¹ Principal Datum

Figure 1. - Rock profile from Tat Chee Avenue to KCR Kowloon Tong Station direction.

Photo 1

2. Site Formation and Diaphragm Wall Construction
The site formation contract required the contractor to excavate and remove the soil material on the terraced site from averaged +34.5m PD on Tat Chee Avenue side down to +19.5m PD on the Kowloon Tong Railway Station side (Photo 1). This involved initially a total of about 180,000cu m of excavation in volume.

To support the sides of the excavation, a 1.2m thick diaphragm wall was constructed around the site perimeter as well as along the sides of MTR tunnels as a cut-off wall between the basement and the tunnel structure. The walls generally extended to the rockhead, which varied between 6m and 65m below ground level. In addition to the using of rather traditional ways to form the diaphragm walls by grabs and chisels, part of the walls was constructed using the Hydrofraise, or the reverse circulation trench cutting machine. With a system of built-in inclinometers and guides, the hydrofraise had proved to be very efficient for excavating moderately to completely decomposed granite and ensured excellent verticality. Besides, the lack of vibration made the machine ideal for the carrying out of ground works close to sensitive structures such as the MTR tunnels. Meanwhile, grabs and chisels were still being used extensively in locations where hard rock was encountered.

Mini-piles were being installed to support the weight of the load-bearing diaphragm walls and to provide lateral restraint at toes where insufficient passive soil resistance is expected. Before the installation of the mini-piles, the rock below was grouted to seal any fissures and prevent water ingress during excavation of the basements. Due to the shallow-laying of bedrock, the toe of the diaphragm wall in many locations were formed well above the final formation level of the basement. Underpinning works to extend the wall down to the final level were thus required, which would be carried out in parallel with the construction of the top-down basement in the following contract. 

Photo 2

Since chiseling was not allowed within 10m of MTR tunnels, "stitch drilling" was used to overcome underground obstructions before the forming of the cut-off diaphragm walls (Photo 2). This was done by drilling a series of 600mm diameter holes in row, so that any boulder or corestone could later be cut into fragments small enough to be removed by grab.

Site formation works were in general phased from the north to south, or from the up-hill to down-hill direction. The main reason was to maintain as long as possible an entrance/exit point, which situated at the south-western corner of the site, for the efficient removal of the overall 460,000 cu m excavated spoil during the course of the site formation and basement construction processes.

Photo 3

Site formation works were carried out in roughly 6 phased, unsymmetrical sections, according to the convenience of cutting and dividing of the complicated building layout with construction jointings. The main strategy in the scheduling of site formation was to have the northern portion completed as the earliest possible, so that it could be handed over to the main contractor for the construction of the basement and superstructure on this portion. The reason was in fact straight forward, a circular ramp leading to the basement carpark would be located here. With the circular ramp completed, it could be used as the access for vehicle to enter into the basement which was constructed using top-down method. This would be critical, for without which, the last phase of formation on the southern tip of site could not be carried out in full scale (Photo 3). As by that time, the only entrance/exit into the site was still in the southern tip.

Photo 4a

3. Bored Pile Foundation
The entire structure of Festival Walk was founded on some 150 bored piles with diameters ranging from 1.8 to 2.65m, which were being drilled from ground level at the same time with the site formation work to the rock level below the final basement. Bell out was formed at the base of the piles to bear on rock with allowable bearing capacity of 5 or 7.5MPa. The cut-off level for each pile was just below the lowest basement level, with permanent casing installed on top. Due to the shallow-laying rockhead, sometimes up to 25m of hard rock had to be drilled to form the piles, although the concreted length could be as little as 3m in some areas. The deepest record it made for pile in the site was 75m. Piling works were carried out in traditional way: steel casing driven to the rockhead, soil removed by grabs, obstacles broken by chisels, and finally the reverse circulation drill was used to penetrate the rock and form the bell out (Photo 4a). Casing was generally driven by large top-mounted vibrators, or by the use of oscillator and rotator to reduce vibration where worked close to the MTR structures (Photo 4b). Some piles are being installed between the two MTR tunnels, at a separating distance of slightly more than 3m (Figure 2). 

Photo 4b

In the northern part of site where an office block was to be constructed above the podium structure, hand-dug caissons of 3.25m diameter were employed, so as to produce a series of large diameter piles to take up the huge superimposed loads. Steel stanchions were later installed onto the top of piles as support to the basement structure during its construction using the top-down method.

Figure 2. - Typical Section through MTR tunnels.

4. Construction of the Basement Structure
Based on the initial formation level on +19.5m PD, there were 4 levels of full basement below, 3 levels of semi-basement above with one side leaning to the diaphragm wall on Tat Chee Avenue and the other side open to the KCR direction, to be constructed. In addition, there were 5 more levels of superstructure above the Tat Chee Avenue. Except for the 4 levels of full basement which used mainly as car parking and other servicing areas, the rest of the areas would be used as retail purposes. Furthermore, on the northern portion of site, a 4-storey office block with a total built area of about 15,500sq m, would also be provided, which located partly above the 48m span atrium with the ice skating rink underneath.

Photo 5

As for all top-down basement construction, the first floor slab to be constructed on grade is important for it signifies the commencement of the basement work by providing the separating plate and lateral tie such that basement excavation can be started from there on. For the Festival Walk project, this first plate was on the +19.5m PD level (Photo 5).

The basement construction also followed the 6-phased arrangement in conjunction roughly with the site formation sequences. Instead of working with the basement and superstructure at the same time as most top-down construction do, in this project, the superstructure was constructed in an advanced stage from the 1st and 2nd portions in staggered section onwards. This arrangement was to provide sufficient working space until the basement excavation could be started in a more efficient manner, as well as to allow the additional weight of the upper structure to counteract the buoyancy effect during the excavation. With the first two to three levels of the top structure being maintained at its typical cycle, an entrance point as access to the basement below was then formed between portion 2 and 3 (Photo 6).

Photo 6

From this entrance point, excavation to the basement and the construction of the semi-basement structure above the +19.5m PD plate had proceeded simultaneously according to the preliminary 6-phased arrangement, again, in staggered sections that joined with carefully located construction jointings. For the lowest 4 levels of basement where headroom averaged at 3.1m that used as parking spaces for private car, part of which were excavated and constructed using a "Double Bit" method2. This could produce a higher headroom such that excavation could be done using normal-sized excavating machines, as well as to allow the entrance of dumping vehicles for removal of spoil. 

Photo 7

To provide the required protection to the MTR tunnels and to prevent heaving while large volume of soil were being removed, some of the basement slabs, especially those around the MTR tunnels, were deliberately thickened for the purposes. In views of large amount of structural jointings were required to match with the complicated phasing arrangement, this made the casting process rather difficult and time-consuming (Photo 7).

² Please refer to item 4 of reference

There were some locations where the progress of the basement construction works had been significantly interrupted. At the northern edge and the adjoining corner along the Tat Chee Avenue where the rockhead was laying shallowly well above the final formation level of the lowest basement, the toe of the diaphragm wall panels had to be extended further downward during the top-down construction until it reached the final level below the lowest basement. This was done by in-situ underpinning method. Sections of the diaphragm wall panels were constructed after the removal of the bedrock, layer by layer, with vertical junctions being connected by the provision of steel couplers.

Photo 8

In order to minimize the delay caused by the cutting of large volume of hard rock and the associated underpinning works to the toe of diaphragm wall, a section of basement which measured about 100m x 15m along the mid-northwestern corner of Tat Chee Avenue, was being isolated as the 7th portion in the basement construction process (Photo 8). To allow for the continual progress to the upper structure and to stabilize the effect of isolating this part of the basement structure, a row of structural steel strut was erected, which provided the lateral support between the diaphragm wall and the base plate at the +19.5m PD level.

Photo 9

The third area of interruption came from the works around the MTR pedestrian entrance and ventilation shaft. This sensitive structure of the MTR was initially protected by a row of in-site bore piles. A 4-layered temporary steel strut and shoring system was erected to stabilize the structure while excavation proceeded (Photo 9).

Photo 10

5. The Main Structure
Majority of the structure ranged between 8.5m to 16m span in regular grids. However, as a clever design to avoid the awkward passing of the buried MTR tunnels, an elegantly curved, irregularly grided atrium was provided in the design. Under such arrangement, some beams were to be located in the structure with spans close to 35m. Besides, this atrium space, which averaged to be 32m high, 120m long, had actually sub-divided the main structure into four structurally isolated islands, with height ranging from approximately 16m to 38m. This demanded a very strong building frame which should be rigid enough to free-stand itself with limited lateral tie in between (Photo 10). This resulted to a heavily reinforced in-situ structure, partly post-tensioned, and with the use of grade 40 and 45 concrete throughout. 

Formwork used was traditional manual erected timber panel shutters, with standard modulated tubular props for support to the soffits, and the use of UPVC tube as formwork for the circular-sectioned columns. As the overall building layout was very complex with varying levels, spans and irregular grids, as well as the requirements to allow post-tensioning works to be carried out at prescribed stages, and the provision of countless number of construction joints to cope with the phasing arrangement, the difficulties encountered in the erection of formwork can be easily imagined. 

Photo 11

The phasing arrangement of the main structure also followed the 6-phased concept, but with flexible refinements into numerous sub-phases according to the actual progress or practical completion of the consequent sections, or according to the availability of access or spaces into the scheduled work spots (Photo 11).

Photo 12

There were 7 pieces of 48m-span beams above the void of the ice skating rink. These beams were simple-supported in design, cast in-situ and sat on structural bearers on top of columns on each end. The beams were further tied by a 3.5m deep transverse beam on which the southern portion of the 4-level office block was supported. All beams were post-tensioned in order to enhance their structural performance as well as to reduce their size and weight (Photo 12). 

6. The Skylight and the Glass Wall
One very eye-catching feature in the design of the Festival Walk is the provision of two skylight systems which the architect called the "River" and the "Canyon". These skylight systems appeared on the roof with a layout somewhat look like a "X", which measured totally 180m long, averaged 22m in span, and with glazed area close to 3900 sq m.

Photo 13

The main frame of the skylight composed of eight unidentical structural steel trussed girders, each with ends simple-supported and stabled by bolts to the seating plate that anchored firmly in the concrete of the side wall (Photo 13). The girders were erected from the sides outward, producing two cantilevers at the beginning, but supported temporary by tubular props, until the two sides met in the middle. Steel beams running from girder to girder direction were erected afterward to fill up the space in-between. 

Photo 14

There were four main glass wall units on the east and west elevations of the building, totally with a glazed area of about 3500 sq m. Two of the glass wall units were of full height stretching from ground to roof that bounded the atrium space of the "Canyon". The wall units leaned slightly outward at 100o and 93o respectively at the east and west facing elevations. The main frame of the glass wall was constructed in box-sectioned steel of varied sizes, with some member deliberately being tilted to provide added architectural interest (Photo 14). 

 CONCLUSION 
The first contract for site formation and foundation of Festival Walk commenced in April 1994 and with practical completion of the main contract in December 1998. It took more than four and a half years to have the building finished. Yet with consideration to the complexity of the job both in the term of engineering and architectural requirements, and the extraordinary vast scale of work involvement, the project for Festival Walk is still regarded as a great success. Festival Walk marks a lot of unprecedented records for projects of similar nature in the construction history of Hong Kong. The 36m deep cut-off wall below Tat Chee Avenue, the 48m in-situ beams above the ice skating rink, the total 180m long structural steel frame of the skylight that formed the "River" and "Canyon" with the large volume of atrium space within, are some of the examples. It is needless to mention the sophisticated building services systems that provide the comfort environment for a world-class shopping mall.

As the scale of construction projects are getting bigger and bigger, and with the imposing of more and more challenging architectural, engineering and contractual requirements that one can imagine, engineers and builders in Hong Kong have no choice but to transform themselves into an adequately trained, well-experienced and highly competent entities, whom they should be able to foresee, manage and coordinate complex situations to achieve demanding and stringent objectives. It should be of no doubt that the Festival Walk is an excellent training ground.

Reference
1. Mass Transit Railway Corporation, Railway Protection Manual, July 1992.
2. Construction and Contracts News, January/February issue, 1995, page 72-78, China Trend Building Press, 
    Hong Kong.
3. Wong WM, 15 Most Outstanding Projects in Hong Kong, page 19-31, China Trend Building Press, Hong Kong.
4. David Scott, Goman Ho and Hayden Nuttall (June 1999), Design and Construction of 62-storey Cheung Kong
    Center, Symposium on Tall Building Design and Construction Technology.