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1、BAKUN HYROELECTRIC PROJECT, MAIN CIVIL WORKSMAIN DAM AND POWERHOUSE EXCAVATION1.MAIN DAM FOUNDATION EXCAVATION1.1 OBJECTIVEThe main objective of this method statement is to describe in detail the sequence of works and establish priorities of work to allow embankment filling to start by scheduled in 2、the baseline programme on 17-Jan-2003. An immediate objective will be to expedite works and expose the plinth at the dam foundation for geotechnical investigation. This method statement and includes the following: Haul roads and disposal areas, Preparation of secondary river crossings, Dewatering, P3、reparation of loading platform, and Earth/rock excavation for integrated cofferdam foundation.1.2 ZONING OF WORK AREAS:From the drawing, by drawing a line connecting the highest point of the ridges of both left and right banks, the main dam work area is divided into the following work zones: Area 1:4、 From downstream face of auxiliary cofferdam to the dam axis of integrated cofferdam; Area 2: From the dam axis of integrated cofferdam to the main dam axis, below elevation 130m for the right bank, below elevation 140 for the left bank, and including the riverbed. Area: From the main dam axis to th5、e down stream limit of Main Dam, below elevation 130m for the right bank, below elevation 140 for the left bank, and including the riverbed downstream of MD Axis. Area 4: From the dam axis of integrated cofferdam to downstream limit of the Main Dam, above elevation 130m for the right bank, above ele6、vation 140 for the left bank.Each work zone is is further divided into sub-areas, each subdivision is basically defined into left abutment, riverbed and right abutment sections. Please refer to drawing no. MDEX-MS-001 (Attachment 1)1.3 METHODOLOGY1.3.1 Preparation of Haul Roads & Disposal Areas1.3.17、.1 Haul RoadsThe following haul roads shall be prepared for haulage of excavated material: Road R1: Main haul road Road R1-1: Connects right abutment at EL 210 to R1. Road R1-2: From loading platform at Right Abutment EL 130, to the upstream face of auxiliary cofferdam, and connected to R2 at Down S8、tream. Road R2: From loading platform at Right Abutment EL 70, to the disposal areas near W4, and Northern side of W6. Road L2: From riverbed EL 50 to disposal areas near W4, and Northern side of W6.Please refer to “Access Road Arrangement for Main Dam/Power House Excavation- First Stage” (Attachmen9、t 2) 1.3.1.2 Disposal AreasThe designated disposal areas will be as follows: Upstream face of auxiliary cofferdam: Designated as disposal area and at the same time a 20m-width access road across the river is to be constructed adjacent to the auxiliary cofferdam. Near W4: Disposal area at low line ar10、eas adjacent to W4. Northern side of W6: Disposal area at low line areas at Northern side of W6. W 14, and W 15.Please refer to Attachment 3”Disposal Area & Temporary Stockpile for Main Dam and Powerhouse Excavation” for the layout plan of the aforesaid disposal areas.1.3.2 Method of Overburden Exca11、vationThe following loading platforms shall be prepared to collect the material (earth/rock) pushed from the higher elevation: Top platform for the interfacing with the main dam excavation:o Right Abutment:EL 130o Left Abutment:EL 140The above platforms shall be at least 15m width. It shall be prepa12、red by forming temporary slope from higher elevation and shall be benched deeper into the design formation of slightly weathered/sound rock layer.As for the Main Dam excavation above elevation EL 130 (Right) and EL 140 (Left), the aforesaid platforms will serve as loading platforms to collect excava13、ted material pushed downslopes from higher elevation working areas. Intermediate platform This will serve as an intermediate collecting point for excavated material to control the overall dozing distance within 30m 40m.o Right Abutment:EL 100 and EL 70Following the topography and terrain at the righ14、t abutment where there are well-defined mountain ridges and gullies, intermediate platforms will be constructed in between ridges. The area will be cleared from logs or undergrowth, slopes trimmed to provided wider area, and filling to form the platform.o Left Abutment:NilThere are no apparent ridge15、s at left abutment. In addition, the thickness of overburden at the left bank is comparatively thinner and the original slope is comparatively steeper than the right bank. Hence, intermediate platforms becomes unfeasible at this area. Final platform Located at the riverbed at approximately EL 50. Fo16、rmed by the excavated material pushed towards the riverbed during the process of foundation excavation, will served as the final loading platform before the riverbed is to be excavated up to the required rock strata. This platform will mainly serve to load and haul overburden pushed downslopes from 17、the leftbank excavations.Pleas refer to drawing MDEX-MS-001 (Attachment 1).1.3.3 Filling of Secondary Crossings/Bunds.Secondary crossings will be constructed across, by filling the riverbed with suitable material taken from excavation works forming a crossing to both banks, and, also, sectionalise t18、he riverbed work areas. This is to minimize interference of works when excavation works of riverbed and upper elevation are carried out simultaneously at alternate zones. These crossings will also serve containment bunds to prevent excavated material from contaminating the integrated cofferdam work 19、areas.By forming these crossings, the river is “transformed into ponds”, and these allows dewatering of the river to be segmentalized.1.3.4 DewateringA combination of 10 and 14” water pumps will be stationed at separate segments of riverbed. Two -720,000 gallons per day pumps will be installed at th20、e downstream side of the river. Smaller pumps may be installed at the river sections formed by the riverbund, to keep and maintain the area dry during riverbed excavation and plinth construction.1.4 IMPLEMENTATION PROGRAM1.4.1 Time FrameThe time frames for respective zones are summarized as follows:21、o Zone 125-Aug-03 to 07-Oct-03o Zone 220-Aug-03 to 19-Dec-03o Zone 301-Oct-03 to 15-Jan-04o Zone 401-Dec-03 to 20-Dec-04.Please refer to Work Program on Main Dam Excavation (Attachment 4) for details.1.4.2 ResourcesRefer to Attachment 5: Table of Work Areas and Processes” and Attachment 6: “Main Dam22、 Excavation Equipment”1.5 SAFETY As a safety barrier, gabion wall in 2-1 arrangement shall be built (if necessary) as protective armor at the upper platforms (mentioned in Section 1.3.2) to prevent materials pushed downslopes from causing injuries to works at the lower sections of the dam. These gab23、ions will also serve to contain excavated materials, sludges, debris and other objects from contaminating embankment work and permanent excavation slopes. The gabion walls will be “wrapped” with geotextiles to allow clean water pass through to the cut off drains.Also, cut off trenches, of sufficient24、 shall also be provided at the upper platforms which will also prevent runoff carrying silt from contamination the dam foundation at lower elevation.Kindly refer to Attachment 7: Section View of Typical Work Sequence.2.POWER HOUSE FIRST STAGE EXCAVATION ABOVE ELEVATION 70M2.1 OBJECTIVEThis method st25、atement is prepared to strengthen the Overall Method Statement that submitted earlier for further elaborating in detailed the sequence of excavation works that related to the overburden and rock excavation for the Powerhouse. This method statement is to cover the following contents:i. Haul roads;ii.26、 Overburden excavation;iii. Rock excavation;iv. Slope protection works;v. Dewatering;ix.Foundation preparation;2.2 ZONINGBy considering the accessibility and haul road arrangement, the powerhouse can be divided in the following work zones: Area 1: From elevation 190m to elevation 145m; Area 2: From 27、elevation 145m to elevation 115m; Area 3: From elevation 115m to elevation 70m.Please refer to drawing no. PHEX-MS-001 (Attachment 8)2.3 METHODOLOGY2.3.1 Preparation of Haul Roads & Disposal Areas2.3.1.1 Haul RoadsThe following haul roads shall be prepared for haulage of excavated material: Road L2-28、2: Trunk haul road for Powerhouse excavation at Area 3 (Downstream) Road L1-1: Trunk haul road for Powerhouse excavation at Area 2 (Upstream)Please refer to “Access Road Arrangement for Main Dam/Power House Excavation- First Stage” (Attachment 2) 2.3.1.2 Disposal AreasThe designated disposal areas w29、ill be as follows: Near W4: Disposal area at low line areas adjacent to W4. Northern side of W6: Disposal area at low line areas at Northern side of W6.The extracted rock from Powerhouse that suitable for embankment fill for the Main Dam will be directly utilized for the main dam filling or will be 30、temporary stockpile at the following designated stock yards: Platform next to the Adit for the diversion tunnels; The platform between W4 and existing access road.Please refer to Attachment 3”Disposal Area & Temporary Stockpile for Main Dam and Powerhouse Excavation” for the layout plan of the afore31、said disposal areas.2.3.2 Method of Overburden/Rock Excavation2.3.2.1 Overburden/Rock Excavation Area 1At Area 1, the excavation of overburden will be started from the twin peaks apart; the working platforms are relatively smaller and more congested. The most suitable method for overburden excavatio32、n at this stage is suggested to be by way of throwing down the said materials by hydraulic excavator and bulldozer from the top of the cut slope in stages to form the benches for rock blasting. The materials will be subsequently pushed down to lower point el. 145m and pile up for collecting and load33、ing by hydraulic excavators then disposed off using 20 tons/30 tons dump trucks in later stage.At this stage, all materials including earth and rock will be pushed down to lower elevation, and due to no access road (due to very steep terrain,), the segregation of usable and unusable material is not 34、feasible. It is inevitable that all excavated materials from area 1 will be contaminated and will therefore be hauled to disposal area at W6 or W4.2.3.2.2 Overburden/Rock Excavation Area 2At Area 2, excavation will be done by the conventional method of direct load and haul. The excavated material sh35、all be collected by bulldozer, loaded using hydraulic excavator/wheel loader, and hauled to by 30t/20t dumper.Haul road L 1-1 will be used as trunk access road for the haulage. The profile and alignment for haul road L 1-1 may be branched from time to time in accordance to the overall elevation of e36、xcavation.2.3.2.3 Overburden/Rock Excavation Area 3With reference to the drawings, the twin peak of powerhouse excavation will be merged as a larger area of excavation at el. 130m. As such the excavation can be done by the conventional method of direct load and haul. The excavated material shall be 37、collected with bulldozer, loaded with hydraulic excavator/wheel loader and hauled by 30t/20t dumper.Haul road L 2-2 will be used as trunk access road for the haulage. The profile and alignment for haul road L 2-2 maybe branched from time to time in accordance to the overall elevation of excavation.I38、n order to expedite excavation of overburden at this stage, there will be substantial degree of pushing down of overburden material downslopes, to the loading platform at riverbed area. These overburden materials shall be disposed of at a later stage to W6. Rock that can be used for embankment filli39、ng or aggregates will be will be salvaged and transported separately to the designated stockpile areas, crushing plant or directly be utilized as main dam filling.2.3.3Temporary Diversion and DrainagePrior to the excavation of spillway, the existing creek between two ridges of this area will cause t40、he major impact to the powerhouse excavation. With reference to the design drawings, in order to integrate with the permanent berm drain design, a temporary drain is to be provided to join with the permanent berm drain at el. 130. The runoff of the said creek will then be collected and diverted thro41、ugh the slope drains and discharged to Batang Balui through the berm drain at el.70m.Should the said berm drain have failed to cater the runoff from the existing creek, dimension and size of the berm drains at el.130 shall be reviewed and modified to suit the actual site condition and to satisfy the42、 drainage requirement.Precipitation and wastewater during the period of construction will be discharged through the berm drains and slope drains. Reference is made to Attachment 8: Drainage System Layout 2.4 SLOPE PROTECTIONWorks for slope protection will follow closely excavation works. As a genera43、l rule, as soon as one berm layer is excavated, slope protection works will be carried out as per construction drawings prior to continuing with excavation of the next bench. As a general method, each berm (15meters) will be bench blasted into 3 stages or layers ( 5 meters per layer). This is to all44、ow a working platform for the equipment, and slope protection works can proceed without need of scaffolding. In cases where the slope has been cut into its full bench, scaffolding will have to be installed for slope protection works.2.5 IMPLEMENTATION PROGRAM2.5.1 Time FrameThe time frames for respective zones are summarized as follows:o Area 110-Aug-03 to 06-Oct-03o Area 204-Oct-03 to 20- Nov-03o Area 304-Nov-03 to 18-Jan-04Please refer to the attachment 9: WORK SCHEDULE2.5.2 ResourcesRefer to Attachment 10: Manpower and EquipmentPage 9 of 9