Need Drainage Advice--Paver Sidewalk

mrmambo(zone 7b; Atlanta, GA)April 27, 2010


I live in Atlanta, GA in an older neighborhood that uses 18" hex concrete pavers for sidewalks; our sidewalk is 80 years old. I've recently pulled up the pavers to remove broken ones, reroute others around tree roots, etc. As part of this project, I'm also trying to remedy a drainage problem where water collects after heavy storms.

(And, yes, the sidewalk is the homeowners responsibility in our area. The City doesnÂt take care of them; nice, huh?)

The problem area originally was due to two issues:

1. the pavers were sitting in a depression, possibly caused by erosion over the years

2. incorrect slope on the sidewalk, both along the run and down the hill and from the house towards the street

Additionally, roots from a large pecan tree in the center of the walk pushed up the pavers, creating a rise in the middle of the run. Water was both coming down the hill from the corner and then back down from the other direction due to the rise in the middle. Even though the sidewalk is properly sloped (about a 2' drop over 50' or 4%), the rise was causing the water to puddle.

I've now leveled out the ground and tried to ensure a constant slope:

a) from the house to the edge of the sidewalk

b) from the top of the sidewalk to the bottom

and c) I also created a perimeter around the tree to protect the tree roots, narrowing the sidewalk there

Doing c), though, created a barrier wall in essence, as the ground around the tree is now higher than the sidewalk pavers. I think the water will now run down the sidewalk correctly, both from the top and away from the house, but will collect in that new corner.

Also, by excavating the sidewalk edge (by the street) to create a decent slope down from the house, I've now exposed the dirt below the curb edge and I'm concerned about that eroding.

A neighbor suggested building a french drain to route the water around the corner/tree and on down the hill.

My questions:

1. Is a french drain the correct solution?

1a. If so, do I need to build it from the top of the sidewalk all the way to the bottom (approx. 50') or can I start it further down near the problem area?

1b. If I build a french drain, do I need to use 4" pipe or can I go narrower?

2. Do I need to build a retention wall out of 1x6" board along the drain to protect the exposed dirt wall caused by my excavating the walk edge?

Some photos (below) to explain better plus elevation map.



1. Problem AreaÂMarked

2. Elevation Map

3. Problem AreaÂFlooded, Before Resetting Pavers

4. SidewalkÂAfter Resetting Pavers, Looking Down

5. SidewalkÂAfter Resetting Pavers, Looking Up

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mrmambo(zone 7b; Atlanta, GA)

Another shot of the area in question, closer up.

Please let me know if you need additional photos or info.

    Bookmark   April 27, 2010 at 10:40AM
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Since the city is not responsible for the sidewalk, then I'm assuming they won't care if you drill a drainage hole thru the crub?
I'm no expert but it seems the easiest solution would be to drill thru the curb and run some type of (covered) french drain from the low area out to the street.

    Bookmark   April 27, 2010 at 12:24PM
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mrmambo(zone 7b; Atlanta, GA)

Thanks--I had considered this, but the lowpoint is lower than the street level because of my excavations. I wonder if it could still drain into the bed beneath the roadway, but I'd guess not.

    Bookmark   April 27, 2010 at 1:01PM
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That was a much better presentation of the situation compared with the usual request we get on this forum. But very flat properties usually require precise elevation data to develop a detailed workable plan. In your case that would include accurate elevations of several points along the top of curb as well as selected spots of the front yard and grade at the house.

It is unfortunate that you did not come to the forum before doing the reconstruction of the walk, as it will need to be removed and re-laid. I canât give you a grading plan for lack of the data stated, but the correct grading concept is shown in the graphics below. The plan shows the location of the profile graphic.

For that problem area between the street corner and where the tree is, the walk and ground strip to the curb should be adjusted for positive drainage over the curb into the gutter. This will establish the elevation of the yard side of the walk. Care should be taken that the fill needed to bring the walk up to grade is well compacted or settlement will cause the problem to come back.

Once the elevation at the walk/yard is set, consideration is given to the grade across the yard to the house. If the grade is sufficient for good surface drainage, no regarding will be required on the lawn. Refer to the green line in the profile graphic above. On the other hand, fixing the walk may result in a new problem area between the walk and house if the grade is too flat (see yellow line). In this case the yard should be regarded as shown in red. The development of this new low area will require either a swale across the walk to the low corner of the lot or a surface drain and a pipe under the walk to a discharge point near the lot corner (upper right in the plan drawing).

    Bookmark   April 27, 2010 at 4:38PM
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mrmambo(zone 7b; Atlanta, GA)

Thanks, pls8xx, for taking the time to respond and develop the diagrams; much appreciated.

- any suggestions on getting more detailed elevation measurements myself? I assume the pros use GPS or differential leveling; I know I can measure the relative elevation difference using line and a line level

- there may be a challenge elevating the problem area back up to be level with the curb:
- Right now I'm calculating a 4% slope (2.75" over 68"), which I thought was proper, but looking around online I'm seeing a 2% maximum cross slope on sidewalks for ADA requirements--do you know what the minimum is that I use? It would help, but probably not enough. For example, going to 1.5% cross slope would give me only 1.75" back out of the 4" drop (seen in photo #1, above)
- The yard is not even with the walk; as you can see in image #5 (above) the yard is actually about 6-12" higher at various points along the walk and is about 11" higher at the problem area.
- The top of the walk is even with the corner slab that the City laid. I can't elevate that end to bring up the entire walk; that's why I dug the overall bed out deeper to get an even and proper slope from top to bottom along the sidewalk.
- We don't get significant run-off from the yard; it's never been a problem. So, right now I'm not concerned about the Sidewalk/Yard elevation, more the Sidewalk/Curb and Sidewalk Top/Sidewalk bottom elevations.

Thanks again and please let me know what additional specific data would be helpful.


    Bookmark   April 27, 2010 at 5:45PM
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Refer to the above graphics. Start at point âAâ on the corner pad and run a level string line along the edge of the walk. Find the point where the string line is 2.5 inches higher than the corresponding top of curb elevation. Mark this location as point âBâ where you will change from transition grading to grading as shown on the walk detail.

For that area from point âBâ toward the tree, use a string line from the curb across the walk to set the street edge of the walk 1 inch above the curb elevation and the other edge 2.5 inches above curb elevation (1/4 â per foot cross slope for walk). Continue this grading toward the tree stopping 4 to 8 feet before reaching the tree. If the new walk grade differs from the existing walk grade at the tree by more than 3 inches, then use an eight foot transition to match the two together. If the difference is less than 3 inches you can run the standard grading detain to within 4 ft of the tree.

Once the grading has been done for the middle portion by the standard grading detail use a string line from the new walk edges to the existing walk or pad for the transition areas.

Finish by grading the front lawn to the new walk grade.

    Bookmark   April 28, 2010 at 8:11AM
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mrmambo(zone 7b; Atlanta, GA)

Thanks again, pls8xx; I think I understand.

It looks like you're incorporating a hump at the tree as a transition grade from the new walk elevation (above the tree) to the existing walk (down past the tree). I'm assuming this will then route water in two directions: down from the top of the walk and down from the tree from the other direction.

In the past this created the pool seen in photo 3 (above), but I'm assuming that's because the pavers closest to the curb were not graded sufficiently to direct the water out towards the curb.

Should I be concerned that this water is all dumping towards the dirt and plantings between the curb and the sidewalk (on the street side)? I'm guessing that's the proper routing--get it off the sidewalk and towards the street, even if there's vegetation in-between.

Thanks again!

    Bookmark   April 28, 2010 at 8:38AM
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Since all the walk will be above the curb and sloped toward it, all water will be discharged to the street. The flow from the walk across the strip to the curb will be a sheet flow, though it may be heavier at some points. Probably best to have grass in the strip to prevent any chance for erosion.

    Bookmark   April 28, 2010 at 9:00AM
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mrmambo(zone 7b; Atlanta, GA)

Thanks again, pls8xx:

Well, I went out and started planning as you suggested; all was going well until I got to the tree. I realized that the current grade along the sidewalk there matched your suggested grade, so no transition was needed.

I then started looking at the whole sidewalk and realized if I had simply followed the curb as a guide all along, I wouldn't have had any problems. The curb falls at an overall 4.5% grade; it's a little steeper in some areas, but fairly consistent.

If I build the sidewalk bed back up to where it was originally plus add some more at the yard side to allow proper cross-slope, I should be okay. I may have to remove some dirt in the median btwn sidewalk and curb as it's not even, but that's a small issue. I also may need to reduce the cross-slope at the tree due to its having elevated the dirt there, but I still think I can get at least a 1% cross-slope (and it's only for a few feet).

So, thanks again (again, again...)...this whole issue may have been more a case of poor pre-planning and premature execution rather than existing grade problems.

    Bookmark   April 28, 2010 at 12:24PM
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Glad I could help. It's always a pleasure working with those that are willing to take the time to provide a plan drawing with photos.

A small flat area on the raised part of the walk by the tree will not cause a drainage issue. Remember to do a good job of compaction on the new fill under the walk.

    Bookmark   April 28, 2010 at 12:54PM
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mrmambo(zone 7b; Atlanta, GA)

I've measured and marked-up the overhead view based on your advice, but incorporating all downhill slope rather than the bump at the tree that you thought I might need.
(See diagram below; the delta symbol indicates the grade between 2 given points.)

It's quite shallow at the beginning (1% slope from A to B), but B was the first point at which the string from point A was 1" (at the curb side) above the curb. The starting point of the walk is actually a little below the curb level due to the way the City poured the corner pad. (see images below)

One final question:
How can I best prep the bed myself using the existing dirt bed?

I understand the best way to lay a paver path is to dig at least 6-8" down, lay down landscape fabric, add gravel, and add underlayment/leveling sand. As I'm out of work right now, I can't afford to spend $1000 or so on the materials (plus delivery).

Plus, all these sidewalks were laid back in the 1920s and only have fill dirt under them. They've been relatively stable for 90 years.

So far, I've been using a 2x4 as a screed and then compacting the dirt with a hand 12" square tamper; it's a mixture of GA clay and dirt and packs down decently. I can't really add landscape fabric as I often have to slightly adjust the dirt under the pavers. It's very hard to do it evenly.

Any advice on handling this best myself?

    Bookmark   April 28, 2010 at 3:17PM
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Please note that the walk grade slope along its length does not matter at all, only that it should be a smooth walking surface. The important part is the cross slope that carries the water over the curb to the gutter, which should closely match the grading detail I provided. The transition areas can deviate from the standard cross slope if the area is higher in grade than the adjacent areas with the proper cross slope.

Looking at your latest graphic I see a 10% slope from point B to C. This looks suspect to me, since this area should be one conforming to the standard grading detail and the lengthwise slope of the walk edges should be the same as the slope along the curb. Since the overall street slope is around 4 to 5%, it seems unlikely that any section of the curb would have a 10% slope.

On the subject of cost, I would think you might need around 2 cy yd of fill to bring the walk up to grade. The best fill would be a something like a select road base material. In my area the cost runs around $15/yd from the crusher and up to twice that from a material supplier, plus trucking cost. Here we have some mineral soil types with clay (not top soil!) that can be compacted to support walks and drives that only carry light loads. Never use a clay with lumps. It will leave voids and result in settlement. You might want to look at renting a plate compactor for this project. A thin layer of sand on top of the base material can facilitate getting the pavers set for a smooth surface.

    Bookmark   April 29, 2010 at 11:47AM
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mrmambo(zone 7b; Atlanta, GA)

Thanks, pls8xx--in terms of the road base material, is that a particular blend of gravel, etc.?

I just spoke with a local supplier and they weren't sure what that was. Are any of these gravels similar?:
Georgia Landscape Supply--Gravel

    Bookmark   April 29, 2010 at 11:57AM
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Go with the Granite CSB (crushed rock with fines). It looks kinda pricey at $26 a ton.

    Bookmark   April 29, 2010 at 12:19PM
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mrmambo(zone 7b; Atlanta, GA)

Hey, pls8xx:

Would really appreciate your advice on drainage problems in the dirt area around the tree.

I installed the sidewalk and it came out great; neighbors all thanked me for improving it so much and it looked really nice. In the dirt area around the tree (approx. 3'x15') I filled it in with more dirt, added some hostas and liriope, and then mulched it.

The problem came today when it rained--the water ran down the hill and to the street, as planned, but it washed out all the mulch, some of the plants, and eroded some of the sand under one of the pavers, compromising the sidewalk. Not pretty. (photos--below)

I'm thinking I need to build a retaining wall:
- maybe a stacked timber wall--4x4s or smaller(?)
- make it level, rather than sloped, to prevent water running though it
- if level, it'll have to be about 7-8" higher on down side due to 4% slope
- maybe a french drain (but would prefer not to have to do this unless necessary)

- Does this make sense?
- Would another type of wall or barrier be simpler and/or better?
- Should the "box" of the retaining wall be level to avoid run-off?

Thanks in advance!


    Bookmark   May 21, 2010 at 7:11PM
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