Useful Notes on Method Statement for Drainage Works – Civil Engineering
1. Scope:
The intention of this “document is to define the activities involved in the drainage works.
Image Source: 1.bp.blogspot.com
ADVERTISEMENTS:
2. Relevant Process/Specification:
Contract specifications
3. Tools/Equipment Required:
Theodolite, leveling staff, plate’ compactors, compressor, jack hammer, pump, measuring tape,
4. Procedure/Method:
All materials used shall comply with the requirements of the contract specifications.
ADVERTISEMENTS:
The locations and level of the drainage system to the dimensions and levels shown on the contract drawings shall be set out first.
Then the excavation in trench shall be carried out to ensure that the drains can be laid in straight lines as shown on the drawings.
Reusable excavated materials shall be set aside for reinstatement. .
Sides of trenches and other excavations greater than 1.2 m deep shall be suitably shored or shall be battered back to ensure stability of the excavation.
ADVERTISEMENTS:
Trench widths shall be kept to a minimum, allowing 400 mm over the pipe diameter for pipes up to 750 mm dia. and 600 mm for larger pipes. An allowance of 150 mm shall be added where timbering is required.
Trench widths shall be excavated 100 mm lower than the required bottom level elevation of pipe to allow for bedding.
The trench base shall be free from any protrusions and hard spots. Any unsuitable material shall be removed and replaced with suitable material.
Granular material for pipe bedding shall be hard, durable, and free from silt and clay.
The pipe bedding material shall be placed evenly on the prepared trench base, so that the pipes receive continuous support at the required gradient.
All pipes shall be laid true to line and level as indicated on the drawings. Each pipe shall be checked for correct position before laying the next pipe section.
No pipes shall be jointed together before being laid in the trench.
All pipes and fittings shall be jointed strictly in accordance with the manufacturer’s instructions.
Branches and drain line connections for future use shall be sealed off with proper stoppers, to prevent the ingress of extraneous matter.
Septic tanks, manholes and inspection chambers shall be installed as shown on drawings.
Backfilling shall not commence until the system has been successfully tested and accepted. This shall be carried out in sections as work proceeds.
Selected fill material in maximum 150 mm layers shall be placed up to the crown of the pipe and compacted by hand ramming.
A cushion layer 300 mm deep of backfill shall then be placed above the crown of the pipe and well compacted.
The remaining backfill shall be built up to ground level with suitable fill in well compacted layers.
The backfill in trenches crossing permanent roads shall be thoroughly compacted.
Backfilling around inspection chamber, septic tanks and manholes shall be carried out as specified.
The pipe lines, manholes & inspection chambers shall be tested again.
Test methods:
The pipe line shall support a water head of 1.2 m at its upper end and not more than 2.4 m at its lower end for a period of 10 minutes or as specified.
Manholes & inspection chambers shall be inspected for leaks by stopping the pipes and filling with water. The fall in the water level shall not exceed 25 mm over a 48 hours period or as specified.