Construction of Site Formation Works for the South Soko Location

 

A1          Construction of Site Formation Works

A1.1            General

In order to construct the proposed LNG Receiving Terminal facility it is necessary to form at least 20-25ha of land.  Where the available land area is insufficient it is necessary to undertake reclamation to make up the difference.  A comparison of the offshore reclamation requirements for the site layouts is given in Annex B1.

In order to remain clear of the tidal effects in Hong Kong a minimum platform level of +6mPD is proposed although higher levels may be considered during detailed design stage to reduce cuttings and effects of wave overtopping where necessary.

The site formation layout has been largely dictated by the following criteria: -

·           Maximisation of the use of the existing reclamation area at the site which was formed in the early 1980’s for the Vietnamese refugee detention camp to reduce further disturbance to the island.

·           Reduction of reclamation in the environmentally sensitive waters around South Soko Island

·           Creation of sufficient space for two tanks with provision for a third future tank.

·           Maintenance of safe distances between the storage tanks and associated process facilities.

 

A1.2            Assessment Parameters

In order to assess the engineering implications of the on-land formation works at each layout option at South Soko the following engineering assessment parameters have been considered: -

·            Volume of excavation in soil

·            Volume of excavation in rock

·            Volume of soil and rock to be removed from site

·            Impact of formation works on the overall construction programme

·            Extent of slope stabilisation measures required

·            Slope maintenance requirements

·            Potential future hazard from slopes

·            Blasting restrictions

 

A2          Volume of Excavation in Soil and Rock

A2.1            Option 1 – Base Case 

The excavation for this option will be essentially undertaken within the slopes on either side of the existing reclamation platform, which is at a level of between +5mPD and +6mPD. 

The excavation on the northern side of the site will be undertaken to provide sufficient land area for the two initial tanks with provision for a third tank in the future.  The excavation is to be undertaken completely within the hillside for two purposes: -

1)       To enable the tanks to be founded directly onto rock which will permit the use of raft foundations thus negating the need for deep foundations.

2)       To screen the tanks from the visually sensitive receivers on the south side of Lantau Island

The excavation on the southern side of the site will be undertaken to provide sufficient land area for the process plant and associated facilities to maintain the safe distances from the storage tanks.  The elevation of these facilities will be mainly at +10mPD in order to reduce the volume of cutting.  

On the northern side of the site the existing hillside slopes are steep with average angles of between approximately 60° at the base to 35° higher up.  In order to create a platform of sufficient area to house the three storage tanks on the northern side of the site at a level of +6mPD will require a cutting up to approximately 72m in height.  This is based on the assumption that the existing slope comprises approximately 5m of completely decomposed rock over slightly to moderately decomposed rock as indicated from the available drillhole data on the hillside.  It is assumed that the rock slopes will be cut at an angle of between approximately 60° and 80° with 1.5m wide benches typically every 10m in accordance with local practice and supported with rock bolts and dowels as necessary. The soil slopes will be similarly cut to an angle of between approximately 30° and 45° and supported with soil nails as necessary.  The precise slope geometry will be determined during the detailed design stage.  

On the southern side of the site the existing hillside slope is similarly steep at the base with average angles of about 60° but with a flatter profile on the upper levels with angles of between 25° and 35°.  In order to create a platform of sufficient area to house the process facilities on the southern side of the site at a level of +10mPD will require a cutting up to 30m in height.  This is based on the assumption that the existing slope comprises approximately 5m to 15m of completely decomposed rock over slightly to moderately decomposed rock as indicated from the available drillhole data on the hillside. It is assumed that the rock slopes will be cut at an angle of between approximately 60° and 70° with 1.5m wide benches every 10m in accordance with local practice and supported with rock bolts and dowels as necessary.  The soil slopes will be similarly cut to an angle of between approximately 30° and 45° and supported with soil nails as necessary.  The precise slope geometry will be determined at the detailed design stage.

Adopting this arrangement it is estimated that a total volume of approximately 0.44 x 106m³ of soil and 1.63 x 106m³ of rock material will be excavated from the existing slope cuttings. The total excavated volume will be about 2.07 x 106m³. 

A2.2            Option 2 – Full Reclamation

Similar to Option 1, the excavation for the site will be essentially undertaken within the slopes on either side of the existing reclamation platform.  The excavation on the northern side of the site will be undertaken to provide sufficient land area for the two initial tanks with provision for a third tank.  However since the tanks will be located further to the south, the excavation volume for this option is smaller than that of Option 1.

The excavation on the southern side of the site will be undertaken to provide sufficient land area for the process plant and associated facilities to maintain the regulatory safe distances from the storage tanks.  The elevation of these facilities will be mainly at +10mPD in order to reduce the volume of cutting.

In order to create a platform of sufficient area to house the three storage tanks on the northern side of the site at a level of +6mPD will require a cutting up to 64m in height, using the same geological assumption as in Option 1.  It is assumed that the rock slopes will be cut at an angle of between approximately 60° and 70° with 1.5m benches every 10m in accordance with local practice and supported with rock bolts and dowels as necessary.  The soil slopes will be similarly cut to an angle of between approximately 30° and 45° and supported with soil nails as necessary.  The precise slope geometry will be determined at the detailed design stage.

On the southern side of the site a platform at a level of +10mPD will be created to allow sufficient area to house the process facilities.  This is based on the assumption that the existing slope comprises approximately 5m to 15m of completely decomposed rock over slightly to moderately decomposed rock as indicated from the available drillhole data on the hillside.  It is assumed that the rock slopes will be cut at an angle of approximately 70° with 1.5m benches every 10m in accordance with local practice and supported with rock bolts and dowels as necessary.  The soil slopes will be similarly cut to an angle of between approximately 30° and 45° and supported with soil nails as necessary.  The precise slope geometry will be determined at the detailed design stage.

Adopting this arrangement it is estimated that a total volume of 0.34 x 106m³ of soil and 0.97 x 106m³ of rock material will be excavated from the existing slope cuttings. The total excavated volume will be about 1.31 x106m³. 

A2.3            Option 3 – SE Jetty

For Option 3, the excavation profile at the northern side of the site will be similar to that of Option 1 in terms of quantities.  A platform at a level of +6mPD will be formed to house the three storage tanks on the northern side of the site which will require a cutting up to 72m height, using similar ground profile assumption previously. 

On the southern side, a platform at a level of +10mPD will be created to allow sufficient area to house the process facilities. 

Excavation will also be required along the proposed pipe trestle, which will run from the newly formed platform through the southern hills to the jetty located at the southeast corner of South Soko Island.

All of the rock and soil slope will be cut at a profile as in Option 1 and 2, and supported with soil nails, rock bolts and dowels as necessary. 

Adopting this arrangement it is estimated that a total volume of 0.52 x 106m³ of soil and 1.77 x 106m³ of rock material will be excavated from the existing slope cuttings.  The total excavated volume will be about 2.29 x 106m³. 

A3          Volume of Spoil to be Removed from Site

A3.1            Option 1 – Base Case

For site formation to be cost effective and sustainable, a balance between cut and fill quantities is required.  However for the Option 1 layout Island this is not feasible as the reclamation requirement is purposely low for environmental reasons.  The reclamation fill requirement is estimated at 0.52 x 106 m³ (Ref. Annex B), which is significantly smaller than the amount of spoil excavated from the hillside.  Approximately 95% of the soil material will be suitable for use within the reclamation and exportation of this material will not be required.  The remaining 5% is assumed to be top-soil, which is unsuitable for reclamation purposes and will be used for landscaping to the extent practical on the site. 

The excavated rock material will be suitable for use in the following areas: -

·            Beneath the sea walls.

·            As rock armour along the sea walls.

·            Potentially as rock armour protection for the proposed submarine gas pipeline to Black Point Power Station and the new submarine watermain.

Due to the relative timing of the works the rock spoil material will need to be initially removed to leave sufficient working area at the site.  Given the large quantity of rock material being exported from the site a separate stockpile site will need to be established, preferably, nearby to the South Soko Island to store, sort, grade and possibly crush the rock materials to create suitable engineering materials for use on the site. 

A surplus of approximately 0.04 x 106 m³ will be created from the formation works at this site.

A3.2            Option 2 – Full Reclamation

The total fill requirements for option 2 is estimated to be 1.93 x 106 m³, which exceeds the amount of spoil excavated from the hillside.  If all of the generated spoil is to be reused for reclamation purposes, then no surplus material will result.  However, due to the relative timing of the excavation and reclamation works it will be necessary for the rock spoil to be taken off to a stockpile site for crushing and sorting before transporting back to South Soko Island for further reuse as described in Section A3.1.

A3.3            Option 3 – SE Jetty

As in the case of the Option 1 layout, the excavation quantity of the Option 3 layout far exceeds the reclamation and filling requirement although a high proportion of the excess material may be used as protection to the proposed submarine pipeline to Black Point Power Station. The recycling and reusing strategy of the waste material will be similar to that of Option 1.

A surplus of approximately 0.12 x 106 m³ is estimated to be created from the formation works at this site.

 

A4          Extent of Slope Stabilisation Measures

A4.1            Option 1 – Base Case

The cuttings for the tank structures will extend to a height of approximately 72m and will be predominantly in rock.  The face area of the slope is estimated to be approximately 22,000m2.  The rock mass forming the hillside will likely be jointed and fractured for which extensive stabilisation measures will be required including rock bolting, dowelling and buttressing as necessary to prevent toppling, wedge and sliding failures.  The soil slopes will be similarly stabilised with soil nails.  The natural slope above the cut slope will also require inspection and possibly some stabilising works.  Appropriate drainage measures will be required to drain surface run-off away to reduce infiltration into the slopes. 

A4.2            Option 2 – Full Reclamation

The cuttings for the tank structures will extend to a height of approximately 64m and will be predominantly in rock.  The face area of the slope is estimated to be approximately 20,000m2.  The stabilisation measures required for the soil and rock slopes will be similar to Option 1.  The slope stabilisation works in this case may be classified as on a smaller scale than of Option 1 and therefore given a relative score of 5.

A4.3            Option 3 – SE Jetty

The cuttings for the tank structures will extend to a height of approximately 72m and will be predominantly in rock.  The face area of the slope is estimated to be approximately 22,000m2.  The cuttings for the pipe trestle will extend to a height of approximately 20m and the face area of the slope is estimated to be approximately 10,000m2.  The stabilisation measures required for the soil and rock slopes will be similar to Option 1.  The slope stabilisation works in this case may be classified as on a larger scale than Option 1 and therefore given a relative score of 2.

A5          Slope Maintenance Requirements

The cut slopes created for the site formation works are large and extensive i.e., >5m height and will therefore be subject to registration with the government.  The slopes will be classified as Category 1 in view of their consequence to life.  As such the slopes will be subject to Routine Maintenance Inspections each year and Engineer Inspections for Maintenance every 5 years.  The slope maintenance requirements may therefore be considered as being the same for all  options, for the pipe trestle.

A6          Long Term Slope Hazard

The terminal facility will be located adjacent to a high cut slope in soil and rock with an extensive natural slope above it.  Even with the slope stabilisation measures and long term maintenance activities there is a risk of future instability. The risk is classified as being the same for the first two options, with Option 3 having the highest risk as the slope area created is larger due to the additional excavation for the pipe trestle.

A7          Impact of Site Formation Works on Construction Programme

A7.1            Option 1 – Base Case

The construction of the storage tanks is on the critical path for the construction of the receiving terminal facility.  The excavation within the hillside to create the formation for the tanks is therefore also on the critical path.  Assuming an excavation rate of approximately 42,000m3 per week the excavation works will take approximately 270 days.  The impact on the program is given a relative score of 3.

A7.2            Option 2 – Full Reclamation

Since the excavation volume for the Option 2 layout is significantly smaller than of Option 1, the excavation works will take only 160 days.  The impact on the programme is therefore given a relative score of 5.

A7.3            Option 3 – SE Jetty

Similar to Option 1, the excavation within the hillside to create the formation for the tanks is on the critical path.  Assuming an excavation rate of 42,000m3 per week the excavation works will take approximately 290 days.  The impact on the programme is therefore given a relative score of 3.

A8          Blasting Restrictions

The South Soko Island site is approximately 6km south of Lantau Island.  There are no significant residential areas near to the site.  The only restriction to blasting will be with the supply of emulsion explosive to the site, which is controlled by the Mines and Quarries Department of Hong Kong.  However, given the remoteness of the site it is likely that a magazine storage and explosive manufacturing plant will be established on the site, which will overcome this issue.  Restrictions to blasting are therefore considered to be low and equal for all three options.


A9          Summary for Site Formation Construction

A summary of the parameter values and relative scores derived from the engineering assessment for the site formation construction is given in Table A1 below.

Table A1 - Summary for Site Formation Construction

Parameter

Option 1
(Base Case)

Option 2
(Reclamation)

Option 3
(SE Jetty)

Volume of excavation in soil

(106m³)

RS = 3

(0.44)

RS = 5

(0.34)

RS = 2

(0.52)

Volume of excavation in rock

(106m³)

RS = 3

(1.63)

RS = 5

(0.97)

RS = 2

(1.77)

Volume of soil to be disposed of

(106m³)

RS = 3

(0.04)

RS = 5

(0)

RS = 2

(0.12)

Volume of rock to be disposed of

(106m³)

RS = 3

(0)

RS = 3

(0)

RS = 3

(0)

Impact on construction programme (months)

RS = 3

(12)

RS = 5

(9)

RS= 3

(13)

Slope stabilisation measures required

RS = 3

RS = 3

RS = 3

Slope maintenance

RS = 3

RS = 3

RS = 3

Future slope hazard

RS = 3

RS = 3

RS = 3

Blasting risks

RS = 3

RS = 3

RS = 3

RS = Relative Score

A10    Scoring for Site Formation Construction

Each of the parameters summarised above in Table A1 have been scored in accordance with the procedure described in Section 2.1.3.  The results are shown below in Table A2.  The table also shows the total score for each layout derived using the relative weightings given in Table 2.3.

Table A2 – Scoring for Each Layout Option at South Soko Island for Site Formation Construction

Parameter

Weight

Option 1
(Base Case)

Option 2
(Full Reclamation)

Option 3
(SE Jetty)

Score

WS

Score

WS

Score

WS

Volume of excavation in soil

0.05

3

0.15

5

0.25

2

0.10

Volume of excavation in rock

0.25

3

0.75

5

1.25

2

0.50

Volume of soil to be disposed of

0.20

3

0.60

5

1.00

2

0.40

Volume of rock to be disposed of

0.05

3

0.15

3

0.15

3

0.15

Impact on construction programme

0.10

3

0.30

5

0.50

3

0.30

Slope stabilisation measures required

0.10

3

0.30

3

0.30

3

0.30

Slope maintenance

0.05

3

0.15

3

0.15

3

0.15

Future slope hazard

0.05

3

0.15

3

0.15

3

0.15

Blasting risks

0.15

3

0.45

3

0.45

3

0.45

Total Weighted Score

 

3.00

 

4.30

 

2.45

Normalised Score

3.57

5.00

2.98

 

From the result of the assessment of all parameters for site formation construction, Option 2 is the preferred layout.