Groundwater Levels in Shallow Clay Soils

[xlimestonecowboyx]

Hello All,

I'm trying to end an technical dilemma/argument with a colleague that has vexed me for some time regarding the representation of the groundwater surface in shallow clay soils.

Consider a clay soil on top of impermeable bedrock of a thickness of 20 feet. A boring is advance to 19 feet bgs and does not penetrate the clay. Saturated soil is observed in the core samples at 15 feet bgs based on water in core barrel, softening of the soil, discoloration. A 2-inch well is advanced with a screened interval from 9-19 feet bgs. After a period of time, groundwater levels in the well are reported as 5 feet bgs.

My question is: is this water level a representation of the water table present with in the clay (i.e. does WL in the well equal WL in the surrounding sediment)? Or is the WL a representation of some pressure head (potentiometric surface) above the actual saturated zone (observed in boring samples at 15 feet bgs)?

I contend that the water level in the well represents a water table condition and equals water levels/saturated zone in the clay at 5 feet bgs, and the clay acts as an unconfined aquifer. This is supported by a volumetric water content value that is close to porosity in soil samples collected from 5 feet bgs even though boring logs may have noted that interval as moist rather than saturated. My friend argues that pressure is acting on groundwater present in the voids between the clay particles and creates a pressure head that is observed as being shallower (5 feet bgs) than the actual saturated zone (15 feet bgs) similar to a semiconfined aquifer.

I have found no solace in the literature or textbooks that I have reviewed as all unconfined aquifers are modeled as sand and no special attention is generally given to clay as a aquifer, or groundwater-bearing, medium. I have not reviewed any geotechnical text as my library is in my hometown and I'm working remotely for a period of time.

Any insight into this issue is greatly appreciated. Thanks in advance!

 

[aeoliantexan]

I agree with you. If the clay is uniform, I expect that there is a water table at 5 feet and if you dig a hole 6 feet deep, there will be water in it within a few days. From a practical standpoint, my experience is that most of the time, the stabilized water level in the boring predicts the water table.

There may be dynamic conditions such as evaporation from the surface in dry weather that can create negative pore water pressures at 6 feet or deeper. Pore water pressures in clay can be complex. A source of water to replace the evaporation would be needed to maintain such a condition for very long, or the water level in the well will decline.

 

[xlimestonecowboyx]

Thanks for the reply, tex.

I think we all agree that water will be in the hole. The issue is that will water be at five feet or, say, three feet. If the clay is homogenous and at ground surface, then it can be characterized as an unconfined aquifer (not really an aquifer, but a water-bearing zone). Why, then, is it that often times when you install a well in this clay that the water level in the well rises well above where your core observations indicate the 'water table' or saturated zone is apparent? My colleague believes it is a function of confining (or some other type) pressure in the clay acting against hydrostatic pressure of water in pore spaces. When a well (a large low pressure pore) is installed in the clay, hydrostatic pressure pushes water into the well. This pressure may be enough to elevate the well water level above the water level in the clay, thus creating a potentiometric surface rather than a water table... Thoughts?

 

[dgillette]

Interesting question. I don't understand how the water in the well could be higher than the water table in the clay IF things are really hydrostatic as we normally use the term. (Your last two sentences don't seem like a good explanation.) If you define water table as top of saturation, rather than elevation of zero PWP (which should equal the elevation in the well IF it's really hydrostatic), then you would expect the WT to be higher than the zero-pressure elevation because of the capillary rise.

Three possibilities:

1. The clay at the actual water table has been desiccated at some time, so it is not obvious that it is saturated (%sat is high, even though %w is relatively low, due to lower void ratio).

2. There is an artesian condition in bedrock, not exactly impervious, so that the head is higher at the base of the clay and the screened portion of the well. In my work in dams (~25 years), the bedrock has almost always been more pervious than the embankment core due to jointing, fracturing, layering, solution tunnels, interflow contacts in volcanics, etc. Right off, I can't think of any counterexamples. Very often we measure higher head in the bedrock than in the overlying embankment or soil as a result.

3. The clay is not done with primary consolidation.

Any other possibilities?

 

[aeoliantexan]

A few unsophisticated thoughts:

Many drillers note "water entry" only when the spoon comes up wet. It takes a while for this much water to enter the hole in clay (longer in tight fat clay than in a lean or fractured clay), so several samples may be taken between the time the water table is first encountered and the time the driller notes it. The water level in the open boring or in an observation well the next day will be closer to the actual water table.

If there is even a modest aquifer beneath the clay, say a thin layer of sand or a weathered zone at the top of the bedrock,there may be an upward flow of water to evaporation at the surface. The pore pressure can be very negative (tens of feet) at the surface in dry, hot weather, so the depth where the pore pressure is atmospheric (one definition of the water table) may be well below the piezometric elevation indicated by the observation well. In this condition, however, we would not find water in an excavation just a few feet deeper than the water level in the well.

The water table has surely fluctuated a good deal over the years, so some oxidized colors will be seen well below the present water table. the truly unoxidized dark gray color may indicate the lower bound of past water tables.

Perhaps an agronomist or ag engineer will contribute some thoughts.