ABSTRACT: Work within ground is always a nightmare for engineers especially where the site is situated in congested urban environment. For the Lee Garden Hotel Redevelopment project, situations were much more complex for quite a lot of almost unprecedented conditions in ground works were to be faced. This paper tries to highlight those difficulties encountered in the project and the methods taken to tackle the problems.
1. INTRODUCTION
The project located in a 5,750 m2 site, which was abutted on 3 sides to small roads from 12 to 20m wide, and the remaining side adjoining a 17-storey residential building of 28 years old. In addition to the congested environment, the project also required to demolish the 22-storey Lee Garden Hotel, with a 2-level basement structure in it. This paper concentrates on the methods and work arrangement for the substructure only.
In the redevelopment, a new 50-storey office building, with total GFA of 83,860 m2, constructed in structural steel with a RC core, together with a 4-level basement, was to be built within a construction period of about 50 months, from demolition till final completion.
There were quite a lot of technical difficulties needed to be overcome in the project. For example, almost all the new foundations and the required ground strengthening and permanent basement supporting works had to be carried out in the old basement before it could be demolished. As a result the old basement could only be demolished in small sections to allow for room and to cope with other associated works. At the same time when part of the basement was being demolished and cleared, temporary or sometimes permanent supporting structures have to be built as soon as possible to infill the void until the old basement was completely replaced by the new. Meanwhile, in order to gain more time, provisions were also made for the construction of the future building including the central core in RC as well as part of the new basement constructed in top-down method. Due to such constraints, it is comprehensible that limited mechanical plant could be used during the entire process.
2.
DIFFICULTIES AND UNIQUENESS OF THE PROJECT
Though project of this kind is not exactly uncommon in Hong Kong, the following features, however, still make the redevelopment of the Lee Garden quite unique and thus imposed certain technical difficulties.
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2.1 |
A 2-level old
basement structure covered the entire site area was to be
demolished. Basement structure of this size was rather big when
comparing to other of the same type, and demolition was quite a
difficult job in itself. |
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| 2.2 | A 4-level
basement of the same size was to be built to replace the old.
Besides, a 5-level podium covering the entire site area was also
built, that made the site extremely lack of working space during the
construction processes.1 |
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| 2.3 | Due to the old
basement could not support those heavy plants which were required
for the construction of the foundation using other mechanical means,
hand-dug caissons were being used in this case. |
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| 2.4 |
For example, there were at least 4 major phases in the demolition of the old basement, so as to
allow for the construction of the caisson wall, the central building core, the capping beams and the final ground floor slab in small sections at various scheduled time (see attached demolition phasing plan). Therefore, the phasing and junctioning arrangement to accommodate the required works efficiently and safely was thus very complicated. |
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| 2.5 | While doing the major substructure and ground works, provisions and the actual construction of part of the future building structure were carried out at the same time. Examples of some major activities of this included the construction of the foundation and cap for the central core of the future tower, and the erection of steel stanchions on top of the caisson foundation as column support to facilitate the construction of the future building during the top-down basement construction. |
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| 2.6 |
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1 A recent comparable project is the Hilton Hotel Redevelopment, in which a 2-level basement was required to demolish and replaced by a 6-level new basement. However, the size of the site (about 2,600 m2) is smaller and without an overall covered podium. |
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| 3. | WORKS BEING DONE TO FACILITATE THE CARRYING OUT OF THE DEMOLITION OF THE OLD |
| ¡@ | BASEMENT AND THE CONSTRUCTION OF THE SUBSTRUCTURE |
In order to have the old basement demolished and replaced by the construction of a new, as well as to make way and facilitate the construction of the new 50-storey tower block, the following works were being carried out that comprised the whole contract. (see chart below showing the approximate work schedule)
Schedule showing the approximate time relationship between major works/activities
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 Photo 3 |
3.1 Demolition support and ground stabilization
- A series of temporary supporting system mainly in the form of steel shore and bracing were erected before various stages of basement demolition. (see Photo 3)
- Form a grouted wall along the site perimeter down to 3m below bedrock level (average 25m deep) as a means of ground water control during excavation and construction of caissons. To achieve this, 470 nos. of bore holes in two rows were drilled and pressure grouted afterward.
- Demolish part of the basement slab along the perimeter wall to give way for the construction of hand-dug caissons, which were used later as cut-off wall for the new basement structure. Totally 244 caissons of 1.2m diameter were constructed for the purpose. (see Photo 4)
 Photo 4 |
3.2 Construction of new foundations
- Excavate and construct the hand-dug caissons as foundation for the future building within the old basement. Totally 49 caissons with diameter ranging from 1.6m (for podium structure) to 5.0m (for main tower) were constructed. (see Photo 5)
- Demolish the central part of the old basement to provide working space for the carrying out of the foundation and cap for the future building core. (see Photo 3 and 6)
- Construct hand-dug caissons and the caisson cap that supported the central core of the new building. (see Photo 7). Excavation and construction of the 5m diameter caissons was done at the bottom of the pit that formed after the demolition of the centre part of the old basement. After the completion of the caissons, another pit for the construction of the cap would start. The pit was supported by soldier piles and lagged with mild steel angles. The entire system was further strutted and braced by universal beam sections. This cap sat on 7 caissons and was 8m in depth. Due to the massive size and weight, it helped to hold down the entire basement from up-heaving during demolition and excavation processes.
3.3 Changing over and provisional works for the new building
- Demolish the old basement structure along the perimeters and build the capping beam on top of the caisson wall. (see Photo 8)
- Demolish part of the old basement structure for the erection of steel stanchions as column support to future building. These columns could enable the superstructure be constructed at the same time with the basement that was built using top-down method. Rows of couplers were provided at basement floor levels to allow for necessary connection of the basement slab to the steel columns at a later stage. (see Photo 9)
- Construct the building core from the cap up to ground level. Since there was very limited working space within the central pit, traditional timber panel formwork was used in the construction of the core. The core would act later as a lateral support for the ground floor slab which served also as a separating plate to facilitate the construction of the top-down basement. At this moment, only half of the old basement was being demolished. The construction of the core was done in the central area with the remaining basement structure still under demolition at the same time. The problem of accessibility, clearing of debris formed by demolition and the assurance of safety etc. were the most headache part of the works at this stage. (see Photo 10)
- Demolish the remaining old basement, section by section, and covered the space immediately with the new ground floor slab. Phasing and junctioning arrangement was the most difficult part of work here. Finally the floor slab would infill the area between the central core and the capping beam. (see also Photo 1 and 10)
3.4 Works merging with the main contract
- Erect the climb form system for the continual construction of the central core. (see Photo 11)
- Construct the basement using top-down method. In order to allow excavated material could be removed conveniently, two outlet points were provided on site. One was the vehicular exit to the future basement carpark which located at the western end of Lan Fong Road. The other exit point was a temporary opening formed on the ground floor slab near the western corner of Hysan Avenue. A grab lifter was set up on top of the opening for the removal of soil. (see Photo 12). As usual top-down construction does, excavation started from the top level downward until it rearched a certain depth, say 4 to 5 meter depending on the headroom of the basement slab, where it would be shored and strutted. Within that depth, basement slab would be constructed that connected to the central core, steel column and the caisson wall with couplers that were provided in advanced. The works repeated until it reached the bottom level of the basement where the caisson caps and other ground beams were then constructed.
4. CONCLUSION AND REVIEW OF THE METHODS USED IN THE CONSTRUCTION
It can hardly say the basement construction of Lee Garden project was a great success. There is no any perfect solution in the carrying out of this kind of project under such environment and time constraints.
Generally speaking, the substructure of this project could be sub-divided into two distinctive stages. The first was the preparatory stage during which the old basement was shored and protected with the cut-off wall and caisson foundation being carried out. The second was the shifting over stage during which the old basement was being knocked down and replaced by the new ground floor slab that acted also as a lateral support as well as a separating plate for the future top-down basement construction. At this stage it also included the construction of the central core for the future building tower.
The first stage of works was being done accordingly under an usual way as most similar project did. However, the arrangement of the second stage of works was considered a bit too hasty for two fairly dangerous operations (demolition of the remaining portion of the basement and the construction of the central core wall) were scheduled to be done at the same time in close proximity. Since the final stage of the demolition had broken the remaining basement into several practically inaccessible portions, works could hardly be done in a reasonably safe manner. Though contractor had made provisions to ensure safety and efficiency of work, such as to have that section of ground floor slab leading to Hysan Avenue at the earliest possible to make accessibility easier into the central core area, as well as to install bamboo scaffold with dust screen to protect the works within the central core. It still rendered a large area of site unsafe by exposing two actively progressing areas to unavoidable hazardous environment. Some of the attached photos for sure can well illustrate the situation.
