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The construction of deep and complex basements under

extremely difficult urban environment – with 3 representing projects in Hong Kong

This paper was prepared by Raymond W M Wong and was published in the

International Conference on Advances in Building Technology Organized by the Faculty of Construction & Land Use, Hong Kong Polytechnic University

Hong Kong, December 2002

ABSTRACT Hong Kong is famous for its congested urban environment and hilly topography, which impose enormous constraints in the construction of buildings. Cases like constructing very tall buildings, either of residential or non-residential nature, up to 50 or even 60 storeys high, and in close proximity of MTR tunnels, near sensitive slopes, or in the middle of an old town area, are very common. Such building examples are often consisting of very deep basement, with depth sometimes more than 20m. The author has identified 3 project cases in recent Hong Kong which can best illustrate the mentioned features. These projects include the redevelopment of the Lee Gardens Hotel, the construction of the Festival Walk and the construction of the Cheung Kong Center. The author wishes to generalize these 3 projects, each has applied rather unique and innovative concepts and techniques in construction, and highlight how engineers and constructors in Hong Kong tackle such extreme situations and have the jobs neatly accomplished.

KEYWORDS Basement construction, difficult construction environment, common construction problems and solutions

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INTRODUCTION Deep basements are becoming very popular as buildings constructed higher and higher in Hong Kong. Numerous engineering problems are likely to be encountered as construction works are going deeper and deeper down into the ground. Engineers and builders have to face a lot of problems such as the existence of complicated sub-soil, overcoming of tremendous soil pressure, the provision of complicated temporary support works, working in congested underground or sensitive nearby environment. In particular in most modern development projects, the scale of construction tends to be growing bigger and bigger, that makes planning and construction of deep basement become much more difficult and complex. At the same time, the construction of deep basement is very expensive, time consuming, inconsistent and sensitive to the quality of planning and management of individual projects. The most important of all, such works are highly hazardous, both to human operatives working within and the life and properties of third parties that within the vicinity. Though very technical or engineering in nature it seems, to implement jobs of that level of complexity is of no doubt associated with quite a lot of managerial challenges. Such as, in the preparation of a highly efficient working programme, monitoring and rectifying the progress of works in case problems arising, or in resources planning where materials, labours and plant equipment are involved. From the construction point of views, there are many methods to construct large-scaled and deep basement. Deep basement can be constructed using some traditional ways such as cut and fill or bottom up methods. These methods are very effective when dealing with certain jobs which is simpler in nature. On the other hand where basement is going deeper and the surrounding environment getting more complex and sensitive, bottom up method may be a more appropriate option to construct (Fig. 1). Under typical urban environment, the situation where a basement is to be constructed can be very complex. Scenario such as working in congested site (Fig. 2), working very close to sensitive buried structure (eg. adjacent to a section of railway tunnel), or the site is located close to relatively unstable slope, are common examples that can be found everywhere in Hong Kong. Or, in certain cases, the design of a basement structure is very deep, the size of the basement is exceptionally large, sub-soil condition is very complicated, are again typical situations that make basement construction becomes very difficult. Besides, there are situations where a new basement is required to construct simultaneously to replace an old one. Or due to the requirement of working under fast-track schedule, some basement works need to be carried out at the same time with the new foundation or even with part of the future superstructure. This will create very difficult coordination problems that involve various contractors and complicate the contractual position of the entire job. The process to make decision arriving at the best solution is of no doubt an agony journey for both engineers and builders.

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Fig. 1 – Very large basement job with top-down and bottom up approach working at the same time

Fig. 2 – Example of basement constructed in very congested down-town environment

The main objective of this paper is to find a solution for the mentioned problems for there should be no absolute cure or one-step formula for construction works of such scale and complexity. Most often, the extent and scale of work for complex basement projects are usually very broad with a huge contract sum. It is difficult to compare alternatives base on previous data or project cases. Accurate cost analysis or work study is difficult to carry out either for there is almost without standard ground to make comparisons. Every project, though look relatively similar from certain indicating factors, is in fact unique in itself. A great number of random and uncontrollable variances are likely to arise during the courses of work. This makes planning and scheduling almost cannot be exact. The actual effectiveness of works is highly depended on the as-constructed site environments. Besides, the quality of the management and the executing parties, as well as the problems solving ability of the frontline personnel, also seriously affects the performance and effectiveness of works. By the use of 3 recent basement projects, the writer tries to bring up some very representing project cases and demonstrate how works can be arranged to fit in such complicated environment and have the job accomplished in a safe and efficient manner. LEE GARDEN REDEVELOPMENT PROJECT 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 35 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. In the redevelopment, a new 50-storey office building, constructed in structural steel with a RC core, together with a 4-level basement, was to be built. There were quite a number of difficulties to be overcome in the project. For example, almost all the new foundations and the required

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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 sometime 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. 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. Difficulties and Uniqueness

Though project of this kind is not exactly uncommon in Hong Kong, the following features, however, still make the project quite unique and thus imposed certain technical difficulties.

A 2-level old basement structure covered the entire site area was to be demolished.

A 4-level basement of the same size was to be built to replace the old, on top of which is a 5-level podium structure. This made the site extremely lack of working space.

Due to the old basement could not support those heavy equipments (like the RCD) which were required for the construction of the foundation. Hand-dug caissons were thus used in this case.

Because of the inevitable sectioning and phasing arrangement involved in the demolition of the old basement and to replace it with a new structure, complicated planning and construction jointing provisions were required (see attached demolition phasing plan).

Part of the future building structure was carried out at the same time while doing the major substructure works. This included the construction of the foundation and raft for the central core of the future tower.

The congested environment made storage and transportation arrangement within site very difficult. At a result of this, huge temporary loading platforms were provided at different locations (required to relocate from time to time) in order to store the steel stanchions, to station mobile cranes or other excavating machines etc.

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Demolition of the Old Basement and 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. Demolition support and ground stabilization

A series of temporary supporting system in the form of a steel strut frame was erected before various stages of basement demolition.

A grouted wall was formed along the site perimeter down to 3m below bedrock level (av. 25m deep) as a means of ground water control.

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. Total 244 caissons of 1.2m dia. were constructed for the purpose.

Construction of new foundations

Hand-dug caissons were formed as foundation for the future building within the old basement. Totally 49 caissons with diameter ranging from 1.6m to 5.0m were constructed.

The central part of the old basement was demolished to provide working space for the carrying out of the foundation and raft for the future building core.

Hand-dug caissons and the caisson raft were constructed to support the central core of the new building. Excavation and construction of the 5m diameter caissons was done at the bottom of a 800m2 pit that formed after the demolition of the center part of the old basement (Fig. 3).

Figure 3 – A pit formed further down from the partly demolished old basement structure for the construction of the foundation and raft for the future building tower.

Figure 4 –Layout showing the extremely complicated construction phasing as seen at the peak period.

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Changing over and provisional works for the new building

Part of the old basement structure along the perimeters was demolished to allow space to build the capping beam on top of the caisson wall.

Similarly, part of the old basement structure was demolished to provide room 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.

The building core was constructed from the raft up to ground level. Since there was very limited working space within the central pit, traditional timber 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.

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 (Fig. 4).

Construction of the Basement

The basement was constructed using top-down method. In order to allow excavated material could be removed conveniently, two outlet points were provided on site. A grab lifter was set up on top of one of the outlets for the removal of spoil. As usual top-down construction does, excavation started from the top level downward until it reached a depth of about 5m where it would be shored and strutted. Basement slab would then be constructed with connection made to the central core, steel columns and the caisson wall using couplers. The works repeated until it reached the bottom level of the basement where the caisson caps and other ground beams were finally constructed. THE FESTIVAL WALK

The Festival Walk, situated on a 21,000 sq m site on a stump of a small hill near the Kowloon Canton Railway (KCR) and the Mass Transit Railway (MTR) stations. The project exhibited a lot of complications during the course of its construction, basically inherited from the special nature of the job such as its unfavorable topographical and geotechnical environment, working within the railway and tunnel lines of two very busy railway networks, the requirements of constructing a very large building with exceptionally deep basement, as well as some access problems.

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Topography and Geology of Site The site is situated on a narrow terraced strip of land along the Tat Chee Avenue (TC Av.), which measured about 290m x 80m in size. The existing ground levels vary from +29mPD to +36mPD along the west boundary on TC Av. side, and from +19mPD to +26mPD along the east boundary on the KCR and Kowloon Tong (KT) Station side. In order to cope with the aerial height restriction requirements and to achieve the development potential allowed under the condition of sale of the site, there is 4 levels of basement and 3 levels of semi-basement in the development, with the deepest level being some 36m below TC Av. Completely decomposed granite was encountered at ground level, with the presence of many corestones. Rockhead on the TC Av. side varied between 6m to 65m deep. At the northern portion of site, bedrock was very close to the surface, but sloping downward to about 60m deep near the KT Station. The MTR tunnels run directly through the center of the site, thus confined the geometrical design of the sub-structure. As a result, the lowest excavation levels to the north and south of the tunnels are at -1mPD and +8mPD respectively, with the section above the MTR tunnels not exceeding +13mPD so as to ensure at least 3m cover above the tunnel structures. 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 +34.5mPD on TC Av. side down to +19.5mPD on the KT Station side. This involved initially a total of about 180,000 cu m of excavation. To support the sides of the excavation, a 1.2m thick diaphragm wall was constructed around the site perimeter as well as along the MTR tunnels as a cut-off between the basement and the tunnel structure. The walls generally extended to the rockhead, which varied between 6m and 65m below ground level. 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. Since chiseling was not allowed within 10m of MTR tunnels, “stitch drilling” was being used to overcome underground obstructions before the forming of the diaphragm walls. This was done by drilling a series of 450mm 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. The main reason was to maintain an entrance/exit point, which situated at the south-western corner of the site, for the removal of the overall 460,000 cu m excavated spoil efficiently during the course of the site formation and basement construction processes.

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Phasing and Removal of Spoil 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 this portion could hand over to the main contractor for the construction of the basement and superstructure. The reason is straight forward, a circular ramp leading to the basement carpark is located here. With the circular ramp completed, it could be used as the access for vehicle to enter into the basement which constructed using top-down method. Without which, the last phase of formation on the southern tip of site could not be carried out in full scale. By that time, the only entrance/exit into the site was still in the southern tip.

Figure 5 – Entrance arrangement from ground level into the top-down basement.

Figure 6 – A complicated junction where various major phasing sections of the superstructure and basement meet

Construction of the Basement Structure Based on the initial formation level on +19.5mPD, there are 4 levels of full basement and 3 levels of semi-basement to be constructed. As for all top-down basement construction, the first floor plate to be constructed is important for it signifies the commencement of the basement work by providing the separating plate and lateral strut such that basement excavation can be started from there on. In this project, the first plate was located on the +19.5mPD level (Fig. 5). The basement construction also followed the 6-phased arrangement in conjunction roughly with the site formation sequences (Fig. 6). Instead of working with the basement and superstructure at the same time The superstructure was constructed in an advanced stage from the 1st and 2nd portions in staggered section onwards. This was to accommodate enough working room 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 balance the buoyancy effect during the excavation. With the first two to three

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levels of the top structure being maintained at its typical cycle, an entrance point to the basement below was then formed at the edge of position between portion 2 and 3.

Figure 7 – The portion of the basement slab located above the MTR tunnel tubes. Note the thickened slab serving as a transfer membrane spanning over the tunnel and the steel stanchion for the support of the basement structure during top-down construction

Figure 8 – The lateral support system erected during the basement excavation process to stabilize the existing MTR pedestrian entrance/ventilation shaft which is located on the right side of the photo

From this entrance point, excavation to the basement and the construction of the semi-basement structure above the +19.5mPD plate proceeded simultaneously according to the preliminary 6-phased arrangement, again, in staggered sections with the separation of carefully located construction jointing. For the lowest 4 levels of basement with headroom averaged at 3.2m that used as parking spaces for private car, part of which were excavated and constructed using a “Double Bit” method. 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. To provide the required protection to the MTR tunnels and to prevent heaving while large volume of soil was being removed, some of the basement slabs, especially those around the MTR tunnels, were deliberately thickened for the purposes (Fig. 7). 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 TC Av. 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 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. This section being interrupted measured about 100m. 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 steel strut was erected, which provided the lateral support between the diaphragm wall and the base plate at the +19.5mPD level.

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Another major area of interruption came from the works around the MTR pedestrian entrance and ventilation shaft. This sensitive structure was initially protected by a row of in-situ bore piles. A 4-layered temporary steel strut and shoring system was erected to stabilize the structure while excavation proceeded (Fig. 8). CHEUNG KONG CENTER (Redevelopment of the previous Hiltion Hotel) The project situated in a 9,650m2 site in the down-town area of Central. The new building is a 62-storey composite structure, with a 22m x 27m reinforced concrete inner core encased in a 47 m x 47m external steel frame, together with a 6-level basement that constructed over the 2-level old basement of Hilton Hotel. Demolition and Foundation The previous Hilton Hotel was to be demolished at the commencement of the project. After the site was cleared, the works that followed were the construction of the diaphragm walls and the bored foundation for the new building. All the diaphragm walls employed in the project were of 1.2m thick reinforced concrete. The perimeter walls helped to support and stabilize the ground during the construction of the new basement, as well as to act as the permanent basement wall. There was a 37m-diameter shaft pit formed in the middle of the site for the construction of the core wall for the future tower. Large diameter bored piles were used as foundation for the new building. The bored piles were basically in two standard sizes. Eight of the piles were 6m in diameter and dug manually for supporting the superstructure. 20 piles for the support of the 6-level basement structure were of 1.5m diameter and dug mechanically using grabs and chiselling method. Forming a 37m-diameter shaft pit and the construction of the core wall Before the carrying out of the basement construction using a top-down method, the first major work below ground was to construct the central core of the main building tower, the foundation of which rested on bedrock about –28m from ground level. Instead of constructing the central core in a top-down manner, the core was built bottom up. This could be done by the forming of a pit large enough to house the core structure and its foundation (Fig. 9). A pit was thus formed with the sides supported by panels of 1.2m-thick RC diaphragm wall. When the pit was excavated down to the required formation level, a 5m-deep RC raft was constructed as foundation for the core. The core wall on the lowest basement was constructed on top of the raft (Fig. 10).

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Figure 9 – A 37m dia. shaft and eight 6.5m dia. manual-dug caissons were formed to construct foundation raft for the 62-storey building

Figure 10 – The construction of the core wall using a jump-form system inside 37-m dia. shaft.

Construction of the basement After the completion of the bored piles, steel stanchion was erected on top of each pile at its formation level as support to the basement slabs during the construction process using a top-down sequence. In order to allow the core wall and the structural frame to proceed to a safe separating distance, the ground floor slab was cast after the core wall had been completed up to the 9th level. With the temporary diaphragm wall that formed the 37m-diameter shaft gradually being demolished, the basement slab bound by the 8 super-columns was cast and connected to the core wall structure as soon as a stage of excavation completed. This made the basement structure at the center very rigid and from thereon, excavation to the sides continued, with the central part acting as a base to shore-support the newly excavated sides (Fig. 11). The floor system in the basement was of flat slab design with dropped panel around column heads. Average slab thickness was 400mm. As a means to expedite the progress in the basement construction, the “double-bit” method was adopted (Fig. 13). The principle of this method is to have 2 levels of the basement excavated at a same time. At the bottom of the excavated, then cast the slab of the lower basement. And from this level, usual floor form would be erected and have the upper basement slab cast afterward. The basement further below would be repeated using the same principle. Temporary shores were installed in certain positions to stabilize the sides due to the depth of the excavation in the double bit. To facilitate the removal of large volume of excavated materials, several temporary openings were formed on the basement slab so that the excavated soil could be removed by lifting grabs, excavating machines or dumper truck entering into the basement through temporary ramp within the basement (Fig. 12).

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Figure 11 – Center portion of the basement acting as a base to support the excavated sides during the top-down process.

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Figure 12 – One of the openings formed inside the basement slab for the purpose of spoil removal. Temporary ramp would be erected in this opening to allow dumping vehicles entering into the basement for the purpose.

Figure 13 – another openings for spoil removal. The construction of the intermediate slab using “Double Bit” method can be observed

CONCLUSION Besides the 3 cases as referred to in this paper, there are countless other similar basement projects being executed recently in Hong Kong. A brief summary listing some of the commonality of these projects can be drawn as a conclusion to finalize the discussion here.

Basements of this kind are usually very big (say up to 10,000m2) and very deep (below 20m).

Majority of the basements are constructed in a top-down manner. Some other methods such as combining top-down and bottom up, or combining open-cut and top-down arrangement, can sometimes be seen.

Complicated coordination problems and teething arrangement often exist between various major contracts or other major building works.

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Layout planning especially in phasing and sectioning of the job forms a very important consideration mastering the success of the project.

Dynamic layout arrangement is usually required for the removal of the excavated spoil from the basement. This may involve the forming of temporary ramp, provision of special equipment, or the taking over of part of the completed building as temporary access in an advanced stage.

Diaphragm walling is the most common cut-off provision being used.

System formwork can hardly be applied for most basement jobs due to access problems as well as the confined working condition inside the excavated.

Constructing the basement in “double bit” arrangement is becoming common.

Protection and safety measures in particular to the life and property of third parties are highly concerned in basement jobs. Accident in this area is maintained at a relatively very low rate.

Progress of work can hardly be predicted or monitored accurately due to the existence of numerous unforeseeable problems during the construction process.

From the investigator’s point of view the suggestion to improve such situations may be quite simple. Engineers and builders in Hong Kong should have sufficient experience in handling these kinds of complicated basement jobs. Enhancement in the following areas may be simple and straight-forward solutions to improve the performance of such projects.

Spend more resources to improve house-keeping works on site (e.g. better ventilation, lighting, access provision or tidier working place, remember it is working within basement).

Lengthen the duration of the construction period where possible. Differences between fast track and quality should find a balance.

Improve planning and supervision quality, other than just rely on software and log record.

As for most construction jobs, technicality is often the least of the problem. On the other hand, human factors, planning and management concerns are the most determining issues in reality. This is even more factual for basement project.

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RAMOND W M WONG, Construction of a Semi-buried Building – A Super-sized Shopping Mall: The Festival Walk, Megacities 2000 Conference, Dept. of Architecture, University of HK, 2000.

DAVID SCOTT & GOMAN HO, Design and Construction of 62-Storey Cheung Kong Center, Symposium on Tall Bldg Design & Construction Technology, Beijing, 1999.

M J TOMLINSON, Lateral Support of Deep Excavations, Proceedings of The Institution of Civil Engineers, 1970.

G N GILLOTT AND JAMES C K LAM, Construction of Site Formation and Foundation Works in Building Projects, Building Construction in Hong Kong, Building Departments, HKSAR, 1998.

G J TAMARO, Deep Foundations in the Urban Environment, Habitat and the High-Rise, Council on Tall Building and Urban Habitat, Dutch Council on Tall Buildings, Amsterdam, 1995.