24 hour water level readings 4

[Focht3]

I practice in an area where the soils are predominantly clays with some sands and gravels within the profile. The clays are typically highly plastic with some secondary structure, like slickensides, calcareous pockets, etc.

We take 24 hour water level readings in boreholes whenever possible. It's an old habit - and a good one - that was ingrained in me by my Dad and my first employer. I was taught that this represented a part of the standard of care - at least in areas where groundwater (or perched water) might be observed.

I was challenged on this practice last week - one of my local competitors flatly stated that it's not necessary to observe a borehole for even a few hours. For him, a 40 minute observation was "good enough."

In my view, that's not good design practice or exercising sound engineering judgement - and I won't stop my 24 hour observations. But I have to deal with this knucklehead on the same project over the next few weeks (to months), and I need some help.

Does anyone have references regarding the importance of making 24 hour groundwater observations, or at least procedures for demonstrating that the observed water level is "static"? I'm looking for textbooks, building codes, industry guidelines, technical papers - any credible source. Any help will be greatly appreciated. And while I'm dealing with a U.S. project, the existence of "foreign" (international) standards on this topic would be beneficial -



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[VAD]

Hello Focht3:

Interesting question. Generally the 24 hour water level observation has been traditional for geotechnical investigations associated with highway routes where the uppermost level of ground water is commonly used as a criterion for setting height of embankment. Of course this is not the only criterion. In this context and perhaps for some simple foundations this approach is often considered acceptable. However, for slope stability concerns and excavation problems the determination of the GWT gets outside the scope of the normal 24 hour borehole observation. Even in the 24 hour observation, one has to look at the cuttings carefuly to asses where water may be likely to start seeping etc.for proper interpretation.

There are cases if one is familiar with site conditions of a locale that the 24 hour observation may be short circuted. This would, in my opinion, be based on personal observations of the drilling and yes one may be able to assess a shorter time period for observation. Nonetheless,one often tends to revist the site the next day to determine if predictions are satisfactory. Having been involved with numerous testpits and pacing back and forth during and after a pit has been dug, observations of the likelihood of water at certain levels of stratigraphy, despite no visible water at the base of the pit when completed etc can be still used as an indication of where water level would likely to be be eventually. This is often inferred even if no visible water is observed after 24 hours.

Water level observations are complex and in my opinion water levels should be investigated in relation to the design and construction requirements for the site.

Very often, as well, the 24 hour readings can result in erroneous interpretations being made of ground conditions related to foundations when all there may be is water from a perched level filling in a borehole that was advanced too deep in an impervious material.

I am not too sure that you will find very definitive information as you require in the literature as this subject which is of considerable importance has often been taken for granted.

In closing I would be interested to learn the premise for your competitor making the statement as I am sure there is always some basis for someone making a decision whether it be on judgement or experience. I would also say that his observation would not be universal.

For clay shales I have noted that the best clue to determining the presence of water is to examine the structure by breaking the samples or cuttings from the borehole. Here one may not observe free flowing water even after 24 hours. Piezometer tips in these materials are often best suited and would be important for pore pressure determination related to embankment loading and excavation problems re slope stability issues.

The above is related to observations made in unlined boreholes that stand without sloughing. The use of standpipes and piezometers is not addressed.

 

[BigH]

John:

I'll look for you - will be coming to US later this week. Did you put in any standpipes? In almost all of my Canadian jobs, we would do that and we would take readings, in clayey soils, well after your 24 hour reading - sometimes a week afterwards or more. In clayey soils, as you are well aware, it takes a long time for the time-lag to make up the water in the hole - due to the volume needed and low permeability. This is explained clearly in Hsvorlev's book (1951 or so) on Soil Investigations where he has the equations for time-lag. Clearly with a large diameter hole, it will take a much longer time to "equilize" the groundwater level. For a small dia standpipe - this would be less.
r
I suspect your competitor might be placing far too much faith on colouration of the soil as an estimate of the groundwater level.

The point of the 24 hour reading - compared to the end of the boring reading is to get a least two points on the deltaH/H line and you could ball-park estimate the equilized groundwater levels.

Still, let me ponder and look about - not many references here, but I'll look.

 

[BigH]

John: Found some references, excerpts below:

From Tomlinson’s Foundation Design and Construction (1995)
Section 1.6 Ground Water

"Reliable information on ground-water levels within the depth proposed for excavations and pile borings, and within the zone of influence of foundation pressures, is vital to many aspects of foundation design and construction. Regrettably, observations of ground-water conditions are tall too often neglected in site investigation work. Frequently insufficient time is given for the first ‘strike’ of ground-water in a borehole or trial pit to reach an equilibrium level. In wash boring or rotary core drilling no observations of any value are possible while drilling is in progress. Merely to observe water levels on conclusion of drilling is inadequate, since these levels may take days to recover to equilibrium conditions. Ground-water levels should be monitored over as long a period as possible by measurements in one or more standpipes or piezometers installed in boreholes."


From Krynine and Judd Principals of Engineering Geology and Geotechnics (1957)
Section 6.25

"In some types of highly impervious materials, the wash water may remain in the drill hole for several days, and its level may be erroneously taken for a high water table. Therefore, if such a condition is thought to exist, the hole should be thoroughly bailed and time (at least 24h) allowed for the water table, if any, to stabilize. In important cases, some of the boreholes are left open or vertical small-diameter pipes are set in the completed holes; periodic water-table measurements are taken during and after the entire exploration program is completed."


Terzaghi, Peck and Mesri Soil Mechanics in Engineering Practice, 3rd Edition (1995)
Section 11.6

"In less permeable soils the time required to supply the water is so great that the water level in the drill hole does not come to equilibrium within a reasonable time of observation, and in feebly permeable soils such as clays, a state of equilibrium at which the correct groundwater level can be observed may never be reached. It is customary, in any event, to note the water level on completion of the hole and after allowing the hole to stand overnight or for 24 h before backfilling. Use of drilling mud to stabilize the walls of drillholes precludes obtaining this useful information."


And,

Bowles Foundation Analyses and Design, 5th Edition. (1997) – sorry!
Section 3.18


"The GWT is generally determined by directly measuring to the stabilized water level in the borehole after a suitable time lapse, often 24 to 48 hr later. . . . In soils with a high permeability, such as sands and gravels, 24 hr is usually a sufficient time for the water level to stabilize unless the hole wall has been somewhat sealed with drilling mud.

In soils with low permeability such as silts, fine silty sands, and clays, it may take several days to several weeks (or longer) for the GWT to stabilize."

Note: The quotes are not "whole" - there are some gaps in them and I didn't do the ". . ." thing. Sorry.

 

[Focht3]

Thanks for your responses!

[blue]VAD[/blue]:
I appreciate all of your comments - and share them. This case is unusual because it isn't a pure design assignment; it's forensic as well. (It is complicated in many ways that are hard to convey in this short thread.) The structure has apparently experienced heave of expansive clays that were wetted. The dispute is over the source of the water, and how to repair the structure.

The structure is constructed on a slope with about 7 feet (~2.1 m) of relief across the 57 ft (~17.4 m) of structure width and 40 ft(~12.2 m) of depth. The property was apparently developed using cut-and-fill techniques, and is a slab-on-grade foundation with typical grade beams and conventional reinforcement. The slope is primarily downward from "left" to "right" with some "front" to "back" component. A 3.5 ft (~1.1 m) "drop" in the foundation profile occurs about halfway across the structure (from left to right.) The drop was constructed using a deep beam that is 54 inches (~1.4 m) tall. The structure was completed in 1971, and began having problems in 1998.

A total of 10 borings were drilled - 4 in 1999 and 6 in 2002. One of the borings exhibited water - it was taken in the lower slab section near the downslope edge of the slab. The water reportedly came into the boring over an unspecified time, then "stopped." The difficulty is that the stratigraphy beneath the structure includes 3 to 4.5 feet (~0.9 to 1.4 m) of fill, some of which is gravelly clays - and some of which are lean clays. The hole was backfilled after 40 minutes, and none of the other boreholes were monitored for water. Surface drainage outside the foundation is adequate to good. My competitor claims that the observed water level is "the" water level, and I say that he simply cannot make that statement on the basis of such a short observation period. He didn't argue color, mottling, accretions - anything like that. The logging was really quite poor. It appears that his field crew screwed up, and he is loath to admit it. Foolish.

The borings were all push sampled dry - holes were about 4 inches (100mm) in diameter. I think that a small perched water table (pocket) was present at or very near the boring in a gravelly zone, and drained into the borehole. This put water into the 8 ft (~2.4 m) deep completed boring - and stopped when the gravel "pocket" had drained. The water level may have continued to rise over time as water flowed from some of the less permeable soils into the boring.

And get a load of this: my competitor flatly declared that the use of test pits is the best way to evaluate groundwater levels at this site! With soils that have Liquid limits of up to 75... in an area where the summer temperatures frequently exceed 100 degrees Fahrenheit (38 deg. Celsius) with relative humidities of less than 25 percent!
I had a hard time not laughing -
(His background is really CME...) I don't think his client wants to dig test pits through a completed structure...


[blue]BigH[/blue]:
Thank you! Thank you! Thank you!

The references are great - and you found a real use for Bowles' book (aside from a paperweight, that is.) [For those who have not read all the geotechnical threads - I'm not a big fan of Bowles' book.] The references are superb. I also plan to check Harry Cedergren's books - I'll bet that he has said something on the subject as well. And I will also look hard in Terzaghi & Peck. Those books are in a friend's hands - will get them back this week.



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[letrab]

You could also look at BS 5930:1999 Code of practice for site investigations.

It states in para 23.2.2 that obsevation may involve a considerable response time unless the groung is reasonably permeable, which is unlikely considering for clay.
They do not state a minimum time but explicitly cautions against using the water level shortly after the completion of drilling.

 

[DRC1]

As a contractor who uses borings included in the construction documents, I can tell you the quality of borings and investigations have become very poor over the last 10 years. At least here in the Northeast. It is great to see someone fighting to do the job right. I never understood why the least expensive and most important part of building design and construction was the place everyone chooses for a shortcut.
Good Luck.
If I find any references, I will let you know.

 

[Focht3]

Thanks for the kind words!

The really frustrating thing about this situation is that my competitor works for a large multi-national firm that has done some great projects, and has some fine geotechnical engineers on board. I don't understand why this man's boss hasn't yanked his chain for getting involved in problems that he clearly doesn't understand...

I have lots of war stories about this guy - this isn't an isolated event. Very sad for our profession -



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[BigH]

A comment on DRC1's observation that the borings have become "poor" over the last 10 years. I apologise if I go off topic for a second or two. Could it be that with the prices being so low (see other threads on which I and Focht3 have commented along with VAD) that no one can send engineers out on the rigs anymore. I worked for an "old" geotechnical firm in Toronto and my first 4 years out of school (ME(C) and all that) was spent on drill rigs - motorized tripods, CME 45 and 75 power augers, BBS-1 diamond drills, Longyear 24 and 44 diamond drills, even a screw feed diamond drill. Work was all over northern Ontario, New Brunswick, King Christian Island in NWT, etc. In other words, I, as a yound engineer, found myself not sitting behind a desk looking at computer terminals but out in the -30deg temps keeping the shelbies warm by running my car 12 hours per day while my toes froze. This is where many of us "old timers" gained our experience in how field investigations were done - read that should be done. Now, you probably have drillers logging the holes - the engineers running the job might not even look at the samples (no time or money) - etc. Is this the case?? If not, in all cases, I bet a lot more than one would hope.

Anyway - good luck John and thanks for the opportunity of doing a

 

[jheidt2543]

BigH,

I agree with your comments and I think there is one other source of the problem - engineering schools. I remember talking to one of my profs. after my alma mater Midwest Engineering U (the name has been changed to protect the guilty) dropped all the field requirements for its engineering program. He said that engineers are too highly trained to do field work, let the technitions do it. I just about fell out of my chair! If you don't know how to do it how can you oversee it?

Now we suffer with SOME young engineers that just don't have time for those interupting calls from the field asking questions all the time. If they understood the field work a little more, their directions to the field crews would be clearer, they would get back better more complete information and ultimately the final product would be better.

And, truth be told, all the fun and action are in the field.

 

[PEinc]

Focht3,

Foundation Engineering Handbook, Winterkorn & Fang, Chapter 1 Subsurface Explorations and Sampling (by Lowe & Zaccheo), page 29, Section 1.10.2, Observations of Water Levels and Pressures:

"Boreholes It is common to establish the water table elevation at a site by measuring the depth to water in the boreholes. The length of time required for water levels in boreholes to stabilize at the groundwater level is a function of the permeability of the soil. There is no doubt that the water should be allowed to stand for a minimum period, preferably 24 hours, following completion of the hole. However, even under these circumstances, the reading may be accurate only if the soil is pervious. Accurate readings can be obtained if readings are taken over a long period of time. In one-shift-a-day drilling the groundwater level is usually observed as the first order of business in the morning. This gives 14-16 hours time for stabilization and this is frequently adequate. Drilling mud obscures observations of the groundwater level due to filter cake action and due to its specific gravity being greater than water."

 

[VAD]

Hello Focht3:

Thanks for the additional information. I find your problem both intriguing and challenging. Having investigated swelling soil problems, it is often difficult to attribute the source of moisture from ground water despite well intentioned borings since very often water is expected to be the obvious answer and hence the thought of finding it with ready observation in a borehole. As you are no doubt aware many times there is a need to undertake a moisture profile examination to determine the subtle increase in moisture. I presume that this scenario could have been found from evaluating the information from the two investigations done in 1999 and 2002 if done by the same outfit.

I find it intriguing that the problem surfaced after about 21 years, but not totally surprised. This length of time can perhaps be a signature to the behaviour of the site etc. Very often manifestations of swelling are observed much earlier on building sites. The presence of water in one hole is interesting and yes one would have thought of further observations in the other holes. To backfill the other holes without further observation seems to be adding more variables and really would leave the source of water as questionable by any serious practitioner. Seems like a re-investigation is required to ensure that the remedy that will be applied is practical and cost effective.

I will now take a stab at providing some thoughts based on the info that you have provided from my past experience.

1. The fact that this problem has manifested itself to cause problem after 22 years suggests that the condition may not have been directly associated with rising ground water levels unless this can be shown to be consistent in the locale.

2. Moisture movement due to evaporisation cutoff is a possible factor as this is often promoted through the presence of a building.

3. Capilliary movement of moisture from a low water table may be another factor

The perched water level contributing to the water observed may be an important clue which would suggest that the site is fed by some source from outside of the point at which this water was observed. The sugestion of the building being constructed on sloping land would lead me to wonder if the gravel seam was associated with the stratigraphy outside of the building envelope. Evaluation of previous borings may assist in this respect. This source could be seasonal and much longer and depend on climatic conditions re rainfall or other precipitation.

4. Is the swelling concentrated more in one area than the other?. Generally this gives some clue as well in relation to moisture movement in the vapour stage which occurs cyclically through heating and cooling. This may be a factor to consider as well as this process take a long time.

5. Has any thing changed in terms of the site characteristics or building charcateristics to cause a possible increase of moisture adjacent to the building over the period of time?. Very often this aspect can be overlooked in the investigation. I have noted that watering of flower gardens next to the periphery of buildings, installation of planters etc,broken water spouts, underground pipes etc can lead to problems which were not present for many years prior to the event.

Final Thoughts. It would seem appropriate to undertake a structured investigation of the site before prescribing a remedy. This investigation should be designed to address some of the comments made as well as a proper determination of soil water.

Problems of these sort tax the mind and knowledge and provide some of the best tests of our understanding of soil mechanics. One or two of these problems would certainly make those who wish to sit behind a desk really want to go to the field. Perhaps this is one of the types of problems that a young practitioner needs to address to determine if he or she has a passion for geotechnical engineering.

All for now

[Cheers]

 

[BigH]

for VAD!!! You must be Canadian!!

 

[cphi]

Focht3,

Check out Hvorslev, "Subsurface Exploration and Sampling of Soils for Civil Engineering Purposes" (November 1949), Chapter 3 Groundwater Observations. Your Dad should have it or your university library. This book is what I consider one of the bibles of our profession. Never mind it was written in 1949, many of the concepts still are valid. I've got it, if you need a fax.

If your forensic project requires information on "wetting", I suggest a series of piezometer nests be installed. Install piezometers (pneumatic or electric with a short lag time) at different elevations in a certain location to make a "nest" and install similar nests in other areas of concern. Seal the piezometers to measure water pressures within a certain elevation range or layer or lens to evaluate where the water is, and changes if any, over the longest period of time allowable which could give clues as to the source. Also, I recommend you determine soil suction values with depth and horizontal location, over the longest time available. This soil suction study can be used to evaluate the time-rate of wetting amount and movement which is a part of unsaturated soil mechanics procedure. Both or either of these methods should be implemented in a nearby but unaffected area to develop a "background or baseline" benchmark if you will, of the subsurface conditions for comparison with the sub-building data. The soils are probably low permeability so a lengthy time for all this will be needed, also some big amounts of money.

Leaking sub-building pipes in aggregate backfilled trenches, and irrigation of exterior ornamental plants with excess water travelling in aggregate filled utility trenches are always possible sources that requires careful study. Is a parking lot surface nearby that gets washed off intentionally with water hoses?

Hope this helps, and good luck!

 

[BigH]

cphi - thanks for the correct year to my first post - I was going by memory as my copy of the "bible" is in deep storage. I guess 1951 was Lambe's Soil Testing bible!

 

[gandersen]

Hello Focht3 and Others:

I am most interested in the forensic issues embedded in this thread.

I spent several years practicing in the Jackson, Mississippi area where we did multiple forensic investigations for buildings damaged by expansive clays. I found that one way to attempt to identify the source of the moisture causing the expansion is to do a floor level survey and to plot a contour map for the movements. As a first order approximation, the portion of the floor slab that has experienced the most movement is probably nearest the source of the water. Clearly, however, there are other factors such as depth of nonexpansive overburden soils.

I would recommend the floor level survey and a crack survey of the building to postulate a deformation pattern in the underlying expansive clays prior to performing test borings.

On one occasion, it was clear from the deformation patterns of the floor slab and the sidewalk and parking lot outside that the source of the water was upslope in the parking area. We did continuous sampling of the soils to look for a water source and found a very thin seam of silts in a silty clay matrix just above the Yazoo Clay. I don't believe that installing a piezometer would have given an accurate representation of the condition of seepage along a thin silt seam, even if we had waited for several days or weeks to stabilize.

 

[Focht3]

Thanks again, everyone! I deeply appreciate all your efforts - this is a great forum!

[blue]gandersen[/blue]:
Thanks for your thoughts. We have, indeed, reviewed quite a few elevation surveys, and I have looked at patterns of movement. (Five surveys since 1999...) I didn't perform any of the surveys, so none of them are referenced to a deep bench mark ( - a truly "fixed point.&quot The structure's floor geometry (grade transition) has made the evaluation much more difficult. However, I discovered a special use for Surfer's matrix math capabilities: I subtracted one survey from another. The 'subtraction' process allowed me to compare two "relative" elevation surveys and 'see' movements that were difficult to identify. The results were very startling - to the other side...



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[Focht3]

Hmmm,

The case didn't settle - went to trial. My client prevailed, but several items are up for appeal. I can't give the 'condensed' version of events yet.

I have this page saved as a part of my 'Engineering' web page group within the MyIE2 web browser. (It's a front end for IE5 & IE6 - tabbed web pages like Mozilla. Highly recommended.) I see it daily, so there's no chance I'll miss giving you 'The rest of the story' a la Paul Harvey.

But you may have to wait a few years...



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