Table of Contents for Retaining Wall-
- Introduction to Retaining Wall & Uses
- Types of retaining walls
- Gravity Wall
- Cantilever Wall
- Buttressed/ Counterfort Wall
- Pile Retaining Wall
- Crib Wall
- Anchored Wall
- Gabion Wall
- MSE Wall
- Design Criteria for a retaining wall
- Forces acting on a retaining wall
- Points to consider for a design of a retaining wall
- Why do retaining walls fail? Failure modes of a retaining wall
- Bearing Failure
Introduction to Retaining Wall and its’ Uses
A retaining wall is a structure that retains the sloped or vertical cut soil. The soil level is different on both sides of the walls. A retaining wall is subjected to horizontal load and hence designed for the same. The load acting on the wall depends upon the height of fill material, type of fill material, surcharge, water table and drainage conditions.
A retaining wall is an option where the space is limited, and the gentle slopes aren’t possible. Another use of a retaining wall involves the protection of soil against erosion.
In this article, I shall explain the types of retaining walls in brief, forces acting on a retaining wall, points to consider for the design of a retaining wall and the reasons for retaining wall failures.
Types of retaining wall
The types of retaining walls based on their function are explained below-
Gravity retaining wall
- These walls counter the lateral earth pressure by its’ weight.
- The gravity retaining walls are massive in size. Its’ size increases in proportional to its height.
- The base of the wall is generally thicker than its’ top. It helps in lowering the wall centre of gravity. Thus, more safety against overturning failure.
- The gravity retaining wall is of masonry, concrete or rock fill.
- The gravity retaining walls are only suitable for height up to 3m. For wall height greater than 3m, cheaper alternatives are available.
Cantilever retaining wall
- It resists the lateral earth pressure by its’ flexural strength. The cantilever action takes place on the load application. The bottom of the wall is fixed and subjected to load and bending moment while the top is free.
- A cantilever retaining wall is thinner than a gravity retaining wall for the same backfill.
- It is generally reinforced concrete (in-situ or precast). In some places, reinforced masonry optimises the cost.
- The cantilever retaining wall starts over a foundation.
- It is suitable for heights up to 6m.
Buttressed/ Counterfort retaining wall
- It is an improved type of cantilever retaining wall.
- One or more counterforts support the main retaining wall (stem).
- The buttressed retaining wall acts as a slab system where negative moment occurs at the counterforts and maximum moment occurs at mid-point of two counterforts.
- It provides better stability than a cantilever retaining wall against overturning.
- A buttressed retaining wall is difficult and expensive to construct.
- It is suitable for heights greater than 6m.
Pile Retaining Wall
- A pile retaining wall is a temporary as well as permanent lateral pressure resisting system. The piles are adjacent to each other form a wall.
- Piles are driven into the earth up to a depth so they can counteract the lateral earth pressure. Generally, the 2/3rd pile is below the ground, and the remaining 1/3rd pile retains the soil. The exposed pile of up to 6m height is economical.
- It is a suitable solution for low to medium earth pressure only. Hence, they retain the soft soils only.
- The piles are of RCC, steel, vinyl, fibreglass, wood etc. The plate used makes a sheet pile retaining wall.
- Bored piles are better if the vibration from pile-driving is beyond the acceptable limits.
- It uses minimum space as compared to other types of retaining walls. It is common in deep excavation works, cofferdams, marine works, beach erosion protection.
- A crib wall is a system of interlocking blocks.
- It is of steel, wood, RCC etc.
- The crib wall functions as a type of gravity wall as the interlocked blocks provide only the facing and hold the soil in place.
- The wall reinforced with the anchors is called an anchored retaining wall.
- The anchors are placed into the soil slightly tilted downwards and are parallel to each other.
- Under the lateral earth pressure, the anchors are in tension.
- It is a type of gravity wall.
- A gabion wall is of stones packed in wire mesh boxes.
- The Gabion wall is common in highways, hilly regions and river training work.
- The Gabion wall construction needs no specialised skills and is cheap as well.
Mechanically Stabilised Earth Retaining Wall
- The MSE Wall is a system of earth reinforcement and a facing system.
- The earth at the corner is reinforced with suitable reinforcement and compacted in layers. The face material makes a facing system.
- A well-designed MSE wall is durable and resists all types of failure.
- It is a ductile system and stands strong under dynamic loading like an earthquake.
Retaining wall design
For designing a retaining wall, first, we shall go through the forces acting on it. Let us go through them one by one.
Forces acting on a retaining wall
- Earth Pressure: The earth pressure acts horizontally on the wall. It is of three types- earth pressure at rest, active earth pressure, passive earth pressure. Earth pressure at rest corresponds to the soil condition when soil neither pulls nor pushes the wall. The active earth pressure pushes the wall, and the wall moves away from the fill. Similarly, passive earth pressure pulls the wall, and the wall moves toward the backfill.
- Friction: The friction between the wall and the ground beneath it prevents the wall from sliding due to earth pressure.
- Hydrostatic Pressure: Thewater accumulation inside the fill exerts an additional pressure on the wall called hydrostatic pressure. A weep hole in the wall releases the pressure by allowing the water to drain through it.
Points to consider for the design of retaining wall
The retaining wall of up to 1 m height doesn’t require designing. The complications increase with an increase in the wall height.
- For the gravity wall, the structure weight should be large enough to counteract the lateral earth pressure. To prevent the overturning failure, the resultant of the earth pressure and weight of the wall must pass through the base of the wall.
- The cantilever wall and the counterfort cantilever wall should have the necessary heel and toe length to prevent the wall from overturning.
- For protection against the wall sliding, the weight should be enough to provide the necessary friction. The shear key at the bottom of the foundation is a fantastic option to prevent sliding.
- The provision for hydrostatic pressure dissipating arrangements. A system of weep holes is a good solution for it. Hence, the weep holes are designed at suitable intervals.
- The water accumulation below the foundation exerts an uplift force on the wall. The uplift force reduces the friction between the ground and the wall, and hence the wall slides.
Why do retaining walls fail? Failure modes of a retaining wall
A retaining wall fails if it fails to retain the soil in its’ position. The failure may occur due to many reasons such as foundation failure, built-up of water pressure in backfill material, the overturning of wall and sliding of the wall. Hence, the engineering approach for the design of the retaining wall covers all such scenarios.
Bearing Capacity Failure
Soil can bear load up to a certain extent. The load-carrying ability of soil within the safety limit is called safe bearing capacity. The pressure exerted by the retaining wall on the ground beneath should not exceed the safe bearing capacity of the ground beneath the wall. If so, the bearing capacity failure occurs.
The enlarged base of the foundation prevents bearing capacity failure.
Lateral Sliding Failure
The soil tends to push or pull the wall (active & passive earth pressure). A good retaining wall should not move. The two factors responsible for lateral sliding failure are low friction and the absence of external resistance to sliding.
The cantilever and counterfort walls having a shear key at the bottom of the foundation are safer from sliding failure.
Wall Rotational Failure
The overturning of the wall occurs either because of the insufficient weight of the wall (or thin base for gravity wall) or inadequate foundation design (for cantilever and buttressed cantilever wall).
The selection of retaining walls depends upon the purpose and usage. The walls required for highways/railways experience heavy loading and vibrations. The walls constructed for landscaping have only the earth pressure.
The drainage is a crucial factor in the design of retaining walls. The importance of proper arrangements can’t be emphasized more.
The cost and durability are other factors that decide the type of wall. A gabion wall is the cheapest of all the wall types and durable by the time its’ wire mesh corrodes.
The soil quality tests play an important role in the safety of retaining wall. Check out these essential quality tests of soil.
Wikipedia- Retaining Walls.
Failure Modes via Encyclopedia of Geology (2nd Edition).