At 06:39 AM 1/2/01 -0600, n4kg@juno.com wrote:
...
>> Screw anchors are a lot more dependent on specific soil
>> characteristics than concrete anchors. As an example,
>> the concrete anchors on my tower weight about ~1800 lb each,
>> and get the remainder of their anchoring power from the weight
>> of soil above them. Screw anchors weigh what -- maybe 50
>> lb each?
>
> I suspect that much (most?) of the "holding power"
> of the earth is not from the weight but from the
> 'cohesion' of the earth. Loose dirt could weigh
> as much as undisturbed (compacted) earth but
> would have little or NO 'cohesion'. K5RC once
> lost a tower when an anchor (cased in concrete)
> pulled through 'soupy' earth after sustained rains.
> For small anchors, the surface area and depth is
> more significant than the weight. (I agree that
> Pete's 1800 lb anchors are pretty substantial)
Tom's right, of course -- I was short-handing. It's like shoveling sand or
soupy mud vs shoveling clay.
The more I think about this, the more it seems appropriate to consider
using 2 guy anchors per guy set, so that a single point failure of the guy
anchor shaft won't release all of the guys on one side at once. Next
spring, I also intend to dig down to the concrete and inspect the shafts on
mine.
By the way, the ~1800 lb anchors are per Rohn spec, and are only roughly .5
cubic yard each, but resemble a large flat plate on the end of the anchor
shaft, so you get both area and dead weight.
73, Pete N4ZR
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