Soft Clay Consolidation with Lowered Water Table

[jimbo2]

I searched the forums with the keywords soft, clay, consolidation, water table and lower.... and I got into the civil/env/other forum and posted this same query. I thought afterwards that the foundation engineering forum was the proper place, and apologize for any inconvenience.

I was hoping for some kind of reality check for a question I am trying to resolve regarding the potential magnitude of consolidation settlements in thick, soft clays due to lowering of the water table.

Residences and small commercial buildings are being built, and many are experiencing significant differential settlement and damage, in a region with a soft subsurface profile. Among the theories is that development of the infrastructure is lowering the water table in the vicinity of the buildings.

The subsurface is characterized by a loose to compact silty sand or sand (N = 5 to 20) to about 10 to 12' below ground surface, which is underlain by a thick (typically 15 to 40 feet), expansive, very soft, normally to slightly overconsolidated silty clay (w = 40 to 56%, LL = 49, PL = 23, PI = 26, Shrinkage Limit = 18).

I have no consolidation test data, but assuming a Cc = 0.2, and an initial void ratio of 1, I computed a consolidation settlement of just under 4-inches for a 43-foot thick soft clay layer given the effective stress increase due to an 8-foot drop in the water table.

Does that sound within the reasonable range? How much higher than that could it be?

 

[BigH]

Without checking myself, I wouldn't be surprised. Normally, I use Cc/1+eo rather than assuming e0 and Cc. You can get estimates from Lambe and Whitman. I judge that your value might be too low (0.2/2 = 1). I've usually found it to be nearer to 0.15.

Try doing a search on the internet (google, msn, rediff) on Bangkok clay. Bangkok is very famous for groundwater lowering settlements - as is Mexico City. You might find relavent information in such a search. I'll also have look in the next few days of the material I have on hand.

Its an interesting question and of great relevance to practicing engineers where sandy layers are being used as water sources. I realise yours is more a case of normally infiltrated water being cut off by the development's storm water systems. Still, the principal is the same.

Best regards and

You all have a great day!!!

 

[MRM]

Hi Jimbo2,
Just based on your description of the clay itself, it looks to me like your Cc of 0.2 may be a little low. Your clay looks similar to a lot of the lucustrine clays I run across judging by your Atterberg limits and other description. The water content is certainly up there too. It seems fair to say that it may be NC (presumably with preconsolidation effects due to aging to some extent too). Compressibility parameter discussion aside, it looks like your increase in pressure is about 300 to 400psf due to your drop in GW based on my assumptions. If the GW drops that much, I'd say that it would be quite possible to get an ultimate settlement of the amount you stated or more. It would also depend on the area (length and width) affected by the lower GW.

Here's something else to consider too and this is often overlooked I think;
That ultimate settlement figure you’ve developed is unlikely to ever occur in your lifetime or well beyond. Given the thickness of the clay stratum and the clay description, I'd guess that consolidation would take many many years. Now that is not to say that consolidation of the upper fringe of the clay stratum will not create excessive movement below buildings and resulting damage-because it certainly could. I'm just saying I'd be surprised if those buildings were standing long enough to see that ultimate settlement. This is assuming that your clay is somewhat continuous (no horizontal sand drainage paths every 2 feet vertically for example).

 

[SirAl]

I would like to complement jimbo2 on providing the forum with good background information describing his/her site in context to the the question posed. IMHO, the compression index value of the clay as noted might be on the low side. In addition, my gut feel tells me that the initial void ratio of the clay could be closer to 1.1. Where is your site located?

 

[npthao121]

Bangkok city has been affected by water lowering until now. Some research done by AIT (Asian institute of Technology) the value of settlement due to this reason & soil consolidation is about 4in/year.
But the soil parameter of Bangkok is different from Jimbo2’s. Bangkok clay is much weaker.
Jimbo2, if you like to know about the AIT research, try to use Internet to find some papers regarding to the problem. I have here some papers by prof Bergardo in which he summarized his research about soil consolidation, consolidation with vacuum assistant etc.
I think Jimbo2 should provide some more info about his/her site such as is there any fill surcharge applied to the site? Etc, more detail info provided more comments could be given
Regards,

 

[jimbo2]

Thank-you for the great feedback! Sounds unanimous, I will assume a higher Cc and eo, making my number even greater. Thanks for the lead on Bangkok and Mexico City.

MRM you are absolutley right on the 300 to 400 psf increase. Also, I think your comments on drainage and time of consolidation, and the length and width of the area involved may be very important in this instance.

The site might generally be described as an eight mile long and five mile wide area of three or four townships, in a mid-sized metropolitan area in the northeastern U.S.A. Suburban sprawl, especially over the past 20 years or so has resulted in the scattered development of residential areas, malls, highways..... a common situation near a large city. At this time, reports are that 20 to 30 year old foundations and homes/small buildings are cracking, "crumbling" pulling apart, etc. Damage has been reported for hundreds of homes over the past ten years, and the number is increasing. This is a big problem for these townships and is costing millions of dollars.

Sometimes the area is called as an "Old Swamp". A few subdivisions have been built entirely on eight to twelve foot thick fills, some have not. Now, one must have a geotechnical borehole and report to obtain a building permit for an addition or new home construction. I am one of many geotechnical engineers who have been asked to provide these reports, including the specification of bearing capacity, seismic site class and drainage requirements, among others. I have been careful to make sure we have a sufficiently thick and stiff, natural or manmade layer between this soft clay and the bottom of the footer or 5 to 7 feet minimum. Also, I am specifying that an adequate foundation drainage system be installed. Am I increasing settlement of the home by specifying this drainage?

On review of a few other logs for this area, there does appear to be some evidence of horizons in the thicker lacustrine clay with little or little to some silt, fine sand and fine gravel, say eight to ten feet apart, but this does not appear in all cases. Also the clay over the larger area is more likely found at an average thickness of about 15 to 20 feet. Sometimes a desiccated layer of the clay can be found as shallow as 4 or 5 feet. Because water levels are only taken after the end of the HSA boring, static water levels, if found, are typically 10 to 20 feet bgs at completion. I believe the actual static level is higher than that.

I am curious about the time to consolidate and whether the timeframes seem right to you, and the area question. Can these consolidation settlements due to dewatering occur over relatively small (or maybe not so small) areas of between 5,000 square feet to a few acres or more.

 

[BigH]

Remember that time frames for clays can vary widely; you just can't go with Atterbergs and correlations. The most important feature is horizontal layering - even if the layerings are silt rather than sand, these can reduce the amount of time for full settlement to occur. Northeastern US - could the clays be Varved clays - if so, then the horizontal drainage can be larger. I understand the "whole area" concept but . . . These are some points to keep in mind.

 

[npthao121]

It seems to me that cracks “crumbling” pulling apart … of the exsisting building as your mention are due to consolidation settlement. With the soil profile like yours & the fill of 8-12 foot thick for residential area, the condition is matched for the consolidation settlement to be occur. 20-30 year is also suitable for this type of clay.
If you are now developing a new residential area which requires new surcharge fill layer to be applied, consolidation must be considered. If your building is in the area which is developed (filling & leveling) for the last 20-30 years, the full settlement of primary consolidation may be reached.
But, if it is possible, you should perform an consolidation test & others for confirmation as well as to compare the test data with the past data.
Wick drain or PVD (prefabricated vertical data) could be used to accelarate the consolidation speed.
As BigH mentioned, the horizontal layering in your soil profile is very important for the consolidation process. It increase the Kh & Ch values.
The stiff layer between your bottom of foundation anf the soft soil below is not much meaning vs the consolidation.
Regards,

 

[Robert168]

With regard to the damage to the houses or structures, I would like to have the followign comments:

1. It is the amount of differential settlement, rather than the total settlement, that had led to the damage.

2. While the soil needs to be improved, the foundation type for those damaged buildings may not have been chosen correctly to resist the deflection induced by the defferential settlement.

 

[Horseman]

I have low chorma soils (10YR 2/0,2/1 and 4/2), mottled, hydric , silt loams.

What would you suggest as the best foundation for a 36x48 horse barn ?

Cement Pad, Gravel Pad, etc

Keith Conner
917 885 6247

 

[Focht3]

Is there a pattern to the damage - either with respect to building footprints (i.e. the edges are always down) or spatially (the damaged homes appear in zones)?

Frankly, I'm quite skeptical that an 8 foot decrease in the groundwater level would cause the observed damage - while the addition of 10 to 12 feet of fill has not. The source of the depression of the GWT wasn't identified - what has caused it?

I will be the contrarian in this thread - I expect that all of the primary consolidation of the clay due to the addition of fill some 20 to 30 years ago has likely occurred. The clay is neither so thick nor so homogeneous, and the loads are relatively low, and the drainage paths sufficiently short that t₁₀₀ times of more than 20 years would be unlikely. Theoretically possible, but practically unlikely.

What do I think has happened? Since I haven't seen the structures or studied the distribution of damage, I'm at a real disadvantage. My gut says that it is primarily a function of the original construction quality (house and fill), with impact from weather, vegetation, etc. combining to cause the damage. The geometry of the building footprints could be at fault, too.

One other cause to consider: leaking plumbing. Check for leaks - and pinpoint their location(s). This is a particular concern where the structure was built before 1970, and has cast iron sewer lines.



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[SirAl]

A most interesting case study. There may well be variable factors at play. Robert168 presents an important consideration, that being differential settlements.

Given the 20 year to 30 year time frame, maturing tree vegetation adjacent to houses and small commercial structures can play havoc with soil moisture changes resulting in foundation subsidence. I can attest to this as it has caused major damage to my parent's former home - built some 30 years ago - constructed in a lucustrine clay (albeit a modestly over-consolidated clay) environment. Damage to the house really was not identified within the first 15 to 20 years of construction.

It is also common, in my neck of the woods, for housing to be provided with basements, say 5' to 8' below grade, consisting of concrete foundation walls placed on concrete footings. Attached garages, however, are often constructed on a pile and gradebeam system doweled into the house foundation walls. Settlements/movements of the house footings relative to the garage gradebeams can cause major problems, particularly on the house adjacent to the gradebeam tie-in points.

With respect to water table subsidence, I too shared the opinion that water tables within a new development (formerly farmland, other agricultural, or just undeveloped) should, logically, lower. This is due in part to positive lot grading. After all, surface water that would simply infiltrate cultivated lands could now drain positively with positive lot grades, catch basins, etc. In addition I felt that the affect of bedding associated with subsurface sewerage, water distribution, etc. would act as an inter-connected subdrain network. The catch is, I tabled my views at a meeting with a local developer and senior engineering staff of a municipality concerning a development in a high ground water location. I was surprised to learn that studies showed water tables to commonly rise upon development, attributable to lawn watering, etc.

Just a few of my experiences to ponder. They may or may may not be applicable to this situation.

 

[jimbo2]

Thanx for the continuing valuable feedback on this. I have been doing a bit more number crunching in consideration of the targeted input in this thread. One thing that seems to run through your comments is that there are likely many different factors combining to cause the foundation failures observed in the region.

Considering the average clay layer thickness and the presence of more permeable pathways for draiange, my estimates compare well with the 20 year primary consolidation time suggested by Focht3 for the pre/post construction effective stress conditions. I have come to a tentative comfort level that lowering the water table may play some role in the failures, but is not likely the primary, or root cause of the problems. It seems that if this were the root cause, much more evidence would be avaialble in roadway and utility infrastructure performance, etc.

I'm thinking the (8') water table fluctuations that may result from development have likely already occurred in many of the areas over the thousands of years since deposition, lightly overconsolidating this soil within the effective stress range of interest. Even in the face of virgin compression of a thick clay layer, the differential loadings appear to be resovable with a stiff foundation. While I have not had the opportunity (yet) to observe damaged structures, I have spoken with some of the architects and engineers involved in this work. Many of the foundation designs rely heavily on standard charts included in the Building Code. More subtle implications of the design (e.g. building geometry requirements, reinforcing details, balanced footing pressures, etc.) appear less of a focus, if considered at all.

SirAl, your comments describe the situation quite well. The theory offered by some of my collegues that a lower water table may be the root cause of the foundation problems is based on the type of drainage improvements you presented.

From my perspective, this discussion focuses on the need for more information regarding the subsurface conditions. More and better long term water level measurements, additional geotechnical sampling and testing to nail down the compressibility of the clays, and post mortems on the failures should be included in the next steps to resolving this problem.

 

[Focht3]

[blue]SirAl[/blue] opined,

The catch is, I tabled my views at a meeting with a local developer and senior engineering staff of a municipality concerning a development in a high ground water location. I was surprised to learn that studies showed water tables to commonly rise upon development, attributable to lawn watering, etc.

I'd like the references to those studies. Their results coincide with my observation that lawn irrigation is a much bigger factor than we geotechnical engineers commonly acknowledge. Our area rainfall (San Antonio) ranges from about 25 to 45 inches per year, and our St. Augustine lawns need 38 to 60 inches per year (allowing for 3 months of dormant conditions.) So we irrigate a lot. And geotechnical data collected by one of my local competitors indicates that the moisture in the yard is larger than under a house slab - the opposite of what one would expect from the literature. The database is quite large - two or more borings taken beneath thousands of homes and in the yards, with the data collected over a three year period.

I'm willing to bet that a finding that the groundwater table is not at fault wouldn't be very popular with the builders - it would mean that the original design and/or construction of the homes is the likely culprit. That should (would?) lead to higher design and construction standards - and increased building costs.

An interesting problem. Please keep us posted, [blue]jimbo2[/blue].



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[BigH]

Well, one thing that, in the future may not be in the cards is lawn watering - given the current trends in many areas of being in water deprevation in the summer months!
As I've said before, the case histories are highlights of these threads. Focht3 - thanks ; SirAl and the others too!! - best to all.