Method Statement for Dewatering
CONTENTS
- Scope of Work
- Definitions
- References
- Subcontractors
- Materials
- Plant & Equipment
- Safety
- Prior Activities
- Procedure
- Responsibilities
- Attachments
1. Scope:
| 1.1 | This Method Statement covers the various methods utilized to control water in excavation. |
2. Definitions:
| 2.1 | MS: STM Method Statement : Storm Water Drainage Work |
| 2.2 | ITR Inspection and Test Request |
| 2.3 | PPS Particular Project Specification |
| 2.4 | GPS General Project Specification |
| 2.5 | UG Underground |
| 2.6 | TPS Technical Project Specification |
3. References:
| 3.1 | TPS Section 901 :Clause 901.3.3 Pumping and Dewatering |
4. Subcontractors:
| N/A |
5. Products:
| 5.1 | Filter Material (3- 5mm Aggregates) |
6. Tools & Equipments:
| 6.1 | Drilling Machine |
| 6.2 | Well Point, header pipes and discharge lines |
| 6.3 | Pumps, hoses and accessories |
7. Safety
| 7.1 | TPS Section : Site Safety and Security |
| 7.2 | CSM Construction Safety Manual |
| 7.3 | OAJ-JV Safety Manual |
| 7.4 | Safety Hazards and Precautions |
| a. | Strike UG Services: Use locator and hand excavation (trial holes) |
| b. | Excavation Collapse: Batter sides to safe slope or provide shoring (if required) |
| Keep spoil at least 2.0 meter away from edge | |
| c. | Fall into Excavation Provide fencing, guardrails or flagging tape around edges. Provide safely fixed ladders (every 50 meters). Provide banks man when operating equipment near excavation. |
8. Prior Activities:
| 8.1 | A “Dewatering Plan” will be submitted showing typical well points system, sumps, pumps, discharge lines, silt traps and monitoring equipment. |
| 8.2 | Earth berms of cut-off ditches will be provided to prevent surface water from entering the excavation where needed. |
| 8.3 | Proper discharge away from work area will be provided and Permits will be obtained where needed. |
| 8.5 | A checking for existing utilities will be conducted before drilling or excavating. |
9. Procedure:
| A)- 9.1 | Selection of dewatering method depends on the following :- Depending on certain factors, one or more of the below detailed dewatering methods can be adopted. The factors affecting the decision include among others: The soil characteristics The soil hydrology The size and depth of the excavation The proposed method of excavation and ground support The proximity of existing structure |
| B)- | Dewatering Method:- |
| 9.2 | Pre-drainage (Well Points System) Well points will be drilled by augers or high pressure water jet and fitted with riser pipes at sufficient intervals to maintain the surrounding water table below the level of excavation. |
| After well points are driven into the ground they are surrounded with a filter layer (5mm sand) to facilitate suction. | |
| Flexible elbows are used to connect the riser pipes to Header pipes, which in turn are connected to suction Pumps. | |
| The water is then pumped through flexible hoses or discharge lines to the discharge points. | |
| Once the water table has sufficiently dropped, the excavation may proceed in dry soil. | |
| 9.3 | Open Pump (Sumps) |
| Excavation continues below the water table and a collection trench and/or pit is dug to one side of the excavation. | |
| Submersible pumps are then utilized to draw off the water and pump it to a discharge area. | |
| 9.4 | Horizontal Drains |
| Used in conjunction with one of the above mentioned methods. | |
| Prepare drainage ditches filled with gravel or perforated pipes. | |
| The ditches can be near the trench walls or crossing below the blinding. | |
10. Responsibilities:
| 10.1 | It is the Site Manager’s duty to ensure that all necessary safety and protective equipment are provided along with properly trained and qualified personnel. |
| 10.2 | It is the Site Supervisor’s duty to organize the work in safe manner and to ensure that safety and protective equipment are being properly utilized. He must also assure the method statement procedure is followed and must observe any hold-points and notify the Consultant for inspection. |
| 10.3 | It is the worker’s duty to properly wear the protective equipment provided and obey all safety rules. They must also perform the work as described in the method statement procedure. |
| 10.4 | The Site Engineer will ensure that the site preparation activities are conducted according to the approved method statement. |
| 10.5 | The Material Engineer will ensure that the materials being used are in accordance to the project specifications and the approved method statement. |
Md Abu Zaed Khan
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