1. Emer T. Quezon, PhD Graduate School, Cagayan State University, Carig Campus, Tuguegarao City Philippines MSCEM-207 ADVANCED CONSTRUCTION METHODS & TECHNIQUES Master of Science in Civil Engineering (Construction Engineering & Management)

2. Lecture 7 TOP - DOWN CONSTRUCTION METHODS Top - Down Method

3. 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.  To speed up the construction of a building, in 1935 in Tokyo, the top - down construction technique was used.  Over time, this method has been used most of the countries worldwide, including the Philippines. Top - Down Method

4. What is Top - Down Construction Method?  Top - down is defined by the use of the permanent internal structure as the temporary propping to the retaining wall, casted-in from top to down sequence.  The higher-level slabs are cast before the lower- level slabs to act as horizontal frames for wall support as the excavation progresses. Top - Down Method

5. Used of Top - Down Method  The Top - down method is mainly used for two types of urban structures: (1)tall buildings with deep basements; and (2) underground structures such as car parks, underpasses and subway stations.  The method has been used for deep excavation projects where tieback or anchor installation was not feasible, and soil movements had to be minimized.  There is a possibility of saving the overall construction time. Top - Down Method

6.  The construction begins with retaining wall installation, and then load-bearing elements that will carry the future super-structure followed.  The basement columns (typically steel beams) are constructed before any excavation takes place, and rest on the load-bearing elements.  These load-bearing elements are typically concrete piles or diaphragm wall panels. Top - Down Method

7. Design and construction principles for the top - down method primarily call for two major structural elements.  Columns with sufficient capacity must be pre- founded in bored piles or diaphragm wall panels to sustain the construction load, and to utilize as part of the bracing system.  Excavation for basement must be carried out with the support of permanent retaining wall so that basement floor slabs can be utilized as lateral bracing. Diaphragm wall with sufficient embedment in firm soil layers is commonly used as a retaining wall. Top - Down Method

8.  A top - down solution requires access for the removal of soil and the supply of materials, which may be through the ground floor and substructure slabs and ventilation for the work below ground beneath permanent slabs.  Consideration should also be given to safe methods of working and method for the excavation and construction of the substructure that is compatible with the available space and limited access. Top - Down Method

9. Advantages of top - down construction method: 1. The superstructure construction can proceed at the same time as the substructure. 2. Temporary propping is replaced by the use of the permanent slabs. 3. It provides a stiff support system for the wall, minimizing movement. 4. Piling can take place from the existing ground level. Top - Down Method

10. Disadvantages of top - down construction method: 1. The excavation works and substructure construction are slower and more expensive due to the restrictions on the size of the plant and the limited access. 2. Holes may have to be left in the slabs to provide access for the subsequent excavation. 3. Vertical support for the permanent slabs is required in the temporary condition. 4. Inability to install external waterproofing. 5. Stiffer construction during the intermediate construction stages attracts higher loads into the permanent structure. Top - Down Method

11. I. Stages of Top - Down Building Construction Execution Method 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, 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. Top - Down Method

12. Stage 1: Execution of the surrounding retaining wall Top - Down Method

13. 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.  The columns of the main structure are executed on top of the piles in 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. Top - Down Method

14. Stage 2: Execution of piles under the columns, foundations under the columns and columns of the main structure Top - Down Method

15. 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. Top - Down Method

16. Stage 3: Execution of the ground floor slab and starting excavation to the next basement level Top Down Method

17. 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: 1. Condition of soil layers 2. Accurate drilling height. 3. Selecting machines with shorter heights. 4. Selecting machines with more power. 5. After the execution of the slab of this roof, it is possible to execute the structure at high levels. Top - Down Method

18. Stage 4: Execution of the basement 1 slab, start excavation to the level of the next basement and the possibility of starting the superstructure Top - Down Method

19. 5) Repeating stage 4 to reach the foundation level  The process of stage 4 continues until the foundation level is reached. Stage 5a: Execution of the lower floor slab until it reaches the foundation level Top - Down Method

20. Stage 5b: Execution of the lower floor slab until it reaches the foundation level Top - Down Method

21. 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 (Foundation on pile). Top - Down Method

22. Stage 6 Completing the second layer of columns and load- bearing walls Top - Down Method

24. Figure: Execution of the ground floor slab, from Top to down construction Top - Down Method

26. Figure: Connection of the groove and tongue between the diaphragm wall and the concrete slab and the execution of the dewatering Top - Down Method

27. II. Stages of Tunnel & 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: Top - Down Method

28. 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 piled wall. This retaining wall is ultimately the final wall of the tunnel structure. Top - Down Method

29. 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. Top - Down Method

30. Figure: Excavation after strut to the level of the roof of the structure Top - Down Method

31. 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. Top - Down Method

32. 4) Repeating the previous stage until reaching the foundation level  Stage 3 continues until reaching the foundation level Figure: Execution of all roofs until reaching the foundation level Top - Down Method

33. 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. Figure: Completion of foundations and inner structural walls Top - Down Method

34. 6) Final embankment of the upper surface of the tunnel  After the completion of the underground structure, t he embankment operation is performed up to the level of struts and then the struts are removed, and the embankment continues to the ground level. Figure: Completion of foundations and inner walls and embankment of the upper surface of the tunnel Top - Down Method

35. ANY QUESTIONS PLEASE ?? Source: Various reading materials