Structural Guide

Structural loads, structural analysis and structural design are simply explained with the worked example for easiness of understanding. Element designs with notes and discussions have added to get comprehensive knowledge. Also, construction materials, shoring system design, water retaining structures, crack width calculations, etc. have discussed in addition to other aspects. 

Retaining Wall Failures

The retaining wall is a critical structure that provides the stability to the embankment. Retaining wall failures are commonly observed in hilly areas due to the slope stability issue.

However, as discussed in the latter part of this article, there are many causes for the failures of a retaining wall.

What is a retaining wall?

It is a structure built from concrete, random rubble masonry, precast blocks, etc to retain earth and to provide the stability to a soil cut.

Soil cut could be sloping or vertical depending on the nature of the construction.

How do we design a retaining wall?

There are two-stages for the design of a retaining wall.

  • Stability analysis – Design in Serviceability Limit State

The stability of the wall is checked in the serviceability limit stage based on the applied loads.

Overturning, sliding, bearing, slope stability failures, etc are checked in the stage.

There is a requirement of the factor of safety of each of the above cases. It shall be satisfied by the structure to be considered stable.

The article stability calculation of retaining walls could be referred for more information on calculation methods.

  • Ultimate Limit State Design – Desing for ultimate Loads

Factored loads are considered for the design of relating wall in the ultimate limit stage.

For example, if consider a concrete retaining wall, we find the reinforcement in the wall and base for bending and shear as per the applied loads.

Main Causes for Retaining Wall Failures

The following causes can be considered as key factors affecting the stability of the retaining wall. No retaining wall shall be constructed without checking the following stability requirements.

  1. Overturning Failures
  2. Sliding Failures
  3. Bearing Failure of Soil
  4. Slope Stability Failures

Let’s discuss each of those issues in detail and measure to be taken to avoid the same.

Overturning Failures

Overturning is one of the commonly observed cause for retaining wall failures.

Failure due to the overturning could be due to the following reasons.

  • An inadequate factor of safety against overturning
  • Inadequate width of the base
  • Calculation errors in the stability calculation
  • Consideration of the incorrect active pressure coefficient. Incorrect assumptions made during the design when the soil properties from soil investigation are not available.
  • Increase the fill heights with the time
  • Increase the design load than anticipated during the design. For example, excess surcharge loads can be considered.

Attention shall be made to maintain an adequate lever arm that could provide the required stability. Usually, a factor of safety about 1.5 is maintained in the design.

However, depending on the nature of the construction, a higher factor of safety may be considered.

Sliding Failures

Sliding resistance is provided by the base and the shear key provided under the foundation.

The article written on the shear key could be referred for more information on the shear key constructed for retaining walls.

The shear key generates passive resistance against the lateral loads applied to the retaining wall.

We should be very careful when the passive resistance of the shear key is taken into account when the sliding factor of safety is found. There should be adequately compacted soil behind the shear key to mobilizing the passive pressure. If there is loose soil, expected passive pressure will not be mobilized and as a result, the retailing wall could slide.

The base of the retaining wall also provides considerable resistance against the sliding. Depending on the condition of the ground there are two types of base resistance.

This is applied only when the retaining walls are constructed on the rock surface. If your construction is on the soil, we can only take the friction component.

  • Friction between the base material and the rock

Friction is the resistance to movement one surface against another. Friction is evaluated based on the friction angle between the two surfaces. And it is a function of the reaction between the surfaces.

Resistance = R.tanδ, where the tanδ is the friction angle, and R is the reaction on the surface.

  • Resistance provided by the Cohesion in the Rock Surface

The unevenness of the rock surface provides the resistance against sliding. It improvises the factor of safety against sliding considerably.

Resistance = C.A, where C is the cohesion and A is the surface area.

This method widely used in dam construction.

Inadequate estimation of friction angle, the weight of the structure, cohesion of the surface leads to the retaining wall failures in sliding.

Bearing Failure of Soil

Pressure under the base of the wall is not uniform due to rotation in the base induced by the lateral earth pressure. An increase in the soil pressure than the allowable bearing pressure could lead to retaining wall failures.

Therefore it is very important to check the soil pressure under the base in a retaining wall

Stress could be checked from the following equation.

σ = F/A ± MI/y

where, σ – soil stress under the base, F – vertical force, A – area of the base(calculation and be done for 1m length; A = 1xB; B- width of the base), M – Moment, I – second moment of inertia of the base and y – depth to the neutral axis.

From the above equation, we should check whether it increases the allowable bearing pressure.

Slope Stability Failures

Slope stability is one of the key factors that need to consider in the retaining wall design. If there are is a slope stability issue, even other failure modes have higher factors of safety, we could not be able to avoid the failure of the retaining wall.

The above figure clearly indicates the failure mode of the soil. So whether there retaining wall is stable or not, if it subject to this type of failure, the whole thing will collapse.

There will be a similar type of failure planes from the top to bottom of the wall. The factor of safety against each failure mode needs to be calculated. If the required factor of safety is meet by each failure plane, these types of retaining wall failures could not happen.

It could be difficult to do the analysis manually as it a long process and it will take time. If we use software like Slope W, we can find the minimum factor of safety very easily.

In addition to the above failure modes, there are many other reasons for retaining wall failures.

Other Retaining wall Failure Causes

  • Lack of Reinforcements

Dud to the errors in the design calculations or errors made during the reinforcement detailing, inadequate reinforcement could be indicated in the drawing, which could result in retaining wall failures.

Even if the bar diameter and the spacing are correctly indicated in the drawings correctly, it may not be placed during the construction.

Further, it is required to provide shear links close to the base of the wall when the height of the increases. Incorrect calculation or construction error could avid placing these reinforcements will load retaining wall shear failures.

  • Excessive Pour Water Development

In general, weep holes are provided to avoid generating the pour of water pressure behind the retaining wall. They are placed in regular intervals to drain the water collected behind.

Poor construction or improper method of the installation could block the pipes could lead to an increase the porewater pressure. It will be considerable pressure on the retaining wall.

For example, when we take soil pressure for friction angle 30 degrees, it is 0.33 x 20 h = 6.67h. The water pressure will be 10h. Here, soil density considered as 20 kN/m3.

Sol pressure without water = 6.67 h

With water = 0.33 (20 -10) h + 10 h = 13.3 h

When there is water, pressure increases significantly. This will cause retaining wall failures.

  • Incorrect estimation of earth pressure coefficients

Earth pressure coefficients are calculated from the soil properties. Incorrect representation of the soil in the analysis and the design could lead to retaining wall failures.

The best practice is to obtain soil parameters is soil investigation and subsequent testing of soil. However, it is observed that many designs are carried out based on assumed values.

These practices could increase the risk of failures.

  • Shallow Foundations

Depth to the foundation is very important in sloping ground. In addition, an increase in the depth of the foundation improves the stability even if it is constructed on flat ground.

When it is a sloping ground, there is a high chance of soil erosion. It could lead to exposing the foundation base reducing the resistance to overturning and sliding.

These actions could lead to failures in retaining walls.

  • Excessive Settlement

If the foundation is placed on weak ground, creating a wall could settle when the soil pressure is applied on the wall.

Since the pressure under the foundation is not uniform due to the overturning, one side will settle more than the other side.

It will adversely affect the stability of the retaining wall and excessive deformation will occur.

  • Unexpected Loads

Usually, retaining walls are designed for some surcharge loads depending on the design requirement.

The unanticipated load could lead to failure of the retaining wall. For example, one might construct a building close to the retaining wall. It will increase the pressure on the retaining wall.

As a result, retaining wall more laterally while rotating about the base. In addition, a new building constructed close to the retaining wall could collapse.

  • Seismic Loads

Depending on the location of the retaining wall and according to the guidelines of the particulate area, retaining walls may desing to carry the forces induced by the seismic events.

Whether it is constructed in an earthquake-prone area or not, if the structure is very important like a dam retaining wall, they consider some seismic acceleration in the desing considering the safety of the dam.

There are special formulas developed by the US Army Corps to find the seismic earth pressure coefficients. They can be used in the design. The guideline Retaining and Flood Walls, EM 1110-2-2502 could be referred for the method of calculation of the pressure coefficients.

  • Reinforcement Detailing Issues

Clear reinforcement drawings need to be provided for the construction other than providing typical drawings.

Drawings indicating typical arrangement could not address the side condition accurately and the construction will be done incorrectly.

Lap positions, lap length shall be correctly provided for a better understanding of the construction team. Providing incorrectly lap lengths, lapping the reinforcements where there are very high bending moments, etc could lead to retaining wall failures.

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