Excavation stabilization methods and types of deep excavation methods in buildings

There are several ways to stabilize the excavation. The increasing growth of construction projects and the increase in the price of buildings and real estate on the one hand and user needs and the need to provide more public spaces and benefit from higher levels of social welfare on the other hand has increased the interest of builders in the country to use underground spaces to provide parking and communal with social, sports and welfare uses.

In recent decades, with the development of large cities and rapid population growth, the need to build high-rise buildings, which often have several basements, is strongly felt. Due to the density of construction in cities, the implementation of excavation operations alongside existing buildings requires the provision of safety principles, including the implementation of excavation stabilization methods.

Unfortunately, in this period of time, due to not choosing the appropriate methods and anticipating safety measures, we have witnessed the collapse of the excavation walls, the destruction of nearby buildings, and financial and human losses. Therefore, the development of scientific resources and design and implementation of retaining structures, which are known as excavation stabilization methods or lateral protection systems of excavation walls, such as diaphragm walls, active or passive anchors, shotcrete, and micro-piles, etc. has a special priority to ensure the stability of excavation walls, which is also discussed in the new edition of the seventh issue of national building regulations.

Lack of appropriate analytical and practical resources (even in English) is another problem of our country’s civil engineering community.

Dear Dr. Majid Mohagheghi, who are experienced experts in the field of geotechnical issues, compiled and translated books for various stabilization methods using their valuable experiences and in collaboration with Dr. Mohammad Reza Atrchian and Gholamreza Niazi.

In these books, various topics such as types of nailing system implementation methods and geotechnical studies and field experiments, materials and executive considerations, methods of analysis and design of nailing system, contractual considerations, and the important matter of inspection and construction supervision have been studied.

Due to the way of compiling the contents and description of design and implementation details, which is due to the special attention of translators, these useful and scientific collections can be used by the engineering of the country. By applying its instructions, the important issue of providing safety in the excavation stabilization is removed from the state of challenge and concern in the executive plans and the country’s engineers can have more success in the implementation of large and infrastructure projects.

It should be noted that not paying attention to the proper drainage method in stabilization operations alone can cause irreparable damage.

Implementing points in excavation and stabilization of excavation projects by nailing method

  • Regulate the walls up to a distance of 20 cm using an excavator and manually.
  • Panel removal in each stage is subject to the completion of the stabilization operation of the upper level stage
  • Panel removal should be done in the form of teeth and from the third row onwards with a maximum length of 12 meters (with the opinion of the designer and supervisor)

Drainage

The following points are included drainage in excavation and stabilization of excavation projects by nailing and anchoring methods:

A set of executive tips related to wall drainage

Execution of drainage strips and deep drain pipes (weep hol)e in all walls in excavation and stabilization of the excavation projects by nailing and anchoring methods is required.

  • Drainage strips should be installed at horizontal distances of up to 5 meters.
  • Minimum overlap length of strip drains: 50 cm
  • Minimum width of drainage strips (geocomposite): 20 cm
  • Minimum diameter of deep drain pipes: 2.5 inches
  • Drainage pipes should be installed at horizontal distances of up to 6 meters (if there is water flow and groundwater level inside the excavation, these distances will be reduced).
  • The allowable tolerance for the length of drainage pipes is 25 cm.
  • It is necessary to pack the ends of deep drains with a cap or geotextile.
  • Gouging on deep drainage in the upper semicircle of its cross section is required.
  • To prevent the slurry from settling due to the injection of reinforcement, deep drainage should be done at least 2 days after the injection of the boreholes of stabilization plan in the upper and lower rows.

A set of executive points related to the complete drainage of the project:

  • The presence of a specialized drainage consultant in large excavations is recommended.
  • The conventional method of complete drainage of the excavation and stabilization of the excavation project by nailing anchoring methods to a depth lower than the groundwater level, is the implementation of several rods and walls in the form of drainage rings. Stabilization of rods and walls using installation of kaval and filling the gap between the soil and the kaval.
  • The rate of water pumping from the excavation is very important and its increase leads to an increase in settlements.
  • In large excavations, it is recommended to start the stabilization operation after completing the drainage.
  • It is mandatory to report a water level on different sides of the project to the designer and supervisor by the drainage project agents.
  • The presence of a reservation pump in the workshop is necessary.
  • Water drainage in sandy layers fine-grained clay soils is fast and time consuming, respectively.
  • Any change in the drainage design of the excavation should be in coordination with the designer.

Bracing and stabilization of the excavation using the diaphragm wall method

Construction of a diaphragm wall or slurry wall is another method of stabilizing the excavation.

Simultaneously with drilling, bentonite slurry is used to stabilize the drilled wall and prevent local micropiles.

The formation of bentonite cake inside the drilled wall and the penetration in the grain layers of the wall makes the wall always stable. In this type of retaining structure, after reaching the desired depth, reinforcement and finally concreting is done. Bentonite cannot be used if we are sure that the wall does not fall. This stabilization method is also used under the core of earth dams and prevents any leakage. The use of this technique of retaining structure and stabilization in urban areas also has limitations such as the use of horizontal and inclined bracing and tensile elements.

The stabilization of the excavation is done by constructing a diaphragm wall or slurry wall in the following way:

  • Drilling the wall step by step using special devices (hydrofraise or grab)
  • Fill the drilled site with bentonite (drilling fluid)
  • Installing reinforcement cage
  • Concreting the wall

Advantages of stabilizing the excavation using diaphragm wall method

  1. Very fast execution
  2. Very high degree of work safety
  3. The diaphragm wall acts both as a retaining structure and as a retaining wall during operation.
  4. Diaphragm wall is especially suitable for drilling and deep excavations

Disadvantages of excavation stabilization using retaining structure of diaphragm wall method

  1. In low volumes, the cost of performing the work in this method of stabilization is very high, but in high volumes, the overall cost of the work can be less than the simpler methods.
  2. In this method of retaining structure, the related drilling machines require more work space, and if we have limited space on both sides of the wall, the work will be impossible, or difficult.
  3. This stabilization method requires special drilling machines.
  4. In this method of excavation stabilization, highly specialized experts are needed to work with the desired devices and other items.

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