TOP CONSTRUCTION TECHNOLOGY – DOWN

Currently, High-rise buildings are springing up more and more, It is not only convenient for people but also suitable for large cities with limited construction area.

So, Basement jobs in high-rise buildings are indispensable.

Today, ACI would like to share with customers about Top Construction technique – Down is applied in the construction of the basement!

1) TOP CONSTRUCTION TECHNOLOGY – WHAT IS DOWN??

Construction Technology Top – Down (from top to bottom), English is Top – Down Construction Method. Is the construction technology of the underground part of the house, top-down method. Different from the traditional method: Construction from the bottom up.

Construction Technology Top – Down

In construction technology Top – Down, one can simultaneously construct the underground floors and the foundation of the building, just built a limited number of floors, belonging to the body.

This technology appeared in Vietnam a few decades ago. The first project is Harborview - Nguyen Hue built at the end of the year 1993 and the beginning of the year 1994 then to Saigon Center and many other works.

2) ADVANTAGES OF TOP CONSTRUCTION TECHNOLOGY – DOWN

Construction Technology Top – Down brings the following advantages:

• Because there is no need for a large foundation excavation area, it is easy for construction whether the site is large or small. Furthermore, This technology also helps to speed up the construction progress and save the cost of making diaphragm walls.
• Top-down construction technology helps to re-establish the road surface soon, Avoid affecting traffic.
• Can be constructed in combination with Up – Up body and Top – Download the underground part. This saves construction time, speed up construction. Reality show, with Top . construction technology – Down can help shorten the progress 30% – 50% total construction time.
• There is no need to use a temporary support system to support the tunnel wall during the excavation and construction of the basement and there is no additional cost for the auxiliary system..
• No spear system cost, formwork for floor beam structure.
• Solve foundation problems such as in high-rise buildings that need to be excavated with diaphragm walls, The foundation is deep and the groundwater level is lowered to construct the underground part that traditional construction has not been able to do.

3) Disadvantage of TOP CONSTRUCTION TECHNOLOGY – DOWN

In addition to the outstanding advantages, the construction technology of Top – Down still has some limitations:

• Complex basement column structure.
• The connection between floor beams and columns is difficult in construction.
• Requires construction workers to have a lot of experience.
• Difficult to implement mechanization due to construction in closed space.
• Construction in closed space should greatly affect the health of workers.
• Ventilation and artificial lighting must be installed.

4) EQUIPMENT AND MATERIALS NEEDED FOR TOP . CONSTRUCTION – DOWN

4.1) CONSTRUCTION SERVICE EQUIPMENT

With underground excavation work including earthmoving machine, The leveling machine and the ground roller are both small, manual excavation tools and drilling machines.

For transportation work, it is necessary to prepare two KOBELCO cranes 7045.

For other work, preparation is needed 2 pump, 2 Steel ladders are located at both up and down entrances, Underground lighting system.

With concrete construction work: concrete pumping station, Commercial concrete trucks and other concrete construction equipment.

4.2) MATERIAL

Temporary Steel Support

When constructing a basement using the top down method, steel columns must be used to support the basement floors. If both the body and the tunnel are constructed simultaneously, the temporary reinforcing steel must be able to withstand the additional load of the body.

The temporary supporting steel bars will be covered with concrete to become the load-bearing columns of the project. Therefore, it is necessary to calculate reasonably and must have separate regulations. Steel columns must be properly placed in the bearing position.

Temporary support steel columns in Top . construction – Down

Concrete

Due to the requirement of continuous construction and the need to remove the formwork early to continue digging, it is necessary to use additives to accelerate the hardening ability of the concrete..

Methods to help concrete quickly reach the required strength in a short time such as::

• Using plasticizers, Superplasticizer to reduce water ratio but keep slump required to increase concrete's strength.
• Use growth additive with rapid strength to achieve above 90% design strength within 7 day.

When constructing columns and hard walls, it is necessary to use concrete with expansion additives to patch the ends of columns, core bridge where adjacent to floor beams. The amount of swelling additive used a lot should be 5% arrive 15% compared to the amount of cement. Besides, Do not use group powder or gas-generating agents to expand concrete because it causes corrosion of reinforcement.

Concrete floor must be waterproofed by effective methods because this is the place adjacent to the basement. If it leaks after being put into use, it will be very difficult and expensive to repair.

5) TOP CONSTRUCTION PROCESS – DOWN

Construction process of Top – Down

We would like to introduce to you: Top Construction Process – Down applies to Works with 2 basement B1 and B2.

If the Construction has a greater number of basements 2, then the Construction Process of basements is similar to basement B1 and the last basement is similar to basement B2.

Step 1: Construction of Pre-Placement of Shaped Steel Temporary Support Poles

• The temporary pole is designed with a combination of 2l-55 13.7m long and must be placed in front of the column position when constructing bored piles..
• Relocating the center of the column on the ground after the construction of bored piles.
• Use the crane to slowly lower the shaped steel column into the borehole, The crane arm does not move but only rolls the cable drum to avoid touching the column against the borehole wall.
• Shake or use a crane to press the steel column so that it is submerged about 1m deep in the concrete pile.
• Adjust the vertical axis of the steel column so that it coincides with the column axis and fix the vertical column with a temporary support system.
• Pour concrete into the hole to fill the hole about 1 meter and then fill the rest of the hole with sand.
• After being buried, the pile must protrude 2m above the ground. Besides, Construction cranes must always use KOBELCO cranes 7045.

Step 2: Floor Beam Construction 1 (Ground floor)

• Carry out excavation for floor construction 1. Depending on the project, the depth and height needed to dig to place the spear will be different, it depends on the design, demand.
• When conducting tunneling, it is necessary to dig two layers but only move the machine once and each furrow must be 5m wide.
• When constructing floor beams 1 then use the reverse bucket excavator E must be correct with the following parameters: bucket capacity q=0.25m, digging radius R=5m, pouring height H=2.2m, Machine weight Q=5.1T, machine width b=2.1m, digging depth is 3.5m, time 1 period tck=20s.
• Carry out the installation of the formwork of the top down construction technology the same way as for normal floor beams. However, During construction, it should be noted that poles are only used 1 anti-PAL spear is enough, The base of the spear must be placed on a cushion to ensure it does not sink when placed on soft natural ground.
• Concrete must be poured in each subdivision using pumps and must be of the type with quick-setting additives. Therefore, Must check slump before pouring and test sample strength before use.
• The reinforcement system must comply with the design so that the structure can withstand the force. At the same time, pay attention to preservation, hygiene and quality specifications of joints.

Step 3: Construction of Underground Floor B1

• When excavating soil to construct the B1 underground floor, you should use a previously prepared small excavator.
• Construction using top down technology will cause the construction site to lack light, That's why it is necessary to arrange a lighting system to work better and safer.
• If the depth of the land is greater than 4m or more, it must be divided into sections 2 The first round of excavation is 2.5m thick and the next round is 1.5m thick. One thing you need to keep in mind when constructing is that the underground water level is 2.2m deep and must be arranged with a drainage ditch., water collection wells and pumps.
• Construction techniques for the first underground floor are similar to other floors, but space is limited, so care must be taken when transporting concrete.: Weld the structural steel plates of the frame nodes exactly according to the design, Arrange the concrete pump line so that when changing direction suddenly, the pipe does not become clogged.
• After completing the construction of the floor and beams of the B1 underground floor, proceed to assemble the formwork and construct the columns and core of this floor..
• The steel core in the column must be soft, capable of tying each tree to the waiting steel.
• Fabricated belt core includes: 2 half for ease of construction, Therefore, the joints must be long enough and strong.
• Formwork is assembled first 3 thin but must leave the concrete pouring door in the middle and at the top of the column.
• When handling dry joints in columns, pay attention: grout used for joints must have the designed expansion rate, joints must be cleaned before construction and mortar with additives used to spray on joints must be used with a small concrete sprayer with a diameter of 100mm.

Step 4: Construction of Basement B2

• After construction of beams, The floor of underground floor B1 is completed 10 Today, it is possible to remove the load-bearing formwork and construct the B2 underground floor.
• When the floor, Underground beam B1 has enough strength, this system will work as a horizontal bracing system to support the Barrette wall. The calculation checks the strength, The deformation of this system is done when the underground layer B2 is hollowed out.
• Transporting from the bottom up is mainly done by rolling the cable drum, meaning two workers stand in the pit while hooking the crane bucket while guiding it so that the crane bucket does not touch the sides..
• Two pumping stations are located right at the two transport gates on the B1 underground floor to serve ground drainage work. The pump will be placed as follows: One end of the straw is dropped into the hole to collect water, A pipe will be arranged to allow the water to drain safely.
• The drainage ditch system is arranged with a row of piles with a slope of i=1%, 0.5m deep and facing the water collection holes.. The water collection pit is reinforced with wooden boards and pillars, Pour a layer of grade concrete at the bottom 150 200mm thick.

The number of pumps depends on the pumping test method:

• Case 1: The water level in the foundation pit drops quickly, indicating that the pump is too large. Therefore, it is necessary to close the pump to limit the amount of water pumped out. Turn off the pump until the water level lowering speed is consistent with the stability of the soil roof.
• Case 2: The water level in the hole does not decrease, indicating that the amount of water pumped out is low. Therefore, it is necessary to increase pumping station capacity.
• Case 3: The water level recedes to a certain depth and then no longer drops because the water level difference increases.

Before placing the water pump, it is necessary to determine the type of soil on this layer to have appropriate treatment measures. If drainage measures are not effective, a system must be designed to lower the underground water level using a filter system around the structure.

People often use centrifugal pumps because they are suitable for changing working modes.

When digging this layer, you need to pay attention:

• The soil must be excavated from the transport gates first, then expanded horizontally and vertically and must ensure that the ground always slopes towards the water collection hole.. Dig soil from the soil layer close to the floor to avoid falling rocks and soil, causing danger to the construction workers. When you get to the concrete layer lining the upper beam floor, use a crowbar to pry off each piece and then pull the polymer sheet down.
• When digging, ensure a safe distance between workers to avoid accidents.

When constructing the pile cap:

• The core must be transmitted to the underground floor B2 and then destroyed at the top of the pile to the core at the bottom of the station.
• Next is cleaning the rebar waiting for the pile head and shaped rebar. Carry out waterproofing of the pile cap using the concrete grouting method, bitumen or use liquid glass.
• After waterproofing is completed, it is necessary to pour concrete to line the bottom of the station and place steel reinforcement for the pile cap, Welding steel plates to connect shaped steel columns.
• Build the formwork for the pile cap and proceed to pour concrete for the pile cap, Pour columns to the level of the second underground floor floor 2. After the concrete dries, it is necessary to waterproof the basement floor.
• The breaking of the pile head and cleaning of the reinforcement must be done quickly and then organize the waterproofing immediately to avoid leaving it for too long in a humid environment., Cavitation will cause difficulties in construction and affect the quality of joints.

The surveying work of transferring the main axis of the project to the basement is extremely important, so the surveying department only does it correctly with errors within the allowable limit.. To do so, we must use Ni leveling meters 030 of Germany, NA machine 824 of Switzerland.

Top Construction Technology – Down umbrellas are suitable for all construction areas and help speed up construction progress.

Reference links: https://theconstructor.org/construction/top-down-construction/567534/

With the above sharing, We hope to help customers gain more useful knowledge about Top Construction Technology – Down.

Customers can refer to ACI's "Field of Operation" for more information:

Link: https://acicons.vn/linh-vuc/

If you want to know more detailed information or need advice on any other information, please contact ACI Construction Co., Ltd for quick advice and support.!