Saturation of clay below a water table
Saturation of clay below a water table
(OP)
Gents,
A site in an alluvial deposit has a water table ranging in depth from about 18 to 24 ft (based on boring logs). The soil from a depth of 10 to 30 ft is a clay, generally consistent in color (munsell), water content (25 - 35%), dry density (85 - 95 pcf) and fines content (75 - 95%). Below 30 ft is a stiff sandy lean clay. Can a clay below a water table have unsaturated zones, or at least appear unsaturated?
The adjacent property was dewatered to a depth of 40 ft during conctruction of an underground parking lot using dewatering wells. Two years following construction the adjacent site was drilled using a hollow-stem auger boring. Measured depths of ground water table (prior to dewatering and about 3-4 years later) were consistent. However, upon examing samples below the water table they did not appear saturated. Using an assumed specific gravity for each sample the degree of saturation ranged from 90 to 100%. It is odd considering the dewatering contractor on the adjacent site had problems keeping up with the rate of water flowing into the wells, and that it was dewatered to 40 ft.
Thanks for any comments.
A site in an alluvial deposit has a water table ranging in depth from about 18 to 24 ft (based on boring logs). The soil from a depth of 10 to 30 ft is a clay, generally consistent in color (munsell), water content (25 - 35%), dry density (85 - 95 pcf) and fines content (75 - 95%). Below 30 ft is a stiff sandy lean clay. Can a clay below a water table have unsaturated zones, or at least appear unsaturated?
The adjacent property was dewatered to a depth of 40 ft during conctruction of an underground parking lot using dewatering wells. Two years following construction the adjacent site was drilled using a hollow-stem auger boring. Measured depths of ground water table (prior to dewatering and about 3-4 years later) were consistent. However, upon examing samples below the water table they did not appear saturated. Using an assumed specific gravity for each sample the degree of saturation ranged from 90 to 100%. It is odd considering the dewatering contractor on the adjacent site had problems keeping up with the rate of water flowing into the wells, and that it was dewatered to 40 ft.
Thanks for any comments.





RE: Saturation of clay below a water table
Regarding water table observations when drilling through a clay layer, you can easily remove clay faster than water can enter the boring (i.e., if you remove 20 gallons of soil in an hour and the water table only flows into the boring at 5 gallons per hour, it may appear that you encountered water in the last few feet of a boring that is well below the phreatic surface.
Consider a sand layer in the near surface: Often folks will claim a perched water table even when the sand and the clay share the same phreatic surface. Short-term field observations may indicate that the clay does not have a phreatic surface as standing water may have been seen in the sand layer. It really takes a piezometer to figure this stuff out.
Regarding the dewatering contractor: There may be water bearing sand layers or some other such condition that would provide for water flow from something other than the clay.
Good luck.
f-d
¡papá gordo ain't no madre flaca!
RE: Saturation of clay below a water table
Groundwater levels should be check at significantly later times than 24 hours, when dealing with clayey materials.
RE: Saturation of clay below a water table
RE: Saturation of clay below a water table
Actually the water readings were easily approximated during drilling (wet rod, sampler) - there was no option to leave the holes open for an extended amount of time - so I am confident in the location of the water surface.
So if fat clays are often not saturated then time-rate consolidation curves are not applicable. I ran several time-rates over a range of depths (including samples that appeared not to be saturated) and each curve had the classical shape with obvious transition from primary to secondary consolidation.
RE: Saturation of clay below a water table
Don't agree with that deduction. Consolidation will require that water moves around soil grains and blebs of air. Do the consolidation test and interpret the results. This should reflect field conditions.
f-d
¡papá gordo ain't no madre flaca!
RE: Saturation of clay below a water table
RE: Saturation of clay below a water table
RE: Saturation of clay below a water table
RE: Saturation of clay below a water table
RE: Saturation of clay below a water table
f-d
¡papá gordo ain't no madre flaca!
RE: Saturation of clay below a water table
RE: Saturation of clay below a water table
If you take a clay sample from the ground and put it into an odometer, cover it with water and run a consolidation test, is the sample saturated? Doubtful. Will you get data that you can interpret? I think so. I think you'll see classic time curves and a classic e log p curve.
I expect folks may chime in with some experience in a desert or such and maybe then I'm off base, but in the parts of the world where I've worked, other than some unusual situation, this is what I'd do.
Not that I'm always right, that is - ha.
f-d
¡papá gordo ain't no madre flaca!
RE: Saturation of clay below a water table
RE: Saturation of clay below a water table
Getting back to the original post - How long was the dewatering going on for? Days/weeks/months? How far away was the dewatering wells? - deep wells? or staged well points? If you had such tight clays, why was dewatering even necessary? Did you have sand seams or lenses?? I remember a job where the contractor talked his way into well points but it wasn't really necessary as the sands were only pockets with limited amounts of water. Obviously, you are trying to determine if the dewatering caused cracking of the structure nearby?? Any condition survey before dewatering/excavation? Don't forget that you will have surface settlements due to the excavation even if you put in "tight" excavation support - see Peck's book - he has a graph of depth of excvation and settlement - three zones. I'll try to find it later. This might be the cause moreso than the effect of dewatering. One would need to know the coefficient of permeability and try to determine if the dewatering could actually lower the water table in the clay - but you have indicated your clay to be stiff which means it would be somewhat overconsolidated - then, any settlements due to dewatering would likely be within recompression. - Just some additonal things to think about - which most likely you already have.
RE: Saturation of clay below a water table
I am very interested in the OP's response to BigH's points as I share them. There may be elastic properties (and inadequate shoring design) to consider.
f-d
¡papá gordo ain't no madre flaca!
RE: Saturation of clay below a water table
Still waiting for additional dewatering info (different consultants for underground structure). Basically point wells were used for months at 60' spacing directly behind pile-supported excavation (with tie-backs) to depth of 40 ft. 26 ft excavation ~7' from existing adjacent building. Not sure why dewatering went to 40 ft. Agreed - it does not appear that dewatering would create enough additional stress to push it over onto the virgin side of the consol curve.
Inadequate shoring also being considered. I would be interested in the Peck reference BigH.
RE: Saturation of clay below a water table
Generally, less permiable layers underlying more permiable layers are considered to be saturated.
My guess is that most of the recharge is from a rising water table. The material is very impermiable and the water table has not recovered from its depressed state. In tighter materials, stabilized piezometers provide reliable and usable water table data. Driller observations and bore hole measurments at completion of drilling are not usable data.
RE: Saturation of clay below a water table
RE: Saturation of clay below a water table
For a dry density of 85 pcf, saturation moisture content (Gs=2.7) is 36.5 percent (i.e., pretty close to your 35 percent). For a dry density of 95 pcf, saturation moisture content is 28.5 percent.
Your data seems to indicate that you are seemingly close to saturation moisture contents.
f-d
¡papá gordo ain't no madre flaca!
RE: Saturation of clay below a water table
I am working on a phd project where we \re using Lambeth group clay (LL 60% PL 26% OMC 19% particle density 2.7)I have collected disturbed samples from site and remolded them into 100mm dia 200mm length mould applying specific compaction effort. I am trying to saturate the sample in triaxial equipment aplaying 50kpa cell and 30 kpa back presure. I have been waiting for few days and there is no hope. Anyone has any saggestions for making saturation process faster.Concern is that during compaction samples from the site chopped into 10 to 15mm small pads and brought to OMC inorder to simulate site condition. I think this is causing some discontinuitys within the sample and not allow full saturation. Any comments, new ideas can help.
Thankyou
RE: Saturation of clay below a water table
f-d
¡papá gordo ain't no madre flaca!
RE: Saturation of clay below a water table
RE: Saturation of clay below a water table
You're fortunate that fattdad is a regular in the forum and answered your first question....
RE: Saturation of clay below a water table
Is this a compacted sample? If so, you can compact it as wet as possible.
f-d
¡papá gordo ain't no madre flaca!