Water Table Variations
Water Table Variations
(OP)
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
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





RE: Water Table Variations
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.
RE: Water Table Variations
RE: Water Table Variations
RE: Water Table Variations
RE: Water Table Variations
- 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.
RE: Water Table Variations
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.
RE: Water Table Variations
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.
RE: Water Table Variations
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.
RE: Water Table Variations
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.
RE: Water Table Variations
RE: Water Table Variations
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%20V5-C7.pdf
RE: Water Table Variations
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.
RE: Water Table Variations
RE: Water Table Variations
This is a VERY general method of estimating long-term groundwater level. However, the levels still may fluctuate.
RE: Water Table Variations