Water Table Variations 8

[IJR]

My dear friends

Can anyone give us a general statement on changes in elevations of water table.

Let me clarify:

The geotechnical report confirms water level 2.00m (6ft) below ground level. And I am going to put A BASE TO a structure 2.50m(7.5ft) below ground level. The base now floats and is subject to water head h= 2.5-1.5m = 0.5m(1.5ft). So far so good.

But then I always have to worry- will the water rise by how much? and I go about using the full head as if water table will strike the ground(some conservative bad judgement)

Geotech reports dont usually cite statistical data on water table variations, but it will be nice if someone mentions extreme cases so that we can get a better feel.

My respects
ijr

 

[VAD]

Interesting question. I am not sure how you are doing your design and the implications of knowing or not knowing effect of the seasonal variation in water level on the design outcome.

Depending on the above your conservative bad judgement may be appropriate for your conceptual thinking.

Most geotech reports tend to provide the statement that water levels are seasonal etc etc . On many jobs the water table is evaluated through standpipe piezometers. This approach is perhaps reasonable for many projects.

If the geotechnical engineer has done sufficient projects in an area he may venture to provide the likely possible rise or if he has taken care in examining the soil when the drilling is being undertaken. However, the latter costs money and very often this work is relgated to someone who can log the holes, note where seepage is first observed, place a standpipe and monitor same after 24 hours. Very often longer term monitoring is required for better information This, however, adds to the cost.

Backhoe testpiting can allow one to make some judgements as well as a better examination of the soil can be made and the structure allows deductions to be made. This has to be done by a trained eye. The Engineer on site is the one who can make this judgement.

There are other reasons as well but I will leave for others to provide their comments etc.

I should have asked to know the types of projects you are speaking about. i.e buildings etc. Also it is a good idea to sit with the geotech engineer and discuss the manner of how you are approaching your design. He may be able to provide you with the answers you need.

 

[cvg]

contact your local government (usually county or state) department of water resources. They should maintain a record of wells and groundwater levels. You may be able to locate several wells in your area. With that data you could estimate historic water levels.

 

[BigH]

VAD hit much of the question on the head. I would like to put in a baht or two worth of points. Unless your structure is very very light, even if the water pressure rises to the surface, it is highly unlikely the uplifting pressure will exceed the actual bearing pressures on the footing - hence your foundations will not float. However, if you have a floor level, at say a metre or so down (say a partial basement), then the rise to the ground surface may be enough to uplift the thin lightly loaded slab - just a possibility. Much of the seasonal variation in water levels is critical to the drying out or wetting of clayey soils - hence shrink and swell. Case histories by Bozozuk (effect of this augmented by tree roots) might be appropriate to study. As Focht3 would point out - in moderatly high to highly expansive soils, the wetting and drying might have great influence due to swelling pressures. So, in the end, you need to clarify the nature of the problem that a rising/falling seasonal water level might have. Then, you can make a judgment on critical, not critical or maybe critical.

 

[Ron]

Groundwater levels do vary seasonally, but it is usually possible to determine maximum past groundwater levels by soil stratigraphy. Soil scientists are better at this than Geotechnical engineers, simply because they do it a lot. On the Geotech side, we usually state that the groundwater level will change seasonally and that there has been a (excess/deficiency) in rainfall over the last (few months, years, etc.), thus leading the reader to their own conclusions. Most groundwater level readings are taken at the time of boring and then 24 hours after boring. This is a reasonable indicator of GWL at the time, but it can be expected to be higher or lower at other times and relative to rainfall, local development, etc.

 

[dirtguy4]

Interesting topic. My thoughts go directly to what would cause flucuations in the water table. This could be caused from various situations, ie:

- a prolonged rainfall event (water table rises)
- not enough rainfall (water table lowers)
- clearing of surface plants and shrubs thus decreasing evapotranspiration (water table rises)
- paving the ground surface, less water in/less water out (water table should find some sort of equilibrium and remain somewhat constant)
- placing a building over the site with proper positive drainage , less water in/less water out (water table should find some sort of equilibrium and remain somewhat constant)
- new localized event like building of a dugoutor sewage lagoon (water table rises), etc.

As VAD and BigH have stated you have to design for the worst case scenario and that is probably going to occur when the water table is at or near surface, but it could also be a problem should the water table drop significantly and shrinkage occur as BigH has stated.

cvg also has some excellent points, check out the local weather stations or facilities that would measure rainfall events in your area. This may give you some indication as to what type of water table fluctuation may occur and what type of storm to model your design after (50 yr, 100 yr, etc.)

Finding your upper and lower limits of the groundwater flucuations can be expensive and time consuming and you are right, they generally are not mentioned in a geotechnical report because of these reasons. Perhaps trying to control some of the boundaries that allow for flucutations in your water table may be beneficial. Discussing these various options with you geotechnical engineer may give you a better overall level of comfort with your final structural design.

 

[flamby]

Hi IJR,
Look at the rainfall record of the area for further clues. An inch of rainfall infilterated will lift the ground water table by 3 or 4 inches depending upon the porosity of soil. By looking at rainfall amounts, seasonal or daily, and assuming a handsome 70% (or is it ugly?)will be soaked up by the ground, you get a wild idea, how much it rises.

 

[cvg]

flame
this would be true if there were no runoff. much of the water may flow to the nearest stream where it will eventually end up in the ocean, never impacting the local water table. I suggest that looking at historic records of water levels is likely a better approach.

 

[flamby]

cvg,

I fully agree with you. flame's method is just a wild guess and may or may not work. That's why I am still looking for a publisher for my method.;-)

As to runoff, I have already taken it into consideration as 30% which is a controversial point. In cities it may reach upto 90%. IJR known his area and I leave it to his judgement how he chooses to use the equation or even throw this method out of the door.

Adding to the complexity is the movement of ground water. But I think short term analysis should be close to reality if you have some idea of the runoff coefficient of the area you are dealing with.

I accept that if any records of water table are available, the method is not worthy of attention.

 

[DRC1]

Water tabels ue fluctuate for a varrity of reasons. One big question is how was the water table established? If it is measured from test bore holes, the depth should be measured immeadiately after drilling and then again 24 hours later. All too often the water level is only measured right after drilling before it has had a chance to stabilize, thus providing potentaly unreliable readings. Generaly, the more porus the soil, the less fluctuation in water table. If your geotech can't provide any insight, perhaps a local geotech can. I would also talk to the town engineer or building dept. or local agricultural extention service.
Another option may be to provide adequate drainage at the bottom of the excavation, if you can drain the water to daylight or a storm sewer. This will control the water table locally at your project.

 

[BigH]

For measuring water levels - there is only one good way after drilling; namely to install a monitoring well (standpipe) and have the ability to return and measure the water level after a week or so - even longer in soft low permeable clays. This "24" hour routine is okay for many soils, but not clays. If you took measurement at the end of the day's drilling, the next morning, then 24hours later, you might be able to use falling (or rising) head formula to estimate the true water table.

 

[dirtsqueezer]

Hey cool- I didn't see mine. If soil profiles from individual test pits/boreholes were provided with your earthwork report, it may be possible to get an idea of water table fluxuations. I would actually look for this in the interpretation, but in case your report didn't go into detail, average maximum height of a water table in my locality can be inferred by orange/reddish mottling in the soil profile.

I believe the idea is that corrosion of iron in the soil is encouraged when the water table rises (such as during the winter), then falls again, creating a zone where air and water oxidize the soil at an accelerated rate.

Here's some wording I found on the web that has a few diagrams:

http://www.ci.tacoma.wa.us/waterservices/permits/Volume5/SWMM V5-C7.pdf

 

[arabianhammerhead]

Its important to keep in mind what a change in the water table elevation will influence, whether it will influence Factor of Safety concerns such as bearing capacity or slope stability, or activate mechanisms such as swelling/collapse.

If its a factor of safety concern, there are a number of different programs that will allow easy re-calculation of the factor of safety so that you can "play" with the elevation of the water table once you have set up the scenario and determine which is the most critical case, which would represent a conservative solution, and which most like represents "truth". So I would try to estimate the water table for different times during the year, enter them all into my analysis, and try to determine .

Any comments? I would say my approach is most important for retaining wall problems, and then, depending on the magnitude of change of water table eleavation, important for shallow foundations and slope stability problems, and finally (but probably not) for pile design problems.

 

[BigH]

For the most part unless you are dealing with "tricky" soils, normal fluctuations of groundwater level (say +/- 1m or so) would have little real impact EXCEPT for soils sensitive to moisture change - i.e., swelling clays such as in Texas (Focht3 area) and elsewhere. In these cases it is critical. But remember that for the most part in relatively stable soils, this is a yearly phenomena and I just don't see it being too too critical except under special circumstances - and that,is what you are to judge as an experienced engineer.
- Happy July 4 to all our Yankee (or Rebel) friends!

 

[eric1037]

Oftentimes, a change in soil color from brown to gray will indicate the long-term groundwater level. This is due to the fact that the soil above the groundwater will oxidize and turn brown or red and the soil below will not.

This is a VERY general method of estimating long-term groundwater level. However, the levels still may fluctuate.

 

[lallatin]

Start with the basics: What type of drainage system can you use? It shouldn't be a problem to make your structure independent of groundwater fluctuations.