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Piles are used to distribute loading of a structure to the ground. While different pilling systems can be chosen for the foundation work. With an appropriate selection of pilling system, construction can be completed in an effective and efficient way.
This report first investigates the selection criteria of pilling methods and then studies some commonly used pilling systems in Hong Kong. In addition, case studies are provided for the pilling systems, including driven H-pile, large diameter bored pile, mini-pile and pre-bored H-pile, discuss in the report. Figure 1 shows the general classification of deep foundation.
Bored pile is a kind of replacement pile. It usually formed in-situ and is rotary or percussive in nature. Bored pile often uses to construct medium to high-rise buildings in Hong Kong. There are two types of bored pile, including small diameter bored piles with a diameter of equal or less than 600mm and large diameter bored piles with more than 600mm in diameter. Large diameter bored pile is commonly use in Hong Kong. Its typical sizes are 800mm, 1m, 1.2m, 1.5m, 1.8m, 2m, 2.2m, 2.5m, 2.8m & 3m. Its length up to about 80m and can load up to about 45,000kN.
Before construction of a pile, pre-drilling site investigation needs to be conducted to check the founding level of the pile with the designed one. If there is no significant differences between them, construction of the pile can be started after getting the approval from the Buildings Department.
In general, the construction process of a bored pile can be divided into seven steps as follow:
Forming a bellout
Cleaning with a cleaning bucket
Integrity tests and loading test
Firstly, temporary casing will be pushed into the ground either by i. sinking with a vibrator, ii. an oscillator or iii. a rotator. Secondly, boring can be achieved by a rotary auger, grab and chisel or reverse-circulation drill (RCD) until the designed depth is attained. Thirdly, a bellout will usually be formed at the bottom of the pile to enlarge the surface area for distribution of loading. After cleaning the borehole with a cleaning bucket, reinforcement (i.e. steel reinforcement cage) can be placed with spacers to maintain concrete cover. Concrete will then be poured by tremie pipes to prevent segregation. Figure 3.2.2a illustrates the construction process of a bored pile using grab and chisel.
Figure3.2.2a Construction process of a bored pile using grab and chisel
Lastly, Integrity tests and loading test need to be conducted to inspect the quality of the pile. Non-destructive integrity tests such as Sonic logging and Echo tests can help to find out structural soundness, for example existence of honeycomb and compressive strength, of the pile. Figure 3.2.2b shows the patterns of the reflected waves from different pile situations. Loading test should also be set up to identify the settlement of the pile under loading.
Figure 3.2.2b Examples of Sonic logging test results
Source: Foundation Design and Construction, 2006, p. 261, GEO, CEDD, HKSAR
Application in Hong Kong ââ‚¬" Large Diameter Bored Pile
Large diameter bored pile is often used for foundation in Hong Kong. This part investigates the construction process, advantages and disadvantages as well as the reasons of using it with a case study.
Bored piles are used for foundation of tall buildings and viaducts in Hong Kong as to support the heavy column loads of buildings and highways structures. In most of the cases, large diameter bored piles are used because of its huge load-bearing capacity and low environmental impact. Some recent projects using large diameter bored pile include:
Type of pile
Tung Tai Cottage Area East
Large Dia. Bored Pile
China State Construction Engineering (HK) Ltd.
Large Dia. Bored Pile
Route 9 Lai Chi Kok Viaduct
Large Dia. Bored Pile
MTRC Kowloon Station Development
Large Dia. Bored Pile
Chun Wo Foundations Ltd.
Figure 3.2.3 Recent projects using large diameter bored pile in Hong Kong
Advantages and Disadvantages
There are several advantages that make large diameter bored pile an effective pilling system. In term of flexibility, one of the advantages is that it can almost be constructed in any type of soil or rock due to the mass use of machines. Different kinds of obstructions underground can nearly be broken up easily by machinery. In addition, length of the pile can be varied easily and instantly on site. Moreover, large diameter bored pile can also be installed in conditions of low headroom.
Concerning the environmental aspect, the pilling process of it generates less noise, vibration and no ground heave compared to displacement piles (i.e. driven H-pile). Neighboring buildings and facilities such as dwellings, school and hospital will not be affected by the noise and vibration during pilling.
On the other hand, there are disadvantages which restrict the efficiency of pilling when using large diameter bored pile. One major problem is that the installation of it is much more complicated and time-consuming than H-pile, for instance, the boring process takes around. Furthermore, it occupies a large site area during construction and obstructs others construction process of the structure. Settlement may also be occurred due to the ground loss during boring.
In order to have a deep investigation on the application of bored pile in Hong Kong, a construction site that undergoing installation of large diameter bored pile was studied. The site is located on Lee Tung Street and McGregor Street in Wan Chai. Site inspections took place on 1st, 2nd, 4th, 6th and 13th October 2010. The site is undergoing diaphragm wall works and foundation works during site inspections. Photos and videos are taken in buildings and park nearby. Figure 126.96.36.199 shows the general views of the site.
Figure 188.8.131.52 General views of the site
The Figure below states the location, size, total gross floor area, completion date, proposed land uses, developers, contractor and pilling method used of the site.
Lee Tung Street/ McGregor Street (URA project)
Lee Tung Street/ McGregor Street, Wan Chai, HK
on or before 2015
Residential, Commercial, G/IC and Open space
Sino Land, Hopewell Holdings Ltd. and URA
Gammon Construction Ltd.
Large Diameter Bored pile
Figure 184.108.40.206.1a Site information of Lee Tung Street/ McGregor Street
The site is divided into two parts, which are Site A and Site B. Site A is located on lee Tung Street while Site B is located on McGregor Street as shown as Figure 220.127.116.11.1b.
Figure 18.104.22.168.1b Site Plan of Lee Tung Street/ McGregor Street
This part investigates on the construction of the large diameter bored piles in the site. The first observed step was to insert temporary steel casing by pushing it into the ground using oscillators and rotators. The oscillator oscillates in a to-and-fro manner by its two arms while the rotator rotates the casing to reduce frictional resistance between the soils to push the casing down as shown in Figure 22.214.171.124.2. Casing is used for temporary support, drilling rod, alienation of the drilling process and prevention of the soil to collapse and water to penetrate.
Figure 126.96.36.199.2a Oscillator and Rotator used in the site
The second observed step was boring by casing support technique and slurry support technique. The casing support technique uses a rotary rig or, in this case, grabs and chisels to excavate within the steel casing. After removal of the soil, rock was excavated under a drilling fluid (bentonite slurry) with the use of a reverse-circulation drill (RCD). Reverse-circulation drill uses drilling rod and auger head to break up the rock and remove them from the borehole by pumping out the slurry. Figure 188.8.131.52.2b shows the reverse-circulation drill and the auger head used in the site. Slurry re-circulates back to the borehole after sedimentation using a sedimentation tank as demonstrated as Figure 184.108.40.206.2c. Normally, a bellout will be formed at the base of the pile after drilling, to increase the bearing area of the pile and distribution area of the load.
Figure 220.127.116.11.2b The reverse-circulation drill and the auger head
Pumping out the slurry
Re-circulating back the slurry
Figure 18.104.22.168.2c The sedimentation tank
The third studied step is to place steel reinforcement cages inside the borehole to provide resistance to tension. Reinforcement cages are usually constructed in situ. Concrete is then poured to the borehole using tremie pipes. The tremie pipe outlet should place below 2m from the concrete surface to prevent concrete contamination. Temporary casing needs to be extracted while concreting. Figure 22.214.171.124.2d shows the tremie pipes used during concreting and Figure 126.96.36.199.2e illustrates the removal of casing.
Figure 188.8.131.52.2d Tremie pipes used in pouring concrete
Figure 184.108.40.206.2e Removal of temporary casing
Special Application - Basement
In the present case, basement will be constructed after completion of foundation work in some of the site area. In addition, it is observed that diaphragm wall works are under progress in part of the site area. Because of the future basement construction, the installation process of the large diameter bored piles in the basement area is different from non-basement area. For instance, concrete is poured at the bottom to form a base rather than placing a bellout. Steel stanchions replace steel cages to place in the borehole after boring as shown in Figure 220.127.116.11.3. Moreover, backfilling instead of concreting proceeds after placing steel stanchions as soil will be excavated in the future.
Figure 18.104.22.168.3 Steel stanchion used in basement construction
Reasons of using large diameter bored pile
In the current case, large diameter bored pile was used as the foundation pilling system. Despite its time-consuming and expensive price, there are several reasons to support the decision. The major reason is the low impact to the environment and neighbor. The pilling process of it generates less noise, vibration and no ground heave compared to driven H-pile. This feature is significant to such a large site which many piles need to be installed and surrounded by considerable amount of buildings and communal facilities such as residential, school and park.
In term of flexibility, large diameter bored pile can almost be constructed in any types of soil or rock due to the mass use of machinery which can easily break up different kinds of obstructions underground. Moreover, the high load-bearing capacity of large diameter bored pile is efficient in construction of high-rise building. Concerning the construction of basement, piles with large diameter need to be installed to bear the huge loading and place in the steel stanchions.
In short, large diameter bored pile is one of the most commonly used pilling system in Hong Kong due to the often constructions of skyscraper and highway which required vast load-bearing capacity.