Foundation on granular soil

[ONENGINEER]

I am designing a single foundation (preferably a pad. The site investigation up to 15 m indicated that the ground is made of sand with some gravel. The sand is generally clean, medium grained but with some fine at top changing to coarse towards 15 m. The SPTs are generally around 30 with a range of 20 to 90 and groundwater more than 9 m deep.

Could someone let me know what foundation design issues should be taken into account for the above 40 m high pole. Comments on seismic design would also be appreciated.

 

[oldestguy]

The reason that no one has attempted to answer is that it would appear that they recognize this is a routine design situation on a great site. The aim of this site is not to design things, especially when there are no indicated problems.

 

[Ron]

OG...agree. This is the type of site we would love to have in our coastal plains, but don't always get!

 

[ONENGINEER]

The main intent is about the scope of seismic design for a pole foundation supported on a liquefiable soil as compared to a building foundation that would be governed by building codes/regulators. One could assume that a pole (with its accessories, antenna, etc.) if failed due to a seismic event, could fall upon pedestrians, populated areas, or the nearby buildings/amenities that accommodate people. But this argument may not necessarily be accepted by some others including owners.

Other professional view points or a reference to codes/standards on this matter, would be appreciated. Thanks for the notifications and agree that the original post general and non-specific.

 

[BigH]

Have you checked to see if your cohesionless soil is really "liquefiable"? Seems to me that the N values are pretty good. See Kramer's Geotechnical Earthquake Engineeering Chapter 9, Section 9.5.3.1 Cyclic Stress Approach. This is a pretty extensive book and should be readily available. If the soil is not, then it should not be of a concern - unless you are of the opinion that, regardless, SH.

 

[Ron]

ONENGINEER....the fall radius of a tower is always a concern...without regard to the soil type. A monopole has a limited fall radius that is much more predictable than a guyed tower. A monopole is much shorter than a guyed tower or a lattice tower. Many municipalities control the fall radius as much as possible through ordinances. I have had to do fall radius evaluations for guyed and lattice towers and make presentations to city/county commissioners to explain to them.

As both you and BigH noted, you are right to consider liquefaction in a high seismic area. If the liquefaction potential is not high, then you appear to have a good site. You might also consider that your alternate foundation could be a deep foundation such as a drilled pier or a group of augered, concrete piles with a pile cap. The cost differential might be less than you think, assuming you have a good, accessible site.

 

[oldestguy]

One Engineer: Sorry, I have the same problem with my wife. A question rather non-specific, not fully explained, leaves me wondering what the heck she means. With the wrong answer from me then, the roof falls in.

 

[ONENGINEER]

BigH - The corrected N value is 14 in a SP-SM materials with 7% fines from 9 m to 11 m. this seems to be liquefiable. The rest of the soil profile is dense and one would not worry much for liquefaction. The top 7 m below the future foundation would not be saturated and wonder if this could diminish the potential liquefaction that is expected in the limited soil interval underneath. Thanks for the good reference.
Ron - Radius of fall is an interesting criteria. Is the radius of fall simply the height of the monopole or a multiple of the height. Or are there more into it? Please let me know if any report/publications?
Oldestguy - You are right that the original post was general.

 

[Ron]

OE... Google "tower fall radius" and you'll see a bunch of local ordinances that will give you an idea of what groups around the country are doing.

 

[BigH]

I know it might be a bit of an overkill, but you could drive some short timber piles near the pole foundation to densify the sand from 9 to 11 m - maybe you could do another set of SPT counts - or drive a pentest dynamic cone.

Yes @oldestguy - there are many of us out her that have that problem too - and I have to try to understand English with a strong Javan accent!!

 

[ONENGINEER]

BigH, thank you. I checked again, and the liquefiable zone is only 1.3 m thick. The soil densification by piling seems an interesting option but the cost could be justified if the hazards associated with the liquefaction of the 1.3 m medium dense sand at a depth of 9.5 m to 10.8m and the adverse effects on the pole are demonstrated.. To be more specific, what happens to the pole if the liquefiable zone at depth is not densified.

 

[Ron]

OE...the pole fails under your scenario.

For subsurface densification, consider vibratory densification using an open ended 24 to 30-inch diameter pipe vibrated at 5-foot grid points over the entire foundation influence area and extending to a depth significantly beyond the anticipated liquefaction zone. I've used this method several time with excellent results.

The entire cruise ship berth in Nassau harbor (Nassau, Bahamas) was done this way and densification was carried to about 50 feet below grade to mitigate potential settlement on loose sands. Has been in place about 20 years and is performing nicely.

 

[BigH]

. . . or drive the piles I mentioned and put your foundation on them making sure they go "deep" enough so that the thin zone would have little if any effect. The "jelly" effect on a deep enough pile on flat land (assuming that the liquefiable soil cannot "slide" would be rather small, I'd think. Ron - like your point - although vibroflotation could be considered.

 

[ONENGINEER]

Just to understand better before I can justify the cost for ground improvement. I assume the liquefiable zone is 2 m to be on conservative side. Let us assume that the aforementioned layer at 10 m depth soil liquefies. Some preliminary calculation showed that the soil would only deform vertically about 66 mm if this 2 m layer liquefies. There is no indication of differential settlement. Can this deformation cause failure of the single footing at surface or the fall of the pole, while there is a thick crust of soil underneath the footing.