars. Cantilever walls are generally constructed for retaining height 10ft – 25ft of the elevation difference.
These walls have large base footing, which is anchored over the soil acts as fixed support and the vertical stem suspended upright position connected to base acts as the free end and supported lateral force of confinement; as in cantilever beam. Thus are known as cantilever wall.
Parts of Cantilever retaining wall
- Stem: It is the vertical upright portion of the cantilever wall which supports or restrain the lateral confinement. The stem has a greater slender ratio. Sometimes the stem is made of the same thickness throughout and sometimes they are made thicker at the base.
- Toe: it is the base footing embedded on the soil to the outside part of the wall where the soil mass has not to be retained.
- Heel: It is also part of the base footing embedded on soil but to inside part of the wall where soil mass has to be retained. The length of the heel is relatively larger. This is done in order to increase the self-weight of the wall. In doing so, the soil above the heel acts as the part of the wall and thus provides weight at the base of footing, which increases the restoring moment against the moment due to the lateral force of the retaining mass.
- Key: It is the small intrusive structure of a retaining wall, sometimes build at bottom of the base in order to increase the passive lateral force and resisting sliding and shear.
Types of cantilever retaining wall
1) Cantilever wall without toe(L shaped)
2) Cantilever wall with another heel (Inverted L shaped)
3) Cantilever wall with the key
4) Cantilever wall on abutment the (Wing wall)
Design of cantilever walls
The design of the cantilever retaining walls depend on various parameters and factors such as soil parameters: liquidity limit, water content, the density of soil, cohesiveness property of soil, bearing capacity, angle of friction to the wall, angle of shearing resistance in soil; the difference in elevation; the presence of water table, etc.
Before the design of the wall, the in-situ soil property must be analyzed properly and the values of parameters are needed to be determined. Then the following procedures may be continued for the design of the cantilever wall.
1) Determine the Dimension using General proportion
2) Determine the lateral earth pressure
Rankine theory and its formula are adopted for calculations of lateral earth pressure for cantilever walls. (Columb theory is adopted for gravity and semi-gravity wall)
For cantilever wall, Pa is active pressure that acts on vertical plane AB that passes through heel of the wall. The applied pressure Pa is parallel to the backfill surface that is retain by wall and acts at height H/3 from base of wall. Note: H is height of plane AB
Thus resultant force PR is obtained by combining Pa and self-weight of soil Ws between AB and wall at outer failure plane. (Refer fig 8)
3) Check for stability
Determine the required bearing pressure , resultant forces, resultant moments, resisting force, frictional force, sliding force , resisting moments ,overturning moments using appropriate formula and techniques.
If the Cantilever wall doesn’t lie with in safe permissible limit, then the key must be added in design so that it increases the passive lateral force and resistance against sliding.
e. Base failure of foundation
Circular slip of the whole soil mass including whole retaining wall may occur due to inappropriate design. This generally happen if the foundation is mostly soft clay. Determination of critical position of retain wall must be designed according to specifications as shown in fig 10
If any of the above checks gives the resulting failure then the design must be repeated with greater assumption values in order to create a safe design.
4) Design for the Reinforcement bars
The RCC design of the cantilever wall is adopted as the design basis for slabs and beams. The three elements of the retaining wall are designed as a cantilever slab to resist the factored moments and shear forces.
The stem of the cantilever wall is treated as the vertical cantilever beam or slab. And, the base as footing slab. This can be done either directly by designing and analyzing apps or by traditional hand worked out the method with the help of IS code, checking safe design for every part.
I hope this article on “Cantilever retaining wall” remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Rajan shrestha