3                                  Consideration of Alternatives

3.1                            Introduction

In accordance with the requirements of Section 3.3 of the EIA Study Brief, this Section describes the need for the Project and the consideration of alternative construction methods and work sequence. 

3.2                            The Need for the Project

As highlighted in Section 1, traffic queues are already observed on the northern and southern approaches of POR during the evening peak period.  The situation will likely deteriorate further when the nearby development is in place.  The tailback of traffic on the southbound carriageway of the northern approach may block the through traffic from Tai Lam Tunnel / NT North to Tuen Mun / Tin Shui Wai / the Hong Kong – Shenzhen Western Corridor.  It is envisaged that POR will become saturated progressively with the increasing population intake in the nearby area. 

The objective of the Project is to relieve the traffic pressure and traffic queues of the existing POR so that its design flow / capacity ratio can be maintained at a reasonable value acceptable to the Transport Department (TD).

In accordance with the Traffic Impact Assessment Report([1]), the Ratios of Flow to Capacity (RFC) of POR are 0.84 and 0.99 during AM and PM Peaks for the Year 2006, respectively.  As demonstrated by the RFC, POR has almost reached its capacity in Year 2006.  With the proposed improvement works, the RFC could be reduced from 1.05 to 0.57 and 1.22 to 0.78 in Year 2011 for the AM and PM Peaks, respectively.   The proposed improvement works were shown to be capable of relieving the queuing problem at POR.

3.3                            Considerations of Alternative Construction Methods and Work Sequence

Different construction methods for the foundation works, erection of the new flyover deck and retaining structures are examined based on the following criteria:

·           Severity and duration of the construction impacts on the nearby environmental sensitive receivers;

·           Traffic impacts on the existing carriageways;

·           Site constraints, such as limited working spaces, unforeseen ground conditions and potential impacts on the existing structures;

·           Satisfaction to the design requirements, such as loading requirements and retained heights; and

·           Tight construction programme.

3.3.1                      Options for Piling

Four piling options have been assessed during preliminary design, namely,

·           Option A: Continuous Flight Auger Piles;

·           Option B: Large Diameter Bored Piles;

·           Option C: Pre-bored Concrete Piles; and

·           Option D: Driven Steel H-piles.  

The above options are considered common piling methods in Hong Kong and therefore the required machinery should be available in the Hong Kong market for foundation construction.  Benefits and drawbacks of each option are discussed below and summarised in Table 3.3a:

Option A: Continuous Flight Auger (CFA) Piles

In this construction method, continuous flight auger is used to form the pile holes, which will be infilled with concrete or grout after the placing of steel reinforcement which would either be steel H-piles or rebar cages.  The sizes of CFA piles range from 300mm to 700mm in diameter and the most common pile size is 610mm in diameter.

Benefits

·           Piles can be installed without appreciable noise and vibration;

·           Generally ground loss/disturbance is minimal during pile hole excavation and therefore less disturbance on the adjacent structures and piles;

·           Compared with Options B and C, it is a more economic pile construction method under suitable ground condition (i.e. for cohesive soils) and satisfying load requirements;

·           Less working spaces are required compared with Option B;

·           Construction time is relatively fast; and

·           There is no restriction on the normal working hours.

Drawbacks

·           Pre-excavation or pre-boring may be required to cope with any underground obstruction such as boulders within the soil matrix;

·           Compared with Option B, this pile type has less structural capacities to resist lateral and vertical loads;

·           Specialist contractor may be required to ensure workmanship; and

·           Designed pile length is limited by the construction plants and it is generally less than 40m.

Option B: Large Diameter Bored Piles

Shafts of the large diameter bored piles are constructed by traditional boring machines with temporary steel casings or drilling fluid as a supporting system. Chisel and grab system with casings, and Reverse Circulation Drilling (RCD) are the common large bored pile construction plants. Typical sizes of large diameter bored piles range from 1m to 3m in diameter. In Hong Kong, large diameter bored piles are usually designed as end-bearing piles founding in bedrock. For friction pile design, shaft-grouting may be provided to enhance the shaft frictional resistance.

Benefits

·           Extensive experience of using this type of construction method has been developed in Hong Kong;

·           Compared with Options C and D, less noise and vibration will be generated during pile installation;

·           It is relatively easy to overcome underground obstructions;

·           Compared with Option A, this construction method has higher flexibility in  designing longer piles to suit design requirements;

·           This pile type has larger structural capacities to cater for lateral and vertical loads; and

·           With provision of temporary casings during pile excavation, shaft collapse and over-excavation can be minimized.

Drawbacks

·           Comparatively larger working spaces are required due to larger piling plants required;

·           It may be susceptible to bulging or necking during pile concreting in unstable ground due to the larger pile size;

·           Compared with Options A and C, risk of loosening of surrounding soil is higher, causing ground movement and structural impacts on the adjacent structures; and

·           Extension of working hours to the restricted hours may be required for concreting of very long piles.

Option C: Pre-bored Concrete Pile

Pile shafts are formed by drilling rigs with the use of down-the-hole hammers.  Pre-bored concrete piles are considered as small diameter bored piles.  Therefore, similar to the large diameter bored pile construction, the pre-bored holes will be inserted with rebar cages and in-filled with concrete.  Shaft-grouting can also be provided for enhancing shaft resistance for friction piles.

Benefits

·           It is relatively easy to overcome underground obstructions by down-the-hole hammers;

·           Compared with Option A, this construction method has higher flexibility in designing longer piles to suit design requirements;

·           Compared with Option D, less ground borne vibration will be induced;

·           With provision of temporary casings during pile excavation, shaft collapse and over-excavation can be minimized.

Drawbacks

·           Pile excavation by down-the-hole (DTH) hammer will cause disturbance affecting the adjacent structures and foundations;

·           Construction Noise Permit (CNP) under the Noise Control Ordinance (NCO) will be required for the use of DTN hammer. Longer construction period may be required due to the restricted working hours imposed by the Authority;

·           It may be susceptible to bulging or necking during pile concreting in unstable ground; and

·           Possible collapse of the annulus space (over-cut) between the side wall and temporary casing before pile concreting would reduce the skin friction.

Option D: Driven Steel H-Pile

This construction method is to pitch H-piles using percussion method until final sets are achieved.  A hydraulic hammer is commonly used for pile driving in Hong Kong.  In bouldery grounds, pre-boring can be carried out before the insertion of the pile.  The bored holes will be subsequently infilled with sand and the H-piles driven to design founding levels.

Benefits

·           Pile installation method is relatively simple and degree of redundancy can be easily incorporated to provide flexibility to deal with any unexpected ground condition;

·           Extensive experiences of driving H-piles within marble areas in Hong Kong;

·           Installation is generally unaffected by groundwater conditions;

·           Compared with Options A and C, this pile type can be designed to withstand high bending and tensile stresses; and

·           Compared with Option A, this construction method has higher flexibility to design longer piles to suit design requirement.

Drawbacks

·           The driving operation will generate relatively high noise and vibration levels, affecting the nearby environmental sensitive receivers;

·           Construction Noise Permit (CNP) under the Noise Control Ordinance (NCO) will be required for percussion pile installation. Longer construction period may be required due to the restricted working hours imposed by the Authority;

·           Higher ground borne vibration and movement induced from the driving operation may cause damage to the adjacent piles, structures and utilities installations;

·           Pre-boring may be required to overcome underground obstructions and require longer construction period.

3.3.2                      Selection of Pile Construction Method

By comparing the above four options, Option A - Continuous Flight Auger (CFA) Pile is preferable in view of the following:

·           Relatively less noise and vibration impacts;

·           No restriction on normal working hours;

·           Relatively fast construction rate;

·           Less impacts to existing structures; and

·           Satisfying the loading and ground condition requirements.

 

Table 3.3a  Comparison of Alternative Pile Construction Methods

Options

Alternative Construction Methods

Benefits

Drawbacks

A

Continuous Flight Auger Pile

 

·     Piles can be installed without appreciable noise and vibration;

·     Generally ground loss/disturbance is minimal during pile hole excavation and therefore less disturbance on the adjacent structures and piles;

·     Compared with Options B and C, it is a more economic pile construction method under suitable ground condition (i.e. for cohesive soils) and satisfying load requirements;

·     Less working spaces are required compared with Option B;

·     Construction time is relatively fast; and

·     There is no restriction on the normal working hours.

·     Pre-excavation or pre-boring may be required to cope with any underground obstruction such as boulders within the soil matrix;

·     Compared with Option B, this pile type has less structural capacities to resist lateral and vertical loads;

·     Specialist contractor may be required to ensure the pile construction workmanship; and

·     Designed pile length is limited by the construction plants and it is generally less than 40m.

B

Large Diameter Bored Pile

 

·     Extensive experience of using this type of construction method has been developed in Hong Kong;

·     Compared with Options C and D, less noise and vibration will be generated during pile installation;

·     It is relatively easy to overcome underground obstructions;

·     Compared with Option A, this construction method has higher flexibility in designing longer piles to suit design requirement;

·     This pile type has larger structural capacities to cater for lateral and vertical loads; and

·     With provision of temporary casings during pile excavation, shaft collapse and over-excavation can be minimized.

·     Comparatively larger working spaces are required due to larger piling plants required;

·     It may be susceptible to bulging or necking during pile concreting in unstable ground due to the larger pile size; Compared with Options A and C, risk of loosening of surrounding soil is higher, causing ground movement and structural impacts on the adjacent structures; and

·     Extension of working hours to the restricted hours may be required for concreting of very long piles.

C

Pre-bored Concrete Pile

 

·     It is relatively easy to overcome underground obstructions by down-the-hole hammers;

·     Compared with Option A, this construction method has higher flexibility in designing longer piles to suit design requirements;

·     Compared with Option D, less ground borne vibration will be induced;

·     With provision of temporary casings during pile excavation, shaft collapse and over-excavation can be minimized.

·     Pile excavation by down-the-hole (DTH) hammer will cause disturbance affecting the adjacent structures and foundations;

·     Construction Noise Permit (CNP) under the Noise Control Ordinance (NCO) will be required for the use of DTN hammer. Longer construction period may be required due to the restricted working hours imposed by the Authority;

·     It may be susceptible to bulging or necking during pile concreting in unstable ground; and

·     Possible collapse of the annulus space (over-cut) between the side wall and temporary casing before pile concreting would reduce the skin friction.

D

Driven Steel H-Pile

 

·     Pile installation method is relatively simple and degree of redundancy can be easily incorporated to provide flexibility to deal with any unexpected ground condition;

·     Extensive experiences of driving H-piles within marble areas in Hong Kong;

·     Installation is generally unaffected by groundwater conditions;

·     Compared with Options A and C, this pile type can be designed to withstand high bending and tensile stresses; and

·     The pile installation method has less settlement concern, which is essential for bridge structures.

·     The driving operation will generate relatively high noise and vibration levels, affecting the nearby environmental sensitive receivers;

·     Construction Noise Permit (CNP) under the Noise Control Ordinance (NCO) will be required for percussion pile installation. Longer construction period may be required due to the restricted working hours imposed by the Authority;

·     Higher ground borne vibration and movement induced from the driving operation may cause damage to the adjacent piles, structures and utilities installations;

·     Pre-boring may be required to overcome underground obstructions and require longer construction period.

 

 

3.3.3                      Options for Construction of New Flyover Deck

Three methods for construction of the new flyover deck have been examined.    The benefits and drawbacks of each method are discussed below and summarised in Table 3.3b:

Option 1: Precast Segmental Balanced Cantilever Box Girder

Due to limited working areas on site and heavy traffic flows at the existing interchange, concrete box girder segments is likely to be used for construction of the flyover. They are usually cast in casting yards in Mainland China, and shipped to Hong Kong to the contractor’s work area.  The segments will be placed in a symmetrical manner about the pier head so as to create a balanced cantilever.  The segments will be then fixed by post-tensioning.  The same operation will continue until the box girder reaches the abutment or the adjacent completed balanced cantilever.  In-situ stitch joint will be formed between the cantilevers to provide continuity of the bridge deck.

Benefits

·           Conventional falseworks and formworks are not required and therefore extensive temporary road closures/diversions at the interchange can be avoided;

·           In-situ concreting works are significantly reduced compared with the other methods, minimizing the potential impacts to the environment, such as noise and air quality.  In addition, the operation is quieter compared with other methods; 

·           Construction time is shorter and better construction planning can be achieved;

·           Although not labour intensive as most of the works can be achieved off site. Even so, more experience and mature skilled labourers have been developed in recent years in Hong Kong for this method of construction, and therefore local resources should be readily available.

Drawbacks

·           Construction cost is relatively high for an approximately 140m long bridge; and;

·           The gantry girder cannot be very long thus restricting the span length to a maximum of around 60m currently in Hong Kong.

·           Worldwide specialist contractors may be required.

Option 2: Cast In-situ Deck with Conventional Temporary Works

Falseworks/scaffolding will be erected across the existing carriageway for placement of formworks.  Rebar fixing will be subsequently carried out before in-situ concreting of the deck structure.

Benefits

·           It is a simple and straight forward construction method;

·           Extensive experiences have been gained among the local contractors.

Drawbacks

·           Temporary road closure is necessary for erection of falseworks and during concreting, causing significant impacts to the existing traffic;

·           Construction time may be taken longer for the span-by-span construction method;

·           More working areas are required; and

·           More construction plants will be involved for the in-situ works and the longer construction period will induce higher environmental impacts to the surroundings, such as noise and air quality impacts.

Option 3: Cast In-situ Balanced Cantilever Box Girder by Form Travellers

This construction method is a combination of Options 1 and 2.  First pair of the box girder segments will be cast in-situ symmetrically about the pier head by conventional falseworks and formworks.  Form travellers will be launched and anchored on the previously cast segments for rebar fixing and concreting of the subsequent pair of segments.  After stressing the new segments to the deck structure, the travelling formworks will be moved forward and the construction procedures will be repeated until completion of the bridge deck.

Benefits

·           Requirements of the conventional falseworks and formworks is greatly reduced; and

·           Heavy lifting operations are minimised.

Drawbacks

·           It is time consuming for steel fixing, concreting of segments and adjustment of the form travellers as compared with the other two options, causing longer construction period and higher environmental impacts;

·           Comparatively, more construction plants will be involved for the in-situ works, which will also induce higher environmental impacts on the surroundings, such as noise and air quality impacts.

·           Temporary road closure during day-time cannot be avoided, particularly for concreting above existing carriageway, which cause disturbances to the traffic;

·           There is limited experiences for highways construction using form travellers in Hong Kong; and

·           Worldwide specialist contractors may be required.

 

3.3.4                      Selection of Construction Method for New Flyover Deck

The potential environmental impacts to the NSRs in the vicinity, disturbance to existing traffic flows and construction speed are all important factors in the selection of a preferred construction method for the new flyover deck.  Option 3 is considered to be inferior in all the above aspects, and therefore has not been considered further.  Both Options 1 - Precast Segmental Balanced Cantilever Box Girder and Option 2 - Cast In-situ Deck with Conventional Temporary Works are found to be acceptable in terms of the potential construction noise impacts, as demonstrated in Section 4.  Option 1 is a common method for the construction of flyovers/viaducts in Hong Kong in recent years, and therefore the required construction equipment should be readily available.  Option 1 however has an advantage over Option 2 in its faster construction speed, and therefore helps to reduce the duration of potential environmental impacts arising from the works of the Project.  Based on the above, Option 1 has been adopted as the method for deck construction for the purpose of the following environmental assessments but the prospective contractor may still adopt Option 2 where site and traffic conditions allow.

Table 3.3b Comparison of Alternative Construction Methods for New Flyer Deck

Options

Alternative Construction Methods

Benefits

Drawbacks

1

Precast Segmental Balanced Cantilever Box Girder

·     Conventional falseworks and formworks are not required and therefore extensive temporary road closures/diversions at the interchange can be avoided;

·     In-situ concreting works are significantly reduced compared with the other methods, minimizing the potential impacts to the environment, such as noise and air quality; In addition, the operation is quiet compared with other methods. 

·     Construction time is shorter and better construction planning can be achieved;

·     Although not labour intensive as most of the works can be achieved off site.  Even so, more experience and mature skilled labourers have been developed in recent years in Hong Kong for this method of construction, and therefore local resources should be readily available.

·     Construction cost is relatively high for an approximately 140m long bridge;

·     The gantry girder cannot be very long thus restricting the span length to a maximum of around 60m currently in Hong Kong; and

·     Worldwide specialist contractors may be required.

2

Cast In-situ Deck with Conventional Temporary Works

·     It is a simple and straight forward construction method;

·     Extensive experiences have been gained among the local contractors.

·     Temporary road closure is necessary for erection of falseworks and during concreting, causing significant impacts to the existing traffic;

·     Construction time may be taken longer for the span-by-span construction method;

·     More working areas are required; and

·     More construction plants will be involved for the in-situ works and the longer construction period will induce higher environmental impacts to the surroundings, such as noise and air quality impacts.

3

Cast In-situ Balanced Cantilever Box Girder by Form Travellers

·     Requirements of the conventional falseworks and formworks is greatly reduced; and

·     Heavy lifting operations are minimised.

·     It is time consuming for steel fixing, concreting of segments and adjustment of the form travellers as compared with the other two options, causing longer construction period and higher environmental impacts;

·     Comparatively, more construction plants will be involved for the in-situ works, which will also induce higher environmental impacts on the surroundings, such as noise and air quality impacts;

·     Temporary road closure during day-time can not be avoided, particularly for concreting above existing carriageway, which cause disturbances to the traffic;

·     There is limited experiences for highways construction using form travellers in Hong Kong; and

·     Worldwide specialist contractors may be required.

3.3.5                      Options for Construction of Retaining Structures

Three types of retaining structures are considered for the formation of road embankments, namely reinforced concrete (RC) cantilever wall, piled wall and reinforced earth (RE) wall. As these are widely adopted as solutions for retaining structures in Hong Kong in recent years, the required machinery should be available during construction.

Construction of RC cantilever walls involves excavation, formwork erection, rebar fixing, placing of wall drainage system, in-situ concreting, temporary works removal and backfilling of the walls.  It is a relatively straight forward construction method and extensive project experiences have been gained in Hong Kong.

Piled wall utilizes the lateral resistance of the piles for earth retaining purpose.  It is commonly used in areas where excavation for RC wall construction is difficult or technically infeasible. Piled wall construction involves pile shaft excavation using piling plants, placing of rebar cages/H-piles, in-situ shaft concreting/grouting, excavation and construction of lagging between piles.  The retained height would generally be higher than the RC wall, and excavation works would be much less. However, pile installation usually induces noise and vibration with higher degree of environmental concerns.  Moreover, for the works immediately adjacent to the existing carriageways, temporary road closure or traffic diversion may be required to provide larger working spaces required for the piling plants.

Construction works for RE wall comprises excavation, and placing of fill materials and reinforcements in compacted backfill layers until the design road formation level is reached.  The construction sequences are relatively simple and construction rate is usually faster than the conventional RC retaining wall construction. Experiences in the use of RE wall have been gained in recent years to form embankment slopes for highway projects in Hong Kong.  Moreover, this wall structure is flexible and can accommodate higher differential settlement that is particularly suitable for the Project site, which has the presence of compressible layers.  From environmental point of view, the construction method comparatively generates less noise, and consumes less energy. 

By comparing the above three construction method of retaining structures, it is considered that the RE wall option is more preferable in terms of environmental consideration, construction speed and flexibility.  However, RC cantilever walls at some areas are still required due to small retaining height required or site constraints, such as tie-in issues with the proposed/existing RC structures.

In conclusion, the optimum solution of retaining structures is provision of RE walls, associated with RC walls, where appropriate.

3.3.6                      Consideration of Construction Sequences

The interchange improvement works are broadly grouped into three main tasks as follows:

Task 1: Provision of southbound carriageways, including construction works for Road Resurfacing & Remarking A, Left-turn Lane A and Slip Road C; and

Task 2: Provision of northbound carriageways, including construction works for Slip Roads A and B, new flyover and Road Resurfacing & Remarking B.

Task 3: Earmark an approximate 200m long and 2.5m high vertical noise barrier along the southern arm of the northbound carriageway of Yuen Long Highway to the east of the planned schools (refer to Annex C11-2 for noise barrier location).

Overall Planning

Task 2 is scheduled to start following the commencement of Task 1 for alleviating the cumulative environmental impacts to the surroundings and to enable early handover of the carriageway. 

Construction sequences are also highly dependent on the selection of construction methods.  Consideration of different construction methods for piling works, erection of the new flyover deck and retaining walls has already been discussed in above sections.  However, one of key approaches is to maximize the use of pre-cast units, and therefore minimise in-situ works and the consequential environmental impacts on the Project site.  Other preferences in the construction methods include fast construction progress and the use of relatively silent plant.

Construction Works for Task 1

The existing alignment of the Road Resurfacing & Remarking A is to be locally adjusted to increase the number of carriageway lanes from three to four. Works include resurfacing of the existing carriageway and reprovision of road markings. 

Left-turn Lane A construction includes dismantling of the existing noise barrier, construction of a subway extension, and integrated planter and retaining structures, formation of fill embankment slopes, construction of pavement and the associated drainage, watermain, utilities and landscaping works.

Construction of Slip Road C involves retaining walls construction, re-profiling of the existing fill embankment slopes, construction of pavement and the associated drainage, watermain, utilities and landscaping works. 

As shown on the construction programme in Annex C2, it is anticipated that construction of Task 1 will take around one and half year.  Other than the construction methods and sequences described above, there are limited alternatives due to the relatively straight-forward works sequences and the tight construction programme.

Construction Works for Task 2

Construction of the new flyover involves pile installations, construction of pilecaps and piers/columns, erection of deck, installation of parapets and planters, pavement works and the associated drainage, watermain, utilities and landscaping works.

Similar to the construction of Slip Road C, works for Slip Roads A and B comprise construction of retaining walls or formation fill embankment slopes, installation of parapets, construction of pavement and the associated drainage, watermain, utilities and landscaping works.

The existing alignment of Road Resurfacing & Remarking B is to be slightly shifted westward to accommodate the additional lane at the northbound carriageway.  Works mainly consist of road formation and re-surfacing, and reprovision of existing drainage and road furniture along the realigned carriageway.

Noise induced from piling works is one of the major concerns; construction works for Slip Roads A and B are therefore planned to commence after the completion of the pile installation works. Moreover, pavement construction of Slip Roads A and B, the new flyover and Road Resurfacing & Remarking B is planned to be conducted in a similar period so that paving of the southbound carriageway can be carried out within the same time frame. Consequently, it can reduce the numbers of overlapping joints and mobilization of construction plants. In addition, it can mitigate the overall level of disturbance to the environment and traffic.

Construction Works for Task 3

Construction of the proposed noise barrier will be earmarked as a provisional work of the Project as the construction schedule of the planned schools has not yet been confirmed. Moreover, it is anticipated that the noise barrier will be designed in form of a retaining structure with an extended wall plinth for the noise barrier. Therefore, construction of the noise barrier will involve site formation, retaining wall construction, backfilling works and reinstatement of the existing street furniture. Furthermore, the noise barrier will be positioned at the crest of existing fill embankment slope of Yuen Long Highway, and therefore re-profiling /reinstatement of the existing fill slopes may be required during the noise barrier construction.

 


 

([1]) Traffic Impact Assessment Report prepared under the Agreement No. WD 3/2006 for Improvement to Pok Oi Interchange - Traffic Impact Assessment and Alignment Design Study dated July 2007.