"Temporary" Phreatic Surface

[eric1037]

Please refer to the crude sketch below.

I am trying to determine the phreatic surface between a below-grade stormwater infiltration system and a vegetated, mechanically stabilized earth wall.

The wall will be about 15 feet tall. The wall will consist of layers of clean sand (SP) and geogrid, with a wire basket facing. The facing will be vegetated.

The stormwater infiltration system is the Hancor "Landsaver" product and will be about 23 feet away from the wall (as shown on the drawing).

See

http://www.hancor.com/product/landsaver.html

for details.

For the 100-year storm event, the entire depth of the system will be full. (Up to the 712.0 ft. elevation.)

This is a fill situation and the groundwater level is well below the bottom of the wall.

I have 2 concerns:

1. How do I design the reinforcement for the wall?

2. Will water seep out the face of the wall?

I know I should account for some water that will be released from the infiltration system. However, all of the literature that I have read only describes techniques for steady-state conditions.

How do I estimate how the groundwater will infiltrate and its hydraulic gradient?

Intuitively, since this will relatively clean, unsaturated sand with some gravel, I would think that most of the water would go straight down due to gravity. However, I understand that it will spread horizontally as well.

Does anyone have any experience with this kind of analysis?

Thanks in advance!

 

[jdonville]

eric,

I would think that unless you had some finer matrix at the face of the wall, you would find it difficult to establish the vegetation. This finer matrix would, I think, tend to inhibit the flows out of the wall SP medium, although you could leave the outer toe as an outlet.

Maybe you could create a SEEP/W model to look at the transient GW conditions? Run a lab perm on compacted SP material to verify assumption for k (for model calibration)? I would think, depending on the degree of face batter, that the anisotropy of the permeabilities (kh vs kv) would have to be rather large in order to have the water leak out of the face rather than the bottom of the face.

Jeff


Jeff

 

[eric1037]

Jeff:

Thanks for the tip. Unfortunately, I don't have SEEP/W. I'm looking for some shareware or freeware software for seepage or groundwater modelling right now.

 

[BigH]

Where is the curret phreatic surface? You could probably look up solutions in book by Harr and others - for mounded water in transient situations. This is also a situation where, in my view, you will get some 'outward' flow, but when the source of water for the pit dries up - that is no longer has runoff coming in, then the dynamics will change and downward flow would be greater. Have you done a check on various "phreatic" surfaces within your fill to see what the effects are on your wall stability? Perhaps, the effects would be so minimal the pit doesn't affect anything anyway.

 

[eric1037]

BigH:

That was my next plan. I was going to see at what level the water affects the design of the wall. If it's not significant, then I will probably not do the analysis and just be a bit conservative.

The current groundwater level about 30 ft. below the bottom of the wall at about elevation 670.0 feet.

 

[dgillette]

Although the material is SP, it is likely that there are strata of lower permeability and higher permeability that would, in effect, give you anisotropic permeability of the fill as a whole. (A little more fines, a little too much compaction...) I would not be at all surprised if you did get some water exiting the wall. The soil science (agriculture) literature has a fair amount about travel of wetting fronts due to capillarity in unsaturated materials.

Any chance you could get some gravel to build a thin chimney drain some distance in from the face? That would make the issue go away.

With a drain or without, be careful not to overcompact it because that can break down particles right on the compaction surface, giving you a less permeable surface at the top of each lift.

 

[emmgjld]

I have done several Stormwater Retention (Infiltration) designs adjacent to structures. It was important that seepage under these structures not occur. I use SEEP/W for the modeling. I am assuming you have more than a passing understanding of the site geology (which includes the unconsolidated soils) conditions and the MSE wall will be properly drained.

Two schemes have worked for me;
1 --- Provide some protection between your infiltration and the structure. I usually specify a 'soil dam', which is a trench in which the native soil is excavated and immediately replaced with either the same compacted material, possibly with some finer grained soils mixed in or import a finer grained soil for backfill. The local contractors use a 'wheel' on the excavator arm. The amount of compaction does not need to be high, only enough to somewhat inhibit the horizontal factor of seepage. No need to go overboard.

Please note that the construction of a 'soil dam', even with the native soils placed in a slightly more than native density, may remove much or all of the considerations for the MSE Wall. Of course, the wall is designed with a subsurface drainage system.

2 --- If the soil stratification tends to inhibit rapid, vertical seepage, I oftentimes provide a sand filled trench beneath the pond area to direct much of the seepage to lower levels. Design of the trench material to coordinate with the native soils is assumed. The judicious use filter fabrics should be considered.

 

[jdonville]

eric,

geoslope offers a "student" version of their analysis suite for download (including SEEP/W). There are restrictions on boundary conditions and the number of soils you can use in the model, but it's a start. The GeoSlope manuals are chock full of really good modeling tips and examples, too.

http://www.geo-slope.com/downloads/

http://downloads.geo-slope.com/installs/geostudio2004/geostudio_dl_620.exe

Warning! 70-odd MB download!

Jeff

 

[eric1037]

Jeff:

Thanks for the links. Unfortunately, the student version doesn't support transient conditions.

It may be useful for other preliminary purposes though.

 

[fattdad]

don't forget to do a total stress analysis. During the rapid inflow of water to the infiltration basin, you will have a surcharge pressure develop by the weight of the water. Just because there is a water elevation in the infiltration basin does not mean that everything below it will "submerged" bouyant stress conditions.

good luck.

f-d

¡papá gordo ain’t no madre flaca!