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Residential French Drain Question/Problem

Residential French Drain Question/Problem

Residential French Drain Question/Problem

(OP)
Hi everyone,

I have this project where the client is insisting that I do drainage and I'm a structural engineer and don't do that and I could use some input.


This is what I have at the moment. What is going on is that the contractor guessed and built a bunch of stuff w/o engineering and the owner wants me to tell them how to fix it. The problem is that I'm structural and don't know enough about drainage to be able to determine if I can think of a quick/cheap/easy fix or if I should have them demo all of it and do something completely different.

Most of the stuff on this plan is existing except for the drain between the catch basins. Does this look okay? Should I just have one drain looping around the house and have them demo the catch basins? What do you think?

Thanks in advance :)

RE: Residential French Drain Question/Problem

starting with the obvious
there must be a drainage problem, what is it?
why build a french drain?
i don't see any elevations on anything?
how does the water drain out of the catch basins?
why are the catch basins grated and should they be separated from the french?
should you decline since you have no experience?
how deep is the water table?
is this supposed to handle surface runoff?
what kind of soil?
Maybe a grading plan would help?

RE: Residential French Drain Question/Problem

(OP)
Thank you for your reply. To answer your questions:

1. There was a minor amount of water (1-2" deep) in the bottom right section of the crawlspace when the owner purchased the home recently. The contractor on this job is one of those paper-pusher salesman types and told the owner she needed to build a bunch of french drains. It's possible the water has been there for years or there's a leaky pipe or something and there is no true drainage problem at all.

2. That's a good question for the contractor who decided it was needed. Do you think building french drains on this was a stupid decision? It's all already built and I'm trying to decide how to handle it.

3. There's no survey so I don't know the elevations. The site is flat though all the way through with the exception of the front yard and driveway (top of image) slopes down towards the street like 5ft drop in 20ft length approximately.

4. There's currently a pump in each catch basin. Right now the catch basin on the left doesn't drain to the right, as shown. It pumps straight back up the page to the street and they made it so it empties out all over the neighbor's driveway lol. So I changed it to go to the other catch basin on the right. The one on the right currently pumps all the way to the street which the city said was illegal so I changed it to be an outfall in the middle of the front yard a couple feet down. Not sure if those two changes are good ideas or not.

5. The catch basins are grated because they have a metal grate that's sitting on top of them. They are basically just a drum that's dug into the ground and they cut a hole in the side and the french drain pipe is sticking into the side of it draining into it. I'm not sure if they should be separated from the french or not. One problem with that would be that a 1/8" per foot drain slope would put the drains below the water table after any reasonable distance so that's why I believe the pumps were installed.

6. Probably. I already told the owner I don't do drainage but she still insisted I handle it. I still have the option to either get a peer reviewer to help or sub it out completely if it sounds too hard for me to handle with no experience. I want to learn as much as possible and at least wanted to look into it a bit first and try to learn something about it before I have someone else handle it. I guess I'm trying to figure out if it's easy and I'm making it harder than it really is or if it's hard and I should let someone else do it. What do you think? Is this an easy drainage project or a pain in the butt one?

7. The water table is like 2-3ft down. It's a high water table. The catch basins are existing and built too low and are currently filling up with groundwater and the pumps are just turning on all the time and pumping groundwater out to the street lol. That's how the owner figured out the contractor was being stupid and wants me to fix it.

8. Yeah, I believe it is supposed to handle surface runoff. I believe the original idea with it was that there was water in the crawlspace so lets make sure it doesn't get down there again. It has rained since they built the drains and no water made it into the crawlspace so it seems like what they have is at least kinda working, assuming the problem wasn't leaking pipes that slowly accumulated over the years.

9. I don't know what kind of soil. Is there any nasty soil conditions I should be aware of that have a negative effect on catch basins and french drains?

10. I'm assuming by grading plan you mean grading the site so the water drains in the desired direction (away from the house)? I don't think that's feasible on this one since the backyard has planter boxes and grass and stuff and the site is flat. It's not like there's a hill nearby flooding the house or anything.

RE: Residential French Drain Question/Problem

If I were you, I would hire a geotech to reevaluate the situation. It appears that you have a whole sort of issues, therefore a geotech would be the most expeditious way and the least costly for solutions. If this is not resolved soon, the contractor may be in a liability lawsuit and that would be very costly. As far as you are concerned, you should not make any recommendation.

RE: Residential French Drain Question/Problem

Aside to your issues, I usually build French drains with a sloped bottom, draining to the sock covered perf pipe and fill to 1' from grade with clean gran crush that is free draining. The crush is encapsulated in non-woven geotextile. 12" of topsoil over (usually use 6", but, due to the drying effect of a French drain, I like to see a foot of topsoil... sod over the topsoil. I don't provide drainage beneath the perf pipe.

Dik

RE: Residential French Drain Question/Problem

1. There was a minor amount of water (1-2" deep) in the bottom right section of the crawlspace when the owner purchased the home recently. The contractor on this job is one of those paper-pusher salesman types and told the owner she needed to build a bunch of french drains. It's possible the water has been there for years or there's a leaky pipe or something and there is no true drainage problem at all. so the obvious question remains, do you really need a french drain? maybe a sump pump in the crawlspace is adequate.

2. That's a good question for the contractor who decided it was needed. Do you think building french drains on this was a stupid decision? It's all already built and I'm trying to decide how to handle it.

3. There's no survey so I don't know the elevations. The site is flat though all the way through with the exception of the front yard and driveway (top of image) slopes down towards the street like 5ft drop in 20ft length approximately.

4. There's currently a pump in each catch basin. Right now the catch basin on the left doesn't drain to the right, as shown. It pumps straight back up the page to the street and they made it so it empties out all over the neighbor's driveway lol. So I changed it to go to the other catch basin on the right. The one on the right currently pumps all the way to the street which the city said was illegal so I changed it to be an outfall in the middle of the front yard a couple feet down. Not sure if those two changes are good ideas or not. So wouldnt it have been better to just slope the french drain towards the front yard and let it drain naturally without a pump since there is a good slope there?

5. The catch basins are grated because they have a metal grate that's sitting on top of them. They are basically just a drum that's dug into the ground and they cut a hole in the side and the french drain pipe is sticking into the side of it draining into it. I'm not sure if they should be separated from the french or not. One problem with that would be that a 1/8" per foot drain slope would put the drains below the water table after any reasonable distance so that's why I believe the pumps were installed. If you have 4 foot drop in the front yard, you should confirm that your water table is actually 2-3 feet deep. or do you have springs next to the driveway?

6. Probably. I already told the owner I don't do drainage but she still insisted I handle it. I still have the option to either get a peer reviewer to help or sub it out completely if it sounds too hard for me to handle with no experience. I want to learn as much as possible and at least wanted to look into it a bit first and try to learn something about it before I have someone else handle it. I guess I'm trying to figure out if it's easy and I'm making it harder than it really is or if it's hard and I should let someone else do it. What do you think? Is this an easy drainage project or a pain in the butt one?

7. The water table is like 2-3ft down. It's a high water table. The catch basins are existing and built too low and are currently filling up with groundwater and the pumps are just turning on all the time and pumping groundwater out to the street lol. That's how the owner figured out the contractor was being stupid and wants me to fix it. if you have perf pipe running into a catch basin that is below the water table, than yes, they will always run. do you really want these pumps running full time trying to lower the water table?

8. Yeah, I believe it is supposed to handle surface runoff. I believe the original idea with it was that there was water in the crawlspace so lets make sure it doesn't get down there again. It has rained since they built the drains and no water made it into the crawlspace so it seems like what they have is at least kinda working, assuming the problem wasn't leaking pipes that slowly accumulated over the years. surface runoff should always be handled by surface drains, not by french drains. if it is surface water, proper site grading is the preferred solution

9. I don't know what kind of soil. Is there any nasty soil conditions I should be aware of that have a negative effect on catch basins and french drains? ground water flows through the soil, so knowing what you are dealing with is important for proper design.

10. I'm assuming by grading plan you mean grading the site so the water drains in the desired direction (away from the house)? I don't think that's feasible on this one since the backyard has planter boxes and grass and stuff and the site is flat. It's not like there's a hill nearby flooding the house or anything. sounds like you should evaluate surface drainage before you make that assumption.

RE: Residential French Drain Question/Problem

Tread carefully, 333OnlyHalfEvil. Just as I wouldn't attempt to design a deck for somebody as a water resources engineer; you might want to reconsider designing drainage solutions as a structural engineer.

I'd suggest telling your client you are going to have to sub this part out. While it seems relatively straightforward (and should be cheap[ish] to have designed), the challenges you are having in effectively describing the problem (and that's not meant to be a dig) indicate that you might be a bit over your head.

I'm assuming you are in the States. And if so, this sort of thing can get you in trouble with a lot (all?) of the licensing boards.

RE: Residential French Drain Question/Problem

(OP)
Thanks for the replies everyone. I took the advice I seem to be getting along the lines of not helping with this and just told the owner this was too much for me and to consult with someone specialized in this.

That being said, can we keep the conversation going a little longer for my own knowledge?

I talked to the owner a little more and I guess I was slightly mistaken about the problems. She said that when it rains the backyard basically floods and some of that floodwater goes into the crawlspace. Does that change any of your responses at all? Also, the front yard slope is more like 1' in 20', I remembered it being steeper than it actually is.

I wouldn't be able to slope the french drain towards the front yard because it's too long of a run and the french drain would end up below the water table.

By surface drains, do you just mean like a gutter or a V-shaped indentation that directs the water?

Sump pump in the crawlspace seems like now is a no-go since the problems are also backyard flooding.

Would any of you civils mind recommending some literature that explains it really well and also letting me know what code books you refer to on a regular basis regarding this stuff?

Thanks.

RE: Residential French Drain Question/Problem

thats a surface drainage problem. handle it at the surface without using perforated drains. the water should be drained off before it seeps into the ground. perforated french drains are for dealing with groundwater.

keep the rainwater out of the crawlspace. once it gets in, a sump pump may still be required

if the yard floods than site grading and drainage is a problem as I originally speculated. you should do the following:

1. complete drainage analysis to determine rainfall runoff which impacts the site from both offsite flow and onsite flow
2. determine flow rates and volumes of rainfall runoff and determine how it is currently conveyed across the site. this will confirm ponding and flow depths that impact the crawlspace. You should be looking for the low point which is where the water is getting in. this will require a site plan with elevations that I recommended originally
3. recommend grading and drainage improvements to convey the water away from the building and off the site. this will probably require removing the grass and planter boxes and doing some earthwork. we are talking berms, swales, gutters, pipe drains, retention basins, etc.

suggest the following might be a helpful references:
https://portal.hud.gov/hudportal/documents/huddoc?...
https://www.fhwa.dot.gov/engineering/hydraulics/pu...

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