Retaining Structures - Design and Assessment from a Forensic Engineering Perspective

By: Pablo J. Robalino

2020.05.28

Retaining structures are used to laterally support (hold back) material, typically soil, and allow 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, abutments walls can be used in bridges and traffic underpasses to support the bridge or underpass as well as to retain the ground at the approaches. Another example are 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.

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. 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 knowledge and expertise, forensic engineers identify deficiencies that can be traced back to the design or construction processes. In this article, I 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 structural and geotechnical 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, who will 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 may 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 result in relative vertical displacements between sections and cause damage such as cracking of concrete, failure of joint or connections, and deterioration.

Factors that Contribute to 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 a 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.

Regulations

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 1.1.2.2. (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 a private property caused significant material damage and had the potential to cause injury or death. 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 engineering and reviews 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). 

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. 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.

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Pablo J. Robalino is a structural forensic engineer specialist and project manager, and a Senior Associate at 30 Forensic Engineering in Toronto, Ontario.

Contact: probalino@30fe.com

Silo failures caused by unrealistic design considerations

 Many silo failures have occurred as a result of unrealistic loading conditions assumed during the design phase. Other failures have been attributed to changes in the use of silo structures without proper consideration of the implications. In some cases, more stringent loading conditions were introduced with changes to the stored material properties or the implementation of different discharge mechanisms. The limited existing design guidelines have contributed to oversimplified designs, as they fail to consider complex loading conditions, such as those caused by the asymmetric flow of stored materials.

Silo failures can cause loss of life and significant damage to adjacent structures and equipment when they occur at industrial plants or storage facilities (Figure 1). Silo structures tend to lack redundancy, and their failures can be catastrophic. Contributing factors may include poor design considerations, construction deficiencies, change of use, deterioration and lack of maintenance, and others (Figure 2). The cost of remediation after a catastrophic failure can be significant as it may include forensic investigations, temporary emergency response measures, shoring, demolition of the damaged structures, repair designs, and reconstruction.

During remediation of a catastrophic silo collapse, the permanent repairs may require the reconstruction of a silo as well as the repair or reconstruction of adjacent damaged structures. The repair or replacement of damaged mechanical equipment, electrical instrumentations and control systems, and other components may significantly increase the remediation cost. An insurance claim may also need to cover significant expenses or losses related to the business interruption of an industrial plant or storage facility that may need to remain closed for a significant period after a catastrophic failure of a silo structure. In some cases, litigation may occur after a silo failure, causing a possible surge in the cost of remediation as well as significant delays. In some cases, the litigation occurs after remediation of the structures to curtailed rising business interruption cost.

 Figure 1. Partial view of a report by “fin24” of a coal silo and conveyors that collapsed at Eskom Majuba Power Station in Cape Town, South Africa[1].

 Figure 2. Chart of possible consequences resulting from a silo failure

The design, construction, operation, maintenance, and repair of silos requires the involvement of many stakeholders. The obvious stakeholders include the owner, the design engineers, the construction and maintenance contractors, and the operational managers and personnel. Other stakeholders include the insurance companies that provide coverage in the event of damage or failure, the banks or financial institutions that finance the construction or maintenance of projects related to the use of silos at industrial or storage facilities, the owners of neighbouring properties that could be affected by silo failures, and others.

To prevent silo failures, regardless of the age of the structure, the property owner and stakeholders should assess the condition of silo structures and monitor any changes on a scheduled basis (e.g. semi-annually or annually). Qualified personnel should verify if the original design considerations are consistent with current loading conditions, and ensure that the silo structures are capable of supporting the current loading conditions including the possible asymmetric flow of materials. The loading conditions related to the specific industry can be complex, and failures can occur due to failure to perform the proper analysis and testing that goes beyond the limited recommendations provided in the available industry standards and guidelines. A preventive engineering assessment can significantly reduce the risk of failure and avoid the corresponding remediation cost (Figure 3).

Figure 3. Possible Stakeholders involved in preventing a silo failure on a regular basis

Silo structures are used in many industries as an essential component of process plants and storage facilities. For instance, silos are commonly used to store bulk raw and processed materials. The following list includes a few examples among many other applications:

• Grains and other food-related products in the agricultural and food industries;
• Cement, clinker and other materials related to the cement manufacturing industry;
• Materials used for energy generation, such as coal and woodchips; and carbon black used in the production of rubber and other processed materials.

Although storage structures have been used for many centuries, the first modern upright silo was built in 1873 by Fred Hatch in Illinois, USA, for the storage of grain for the agricultural industry. Since that time, use of the upright silo for storage has spread across many industries.

Initially, most silo structures were relatively small and simple regarding the design, materials, and loading conditions. Over time, the need to increase production and store greater amounts of bulk materials has pushed the innovation and design limits to build bigger silos with more complex configurations, loading conditions, and material handling technologies. However, the research and development of codes and standards governing the design of silo structures have not advanced at the same pace, leaving a gap between the challenges imposed by more advanced technologies and the limited existing design guidelines and regulations. Thus, silo failures have occurred because of unrealistic design considerations.

Through most of the 20th century, the traditional design of silo structures assumed a concentric flow of the stored material through discharge mechanisms which, in many cases, included concentric hoppers. Therefore, the structural design was performed by estimating concentric material pressures at specific heights. The assumed lateral pressures were to be resisted by the cylindrical silo walls, mainly through uniform hoop tension. The vertical traction loads were to be resisted through uniform compression of the silo wall in the vertical direction. Since the flow of material was considered concentric, the material pressures were assumed to be uniform along the perimeter of the silo wall cross‑section at any given height. The assumption was that the silo would be subjected mainly to axial stresses. While this is true of small silos with concentric flow of material, increasingly larger and more slender silos were constructed, which necessitated an asymmetric flow of material.

With the increase of storage capacity and industrial applications, more modern and sophisticated discharge technologies utilized in large silos required eccentric discharge outlets, further generating asymmetric flow and eccentric pressures (Figure 4). Problematically, in some jurisdictions, the code provisions allowed for overly simplified designs which failed to take into account the critical loading conditions caused by asymmetric flow. The lack of detailed provisions to account for the effects of eccentric material flow was evident in North America and other parts of the world where the applicable standards, such as the editions of ACI 313 before 2016 (e.g., ACI 313-97[2], and previous editions) did not provide proper guidance. For instance, ACI 313-97, “Standard Practice for Design and Construction of Concrete Silos and Stacking Tubes for Storing Granular Materials,” provided little guidance to the design engineer to properly estimate eccentric loads.

Figure 4. Schematic plan view of silo cross-sections with concentric (left) and asymmetric (right) flow of materials

 Only in the last few decades, more advanced material handling testing and modelling engineering tools, such as the relatively recent development of linear and non-linear finite element analysis software, have allowed for further research and development of more realistic models of silo structures. In the last couple of decades, some standards have already incorporated design provisions to account for more realistic loading conditions, such as those concerning the design of silos that would likely exhibit an eccentric flow of the stored material. Asymmetric flow results from the formation of flow channels inside slender silos or in silos that incorporate eccentric discharge mechanisms, such as eccentric discharge outlets and inverted discharge cones.

For instance, the European standard for the design of silo and tank structures, EN 1991-4:2006[3], became one of the first and most comprehensive codes, providing detailed guidelines for design engineers to estimate most loading conditions including eccentric pressures (Figure 5). In North America, the latest version of ACI 313, published in 2016[4], acknowledges the need to consider the effects of asymmetric flow in silos and provides the corresponding guidelines.

Figure 5. View of estimated eccentric horizontal pressures for a silo structure in accordance with EN1991-4:2006

The pressures resulting from an eccentric material flow pattern would adversely change the loading conditions imposed on a silo originally designed and constructed to operate under concentric pressures. An asymmetric flow would impose axial stresses combined with flexural and shear stresses (Figure 6), instead of only axial stresses such as the case of concentric flow. For instance, a silo wall would exhibit hoop tensile plus flexural and shear stresses in the horizontal direction, and compressive plus flexural and shear stresses in the vertical direction. The combined stresses could increase the demand versus capacity ratio of critical structural elements beyond a serviceability limit state and result in partial damage or collapse of a silo structure. 

Figure 6. View of a finite element model showing the typical uneven distribution of stresses and deformation as a result of asymmetric flow of the stored material in a silo structure

When an eccentric flow channel is formed within the stored material, and its cross-section strikes the silos wall (Figure 7), the horizontal pressure distribution along the perimeter wall cross-section will vary because of an arch effect of the material around the flow channel. The uneven distribution of pressures will then impose flexural and shear demands to the silo walls, in addition to the axial stresses (e.g., hoop tensile stresses, Figure 6).

Figure 7. Schematic plan view of a flow channel resulting from asymmetric flow inside a silo cross-section.

Bulk materials and silo wall surfaces have unique material properties and behave differently under different material handling scenarios (e.g., at rest, during material discharge, etc.). The storage of advanced materials for new applications, the introduction of new material handling technologies, and the use of new design configurations and silo construction materials have also increased the loading complexities.

For instance, fluidized loading conditions (e.g., quasi-hydrostatic pressures) may develop when gases are added to change the temperature of the stored materials or to prevent or trigger chemical reactions. In some industries, such as cement processing and storage, the loading conditions would also change because of complex discharge mechanisms used to fluidize the material. For example, cement silo aeration systems, silo fluidizing pads, and inverted discharge cones are used to keep the material flowable at the base of cement silos. Fluidizing the material changes the loading conditions, and may cause impact loads as a result of arching and collapsing of the material above and around the fluidized material or flow channels (e.g., “rat hole formation”). Other industries may incorporate different equipment, such as pneumatic vibrators to improve the discharge process. The additional loading conditions including the effects of vibration, impact, and uneven distribution of pressures must be considered in the design of silo structures and their foundations.

In complex scenarios, the available code provisions do not cover all the realistic loading conditions, and further testing and analysis might be required. In addition to the safety risk, a silo failure can cause significant damage to adjacent structures and equipment in a process plant or a storage facility. Preventive engineering assessments and monitoring of silo conditions can greatly reduce the risk of failure and avoid onerous remediation expenses.

The remediation cost for a property owner or insurance company can be significant as it may include the initial remediation and emergency response, business interruption, structural repairs, equipment and material losses, as well as other costs. Therefore, it is important to prevent silo failures by verifying that the original design considers all the realistic loading conditions for the current use of the structure. It is also essential for design engineers to consider all the loading conditions related to the specific industry, stored material and silo wall properties, silo configuration, and discharge systems.

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Pablo J. Robalino is a structural forensic engineer specialist and project manager, and a Senior Associate at 30 Forensic Engineering in Toronto, Ontario.

Contact: probalino@30fe.com 

References:

[1] Eskom knew about silo problem in January - Solidarity. Fin24, 3 Nov. 2017,  https://www.fin24.com/Economy/Eskom-knew-about-silo-problem-in-January-Solidarity-20141103

[2] American Standard ACI 313-97, Standard Practice for Design and Construction of Concrete Silos and Stacking Tubes for Storing Granular Materials, American Concrete Institute, January 1997.

[3] European Standard EN 1991-4:2006 (E), Eurocode 1 – Actions on structures – Part 4: Silos and tanks, European Committee for Standardization, May 2006.

[4] American Standard ACI 313-16, Design Specification for Concrete Silos and Stacking Tubes for Storing Granular Materials and Commentary, American Concrete Institute, 2016.

Reference Materials:

1.      J. W. Carson, Silo Failures: Case Histories and Lessons Learned, presented at the Third Israeli Conference for Conveying and Handling of Particulate Solids, Dead Sea Israel, May 2000.

2.      J. Chavez Sagarnaga, Eccentric Discharge From Silos: Bulk Solids Behavior, Loads and Structural Implications, presented at the ASCE Structures Congress 2014, pp. 1729-1740, 2014.

3.      J. W. Carson and T. Holmes, Silo Failures: Why do they Happen?, Task Quarterly 7 No 4, pp. 499-512, 2003.

4.      A. Dogangun, Z. Karaca, A. Durmus, and H. Sezen, Cause of Damage and Failures in Silo Structures, ASCE Journal of Performance of Constructed Facilities, March/April 2009.

5.      A. J. Sadowski and J. M. Rotter, Study of Buckling in Steel Silos under Eccentric Discharge Flows of Stored Solids, ASCE Journal of Engineering Mechanics, June 2010.

6.      J. W. Carson, Limits of Silo Design Codes, ASCE Practice Periodical on Structural Design and Construction, Vol. 20, Issue 2, May 2015.

7.      H. McKay and J. Durack, Implications of the new Eurocode EN1991-4 on the design of Cement and Raw Meal Storage Silos, Cemtech, September 2006.

8.      J. W. Carson and R. T. Jenkyn, Load Development and Structural Considerations in Silo Design, Presented at Reliable Flow of Particulate Solids II, Oslo Norway, August 1993.

9.      J. Johnson, Deterioration of Concrete Tower Silos, Ontario Ministry of Agriculture, Food and Rural Affairs, December 2008.

10.  European Standard EN 1991-4:2006 (E), Eurocode 1 – Actions on structures – Part 4: Silos and tanks, European Committee for Standardization, May 2006.

11.  American Standard ACI 313-97, Standard Practice for Design and Construction of Concrete Silos and Stacking Tubes for Storing Granular Materials, American Concrete Institute, January 1997.

12. American Standard ACI 313-16, Design Specification for Concrete Silos and Stacking Tubes for Storing Granular Materials and Commentary, American Concrete Institute, 2016.

A collaborative success story: Explosion response

Forensic investigators are both scientists and detectives, fueled by the commitment to pursue knowledge and the drive to successfully solve each case. Though a forensic investigator may specialize in areas as diverse as Building Science, Fire Investigation and Materials Failure, they all share the common goal of uncovering the true cause of an incident and making our world a safer place through sharing this information and empowering people to make informed decisions based in science. Work fulfillment is gained by confronting a challenging problem and finding a solution. It’s this problem-solving in service of a greater good that drives a forensic investigator in their work.

But many losses are complex, and a single investigator, regardless of skill and commitment, cannot always reach a conclusion alone. Loses often involve multiple factors and require the expertise of more than one investigator to uncover origin and cause and develop practical solutions. In such cases, it is ideal to work within a team of specialists, with each investigator focusing their efforts in the area where they have a deep understanding of the science involved, and the team combining their shared knowledge and experience to uncover the complete picture. It is this approach that will lead to conclusions on challenging cases and allow the forensic investigator to deliver useful solutions to clients.

One case involved an explosion in an industrial building. The explosion occurred on the 3rd floor of a 4-storey structure and caused severe damage to one side of the building (Photograph 1). It was determined that the structural integrity and safety of the building had been compromised and immediate action was required to ensure safety in the areas at risk of structural collapse. An emergency response was necessary.

Photograph 1 – View of explosion damage and temporary shoring

A team of Civil/Structural engineers and Engineering Technologists quickly developed an emergency response strategy, which involved an initial structural assessment, 3D scanning of the damaged structure to establish a baseline condition for continuous monitoring, design and installation of temporary shoring to support the damaged building, and the use of a 90 ton mobile crane to temporarily support the top portion of the building while workers accessed the severely damaged area (Photograph 2). 

An ‘Emergency Response Proposal’ was developed, which included a scope of work detailing the design of the emergency shoring and support system as well as installation and safety procedures to be adhered to throughout the emergency response. The plan evolved as required throughout the process and was implemented in cooperation with the Restoration Contractor, with members of the emergency response team present throughout the installation to monitor progress and provide assistance when necessary.

Photograph 2 – View of 90-ton crane operation and set-up

The completed shoring system rendered the building temporarily safe for a thorough inspection by the Office of the Ontario Fire Marshal, which was investigating the explosion. At the same time, Fire and Electrical experts were permitted to complete an investigation of the explosion and were able to provide conclusions regarding origin and cause.

During the development and implementation of the emergency response, additional assessments of various environmental and cleaning requirements were also identified and undertaken within the damaged building. The explosion had caused significant particulate accumulation as well as some minor smoke and soot damage. An Environmental Health and Safety team was able to provide a cleaning plan to remediate the identified environmental factors. In addition, an Electrical Investigations team became involved in the case when asked to assess damage to the building’s electrical room and power distribution system and provide recommendations for remediation. At this point, a diverse team of experts was onsite and working together to support the successful remediation of the damaged building.

Following completion of the origin and cause investigations, the emergency response team was asked to return to the site to thoroughly investigate the explosion‑related damage and provide recommendations for restoring the building to its pre‑loss condition. Following the investigation, a detailed assessment of the explosion-related damage, accompanied by a detailed scope of work and drawings to facilitate the remediation process was completed and the building permit to allow remediation efforts to commence was secured.

Damaged structural elements of the building were selectively removed and replaced, restoring the structural integrity of the building, and new siding, roofing and additional finishes were installed. The cleaning measures proposed by the environmental team were implemented and the building’s electrical system was repaired.

Following completion of the remediation work, the affected structure had been reinstated to its pre-loss condition, and the owner of the building was able to resume operations with the confidence that the structure was safe for its intended occupants and uses. Ultimately, it was the quick initial response and the innovative solutions for temporary shoring that prevented the necessity of a full demolition, and allowed for the completion of the origin and cause investigations and repair of the damaged structure. Repairing the damage to the building was done at a fraction of the cost of rebuilding, as the construction of a new building would have required the removal and replacement of the entire structure, including all the mechanical and electrical equipment, and would have involved a significantly greater business interruption. Through saving the damaged structure and eliminating the necessity for a costly demolition and rebuilding process, this case was closed knowing that the building was safe and the most practical and cost‑effective solution was implemented.

Success on a case such as this can be attributed to a tightly-coordinated effort utilizing specialists across a number of disciplines. No one expert could have achieved this success alone. This case exemplifies the power of deploying a multidisciplinary team in the face of a complex loss scenario, and fuels the curiosity to discover what greater innovations and successes are possible when experts combine forces.

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Pablo J. Robalino is a structural forensic engineer specialist and project manager, and a Senior Associate at 30 Forensic Engineering in Toronto, Ontario.

Contact: probalino@30fe.com