One of the excavation stabilizing methods is Top-down method, which different from methods such as the tieback, bracing by cast in-situ pile, and sheet pile, etc.
It is one of the new executive methods that is a safe and applicable method in cases where the tieback methods are not practical (it is not possible to obtain satisfaction from the neighbors, etc.).
This method has a great role in reducing the execution time due to its high construction speed, because it is possible to implement structures above the ground level with the advancement of basement floors simultaneously in this method.
In this method, the basement retaining walls are placed first. In most cases, these retaining walls are diaphragm walls. Excavation is then done under the roof slab and strata are installed to keep the retaining walls. The roof slab is then built and an open space is provided on the slab to continue the work. The roof slab not only provides an anchor on top of the excavation but also acts as a noise barrier. The next slab is made and this process continues until the end to complete the floor slab. The side walls are then raised to the top, the middle struts are removed and the open access space on the roof slab is covered. (Land transport and authority www.lta.gov.sg)
Explain the method or top-down construction:
Top down method is a combination of executive technique along with geotechnical and structural calculations. The main structure of buildings and underpasses and ramps can be designed and implemented without the need for a temporary maintenance system using this method. This design can be used in urban structures for buildings with deep and semi-deep excavations at the level under the foundation of adjacent buildings, as well as trenches from the construction of ramps and underpasses.
The purpose of this method is to use a system that can be part of the main structure and at the same time provide the stability of the trench during and after the excavation operation and also have a relatively high ex
ecution speed. In this method, the main columns of the structure, with any distance from each other, act as a trench support system, and also the composite wall that is placed between the columns acts as a permanent retaining wall.
The use of behavior mapping methods allows the actual behavior of the soil to be controlled and if there is
an error in the results of geotechnical experiments, according to the results of be
havior mapping and compare it with the design results, it is possible to react quickly and take the necessary measures. One of the unique features of this plan is the flexibility and compatibility of the system with the surrounding environment and the space inside the main structure. Most importantly, after the completion of the excavation and the implementation of reinforcements according to this design, in fact, most of the structure has been executed, which will significantly increase the execution speed and thus reduce the subsequent costs.
Top-down Computational Basics
In this method, by simultaneously using geotechnical structural calculations, the design needs are met.
Simultaneously, geotechnical calculations are performed based on the criteria of providing sufficient reliability to control the displacement of adjacent walls in different stages of this method.
The advantages of the top down method
1- Simple execution capability and no need for special equipment
2- High execution speed and reduction of executive costs
3- Ensuring safety during implementation and after implementation
4- Do not interfere with the main structural system
5- Improving the performance of the main structure and meeting the design needs of the project
6- Starting the execution
of the upper floors before the complete excavation and execution of the foundation
7- It has a higher reliability than the usual methods because of the implementation of the retaining wall from top to down.
The implementation of the system designed in this method includes 7 main parts and 4 sub-parts. The main components of this system include:
1-Wells under the main columns,
2- Pile reinforcement cage,
3- Column confining area in the pile,
4- Composite beams between the columns adjacent to the trench, 5- Wall reinforcement mesh, 6- Shuttering the wall, 7 – The main beams of the structure.
Sub-components of this system include:
1 – Bracing gusset plate, 2 – Bracing members, 3 – Waterproof insulation layer 4 – Geodetic pins on the wall. Each component has a specific application and is designed with a specific purpose in the system
The basis of the work is such a way that the main struct
ure is drilled under all the columns to a certain depth from the foundation (which is determined by geotechnical calculations) like cast in-situ piles. Reinforcement cage of piles are then installed up to the foundation and concrete is poured to a certain depth from the foundation. After that, the main columns of the structure, in which the shear key elements are installed in the buried part, are placed inside the pile along with a column base plate. After that, the excavation operation begins at depths of 1 to 1.5 meters. In each stage, the area between the piles is shuttered and concreted
by installing horizontal beams and also installing reinforcement mesh between the trench and the beams and columns. After two stages of drilling and consolidation, the roof beams are installed between the columns so, all the columns are connected to each other.