2. CONSIDERATION OF ALTERNATIVES
2.1.1 Four alignment options, straight-type bridge, curved-type bridge, immersed tube-type tunnel and drill and blast-type tunnel option, are presented below:
Option A - Bridge Option (Straight Type) Landing at Ngau Hom Shek
2.1.2 Option A is a straight-type bridge option connecting Shenzhen at Dongjiaotou and Hong Kong at Ngau Hom Shek. Figure ES2.1 shows the horizontal and vertical alignments for Option A. This alignment is the shortest of the four landing options and relatively straightforward. The alignment beyond the boundary of HKSAR entirely follows the proposed alignment in the Mainland Feasibility Study Report dated July 2001, and is the most preferred option by Mainland.
2.1.3 At the northern and southern navigation channels, a cable-stayed bridge had been proposed to provide a landmark view to the Deep Bay, and was also recommended by Mainland.
Option B - Bridge Option (Curved Type) Landing at Ngau Hom Shek
2.1.4 This option is more or less the same as alignment Option A. This alignment is shown on Figure ES2.2. The curved shape of the alignment was proposed from an aesthetic point of view. Although the alignment is slightly longer, this option would reduce ship impact risks, lower accident rate and enhance the aesthetic of the bridge. The vertical alignment is the same as Option A.
Option C - Tunnel Option (Immersed Tube Type) Landing at Ngau Hom Sha
2.1.5 An immersed tube tunnel was proposed for this tunnel alignment landing at Ngau Hom Sha. The tunnel would have to climb up to the land and reach the valley in YL/57 with a maximum gradient of 3%. The tunnel portal would be located slightly south of Deep Bay Road just above the ground surface. Due to the required sight distance, the radius within the tunnel is large (around 1000 m) and due to the required gradient, it would have to utilise the area between Deep Bay Road and the hill identified as Permitted Burial Ground YL/57.
2.1.6 To connect the Hung Shui Kiu New Development Area (HSK NDA), which is a requirement, the road has to curve around the valley through the BG YL/57 via the Ha Tsuen Interchange. The alignment is shown on Figure ES2.3.
2.1.7 The tunnel box would be composed of four cells, in which two cells would be used for ventilation. The cross-section of the ventilation cells required is comparable to the carriageway cells, as the length of the tunnel would be approximately 5.5 km, i.e. almost four times the length of the Western Harbour Crossing.
Option D - Tunnel Option (Drill and Blast Type) Landing at Sheung Pak Nai
2.1.8 This option is similar to Option C, but proposes using the drill and blast construction method. The tunnel would be located further down in rock, thus requiring this construction method, and hence it would be very deep for the section within Deep Bay. It would have to climb up to the land and reach the portal near Ha Tsuen with a maximum gradient of 3%.
2.1.9 To allow for sight distance, the turning radius for the tunnel would be very large (1000 m). To provide adequate gradient (less than 3%), the road would have to utilise the entire area south of Deep Bay Road. To connect HSK NDA, the road would be aligned to connect the Ha Tsuen Interchange.
2.1.10 Due to the substantial length of the tunnel, a ventilation tunnel is required for the whole alignment and more than two ventilation buildings may be required. This alignment is shown on Figure ES2.4.
2.1.11 For drill and blast tunnel excavation, it is preferred to locate the tunnel within competent rock of Grade III to Grade II to avoid extensive temporary stabilisation. Towards both ends, where the tunnel would have to rise to meet ground level, the tunnel would inevitably need to go through less competent rock. In such cases, careful staging and specifically planned excavation techniques with primary support requirements are essential.
2.1.12 At the HKSAR end, it would be possible to excavate mostly within decomposed rock of varying degrees of decomposition. At certain locations, the ground cover above the tunnel would be insufficient for the drill and blast technique. In such cases, a different construction technique would have to be employed.
2.1.13 At the Mainland end, it would have to pass through sediments and eventually marine deposits near the reclamation. Obviously, the use of drill and blast would not be appropriate here. It is envisaged that this section would be constructed by cut and cover techniques. Various environmental factors would have to be considered in determining the appropriate cut and cover method, if this option were chosen.
2.2 The Preferred Alignment Option
2.2.1 Alternative bridge and tunnel alignment options were evaluated and ranked to assess the alignment options and identify the preferred option. Evaluation of alignment options involved consideration of traffic, engineering, environmental, marine, land, implementation programme, public perception and cost issues. The option with the highest score in the evaluation was chosen as the preferred alignment option for the Shenzhen Western Corridor.
2.2.2 The score of each alignment option was the summation of the weighted scores for different factors. The maximum score was 320. Based on the evaluation results, Options A and B (the bridge options) had similar high scores of 248 points. There was a marked difference in the scores between the bridge and tunnel options (Option C had 146 points and Option D had 160 points).
2.2.3 From the sensitivity test results, it was identified that although the relative positions of Options A and B in the ranking alternated in the sensitivity tests, the bridge options (A and B) were always higher in ranking than the tunnel options. It could therefore be concluded that the bridge option would be preferred.
2.2.4 Options A and B had close scores in this alignment comparison exercise and their ranking alternated in the sensitivity tests. In comparing the straight type bridge option and s-curve bridge, the latter would be slightly more expensive than the former. The s-curve was introduced to improve the angles between the bridge alignment and the navigation channels. This would reduce ship impact risks.
2.2.5 Another advantage of the curved alignment over the straight alignment would be the varying and interesting views afforded to the drivers and passengers travelling on the bridge. A straight alignment is not aesthetically pleasing and would not afford appropriate views of the main span bridge. For example, in the event of adoption of a cable-stayed bridge, the driver on the approach spans would not be afforded views of the cable arrangement. On the other hand, a driver on an s-curve alignment would be afforded not only side views of the main navigation span bridges, but also inward towards the Deep Bay inlet and outward towards the western waters and the open seas.
2.2.6 A drawback of the straight alignment is that experience has shown that driver concentration level tends to drop on long straight sections of road due to the lack of steering required. This could result in accidents.
2.2.7 In view of the advantages of the curved alignment over the straight alignment, the curved alignment was selected as the preferred option, and would be subject to fine-tuning of the alignment in the course of the design. The concept of the curved alignment was also presented to and accepted by the Mainland authorities. This EIA Study would be based on the s-curve bridge alignment.
2.3 Consideration of Different Built Forms
2.3.1 Various built forms considered in the selection of the conceptual design covers for the typical spans and main spans are:
Typical Spans
· Option 1 split deck with inclined vertical face box girder, and vase shaped
column;
· Option 1a split deck with double curved soffit box girder and elliptical
columns;
· Option 2 split deck with single curved soffit box girder, and oval columns;
· Option 3 twin cell box girder, with Y-shaped column.
Main Spans
· Option 1 Cable-Stayed bridge with Inclined Tower;
· Option 2 Cable-Stayed bridge with Kinked Tower;
· Option 3 Beam Bridge;
· Option 4 Twin Arch Bridge.
2.3.2 The various built forms were joint developed with the Mainland designer. Both Hong Kong and Mainland authorities had agreed in the Joint Assessment Panel held on December 2001 that Option 1a at 75m span spacing to be adopted for the typical span and Option 1 to be adopted for the main span, see Figure ES2.5 and ES2.6.
2.4 Consideration of Alternative Construction Methods and Sequence of Works
2.4.1 Different construction methodology and sequences of works were studied, given careful consideration on environmental impacts including noise, ecological, water quality etc.
2.4.2 The recommended construction methods for constructing viaducts over the mudflat area with limited water depth is using a temporary bridge platform supported by small diameter pipe-pile with steel deck running alongside the SWC alignment. In order to meet the tight construction programme, the access bridge will be built in two sections, one section starting from the shoreline and the other section starting from the deeper water section, the two sections will meet over the mudflat area. This temporary platform could serve as a marine construction access by barge and also as an easy and quick access from landside. A large temporary storage platform will be formed using two barges secured in position to provide a large storage area for construction materials and storage of the precast segment before they are lifted to place. A berthing area outside the southern navigation channel would be provided for loading and unloading of the barges. Tee-off platform at each pier location would also provide an area for substructure construction. The employment of the barges would include safe mooring arrangement during inclement weather conditions and typhoon.
2.4.3 Foundation for the SWC is proposed to be large diameter bored pile, which is very common for bridges over water. The piles are constructed from barges or temporary tee-off platforms in the regions of shallow water. All pile caps would be buried in the seabed except for the cable-stayed bridge section, where the pile caps would be exposed just above the highest tidal level. Excavation would be carried out within the de-watered sheet-piled cofferdam.
2.4.4 Precast segmental balanced cantilever construction method is proposed for the approach viaducts construction. This is a common, fast and mature method adopted worldwide. It is carried out either using launching gantry from deck level or by crane from a barge. Temporary supports are required only for the first segment to be placed at the pier with subsequent segments placed either side of this initial segment in a progressive manner. These segments are fixed by means of epoxy and temporary prestress until an in-situ stitch is formed at the centre span between the last two segments.
2.4.5 Construction of the cable-stayed bridge would involve the installation and later retraction of temporary supports in the back spans to facilitate in-situ casting of both back spans before erection of the main span can commence. After completion of the back span, the inclined tower can start construction using 4m climbing form. After completion of certain height of the tower, the main spans and cables can be constructed and installed progressively with the tower construction.
2.4.6 Other alternative construction methodology considered including constructing a temporary bund, temporary access platform placed directly above mudflats or the use of temporary pontoon platform for temporary access, construction plant and storage of precast segments before lifted to place. However, these methods and sequences of work will have direct disturbance to the mudflat area. Therefore, they are not recommended.
2.4.7 Other construction methodology considered for the construction of typical span includes 'in-situ Balanced Cantilever Box Girders', 'Incrementally Launched Box Girders' and 'In-situ Span by Span Construction'. However, these methods are less suitable for marine based environment due to the labour intensive work. These methods are therefore not recommended
2.5 Selection of Preferred Scenario
2.5.1 Based on the discussions given in the above paragraphs, the S-curved alignment with inclined tower cable stayed bridge for the main span and split deck with double curved soffit box girder and elliptical columns for the typical span are chosen. The construction methodology and sequence of works recommendation have been carefully considered to avoid or minimise environmental impacts to the area, in particular on the ecology and water quality of Deep Bay.