Help in Permeability testing using GDS system 2

[studyphd]

Hi
I am trying to find out the permeability of a stabilised soil (natural clay + lime + flyash). The samples are made by compacted stabilised soil at OMC to get MDD. I am using stress path cell for 38mm dia samples with 3 controllers from GDS (back pressure, base pressure and cell pressure controller). The problem is i am getting high values of permeability. I dont know what is the problem. May be i am calculating the values wrong or doing the test wrong. Firstly i am saturating the sample, check for B value and then I am initiating the flow from the base of the speimen by doing constant head test. Please see the file attached of the results and my calculations for permeability. I am really in a fuss because there is no body in the department who knows much about it. I am also confused which test to select from the drop down list....constant head or constant flow. Constant head allows me to initiate a flow from base and ask me to give desired base and back pressure and hydraulic conductivity value(i). But if i do it on option of constant flow then it asks me flow rate which i dont know and also i think it initiates a flow from top which according to my thinking is not right.

Is there anybody working on such problems. Any help would be highly appriciated. Sorry for taking so much time.

 

[iandig]

I will look through your data, as this is something we do all the time as a joint stabilisaiton/solidification of contaminated material and stabilisation for geotech purposes on sites we are remediating. I would also suggest you look at the actual sample data, and in particular the air void content against a measured particle density. Also the smaple curing after compaction is key to these materials, along with when the compaction was doen after addition of binder. Just downloading the file now, so will review and reply later once I have gone through the numebrs, If there is anything else I need, will let you know.

 

[studyphd]

I cured the specimens for 1 week and now i am going to do testing on 28days old specimens. Any help would be useful. Waiting for your reply.

 

[fattdad]

(I'm working in cm and seconds and confirmed your gradient calculations separately.)

Referring to your line 36 (just an arbitary selection).

Q=kiA, where,

Q= 1.6 cc/s
i=1.23.8
A=11.58

rearrainge to get:

k=1.6/(23.8*11/58)= 5.8x10^-3 cm/sec.

This value confims the working of your spreadsheet, other than the fact that my area calculation is just slightly different from yours (A= (pi*D^2)/4

You'd have to tell us more at this point:

What is the precent passing the No. 200 seive?
What are the Atterberg limits?
What is the maximum dry density and optimum moisture content of just the soil (i.e., prior to adding the flyash and lime)
What is the maximum dry denstiy and optimum moisture content of the soil+lime+flyash misture?
What was actual compcation moisture content for the test sample?
What is the specific gravity of the tested mixture (so we can indirectly calculate the percent saturation of the test sample at the time of compaction).

On a separate note: What is the overall purpose of knowing the permeability of this mixture? Are you designing a dam or someother hydraulic barrier? Just what's the end game?

On another separarte note, I'd never blend flyash, lime and soil together, just wouldn't do it! If there is ANY natural sulfur in the soil, the presence of free lime and flyash (in addition to natural soil moisture) will lead to the formation of ettringite, which can cause damaging soil expansion. I've personally been involved in million dollar claims owing to the formation of ettringite in lime-stabilized flyash mixtures. There are current publications on this matter and I refer you to the Geo-Institute's GeoStrata publication from earlier this year for more information.

f-d



¡papá gordo ain’t no madre flaca!

 

[iandig]

Hi
I have gone through your numbers, and I have one BIG question, have you got the units correct for the flow. According to the spreadsheet, the flow is averaging 1ml per 10 seconds, that is 6ml per minute, = 360ml per hour = 8.64 litres per day. Based on the spreadsheet, the flow has been 9.630 litres for around a 24 hour period. Do you have an inexhaustable supply of water or is there a decimal missing?
If the flow of water through the sample is correct, then the permebaility is correct. If the flow of water is not in ml but in µl, then this puts a completely different perspective on the results, you end up with a value in the order of 3.65 x 10-11 m/s
Hope that helps.
As a check, 'see' how much water has been used over the period of time.

 

[studyphd]

Hi,

First of all i apologise for my silly mistake. The units for the volume are mm3 rather than ml. So the permeability values are less than what I calculated. Thanks a lot to iandig for asking me to check this.

Thanks a lot to fattdad also. I am working on etrringite induced heave of the soil actually that is why I am doing these tests. I really want to confirm that what I am doing is right or not. I have attached a word file with the message explaining the purpose of my project, material and sample preparation etc. I have also put in step wise procedure of how I did the test with screen shots and my queries. Any help in this regard would be useful.
Sorry for bothering.
I have put my testing to half and would proceed after your advice.

Thanks a lot.



Questions regarding the procedure

1.Is it necessary to get a B-Value to be 0.99? Because I am using a soil stabilised with fly ash and lime and the B-Values are coming low. The literature says they can be low if some cementatious material is present in the sample. Do you also get low B-values?

2.Am I required to do consolidation as well? I am not sure do I need to do it after saturation or not.

3.Which permeability test should I follow? Constant head or constant flow. If I do constant flow then I have to give a value of back flow rate which I don’t know.

4.I gave negative value for base pressure differential (in case of constant head) to initiate a flow from base. Is this right? Also I am unable to initiate a flow from base if I do constant flow because the space for maximum hydraulic gradient does not take a negative value.

 

[fattdad]

on the matter of "B" value, if you get a value greater than 0.95 consider it saturated. There is no real linear relationship between percent saturation and b-value (i.e., a b-value of 0.75 may actually relate to 90 percent saturation).

f-d

p.s., I trust that you are actually working and not a student. . .

¡papá gordo ain’t no madre flaca!

 

[iandig]

Interesting Subject!!
I have been involved with a couple of investigations into sites where heave has occurred, even after the addition of GGBS to 'combat' the effect of sulphates. Unfortunately the sulfate content was so high, it ate the GGBS as well as all the lime and I have some great Petrographic shots to show this.
Don’t forget that it may not just be ettringite which forms, you may also get thaumasite. I can never remember which way round they go but I know one uses the silica and one the alumina, plus one reacts quicker in warm weather and one in colder weather. On the site mentioned above, we had a combination of both.
Not sure what the purpose of the permeability is unless of course you are looking at the beneficial effect of reducing the permeability, thus reducing the availability of water and as such removing/reducing one of the key components to heave. One of the earliest remedial actions we instigated on the above site was to put in place competent surface drainage on the affected ground to prevent inundation of water. The contractor had not included this originally (not sure the reason) and heavy rain had hit the side of the building, run down the wall and caused the edge of the floor to heave up. The more it heaved, the more water got in the bigger the heave etc…
If you are perming the material and ettringite/thaumasite form, this will cause the sample to heave, changing the volume of the sample so keep an eye on this. Also as the reaction will use water to bond with the sulfate, lime, alumina, silica, the inflow and outflow through the sample will be different IF sulfates are reacting. Steady state of flow is where what goes in comes out at the same rate, but if you know your samples are going to swell/heave, this will not be the case, so I would look at the flow out of the sample. Once all the sulfate has used up the lime, then the sample should settle back down again although it will depend on how much lime, sulfate, alumina and silica you have in the soil.
Which country are you in, if UK I will try and get some info sent across to you, plus point you in the direction of a couple of very good technical resources.

 

[fattdad]

To the OP and regarding heave of sulfate laden soils, I direct you to the following:

Heave Distres Problems in Chemically-Treated Sulfate-Laden Marterials, Puppala, A.J., and Ceranto, A., Geo-Strata, Vol. 10, Issue 2, p. 28.

I'll also republish my response to this article in this thread:

"Dear Doctors:

With great interest I just finished reading your article, “Heave Distress Problems in Chemically-Treated Sulfate-Laden Materials,” presented in the March/April issue of GeoStrata. I work as a consulting geologist and geotechnical engineer in Central Virginia, where 12 years ago, we had a HUGE problem that was chemically identical to your article’s description, but geologically very different.

In light of today’s growing interest in the beneficial re-use of industrial waste (i.e., for LEED Credits), I’m writing to share an overview of Central Virginia’s multi-case history.

The use of CCBs (coal-combustion byproducts) in civil/structural engineering projects has some very tangible advantages, when properly executed. During the mid-‘90s many projects were favorably developed using a mixture of lime-stabilized fly-ash and bottom ash for building pad and pavement subbase construction. At some point in 1997, a series of new cogeneration facilities came on-line within Virginia. These facilities burned high-sulfur coal and were designed with FGD (flue-gas desulfurization) systems for compliance to the “Clean Air Act.” These facilities were also supplying the principal CCB materials to the local broker.

With the accumulation of FGD materials at the cogeneration facilities, the local broker began to receive FGD residue in addition to the other bulk CCB materials. The result was a commingled inventory of glass (i.e., ash) and sulfur, which when stabilized with LKD (lime kiln dust) became highly reactive when exposed to moisture.

Damage to construction projects was dramatic. Building slabs heaved, pavements became hummocky and damage claims were in the millions! I’m attaching a photo that illustrates the nature of heaving within a small warehouse building. Interesting to note: In many instances the heave at the perimeter of the structures was less as the sustained foundation loads countered the heave stress. Interior of buildings were not so fortunate; however. An entire cottage industry of engineers and lawyers developed. In many instances entire buildings were razed and reconstructed just to remove the reactive materials. Words like ettringite and thaumasite become known to many as these were the bad actors shown by XRD (x-ray diffraction) and SEM (scanning electron microscope).

What have I learned? Well, to this day when somebody references the use of CCB for building pad construction or as a substitute for dense-graded aggregate below floor slabs or pavements, I look closely at the blended makeup including the presence of free lime and sulfur. Once burned, twice shy, eh?

Just thought you may be interested in a similar, but distinct example of your well-written article."

f-d


¡papá gordo ain’t no madre flaca!