Increased Sump Usage Near Pond

[RFreund]

I'm really just looking for any general comments here. More specifically related to the flow of water through various materials i.e. clayey/sandy or sandy/gravel or a granular material similar to a CA7 (3/4" crushed stone). Feel free to reference actual soil designations unfortunately I'm no geotech so I really can't get too specific. Which is also why we may get some borings.

Scope: Determine what is causing increased sump usage at residence home.

Background: (See sketch).
The retaining wall was replaced approx, 1.5 years ago. The existing wall failed due to lack of geogrid. Geogrid was placed in the new wall. It is not know if site soils (sandy / clayey material) which it was designed for were used as reinforced back fill or if granular materials were used. Ever since the replacement the sump usage has increased. It used to run every 30mins - 1hr, now it is down to every 3min. There is a point at which the water 'rushes' up to then it fills fairly slowly. However if you raise the pump, this 'slow' down point also raises.
Back-feeding in the system has been checked for.
A possibility maybe some old abandoned drainage pipes that used to run across the lot. Possibly these were disturbed during construction. For now this is unlikely and lets assume it is not the problem.
For now I am wondering what sort of flow could be expected into the sump from the ground water level / pond level. I'm thinking that the water level tapers off as it approaches the home and now it is getting closer and thus higher???



EIT

http://www.HowToEngineer.com

 

[fattdad]

Figure out the soil used for the wall backfill. If you are correct (i.e., that they used open-graded aggregate), then the reconstruction of the wall resulted in moving the boundary condition of the pond closer to the sump. I say this because it just seem obvious that the hydrostatic pressure from the pond would be in direct communication with the open-graded aggregate.

I'm sure you've noticed that the sump elevation is lower than the pond level. If the boundary condition of the pond was shifted closer to the sump then there'd be less distance of flow through native soil to attenuate the pressure head. This would result in increased flow under Darcy's law.

If you are looking for a "fix" you'd have to install hydraulic barrier to limit flow in plan dimension. Sheeting works. So does a clay filled trench. May be hard to install it though. . .

f-d



¡papá gordo ain’t no madre flaca!

 

[cvg]

unless absolutely required, no contractor would have imported gravel to backfill and then exported native soil. i would bet dollars to donuts they used native material. i find it interesting your sump outfall is lower than nwl. could that be a problem? assume the check valve is not holding or possibly the pump is not performing well. try a bigger / new pump?

 

[RFreund]

Thanks for the responses.

We did look at the possibility of 'back-feeding' and the check valve but it really did not seem to be the problem. Essentiall the water is pumped up and outleted above grade (766') then flows down hill to the pond. You could see the water discharge into the pond and it is basically right at the NWL. I'm not sure I can totally rule it out but I'm pretty confident.

As for the backfill, normally I would agree that they would use the site soils when allowed, but we do know a couple of contractors that really prefer to use granular fill. I'm looking into this further.
There also would be 12" of drainage aggregate behind the wall that was not there prior, do you think that would be enough to make a difference?

EIT

http://www.HowToEngineer.com

 

[GeoPaveTraffic]

Based on the sketch and the discussion, it would certainly appear that reconstruciton of the wall caused increase flow of the pond water toward the house. This could be caused by the wall backfill allowing more pressure head and flow toward the house. As for backfill type, granular would certainly do this, but sandy clay backfill could as well depending on how the fill was placed, compacted, etc.

I expect that your only option is to install a barrier between the wall and the house. Sheet piles might work, a clay filled trench would work better. I expect that the barrier will need to extend on both sides of the house (and a little further) between the house and the geogrid.

Going to be a pricey fix.

Mike Lambert

 

[cvg]

was the pond lined? new construction along the edge might have damaged an existing pond liner. the lining could have been a clay layer which was removed and then replaced with native or it might have been a membrane that was cut

 

[motorbiketocrank]

I agree that to investigate the subsurface and implement a cutoff would be a very large expense with no guarantee of success. If the grades at the outfall and the depth of the pond allowed, I'd consider lowering the pond surface elevation about 4' by modifying the outfall as more foolproof fix.

 

[RFreund]

We have come across some other factors which would be difficult to explain so I will hold off for now, but I was hoping for some help with the following.
Is this statement 'technically' correct:
"The hydraulic gradient is proportional to the water level elevation. Thus as the water travels through the soil the hydraulic gradient is reduced according to Darcy's law. Therefore the water elevation is reduced as you travel further from the pond."

Basically trying to say that the water level closer to the pump is not actually at the same elevation as it is in the pond but moving the boundary closer will then raise this level.

Thanks again!

EIT

http://www.HowToEngineer.com

 

[fattdad]

RFreund,

The statement you're making is true, because the pump controls the hydraulic gradient. If there were no pump, the phreatic surface would "flow" toward the point of lowest energy, which may or may not be toward the pump.

So, yes, when the pump is controlling the hydraulic gradient, the flow of water toward the pump looses head elevation (thus energy), based on Darcy's law.

f-d

¡papá gordo ain’t no madre flaca!

 

[GeoPaveTraffic]

What you are saying is generally true as long the ground is homogeneous and there is flow. If there is no flow, the water level will rise to match the pond level.

Note that we generally do not reference Darcy's law when talking about groundwater flow.

I would say something more like: Groundwater elevations are expected to vary with changes in pond level and precipitation as well as the distance from the pond with levels higher near the pond. Groundwater elevations are expected to approach normal static elevations at some distance from the pond. This distance varies with soil properties, flow rates, precipitation, among other factors.

Mike Lambert

 

[cvg]

I think Mike stated it pretty well and in addition, running the sump pump will tend to create a "cone of depression" similar to a water well where the groundwater level is drawn down lower as you get closer to the pump.

other sources can raise or lower the groundwater level, for example a leaking water or broken sewer line

estimation of flow rates through the soil or gravel can be estimated using Darcy's law

 

[RFreund]

Ahhh, I see.

I think I was understanding the basics but I see now that my wording is a little off/confusing perhaps.

Thanks!

EIT

http://www.HowToEngineer.com

 

[fattdad]

Note that we generally do not reference Darcy's law when talking about groundwater flow. /quote]

On the surface, I just don't agree with this statement at all! When ground water is flowing, its flow is determined by boundary conditions and Darcy's law. Modflow, Plasm and all numerical solutions for ground water modeling have Darcy's law at the core. The derivation of the LaPlace equation (the basis for flow nets) is rooted in Darcy's law. I can go on. . .

Now if there is no flow (i.e., the boundary conditons) then Darcy's law is irrelavant. I'm just saying the quoted text says, "ground water flow" and to my knowledge, Darcy's law is key to such understanding.

The vertical flow of a drop of rainwater is governed by Darcy's law.
The flow of water to a well, sump or perf pipe is governed by Darcy's law.
Horizontal flow (i.e., ground water flow that mimics topographic trends) is governed by Darcy's law.
Etc.

Now I may be missing some intent of the quoted text and I don't want to imply that I'm some scholer. I just don't know how to get to the place where I'd agree with GeoPaveTraffic.

Insight please. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[77JQX]

A couple of points strike me - the first is that the water has an initial high inrush after the pump stops, slowing thereafter. The OP says the pump check valve is working, and there isn't backflow through the pump. How about water flowing through the sump pipe bedding? Could the construction of the new wall have disturbed a cutoff wall in the pipe trench, allowing faster flow into the sump?

Second, all the discussion so far assumes groundwater flow is from the pond toward the house. However, I'd like to point out that the pond may be the low point in the phreatic surface, and that groundwater flow may be in the direction from the house toward the pond. If so, installation of the wall could have impeded GW flow toward the pond, resulting in a higher phreatic surface at the house and increased sump usage.

 

[RFreund]

Hmmmmm, an interesting turn of events....

How would you know if the pond is the low point? Borings?
The pond is part of a drainage easement and is the low point for surface runoff, fyi.

I'm not sure I follow - "Could the construction of the new wall have disturbed a cutoff wall in the pipe trench, allowing faster flow into the sump"

Are you referring to the perimeter drain around the home? That there would have been a wall or plug to stop the flow of water collected around the home to the sump?

EIT

http://www.HowToEngineer.com

 

[RFreund]

Having thought about this a little more....
Say the water were flowing toward the pond and the new wall acts as a barrier. Would the argument be that the groundwater elevation rises behind the wall and thus the water elevation at the sump is higher. As if the water was 'backing up' at the wall?

EIT

http://www.HowToEngineer.com

 

[77JQX]

RFreund - By "Could the construction of the new wall have disturbed a cutoff wall in the pipe trench, allowing faster flow into the sump" I mean that pipes are commonly placed in a material (sand, pea gravel, etc.) that has a relatively low resistance to flow, and that it is fairly common to have water flow along the outside of a pipe through the bedding material. To prevent this, a "cutoff wall" of clay or concrete is sometimes used. I was wondering if the retaining wall construction could have disturbed one if it was present.

And yes, I was suggesting that it might be possible that water was backing up at the wall. In theory, the retaining wall should have weep holes and good uphill drainage. But practice doesn't always match theory.

 

[GeoPaveTraffic]

fattdad,

You are correct that Dary's law forms the basis of the analysis, my point in the post was that, at least in my experience, that the discussion doesn't typically get that far into the weeds.

Mike Lambert

 

[RFreund]

This pond is located in 'midwest usa' where we had a substantial drought this year. There was another pond in the area that was down approx 3'-4' (based on others observations). However this pond was not (noticeably) down at all. Also I found out that granular materials were used for reinforced backfill.

What does the fact that a pond "1.5 blocks north, down 3-4 feet" and this pond not down at all say about this pond? Or does it tell me anything?

Thanks!

EIT

http://www.HowToEngineer.com

 

[RFreund]

I should say almost all ponds are down.

I'm assuming this means that there is some source of underground water supplying the pond?

EIT

http://www.HowToEngineer.com