Dewatering Method Statement
This method statement details the installation, operation,
and maintenance of deep well dewatering system for the construction site.
- Dewatering system aim.
Dewatering is defined as the removal of the natural ground
water to a level below the normal water table. The target water level for this
project is 1.00 meter below excavation level. This level may change according
to your site condition.
- Proposed dewatering system.
It is proposed to install 28 deep wells plus 2 deep wells as
temporary at the site to depth 10.00 and – 13.00 from sea level, respectively.
Deep well dewatering involves the drilling of a well and installation of a
perforated well screen into which a submersible borehole pumps can be fitted.
The pumps are connected to a common discharge header through which the water is
pumped to a discharge point.
- Deep well construction.
- Drilling & Temporary casing installation.
A temporary steel casing is drilled/rotated into place by a
pilling rig. An auger is then used to remove the materials inside the casing.
This process of casing rotation followed by material removal continues until
the casing is at the required depth. Polymer or Bentonite will be used during
the drilling along with the steel casing in order to ensure the stability of
the borehole’s sides. Additionally, Bentonite or Polymer is used in drilling
fluids to increase viscosity and reduce filtration losses. When Bentonite
interacts with water it swells because it is hydrophilic and adsorbs water.
Good quality Bentonite when used in drilling fluids will give the required
viscosity and acceptable loss.
- Well screen installation.
Following completion of drilling and material removal from
the steel casing, a well screen is installed with centralizers to position the
pipe in the center of the temporary steel casing. The screen is hoisted using a
crane.
The annulus between the well screen and the deep well casing
is filled with 3/8” or 3/16” aggregate to allow drainage into the screen and
prevent excessive fine materials passing through into the pumping area. A
temporary cover seals the top of the screen pipe to prevent accidental
insertion of aggregate.
After the aggregate has filled the annulus to surface, the
temporary steel casing is removed from the well by the piling rig.
- Well development.
- Pump installation.
- Pump control and discharge arrangement.
- Commissioning.
Once constructed, the dewatering system shall be started with the approval of the client’s nominated representative. The system will be commissioned to ensure it is operating correctly as per the dewatering design.
- French drains (if required)
The French drains may be constructed as an integral part of the excavation works. A network of French drains can be installed depending on ground conditions. French drains shall be interconnected with deep wells & the collected water in the French drains will be channeled to the deep wells and will be pumped out using the existing submersible pumps. The added advantage of French drains is their ability to capture water ingress through any fissures that are encountered. The flexible nature of the system allows for dewatering to be developed in parallel with the excavation.
- Decommissioning.
Decommissioning of the dewatering system requires several steps to make sure that no water leakage occurs in the basements due to the termination of dewatering system. Internal pumps shall be turned off and well pits shall be closed prior to switching off peripheral pumps.
- General sequence of decommissioning.
Following are the sequence for decommissioning of dewatering system:
Once the structure obtained enough weight to counteract uplift pressure, main contractor’s engineer shall give written instruction to start decommissioning work. Once the instruction issued by main contractor’s engineer the following will be implemented on site.
Carry out recovery tests on site for interior wells to assess the recovery rate for each deep well. This exercise shall be done for every individual well and group of wells.
According to the results achieved from the test, a report will be issued by dewatering subcontractor is explaining the sequence od decommissioning showing how many phases will the plan be, wells shutdown distribution and each well case whether it will be decommissioned or sacrificed. Another recovery test shall be conducted during each phase of decommissioning.
Consultant to acknowledge and approve the report stating the sequence of decommissioning.
After the report is approved by the consultant the following will be implemented at site:
Case 1 – deep well decommissioning.
- Switch off the pump and disconnect the pump from discharge line.
- Remove submersible pump, riser pipes, etc. from the deep well casing.
- Deep well casings can be closed using an end cap to be placed at the top of the deep well casing.
- After closing the top of deep well, remaining works like filling of well pit, blinding, waterproofing, raft concreting, etc. can be executed.
- Case 2- deep well sacrificing.
- If the recovery is quick, pumps need to be sacrificed.
- Deep well casings can be closed using an end cap to be placed at the top of the deep well casing.
- After closing the top of deep well, remaining works like filling of well pit, blinding, waterproofing, raft concreting, etc. can be executed.
- After completion of the above, pump can be switched off based on main contractor’s engineer instruction.