The collapse of retaining walls may result from several factors, including:
Soil instability (GEO): If the soil surrounding the wall is unstable, for example, due to telluric movements, accumulation of water, erosion, etc.
Structural settlements (STR): If the wall has been improperly designed or built, for example with inadequate structural stability or poor quality materials, it may cause the wall to collapse.
Maintenance defects: If the wall isn’t regularly maintained and monitored, it can deteriorate over time and lead to collapse.
Generally, the collapse of retaining walls is primarily attributable to a combination of these factors.
How to design a retaining wall
The design of a retaining wall depends on the specific conditions of the site, including the terrain topography, seismic zone, soil type, wall height, the expected load on the wall, and the surrounding environment.
Generally, the design of a retaining wall involves:
- Site Evaluation: Site conditions such as terrain topography, soil type, and surrounding environment are assessed.
- Slope: A high slope can make the construction and stability of the wall harder.
- Surrounding environment: The presence of waterways, vegetation, or other structures may affect the stability of the wall.
- Wall type selection: Based on site conditions, the most appropriate retaining wall type is chosen. There are various types of retaining walls, such as gravity walls, reinforced concrete walls, or reinforced ground walls.
- Calculation of pushes: We calculate the load that the wall will have to support, so as to properly size it.
- Structural design: The wall design is based on the most appropriate materials and construction techniques.
- Stability check: The stability of the walls is checked during the design stage, using calculation models and simulations.
- Wall construction: The wall is built to design specifications, with a special focus on soil compaction.
What inspections are planned for the retaining walls
In the case of retaining walls or other similar structures, checks must be carried out with reference to at least the following limit states:
- SLU of geotechnical type (GEO) and the balance of the rigid body (EQU):
- global stability of complex ground support work;
- sliding on the laying surface;
- collapses as a result of the low load of the foundation-to-ground assembly;
- SLU of structural type (STR):
- reinforcement of structural elements.
- In all cases, the following requirement must be fulfilled:
Ed ≤ Rd where:
- Ed is the design value of the action or effect of the action;
- Rd is the design value of the resistance of the geotechnical system.
The overall stability check of the support-ground complex shall be carried out according to Approach 1, Combination 2 (A2 + M2 + R2), taking into account the partial coefficients given in the tables of standards for geotechnical actions and parameters.
The remaining verifications shall be carried out according to at least one of the following approaches, taking into account the values of the partial coefficients set out in the relevant standards:
- Approach 1: Combination 1 (A1 + M1 + R1),
Combination 2 (A2 + M2 + R2);
- Approach 2: (A1 + M1 + R3).
The rollover limit state doesn’t provide for the mobilization of the resistance of the foundation ground and must be treated as a rigid body equilibrium limit state (EQU), using partial coefficients on the actions and using partial coefficients of the group (M2) for the calculation of thrusts.
MDC is a software product for designing and analyzing reinforced concrete retaining walls resting on their foundation or piles, optionally supported by tiebacks.
The geotechnical computation employs the standard geotechnical methods subject to user choice and carries out the verifications prescribed by the selected standard, amongst which global stability even in seismic conditions. Structural evaluations perform reinforcement sizing and verification with Ultimate Limit State or Allowable Tensions.
MDC is simple and intuitive but with unique features, including pre-design work, easy management of the combinations: GEO, STR, GLOBAL STABILITY, localization and automatic import of seismic parameters, displacements analysis in a dynamic field, reinforcements editor, import and export of works that should fall in stabilization operations.
Example collapse of a retaining wall in Cluj-Napoca (Romania)
A retaining wall collapsed above the cars in Via Lombului in the city of Cluj-Napoca (Romania).
Following a report, the local police officers of the urban and construction control and traffic control services found the collapse of a retaining wall, for a length of about 35 meters. Five vehicles and one van were damaged.
“Please note that the retaining wall was provided in the technical documentation authorized by AC No. 576/2020 – “Construction of a mixed building, external improvements and fences” – private land, the work was not accepted”.
The local police have drawn up control during the security of the area concerned and the delimitation of the site, as well as the adoption of all legal measures necessary to remedy the situation.
In addition, the Construction Control and Discipline Service have revered the matter to the State Construction Inspectorate of Cluj to verify the “quality of the works”, says the Press Office of the Municipality of Cluj-Napoca.
The investigation that will be carried out will verify if the plan realized by the structural engineer has been respected and if the job had at least such documentation. Depending on what is discovered, the responsibility will lie with the contractor or those who have carried out the studies.