Construction of the Site Formation for the Black Point Location
A1 Construction of Site Formation Works
In order to construct the proposed LNG Receiving Terminal facility it is necessary to form at least 20-25ha of land area if an on-site batching plant is required, otherwise between 15ha to 20ha is sufficient. 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 2-A-B.
In order to remain clear of the tidal effects in
The location of the proposed site at Black Point was originally discussed with the Government in 1992 as reserve for future power plant expansion. The possible site formation works at this site can undertaken through a combination of cutting into the existing hillside and / or reclamation along the coast.
A1.2 Assessment Parameters
In order to assess the engineering implications of the on-land formation works at each layout option at Black Point 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 area of excavation for the site is limited to the provision of sufficient land area for the two initial tanks to enable them to be founded directly on rock which will permit the use of pad/raft foundations, thus negating the need for deep foundations. However, the existing slopes of the hillside are steep, therefore, in order to create a platform of sufficient area to house the tanks at a level of +6mPD a cutting up to 74m height will be required. This has been based on the assumption that the existing slope comprises approximately 5m to 10m of completely decomposed rock over slight 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 60-80° 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 30° and 45° and supported with soil nails as necessary, the precise geometry of the slope will be subject to detailed design. Adopting this arrangement it is estimated that a total volume of approximately 0.22 x 106 m³ of soil and approximately 0.77 x 106 m³ of rock material will be excavated from the existing slope cutting. The total excavated volume will be approximately 0.99 x 106 m³.
A2.2 Option 2 – Full Reclamation
No excavation will be carried out for this option. All the land required for the terminal facilities and the LNG tanks will be reclaimed in front of the Black Point headland.
A2.3 Option 3 – Full Excavation
Option 3 is the opposite of Option 2 as there is no reclamation proposed. The required site area will be provided by cutting into the Black Point headland. In order to create a platform of sufficient area to house the tanks at a level of +6mPD a cutting of up to 130m in height will be required. This is based on the assumption that the existing slope comprises approximately 5m to 10m of completely decomposed rock over slight 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 60-80° 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 30° and 45° and supported with soil nails as necessary, the precise geometry of the slope will be subject to detailed design. Adopting this arrangement it is estimated that a total volume of approximately 3.11 x 106 m³ of soil and approximately 10.9 x 106 m³ of rock material will be excavated from the existing slope cutting. The total excavated volume will be approximately 14.0 x 106 m³.
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. The reclamation fill requirement is estimated at 3.6 x 106 m³ (Ref. Annex 2-A-B) which is significantly larger than the amount of spoil excavated from the hillside. Approximately 95% of the soil material will be suitable for use within the reclamation. 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. 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, near to Black Point to store, sort, grade and possibly crush the rock materials to create suitable engineering materials for use on the site.
The excavated rock material will be suitable for use within the reclamation in the following areas:-
· Beneath the sea walls.
· As rock armour along the sea walls.
Given the large quantity of rock material being exported from the site a separate stockpile site will need to be established nearby to the Black Point to store, sort, grade and possibly crush the rock materials to create suitable engineering materials for use on the site. Given the limited space on site it is likely that all of the rock material will need to be initially exported from the site to the stockpiling area and then imported back to the site for reuse within the works once it has been suitably graded. No surplus of rock will be generated.
A3.2 Option 2 – Full Reclamation
As there is no excavation for this layout option, no surplus of excavated material will be exported off site.
A3.3 Option 3 – Full Excavation
As the excavated material cannot be used within site as there is no reclamation works, all the generated excavated material (approximately 14.0 x 106 m³) will be surplus and needs to be disposed off 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 74m and will be predominantly in rock. The face area of the slope is estimated to be approximately 20,000m2. The rock mass forming the headland will likely be extensively 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 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
Slope stabilisation measures may be required on the existing NW-facing rock slopes. The maximum height of the slopes is approximately 135m. The rock mass forming the headland will likely be extensively 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 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. The slope stabilisation works in this case are classified as being similar to the base case.
A4.3 Option 3 – Full excavation
The cuttings for the tank structures will extend to a height of approximately 130m and will be predominantly in rock. The face area of the slope is estimated to be approximately 60,000m2. The stabilisation measures required for the soil and rock slopes will be similar to Option 1 and 2. The slope stabilisation works in this case may be classified as being more significant than Option 1 and 2 and therefore given a relative score of 1.
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 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 maybe considered as being the same for the first two options, with Option 3 having greater risk as the slope area created is larger.
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 small risk of future instability. The risk is classified as being the same for the first two options, with Option 3 having the highest risk of all as the slope area involved in larger in comparison to Options 1 and 2.
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 15,000m3 per week the excavation works will take approximately 300 days.
A7.2 Option 2 – Full Reclamation
As there is no excavation works, the impact of site formation works on the programme is the lowest of all three options and therefore given a relative score of 5.
A7.3 Option 3 – Full Excavation
Similarly 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 approximately 15,000m3 per week the excavation works will take approximately 4,360 days. The impact on the programme is classified the highest of all three options and therefore given a relative score of 1.
A8 Blasting Restrictions
A8.1 Option 1 – Base Case
The site is located close to the existing Black Point Power Station and stringent blasting restrictions can therefore be expected. Similarly, there are residential areas on the other side of the headland less than 2km from the blasting area which may also affect the magnitude and progress of the blasting works. It is expected that a maximum excavation rate of approximately 15,000m3 per week will be achievable given these restrictions.
A8.2 Option 2 – Full Reclamation
As there is no excavation works, there are no blasting concerns.
A8.3 Option 3 – Full Excavation
Blasting restrictions are considered to be the same as those described for Option 1.
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 |
Option 2 |
Option 3 |
|
Volume of excavation in soil (106m³) |
RS = 3 (0.22) |
RS = 5 (0) |
RS = 1 (3.1) |
|
Volume of excavation in rock (106m³) |
RS = 3 (0.77) |
RS = 5 (0) |
RS = 1 (10.90) |
|
Volume of soil to be disposed of (106m³) |
RS = 3 (0) |
RS = 3 (0) |
RS = 1 (3.1) |
|
Volume of rock to be disposal of (106m³) |
RS = 3 (0) |
RS = 3 (0) |
RS = 1 (10.90) |
|
Impact on construction programme (months) |
RS = 3 (10) |
RS = 5 (0) |
RS = 1 (145) |
|
Slope stabilisation measures required |
RS = 3 |
RS = 3 |
RS = 1 |
|
Slope maintenance |
RS = 3 |
RS = 3 |
RS = 1 |
|
Future slope hazard |
RS = 3 |
RS = 5 |
RS = 1 |
|
Blasting risks |
RS = 3 |
RS = 3 |
RS = 1 |
RS = Relative Score