Generally, the term retaining wall failure do not referred to the total failure or collapse but rather describes signs and indications by which failure possibilities and wall instability could be predicted and can be saved if dealt with properly.
Retaining wall sliding, toppling, overturning are types of total collapses that cannot be rehabilitated, therefore rebuilding the wall the sole solution for these failures.
However, full collapse of retaining wall is unlikely in addition to show signs of problems that could be observed and rehabilitate the wall before the wall fail entirely. By and large, most of retaining walls could be rehabilitated and saved after evaluating and specifying reasons of deficiencies.
In this article, various parameter, which might go wrong and consequently provide signs of deficiencies, are discussed in the following points.
Causes for Retaining Wall Failure
Following the common causes for retaining wall failures:
- Improper reinforcement placement
- Saturated backfill
- Weep holes that do not weep
- Design error
- Calculation errors
- Unanticipated loads
- Mistakes in utilizing software
- Detailing errors
- Foundation issues
- Inadequate specifications and notes
- Shoddy construction
- Retaining wall age
Following are the explanation of various causes for retaining wall failure:
Retaining Wall Failure due to Improper Reinforcement Placement:
Reinforcement size, depth, and spacing should be checked when wall stem exhibits sign if issues such as cracking and extreme deflections. Reinforcement size and depth can be determined either by devices for example magnetic field measuring pachometer.
This device is used to determine reinforcement position and depth up to around 100 mm with acceptable accuracy or to achieve more accurate measurement. This device can also locate steel bars and chip out concrete to find out the precise reinforcement size and depth.
Surprisingly, there are situations where reinforcement was installed in wrong wall side that might be resulted from contractor error or detailing error.
After actual steel reinforcement size, depth, location, and sometimes testing stem concrete strength by taken core sample, back design computation is employed to estimate actual design capacity then provide remedial measures.
Retaining Wall Failure due to Saturated Backfill
It is assumed that backfill is granular and well drained during the design of retaining wall. Pressure against the wall is substantially increased if surface water is permitted to infiltrate into the backfill.
This can be avoided by grading backfill surface that direct water away from the wall or by diverting water to disposal through drainage channels close to the retaining wall.
Furthermore, poor backfill such as those containing clay swells and lead to increase pressure considerably. Finally, crushed and pea gravels are examples of good backfill that provide proper drainage and avoid creating pool water behind the wall.
Figure-1 illustrates drainage system which can be used avoid hydrostatic pressure.
Figure-1: Retaining wall with proper drainage system
Retaining Wall Failure due to Weep Holes that do not Weep
Due to lack of filters, for instance line of gravel or crashed stone positioned along the base of the wall weeds turn into clogs and create problems for water draining.
In masonry retaining wall, weep hole is made by removing mortar at the side joints and distance between weeps is around 80 cm. Weep hole in reinforced concrete retaining wall is at least 7.5 cm in diameter and spacing should not be more than 1 m or it can be specified by the designer.
Figure-2 and Figure-3 show weep hole in retaining walls.
Figure-2: Weep hole in retaining wall
Figure-3: Providing weep hole in retaining wall to drain water
Design Error Resulted from Misinformation
Retaining wall failures due to design errors are rather exceptional case provided that an experienced structural designer carried out wall design. Nonetheless, there are situations where designers are provided with inadequate or faulty information that can be extremely detrimental.
Retaining Wall Failure due to Calculation Errors
These errors could be noticed easily by experienced designer. However, there are possibilities when new designer carry out the computation and it is extremely significant to double check the design. This can avoid costly fixing of the wall after construction.
It is client to designer information issues so it is essential to have good communication between different people who involve in the design. Unexpected loads might be resulted from a surcharge that designer did not about it. Moreover, it could have been a steeper slope backfill or wind load.
Mistakes in utilizing software
Designers need to input data precisely and familiar with capabilities, outputs, and limitation of the program when a software is employed for designing. Moreover, it is recommended to check and perform quick calculations for verification especially when there is doubt about outputs.
Retaining Wall Failure due to Detailing errors
Detailing should be clear, conforming design calculation and prevent doubtful interpretation. Vague detailing could lead to inaccurate reading of information for instance there were cases that dowel extended 0.15 m instead of 0.6 m into the stem.
There are guidelines for foundation design that designer can use with the help of site investigation report, but there could be cases where this investigation is not provided.
Lack of site investigation could lead to foundation problems because Codes restricted soil bearing and designers should use conservative values. Additionally, designers should be aware about compressible soil, backfill material, water table, and other factors that might decrease sliding resistance or lead to large differential settlement.
Inadequate specifications and notes
If discrepancies come across between site conditions and drawings, or unexpected conditions is encountered the engineer should be contacted to for the steps to be followed. If there are conflicts between standards and details, the most restricted must govern.
Finally, all changed instructions must be conformed and affected parties should be informed. These measures are taken avoid problems that could lead to detrimental effect on the retaining wall.
Retaining Wall Failure due to Poor Construction
Poor construction practices might be due to unscrupulous or inexperienced contractor works that is carried out as per standards and specified plans.
Inadequate mortar, or grouting, or improper steel reinforcement placements are compelling examples of poor construction. it is recommended to understood construction requirement and conditions and review the plan properly.
Retaining Wall Failure due to Age
When a retaining wall is stood for about fifty years or more without showing distress indication, therefore there are possibilities that it may stand for another fifty years or more in the future and will not need to take any actions.
However, this is not the case in seismic regions, or adding new surcharges, or drainage change above the wall, so maintenance or seismic evaluation would be suitable to verify whether the wall can take new loads or withstand another earthquake.