How to estimate the available moment resistance for monopoles 1

[shane]

I am trying to determine the available moment resistance for direct buried power transmission monopoles. The typical profile is either 5-8 clay overlying loose sands or predominately loose sands. I evaluated the poles using LPile, but the results seem very very conservative.

Does anyone have any suggestions on modifiying LPile to realistically estimate the capacities, or another method. Also, any thoughts on modeling the backfill, since it generally consists of compacted cuttings (in this case).

Thanks for your thoughts
Shane

 

[pmkPE]

Your state's DOT should have nomographs for utility pole foundations based on the local of the state in which you would like to design the foundation it would include typical soil types. The state of Georgia has one for three different scenarios. I'll email this to my office and look up the data when I return. Any additional info such as diameter and height and applied moment would be helpful.

 

[pmkPE]

GDOT has three different scenarios one each for 1) Coastal Plain Region, 2) Piedmont Region, and 3) Valley and Ridge Region. The corresponding soil type/ and characteristics are:1)Unit Wt= 120 PCF, C=0 PSF and internal angle = 32;
2)Unit Wt= 120 PCF, C= 300 PSF and internal angle = 25 degrees; 3) Unit Wt= 115 PCF, C= 750 PSF and internal angle = 18 degrees. The nomographs previously mentioned are based on depths from 5 - 15 ft for cases 2 and 3 and 7 -17 ft for case 1. The diameters for the caisons range from 2'-6" with 6" increments up to 4'-0". Bending moments covered are between 25 and 175 ft-kips. Various reinforcement sizes are specified according to the applied moment. If you want a copy from the GDOT ask for drawings TS-01 through TS-09. TS -06 has the information that is referenced above, its title is Strain Pole and Mast Arm Pole Foundations. You can use these charts to determine if your other means of analyses are reasonable.

 

[shane]

Thanks for the help and information, I will see where that leads me.

 

[Focht3]

[blue]shane[/blue] -

Sorry to be so late in commenting - busy month! I've done a good bit of transmission line work in south Texas, and the profile you have described is similar to many that I have dealt with over the years.

Did you look at the GDOT approach? What was your conclusion? What pile/pier sizes did you get from LPILE, and what sizes did you expect?

I see two common mistakes when using LPILE:[ol][li]Choosing too small a k-value, and[/li][li]Applying a factor of safety to the soil parameters instead of the loads.[/li][/ol]Be sure that you use reasonable - not overly conservative - soil parameters with LPILE. [ε]₅₀ values are also important - if they are too big, it will screw things up. You check for stability by doubling the loads - if it fails, then you need to deepen the pile/pier and try again.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"