Festive Walk

 

Festival Walk

Festival Walk, situated on a 21,000 sq m site at Yau Yat Chuen, involved complicated site formation and basement construction processes due to its terraced land form as well as its narrow land shape. The special problems encountered in the construction of the project are illustrated by the following:

  • The tunnels of the MTR Kwun Tong Line were laid about 9 m below the formation level and cut through the centre of the site.

  • A total of about 460,000 cu m of soil were excavated, of which about one-quarter was hard rock in the form of slightly decomposed granite.

  • The site has to be formed from an initial level of 39 m to 24 m on the western side along Tat Chee Avenue, producing a 15-metre high vertical cut.

  • The site also required the construction of a four-level basement continuing downward from the 24-metre formation level.

  • Since the only site access was from the Tat Chee Avenue side, the removal of the excavated material caused many problems both in planning and in the carrying out of works.

  • Due to shallow bedrock in many site areas, a large amount of underpinning works were required to build the bottom of the diaphragm wall during the top-down construction process of the basement. The diaphragm wall was used as the major soil support system for the basement construction. This caused serious interruption and delay to the basement construction works.

  • The exceptionally large size of the project, both in the construction of the basement and the superstructure, required very complicated phasing arrangements.

  • Due to special design requirements, the layout of all floor levels are non-typical in nature: The structural framing layout, partitioning and other internal space arrangement are different without significant repetition. The 35 m x 120 m atrium within the shopping mall made the spatial situation much more complicated in terms of construction.

This series of photos illustrates how these difficulties were tackled.

main contractor
Sanfield Contractor Co Ltd

foundation and site formation
Gammon Construction Ltd/Dragages et Travaux Publics – joint venture

A Project with Complex Site Formation,
Foundations & Basement Construction

September 1994: this photo shows the overall layout of the site in the early stages of site formation. The site was congested with construction equipment: large cranes, drilling rigs and excavating machines of all kinds. Though it looks quite a mess, this is typical for this type of project.
The site as seen in October 1995. The site level was further reduced, and the phasing of work is basically arranged from north to south: from the hillside working downward. As can be seen here, excavation work on the northern part of site had already been completed down to the formation level, while the southern end was almost remain untouched. A temporary access ramp had been reserved at this end for the removal of excavated materials.
The site as seen in April 1995. The majority of the diaphragm wall construction work had beencompleted by this point. The road at left is Tat Chee Avenue. The top three or four metres of the diaphragm wall along the road was exposed by the first stage of excavation for the site. Excavation halted at this level to wait for the construction of the first row of ground anchors. A temporary access roadway winding up from the bottom of the site to the exit on the north-western end corner can also be seen. The plastic sheet covering a section of a newly cut embankment is to protect the soil from rain water.
The hydrofraise, or circulation trench cutter, was used at the site for the excavation of the diaphragm wall. This machine was used mainly in some sensitive locations such as areas near, or on top of, the MTR tunnel.

A closer view of the hydrofraise with the drum cutters resting on ground. The cutting rack was hung on the rig of a specially designed crawler crane and further secured to the crane chassis by a hydraulic arm to protect the rack from unnecessary swinging during digging process.
Another method employed to form the trench of diaphragm wall used the clamshell or the excavating grab.

The bentonite treatment plant that was used to separate soil and gravel materials brought to the plant by the circulating bentonite slurry. This equipment usually works together with the hydrofraise. The soil or gravel cut by the drum cutters of the hydrofraise are pumped up together with the bentonite slurry, which is placed in the trench of the diaphragm wall as a support to counteract the soil pressure. After the soil materials are filtered out, the slurry is circulated back to the trench so that the slurry level will remain constant.
A drilling machine used to cut through, and form a series of holes in, a thick layer of boulders. The drilling required in this situation for the diaphragm wall was very close to the MTR tunnels, only a few metres from the construction line. After the drilling was completed, the trench for the diaphragm wall was then formed using the hydrofraise. This assured that minimum disturbance occurred during the construction.

This photo shows the drilling rig for the
formation of the caisson shaft. The rig is
being tilted sideways by a built-in hydraulic arm to allow for the placement of the drilling shaft and auger head.
Another caisson shaft formation method; this one uses a grab. A hydraulic operated rotator is placed at the bottom of the steel casing which supports the soil during the digging. The rotator can lower the casing down into the soil or jack it up for the caisson during the concreting process.

The boring machine is drilling a hole in the ground and through the capping beam of the diaphragm wall. A post-tensioned tendon will be inserted into the hole later to form the anchor.
Steel stanchions of various size, partly fabricated on site, awaiting placement into the caissons as temporary support to the basement structure during the top-down construction process. The stanchions were encased with reinforced concrete later to form permanent columns.   

A typical way to reduce the site level is by sequentially cutting the ground into terraces at various levels. In the photo, the vertical structure at left is the diaphragm wall on Tat Chee Avenue. On top of the panelled wall is the capping beam. The upper row of ground anchors tying the diaphragm wall has been completed, while the lower row is still in progress. In the foreground, the site has been formed to its required level of 15 m. On the middle terrace, a boring machine performs the drilling for the ground anchor work.

In the photo, the north-western corner of the site is being reduced to the required formation level. The mechanically dug caissons, each protected by a steel casing, are waiting for the placement of a steel stanchion. The stanchion will support the top-down basement to be constructed later by the main contractor for the main structure.
The site in November 1995. Half of the site on the northern side was essentially formed by this time. The rock stratum in the centre of the site was close to the surface. As a result, quite a lot of time was spent in cutting the rock. Some hand-dug caissons, mostly of 3.2 m diameter, can be seen here.

The northern end of the site in March 1996. The first portion of the basement slab (Level LG2 at PD +19.5m, approximately level to the KCR) had been cast at this stage. The formwork for columns and slabs for the floor above (Level LG1) was in progress.
The northern end of the site in May 1996. The first portion of the main structure had been completed up to the G/F Level. Excavation work has also commenced under the slab of basement Level LG2 forming the void for the basement at MTR Level. In the photo, dump trucks are removing the excavated material from the basement void for disposal.

The site in July 1996. Work on the second portion of the basement structure had commenced. At the same time, the main structure of the first portion was steadily progressing up to U/G level, which is equivalent to the first floor from the Tat Chee Avenue perspective. The entire site was roughly subdivided into six main portions for the convenience of construction.
Erection of the first floor slab (Level LG2) on the fourth portion formation level at +15 m. The rows of steel posts and joists in the middle of the floor formwork formed a tunnel for the passage of dumping vehicles into the basement. The steel stanchions as temporary support for the top-down basement were already in place for connection with the first slab.

Progress of work: August 1996. The site was becoming quite congested and a temporary access ramp leading to the south-western corner of site was the only entrance/exit for the required equipment and materials at this stage.
An active work frontage: substructure and superstructure work meeting at the fourth and fifth portion interface.

The connection between the stanchion and the first slab at Level LG2. This slab was used as a separating plate; above it, the building will be constructed in a typically upward fashion, while underneath the basement will be constructed using the top-down method. The rebate around the head of the stanchion is a dropped panel, which is a typical feature for a floor structure using flat slab design. However, the floor system for this project is quite complicated, as flat slab is only part of the overall floor design; large-span beams and post-tension beams are also used in the project. 

October 1996: Along the bottom of the diaphragm wall on the Tat Chee Avenue side, a long exposed strip of bedrock lay close to the surface. The existence of shallow rock here caused a significant delay to the overall progress, particularly for the basement structure which required much more time as a result.
December 1996; As seen here, the MTR tunnel runs from the south-east to the north-west about 12 m below the formation level. The access ramp leading to the south-west end of the site was closed permanently from this point onward. The only entrance/exit to the site at this stage was a spiral ramp which is part of the permanent structure for vehicular basement access on the north-east portion of the site.

A slender stanchion supporting a heavy floor slab; typical for a basement constructed using the top-down method. The steel casing for the caisson will be removed, leaving a lower section of the stanchion exposed, while the excavation goes further downward.
An overall view of the site in September 1996. At this stage, the first two portions of the building structure in the overall phasing arrangement have already been shaped, while the first slab for basement at LG2 level at portion 3 and 4 has only partly been cast. The provision of the temporary access ramp offered some hindrance to the progress of work though its function for site transportation was essential.

Close-up of the preliminary stage for the top-down basement excavation work.
Progress of work in July 1997, showing the gigantic scale and the complexity of the building work.

The floor slab at the top of this photo is the slab for basement level LG2. As soon as a basement depth of about seven metres is reached, a section of the slab below will be cast (the partly completed floor slab in the middle level). Casting a slab in this situation can be accomplished in one of two ways; One is to erect the floor formwork with the usual propping support, though this requires a deeper excavation in order to allow for the erection of the props. The other method is to trim and level the surface of soil and shape it according to the beam layout, then do the reinforcement fixing and concreting to form the slab.


Another view from the MTR Level and Level P1 of the basement. Here, the slab on the lower level is constructed using the usual propping support method. The space provided in the upper slab (from which the author took this photo) is a pre-determined opening for the access of excavated materials and other required equipment.

The temporary access ramp leading to the entrance/exit at the south-western corner of site. It served as the major entrance/exit to the site from the commencement of the formation work in July 1994. The entrance/exit was permanently closed in May 1996.
The spiral ramp from Tat Chee Avenue (U/G level) leading down to the basement carpark. Festival Walk has four levels of carpark from the lowest basement (Level P3) to the MTR Level, providing parking space for about 600 vehicles. During the construction of the basement and substructure, this ramp served as a major access point for the removal of the excavated materials and delivery of building materials.
The bedrock laid quite close to the ground surface along the Tat Chee Avenue side (at approx. PD + 18 m). In order not to delay the rest of the building and basement construction works, a long narrow strip of building space, as shown in areas with rows of steel struts in the photo, was isolated as a separate portion. This area stretched from portion 3 to 5 in the overall phasing plan. The steel struts erected were to stabilise the diaphragm wall which rested only on top of the surface of the bedrock and required underpinning to extend it to a much lower level below the basement. Excavating and rock breaking machines will carry out the rock cutting work after the erection of the strut.

 

A look at the bottom part of the diaphragm wall where underpinning work is to be carried out. As seen on the underside of the wall, rows of couplers have been inserted here with the placing of reinforcement cage. The couplers are used to extend the reinforcing bars. Several groups of steel bars can also be seen; these are the mini-piles which protect the diaphragm wall from lateral movement. When the rock under the wall has been cut and cleared, reinforcing steel will then be fixed and formwork erected. The extension to the wall will be completed after the placement of concrete. The work is repeated until it reaches the required level: the lower basement.
Since the construction is subdivided into many main portions and sub-portions, numerous construction joints are required to connect these portions. The details of the joints should be carefully planned and positioned so as not to interrupt the construction works as well as to obtain the best structural effect. The photo shows a typical view with over five sub-portions in active progress.

Another look at a series of construction joints at the external edge of the building.
A construction joint on the floor slab. Starter bars protruding from the slab and beams make the joint and future connection work extremely difficult.

After 18 months of difficult work, the superstructure almost take shape. The major operation as seen here is the erection of the floor formwork on the southern portion at the Lower Ground level, from which the size of the building can be easily comprehended.
One prominent feature of the Festival Walk is the 6-level high, 120 m long and 30 m wide atrium that cuts longitudinally through the interior of the shopping mail. Above which, there is a glass skylight providing natural light to the interior of the building. The photo as seen here is the atrium before the skylight being erected.

A panoramic view seeing the overall layout of the atria with the truss frames basically erected. To provided a more impressive identification, the longitudinal-run atrium is called the River, the transverse one the Canyon.
Strong structural steel trusses were erected to form the supporting frame of the skylight of the atrium. Looking into the atrium below, escalators spanning the floors had already been placed using the tower crane before the closure of the atrium by the truss frame.