Retaining structures are used to laterally support (hold back) material, typically soil, and allow for a difference in grade elevation by preventing the material from eroding or sliding. Retaining structures are designed to resist the pressures of the retained materials, support loads imposed on the retained soil surface, and in some cases, support buildings or other structures. For instance, abutment walls can be used to provide support for bridges and traffic underpasses as well as to retain the ground at the approaches. Alternatively, retaining walls can be as simple as basement foundation walls, used to laterally support the ground while supporting a building above. There are numerous applications of retaining structures, including shoring of building excavations, retaining of soil and water for marine structures such as docks, underground structures such as tanks, building foundations, among others.
It should be noted that a great deal of planning is required for the establishment of retaining structures as there are varying factors that need to be considered, and multiple modes of failure that can occur. Further, when considering the cause of failure of retaining structures, there is room for improvement in the industry through regulation. Considering the vast range of structural configurations, materials, and site conditions, improving regulations related to the design, approval, and construction of retaining structures can prevent failures and ensure safety.
Forensic engineers investigate structural and geotechnical failures, including those of retaining structures. During any forensic investigation, structural and geotechnical experts review design and construction records, perform site examinations and testing if required, identify factors that contributed to the failure, and analyze possible failure modes to determine the cause of the retaining structure’s failure. Based on their knowledge and expertise, forensic engineers can identify deficiencies that can be traced back to the design or construction process.
In this article, I will discuss the importance of analyzing the possible modes of failure as part of the design and assessment of retaining structures, the possible factors that can contribute to a failure, and the importance of sound regulations and proper expertise (e.g., structural and geotechnical) for the assessment, analysis, or design of retaining structures.
Retaining structures can be constructed of concrete (reinforced or unreinforced), masonry, wood, steel, or reinforced soil. In terms of geometry and configuration, there are many types of retaining walls, including gravity or cantilever walls, mechanically stabilized earth walls, temporary or permanent sheet pile walls, braced walls such as bridge abutments or basement walls, laterally restrained walls with tiebacks or anchored bulkheads, and crib walls. Therefore, multiple failure modes need to be analyzed in detail to ensure the stability and structural integrity of the system. In the case of existing structures, the assessment must consider varying factors and site conditions that may trigger or contribute to a failure mechanism.
Types of Failure Modes
In general, the proper engineering expertise, such as from a structural and/or geotechnical engineer, shall be assigned to assess an existing retaining wall or to perform the design of a new retaining structure. In the case of large and complex projects, it is essential to engage qualified structural and geotechnical professional engineers and contractors to analyze all possible failure modes. For instance, instability of a retaining system can result in failure mechanisms such as sliding, overturning, bearing failure, or overall slope instability (Figures 1 to 4).
Failure of retaining structures also occur when structural components are overstressed, and the capacity of the materials to resist internal forces is exceeded (Figures 5 to 10). For instance, retaining walls may exhibit a shear failure at the base with cracking of concrete or dislodging of masonry blocks. A flexural failure due to bending may occur and cause cracking, crushing, and spalling of concrete, and yielding of reinforcement steel. Failure of tiebacks or mechanical stabilization systems can result in catastrophic failures due to a failure of their structural components (e.g., tensile failure of tie-rods or pullout of anchors). Failure of connections or joints between wall sections can also contribute to a failure of the system (e.g., connections between framing or bracing elements, failure of shear keys or dowels). In general, a proper assessment shall include a detailed structural analysis of the retaining system and its components.
Investigating the site-specific conditions and performing geotechnical studies and testing can be critical to achieving a safe design or to accurately assess an existing retaining structure. Site-specific data is gathered to verify assumptions and parameters and to identify unforeseen conditions that can be the cause of a failure. For instance, changes in soil properties along the length of a retaining structure may cause differential settlements or sinking of wall sections. Also, if local faults are present across retaining structures, they can cause relative vertical displacements between sections and damage such as cracking of concrete, failure of joints or connections, and deterioration.
Factors that Contribute to the Failure of Retaining Structures
In the case of a retaining structure failure, a thorough forensic investigation shall be performed. Qualified experts such as structural and geotechnical professional engineers, assigned to the investigation, identify and analyze the contributing factors of the failure, which vary from incident to incident.
Although case-specific, contributing factors may include:
- Incomplete or deficient design (e.g., if the design neglected to consider all possible failure modes such as global instability or structural failure);
- Incomplete, deficient, or erroneous site-specific data (e.g., lack of geotechnical investigation);
- Change of the use of the area adjacent to the retaining wall (e.g., additional surcharge load);
- A lack of or improper drainage;
- Construction deficiencies;
- Impact or vibration (e.g., vehicle impact);
- Extreme weather events (e.g., changes to water table levels, flooding, higher hydrostatic pressure);
- Increased pressures due to frost action;
- Deterioration due to lack of maintenance (e.g., corrosion of reinforcement); and
- Unforeseen seismic activity.
The design and construction of retaining walls are regulated by building codes, local regulations by the authority having jurisdiction (such as municipal bylaws), industry standards including structural, geotechnical, materials and manufacturing standards, and best practices.
According to the 2012 Ontario Building Code (OBC), Clause 18.104.22.168. (2), “a retaining wall exceeding 1000 mm [one meter] in exposed height and adjacent to, (i) public property, (ii) access to a building, or (iii) private property to which the public is admitted” is a designated structure and requires a structural design. However, retaining walls on private properties are currently not regulated by the OBC. Certain municipalities in Ontario would not accept or review any design or construction records related to retaining walls on a private property where the public is not admitted, even if a request is presented before construction.
As a forensic engineer, I have witnessed cases where the failure of a retaining wall on private property caused significant material damage and had the potential to cause injury or death (Figure 11). In some cases, it was clear that the design was inadequate, while in others, the construction deficiencies revealed a lack of quality assurance and control. In my opinion, retaining walls on private properties should be regulated to ensure proper design and approval before construction, as well as be reviewed by an engineer during construction. Future regulation improvements may also include minimum requirements for the design and construction of the different types of retaining structures as well as limitations related to the type of material and system (e.g., height, slenderness, etc.).
Retaining structures can exhibit varying failure mechanisms, and failures can be caused by a variety of factors. Analyzing the possible modes of failure is paramount during the design or assessment of a retaining structure. Further, even when considering the different structural configurations, materials, and varying site conditions (depending on the location), retaining structures require sound regulations to prevent failures and ensure safety. The analysis, design, construction, or assessment of retaining walls requires qualified experts such as structural and geotechnical professional engineers to ensure a safe design or a successful investigation. The betterment of current regulations related to the location, material limitations, and type of retaining structural system is possible and can reduce the risk of failure of retaining structures in the future.