The purpose of this method statement is to define the procedure for sequence of works for dewatering in construction works at site to complete the excavation from the existing levels down to the formation level considering all safety and quality requirements.
This method statement covers dewatering works at construction site having 6 meter basement structure, to allow for a dry surface at formation level for basement including lift pit levels.
References
- Project Specifications for dewatering system
- Contract Drawings
- Project Quality Plan
- Project Safety Plan
- Material Approvals / Material Submittals
Necessary Equipment’s / Tools / Materials
- Augers
- Dewatering Pumps / Sump Pumps
- Shovels
- JCBs
- Settlement Tanks
- Total Stations
- Auto Levels
- Header Pipes
- Slotted pipes
- Flexible hoses & clamps
- Sand & gravel
Roles and Responsibilities
Project Manager
Ensuring that the project works in his zone are carried out in accordance with company policies and in accordance with the requirements of project quality and HSE plans.
Also to ensure all subcontractor works to comply with the requirements of the quality and safety plans.
To ensure that all the required equipment to execute the dewatering works according to the construction program are available, in good condition, and provide any additional equipment that might be required.
Co-ordinate with the Construction Manager, Project Engineer, Safety Engineer, Foreman and Surveyor for a safe and proper execution of the works.
To guide specific attention to all safety measures in co-ordination with the safety officer/engineer.
Construction Manager
Ensure area is ready and safe to start the works.
Ensure all the required approvals/ NOC are obtained from the relevant authorities prior to the commencement of works.
Set up the required equipment and allocate required manpower through discussion with the Project Manager and Project Engineer / Works Foreman.
Ensure the works are carried out according to the specification, quality plan, approved shop drawings and the construction program.
Provide sufficient and safe access for operatives, trucks and pumps.
Ensure that the dewatering works are carried out well outside the building line and will not cause any hindrance for concreting or waterproofing works.
Project Engineer
Set up the required equipment and allocate required manpower through discussion with the Project Manager and Project Engineer / Works Foreman.
Ensure the works are carried out according to the specification, quality plan, approved shop drawings and the construction program.
Provide sufficient and safe access for operatives, trucks and pumps etc.
Ensure that the dewatering works are carried out well outside the building line and will not cause any hindrance for concreting or waterproofing works.
QA/QC Engineer / Inspector
Ensure that Consultant/ Client inspection requests are implemented.
Compilation of all necessary quality control check lists.
The control of work performance by means of checking the work before consultant’s inspection and issuing RFI’s, punch lists and assisting consultant during the inspections.
Completion of documentation to verify the work performed.
Controlling all inspection activities on site in line with ITP’s.
Ensuring that all test equipment including surveying equipment is calibrated and suitable for use at site.
Surveyor:
Establish benchmarks from agreed reference points, provide required setting out and level markings and follow up with regular checks.
Co-ordinate with the Project Engineer / Foreman and ensure the approved shop drawings/construction drawings shall be implemented properly.
Maintain survey details and reports, periodically check the progressing works and advise the project manager of any deviation from the drawings.
HSE Engineer/Officer
Ensure implementation of all safety measures related to the nature of works being carried out, and in accordance with the Project Safety Plan.
To ensure that all the persons involved in the works are aware of their responsibilities, and that they have enough understanding of the safety procedures.
Prepare, provide and distribute risk assessment for the activity on the site.
The safety officer in co-ordination with the Project Manager shall ensure that all the implemented safety measures are effective enough to maintain safe working on the site.
To maintain continuous inspections of the site activities, advise and train persons on a daily basis to prevent accidents and personnel injury.
Give special attention to housekeeping, and ensure that the site is maintained clean and tidy.
To ensure all the relevant safety sign boards for different works are in place.
Foremen/ Works Supervisor
Ensure the works are progressed in the sequence as agreed with the Project Manager.
Liaise with the Construction Manager/ Project Engineer for the required plant and material and allocation of the required manpower.
To liaise with the site manager to ensure all the required plant / materials are available to perform the dewatering work properly.
Ensure full time supervision, and the ground water level has to be monitored throughout along with running pumps.
Well Points Drilling
The location of well points has to be set out, the well holes to be drilled or augured at the specified interval to the required depth, and slotted pipes to be placed and surround to be filled with gravel.
Header Pipes and Pump Connections:
Once the well points are being constructed the header pipes should be connected with flexible hoses and clamped.
The header pipe connections shall be terminated as per design length and connected to the dewatering pump.
Dewatering pumps should be placed on stabilized ground.
Connecting to External Strom Water Manhole:
The Dewatering Pumps should be connected to Strom Water Manholes via the settlement tank by flexible hose.
Prior to commissioning the pump, the dewatering foreman has to confirm with Project Engineer / Construction Manager that the required approval / NOC are obtained from the relevant authorities.
Completion of Dewatering System:
The 1st stage dewatering ring can be completed via repeating above steps all around the excavation area.
All the dewatering pumps are to be placed as per the design at correct location and level, in order to lower the ground water level at least 60 cm below formation level.
It should be away from building line without hindering the subsequent concreting and waterproofing works.
Dewatering for Lift Pits
The 2nd stage dewatering system can be installed around Lift Pits and other deep excavations if required by using same as in 1st stage or by sump pumping.
Stand by Pump
In addition to the Dewatering Pumps installed as per design, an additional Stand by pump should be maintained at site.
Monitoring Dewatering System
The dewatering system should be monitored 24 hours x 7 days throughout until the completion of tanking of basement and the hydrostatic uplift is balanced by the weight of structure, and approval of consultant to be obtained prior to terminating or decommissioning the dewatering system.
Monitoring the Ground Water Level:
Ground water level has to be monitored via installing piezometers and / or by measuring via well points at different locations around the site throughout the duration of dewatering.
These details should be recorded on daily basis and submitted to consultant on every week.
Any adverse increase of water level noticed should be brought to consultant’s attention and immediate remedial action to be taken.
Decommissioning of Dewatering System
Following equipment and tools may be used as required during the decommissioning procedure of dewatering system.
- Generators (Working & Standby)
- Control panels with audio and visual alarm systems
- Welding Machine
- Gas Cylinders etc.
- Other Hand Tools
- Other necessary cabling and electrical equipment
DECOMMISSIONING PROCEDURE
Contractor will submit inspection request for switch off pumps for monitoring as per contract dewatering can be stopped once ground floor been casted but as per consultant instruction dewatering will be stopped once structure load reach 1.5 the uplift force from water, calculations will show the stage at which dewatering can be stopped.
Decommissioning of the dewatering system requires several steps to make sure that the excavation stays dry and safe for construction.
First, pumps to be switched off progressively and monitor the structure for any leakages, after which pumps shall be safely removed or otherwise, remain operational if the situation requires.
The following describes the steps that will be followed for the recovery test.
- Prior to switching off the pump to be decommissioned, official instruction need to be provided confirming that it is safe to decommission the dewatering system.
- After official instruction is received, subcontractor will start switching off the submersible pumps progressively.
- Meanwhile, the structure will be monitored for any leakages for a period of approximately two weeks.
- In case of leakages are observed, it is recommended to switch on the pumps to enable to lower the water table and to commence the repair works.
- After completion of monitoring period, contractor will confirm for full decommissioning and removal of the dewatering system.
Following steps will be followed for the removal of pump from the deep well.
The submersible pump, discharge hose etc. shall be removed from the well casing.
Well casings then shall be closed using a suitable steel end-cap to be placed at the top of the casing.