There are different types of retaining structures. In addition to the inherent resistance of the soil, the retaining structure system is used to increase the safety and stability of the excavation walls and earth trenches as much as possible. Retaining structures generally consist of the following:
- Facing which is used for covering and spreading force (such as shotcrete walls, or a system consisting of beams and columns, or reinforced concrete walls, etc.)
- Techniques that provide the forces required for the stability of the above walls and are divided into the following three categories:
Providing a stable force from inside the project land: in the form of stub or diagonal piles or earthen panels (such as traditional retaining structures) which are less used due to disturbing the construction of the structure and inability to properly control the displacement of the tip of the excavation wall. But today, truss retaining structures are increasingly successful due to the lack of neighbors to allow them to dig.
In all types of retaining structures, truss retaining structures are recommended for excavation up to 12 meters deep. Details of truss retaining structure and truss retaining structure maps are given in the reference book, but it is recommended to use modeling in software for accurate and optimal design. For this purpose, it is necessary to watch the video of the construction of the truss retaining structure carefully.
Providing a stable force by extending the wall inside the ground: in this case, the wall acts as a cantilever and provides its constraining from the soil of the project bed. Like the soldier pile or pile sheet method, this method is also a kind of retaining structure and is not widely used due to the inability to control the displacement of the tip of the wall of trenches and excavations. This method is used for excavations up to 2 to 6 meters deep, this method is also part of the implementation of truss retaining structure. The micropile method is a part of the lateral stability system.
Providing a stable force from the soil behind the wall: reinforcements such as rebar, strand, self-drilling anchor bar, plastic/fiber reinforced polymer (FRP), etc. are installed in the soil behind the wall and are strengthened by injecting cement or chemical slurry or physical restraint into the soil. This method is called nailing and anchorage. The design of these methods is also fully described in the articles section.
What should be considered by the supervising engineer before implementing the truss retaining structure?
To safely execute the retaining structure, it is better for the supervising engineer to consider the following executive steps:
Field visit with the aim of identifying the technical and qualitative status of the neighborhoods
At this stage, the supervising engineer should visit the adjacent buildings, evaluate the static condition of the existing buildings, the quality of construction, and the type of skeleton and so on. In this visit, the status of absorbing wells and their location should be examined and identified in the form of field negotiations with neighbors.
If there is a history of Qanats, installation canals or old sewers in the project construction site, the items should be identified and registered by the supervising engineer.
It is always recommended that the supervising engineer negotiate with the project neighbors to provide the necessary explanations for the engineering programs that are required to increase safety during excavation.
This makes them more confident in the role of the supervising engineer in the project and the seriousness of the engineering programs in the project, and reduces the creation and development of rumors and mental perceptions in the neighbors. At this stage, it is recommended that the supervising engineer provide his contact number to the mentioned people for more communication with the managers of the neighborhoods, because with this method, the accuracy and supervision in the project will be continuously increased.
Adaptation of the retaining structure map with field studies
- Based on the results of field studies, it is necessary to review the approved maps of the retaining structure with the current situation and identify possible discrepancies in the maps before starting the executive operation. Some common inconsistencies in maps are as follow:
- Impossibility of implementing the map with the executive coordinates of the project land
- Lack of anticipation of the retaining structure in the areas that lead to the passage and the need for wall stabilization is felt in them.
- Use of non-specific materials such as lumber in executive plans
- Lack of connection details
- Lack of anticipation of transverse anchors between trusses
- Lack of effective embedded length and proper height of vertical truss element
- Non-compliance of truss retaining structure method with observed soil material in the project
If the supervising engineer observes any of the ambiguities raised, he should provide an appropriate way to solve the problem. Obviously, those problems that have a computational origin should be provided to the accounting engineer through the owner / executor or directly by the supervisor to review the maps and finally re-seal the maps.
The supervising engineer should check the ambiguities in the maps of the retaining structure before starting the executive operation to minimize the possibility of incorrect retaining structure or unwanted stoppage during excavation due to improper design of the retaining structure.
Other problems include the desire of some owners to make changes to the map (increase in size or number of basements).
Applying for changes in the depth of excavation when the executive operation has started or ended according to the initial plan, is very effective in ensuring the safety of the excavations.
The change often causes the supervising engineer to stop work when the initial excavation operation is completed so that the process of changing the maps can proceed legally. So, considering that such changes after the start of excavation, lead to a forced stop in the excavation operation of the project, so it is strongly recommended that the supervising engineer conduct negotiations with the owner before starting the excavation operation to become more familiar with his latest views and wishes. In such cases, it is recommended to avoid issuing the start-up form until the finalization of the excavation plan, if the supervising engineer is faced with a request to change the plan.
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Selecting the appropriate retaining structure (appropriate method of stabilizing the excavation)
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