Self-Supporting Tower Foundation Dig - still digging

[SparWeb]

Hello,

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.   

Someone with more experience with excavations might have some suggestions to improve what I've got here.

Reason #1 for the tower tower in my backyard 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.  Reason #2... well, a small wind turbine can't hurt, right?

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?

[dnix71]

Do you have a picture showing the lay of the land? If the back yard rests on muck, what is under your home? I live in south Florida. That looks pretty normal to me. Dig down a few feet and rain or not, the hole will fill with water. Our water table is high. If you try to install a basement it become an indoor swimming pool.

Proper building down here is to scrape off the muck until you reach coral rock and then backfill with rock/sand until the house pad is a certain feet above sea level. Calgary sits at the junction of 2 rivers, so how far above the water table are you?

You could dig a small test well to see what layers of clay/rock/sand are beneath the place you intend to plant this tower. You may need to dig down a bit until you reach something solid, or maybe even build a pad a few feet above grade to insure water doesn't pool around the pad itself.

[dnix71]

https://www.youtube.com/watch?v=N3msAkpUumI

Floating versus caisson foundations. Short course.

[SparWeb]

Here's one more picture, for context:



There are 3-4 feet of space between garage and the pit.  When filled, the concrete base will have 5 feet between garage concrete pad.  Garage pad is nothing special - slab on grade.
The slope goes down from right to left.  The garage roof drainpipe is what filled the hole on the rainy days.

My house is on concrete piles into the same earth, and none of them have sunk in 20 years.

"caisson" foundations...  See, I don't even know what the terminology like that means.  I'll watch the vid when I get to work on Monday.

I'm really far from Calgary.  There are nearby wetlands, but I'm on a hill well above them. 

[Mary B]

I don't think you have a problem, just the rain saturated the bottom of your hole. Long as the sides are firm it should support the tower just fine.

[dnix71]

Your home sits on a floating slab, that's how most homes are done if the land is level. That works fine because the weight is spread long and low.

Stick a tower way up and put a mill on it and then it's not okay. If it settles a little off level it will fall over.

I worked for the TVA for a year in water quality. There was a deep pit on top of Missionary Ridge north of Chattanooga that had been a coal mine, then was filled with garbage and levelled off with dirt and converted to a farmer's market. The pit was full of coal bottom distillates and rain water from it's original use as a waste pit from distilling coal into lamp gas (before WW1). The underlying rock kept it from leaking down into the Tennessee River for over 75 years. The EPA had to pump it out to clean it up. The TVA had sunk wells into it looking for cold ground water for air conditioning buildings and the owner on the other side had sunk wells for water for an ice skating rink. The chemical stew was so foul the well drillers had to go to the hospital after getting it on them. When I was asked what to do I told the boss plainly "cap the well and tell the EPA" and consider yourself lucky you didn't pierce the confining layer.

If you live over a confining layer that prevents vertical drainage, then you need to pierce the confining layer with caissons and then pour your slab on top of those. Friction piles would work fine as well, if you can't easily get to a rock layer.


My mother's condo in Dunedin, Florida was just shored up after one end began to settle into a sink hole. The repair was 90+ caissons drilled and filled with concrete around the perimeter of the building slab. I talked with the supervisor on the project and he said they had to go up to 40 feet to find rock in some places. The building is only 1 floor, but would have been unsafe to occupy without the fix. They had already dumped 150 loads of rock fill under the building and that failed to stop the settling. The caissons had screw jacks on top of them to allow the building to be relevelled before pouring the final concrete in each hole.

[dnix71]

Here is a picture of a small caisson drill rig like they used on my mother's condo. They are very small and portable. The drill bits can be linked together for making deeper holes.

[ontfarmer]

On the farm a perforated sub surface tile would be installed