×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Stuck in a hole, still digging

Stuck in a hole, still digging

Stuck in a hole, still digging

(OP)
Hello,
I started putting up an antenna tower in my backyard. Reason #1 for the tower is to get high-speed internet in my remote rural location for less than 200 dollars a month. From the ground I can see only one cell provider's tower (the expensive option). At 45 feet up, I should get a signal from a lower-cost provider with unlimited bandwidth for 1/4 the cost.

The tower is a self-supporting steel truss, 40 feet tall with a 6 foot extension tube on top. The manufacturer of the tower provides an installation sheet for the do-it-yourselfers like me. The minimum required base is about 5' x 5' x 4'deep. For those who want to see more tower details, a google search of "DMXHD-40N" will give multiple hits from the OEM and various distributors of the tower.

Things started to go wrong on the day I excavated the hole. The actual excavation went smoothly (mini-excavator from Home Depot did the job in 1.5 hours). But it was pouring rain the whole time. Following that downpour came another heavy rain that added up to about 60 mm of rain. This is very unusual for Calgary, by the way. Just my luck. The pit filled with 2 feet of water, receiving not only the direct rainfall but also run-off from the garage roof which I didn't think of diverting. I sump-pumped it to 6 inches and allowed another week for it to dry out.



This weekend I tried putting in the gravel base, but I was dismayed to find that the gravel never became firm. It just adopted the texture of mud. By hand, I dug down to see what was underneath. The gravel/mud mix was about 6" deep, below that was like clay, and below that seemed to be sand. Then why did it take a week to drain the last few inches of the water? I'm thinking the sand is so fine, and mixed with clay, that it can't drain quickly. Even though it looks and feels like sand. Pressed between my fingers the deepest sand is granular, and does not smear mud on my fingertips. Why didn't it drain water from above? I do not know where the water table is, but this hole at the bottom of the excavation added up to 6 feet below grade and it did not fill with water.



Another thing that happened while packing the gravel, where it would pack, is that it seems to "float". This is hard to describe - it's like I'm standing on a waterbed and when I push my foot down, some other spot pushes up. This is hardly a solid footing for a 10,000 pound concrete block!

Is there a way to improve this gravel base?
What are the odds that by excavating the hole deeper I will find more solid footing?
Should I abandon the hole entirely and locate the tower elsewhere?

This isn't my first tower base excavation, but this is the first time I've had difficulty with water - and exposes how little I know about civil engineering. sad

If I look for a professional to help on site, what should I look for? Architects with foundation wall / retaining wall experience? Excavation contractors? Most of the options I find seem like turn-key project managers who won't be interested in a small project that's already half-finished ("half-botched?"). I'm willing to pay a pro to set me back on the right course, but I fear that none will want to "own" the result of their advice.

STF

RE: Stuck in a hole, still digging

There should be local 'tower contractors'.

In my area, I know of two. There's one with an office and yard only a few miles away, and another one is run by a neighbour's father.

It won't be cheap.

Did you confirm that you're within radio range? The inverse square law for signal strength is a killer.

RE: Stuck in a hole, still digging

Anecdotal, as you might expect from an electrical guy.

When digging my pond in the back garden I encountered similar ground conditions with clay, sand and gravel in layers. My observation was that the clay layers form an impervious barrier which prevented the water raining to a more porous lower layer. A couple of times I broke through clay to find that standing water in the hole vanished almost without trace. Weird to watch, but most welcome. The pond excavation was a pig of a job overall and in hindsight I wish I'd hired a back-hoe or a mini-digger. Moving nine tonnes of material by hand is hard work. You made a wise choice.

RE: Stuck in a hole, still digging

Clay is waterproof. I'd dig down a bit further.

Also, I am not a huge fan of gravel as a base, round here we usually start with baby's heads and then 2" and then gravel. Stone is cheap, digging and concrete is not.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Stuck in a hole, still digging

(OP)
VE1BLL:
I know it might not be cheap, but here's how my project's budget works: For every month of internet usage on the new provider, I save 100-150 dollars. The current plan for the tower will "spend" 14 months of savings. My expenses on the tower project are amortized by surfing the net more!

An installation crew came from the company and couldn't get a signal from the ground. I have two neighbours who subscribe to this company, get good service, but have less trees around their houses. I have no line-of-sight to their tower. Believe it or not, cutting down enough trees will cost more than raising the tower. I don't own all of the culprit trees, either.

Scotty:
These mini-excavators are VERY useful. I could have spent the whole weekend digging that by hand - and in the rain I wouldn't have enjoyed it at all.

Greg:
You are cruel, but your voodoo tricks may help. I'd have to dig up the gravel I've already layed down (1 cubic yard is now more than 6 inches deep in there) for that to work...
Is round 2"-3" rock good enough? Shouldn't it be angular? The local DIY garden/landscape center doesn't have large angular crushed rock, but they do have round "river" rock.


STF

RE: Stuck in a hole, still digging

Well to be honest I am strictly amateur on this stuff, and what we use suits our local conditions, which is cold climate rainforest, rainstorms of 90mm in 24h are the dominant design condition, on sand over clay, somewhat adrift from Calgary's requirements I suspect. I basically just ask the local quarry guys what they sell to everybody else.

What bearing pressure are you using? 75 kPa?

Looking at various foundation designs it seems like the voodoo method is overcomplex, my only excuse is that it (a) works and (b) doesn't cost any more than plain gravel, for us, as cartage is the dominant cost, not the material. No idea on pebbles, we get crushed rock.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Stuck in a hole, still digging

A civil I used to work with told me that clear stone dumps to about 95% of its full compacted density- no need to compact it. Mixed roadbed material such as what you apparently were sold as "gravel", if the picture isn't lying to my untrained chemical engineer's eyes, does NOT dump to 95% of its fully compacted density- it needs to be beaten violently to compact it, but when compacted it makes an even more stable base to build on. It appears to have mixed material which varies from the size of dust to about 3/4" diameter.

Doubt you want to rent a jumping jack and lower it into that hole, or be in the hole at the same time as it is running, and doubt even more that you want to haul it back out again after- if you're still alive! And manual compaction isn't going to do much. Doubt also you want to rent a machine again to mash that stuff down, and it's doubtful how good a job it'll do. Others may be able to comment on a rental machine which would do this job for you.

What we use here is 3/4" "clear stone", and no compaction. But in a hole like that, once it's dewatered and all the spoils of digging are out, leaving undisturbed soil at the base, it is also common to just start dumping concrete in. There's no need for a capillary break under a foundation like that- no different than the footing of a house- so no need for the clear stone. My addition's footings are on clay till that turns into World War 1 trench gumbo when wet, but there's been the central bearing wall for my two-storey house sitting on it for ten years now and no evidence of sagging. The 24" piers that support the grade beams my garage/shop bears on are similarly rock solid, and were poured right on the undisturbed clay.

Don't know what the frost penetration is in Calgary- here you need to go four feet for anything to remain stable against frost heave. If you're down six you're probably golden.

RE: Stuck in a hole, still digging

I think Moltenmetal might be homing in on the truth. I am not a civ eng but I have done a bit of heavy industrial construction and the stuff in your hole looks like crap to me as a load bearing surface. I suspect that digging it out , possibly getting down into one of your sand layers and then filing with a more free draining gravel mix might be a good start. Out of curiousity what part of Calgary is this site situated in. Thee are a variety of geological conditions within the general area.

RE: Stuck in a hole, still digging

So, what if he digs out the "loose" gravel above the clay sufficiently to allow a 6-12 inch diameter rented post-hole digger to rotate through the gravel and lift out the gravel and clay?

Then, once the clay layer is through, put a concrete mixture into the bottom to form the base?

RE: Stuck in a hole, still digging

(OP)
Thanks for all the comments!

Greg:
I think you mean the bearing pressure on the soil - the concrete block really will weigh 10,00 pounds, so at most 500 psf to the soil, which is roughly 20 kPa. If that's a lot or not much at all, I wouldn't know.
Maybe you're referring to the concrete, or "ready-mix" we call it, which is maybe 20MPa. I haven't really worried much about this stuff. I don't know what to do with the information, to be honest. Is concrete strong enough to hold up the tower against overturning forces? Heck yeah, that's pretty obvious. Is the soil steady enough to hold up the concrete block? Pfft I have no idea. It still gives under my shoes. My weight / my shoe size applies about 500 psf, too, so by that measure alone, I worried about the soil.


Moltenmetal:
I just did, in fact, rent a vibrating compactor. Breezy enough today that I could breathe beside it. After about 1/2 hour it didn't seem to be doing much good, as you say. I got spots with a seemingly hard surface but when jumping on it, the surface was clearly moving like a trampoline. It seems I was packing the top 2" and the mix underneath was as fluid as ever.

I used gravel that's called "3/4 road crush" which is pretty obviously the product of the quarry rock crusher and all the debris, fines, and sand that came with it. I've been filling potholes on my driveway for years with the same stuff. By using "clear stone" instead, I believe you mean a rock of the same size but without the smaller particles. That would drain better yes. If angular, it could be packed and stay packed. If packed into existing soil it would be more likely to add structure, rather than more soup sandwich.


Mininman:
I'm Northeast of Calgary. Researching my problem, I have gone in all directions, and now I have found a detailed soil survey report from the province. More info than I know what to do with, or can understand. My topsoil is 15cm of "Chernozem", and the underlying soil structure is, as follows:

Quote:

Continuous veneer of coarse textured (sl, ls) glaciofluvial materials overlying medium textured till or glaciolacustrine sediments 30 to 100 cm below the surface; undulating.

I tried to spend some time talking to a geologist friend to explain to me what all that meant. Since she works for an oil-sands company, the first thing she said was like "oh, that's just the overburden stuff we scrape off first". Sigh. Anyway eventually she was able to convince me that it means "mostly lakebed sediments" which is all fine sand and clay. Word of advice, don't bother asking a geologist about the stuff on top of the pretty rocks.

This might be relevant: In the report is a table of data that measures the following parameters at numerous depths:
Base Saturation
H2O at 33kPa
H2O at 1500kPa

If anyone thinks think these data are of use, I will extract a page of the report and attach it here.

STF

RE: Stuck in a hole, still digging

(OP)
Yeah you can tell, I haven't played in mud since I was 9.

STF

RE: Stuck in a hole, still digging

(OP)
Huh. I thought "puddling" was an old wrought-iron production process.

STF

RE: Stuck in a hole, still digging

Well 'I'm no geologist so take these comments as such. The comments from your female geo colleague jives with my oberservations in Fort Mac. That stuff really is crap and is removed not only to gain access to the oilsands but also because nothing can be built on it. For confirmation check out the reports on the recent failure of the tailings dam at Mount Polley. Those big long geological words are awefull similiar to the words used in the analysis of the failure , being an unidentified layer of this crap finally having its limited strength overloaded. I will go out on a limb and suggest you cant build anything on this materioal.......... either excavate all of it or come up with a different construction technique.

RE: Stuck in a hole, still digging

(OP)
Any experience with Screw Piles?

Sounds like a better way to do this in my soil.

STF

RE: Stuck in a hole, still digging

I have a client in Ft Sask which admittedly is nowhere near you, but they put everything on screw piles.

Do you have a house with a basement? What is its foundation?

RE: Stuck in a hole, still digging

(OP)
The house is on concrete piles, no idea how deep they are, or if anything was embedded at the bottom.
The garage is on a thin flat slab on the grade (sort-of visible in the first photo). Not a good example here.

Lots of temporary building in Alberta go on these. "Portable" school building units. Atco trailers, etc., etc. I've found a contractor in the area who installs screw piles. Talking to them is a little tricky because they are used to just taking stamped drawings from a civil engineer and then bringing their equipment to the site. Not answering a hundred questions from someone like me.

STF

RE: Stuck in a hole, still digging

(OP)
Done!
Found a contractor willing and able to install screw-pile anchors, and as I was on the phone with their office, the engineer checked my numbers for me, and made some recommendations to get the torque right. Finally, I can get back to welding up the interface to the tower - the project is fun again.
Thanks to everyone for their suggestions and corrections.

And why not a picture, just for posterity:


STF

RE: Stuck in a hole, still digging

Whoa whoa.. We need educationing.

What is a screw-pile anchor?

What's torque got to do with it?

Does this have nothing to do with the disastrous "Muck Hole" you started with? Different location?

Thanks.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Stuck in a hole, still digging

(OP)
:)
Muck hole is 10 feet away. This is undisturbed ground. The piles I used are 9-1/2" feet long, the helix is about 8 feet down.
Screw-piles are like the graphic below, but mine didn't need multiple helices. In the photo, all you can see is the cap that slid over the top - so yeah, it's not painted.
I have read a LOT about how torque is not a perfect indication of the ultimate strength of such piles. However, perfect is the opposite of good enough. The engineer talking to me about the installation was working with a safety factor of two, and I didn't mention that my tower leg loads also have a safety factor of about two. My calculations also ignored the effect of the interface frame that I am building to mate the tower legs (30 inch spacing) to the anchors (48 inch spacing).

STF

RE: Stuck in a hole, still digging

Very neat device. I think I've seen installed ones before, I just didn't realize what they were. I thought they were depending from buried concrete.

Thanks for the info. Hope we see a finished picture top and bottom sometime.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Stuck in a hole, still digging

I've used them for anchoring cable-stayed pipe bridges and they work great. Torque gave us a good enough indication of the kind of soil we were in. one pair of them has been in the ground with zero pull out for 15 years, the other for 10. I used them on another bridge where the geotech guy was concerned that the foundation for an existing bridge was inadequate for the additional load my second pipe was going to add. The piers worked just as well in compression.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Stuck in a hole, still digging

Just saw some being installed on a site north of Edmonton as miscellaneous foundations for temporary equipment and piperacks- basically a crane-mounted screwdriver. Fast and easy.

RE: Stuck in a hole, still digging

(OP)
The happy ending to the story:



Antenna dish at the top of the tower, at sunset yesterday. The guys came to install their dish and test the set-up, while I operated the tilt-up of the tower (people don't like to climb towers any more, gosh). The fellow was getting a -74dB signal when he tested the strength with his own laptop. Sounds good to me... what does that really mean? Is there an absolute value over which the ratio is divided, or is it a ratio between background noise vs. signal?

The fun part was, for me, making a safe and simple tilt-up system for the tower. It was very convenient to use when the installers were here, and they appreciated the ability to attach their antenna and run the cable, with the tower at ground level. Tilt-up took about 20 minutes. I had already buried conduit from tower to the house, so we just had to pull their Cat-5 cable through the conduit and finish the hook-up with connectors and their little black box.

Testing the tilt-up:





I tested it at first with just the lowest section of the tower, light enough to move just by hand with a single cable and gin pole. Everything moved freely, and didn't need much shimming to make it plumb. Then I located and drilled the attaching bar for the 3rd leg. This is where I also tested the "feel" of the stop when the 3rd leg comes into position. See, if there's nothing stopping the tower from going all the way over, then the 3rd leg doesn't stop the tower from flipping all the way over and crashing down. The wood block seen between the 3rd leg and the base stub was too hard, and as the tower came up vertical, it banged down on the block hard enough to impress a groove in the wood. Imagine what the whole tower would do with a stop like that. So stopping the tower gently, before the 3rd leg came up short was critical. I settled for a "back-haul" rope, though it's a little fussy to set up and tie off at the right spot. You can see the back-haul rope as the red rope in the photo below, on the left.



The rest of the system: A winch, attached to the trailer hitch of the truck, and a 20-foot long gin pole, to direct the pull from the truck into a pull at the top of the tower, with cables.

The bottom line: The tower, winch and materials cost a bit more than 1000 dollars, the installation of the screw piles another 1000 dollars, and other stuff in the system like hardware, buried conduit, steel cables, and a new bottle of MIG shielding gas added another few hundred bucks. The benefit is that today I'm posting on an unlimited-bandwidth high-speed internet connection which costs me 60$ per month, and I have turned off my cellular-net wireless internet receiver, which cost me 250$ per month for 30 GB. The pay-back time will be less than 2 years. I'm also cancelling satellite TV and getting a UHF antenna, which will save another 50 bucks a month. My 14-year old son has been watching youtube videos for the last hour - something he hasn't been allowed to do from home before.

Dad-cred: level up

STF

RE: Stuck in a hole, still digging

Love it when a plan comes together.

Your -74dB is probably dBm (so referenced to 1mW)

A.

RE: Stuck in a hole, still digging

Love the backyard millwrighting! The gin pole shows some skill for sure! Well done!

RE: Stuck in a hole, still digging

VERY nice. Got lots more cred from us all.

How the heck did you point the dish accurately without someone up the tower to gain-it-up?
Have you done anything to keep the lightning out of the house?

Thanks for the great pics too.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Stuck in a hole, still digging

(OP)
Never thought of taking a picture of it, but I put an antenna rotator motor on the top stub. The kind folks used to use to point their TV antennas. It even says "Radio Shack" and "Archer" on it. Like, 1980's or something. A neighbour of mine had one when I was a kid (my folks got an antenna with good enough omni directional reception not to need to rotate). Anyway it still works, and it came with the tower when the guy sold it to me (he was moving and setting up his HAM stuff elsewhere).

It turns out that the guy that came to the house knew roughly the right direction to point it (10 degrees west of north) and he got it first try. I didn't actually use the motor. Shucks.

Gain was displayed by a program on the guy's laptop computer. There was some interaction with the transmitter station, but the response was almost immediate after he plugged in the Cat-5 cable. I didn't ask a lot of questions to which I wouldn't understand the answers.

There's an 8-foot deep ground rod in the earth beside the tower. There's no urgency to ensuring complete isolation, but you're right. Before summer comes, I have to find out if the "point of entry" device that connects to their CAT-5 from the tower can also suppress surges before sending them to the router. If lightning gets into the router then the surge could fry my computer or my weather station display panel (both of which are wired in).

STF

RE: Stuck in a hole, still digging

Hi Spar. Tried to respond yesterday but ET was down for hours. Of course I had half a response written when I discovered it.

Great application for a rotator. Archer??!! Radio Shack? No such thing.

As a kid I built a lightning receiver to see if I could detect storms on the west side of the Rockies. It was in the garden out my window. The receiver was in my bedroom. We never got lightning storms until I got that antenna put up, then we were ground-zero for a doozy. I remember the shaking hands and clammy sweat as I cut the antenna lead and threw it out the window into the garden. The antenna was struck about 5 minutes later.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Stuck in a hole, still digging

(OP)
And you've been known as "it almost smoked" ever since!

STF

RE: Stuck in a hole, still digging

I like the truck. I wish my truck looked as good.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Stuck in a hole, still digging

(OP)
This is Alberta - it's "small" compared to most trucks 'round these parts.

Last night I noticed, for the first time, that the antenna has two LED's on it. One red, one green. Aircraft anti-collision lights!

STF

RE: Stuck in a hole, still digging

I was at a hearing when the witness said;
"I don't know if it was him, but it was a white pickup truck and he has a white pickup truck!"
Lawyer;
"Do you have any idea how many white pickup trucks there are in Alberta?"
It's not unusual to see 15 or 20 or more white pickup trucks in a parking lot.
The joke, along the lines of the check's in the mail.
"Look for me in the Wal-Mart parking lot. I'll be in the white pickup truck."

I agree. A full size extended cab 1/2 ton is small here. The young rednecks like the diesel powered 3/4 tons and 1 tons. Dualies are popular with the younger set also.
And don't forget the cowboys. A heavy duty 3/4 ton truck is a little light to pull a 3 or 4 horse trailer with living quarters.

Bill
--------------------
"Why not the best?"
Jimmy Carter

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources