Pre Bored Steel H Piling System
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Published: Thu, 11 May 2017
Although Hong Kong is featured of its Skyscrapers all over the world, and it seems that H-Piling system may not have enough bearing capacity to their superstructure, there are still many servicing buildings and small-scale infrastructure developing, the loading of which is capable to be borne by H-Piling system, in the city. In the following words, a brief introduction over Pre-Bored Steel H-Piling system will be presented associated with related cases applying such kind of piling system in Hong Kong in order to draw a more detailed picture over the application of Pre-Bored Steel H-Piling system in Hong Kong.
The principle of Pre-Bored Steel H-Piling system is similar to the Bored Piling system. Both of them are replacement pile and require boring the initial hole on the site for the insertion of the temporary or permanent steel casing, depending on its design. After the completion of the boring process, the soil and rock inside the steel casing will then be extracted and the workers will place the reinforcement material inside the steel casing. That is the major variation between Bored Piling and Pre-Bored Steel H-Piling System. For Bored Piling, the reinforcement bars will be bent in order to form the circular cage (Figure 1). However, H Steel Pile will be the reinforcement material in Pre-bored H-Piling system. Grouting is then started after placing reinforcement material to form the pile.
Since the diameter of Bored Piling can be larger than Pre-Bored H-Piling, the former one can typically take up more loading than the latter one. Therefore, small-scale construction project is most likely to use Pre-Bored Steel H-Piling system in Hong Kong. The cases introduced later are small in scale, but they do play an important role in our city.
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Figure 1: Reinforcement Bar for Bored Piling
Figure 2: H-Pile for Reinforcement in Pre- Bored H-Piling system
The first case being examined is the construction of Queen Mary Interchange near Queen Mary Hospital and Pok Fu Lam Road. It is a road improvement project initiated by the local government at the two junctions, which are in Sassoon Road and Bisney Road with Pok Fu Lam Road, on Hong Kong Island. The aim of this project is to relieve the congestion problem existing in these roads by constructing a new interchange, comprising of several bridges.
Figure 3: Model of proposed development
Figure 4: Queen Mary Interchange ââ‚¬” Still in the progress of construction
As we can see in the model of proposed development (Figure 3), the interchange, which located at the bottom of the figure, is locating on a slope. Since the the soil in slope exert a considerable amount of bending and tensile stress, according to Tomlinson (1994), H-pile is able to withstand such kind of extreme situation due to its unique H design. Therefore, the authority concerned decided to build the interchange with Pre-Bored Drill-in Steel H-Piles. Moreover, the short erection time and economical construction cost are also the reason to make use of such kind of piling system in this project. According to Kumaraswamy, Mohan (2002), the whole process of developing a Pre-Bored drill-in H pile, i.e. pre-drilling, drilling and grouting, can be finished in 3 working days. We can also see that the site is not so spacious enough to place some large-scale machinery during the pile development. The development of Pre-Bored Drill-in Steel H-Piles can overcome this problem because the diameter of this pile is only 600mm in this case The size of drilling techniques can be controlled and fitted into the congested site.
Figure 5: The congested site on a steep slope
in this construction of Queen Mary Interchange
Before the development of pile in this project, several raw materials and tools are required to present. Here is the table showing what are needed,
Circular steel casing of internal diameter 600mm and thickness 25mm
Steel H-pile, which will be inserted in the steel casing
Hydraulic hammer mounted on crawler crane
Table 1: Major Material list for constructing pile of Queen Mary Interchange
In the very first stage of developing Pre-Bored Drill-in Pile in this project, engineers will have a pre-drilling through the soil layers and rock within the site in order to identify the underground condition, such as the rock head levels and the strength of the soil. It is an important procedure since it will determine the type, length, etc of the pile. Any wrong measurement, especially overestimate the soil/rock supporting capacity, will make the whole project lose ground because the whole structure will be ruined, the typical case is the Block E and F of Yu Chui Court, demolished after discovering the corruption scandal, which developed by piles shorter than the standard and cause possible dangerous to its users .
After the completion of soil investigation and analysis, the engineers would then design and choose the most suitable piling system for different project. In the construction of Queen Mary Interchange, the engineers found out that the Pre-bored piling system is suitable for construction of high-way bridges at the steep slope on the site. The site-workers firstly pressed the circular steel casing inside the ground through the help of Hydraulic Hammer which is mounted on a crawler crane. The reason to plug the steel casing into the ground was to prevent the soil from collapsing inside the hole, which would be placed H-pile inside it, during the process of drilling.
Figure 6: Inserting circular steel casing into ground by hydraulic hammer mounted on crawler crane
Figure 7: Connecting two steel casings by welding when the length is not enough
The process in Figure 7 happened when the steel casings being installed were not long enough to reach the target depth into the soil. By welding the end of two steel casings, the total length of it can be extended. For the inadequate length of H-Pile, their length can also be extended through welding.
When the steel casing reached the bed rock level and formed a socket, the next stage is to remove those soil and rock inside the steel tube, as shown in Figure 8. The ground was drilled with a hammer mounted on a drilling rig so as to remove the obstruction underground. This process could enable the hard rock and pressed soil to become extractable out of the steel casing. In this project, the obstructions were then extracted out by flushing the case with water and compressed air. The compressed air acted at the initial force and the water acted as a transferring medium to carry out the obstruction out of the earth. The above two procedures will take turns to start until all of the obstruction inside the tube were wholly carried out.
Figure 8: Removing obstruction by drilling with down-the-hole hammer mounted on drilling rig
Figure 9: Withdrawing soil from inside the casing by flushing with water and compressed air
The H-Piles would then be installed inside the empty steel casing after the clearance of obstruction inside. Meanwhile, a 20mm diameter grout tube was also placed to full length of the pile in order to prepare its grouting process later on.
Figure 10: Installing steel H-pile inside the empty steel casing
The grouting process is to have a re-bonding between the soil and the H-piles with the suitable grade of concrete by tremmie method from the bottom to the top of the casing. Tremmie method is a concrete-pouring method in some dry spaces underground deeply. Since a tube is used to pour the concrete from the bottom, the force of falling concrete can be controlled through shortening the distance of freely movement of the concrete. It can prevent the concrete from improperly mixed when it is placed at the bottom of the casing which result in voids and air packet which can cause possible danger to the whole structure.
The temporary steel casings were extracted gradually when the fresh concrete was pouring inside since the bonding force of the concrete is not strong at this moment. However, engineers maintained the bottom of the casing at a level of at least 2 meters below the grout surface to ensure that there was enough time range for the concrete to form the circular shape. The grouting operation was stopped when the uncontaminated grout flowed out of the hole to ensure the purity of concrete poured inside the casing.
Figure 11: Grouting the steel casing with suitable grade of concrete by tremmie method
Figure 12: Extracting the steel casing while grouting is going on
Figure 13: Grouting the steel casing with suitable grade of concrete by tremmie method
Figure 14: Extracting the steel casing while grouting is going on
The finishing of grouting process means that the whole development of Pre-Bored Drill-in H-Pile system is come into the end soon. After the dewatering process of the concrete attached to the H-Pile itself, the drilled-in H-Pile was finished. Figure 13 shows the product of the above stages. It is a group of drilled-in H-Piles that were formed a combined pile cap for the highway bridges in Queen Mary Interchange. To verify its bear capacity, loading test was taken place in order to test its vertical deflection. Once the test was passed, the superstructure developments were then taken place.
Pre-Bored Steel H-Piling system is not only applied in local road constructions, but it is also applied in some small-scale construction of buildings in Hong Kong. There is a typical example, which is the construction of Sheung Wan Stormwater Pumping Station (SWSPS) on Hong Kong Island. It is a single-storey building with a underground water tank with 11 meters deep at the Chung Kong Road, which is the waterfront of Sheung Wan. This project was to alleviate the flooding problem in Sheung Wan.
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Figure 15: Sheung Wan Stormwater Pumping Station
Figure 16: Location Plan of Sheung Wan Stormwater Pumping Station
According to the paper presented by Leung,Li and Leung (2009), the site of the Sheung Wan Stormwater Pumping station was reclaimed in 1982. The material used for reclaimed it are as followed,
Fill (medium grained granite)
Completely Decomposed Granite (CDG)
Originally the engineers decided to build up this service station with 60 numbers of large diameter bored piles, and the diameter of which are ranged from 1.2 meters to 1.5 meters.
Figure 17: The Original Bored Pile Layout Plan of SWSPS
Figure 18: The Geological Section Across the site (LandÃ¯Æ’ Seawall)
But later on, the contractor of this project advised the government to build up the superstructure and sub-structure over a combination of rock socketted H-piles, mini-piles and raft foundation. The government finally accepted the advice from the contractor and starting the construction process. Referring back to the Figure 17, the PC5 Zone, all of the piling would be changed into the Pre-Bored H-piling system. For the other zoning, mini-piles were used.
The major reason to change the piling system, as the paper disclosed, was because of the cost effectiveness. First of all, since the diameter of the steel casing used in Pre-Bored Steel H-Piling system is far smaller than the bored piling system. The grouting process of the former one will consuming less amount of concrete than the latter one, which can reduce the cost of development.
Table 2: The difference in pile foundation between the initial and final plan
Besides the cost consideration factor, the time used for developing these two different kinds of piling systems is different. As what Kumaraswamy, Mohan (2002) said, the whole process of developing a Pre-bored H-piling system only cost around 3 days. However, the development of a foundation pile normally takes up more than 3 days. Since the amount of pile developed in this project is large, i.e. nearly 50 piles are changed from bored pile to pre-bored H piling system. The time cost saved through the final plan is considerable.
In the aspect of technical operation, the required pile capacity of the pumping station is 6900 kN. For bored piling, For bored piling (The government of HKSAR,2006), typical sizes of these piles range from 1 m to 3 m, with lengths up to about 80 m and working loads up to about 45 000 kN. When high grade and heavy steel H-pile section is used, the load-carrying capacity can exceed 5 500 kN. We can see that the loading capacity of bored piling is far more than the loading requirement of the pumping station. However, we can also discover that a maximum of 1400kN was not be able to supported by Rock Socketted Steel H-Pile. The solution, as shown in Figure 18, is to place in 4 numbers of T50 steel bars inside each of the casing for reinforcement so as to remove the discrepancy.
Therefore, the final plan can enable a more efficient use in the resource than the initial one. That is why it is reasonable for the government to change its decision.
Figure 19: Typical detail of Rock Socketted H-piles of Alternative Design
For the developing processes of the Pre-bored socketted H-piling system, most of them are the same as the Pre-bored Drilled-in piling system. The major difference between them is that the steel casing used in Pre-bored socketted H-piling system is permanently fixed inside the soil, while the steel casing used in Pre-bored Drilled-in piling system was temporary in used. They are removed during grouting. In this project, the rock socket H-piles were installed through the help of concentric overburden drilling system with air-flushing. It uses a rotary-percussive action for drilling and uses air flushing for removal of debris before casting in the H-pile inside the tube and grouting.
In conclusion, the loading bearing capacity of Pre-Bored Steel H-Piling system is relatively lower than the other common piling system used in Hong Kong. However, due to its mobility during installation and its strength, it is still also commonly applied in some small-scale development in local projects.
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