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.
1. Title
Method Statement for dewatering works
2. Purpose and scope
- 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.
3. Work procedure
- 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.
- 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.
- 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.
- For safety purpose, provide safety barrier along the perimeter of the excavated area to prevent fall hazards.
- 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.
4. Safety
- All works shall comply with the safety procedure or instruction as set out in project safety plan and municipality rules and regulations.
- Basic Personal Protective Equipment ( PPE ) shall be worn by all staff or laborers.
- All operatives shall be inducted before entering the site.
- Tool box talk shall be conducted before the activity.
5. Equipments and Tools
- 1 no. of 6″ Centrifugal Vacuum assisted pumps
- 8′ Header pipes approximately equal to 120 linear meter with fittings.
- 3000 gallon water tank
- 1000 gallon settlement tank
- Rotary auger drilling machine
- 6 meter pipes as well points 2′ diameter
- High pressure water jetting pump
- 10″ flexible hose for discharge
6. Materials and References
- Approved Shop Drawing or Sketch
Great article,Thanks for sharing such an amazing article.
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Hi sir,
Can you share an ITP for dewatering?
Thanks
What is the right time for stopping dewatering?
When you reach 85 percent of the total works of the project, especially high-rise building. But this depends on the situation.
We can actually stop the dewatering works in the early stage of construction provided that the uplift pressure due water have been neutralized by at least 25% of the weight or mass of the structure. There are calculations apply to determine and check when is the best time to stop the dewatering works. You can refer to this article for further details.
https://www.thestructuralworld.com/2019/02/22/hydrostatic-uplift-check-in-basements-substructures/
Hi The Structural World,
Thanks for your in-depth explanation and to supplement my answer. A very well explained reply indeed.
Regards,
Can you please suggest a method on how to close a deepwell encasement. We are going to cast a Pumping Chamber base slab (volume=2,300 m3), but the consultant rejected our proposal by providing a blockout (dimension-1 m x 1 m) on the area where the deepwell is located. They rejected because after casting the wall of the Pumping Chamber, we proposed to close with concrete, but did not meet the splicing condition as per our project specifications (to be 2.90 m long). Please provide suggestion and reference from either ASTM or ACI standards to support the idea.
Thanking you in advance.
Hi Noel,
Have you heard about jetted pump method, I don’t know if this is the right name for it, whereby the use fewer pump than well point method.
Hi Noel, can you please help me with the method statement and risk assessment
how to follow safety procedures for dewatering in Manhole with underground utilities?
It is really difficult to figure out what are the potential hazards that we could consider. You better send to me the drawing and photos of the actual situation of the site if you can.
i need method statement of dewatring
Engr. Noel, Can you discuss further how to terminate dewatering pipes. Thanks.
Alex
In the case of the method statement above, we have just pulled out the pipes after casting of the water tank wall and backfilled almost two third of the height of tank. But in the case of our project in Dubai, that’s different type of shutting off the Dewatering System.
Sir Noel, at which stage of construction a building we can stop the dewatering process? Please, share also the Method of Statements for this. Thanks a lot.
We had a project in Dubai, it consists of 9 buildings in one common basement parking. It was a long and wide raft foundation. I remember we stopped the dewatering when we completed the casting of the raft and when we completed the casting of 3.5 meters retaining wall along the perimeter of the project. I’ll publish that ebook soon. Just watch for it.
Would you please share the ITP of Dewatering.
,
I’ve replied to your email.
Can you send ITP to me Please?
CURRICULUM VITAE
SAJJAD HUSSAIN
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PARSONAL DATA
Name: Sajjad Hussain
Father Name: Ashiq Hussain
Date of Birth: 12/08/1986
Marital Status: Married
Nationality: Pakistani
Languages: Arabic, Urdu, English, Panjabi
Academic Education : SSC
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Date of Issue: 18/09/2014
Date of Expiry: 17/09/2019
Skills:
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UAE Experience
Ten(10) year Experience in MABANI DELMA GENERAL CONTRACTING CO. As a Dewatering Foreman.
In Abu Dhabi ADSSC-11557 and Deep Sewer line. Installation of GRP,UPVC and HDPE pipe line and Manhole,Chambers,Connection up to Hand overing From 2011 to till Date