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Truss retaining structure is one of the common excavation methods in urban areas for medium depth excavations. In this method, steel IPE or IPB are leaned against the hollow wall at regular intervals. Inclined members prevent the collapse of the excavation wall by bearing the lateral pressure caused by the soil overhead and transferring it to the ground.

Truss Retaining Structure Design

Due to the lack of need for permission from neighboring neighbors, the truss method is sometimes more appropriate, simpler, more practical, and more economical than other earth retention methods.

Truss retaining structure and its main components in Saadat Abad, Janat Abad and Niavaran projects of Istasazeh Company
Truss retaining structure and its main components in Saadat Abad, Janat Abad and Niavaran projects of Istasazeh Company

The main members of the truss include:

  • Vertical member
  • Inclined member
  • Vertical member root
  • Inclined member foundation
  • Horizontal and inclined truss members
An example of design table of truss retaining structure
An example of design table of truss retaining structure

Design basics of truss retaining structure method

There are two ways to design a truss retaining structure:

1) Use tables that according to:

  • Soil characteristics: adhesion and internal friction angle
  • Depth of excavation
  • Overload
  • Retaining structure distances

provides sections of vertical, inclined, truss members, pile length, foundation dimensions and other details of truss retaining structure.

Design of the truss retaining structure of Janatabad project of Istasazeh Co. using Plaxis 2D software
Design of the truss structure of Janatabad project of Istasazeh Co. using Plaxis 2D software

2) Design of truss retaining structures using finite element software such as Plaxis 2D and 3D, which usually results in lighter results.

In this method, due to the flexural and axial stiffness of the vertical, inclined and truss members, the simulation is performed and the deformation of the complications adjacent to the excavation is calculated.

Design of truss retaining structure using Plaxis 3D software
Design of truss structure using Plaxis 3D software

Truss Retaining Structure Execution

Drilling of wells, execution of vertical member and concreting of wells of Niavaran project of Istasazeh Co.
Drilling of wells, execution of vertical member and concreting of wells of Niavaran project of Istasazeh Co.

1) Drilling vertical members’ wells

As in the case of metal piles in the methods of soil nailing, and ground anchorage, in the method of truss retaining structure, the wells of vertical members must be drilled before excavation begins. The location of these wells is done by surveying and according to the executive plans.

To concretize the roots of vertical members, the floor of these wells is lower than the floor level of the foundation.

2) Execution of vertical members of truss retaining structure

The vertical section is often in the form of an IPE profile or an angle profile. The vertical members are placed vertically in the well and fully adhered to the wall, after which the roots of the metal pile are concreted.

The root of the metal pile is about 25% of the depth of the excavation and the diameter of the wells is about 80 to 100 cm.

For better trapping of the vertical member in the root concrete, corner or stud parts are welded to the vertical member as cutters.

Just as metal piles in the methods of soil nailing, soil anchorage, bracing struts, etc. must be completely supported on the wall of the excavation, for the proper functioning of the truss retaining structure, the vertical member must be completely attached to the wall to minimize deformation. In order for the vertical member to adhere completely, its back is filled with suitable materials that have the least compressive strength of the soil in that part, so that no empty space is left between the vertical member and the wall.

Drilling a well, preparing a vertical member, installing a vertical member and concreting the well
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Drilling a well, preparing a vertical member, installing a vertical member and concreting the well
Drilling a well, preparing a vertical member, installing a vertical member and concreting the well

Execution of inclined members of Janatabad project of Istasazeh Co.
Execution of inclined members of Janatabad project of Istasazeh Co.

3) the Execution of inclined members of the truss retaining structure

After the execution of the vertical members of the truss retaining, the inclined members must be executed.

However, due to the presence of soil, it is not possible to execute inclined members. Therefore, first the vertical members are excavated in a stable slope and then the inclined members are installed.

The inclined members are welded to the vertical members at the top and placed on the foundation by the column plate at the bottom.

In order to integrate the function of the truss retaining structure, the inclined members are connected to each other by a tie.

Forming and concreting the inclined member foundation and installing the inclined member
Forming and concreting the inclined member foundation and installing the inclined member
Forming and concreting the inclined member foundation and installing the inclined member
Execution of horizontal and inclined truss members in Janatabad project of Istasazeh Co.
Execution of horizontal and inclined truss members in Janatabad project of Istasazeh Co.

4) Completion of truss retaining structure and execution of steel mesh and shotcrete

After executing the vertical and inclined members, it is time to complete the retaining structure.

At this stage, the horizontal and inclined members of the truss are welded to the vertical and inclined members.

After installing the horizontal and inclined members of the truss, the excavation operation is done in stages and the steel mesh and shotcrete are done.

Installing horizontal and inclined truss members
Installing horizontal and inclined truss members
Installing horizontal and inclined truss members
Connection of adjacent trusses in Janatabad project of Istasazeh Co.
Connection of adjacent trusses in Janatabad project of Istasazeh Co.

5) Connection of truss retaining structures by bracing members

At the end of the execution of retaining structure, in order to:

  • Proper and uniform performance of trusses
  • To prevent off-plate buckling of trusses

adjacent trusses are connected to each other by bracing members.

Bracing members should be executed as soon as possible and with the progress of excavation operations after the installation of vertical and inclined members.

Execution stages of truss retaining structure
Execution stages of truss retaining structure
3d view of truss retaining structure execution
3d view of truss retaining execution
Combination of truss retaining and nailing methods
Combination of truss retaining and soil nailing methods

Sometimes in the earth retention according to the conditions:

  • Permission and non-permission of adjacent structures
  • Soil stratification
  • Change the width of different parts of the excavation
  • Difference between the depth of the excavation in the walls
  • Excavation overhead

Earth retention methods are combined.

Causes of combining different retaining structure methods
Causes of combining different retaining structure methods
Execution stages of truss retaining structure
stages of truss retaining structure installation

Advantages of truss retaining method

  • Suitable for shallow and medium depth excavations in urban areas
  • No need for neighbors’ permission
  • High flexibility in terms of execution in different situations
  • Possibility of recycling part of truss materials
  • Ease of execution and no need for special machines
  • Do not enter adjacent areas
  • Ability to adapt to drainage methods
  • Relatively high execution speed
  • Ability to execute in projects with irregular geometry
Advantages of truss retaining structure method compared to other stabilization methods
Advantages of truss retaining structure method compared to other stabilization methods

Disadvantages of truss retaining structure method

  • Restrictions on use in deep excavations
  • Occupy the space inside the excavation and the difficulty of excavation and execution of the main structure
  • Difficulty in execution and loss of quality of the dewatering system
  • Impossibility to use to increase the depth of the excavation at the time of execution or after the completion of excavation
  • The possibility of the need to remove part of the soil by manual methods
  • Difficulty of execution and interference of trusses in concave corners
  • Apparent simplicity in the design and method of construction and entry of completely non-professionals in this field
Disadvantages of truss retaining structure method compared to other stabilization methods
Disadvantages of truss retaining structure method compared to other earth retention methods

How to execute a truss retaining structure (with animation)

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