Excavation refers to any excavation at a level below ground level or at a level below the foundation of an adjacent building. In most construction projects, executive activities begin with excavation operations on the site.
The importance of the excavation phase of the project is not only in terms of technical complexity. At the time of excavation, there are always reasonable concerns for adjacent buildings and ensuring that they are safe.
Depth stabilization methods
1. Restraint method:
the wall soils of the excavated area are used by Using special precautions, to control and drift the soil in this method. it is necessary to use the soil body adjacent to the pit wall in this method. Therefore, in cases where the soil adjacent to the pit is under a building or in the neighborhood or in the area of facilities and urban thoroughfares, this method can’t be used or its use is limited.
2- Back sewing method:
In this method, since concrete injection is used, it prevents the movement and slipping of the soil perfectly and in addition, the soil in the wall of the excavated area prevents the movement of the ground and therefore reduces landslides.If the excavated area is adjacent to a building, the use of area soil in this method is limited.
3- Diaphragm wall method:
This method is also widely used under the core of earthen dams and urban facilities such as the metro and prevents any leakage. The use of this technique in urban areas also has limitations such as use of horizontal and inclined restraint methods and traction elements.
4- Method of mutual control:
This method is suitable for narrow pits and is done by digging wells on both sides of the pit and placing metal profiles in them and connecting the opposite profiles with beams or trusses.
5-Method of pile execution:
we run piles at certain intervals around the ground to be excavated in this method. These piles can be made of different types of structural materials such as steel, concrete and wood. Concrete piles can also be prefabricated or in situ.
In many urban projects, due to the problems of piling, prefabricated piles can’t be used and only the piles should be executed in place.
6. Superposition method:
we first hammer shields on both sides of the pit and then start digging in this method. After the sufficient excavation, we install the horizontal back beams in the backs of the shields and on them. The vertical compression brackets are then attached to these horizontal braces in a direction perpendicular to the shield plate. This method is very suitable for running channels, especially with long lengths, as well as for low widths.
7. Truss method :
Truss method is one of the most appropriate, economical and common methods of guard structure in urban areas. Its implementation is simple and does not require high equipment and expertise, and at the same time it has a lot of flexibility in terms of implementation in different situations.
Dismantling of guard structures :
The guard structure can be dismantled when part of the structure is under construction, which can withstand landslides. This structure can be the retaining wall of the structure or all or part of the building frame. If all or part of the building is to be replaced by a guard structure under construction, we must take this into account when designing the building and design the frame for these loads.
Before excavation begins:
Before demolishing the building, the project examines how the adjacent buildings are connected to the building and examines the common boundary walls, the location and manner of connecting the boundary walls, the beams or ceilings of the two adjacent buildings, the presence of openings and the gate shoe. Identify chimneys or installation ducts located in the boundary walls, type of material, wear and the presence of cracks in the wall of the adjacent building.
- By constructing safe roofs using metal scaffolding covered with suitable nets, before demolishing the building, it provided sufficient protection against the possible fall of objects and materials on the roof, walls, yard and passages adjacent to the building.
- The existing sewage wells at the project site are identified before demolition operations are completed and filled with appropriate materials. it is necessary to fill these wells with low-grade concrete or immersed concrete at least 50 cm above the level of the excavation floor and then filled with other suitable materials to the ground level If the depth of these wells is more than the depth of the excavation of the site.
The choice of demolition method should be done carefully and the demolition operation of the project site should be done under the supervision of the supervising engineer. Care must be taken in the selection of demolition tools and equipment so that vertical or lateral dynamic and static forces do not enter the adjacent building during demolition. In particular, it should be noted that the walls bordering the adjacent building should be demolished and removed without impact methods and tools.
it is necessary to cut the branches of mechanical and electrical installations in it with the permission of the relevant authorities and under the supervision of the relevant technical expert Before carrying out demolition operations in the construction site.
The technical factors responsible in the project, especially the supervisor and the supervising engineer, have been fully justified regarding the different stages of excavation and how to build prefabricated and on-site elements and the necessary coordination between the implementing engineers, supervisor and design engineer to deal with anticipated and unforeseen issues.
- Residents of the adjacent buildings should be aware of dangers of demolition and the necessary arrangements should be made for them not to live near the excavation border. Another place for the residents of the adjacent buildings to live and vacate should be anticipated. Also, evacuate valuable and heavy equipment or move it to other parts of the building that are close enough to the excavation boundary.
Nearby buildings should be inspected for the possibility of subsidence, cracks, movement of boundary walls, deformation of the door and window frames or collapse of the roof and
If necessary, new walls from inside the building next to the border wall, reinforcement of the wall through the implementation of reinforced concrete wall and cement plaster, implementation of the filling wall in the border wall openings, closing the border walls and installing suitable piles under the roof beams Suitable places should be implemented inside the adjacent building.
- the necessary warning signs should be installed in appropriate places for pedestrians not to cross and not to park or cross the car around the excavation area with the permission of the relevant authorities. Proper light weight fencing should be created around the excavation wall at appropriate intervals and
If possible, the heavy walls around the pit should be demolished before excavation.
- Prior to excavation, all sewage wells located in adjacent buildings should be identified as much as possible. drain the wells and prevent water from spilling back into them, If the distance of the existing wells from the excavation border is less than the final excavation depth and the water level of the wells is higher than the final level of the excavation floor.
- Canals, street curbs, gutter, Water and sewage transmission facilities along the adjacent excavation passages should be identified and they should be sealed by providing appropriate insulation if there is a possibility of water falling into the excavation wall.
- Identify the flowerbeds of the adjacent building and find a suitable solution to prevent their flood irrigation.
- The route of passage of all vital arteries such as gas line, water, high or low voltage electricity, telephone, optical fiber, internet and its passage in the adjacent passages of the excavation should be identified by inquiring from the relevant authorities.
- Adjacent buildings must be exposed to financial and human risks and civil and third party liability Insured prior to any demolition and excavation.
During excavation :
- Demolition and excavation should be carried out under the supervision of the supervising engineer or the monitoring device, by a competent executor.
- In any case, a person should be appointed to check the existing safety situation and constantly take care of the excavation wall and to warn the workers to escape from danger or take shelter. It is better to install a suitable siren system in the workshop to warn danger and warn workers and residents of nearby buildings.
- Daily statistics of workshop workers should be recorded accurately with registration details, address and telephone number.
- The designed methods must be performed exactly during the excavation.
At no stage of excavation and execution of the guard structure should the excavation wall be left for a long time and the successive speed of the work steps should be maintained.
- Excavation should be done in stages as shown in the executive plans using special machines or manual methods.
- Adjacent buildings and surrounding passages should be continuously inspected during excavation and installation of the structure and thereafter.
Cracking or increasing its dimensions in the walls, ceilings and floors of adjacent buildings and surrounding passages and being pressured or relieving the pressures of doors and frames, breaking or cracking glass, settling or swelling of soil, mosaic or flooring On the floor, wall or ceiling, fracture sounds of structural and non-structural elements of the building adjacent to the excavation may be due to ground movement. In such cases, the issue should be addressed immediately. Identify the weakness of the elements of the guard structure that can be effective in causing this problem and strengthen the guard structure by strengthening those elements or adding new elements
If the color of part of the soil in the excavation wall is darker than the rest of the soil, this could indicate the presence of a hole or sewer near the drilling boundary, and there is a possibility of instability in those areas. Therefore, depending on the type of phenomenon observed, additional stabilization strategies should be considered for that area.
If the moisture content of a part of the pit wall increases during or after the excavation period or water falls from a part of the wall into the pit, indicating the existence of a source that caused this moisture. Probably the presence of absorption wells, water leakage from the sewage network, the presence of irrigating flowerbeds or the passage of groundwater through coarse-grained layers, which is somehow related to the water source. In this case, the possibility of reducing the stability of the pit wall is high and appropriate solutions should be used to eliminate the source of moisture and immediately increase the capacity of the guard structure locally in the same area should be on the agenda.
- If during the installation of the guard structure or after one of the structural elements such as a wall, restraint of the girder, beam, column, foundation of the columns or foundation providing strong compressive forces or horizontal elements reducing the lateral buckling length of the girders Reaching its limit or buckling or rupture indicates the application of forces beyond the capacity of the guard structure, in which case the issue must be investigated immediately and the guard structure strengthened.
- Avoid setting up a room, barracks, residence or resting place for the guard or workers or a material warehouse near the excavation and consider a place that has a suitable distance from the excavation border for these cases.
- Provide a complete lighting system for the excavation site and illuminate all parts of the workshop with sufficient light at night so that the possible failure of any part of the guard structure or excavation wall can be clearly seen from outside the excavation.
- A vehicle equipped with first aid kits should be ready at the project site so that in case of an accident, the injured can be treated or transferred to medical centers.
- If possible, avoid excavation near the excavation border at night and when it is raining. If excavation is required in such cases, the excavation should be performed in the presence and supervision of the supervising engineer.
- Welding operations, construction and installation of guard structures are performed by technically skilled workers. Always, even after the completion of the guard structure, a number of workers with technical skills ready to work and equipped with all the equipment needed to install or strengthen the elements of the guard structure are available.
- In case of rain, if part of the pit wall is exposed to rain and the built-in wall of the guard structure is not completed in that part, it is necessary to cover it with a suitable plastic seal to the bottom of the pit to prevent water from penetrating the wall and its foot.
- When excavating and constructing the guard structure at the groundwater level, the methods of excavating and constructing the guard structure should be appropriate to the situation and considering drainage methods and lowering the water level, piling, spraying. As far as possible, on-site wall methods should be avoided in such cases. In such cases, the occupants of the buildings should evacuate the building immediately and at the earliest opportunity, taking into account all the precautionary aspects of valuable equipment and heavy furniture, move away from the points close to the excavation border.