Determining ground water elevations for dry ponds

[Bwhite7285]

I'm a civil so I'm trying to figure out what the process is for the geotech establishing the shwt. I frequently get conflicting elevations for the shwt from the geotech and the environmental consultant. Currently I have a difference of 5 feet (it is a closed basin however). Just trying to learn what the thought process is and why there would be a difference. Thanks

 

[fattdad]

The geotechnical engineer is likley looking at the ground water observations during the time of the field study. If the field study is done in August, the water may be at one depth, if it's done in February it may be at another depth.

The geotechnical engineer may also be looking at the water table in a boring that extends 15 or 20 ft into the ground and into underlying water table conditions. A static water table may be present at 10 ft (or so) in some sand layer and a separate perched water condition may develop at some higher depth during the wet season.

An environmental consultant (or a soil scientist) will look at other factors. The presence of hydric soils (low chroma), the presence of oxidized rhysosphers (sp) or in some cases the population of hydrophytic vegetation. If you have a ground surface covered by facultative vegetation (or wetter), it's reasonable to assume that the high water table during the year is within 12 inches of the ground surface, irrespective of the well data (albiet it may be a perched condition).

The problem with dry pond design is that the typical design standard is to set the bottom grade 4 or 5 ft above the seasonal high water table. But, after you complete all your civil improvements you will ultimatly focus ALOT of infiltration in that very location. This in turn should (will) lead to a mound on the water table, which could ultimatly reduce the headroom between the design base grade and the pre-construction water table elevation. This is never addressed by design and where I believe is where the body is buried.

f-d

¡papá gordo ain’t no madre flaca!

 

[msucog]

fattdad nailed it. i'd even throw in there things like droughts, changes in surface water flow and land usage will also contribute to changes in the groundwater level. add in things like approximations of elevations from rough topo plans and you could have many feet there depending on the topo scale. if it's surveyed for hor and vert location, then you've dramatically improved your confidence from that aspect.

also is the geotech giving the data from the exploration or are they giving you some estimation for design? a geotech must exercise much caution if we provide an estimation of where we think groundwater might be (cause we don't have a clue where the sucker will be in the future!)...mother nature is nasty, unpredictable and will always throw a curve ball. groundwater seems like a simple enough topic but i think it's much more complex and influential to what we do than many realize.

 

[escrowe]

Also consider the influence of tides and adjacent surface water levels on some sites. For example, in south FL the canals generally fluctuate around established control levels.

One more thing- some folks whip out the NRCS soil book and recite the groundwater depth estimate for the site soil type verbatim. Don't trust that.

If you need very good data, consider installing piezometers and reading them at least through the rainy season

 

[msucog]

indeed, peizometers. by the way, i've seen recent data indicating that groundwater dropped 15' through the drought which hit the hardest during winter (typically the rainy season here). you can almost plot groundwater levels and lake levels together and the groundwater levels follow the lake level trends. a 15' drop in groundwater can add several inches to settlement calculations (say 15'x120pcf-60pcf is added to the area loads--~1ksf area load)...this is why i note that geotechs should not be too "definite" or "all knowing" about groundwater levels. we like to think that it'll follow a trend we typically see...but drought aren't typical (and neither is drawing down a lake). in my opinion, if one must provide some input about groundwater levels, provide the data and indicate the fluctuations that you typically see but note that levels can see variations with seasonal changes or changes in land use.