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.

Prior to commissioning the pumping system, it is necessary to develop the deep well to ensure the efficient performance of the well. Well development provides a means by which to improve the natural permeability of the ground surrounding the well.

A rigid hydraulic pipe connected to a compressor is inserted into the well. The well is filled with water and compressed air injected into the well. The compressed air creates a fountain effect which ensures the circulation of water through the filler material and into the well. This will ensure the removal of any fines in the vicinity of the well. This process continues until the water emerging from the top of the well is free of fines.

- Pump installation.

The pump is powered by a 3-phase electric motor which is mounted on the underside. An operational test is conducted on the pump before installation.

The pump is connected to the riser pipe, power cable and lifting ropes to hoisting the pump. The units will be lifted up using a crane with the pipe strapped to the lifting cable. The pump is centered over the well and then lowered into the PVC casing. Following installation, the lifting rope is tied off at surface for removal of the pump for maintenance.

Following installation of the pump, the well shall be continuously filled by an external water source. Pumping continues until the water being discharged is observed to be free from fines.

- Pump control and discharge arrangement.

The pumps will be powered by a generator. A standby power generator set will be available on site in the event of power failure from the main generator.

Pumps shall be connected to control panels fitted with both Audio and Visual warning alarms.

The control panels shall be connected to a distribution board, which is in turn connected to the generator.

The piping arrangement from the pump will consist of a 2 or 3” diameter riser pipe. The riser pipe will be rigid/semi-rigid pipe to the top of the well. From the top of the well to the discharge header lay flat discharge hose shall be used. Tow valves will be inserted on the 6” discharge line (1 no. NRV,1no. butterfly valve). The butterfly valve will be connected to enable the operator to control the flow of water from the well and allow removal of the pump if and when required.

The collected water produced by the ground water control system shall be discharge into a discharge header main which feeds the water to a settlement tank arrangement prior to discharging to the final discharge point.

- 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.

- Well point construction.

- Preparation of working platform level.

Initial excavation for all structures shall include an installation platform.

- Predrilling (if required).

Boreholes shall be predrilled to the required depth when installing in harder formations or as means of ensuring the integrity of the well point. Pre-drilling shall be executed by the use of a self-propelled drilling rig.

- Jetting.

Well points shall be installed into the boreholes at 1 m intervals. High pressure water shall be injected into the ground through a 6 m lance, forming a borehole into which the well point shall be inserted. Once the well point is installed the lance is held in position until the water being ejected out through the top of the borehole runs clean.

- Aggregate backfilling.

The borehole and well point surrounds are backfilled with 3/16” aggregate, 10-15 cm above the working platform. As the lance is withdrawn from within the borehole, the aggregate shall reduce to 0.0 level by the displacement created by removal of the lance. This procedure is repeated upon installation of each well point.

- Header pipe network.

Upon completion of the installation of the designed number of well points, the well points shall be connected to the 6” header pipe network by means of swing pipe, which in turn is connected to the dewatering pump.

- Discharge.

The collected water produced by the ground water control system shall be discharged into a discharge header main which transfers the water to a settlement tank arrangement prior to discharging sea, in line with specification and municipality regulations. Settlement tanks shall be properly positioned and cordoned off to ensure separation from operatives and vehicles.

- Commissioning.

Once complete, the dewatering system shall be started with the approval of the client’s representative. After the first 12 hours of running, all well point locations shall be observed and adjusted, so that the correct volume is created to achieve full efficiency of the system.