Steps to perform the top-down method:
At first, retaining walls are installed which in many cases are concrete diaphragm walls. Soil is kept on the sides of the structure. After that, the roof slab is made, which plays the main role of a strong support during the excavation. Access openings in the roof slab are installed in such a way that it is possible to work down to the level of the floor slab in the underground structure. As soon as the floor slab is completed, the side walls are built and the middle strata are gradually removed. Finally, the access openings in the roof slab are filled and subsequently the ground surface is embanked and leveled.
1.1 Retaining wall installation:
An underground retaining wall, usually a concrete diaphragm wall, is installed before excavation begins.
1.2 Excavation and installation of steel strata:
The soil is dug below the level of the roof of the underground structure. Strata are installed to protect the retaining walls in order to keep the soil on the sides.
1.3 Construction of underground structures:
The roof slab is constructed with access flaps built into it to allow access to the lower parts. In addition to providing a major support during excavation operations, the roof slab also serves as a sound barrier. It has applications.
1.4 operation Continuation:
Drilling is done under the middle roof to the next level of the slab and this process continues downwards until the floor slab is completed.
1.5 Completing the foundation structure and closing the opening:
After the completion of the foundation of the structure and the exit of the drilling machines from the place of the openings, the side walls are made upwards, then the middle strata are removed and then the access openings of the roof slab are filled.
6.1 Embankment and leveling:
After the underground structure is completed, embankment to the level of the stratum is done before removing it. This stage continues by completing the embankment on top of the underground structure and finally leveling the surfaces.
Top-Down Method advantages:
1. In this method the structure and infrastructure to be implemented simultaneously in design and construction stages and accelerates the construction process.
2. This method saves significant investment costs by removing the temporary barrier structure.
3. reducing costs by saving on construction time.
4. minimize the enviromental pollution levels.
5. The volume of concrete used is greatly reduced and leads to cost savings and reduced carbon emissions.
6. The amount of interference and disturbance in the traffic of the surrounding streets is minimized. Especially in the field of public transport, the use of this method causes the means of transportation to be revived soon.