Would you be comfortable placing a fill downslope from a gravel stormwater sump?

[digger242j]

Just what the title suggests. We took a look at a job today where a builder wants to add some yard by importing fill. The top of the slope is right at the edge of the driveway, and partway down the slope is where the gravel filled stormwater sump has been placed.

The proposal is to cut a key, or several, at intervals coming up the existing slope, and place the fill, with maybe a bench partway up the slope, but the ultimate goal is to add more flat space next to the driveway.

My apologies for the crude illustration, but it serves the purpose.

Brown is existing grade, with the stormwater sump in gray. Outlined in yellow are the proposed keys and fill.

Beyond the fact that you probably can guess that I'm not in love with the idea by the very fact that I'm asking the question, I'll withhold my personal feelings so as not to prejudice the discussion.

What do you think?

 

[grandpa]

Whats the slope of the existing ground? 1to1 or worse? Im assuming the brown stuff is clay(needing a drain) and what type of fill material is to be used?

 

[willie59]

I'm not a dirt contractor so my comments would be much less than a penny for my thoughts, but like gramps stated, need more info on the existing material and the proposed fill material. Aside from that, maybe use large stone for the fill, larger than rip rap stone?

 

[Scrub Puller]

Yair . . .

Digger242j. If your sketch is anywhere near proportion (to me) it seems little more radical than constructing a berm or resheeting/reshaping a dam wall . . . there will always be issues with erosion until the slope is grassed or planted.

I think it's a good idea with the keys

It would need good material, adequate water and compaction and, under the residential circumstances, a legal document absolving you of risk.

Cheers.

 

[biggrader]

if the drain is not a sub surface drain, i dont see any problems with what you are proposing, other than installing a new storm water catch on the outside of the new slope. It must be there for a reason, just find out what the reason is........ But you probably already know that.

 

[digger242j]

Whats the slope of the existing ground? 1to1 or worse?

I only got a real quick look at it, but let's call the existing slope 1 to 1 or less. I expect to get a better look at it tomorrow.

Im assuming the brown stuff is clay(needing a drain)

We have plenty of clay in these parts, but I didn't get the impression that what's there now is all clay anyway.

In case it wasn't clear from what I said, the gray colored part on my picture isn't a "drain" per se, but a pit filled with gravel. Any rain water from the roof or driveway drains gets piped into it, so that it can theoretically percolate into the ground, rather than immediately run off into the local watershed, contributing to flooding. It'd be more like an anti-drain. Actually, there are two of them.

...and what type of fill material is to be used?

There's a borrow site nearby, and the dirt there is supposed to be a sandy/shaley mix, which would hopefully would compact pretty well. (Which would address Willie's suggestion as well.)

On edit: Hopefully what I added in reply to Grandpa clarifies the issue for Scrub and biggrader. Particularly the bolded line.

Thanks for the replies, and keep em coming...

 

[buckfever]

First I have two questions. Is the gravel pit someone's idea or was it part of the original storm water management plan. Second does the local township know about these and have a easement in place for them to get to it for any future maintance. The reason I ask is because we are starting a development with something similar and after we are done the township will own it. Aside for the less then good idea of storing water in a fill I would hate to see you put all that fill in and the local code enforcement show up and make you take it all out.

 

[digger242j]

First I have two questions. Is the gravel pit someone's idea or was it part of the original storm water management plan. Second does the local township know about these and have a easement in place for them to get to it for any future maintance.

It's on a single family residential lot, new construction, and the stormwater retention stuff will be inspected by the municipality, so I'm sure it's a requirement, not just something somebody felt like doing. That means, of course, that any future maintenance is the homeowner's problem.

This isn't the first time I've seen individual homeowners required to install stormwater retention facilities, but of course, I've also seen facilities designed to handle entire developments, as you say you're looking at.

As for the fill I described, I'm not sure whether the potential homeowner is asking for it, or the builder just wants to provide it. The slope off the edge of the driveway is definitely 1 to 1, and it's right off the edge of the driveway. Now, whether the township will want to be involved in approving that additional earthwork, I don't know.

 

[td25c]

Not sure how tall the hill is but if the owner is wanting to buy a little flat ground on top we generally go with a wall of some type .

I might compromise on a job like this .... Instead of working the whole hillside with dirt I might just drop in two rows of gabion rock baskets rite on top of the retention pit to get it up to grade & leave the lower part of the hillside alone .

 

[Tinkerer]

How about installing a curb and catch basin(s) after the fill is in place and move the water retention to the toe of the slope. Then pipe the runoff from the catch basins down to the retention area. That is what is usually done on all the jobs like that I have been on. I would think the curb would be mandatory for safety reasons with the vehicle traffic. Can the toe of the slope be extended so a flatter slope can be built ?

 

[digger242j]

How about installing a curb and catch basin(s) after the fill is in place and move the water retention to the toe of the slope. Then pipe the runoff from the catch basins down to the retention area. That is what is usually done on all the jobs like that I have been on. I would think the curb would be mandatory for safety reasons with the vehicle traffic. Can the toe of the slope be extended so a flatter slope can be built ?

I'm coming really late to this particular party. The job is apparently running a little behind, and the builder is in a bit of a bind. Lots of the work is already in place.

As far as moving the retention to the toe of the slope, that's pretty much out of the question since yesterday's activity was to finish installing it all. I'd have argued to put it down there to begin with, but like I said, I'm arriving late. More on that, below.

There is a curb at the driveway's edge, which is good, but it's a pretty good sized house, so there's a lot of square feet of roof that's going to be fed into it, even if the runoff from the pavement could be eliminated.

Not sure how tall the hill is but if the owner is wanting to buy a little flat ground on top we generally go with a wall of some type .

I might compromise on a job like this .... Instead of working the whole hillside with dirt I might just drop in two rows of gabion rock baskets rite on top of the retention pit to get it up to grade & leave the lower part of the hillside alone .

As a rough estimate, I'd guess the whole slope as it sits now is 30 feet or so tall. Looking at the site on Google Earth prior to construction seems to confirm that. After having scratched around down near the bottom third of the slope yesterday (to un-bury the roots of several trees that they'd like to not kill), I'm not convinced that the retention system is even on virgin ground as it sits now. My gut tells me that the yellow line I added to the picture is about where existing grade was to begin with.

The gabion idea isn't a bad one, if somebody really has their heart set on adding semi-level ground adjacent to the driveway.

What I normally do day in and day out is all the stuff connected with residential excavating. Foundations, footers, the utilities, final grading, etc. I've done a few dirt jobs building fills against slopes, and they've always had a drain specified in at the toe of the existing slope, or the bottom of the key. A couple of them have been to correct slides, where Mother Nature provided the groundwater to get things moving. It seems real counterintuitive to build a fill and then do work to get water to go into it.

In the opening post I said I didn't want to prejudice the discussion, and I still don't, but the truth of the matter is that I'd be happy to hear somebody say, Oh, we've done that dozens of times and never had an issue, but what I expected to hear was You'd be crazy to do that!

 

[Delmer]

You'd be crazy to TAKE RESPONSIBILITY for that. I'm not a lawyer or engineer so I can't tell you what to do...

It reminds me of a time a friend asked me to stop by his jobsite to give my opinion about something unrelated. I commented that I thought the bank he was working on was too steep to hold up in the spring thaw, this was a dry August, he said the homeowner was an engineer, and he was just doing what he was told. A few weeks later there's 10" of rain, a mudslide takes TWO houses down the hill, across the highway, railroad tracks and into the river. But his bank and the driveway were still solid, so what do I know?

 

[390eric]

Seeing as you and I are in the same are digger. I think I'd feel comfortable doing what you talked about. Definitely would run a few key drains off your benches though. And maybe run one right out of the gravel pit through your fill and out the slope. That way if you do get large rain and the gravel pit fills there is a overflow for the water to escape. Feel that shale mix would be perfect fill too. I wouldn't have an issue doing it like that but would probably consult an engineer considering I don't own my own business and have take on that responsibility. I have down fills like this before never thought they would work but as long as you have proper drainage for water to escape there is usually never an issue.

On a side note there is no way to run the downspouts and other runoff into an existing storm water management system?

 

[digger242j]

And maybe run one right out of the gravel pit through your fill and out the slope. That way if you do get large rain and the gravel pit fills there is a overflow for the water to escape.

There is an overflow out of both pits, however, they consist of a tee, laid on its back, and a couple feet of pipe extending vertically out of that. In theory, the gravel will have to be saturated a couple of feet deep before anything goes out the overflow.

On a side note there is no way to run the downspouts and other runoff into an existing storm water management system?

Nope. There's a creek only another 50 or so feet away from the bottom of the slope, so this is it.

Definitely would run a few key drains off your benches though.

Yeah, I think that goes without saying.

I have down fills like this before never thought they would work but as long as you have proper drainage for water to escape there is usually never an issue.

See, that's my concern. As designed, unless the circumstances are extreme, the water's only supposed to escape through percolating into the ground. And if the circumstances are that extreme, who knows how saturated the whole area is going to be?

(For those not around this area, the weather's been crazy. It's hardly rained at all for like two months, but for the couple of months before that, it seemed like it rained almost every day. Landslides of varying sizes are fairly common around here, particularly in the spring when you get melting snow and then a lot of rain.)

You'd be crazy to TAKE RESPONSIBILITY for that.

Fortunately, I'm not the one driving the train, but I am a subcontractor, not simply collecting a payroll check, so, yes, I feel the need to be sure the liability lies with somebody else, preferably with whomever is insisting that it won't be a problem. On the other hand, as a professional, I do feel the responsibility to help everybody else avoid creating a liability issue in the first place, and my imagination sees the potential for a big one here.

 

[grandpa]

I've done it lots of time.... but only using a coarse well draining sand topped with enough topsoil to establish turf. That make ya feel better?

 

[390eric]

Digger I have been thinking about this Don't know if you know where the McCandless crossings are but I did the second phase of it either the movie theater and dicks sporting goods. We installed one of those underground half plastic pipe retention pond drainage things in fill there. It's was like 18in of stone then the half pipe things and then 2 feet of stone all wrapped in frabric. It's designed to drain into the ground. This was in fill wasn't right on the slope but still in fill. Under parking lots for a commercial project engineered that way. We had our doubts but we're asured it would be fine. Can't even begin to tell you some of The strange things we did on that job. Every keyway we put in that place was leaning out not in. That's what the engineers called for. That's just the beginning but everything is still in place two years later

 

[JimBruce42]

My biggest concerns would be how was the current fill slope built? and whether those "sumps" and surrounding slope can handle a big rain or thaw. I think the best bet would be to maybe dig in a rock trench/banket at the toe of slope down to the creek and build up a rock bleeder to the existing sumps. Also tieing into the current slope with steps would be a good call regardless.

 

[digger242j]

Digger I have been thinking about this Don't know if you know where the McCandless crossings are but I did the second phase of it either the movie theater and dicks sporting goods. We installed one of those underground half plastic pipe retention pond drainage things in fill there. It's was like 18in of stone then the half pipe things and then 2 feet of stone all wrapped in frabric. It's designed to drain into the ground.

That's not all that far from this job. It might even be in the same watershed.

I haven't run into a half pipe like you describe. I've seen or built 8 foot corrugated metal retention pipes, or a plain old 4 foot diameter manhole, 12' deep, with a 3" outlet pipe at the bottom. Heck, I even built a 6' tall wall out of mafia blocks at the low corner of a parking lot so that that area would flood rather than letting all the runoff go down the hill at once. (Never even thought until just now about who'd get sued if somebody'd parked their Mercedes in that corner. )

This was in fill wasn't right on the slope but still in fill. Under parking lots for a commercial project engineered that way. We had our doubts but we're asured it would be fine.

That wouldn't worry me too much. Somebody might have to come and repave that lot sooner than otherwise, but it's not going to slide away anywhere.

My biggest concerns would be how was the current fill slope built?

I don't know. :beatsme

...and whether those "sumps" and surrounding slope can handle a big rain or thaw.

Well, as said above, they do have an overflow pipe. But how much will they be saturating the fill beneath and in front of them under "normal" circumstances?

I think the best bet would be to maybe dig in a rock trench/banket at the toe of slope down to the creek and build up a rock bleeder to the existing sumps.

That might be the second best bet. The best bet might be to not put that fill in there at all. (But that's why I went looking for other opinions.)

 

[zhkent]

Covering the rock drain so it can't overflow looks like a bad idea to me.
Keying in the fill would probably hold it. But the drain would always be acting like an auto-greaser
lubricating the fill and lowering it's resistance to slide.