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| GENERAL METHOD STATEMENT FOR DEWATERING |
TABLE OF CONTENTS
1.0. SCOPE
2.0. PURPOSE
3.0. BRIDGING DOCUMENT:
4.0. PROCEDURE
4.1. Pre-Dewatering:
4.2. Groundwater control:
4.3. Deep Well System
4.4. Well Point System
5.0. RISK ASSESSMENT HSE POINT OF VIEW
6.0. MANPOWER REQUIRED
7.0. HEALTH AND SAFETY
8.0. EQUIPMENT REQUIRED
9.0. NO OBJECTION CERTIFICATES AND WORK PERMITS
1.0. SCOPE
This is a general Method Statement for dewatering. A specific statement will be prepared for each activity and submitted for approval by the consultant, contractor, or subcontractor before the work begins.
2.0. PURPOSE
2.1. Ensure that a dewatering system is installed in areas where excavation goes below the groundwater level, providing a clean and dry working surface, as per the specified requirements and approved procedures.
2.2. Safeguard workers operating in waterlogged excavation areas and minimize the environmental impact during the dewatering process
3.0. BRIDGING DOCUMENT
Write Project details with document references.
4.0. PROCEDURE
4.1. Pre-Dewatering
Before starting open dewatering, the following tasks must be completed:
4.1.1. Site clearance has been finished.
4.1.2. The layout has been marked and verified.
4.1.3. The groundwater depth has been properly checked using trial holes.
4.1.4. Necessary approvals have been obtained from the Housing and Environment departments.
4.2. Groundwater control
Dewatering is carried out to reduce the groundwater level to at least 300 mm below the construction level, ensuring the site remains dry throughout the construction process. Depending on site conditions, we propose the following dewatering methods:
4.2.1. Ditches and sumps
4.2.2. Wellpoint system
4.2.3. Shallow well points
4.2.4. Deep well system
4.2.5. Vacuum method
4.2.6. Electro-osmosis method
This General Method Statement deals with the first four of the options stated above.
4.3. Deep Well System
4.3.1. For excavations deeper than 10 meters below the water table, a deep well drainage system is required, especially for shaft construction.
4.3.2. Boreholes are drilled around the shafts at regular intervals, based on the groundwater level and shaft diameter.
4.3.3. A borehole of at least 20 cm in diameter is drilled, and a 15 cm outer diameter casing pipe with end slots is inserted.
4.3.4. A long screen up to 25 cm is provided, and gravel filter material is filled into the space between the casing and the borehole to prevent soil from entering the pump.
4.3.5. A submersible pump with a 10 HP capacity and 7.5 kW power, capable of discharging water up to a 30-meter head, is installed 0.5 meters from the bottom of the well.
4.3.6. Each deep well will have its pump.
4.3.7. Dewatering hoses will be connected to nearby stormwater drains, or extended to the shore as needed.
4.3.8. If no stormwater drainage is available, permission from the Environmental Office is required to create sumps (approximately 2m x 2m, 1.2m deep) for temporary water collection.
4.3.9. Sumps will be located far from the dewatered area, and emptied by water tankers if necessary.
4.3.10. If the sump level is higher than the groundwater table, water will be allowed to percolate naturally while monitoring for environmental impacts.
4.4. Well Point System
4.4.1. A good point is a 6-meter long uPVC pipe, perforated at the bottom 1 meter, with a diameter of 7.5 cm, capped at the end, and covered with a wire mesh screen.
4.4.2. Well points are spaced 1-2 meters apart, based on site conditions, and connected to a common manifold, which directs water to a sump or tank.
4.4.3. Boreholes 15 cm in diameter are drilled 6-8 meters deep, with a 7.5 cm strainer tube inserted and surrounded by a gravel filter.
4.4.4. Each shaft will have about 20 well points arranged in a box or ring pattern, drilled at least 1.5 meters below the shaft's formation level.
4.4.5. Water from well points will be transferred to a header pipe using flexible riser pipes, with a suction hose connected to the dewatering pump.
4.4.6. From the data provided, the maximum water volume from a shaft is expected to be less than 500 cubic meters. A heavy-duty pump with a capacity of 150 cubic meters per hour will be used.
4.4.7. These pumps are suitable for dewatering and can handle sludge and slurry water. They feature automatic priming and re-priming capabilities.
4.4.8. For open trenches, well points will be placed on both sides as needed, and similarly arranged around isolated excavations (shafts).
4.4.9. The maximum allowable drawdown for the first well points series is 4 meters. Multi-stage systems will be used if more drawdown is needed.
4.4.10. Well points will be spaced 1-2 meters apart, with no more than 40 points per pump.
4.4.11. The dewatering system will run continuously until a stable water level is reached, with standby pumps and accessories available to avoid interruptions, which could cause serious issues.
4.4.12 A dewatering system will be used extensively for shaft and open excavation works, with temporary underdrains laid beneath structures, with the Municipality's approval.
4.4.13. Where stormwater drains exist, dewatering outlets will be connected to them, with prior permission.
4.4.14. Where no drainage system is available, approval will be obtained to construct sumps (2m x 2m, 1.2m deep) to collect water, with tankers used to empty them as needed.
4.4.15. Groundwater control activities will be closely monitored, and groundwater level stations will be set up to track changes during the work.
4.4.16. With assistance from the local housing and environment department, nearby wells will be identified, and their owners notified before dewatering starts. These wells will be monitored for any significant changes in water levels.
4.4.17. After completing the work, permission will be sought to remove the groundwater control system, with sumps and stormwater drains restored to their original condition.
5.0. RISK ASSESSMENT HSE POINT OF VIEW
5.1. RA - shaft construction and open excavation
5.2. Electrical Generator and diesel fuel
5.3. Lifting weights using Hi-up
5.4. Corrosive or brackish groundwater.
5.5. Noise
5.6. Settlement monitoring of the buildings in the vicinity where soil strata are weak, If a Condition Survey is required this will be put in hand where necessary in consultation with the Client.
6.0. MANPOWER REQUIRED
1.2. Site supervisor
1.3. Site foreman
1.4. Dewatering helpers
1.5. Driller
1.6. Diesel Mechanic
1.7. Plumber
7.0. HEALTH AND SAFETY
7.1. Personnel Protective Equipment
7.2. Toolbox talk to workers
7.3. Workers within the shaft. Confined space trained.
8.0. EQUIPMENT REQUIRED
8.1. Dewatering pump WAWA WP of 10 HP power two numbers.
8.2. Driller attached to an excavator.
8.3. Jetting rod.
8.4. Filter aggregates
8.5. PVC pipes 7.5 cm dia and 6m long with slits and plugged at the bottom
8.6. Raiser flexible pipes 7.5 cm dia, with screw lamps
8.7. Aluminum Header pipes 18 cm dia, with bush stoppers, and end plugs.
8.8. Flexible hoses of 10 bar capacity 18 cm diameter to connect between pump and header pipe.
8.9. Hand tools.
8.10. Robbin's sump pump of 0.5 HP capacity.
8.11. Installation, monitoring, and decommissioning of the dewatering system shall be carried out by GPS trained workforce associated with these workers.
8.12. As per your project requirements/recommendations
9.0. NO OBJECTION CERTIFICATES AND WORK PERMITS
9.1. Confined space entry.
9.2. Excavation permit.
9.3. Various authority permits like Defense, ROP, Municipality, etc.
9.4. Night work permits (if possible).
9.5. Only workers/persons with confined space entry training will be allowed to enter confined spaces E.g. Shafts, Chambers.
NOTE: All measurements and scope of work are only for reference. You should edit as per your SOW & Requirements along with safety compliance.
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