LPILE and frost 1

[structdtl]

I am modeling the upper frost layer in LPile by making the lateral resistance close to zero (I am using the actual weight of the soil). LPILE runs a report with the warning that my phi value is outside the range. My question is, can the output still be trusted for everything else even if reporting this error?
Thank you!

 

[geotechguy1]

I would either try modeling it with a really low phi to get the program to run, or model it with the pile sticking out of the ground (i.e. you change the top of the 'soil model' to be at the bottom of the frost layer you want to neglect

 

[structdtl]

I am using theta = 5 and k = 5 and received a warning that my value for theta is unreasonable (should be between 28 and 48). The program runs and a typical report is generated with the warning.

If I run LPile neglecting the frost layer, the moment and displacement are larger when compared to the run described above, as it should.

I guess my question now is what happens to the lateral capacity of the soil during the freeze and thaw cycle? Is there a rule of thumb that can be used to back check LPILE for cases where pile is in this layer?

 

[BridgeSmith]

I usually do as you described in your first post - model the frost-susceptible layer with its full weight, but very low stiffness. I haven't tried that in Lpile, but it works just fine in Allpile.

You might try coding the top 5' as a different soil type (maybe soft clay?), and see if the warning goes away.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[structdtl]

I have another question and this is in a more general level and related to the output. I have been told to apply service loads in Lpile for the program to generate the moment, shear and deflection curves. To look at the Mult vs bending curvature graph to determine max moment from soil profile. Apply a factor of safety to this Mult, to compare it to max moment from moment graph and make sure to be below that number. --- Mult being the point at the top of the curve

I am new to LPILE and I trying to check the capacity of an existing pier. I have been reading the technical manual and I keep trying to think of this analysis as ASD and LRFD. One co-worker who has used the program once or twice in the past 5 years, gave me the above suggestion for the evaluation. Is this correct?

 

[BridgeSmith]

Typically, deflection is a service limit state, while moment and shear on the shaft would be strength limit states. It can be somewhat difficult to reconcile the LRFD limit states if the geotech capacities were calculated using ASD. If that's the case, to do it 'right', you really need to know what reduction factors were applied in the geotech analysis, so that you can start with the ultimate capacities and apply the correct LRFD resistance factors. Otherwise, I would suggest assuming the ASD values given are similar to the LRFD capacities with the resistance factors applied.

Soil properties are difficult, sometime impossible, to determine with even a moderate degree of accuracy, so particularly when calculating the max moment, I advise hedging to the low side of the soil stiffness; it usually just means a little extra reinforcing in the shaft - cheap insurance as far as I'm concerned.

Sorry, I just noticed you said you're analyzing an existing pier. For that, I recommend starting at the low end of the possible soil stiffness, and see if it's adequate. If it is, life is good. If not, sometimes more extensive soil testing may be warranted, to narrow down the soil properties.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[structdtl]

Thank you!