Q? Tower base on ROCK?

[WBear2GCR]

As long as I am plumbing the depths of expertise...

My property is on a hill, most of the hill is very little topsoil (or none) and SHALE rock below.

One of the sites where I may put a tower is going to be more or less on the shale rock.
The question is how to best put a footing for the tower?

One of my concerns is that *even if* I drill the shale and insert reybar with appropriate anchor cement/epoxy in the shale that water does infiltrate this rock via cracks and fissures, and I am concerned that water could seep between the concrete pour and the shale, freeze and make a big problem.

I'm also thinking, skip the drilling/reybar and just make a wide base, like 4-6' square. The tower will be guyed, even though it is a crank up/tiltover (at the base) type being likely installed here.

I'm thinking that a big lay-on-the-rock poured guypoint won't cut it, and I will need to drill the guy anchors...?

Grounding is something of an issue, but I suspect that if I lay out enough radials under the soil, I'm ok.

Anyone got experience with this?

          _-_-bear
 

[KL7OF]

As long as I am plumbing the depths of expertise...

My property is on a hill, most of the hill is very little topsoil (or none) and SHALE rock below.

One of the sites where I may put a tower is going to be more or less on the shale rock.
The question is how to best put a footing for the tower?

One of my concerns is that *even if* I drill the shale and insert reybar with appropriate anchor cement/epoxy in the shale that water does infiltrate this rock via cracks and fissures, and I am concerned that water could seep between the concrete pour and the shale, freeze and make a big problem.

I'm also thinking, skip the drilling/reybar and just make a wide base, like 4-6' square. The tower will be guyed, even though it is a crank up/tiltover (at the base) type being likely installed here.

I'm thinking that a big lay-on-the-rock poured guypoint won't cut it, and I will need to drill the guy anchors...?

Grounding is something of an issue, but I suspect that if I lay out enough radials under the soil, I'm ok.

Anyone got experience with this?

          _-_-bear

 

This is an above ground base for a 60-70 ft freestanding cell tower in Alaska...Wind blows at least 80 mph every winter...This tower has been there for 5-6 yrs There is 3 yards of gravel in sacks holding the base down

[KM1H]

One of my concerns is that *even if* I drill the shale and insert reybar with appropriate anchor cement/epoxy in the shale that water does infiltrate this rock via cracks and fissures, and I am concerned that water could seep between the concrete pour and the shale, freeze and make a big problem.

Sounds like you looked at mine

Ive rock at 8-14" down at one tower site and the rebar, epoxy, as well as concrete to rock bonding epoxy has been holding 180' of Rohn 45 for almost 23 years now. With a pier pin base the base support is not stressed and the guys do all the work. For those I went with 1/4" with torque arms and equalizer plates. Guys are at 4 levels with the top 20' open for ease of managing big antenna installations such as the 4el 40M KLM that was there for many years.

Guy anchors are utility company grade rock anchors that required hiring a guy with a big compressor and drill to dig 2" diameter holes 8' deep. The anchors were put in place, tightened and the hole filled with epoxy. Still looks as new.

While the guy was here I had him drill for 2 other towers.

The first tower installed here in 89 was a temporary 60' of 25G guyed to convenient trees and with a concrete base plate just sitting on the ground. Its still in place and has had a wide assortment of antennas on it over the years. Currently has a HB 3 el 20 on a 21' boom and a 35' 8 el for 6M.

Carl

[Chuck...K1KW]

This is what  I did with the same situation you have, years ago.  

Scrape whatever earth and loose rock is on top of the shale.  Then build a form to hold enough concrete such that the restraining force due to the weight alone is enough to meet spec., that would be about 6000 lbs per 1/4" guy line attached. So, for three 1/4" guy lines that would be about 18,000 lbs.  That is about 6 yards of concrete per guy anchor point since concrete is about 3000 lbs/sq. yard.  That would be a form roughly 6 ft X 8 ft X 3 ft high if it all is above ground.  Make sure you use rebar in the concrete to keep it together.  I was able to get about 1 ft below ground so the 6 X 8 foot slab is about 2 ft above ground.  Another advantage is that the guy rod is not in contact with earth at all so it won't rot out.

This is major overkill (by 2 - 3 times) since it doesn't account for the vector force at the average angle of all the guy wires, or any restraining force due to friction of the concrete sitting on the ground, or the likelyhood that all three guy wires will be maxed out at the same time.  Nevertheless, it's good insurance.  I have three towers up this way for over 30 years and have had 100 ft pine trees fall on the guys with no problems!

Chuck

[WBear2GCR]

Chuck,

Ur talking the *weight* of the guy anchor blocks??
Weight of the tower base??
18,000 lbs of concrete is a LOT.

Or are you talking the down force??
They can deliver different "test" concrete. So called "9,000lb" concrete can be had -
higher too, iirc.

I think before I pour 6 yards of concrete, I'm going into the rock with reybar!!
These days 6 years of concrete ain't cheap.

My other problem is that I strongly doubt I can get even one of these small (i've
seen kinda little ones) concrete trucks up the hill as it now stands. Perhaps with the
presence of a bulldozer, as needed. But I'm hoping to keep this within the range of
less than a major multi-thousand dollar project...

Presently, I am hoping to mix the concrete in a gas powered mixer on site and pour in steps... as needed.

Pretty sure I can get a 4wd tractor to haul up a compressor/drill rig ok. Even that isn't
going to be cheap...

A frame on the surface like Alaska might be ok, but I suspect that is in a permafrost area,
so the ground is not moving/squishing.

I was also considering hiring out a well drilling rig, and sinking in some well casings (into the shale), and welding to them... maybe a single with a thick steel plate on top, or three in a triangular layout a bit wider than the tower base... look ma, no concrete!

Although one could fill the well casing (down 10-15') with concrete and reybar and work it that way, I am thinking... well at least thinking is maybe stretching it a bit.

                             _-_-bear

[KL7OF]

s
A frame on the surface like Alaska might be ok, but I suspect that is in a permafrost area,
so the ground is not moving/squishing.



The pictured tower is not on Permafrost...That ground is actually fill put in 10 yrs ago...about 150 ft from the Naknek riverbank...

[K1JJ]

Some tower thoughts:

Speaking of pier pins and guyed tower bases...

My 150' Rohn 45 guyed tower once stood in concrete. (Not good)   I then modified it to a pier pin base. I was actually able to cut the base off and the tower still stood straight and firm as I made the transfer to a pier pin. In other words, the guy cables take up all of the lateral motion, especially the lowest set of guys. (with no wind)

But even though the lateral base force requirement is minimal, the downward force on the tower is tremendous. Each guy (at an average of 45 degrees) can vector down 300 pounds or so - so, with 12 guys or more we have several thousand pounds pushing that base into the ground.  Thus, the need for a good, wide base.   But for a self supporter, it's the opposite.   With no guys, 100% of the lateral  and tipping force requirement is in the base - and we need a HUGE base to equate to the geometric strength and stability of a guyed tower..

Back to guyed towers... the wind can put huge stress on the guy anchors.  There have been situations where it rained so much that marginal guy anchors pulled out of the ground.  Bottom line is I usually use X4  the minimum Rohn recommended concrete mass for base and anchors.  I also use three separate sets of guy anchors that go out 50', 110' and 160' for my 150' tower with 6 point star TORQUE arms. This was designed to handle a 75M 3el rotary Yagi.  

Though it is done otherwise due to space limitations,  the best angle for guy leverage is above 45 degrees.  Using a single set of anchors becomes a compromise for a very tall tower. Either the angle is too sharp and requires large pulling force or the angle is too high and needs longer than normal guy lengths, adding slop and/or more tension.  But this is nit picking.

A good Rohn 45 installation could last 40 years. We may have to paint it once or twice using galvanized zinc paint, but it will last. My 150'er was put up in 1986  (26 years ago) and still looks FB  - and is probably good for another 20 years.  We find the higher sections get "sandblasted" by the wind more than the lower tower sections.  Sections above the treeline get hammered more.

T

 

[KL7OF]

This is one that I did....55 ft freestanding..right in front of my shack

[KL7OF]

NO concrete

[KL7OF]

electric hoist for up and down

[WBear2GCR]

Wow, don't think I'd rely upon just the earth to hold that thing up!
Not with a beam on top for sure!!

Perhaps with a concrete pour over the framework...

Also, I'd want a positive lock on the raising cable, external to the electric hoist.
The gears in those hoists are known to strip and fail...

Just my immediate thoughts and concerns...

                     _-_-bear

[KM1H]

A good Rohn 45 installation could last 40 years. We may have to paint it once or twice using galvanized zinc paint, but it will last. My 150'er was put up in 1986  (26 years ago) and still looks FB  - and is probably good for another 20 years.  We find the higher sections get "sandblasted" by the wind more than the lower tower sections.  Sections above the treeline get hammered more.

Mine is in need of some TLC above the tree line from windblasting, acid rain, and salt water fog at times. The top 2 sets of guy wires are going to need at least 1-2 of the pieces between insulators replaced, mostly all surface rust and this was quality 1/4" EHS as Im sure Chuck remembers. At one point the tower held a 4/4 KLM 4el 40M, a 8el 6M above the top one and 4 4el 40' boom for 20M on sidemounts with individual rotators along with one for the lower 40M. Lots of storms hammered the towers and the only regular failure was that damn top KLM about every 18 months. What a PITA to fix and I finally got rid of it. Went to a 2/2 for awhile and thats now down for some RF performance changes.

There was no way to get a concrete truck anywhere close to all but one of 9 guy anchors and the bases of the 3 towers. I used Manuel Labor, aka 2 healthy boys and myself and wheelbarrows. For the big tower base I thoroughly cleaned and almost polished all the rock that was going to be epoxied and concreted. There are also 6  3/4" holes drilled for 3/4" galvanized rebar that I pounded in and epoxied and then added a one layer cage about 4" below grade. The base extends only 4-5" above grade.

[Chuck...K1KW]

bear,

18,000 lbs is the weight required of each guy anchor point in my example.  The "test" rating of concrete is not the weight of the concrete but the test strength under compression.  Concrete weight will be slightly higher with higher test strength but 3000 lb test concrete is just fine for guy anchors and weighs in at about 3000 lbs per yard.  All my towers are on pier pins and the concrete base is smaller then the guy anchor points.

For getting a truck load of concrete where the truck can't go, use a concrete pump.  They can pump concrete 100's of feet and that is what I had to do here for all of the guy anchor points.  Back when I did it the pump cost an additional $400 and concrete was $30/yd. so 6 yards was not much.

Chuck

[WBear2GCR]

Yeah, somehow I have an aversion to taking money and burying it in the ground!  

And I also have an aversion to spending more on the base of a tower than for the the tower + antennas!!  

Might be how it comes out. I did get the CL33 cheap, and the tower cheap. So...

Concrete now? Dunno, would have to check the current prices, but IF I go all the way up the hill toward the top of the property, it's pull the stuff up there, or nothing. Pump ain't makin' it anywhere near there, unless I spend $$ on a road. And right there, the break-even point starts to look good for buying a big arse used dozer instead of paying for the privilege of someone doing it for me and making payments on theirs... Seems like you can always re-sell the dozer when ur done. What's that you say? You get a 'dozer and you're "never done"?? Ha ha.

And the QTH is about 6-7 air miles from one of the largest cement manufacturing plants on the east coast, the former Atlantic Cement, now owned by LaFarge. You'd think that being so close you could get concrete at a better price? Nope. If you want a barge or cargo ship full... you know they SHIP this powder across the seas in ships?? Last time I checked concrete and water did not store well together.

So, 18,000lbs or 9 tons of concrete, eh? Well THAT'S NOT GOING ANYWHERE!!

Doubt I need quite so much... maybe the rock is a good thing...

I am still concerned about water freezing in the junction between the concrete and the shale.

                         _-_-bear

[KM1H]

I am still concerned about water freezing in the junction between the concrete and the shale.

That is why you should do what I suggested.

Clean the rock and pour this liquid over it that etches and creates a bond between the rock and concrete. I dont remember the name but it came in a plastic jug I got from a store that specializes in outdoor type yard stuff stuff.
You dont get enough cold down that way anyway

[KL7OF]

Is there a problem with the shale freezing and heaving??   Are there any power poles, fence posts, sign posts, or guard rails around that show signs of being affected by freezing and heaving or coming up out of the ground?? 

[WBear2GCR]

I have no way to measure if the shale moves or not.
There is enough water at various times, including if there is a thaw and then a quick freeze
during the winter - we do get as low a -20F - so that I am concerned.

Cleaning and etching the rock for a bond might be enough.

The question is what is the stuff?

                       _-_-

[Chuck...K1KW]

I would not worry about the shale/rock interface at all.  Shale is quite porous so water comes and goes, I doubt you can "seal" anything against it.  I tensioned the guys on my towers 30 years ago and have checked them over the years go and there has been NO change.  Nothing moved, of course with all that weight, I'd be surprised if it did! 

The total cost when divided by the expected life of the installation is very low, even today so a design with lots of margin lets you sleep well.  To that end, several telephone poles here on the shale hill have had their anchors pull out of the rock.  One was helped by a car that hit it but I'd bet that would never have happened with a nice K1KW 18,000 lb guy anchor...(:>)

Chuck

[M1ECY]

Speaking as someone that will never have anything like the towers you guys are playing with, my comments would go this way.

1, It would only be put up once, so it needs to stay up.

2, 9 tons of concrete is way cheaper than litigation for killing someone with a tower collapse.

3, If you do have a rock base, and have no idea of its properties, invest some cash in the opinion of a structural engineer, give that engineer all the variables you can think of, and let them calculate the correct engineered solution - I am betting you will be suprised - even if you choose not to use the rock base, any decent structural engineer will be able to save more than their fee in wasted materials and time.

You have a really nice situation - one that a lot of Hams would like to be stuck with - a big tower, lots of room, and no apparent issues with planning permission!

Cheers
Sean

[WBear2GCR]

Well apparently there is no limit to what can be litigated these days.

My tower site is so far off the road and away from other people's property that there is no way that anyone could be there without knowingly trespassing. That doesn't stop any lawsuits.  But if it fell, it would be no different than a big tree falling here, and they do that quite often. Of course the trees usually fall in bad weather... a tower could fall on a sunny day, I guess. But I'm more likely to wack bambi the deer than any person by a ratio of 1,000,000:1. But stuff does happen...

As far as zoning, they put a dumb tower ordinance in here.
Apparently it was pushed DOWN upon the towns by some unknown source.
It does limit tower height.
It fails to take into account the *location* of the tower - like next to another house on a small lot vs. in a 5 ac field!!
And it fails to discriminate between commercial, amateur, and other uses.
Dumb ordinance.

Imo, ignorant controlling nonsense the way it is written.
Good thing I can't say what I really think about it.

More of the "nanny state" mentality, where personal responsibility is never a criterion.

I guess most people need to be told how, when, where, and what to do... 

                 _-_-bear

PS. the other part is the balance between spending $$ to yak with guys on ham radio vs. benefit... a multi thousand $$ tower install done by pros is very nice indeed, but comes under the heading of being very, very discretionary unless a whole list of other things have the ribbons tied...