The retaining wall is an important structure that provides stability to the dam. Retaining wall failures often occur in mountainous areas because the slope is not stable.
However, as explained in the final part of this article, there are many causes for retaining wall failure.
What is a retaining wall?
It is a structure built in concrete, rubble masonry, prefabricated blocks, etc. to secure the earth and ensure the stability of a section of earth.
Depending on the type of construction, earth excavation can be carried out inclined or vertical.
How do we design a retaining wall?
The design of a retaining wall takes place in two phases.
- Stability check – design at serviceability limit state
The stability of the wall is checked during use based on the applied loads.
In this phase, errors in tipping, slipping, load capacity and slope stability, etc. are checked.
For each of the above cases there is a safety factor requirement. The structure must meet this requirement to be considered stable.
The article Calculation of stability of retaining walls More information about the calculation methods can be found here.
- Design at ultimate limit state – design for higher loads
Factor loads are taken into account when designing the associated walls at the bearing capacity stage.
For example, if we consider a concrete retaining wall, we determine the reinforcement in the wall and base for bending and shear according to the applied loads.
Main causes of retaining wall failure
The following causes can be considered key factors that affect the stability of the retaining wall. No retaining wall can be built without meeting the following stability requirements.
- Reverse errors
- Sliding error
- Failure in the bearing capacity of the soil
- Errors in slope stability
Let's discuss each of these problems in detail and the measures that can be taken to avoid them.
Reverse errors
Tipping is one of the most commonly observed causes of retaining wall failure.
Failure due to tipping over can be caused by:
- Insufficient tip-over safety
- Insufficient base width
- Calculation errors in stability calculation
- Taking into account the incorrect active pressure coefficient. Incorrect assumptions in design if soil properties obtained from soil testing are not available.
- Increase fill levels over time
- Increase the project load than intended in the project. For example, excessive additional loads may be taken into account.
Care must be taken to ensure that there is sufficient lever arm to provide the necessary stability. Typically a safety factor of about 1.5 is maintained in the design.
However, depending on the type of construction, a higher safety factor may be required.
Sliding error
Slip resistance is guaranteed by the base and the shear wedge placed under the foundation.
The article on the shear wedge For more information on the shear wedge designed for retaining walls, see.
The shear pin creates passive resistance to lateral loads acting on the retaining wall.
We must be very careful to consider the passive resistance of the shear wedge when determining the sliding safety factor. There must be enough compacted soil behind the shear wedge to mobilize passive pressure. If there is loose soil, the expected passive pressure will not be mobilized and the commercial wall may slide as a result.
The base of the retaining wall also offers significant slip resistance. Depending on the nature of the soil, there are two types of base strength.
This only applies if the retaining walls are built on rocky surfaces. If your structure is on the ground, we can only consider the friction component.
- Friction between the base material and the rock
Friction is the resistance to movement of one surface against another. Friction is evaluated based on the angle of friction between the two surfaces. It is a function of the reaction between the surfaces.
Resistance = R.tanδ where tanδ is the friction angle and R is the reaction at the surface.
- Strength due to cohesion in the rock surface
The irregularity of the rock surface provides resistance to slipping. Significantly increases the safety factor against slipping.
Resistance = approx. where C is the cohesion and A is the surface.
This method is often used in dam construction.
Inadequate assessment of the friction angle, the weight of the structure and the cohesion of the surface can lead to the failure of the retaining wall during the slide.
Failure in the bearing capacity of the soil
The pressure under the base of the wall is not uniform due to rotation at the base caused by lateral pressure from the earth. An increase in earth pressure beyond the allowable bearing pressure may cause the retaining wall to fail.
Therefore, it is very important to check the ground pressure under the base of a retaining wall.
Voltage can be checked using the following equation.
σ = F/A ± MI/y
where σ is the soil tension under the base, F is the vertical force, A is the area of the base (the calculation is made for 1 m length; A = 1 x B; B is the width of the base), M is the moment, I is the second moment of inertia of the base and y is the depth to the neutral axis.
Using the equation above, we must check whether this increases the allowable bearing pressure.
Errors in slope stability
Slope stability is one of the main factors that must be considered when designing retaining walls. If there is a slope stability problem, we cannot prevent the retaining wall from failing, even if other modes of failure have higher safety factors.
The figure above clearly shows the type of soil failure. Regardless of whether the retaining wall is stable or not, it will collapse with this type of failure.
There will be similar fracture surfaces from top to bottom of the wall. The safety factor for each type of fracture must be calculated. If the required factor of safety is maintained by each failure surface, such retaining wall failures cannot occur.
It can be difficult to perform the analysis manually as it is a tedious and time-consuming process. If we use software like Slope W we can find the minimum safety factor very easily.
In addition to the failure modes mentioned above, there are many other reasons why retaining walls fail.
Other causes of retaining wall failure
- Lack of reinforcement
Due to errors in design calculations or reinforcement detailing, the drawing may indicate insufficient reinforcement, which could lead to the failure of the retaining wall.
Even if bar diameters and spacings are correctly specified on the drawing, they may not be placed correctly during construction.
Additionally, it is necessary to provide shear connections near the base of the wall as the height of the wall increases. Incorrect calculations or design errors can cause these reinforcements to suffer shear failure when placing the retaining wall.
- Excessive formation of irrigation water
Drain holes are usually provided to prevent pressurized water from forming behind the retaining wall. They are placed at regular intervals to drain the water collected behind them.
Poor construction or improper installation can cause clogged pipes and increase pore water pressure. This creates significant pressure on the retaining wall.
For example, if we consider the pressure on the ground for a friction angle of 30 degrees, it will be 0.33 x 20 h = 6.67 h. Water pressure is 10 o'clock. Here the soil density is assumed to be 20 kN/m3.
Sun pressure without water = 6.67 h
With water = 0.33 (20 -10) h + 10 h = 13.3 h
In the presence of water, the pressure increases significantly. This causes the retaining wall to fail.
- Incorrect estimation of earth pressure coefficients
Earth pressure coefficients are calculated from soil properties. Incorrect representation of soil in analysis and design can lead to retaining wall failure.
Soil parameters are best determined through soil testing and subsequent soil testing. However, it is observed that many projects are carried out based on assumed values.
These practices can increase the risk of failure.
- Shallow foundations
The depth of the foundation is very important if the land is sloped. Additionally, greater foundation depth improves stability even when built on level ground.
If the terrain is sloped, there is a high risk of soil erosion. This can cause the base of the foundation to be exposed and reduce resistance to tipping and sliding.
These measures can lead to the failure of the retaining walls.
- Excessive billing
If the foundation is built on weak ground, the wall may settle when earth pressure is applied to it.
As the pressure under the foundation is not uniform due to tipping, one side settles more than the other.
This compromises the stability of the retaining wall and causes excessive deformation.
- Unexpected stress
Typically, retaining walls are designed for a certain load depending on the project requirements.
The unexpected load can cause the retaining wall to fail. For example, you can build a building next to the retaining wall. This increases the pressure on the retaining wall.
This causes the retaining wall to move more laterally as it rotates around the base. There is also a risk that a new building constructed near the retaining wall could collapse.
- Seismic loads
Depending on the location of the retaining wall and the orientations of the specific area, retaining walls can be designed to accommodate the forces caused by earthquakes.
Regardless of whether the construction is in a seismic zone or not, very important structures, such as dam retaining walls, take into account certain seismic accelerations in the project with a view to dam safety.
There are special formulas developed by the US Army Corps to determine the earth's seismic pressure coefficients. They can be used in construction. The Retaining Walls and Flood Protection Directive, EM 1110-2-2502 The following methods can be used to calculate pressure coefficients.
- Problems with reinforcement detailing
In addition to standard drawings, clear reinforcement drawings for the project must also be provided.
Drawings showing a typical arrangement cannot accurately explain the boundary conditions and the design is incorrectly executed.
Overlap positions and lengths must be specified correctly to help the construction team understand them. Incorrect lap lengths or overlapping reinforcements at very high bending moments, etc. can lead to failure of the retaining wall.
