Method Statement for Dewatering

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There are cases that the excavation or any underground works can not proceed because of the water table is at high level or almost on the ground level.

In this case we have underground water tank to be constructed, we found out that the water table just few meters beneath the ground surface therefore dewatering should have to be undertaken by us.

In the case of high-rise buildings, we expect that the foundations of those buildings are deep and we may expect that  water beneath at this type of building is unavoidable. Dewatering equipment would have to be enough in size of pipes, machines, pumps etc. in order to cater the huge volumes of water to withdraw.

Especially if the foundation type is mat foundation for high-rise building, it is necessary that dewatering shall be done before and during underground works.

Here is the method statement for dewatering works.

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1. Title

Method Statement for dewatering works

2. Purpose and scope

  1. The purpose of this method statement is to explain the proper procedure on how to do the dewatering system without any safety violations within our project. The works shall be done as per specification and approved drawing.

Dewatering Photo 1

3. Work procedure

  1. Initial preparation
    • Protect structure, prepare and adjust to the site from damage caused by settlement and hazards created by dewatering.
    • Any closing of roads, streets, or facilities shall be with permission from authority has jurisdiction and then provide alternative access.
  2. Installation of dewatering pipe
    • The header pipe shall be connected to well points placed at 1 meter centers with sufficient depth to ensure that lowering of the water table shall be carried out from a level not less that 1 meter below the excavation bottom.
      Dewatering Photo 2
    • Pre-drilling will be carried out prior to installation of the well point, followed by high-pressure water jetting, to flush out sand and create mini well.
    • Provide extra pump encase of breakdown may happen, it can be replaced immediately and without interruption of dewatering.
    • A filter media of 3/16 inches (3 – 5 mm) screened black sand shall be inserted around the 3/4 depth of the wells to act as barrier against fines and prevent choking of well points.
    • Operate the dewatering system continuously 24 hours a day until drain and structures have been constructed to fill material and until written by authorization to cease dewatering.
    • All the header pipes will be connected to discharge pipe running over land to the approved area by authorities jurisdiction.
    • Dispose the water removed in a manner to avoid endangering public health, environment, and neighboring property.
      Dewatering Photo 3
    • For safety purpose, provide safety barrier along the perimeter of the excavated area to prevent fall hazards.
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4. Safety

  1. All works shall comply with the safety procedure or instruction as set out in project safety plan and municipality rules and regulations.
  2. Basic Personal Protective Equipment ( PPE ) shall be worn by all staff or laborers.
  3. All operatives shall be inducted before entering the site.
  4. Tool box talk shall be conducted before the activity.

5. Equipments and Tools

  1. 1 no. of 6″ Centrifugal Vacuum assisted pumps
  2. 8′ Header pipes approximately equal to 120 linear meter with fittings.
  3. 3000 gallon water tank
  4. 1000 gallon settlement tank
  5. Rotary auger drilling machine
  6. 6 meter pipes as well points 2′ diameter
  7. High pressure water jetting pump
  8. 10″ flexible hose for discharge

6. Materials and References

  1. Approved Shop Drawing or Sketch
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