Top down construction method is a relatively new method of earth retention that is gaining popularity in the construction industry. In the top down construction method, it is possible to execute underground floors up to the ground level simultaneously with the execution of floors on the ground. In the following cases:
- High groundwater level
- Limitation of performance space
- Interference with main street traffic
- Impossibility of obtaining consent from neighboring neighbors and so on
The top-down construction method is one of the suitable methods for Earth retention.
History of top down construction method
For the first time in the 70’s, the top-down construction method was used in the construction of metro stations in Paris and Milan and was known as the Cut & cover method. In top down method, first the tunnel structure is made and then the external environment of the tunnel is filled with embankment materials.
To speed up the construction of a building, in 1935 in Tokyo, the top down construction technique was used. In 1950 in Milan, a combination of a diaphragm wall and top down construction was performed by ICOS. Over time, this method has been used in various countries, including Iran.
Top down construction method regulation
Execution of the top down construction method is provided in UK Regulation BS: 8002: 1994 for retaining structures.
Top down construction execution method
For each project, depending on its type and specific conditions, the top-down construction process may be different, but in general, the stages of top down construction can be described as follows:
1) Execution of surrounding retaining walls
Sometimes at the beginning of the project, the retaining walls are executed to a certain depth below the foundation level. These walls may be executed in one of the following ways:
- In-situ reinforced concrete wall
- Diaphragm walls
- Secant piles
- Continuous candles
- Steel sheet piles
- Jet-Grouting columns
Diaphragm retaining wall is more used than other methods in making surrounding retaining walls.
In order to connect the retaining walls to the roof slab of the lower floors, the necessary equipment for the next connections must be provided at certain levels.
2) Drilling the location of bearing piles from below the level of columns, ground walls and installation of bearing columns
One of the main stages of top down construction method is the execution of the columns of the main structure. For this purpose, first the pile is executed below the level of the foundation of each of the columns, so that after the foundation is executed on these piles, it is possible to execute the columns of the main structure.
The columns of the main structure are executed on top of the piles by one of the following methods:
- In-situ concrete column with square, rectangular or circular sections based on available drilling equipment
- Precast concrete column
- Combining lean columns with concrete jacket during the construction of the main structure
- Composite steel column
3) Execution of ground floor slab
After the execution of the columns of the main structure, considering the location of the access openings to the lower floors, reinforcement and concreting of the ground floor slab at the top level of the piles and the surrounding walls on the columns are executed.
The size and location of the access openings should be suitable for moving drilling machines and accessing the lower floors. In addition, the necessary arrangements must be made for the final reinforcement and concreting to close the opening at the end of the project.
The concrete slab of each basement floor acts as a strut and prevents the excavation from falling.
4) Start excavation from the basement floor
After completing the ground floor slab and trapping the slab concrete, using appropriate machinery, the excavation operation continues at the openings until the lower roof level. After reaching the level of the lower and flat roof, regulating the natural floor and concrete, reinforcing and concreting this roof are started.
The connection of the roof to the columns and surrounding walls is done by using welded or mechanical connections on the pre-installed waiting plates. At this stage, the following should be considered:
- Condition of soil layers
- Accurate drilling height
- Selecting machines with shorter heights
- Selecting machines with more power
After the execution of the slab of this roof, it is possible to execute the structure at high levels.
5) Repeating stage 4 to reach the foundation level
The process of stage 4 continues until the foundation level is reached.
6) Execution of reinforcement and concreting of the complementary layer of the surrounding walls and middle columns
After completing the excavation operation and execution of the foundation and exit of the machines, reinforcement and execution of the second layer concrete around the steel columns and the inner part of the diaphragm wall for permanent load-bearing of the structure will be done. In this method, the use of a pile-based foundation is recommended.
Stages of top down construction method at a glance
The stages of top & down construction method in buildings can be summarized as follows:
Connection of columns to concrete slab in top down construction technology
Due to the fact that the top down construction method is the opposite of the usual mode of project execution, special points should be considered, especially for connections:
- Proper connection of columns and slabs requires special arrangements, some of which are mentioned below.
- Concrete columns require waiting for connection plates on the reinforcement grid.
- The connection of the roof to the main columns and walls is done using welded or mechanical connections.
Execution of dewatering the grooved connection of the roof slab to the surrounding walls
Stages of dewatering the grooved connection of the roof slab to the surrounding walls:
- Placing the foam on the steel bar mesh of the diaphragm wall at the junction
- Removing the foams when the excavation depth reaches the level of the wall and ceiling joints
- Filling the groove with cement mortar
- Executing a waterproof tape
- Crossing the steel mesh through the concrete floor
Stages of tunnel and subway construction by top & down construction method
Metro and tunnel construction stages are different from buildings. The following are the different stages of construction from top to bottom of the subway and tunnel:
1) Execution of surrounding retaining walls
The retaining wall of the tunnel and subway, which is executed before the excavation begins, is usually a concrete diaphragm wall or secant piles. This retaining wall is ultimately the final wall of the tunnel structure.
2) Start excavation after installing steel strut
Excavation should be done below the level of the underground structure. For this purpose, steel struts are applied to prevent the excavation from falling.
3) Construction of concrete slab with installation of access opening
After excavating to the roof of the underground structure, it is time to execute the roof slab. The concrete slab of the roof is connected to the surrounding walls and suitable openings are installed in it. This slab is connected to the surrounding walls and, like a strut, prevents the deformation of the surrounding walls. Also, this slab, like sound insulation, prevents the transmission of machine sound to adjacent structures.
4) Repeating the previous stage until reaching the foundation level
Stage 3 continues until reaching the foundation level
5) Completion of the foundation and structural walls inside the tunnel
After finishing the foundation and leaving the machines from the openings, it is time to execute the double surrounding concrete walls with double reinforcement. The access openings of the roof slab are then closed.
6) Final embankment of the upper surface of the tunnel
After the completion of the underground structure, the embankment operation is performed up to the level of struts and then the struts are removed and the embankment continues to the ground level.
Top down construction Advantages in tunnel construction
- Ability to quickly exploit the high level of the tunnel and not interfere with urban traffic
- Using excavation walls as a wall of permanent structures
- The function of structural slabs as strut and thus reducing the number of struts
- No need to execute a temporary retaining structure and thus reduce the cost of tunnel construction
- Overlap of the executive stages of construction of structures, electrical and mechanical installations and thus reduce the duration of construction operations
- Possibility to equip the workshop above the excavation levels and no need to occupy the passages around the excavation
- Protection of the excavated parts from the effective factors outside the excavation and lack of effect of weather conditions on the activities of the workshop due to its coverage and elimination of relevant hazards
The difference of top down construction method in steel and concrete structures
Differences of top down construction method in steel and concrete structures
- Executive method
- Scheduling
- Shipping method
- Material storage method
- Type of connections
- Basics and requirements of design and execution
For example: to connect a metal column to a concrete base pile and connect it to a concrete slab wall, there are different points with a concrete structure.
Important points about concrete slab openings and temporary ramps
- Determining the position, number and dimensions of concrete slab openings are among the important issues in the top down construction technique.
- Due to the size of the slab, the volume of excavation materials and the depth of excavation, it may be necessary to install separate ramps for entry and exit of machinery.
- In the Hong Kong IFC International Trade Center project, temporary shafts were used to remove excavated material.
- Predicting the reinforcements waiting around the openings for the final execution of the slab and closing the openings, should be appropriate.
Double-Bit Drilling Method
Double-bit drilling method is used to accelerate the construction of basements.
In Double-Bit Drilling method, excavation of 2 basements is done simultaneously. At the lower level, which is on natural soil, a concrete slab is executed and then a high-level concrete slab is executed. Depending on the needs of the project, it is possible to use temporary piles to maintain the upper slab.
Importance of soil type in top down construction technique
In the top & down construction method, in addition to the usual structural design, it is necessary to check the condition of the soil layers.
- Lack of proper knowledge of problematic soil layers
- Lack of study of soil and structural deformation characteristics
- Lack of proper design of retaining structure
- Lack of proper monitoring of deformities
The above items cause the collapsing excavation and damage to adjacent structures.
Top-down construction method Advantages
- Carrying out excavation operations on the natural soil substrate
- No need to obtain consent from neighbors near the excavation
- Removing the temporary retaining structure
- Suitable for excavations located in urban areas
- Suitable for busy areas with limited space
- Reduction of project completion time in case of simultaneous execution of infrastructure and superstructure
- Significant reduction in deformation due to excavation
- Having the function of a permanent retaining structure
- No restrictions on execution in lands with different dimensions and geometry
- Ability to adapt to drainage methods
- Independence of method efficiency does not depend on soil type
Top-down construction method disadvantages
- Complexity of slab connections to columns and retaining walls
- The middle columns deviate from the axis due to increased upper loads if not properly designed and executed
- Restriction of access to the excavation only through openings or through shafts
- Difficulty of excavation and execution of infrastructure
- Relatively low speed of infrastructure operations
- 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 or change of design
- Requiring special equipment and machinery
- Requiring experienced people and strict quality control to execute due to the details and complexities of execution
- The need for design by a structural and geotechnical expert simultaneously
- Limitations and difficulty of concrete structure
Design and execution points of top down construction method
- Proper simulation of soil-structure interaction
- Using appropriate structural and soil reliability coefficients to deal with the causes of failure in the excavation stabilization system
- Considering the non-drained conditions of clay in case of clay and groundwater
- Investigating the possibility of failure of struts connections with surrounding walls at the time of design
- Preventing executive problems, such as:
- Lack of proper pile foundation under the structural columns
- Lack of connection and completion of floor roofs during the excavation process
- Lack of proper drainage and drainage of neighboring sewage
- Improper execution of retaining wall connections to columns and roofs
- Proper dewatering of connections to prevent water from penetrating into the excavation
- Precise adjustment of the placement of the slab on the columns and walls
- Regular monitoring of the amount of excavation deformations
- Considering the dynamic behavior of towers with multiple basements due to the lateral force of wind or earthquake
Video animation on how to do top down construction
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