Water flow into foundation drains

[PEDARRIN2]

I am a plumbing/piping engineer trying to compile a good method to size interior sumps and the pumps to drain/pump water entering foundation drains to the storm water sewer system.

I have found a method that says to use a 10' wide area along the length of the foundation drain for the drainage area. It also says to use 1 gpm/100 sq ft for slow draining soils and twice that for quicker draining soils.

I cannot find the original source material for this method so I am a little leary to use it.

As stated, subsurface drainage is not my specialty so I figured I would ask the question to those who would know about it than I do.

 

[Focht3]

You are asking a pretty site-specific question. And you are right to be leery of using a rule of thumb for design.

What you really need is input from a geotechnical engineer in your geographic area of practice. And site-specific recommendations for each project -



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[PEDARRIN2]

Thank you for your reply.

Our problem is when the project does not have the $$$ for a geotechnical or soils engineer. We still have to come up with a reasonable flow to expect so we can size our system. I was just wondering if anybody had a rule of thumb to use when there is no site information available.

 

[Focht3]

I was just wondering if anybody had a rule of thumb to use when there is no site information available.

Not one that I'd use and then affix my engineering seal to the resulting design - you have too many unknowns.

Evaluating groundwater flow during the usual 50 year design life of a structure is easily one of the more complicated problems around. Think about the various unknowns: weather, site drainage, soil conditions. Remember that you have to design for the extreme events, not the "typical" ones. Entire books are dedicated to the subject. I would suggest that you check with the engineering library at your local university; look for books written by Harry Cedergren. You can also look for the Corps of Engineers manual on dewatering - written primarily by Charles I. Mansur. It's a damn good piece of work, but assumes that you are a geotechnical engineer with some dewatering experience.

Designing the drains is fairly simple because PVC is pretty cheap. If in doubt, use a bigger drain pipe. One of the key issues to address when designing the drains is the design of the filter between the soil and drain pipe. The biggest mistake that is often made by inexperienced engineers is to use too coarse a filter media. If there is any clay in the adjacent soils, use nothing coarser than properly graded concrete sand (ASTM C33.) And a thick filter blanket placed completely around the pipe is also a key construction detail that is often overlooked.

In the end, you will find that designing the sump is clearly the hardest part of the design. You can't pull a pump size "off the shelf." It's the one part of the job that always gives me heartburn.

What kinds of projects are you designing for? Are these new construction, or retrofits?



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[PEDARRIN2]

The projects would be for new construction.

As I said, I am concerned mainly with what comes into the sump. So I am not really interested in the mechanics unless it will help me design my sump. I will try to locate those references to see if they can help me.

Some of our projects we have a soils engineer on board, but not all. It is the latter that I am trying to get a design rationale to handle.

 

[Focht3]

You have to understand the mechanics of the drains in order to size the sump and pump. Period.

What kinds of new construction projects are you involved in that don't have a geotechnical engineer on the design team? Are you preparing a sealed design, or acting as a contractor?



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[PEDARRIN2]

I realize I need to understand the basic mechanics of drain, but I do not have the background education to understand all the particulars of soils and geotechnical engineering. I need something that within limits will give me a good number, which I can add a safety factor to arrive at a reasonable design.

I have worked on a lot of projects where for whatever reason, I did not have all the information I needed to do a good design A lot of the time it is that the owner's budget did not include use of a soils or geotechnical engineer to be a part of the design team. Why they would not include it is beyond me. I guess it happens a lot.

We have to work with what we are given.

I work for a MEP Design Firm which seals the drawings. Since I am not licensed, I work under a licensed engineer.

 

[jdmm]

Pedarrin2

The other approach to Focht2 is to use the emperical approach. Do the sump, make it plenty big, then monitor the volume of water. We frequently use this approach and as long as the mechanical is on board with the idea it is very practical. The theoretical or rule of 'thumb' usually results in rather concervative designs. The reason is obvious... how good is our theory.