Properties of Over/Underconsolidated clays

[hemipower]

hi all,

just a quick question, how do I know if the clay is overconsolidated or underconsolidated based on the following information...

12m of saturated soft-firm clay overlying permeable rock. A consolidation test taken from a depth of 6m gave the following results: Cc = 0.35; Cr = 0.08; Calpha = 0.01; Cv = 3x10-7 m2/sec (for pressures 10 to 100kPa); e0 = 0.95; sigma dash p = 42kPa. Triaxial shear tests provided the following information: Su = 50kPa; c dash = 3 kPa and phi dash = 25 degrees.?????

I understand the policies so if you cannot help thats ok, but if someone could point me in the right direction as to where I could obtain such information that would be great
Thanks

 

[dgillette]

It's overconsolidated if the present effective stress is less than the effective preconsolidation stress, also known as maximum past pressure, underconsolidated if the reverse is true. I'm guessing that sigma dash p = 42kPa refers to the preconsolidation pressure from the test. I'm used to English units, but I'll try:

Assuming a total density of 116 lb/ft^3 or 1840 kg/m^3 (based on e0=0.95 and all assumed approx. saturated), the total overburden stress is 9.81 m/s^2 * 1860 kg/m^3 * 6m = 109,500 Pa = 109 kPa.

Assuming the water table is 2 m down, the effective stress is 109 kPa - 4 m * 1000 kg/m^3 * 9.81 m/s^2 = 70 kPa. If the WT is at the surface, it would be 51 kPa.

Hmmm. Either way, that's more than what I surmise is the preconsolidation stress, 42 kPa. That would suggest that it is underconsolidated, but does that make sense geologically? Only if an embankment fill was constructed over it recently or if the clay is in an active tailings pond. B-(

_________________________________________
Check:

116 * 6 m * 3.28 ft/m = 2300 lb/ft^2 = 110 kPa

62.4 * 4 * 3.28 = 819 lb/ft^2 = 39 kPa

110-39 = 71 kPa

 

[fattdad]

figure the effective confining pressure. Multiply it by 0.2 and compare the result to your undrained shear strength. If it's much greater than it's likely overconsolidated. Su/P for normally consolidated clay is usually about 0.2.

f-d

¡papá gordo ain’t no madre flaca!

 

[dgillette]

Depends on what test he's doing. Often closer to 0.3 in CIUC. If that's from a UU test, 0.2 is probably good.

If they measured Su = 50 kPa, I would guess that the sample was consolidated in the triax cell to something higher than in situ stress. That's 70 percent to 100 percent of my sigma'v estimate, which implies a mighty high strength ratio. Intial void ratio e0 = 0.95 --> gamma-dry ~86 lb/ft^3 and saturated water content ~ 35%. Sounds pretty soft to me.

hemipower - Can you give us a bit more detail when you get in - triax test conditions, suite of three samples, site description, Atterberg limits, other?

Your use of "phi dash" instead of "phi prime" makes me think you don't live in my time zone. You're in (or from) UK, NZ, or Australia, right?

(PS - Does your handle refer to your Dodge truck?)

 

[hemipower]

I do live Australia, I am familiar with the prime notation but everyone here says dash, my handle refers to my valiant regal its pretty much a dodge dart but an aussie version, off the top of my head the site is riddled with faults and bedding defects the site is for a proposed dam location so we are in close proximity to a river as well, it's a country town so there really hasn't been any infastructure in the region, the last I heard about the project is that they are considering moving locations, if this is true there is no need for the calculations so I'll try and find out what is going on in the meantime thanks for the advice, its much appreciated

 

[howardoark]

There's a graphical technique (Cassagrande Construction) which is what you're supposed to use when you have oedometer results. It will give you the maximum past EFFECTIVE overburden pressure at the DEPTH where your sample was collected. You then compare that to the EFFECTIVE stress at that depth.

It's very rare for shallow soils not to be overconsolidated because of dessication.

Sorry to shout, but you really should consult with a soils engineer rather than try to figure these things out for yoursel. You seem like you're smart enough to do the calculations but it doesn't appear that you have enough experience (no shame in that) to know if your results make sense.