In construction projects, excavation is usually done first, and then the foundation and structure are executed. Top Down is a new pit stabilization method that has become popular in Iran in the last few years. This method is called Top-Down construction, because in this method, the construction of infrastructure and superstructure with is also ongoing.

The Top-Down method is considered a structure construction or excavation method. In the Top-Down construction method, unlike other pit stabilization methods, the construction of the structure starts from the middle level (zero) and negative one, negative two, etc. floors are built in order, and at the end, the foundation is implemented.

Pay attention to this point, in the Top-Down method, the structure above the ground level and the structure below the ground level can be built at the same time.

Excavation and implementation of the structure using the top-down method has advantages that have attracted the attention of many construction engineers such as Rooyesh Group. So that this company is currently using the Top-Down excavation method to build its construction projects.

Top Down Excavation in a Simple Word

Top-Down as one of the excavation reinforcement methods has been widely welcomed in recent years. It may seem strange, but this method acts as a stabilizer unlike other methods, and with Top-Down construction, it follows a reverse process, for this reason, it is known as the Top-Down construction method. In other methods of stabilization of the excavation, excavation was done first, and then with one of the methods mentioned above, the reinforcement of the excavation wall is done, but in the method of topping or building from top to bottom, the work process is the opposite, and at the same time as removing the soil, Basement floors are built from the ground level downwards. In this method, the foundation is the last stage of implementation.

History of the Top-Down Method

The Top-Down construction method, as a huge development in the construction of large underground structures (tunnels), was used for the first time in the 70s in the construction of metro stations in Paris and Milan, and was known as the Cut & Cover method. The basis of this method is based on the progress of the tunnel structure inside the excavation and then filling the outer environment of the tunnel with embankment materials after the completion of the tunnel structure. This construction method is more economical and acceptable for the construction of surface tunnels.

In 1935 in Tokyo, Japan, excavation of a building with underground was done with this method due to the increase in construction speed. In 1950 in Milan, Italy, ICOS company implemented the combination of diaphragm wall and Top-Down construction. Over time, many researches were conducted in America, Europe and especially East Asia in this field. This new construction method has been used in different countries such as England, America, Finland, China, Russia, Vietnam, Singapore and Iran.

A Summary of the Steps of Implementing the Structure by the Top-Down Method

The Top-Down implementation method can be implemented in different ways, according to the different conditions of the project, including the soil of the region, time constraints, budget constraints, implementation facilities in the region, etc.

The Implementation of the sections that we will mention in the following can be changed according to the requirements of the plan and the relevant conditions, and similar methods can replace other sections.

In the implementation of the structure using the Top-Down method, we first implement the diaphragm walls that are placed around the pit as a guard structure. If the width of the structure is small, we connect two peripheral walls with the help of steel struts to make each other stable.

If the width of the structure is large, we implement deep concrete piles between the two walls, which appear in the form of columns. Now we connect the struts to the diaphragm walls and the piles.

Then, we have secured the stability of the soil, now we are going to excavate the soil and excavate to the level below the negative one floor. Now we have secured the space for creating the first roof and we can implement the roof and walls of the negative first floor.

We repeat this until we reach the level of the bottom of the pit (the lowest floor). Let’s be careful that in the Top-Down design and implementation, suitable openings must be considered to ensure the access of materials and machines to the lower floors. These openings remain until reaching the floor level and then start closing from the end. Naturally, considering suitable reinforcements and connections for these openings is one of the important points in this execution method.

At the end of the implementation, the struts that were placed to maintain the stability of the soil are also removed.

Advantages of the Top-Down Method

The Top-Down method makes it possible for the superstructure and the substructure to be implemented simultaneously by applying the necessary measures in the design and construction stages, which accelerates the construction process.
By removing the temporary barrier structure, the Top-Down method significantly saves investment costs.
Top-Down saves construction time and ultimately leads to cost reduction.
By using the Top-Down method, the amount of environmental pollution is reduced.
The volume of concrete consumed is greatly reduced and leads to cost savings and reduction of carbon emissions.
The amount of interference and disturbance in the traffic of surrounding streets is minimized. Especially in the projects related to public transportation, the use of this method will make the transportation ways available soon.
Using the Top-Down method reduces the possibility of ground subsidence.
Using this method, there is no need for a large working space for foundation construction, and by eliminating the cost of constructing the excavation guard structure, economic savings will be achieved.
It is not necessary to build temporary struts to protect the walls and build the basement during excavation in this method. Temporary strut systems often complicate work space conditions and are very expensive to build.

The possibility of simultaneous execution of construction activities of structures, electrical and mechanical facilities simultaneously with the increase of floors below and on the ground increases the execution speed and shortens the construction schedule.

Disadvantages of the Top-Down Method

Excavation is not continuous.
Access to the pit is possible only through shafts and openings.
Due to the low height of the floors, special machines are needed.
Due to the specialization of the work, it requires an executive team specialized in the field of Top-Down.
The air conditioning system affects the quality of the construction.
The details of connecting the slab to the columns and walls are complicated.
Low execution speed of infrastructure operations.

Conclusion

With all these explanations and from a general point of view, it can be said that the significant advantages of designing and implementing the structure with the Top-Down method are largely worth coping with these limitations for investors and project managers, and that is why it is always welcomed.

In order to carry out such excavations in different buildings, you can use internal engineers such as Rooyesh Group. Besides being a good engineering advice, they will help you in implementing these buildings with low cost and high speed.