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# Calculating modulus from stress-strain curves in triaxial UU tests

## Calculating modulus from stress-strain curves in triaxial UU tests

(OP)
Hi, for UU triaxial tests (saturated ML), I have 3 stress-strain curves for different confined pressures. In my case, these pressures are around 50 kPa, 100 kPa and 200 kPa. So, I had 3 different modulus.

For example, if I want to calculate the secant modulus at a certain depth, say around 6m. Is it correct to use the modulus obtained from the stress-strain curve for the 50 kPa of confined pressure? Effective stresses at 6m for a typical soil (wet density of 18 kN/m3) with water table at the surface will be around 50 kPa.

Also, should the modulus be similar for these 3 curves since the shear strength is constant at any vertical stress?

I will check my soil mechanics books tomorrow, but if you can provide some guidance here, it will be greatly appreciated.

### RE: Calculating modulus from stress-strain curves in triaxial UU tests

(OP)
I checked my soil mechanics books and for UU tests since there are not increament in effective stresses (not change in strenght), theorically the the stress-strain curves should show similar modulus. So, I took the average of the "E" calculated from each curve.

### RE: Calculating modulus from stress-strain curves in triaxial UU tests

Taking the average should work for most applications, but it really depends on how you are planning to use the data.

Mike Lambert

### RE: Calculating modulus from stress-strain curves in triaxial UU tests

(OP)
Mike, thanks for the reply. I was trying to get the modulus in order to calculate lateral pile capacities/deflections. BTW, is any rule of thumb for maximum allowable pile deflection at ground surface level?

### RE: Calculating modulus from stress-strain curves in triaxial UU tests

For your purposes, an average is likely just fine since the difference in vertical and horizontal modulus might be several time the difference in your data.

As for maximum pile deflection, all depends on the structure. I've seen everything from "well of course none", to 1-inch for structures; and up to a foot for landslide repairs.

Mike Lambert

### RE: Calculating modulus from stress-strain curves in triaxial UU tests

(OP)
This is a marine structure (mooring pontoon), is any literature reference for serviceability requirements for this type of structures?

### RE: Calculating modulus from stress-strain curves in triaxial UU tests

Hi Okiryu
I prefer to use the modulus from 50Kpa confined stress, if you did not use back pressure.
If the sample were homogen, the higher confining stress you will get the higher modulus value as well.

### RE: Calculating modulus from stress-strain curves in triaxial UU tests

(OP)
Hi L0k, 50Kpa of confining pressure is around 2.5 m of soil overburden. Is any specific reason for this? Thanks for your reply.

### RE: Calculating modulus from stress-strain curves in triaxial UU tests

Hi Okiryu
Your sample is 6m depth with the gwl on the surface and wet density of 18 kN/m3, so confining stress of 50 kPa represents the actual condition.

### RE: Calculating modulus from stress-strain curves in triaxial UU tests

(OP)
L0k, thanks for the clarification. Yes, you are right, the 50kPa curve may represent better the actual site conditions.

### RE: Calculating modulus from stress-strain curves in triaxial UU tests

If the sample is supposed to be fully saturated in situ (a possibility given this is apparently a marine environment) then I wouldn't get too hung up on confining stresses as these shouldn't make a difference (at least in theory). Generally I prefer to avoid stiffness data from UU tests as it is often all over the place. You get much more reliable and repeatable results from more sophisticated tests, e.g. CU triaxials. I guess that's not a luxury everyone can afford though!

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