Foundation of Monopole: Shallow or Deep?

[geot88]

The company I'm currently working for has been called to do the Geotechnical design of many monopoles in the city. They gave us a sample study (they have worked like that in other countries) and wanted us to do something similiar here.
The requirements were:
-Do SPT continously up to 6 meters.
-correct N to N60 and get friction angle and Su values from correlations.
-calculate bearing capacity of a shallow foundation (from 5x5 m. to 7x7 m.), Df= 2 m.
They insisted that they have used a shallow foundation 90% of the times that they have contructed monopoles. (Monopole height = 36 m.)

We did our job, SPT 6 meters. Found clay (N= 1 to 7) from 0 to 4.5 meters, then found fine sand (N= 7 to 13) from 4.5 to 6.5 m.
water table was at 2.00 meters down the surface. We got friction angle for sand and Su for clay (correlations)

Then we analyzed it under undrained conditions. we gave friction angle = 0 to the clay (undrained condition) and used Hansen theory. q adm was around 30 kPa.
Later they told us that the Moment of the monopole was 2100 kN-m and the wind speed was 140 Km/h -which I think is really really high. Vertical force was 1400 kN.

Should I analyze eccentricity? I analyzed it and it was (M/V = 1.5 m). and B/6 is 7/6 = 1.16 m. So I suggested deep foundations.
However, they insist in using a shallow one.

My questions are: are those wind values real? Is eccentricity calculated well? I know I have to calculate it for strip and spread footings, does the same rule apply to concrete slabs, mats or rafts?

 

[msquared48]

So they are designing to a 210 mph wind? Where is this monopole? The wind loads could be LRFD values.

Mike McCann, PE, SE (WA)

 

[geot88]

@msquared48, No, it's a 140 Km/h (87 mph) wind. It's in South America, no hurricanes or strong winds here. The maximum annual wind speed in this city is 24 Km/h. However, they said that their client says that's the regional wind speed. That wind causes a really high moment (2100 kN-m, according to our client).

 

[msquared48]

Sorry. Wrong direction.

24 km/her seems really low. What is your governing code there?

Mike McCann, PE, SE (WA)

 

[oldestguy]

What does your contract say? Are you responsible for the job coming out to be safe and and still carry the loads they suggest? If not, I'd cross it off as a lost cause. For one thing why accept the N value correlations for design, if there are such things that are accurate. Going into a job and accepting these limitations I think was rather unwise. Sometimes it is more prudent to back away from imposed risks than to accept them.

 

[Ron]

Agree with oldestguy....
Geotechnical consideration are site specific.....not "one size fits all". Shallow foundations are common for monopoles. I am in a relatively high wind zone in the US and we have used shallow foundations for monopoles, but you have to consider overturning and its effects (eccentricity) and the resulting reduced bearing area (thus increased bearing pressure). With good soil conditions and a large, thick foundation it can be done.

Keep in mind your client is probably looking more at risk assessment than geotechnical/structural design. Don't let your client dictate your engineering judgment.