High Ground Water vs. Laying Sanitary Pipe

[samond]

Could anyone living in areas with high ground water give me some ideas about laying pipe in these conditions?

Almost 2 years ago we laid a sanitary main in an area with high ground water and heavily saturated soil. We lost almost all the road and brought in truckloads of boulders that were pushed into the soil about 8' deep and backfilled with import material. The main was televised for a two year warranty and was found to have sagged - drastically.

I don't know how to fix this. I am hesitant to have the contractor dig up and relay the main for fear it will be a rerun from the first time. I am also afraid that the sags will get worse as time passes.

If anyone could give ideas or share what is done in high groundwater areas I would really appreciate it.

 

[GeoPaveTraffic]

The key is to dewater the site prior to excavation.

If the site had been properly dewatered, likely you wouldn't have had the problems you described.

As for what to do now, dewater the site, lay a new line, and abandon the sagged line.

Your project geotechnical engineer should be able to help you with the details. If you don't have a geotechnical engineer, get one.

Best of luck.

Mike Lambert

 

[cvg]

first of all, you have to re-install the sewer. you just cannot leave a sagging sewerline in, it will plug quickly and be a constant maintenance issue.

there are ways to improve the subgrade in a wet trench, for instance geogrid or geotextile can be layed in the trench to help stabilize. but really you need a good geotech to investigate and make recommendations for your project.

 

[SlideRuleEra]

samond - I agree that a good geotechnical engineer is essential. However, another part of the problem is a Contractor who does not know how to work (successfully) in high groundwater conditions. High water table does not doom the project... having a Contractor without the proper skills does that.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[samond]

Thank you all for your replies! I will check into a geotechnical engineer.

I appreciate it.

 

[bimr]

"Groundwater can be a serious hindrance during excavation, pipe laying, and backfilling. If properly planned for in advance of construction, difficulties associated with groundwater can be minimized. Maintain the water level below the pipe bedding and foundation to provide a stable trench bottom. It is important to ensure the groundwater is below the bottom of the cut at all times, to prevent washout from behind sheeting or sloughing of exposed trench walls. Where feasible, the trench should be dewatered until the pipe has been installed with the prescribed bedding and backfill has been placed to a height at least above the groundwater level. For dewatering smaller volumes of water, the trench may be overexcavated and backfilled to grade with crushed stone or gravel to facilitate drainage of water to the point of removal. Dewatering a large amount of groundwater will require the use of a well-point system consisting of a series of perforated pipes driven into the water bearing strata and connected to a header pipe and pump. Control the running water emanating from drainage of surface or groundwater to avert undermining of the trench bottom or walls, the foundation, or other zones of embedment. Provide dams, cutoffs, or other barriers periodically along the installation to preclude transport of water along the trench bottom. If needed, well-graded materials, along with perforated underdrains, can be used to enhance the transport of running water. The gradation of the drainage materials should be selected to minimize migration of fines from surrounding materials. Backfill all trenches after the pipe is installed to prevent disturbance of pipe and embedment."

Buried Pipe Design
A. P. Moser, Ph.D.

 

[civilman72]

After reading your post, what caught my attention was the "truckloads of boulders" used as backfill, likely to try to stabilize the saturated soil. If the voids in the placed large boulders were not properly filled and compacted, this is likely the reason you are seeing the settling in the pipe.

Make sure you get the backfill compaction checked (per a geotech, as already recommended) before installing the replacement pipe.

 

[LittleInch]

By "high ground water do you mean bog/Swanmore/ past/musket? ?

If so laying pipes is a nightmare unless you fill down to something solid or create a raft or install pulls and bring it above ground.

No details so difficult to make judgement here bit it won't get better on its own. ..

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.