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lebox

French drain or culvert or else?

lebox
9 years ago

Hi Everyone, I was about to install a french drain but read a couple of threads on this forum and have now second thoughts. Since many of you have a lot of experience and may have encountered a similar problem like mine, I thought I'd post a question here to see if anyone can give me some advice. I am happy to add images, maps and dimensions - just let me know what you need to asses the information.

We live in Los Angeles in a house that has been built into the slope of a hillside, with the 1st floor consisting mainly of crawlspace flanked by a garage and guest room and the second floor featuring all the living quarters. The second floor opens toward a narrow patio, which extends along the entire back side of the house. From there the hillside climbs up fairly steeply to the edge of the property and a neighbor's house.

We bought the house recently and did a great deal of remodeling. The house had some issues with water entering the crawlspace before, but we thought it wasn't a problem anymore since we did not see any water after rain storms while we were still remodeling. However, after we poured a new concrete patio our neighbor on the top of the hill kept his sprinklers running for a couple of days, and we noticed water seeping through the seam between the garden wall and the concrete slab of the patio. In addition, water entered the crawlspace along the foundation wall (which is approximately 2 to 3ft tall) and some found its way into the garage. We also had an epic rain storm shortly thereafter, and water entered the crawlspace again.

The contractor who poured the patio slab advised against a french drain, and all we put in were surface drains to collect the water on the slab. I was now planning to put in a french drain behind the garden wall below the elevation of the patio slab, but I am wondering if I should consider multiple culverts on the hill and behind the garden wall instead.

I attached a photograph, which I hope will give a general idea. I am happy to post more and provide more detail. Thank you in advance for your help!

Comments (29)

  • lebox
    Original Author
    9 years ago
    last modified: 9 years ago

    Hi Everyone, here is another photo of the patio seen from the south end.

  • lebox
    Original Author
    9 years ago
    last modified: 9 years ago

    And here is a photo from the north end from the hill.

  • lebox
    Original Author
    9 years ago
    last modified: 9 years ago

    Lastly, here is a photo that shows how the water flows from the neighbor's sprinklers at the top of the hill down toward our house to appear on the patio and in the crawlspace.

  • lebox
    Original Author
    9 years ago
    last modified: 9 years ago

    Here is another photo of the water intrusion in the crawlspace. I hope someone can look at my post and share some thoughts. I am desperate to get this done before the rainy season, in particular since some predict an El Ninjo. I realize that French drains have been discussed ad nauseam on this forum, but it seems to me that every situation is different. Please help. Bueller? Bueller? Bueller?….

  • PRO
    Yardvaark
    9 years ago
    last modified: 9 years ago

    The term "french drain" seems to mean different things to different people here, so you'll need to specify your definition of it.

    There should be a collector drain pipe running at the base, behind the retaining wall. This pipe should carry the water to a lower elevation where it is released. If there is not such a pipe, I don't know how you'll excavate behind the wall to install one, with all that rock in the way. Maybe someone else will know.

    I have no idea what your plan of "multiple culverts" means. You might make a simple sketch to help illustrate your idea.

  • lebox
    Original Author
    9 years ago
    last modified: 9 years ago

    Thank you, Yardvaark.

    When I mentioned "French drain" I was referring to a 4 inch perforated pipe that is wrapped in filter fabric and then embedded in gravel and wrapped again, and I was thinking it would be installed at the base behind the retaining wall. Does this description match what you are suggesting with the collector drain?

    A contractor who gave me an estimate suggested to break the lava rocks by the steps and extend the retaining wall to secure the hillside where the rocks would have to be removed to dig the trench for the drain. The drain would run all the way toward the lower end of the patio and connect to a solid pipe to carry away the water along the side of the house.

    I tried to create a drawing that shows the general footprint of the house and patio as well as the location of the retaining wall. In addition I marked in blue where the water appeared on the patio, in the garage and in the crawlspace (on the inside of the foundation wall) after our neighbor accidentally ran a sprinkler station for a couple of days. We had the same water intrusion in the crawlspace and garage after a heavy rain storm with record rainfall.

    I also marked in pink where we considered to put the "French drain" at the base of the retaining wall and where I thought I could run culverts to collect the surface water. And maybe that would not require to put in the drain at the base of the retaining wall?

    In addition, I created a rough elevation that illustrates how the hillside climbs from the patio. Please note that the drawing of the hillside is not accurate at all. It's just meant to provide a general idea of the layout/conditions.

    Thank you very much for taking a look at this and sharing your opinion.

  • pls8xx
    9 years ago
    last modified: 9 years ago

    Welcome to the forum leBox. Some members here feel that it's almost impossible to do drainage design over the internet. And often that is true. In most cases it requires an accurate and detailed base map to be successful.

    You location in California presents an added complication over that of the eastern US, the possibility of slope failure and a mud slide.

    Your drawings above are a start to those that will be needed to assess the situation. I have studied the photos and drawings. The narrow depth of patio suggest to me that the slope is a very steep one. The drainage problems you describe suggest there is a saturation of the subsoil of the slope.

    My first impression is that your property is one that will be very difficult for you to gather the data needed. The one plus I see is your advanced skill with computer graphics.

    If I understand, California is a highly regulated state. There may be records showing some of the needed horizontal and vertical data. I suggest you seek information from plans for the house or patio construction.

    Some locations have GIS data with aerial photos. Even a photo from google or bing can be helpful. Some data may be available in a property survey. The plans for subdivision development often contain contours and vertical data. Keep in mind, a general knowledge of the slope is needed for all the properties above you.

    Please take more photos. And it will be beneficial if you use some drawing like the one above to show the location of the camera and the direction of the shot. For slopes, it's better to take the shot looking across the slope rather than up or down.

  • lebox
    Original Author
    9 years ago
    last modified: 9 years ago

    Thank you pls8xx and Yardvaark for responding to my post and providing your perspective. Let me try to gather as much of the information you requested today and post it later this evening. And yes, you are correct, the hillside behind our house is classified as landslide area according to the property report.

    One thing I wanted to point out is that it seems that the soil on our property has a high clay content. Would you advise to hire a soil engineer/geologist to get more clarity on soil condition and flow of (sub-surface) water?

    Lastly, I read your entire post to solve OnAHillside's drainage issue and I was struck by both of your comments to avoid underground drains. That made me question my original idea of putting a pipe at the base of the retaining wall. So, I hope that the information I can gather today will give yo a better idea of how to address the problem.

  • pls8xx
    9 years ago
    last modified: 9 years ago

    In my area, 9 out of 10 French drains should have never been installed and some have to be removed. But that is not to say that French drains should never be used.

    The correct use of such drains is to de-water saturated soil where surface drainage and regrading cannot be made to work and where there is a suitable point for discharge. And this may be the case for leBox.

  • PRO
    Yardvaark
    9 years ago
    last modified: 9 years ago

    Lebox, what you are calling a french drain is what I'm calling a collector drain. (To me, a french drain is a hole in the ground with no outlet. Frequently it is filled with rock whereas its volume is the space between the rock; water dissipates by being absorbed into the ground and this kind of drain has very limited use.)

    You would need a drain behind the wall but its purpose it only to get rid of the water that collects behind the wall. It is not intended as a way to drain a yard surface. Is there presently a pipe behind the wall to collect water and run it to a lower elevation? There should be one for the length of the wall. When running such a pipe, care should be taken that it has a uniform positive slope. sometimes these pipes are carelessly installed so that they run like a stock market chart.

    A culvert is an open-ended horizontal pipe that is just below ground surface. It connects two separate parts of a drainage path. An example is where a roadside ditch must be filled so that a driveway can cross it. The pipe/culvert is buried in the driveway fill so that the ditch is able to drain continuous. That said, how are you using the word relative to your sketch above? Please explain the concept.

    Pls8xx, always glad to see you here for these complex drainage issues. If I say something you disagree with, please bring it up for clarification.

  • lebox
    Original Author
    9 years ago
    last modified: 9 years ago

    Thank you, Yardvaark. There is no collector pipe currently installed behind the retaining wall. And I was using the term culvert incorrectly. What I meant was an open trench that runs across the hillside and slopes toward the south end of the property with the idea to collect any surface water and preventing the water to find its way further down the hill.

  • PRO
    Yardvaark
    9 years ago
    last modified: 9 years ago

    Moisture in soil moves downward and sideways. Where a retaining wall is installed, moisture will collect behind it. If there is no drainage mechanism installed, water will accumulate at the back of the wall and eventually seep below the wall and through any cracks that might form in the wall. It looks like this is what is happening now.

    What you are calling a culvert would probably be better called a ditch. A gentle swale (a subtle ditch) is probably sufficient to solve the problem and can be almost imperceptible when planted. Its exact profile would depend on the maximum volume of water it is designed to carry. It might be that a proper collector drain, alone, behind the wall will take care of problem. It depends on if the problem is limited to seepage at the wall or if there is the need to manage water before it gets too close to the house. Also, account for the fact that the house itself forms a retaining wall that should be drained.

  • lebox
    Original Author
    9 years ago
    last modified: 9 years ago

    Thank you Yardvaark and pls8xx. So, it sounds like I may need to install a collector drain at the bottom of the retaining wall after all. Plus maybe a swale on the hillside? Do the soil conditions of heavy clay content affect the type of installation? For example, I read somewhere (can't remember where) that the holes in a perforated pipe, which is installed in clay soil, should face up. Is that correct?

    I drew a topo map and overlaid a satellite image to give you a general idea of the conditions. Hopefully, the drawings are helpful, even though they do not necessarily document the precise slope within the boundaries of our property. I took some measurements and photographs yesterday to capture how quickly the hill climbs from the patio. I am planning to post those later today.

    FYI, the ravine that cuts through the front of the property does not carry water, but is regarded as an "active waterway" by the city and may carry water in a 100-year storm.

  • deviant-deziner
    9 years ago
    last modified: 9 years ago

    Two well experienced California geo-techinical engineers walk into a bar, look at your sketches + site photos on their i-pads and say :

    " How lucky do you feel ?"

  • lebox
    Original Author
    9 years ago
    last modified: 9 years ago

    I am not sure how to interpret your comment, deviant-deziner.

    pls8xx and Yardvaark, following this post I will upload a bunch of photos I took from either side of the hill and the roof. The diagram below indicates the location of the camera in each of the photos. I hope this will give you a good idea of the lay of the land.

    I also took the roof line as a gauge to measure how quickly the hill rises from the patio along the retaining wall. Within 10 to 11ft from the edge of the retaining wall, the hill has climbed about 10 feet (You may be able to see the string I pulled perpendicular to the roof in the photo taken from the north end of the roof).

    The patio itself is overall pretty level, and each of the segments slopes appropriately from all sides toward the drain grates along the middle of the patio. You may also notice the white pipes sticking out along the edge of the patio and at the end of the retaining wall. These pipes are meant to be cut to length to collect surface water. Those pipes also connect to the same drain pipe that is connected to the drain grates.

    Please let me know if you need any other documentation. And thank you again for looking into this.

  • lebox
    Original Author
    9 years ago
    last modified: 9 years ago

    Here are photos 16 and 17 taken from either end of the roof.

  • lebox
    Original Author
    9 years ago
    last modified: 9 years ago

    Here are photos 9 through 15 taken from the north edge of the property looking south. Sorry for the long string of pictures, but garden web doesn't seem to allow you to post multiple pictures with the same post. Therefore, I added them all into a single collage….

  • lebox
    Original Author
    9 years ago
    last modified: 9 years ago

    Last post for tonight. This one is of photos 1 through 8, all taken from the south side of the property looking north.

  • PRO
    Yardvaark
    9 years ago
    last modified: 9 years ago

    In answer to the question of "do I need a collector drain behind the wall?" the answer is yes. It would collect the water that is currently seeping at the base of the wall and disperse it elsewhere. To the question of "do [you] need to place holes in collector pipe up in clay soil?" .... the pipe is going to have holes all around, 360*. Some sediment will eventually accumulate and begin collecting at bottom of pipe. If holes were only there, they would become clogged.

    Swales located at upper portions of the yard will not be that useful since all water below them -- a large amount in a big storm -- would still be going directly toward house/patio. The place to collect water is just prior to all that you wish to protect .... at the back side of the wall. This is separate from draining the water IN the soil behind the wall. Exactly what is the best way to accomplish draining the wall and draining the yard is not something I can say from a distance. Nor can I have knowledge of legal aspects that affect you. Nevertheless, both those goals are valid and various aspects of the approach can be discussed.

  • lebox
    Original Author
    9 years ago
    last modified: 9 years ago

    Thank you Yardvaark. Do you suggest to use a "slotted" pipe (at top in image below) rather than a perforated pipe (at bottom of image below)? And I hear that people advise against a ribbed pipe, since it may be more prone to collecting dirt and clogging. Is that correct?

    And yes, you are correct. Los Angeles certainly has certainly plenty of code requirements. Our house was built in the late 50s, though, and therefore way before a lot of recent building codes were put in place.

  • PRO
    Yardvaark
    9 years ago
    last modified: 9 years ago

    If choosing for myself, it would be slotted -- with more ways for water to enter -- but I can't advise what you should do, with all the variables in play.

  • lebox
    Original Author
    9 years ago
    last modified: 9 years ago

    Thank you, Yardvaark. I realize that you are not familiar with California building codes and I certainly would not expect you to provide me with any kind of planning work. But since I heard different things from different people, I was hoping to obtain some objective advice, which will allow me to better understand how a solution could be found. Would you be willing to comment on a rough drawing of what I had in mind to address the drainage issue?

  • PRO
    Yardvaark
    9 years ago
    last modified: 9 years ago

    I thought I did. Did you see my comments above your most recent picture? If you're looking for something else, please say.

  • lebox
    Original Author
    9 years ago
    last modified: 9 years ago

    I guess, I am not quite clear if you are also suggesting to install a swale to collect surface water in addition to the collector drain at the base of the wall, which would capture the water that seeps through the hill? Are you recommending to install such a swale/surface drain on the surface right behind the patio wall while the collector drain is buried at the base of the wall below it?

    I also wanted to ask you another question regarding the water that penetrates the crawl space. We noticed that it had found its way underneath the foundation and actually eroded some of the soil exposing the footing in a stretch of roughly 1ft (see the picture). The footing itself has not developed any visible cracks. I am not sure what to do here. Reinforce the footing by filling the hole with concrete or gravel? Or install a catch basin of sorts to capture the water and carry it away?

  • PRO
    Yardvaark
    9 years ago
    last modified: 9 years ago

    You need to drain all the water that hits the backyard hill but doesn't soak in. The SUBsurface water that collects behind a retaining wall needs to be drained away by being collected into an underground pipe that is located near or at the bottom back of the wall and run to a lower elevation. this would apply to a retaining wall that is out in the yard or a retaining wall that is part of the foundation of your house. The surface water of the yard is separate and would not be routed through a pipe draining the bottom of any retaining walls. The goal is to get rid of surface water before it soaks into the ground behind the retaining walls. The surface water would be routed (usually by a swale) around the house. Since you have a whole back yard that will be draining toward the house in a storm, you want to collect as much of this water as possible before it gets to the house. If you collect it in a swale at the top of the hill, you're going to leave an awful lot of water --the bulk of the hillside -- uncollected. If you collect it just before it gets to the house (behind the retaining wall) then you'll be collecting as much of it as possible. (Incidental of what falls on the patio itself. presumably, routing water that falls directly on the patio, around the house is already accounted for ... I know you have some surface drains there.)

    I think I have answered your second question ... you should be catching the water that penetrates the foundation wall on the outside of the house before it penetrates the wall. I'm describing ideals and sometimes these are not always possible to achieve. One works toward them as much as all factors allow. One cannot do what is impossible. Wherever erosion has occurred inside the foundation area, it should be filled and and further erosion prevented. I cannot say what is the best material or method you should use. You would be better off with local consultation for that. If a preventative measure is not achieved at the outside of the wall, it will likely be an ongoing repair. i don't know your storms and how much damage can occur in a short time.

  • lebox
    Original Author
    9 years ago
    last modified: 9 years ago

    Thank you, Yardvaark. And sorry for the delayed response. This past work week got the best of me.

    I have created drawings of what I plan to install (collector pipe in green, swale in yellow). It would be great if you can take a look and comment on it. And pls8xx, if you are still looking at this, I would love your input as well.

    The top image below shows a plan view of running a collector pipe all along the far end of the patio at the bottom of the hill (green), carrying the water toward the south end toward daylight. The bottom image shows an elevation of what it would look like behind the retaining wall. I am not sure how deep I will be able to dig behind the retaining wall, so let me know if you think the depth is not sufficient relative to the foundation of the house. Am I correct in the assumption that if I dig below the foundation of the retaining wall that I can not get closer to the wall than a minimum of a 45 degree angle?

    There are a few things I am not clear on. For example, is it ok to put the surface swale along the retaining wall right above the collector drain as shown in my drawing, or should it be slightly offset? And should that swale be formed out of concrete or just dirt?

    Should the slotted pipe sit at the bottom of the ditch right on top of the clay soil, or should there be a shallow gravel bed below the pipe?

    What is the recommended slope of the pipe?

    Is the fabric I use to line the ditch and wrap the pipe the same? Is it a co-called geo-textile or just a simple mesh?

    I was thinking about installing cleanouts, but I am wondering if I should install a bigger access shaft at the end of the retaining wall, half-way down the patio. This is the area where we seem to have the biggest problems, therefore I was wondering if it would make sense to install a drain control shaft. In my drawing, I am just showing a standard 4-inch cleanout.

    Lastly, we have surface drain risers that come up in the dirt between the edge of the patio and the lava rocks. They connect to the main surface drain pipe along the patio. Picture 16 taken from the roof (posted on Sep 29) shows them as white pipes sticking out. We have not cut them to length yet, and I was wondering what the best approach would be to capture the water and not let it seep into the soil. Is it ok to cut them flush with the surface of the soil and not seal the area with concrete?

    My apologies for all the questions. Thank you again for all your tremendous help.

  • PRO
    Yardvaark
    9 years ago
    last modified: 9 years ago

    LeBox, I'm sure you understand this is a complex issue. I can offer comments but not assurance that all important factors are accounted for. I hope you get a local review of your plan in order to prevent work being done that falls short of solving the problems. Let me mention some possible issues.

    In a large rainstorm, the volume of water running off the hillside could be considerable. As it will be caught in the swale at the top, back of the wall, the swale needs to be calculated to handle the volume of water for a large storm. As the swale is located at the north half of the property, would it be better to drain it off to the north side of the property instead of having it go the longer distance around the south end of the house? If possible, I would have the goal of keeping this surface water at the surface instead of putting it into a pipe. (Sometimes it's not possible.) If it's on the surface, one doesn't have pipes clogging in the middle of a storm and failing. If it must go in a pipe, the pipe needs to be calculated to handle the flow of the design storm. Frequently, homeowners think 4" pipe will do everything but it is often inadequate. You would not want to run this water coming off of the hillside into the existing drain system of the patio (feeding a larger pipe -- at least it should be -- into a smaller one.) You might end up doing the reverse ... running the patio drain into whatever drains the hillside surface. The section of the drain behind the wall looks fine.

    The purpose of the collector pipe behind the wall is only to get rid of water that infiltrates the soil behind the wall. You would not want to add any drainage water to it or simultaneously use this pipe to drain any surface water through it. Though you could drain this pipe into a larger pipe or swale that is draining surface water, so long as it is calculated to handle the volume. (The volume of the water collecting behind the wall will be small relative to the volume that is draining from the yard surface.)

    "Arms of surface drains extend to edge of patio w/riser" ... the only purpose I can see for these is as yard drains at the edge of patio, though I wouldn't know what volume of water to expect in this area. As yard drains, inlets would need to be slightly lower (2") than the patio surface with the surround grade pitched downward toward the inlet. This type of drain is subject to clogging from yard debris so requires regular cleaning of the inlet and possibly a cage over the drain to filter out debris. Usually, groundcover of some type helps to keep surrounding soil from washing into the inlet.

    Summing it up, you have 3 separate drainage systems:
    1. swale behind wall ... this is largest volume of water.
    2. existing patio drain w/ attached yard drains ... this is second largest volume of water.
    3. collector drain behind wall ... this is least volume of water.

    lower volumes of water can run into systems handling higher volumes of water, but not the other way around.

  • lebox
    Original Author
    9 years ago
    last modified: 9 years ago

    Thank you for your thoughts, Yardvaark. I totally understand that you are trying to "diagnose" the issue from afar and that a local review is critical to understand some of the conditions, which may not be apparent to you without the benefit of a site visit.

    You are correct in stating that it would be better to drain the water collected in the swale toward the north end of the property. Unfortunately, the property line is right adjacent to concrete steps and a small garden wall, which does not leave any room to install a drain to carry away the water.

    Given the training of the average contractor in our city, I do not expect them to be capable of calculating the correct drain pipe size according to expected storm runoff. Can you guide me on how to do that?

    Regarding the risers of the surface drains along the edge of the patio: They run underneath the patio and connect to the main surface drain that extends along the entire length of the patio. I assume, while not ideal, it's better to keep them functional and monitor them to not be clogged rather than disconnecting them and installing a separate surface drain?

    I wonder if you can answer one important question from my previous post: Should the slotted pipe sit at the bottom of the ditch right on top of the clay soil, or should there be a shallow gravel bed below the pipe? Different people say different things over here, and I am not sure whose advice to follow.

    Thank you.

  • PRO
    Yardvaark
    9 years ago
    last modified: 9 years ago

    It's many times the case that there are things smack in the way of where you need to place something else. And sometimes they are big enough to dictate alterations of the design. So much for taking the shortest drainage route.

    Calculating drainage can be complex. If you want to run some preliminary numbers you might explore using a drainage pipe calculator as I've linked below.

    Regarding the risers ... I agree that it's best to keep them active and monitor. They serve some purpose though not to drain all the back yard.

    The purpose of placing the slotted drain pipe on a thin layer of gravel is so that silt that infiltrates can settle to a lower level down into the gravel, thereby protecting the pipe from clogging. I would call this a "cushion" as if and when enough silt infiltrates, it will eventually clog the gravel, negating its purpose. However, it might give you several more years before any meaningful clogging. I would use it but just a thin layer as the gravel itself behaves as a reservoir to catch water. Whatever water lands in it cannot get back up into the pipe and must infiltrate the ground, so must be limited. An important feature to the system will be to have the pipe pitched so as to be self cleaning, i.e., what gets into it will be carried out in the flow. Min. is 1/8" per ft, but more is better, if possible. All portions of the pipe should be sloped. No portions should be level or run uphill.

    Here is a link that might be useful: drainage pipe calculator