Pipe bedding for RCP in high groundwater 2

[sjp8]

We are installing 12"-36" RCP piping and precast structures for a proposed storm drainage system in an area that has higher than anticipated groundwater. The contractor is dewatering while he is preparing the bedding and installing the pipe and structures. While I realize the high groundwater will eventually stabilize over time to a degree, I am concerned about a capillary action in the trench pulling in large amounts groundwater and negatively affecting the integrity of the RCP pipe or precast structure installations. I was thinking clay dams every 100' or so to help break up the flow of water, but, are there any other measures (e.g. pipe bedding material?) that I should be investigating. I would appreciate any help. Thanks.

 

[gbam]

A couple of questions;
1. Are you concerned with the pipe floating?
2. Have you checked the Conc. Pipe Design Manual?

The link to the conc. pipe manual is

http://www.concrete-pipe.org/manual/START.pdf

Also have considered a cement based bedding?

Good Luck! Out here in the Southwest we don't deal with GW much.

 

[civilperson]

Large crushed rock bedding 12" thick below the pipe is a proven way to install the RCP, (3/4" -3"), keeping the free water below the pipe. If the sections are pushed home, (spigot into bell), with the mastic filler properly placed, no infiltration will occur. Clay dams seem like overkill for a problem that I have never encountered. If the construction is scheduled from low point going upslope, the previous placed rock bedding will dewater the trench and structure subgrade automatically.

 

[LHA]

I have just had a project where perched water tables were discovered durign installation, although our Geotech Report said 10'+ to GW.

Civilperson's fix was proposed in the field by the contractor, and approved by the inspector (Twp Engineer). It appears to be working fine. No trench plugs/clay dams were used.

Engineering is the practice of the art of science - Steve

 

[sjp8]

Thanks very much for the responses. Very helpful.

 

[BigInch]

Wrap the bedding in Geotextile to keep it from infiltrating into the drain, or from washing out if on a slope.

I have used clay or sand-cement sack dams perpendicular across the trench at times, but only when on very steep profiles.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[Scott367]

I generally consider the grounwater flowing through my beeding to be benificial to the site unless the pipe is laid so steeply theat erosion becomes a problem.

 

[BigInch]

I would take it as an indication that the natural flow of the underground water has probably been disturbed by the pipeline and that somebody will be pretty mad about that. Pipe trenches can intercept and divert both surface and underground flows leaving previously well watered fields high and dry. Diversion of water by a pipeline construction project is a common cause of litigation for which high damages are often awarded to landowners. It is best to avoid flow along the pipeline for many reasons of which the above is only one.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[civilperson]

Storm water conveyances and pipelines are two different matters. Free flow along the outside of the storm water pipe is normal and expected. The outlet end daylights and will go to a natural water course. Almost all storm water sewers follow the natural flowlines of the original typography and are owned by the public. Pipelines have none of these characteristics.

 

[BigInch]

Yes, I do realize following and intercepting are two different things. It is also likely that following involves natural hydraulic slopes that are much less than those when intercepting or crossing drainage patterns.

What I didn't consider is that somebody might want to follow one, so that's a good point. Thanks.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[Scott367]

This question is relating to storm systems which are almost always shallow. Any effect on the watertable would be unlikely to have a negative effect on a well which would almost always be quite deep.

 

[emmgjld]

The depths of wells are extremely variable. Depends where you are. In Colorado, wells are not usually the concern (they are usually too deep) but, springs are a concern. Also the concern that shallow ground water levels can be either lowered or raised due to the installation of pipelines.

 

[BigInch]

Changing surface water, shallow groundwater flows, and shallow well water level depths are all valid concerns when installing pipelines. Here is an example, in the extreme, of what can occur when a contractor starts excavation for a 48" D pipeline without first doing a comprehensive preconstruction survey. Shallow groundwater levels were struck filling the pipeline trench with groundwater for a length of 5 km disrupting irrigation and water well production for the surrounding 20 sq km.

Kova, Turkey 2004

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com