The Lee Gardens Redevelopment

The project is located in a 5,750 sq m site, which was abutted on 3 sides with small roads 12 to 20m wide, while the remaining side adjoined a 28-year old, 17-storey residential building. In addition to the congested environment, the project also required the demolition of the 22-storey Lee Gardens Hotel, with its two-level basement structure.

In the redevelopment, a new 50-storey office building with a total GFA of 83,860 sq m was constructed in structural steel with a RC core, together with a 4-level basement. It was built within a construction period of about 50 months from demolition to final completion.

There were quite a few technical difficulties which needed to be overcome. For example, almost all the new foundations and the required ground strengthening and permanent 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 cope with other associated works. At part of the old basement was being demolished and cleared, new basement structures were to be built to infill the void as the old basement was completely replaced by the new basement slab. Meanwhile, in order to gain more time, provisions were also made for the construction of part of the future building, which included the central core in reinforced concrete, as well as the central portion of the new basement. Due to such constraints, it is comprehensible that limited mechanical plant could be used in the carrying out of the works.

In order to have the old basement demolished and replaced by the construction of a new basement, as well as to make way and facilitate the construction of the new 50-storey tower block, the works were carried out that comprised the whole substructure contract.

main contractor
Aoki Corporation

foundation
Leung Kee Construction Co Ltd

 

The Challenging Basement Construction


This photo shows the contiguous caissons used which will be used as the cut-off wall for the basement construction. Instead of using the ground as the working base, these caissons function on top of the basement slab at ground floor level. In the photo, part of the basement slab has been demolished for the caisson works. The building on the left is a 17-storey building which is within four metres of the working line.
Basement view of the contiguous caissons. The slab on top of each caisson shaft has been demolished to allow removal of excavated soil. Steel struts are erected to support the basement structure during the demolition process.

Further extension of the demolition. In the photo, about 40% of the basement demolition has been completed. The central pit has been extended to accommodate the layout of the entire 51-storey office tower.
The central part of the basement has been demolished to make way for the caisson raft of the new building's core wall. The steel strut supporting frame seen in the trapezium-shaped core pit stretches from one end of the basement wall to the other, stabilising the entirety of the previous basement structure.

One of the 5-metre caissons within the old basement during the excavation stage. These caissons are used to support the external columns of the 51-storey building, which is constructed of structural steel.
The digging of the 5 m caisson for the building core progresses inside the central pit.

Further excavation from the trapezium-shaped pit. The 5 m caissons have been completed; Following this, reinforcing bars will be placed and the pit concreted to form the raft cap for the support of the building core.
A mobile crane which rests on a temporary platform is used to place the square-section steel columns for the new building; this photo shows the layout of the 15 steel columns. They are anchored to the 5 m caissons and support the external envelope of the new building.

Demolition of the remaining part of the previous basement. A breaking machine is required to carry out demolition work within the heavily shored space.
Close-up of the 6 m deep caisson raft cap for the 51-storey building core.

This photo was taken in March 1995, about 15 months after the demolition and foundation works commenced. At this point, the foundation and caisson cut-off wall had been virtually completed, indicating that the demolition of the remaining basement portions could continue unhindered. Note that the temporary work platform has been relocated from its location of two months previous.
The most critical stage: The demolition of the main basement structure has commenced, while strips of the old basement structure are retained as a temporary platform to handle the debris. Meanwhile the construction of the building core has started.

When the caisson wall is completed and properly shored, a further strip of basement along the perimeter will be demolished, so that the new ground slab can be cast as soon as possible to stabilise the caisson wall.
A closer look at the relationship between the various phases of demolition. The rows of supporting struts, the newly erected steel columns and the partially demolished basement structure render the site virtually inaccessible.

A critical moment: The old basement is being replaced by the new. The central core has been cast and acts as the main stabilising structure. Meanwhile, the ground floor slab needed to be cast, stage by stage, until it covers the entire site. The ground floor slab supports the free-standing caisson wall and acts as a protective plate which separates the site into upper and lower sections, allowing superstructure work and basement work to be carried out at the same time.
The climbing form for the building core is erected as soon as the central core reaches the ground level. Erecting the custom-designed formwork system often takes weeks or even months to complete. This bird's eye view of the formwork provides a clearer understanding of the construction process.

Looking inside the battlefield-like work site during the final stage of demolition. The new caisson cut-off wall for the basement, at left, is supported by steel struts. Demolition must often be performed within such confined conditions.
A side view of the climbing form. Note that the demolition and the site clearance at this stage is still not fully completed.

An early stage of work showing the layout arrangement for the construction of the superstructure. The top-down construction of the basement will commence shortly.
Concreting work being carried out on the building core. Note that the form has been climbed by one section and a hanging apron-like platform has been erected to facilitate work around the lower part of the formwork system. The crucifix-like structure is the steel member inside the RC core and will be used for the future connection of the steel joint that supports the composite floors.