WC1M

Pier and Guy Anchor Footings

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Ground Work at WC1M
 
This page shows the ground work done in preparation for erecting the new 110' Rohn 55 tower. This included excavating four large holes, building rebar cages and forms, pouring concrete and backfilling.
 
It took about three weeks to complete this work, from September 18, 2006 to October 5, 2006. Even though machinery was used to dig the holes and transport the concrete, it was still a lot of physical work. I never expected to become a ditch digger during this project!
 
 
Hole for the Northwest (downhill) guy anchor
 

Northwest (downhill) guy anchor hole
click on image to enlarge

I went with 4d anchors because I wanted to meet Rohn's spec for 110 MPH. We live in a 70 MPH zone in west-central NH, but the weather folks say we're due for a hurricane. The building inspector told me there's a move afoot to reclassify our area as a 120 MPH zone due to some high speed microbursts that have been recorded. Anyway, I wanted anchors that would withstand any expected wind speed, and would make it feasible to extend the tower and/or mount a bigger 40m beam at the top.
 
The hole is 6'x3'x4' deep. I used a little more rebar than called for in Rohn's spec (they only require bars on the top and tower side.)
 
 
Closeup of the nearly-buried equalizer plate

Northwest guy anchor equalizer plate
click on image to enlarge

The equalizer plate of the GAC3455 anchor is below grade due to the steep slope of the hill. I'll have to excavate a little around the head to attach the turnbuckles.
 
 
Base hole with rebar and Ufer ground

Base hole with rebar and Ufer ground
click on image to enlarge

Obviously, this is a lot bigger than Rohn spec for a pier-pin base, which is only 2.6'x 2.6'x 4'. This is what happens when the excavator you hire only has a two-foot wide bucket. Smaller buckets exist, but they can't deal with the big boulders and ledge often encountered when digging on property like mine.
 
The bottom is about 3'x3', which is what I had planned for, but the top turned out to be 4.2'x4.2'. There were a host of reasons for this. The larger bucket resulted in two of the sides sloping away from the center of the hole. Originally, I didn't plan to use any plywood as a form for the sides, so we overestimated the size of the top form needed to cover the hole. Even at 4.2'x4.2', it wasn't big enough to compensate for the sloping sides. The steep slope of the land compounded the problem, requiring the uphill side of the top form to be almost completely buried and the downhill side to be suspended on a plywood sheet. In the end, we used short pieces of plywood to form the upper portion of the hole. The plywood is a foot or less deep on two sides, but about three feet deep on one side.
 
In this shot, you can see the Ufer ground. This was required by the electrical inspector during the permit process, and he insisted on checking it out before the pour. That inspector is no longer with the town, and the fellow who took his place sort of scratched his head and said, "I wonder why he made you do that." I believe he did so because it's code, which make me wonder about the new electrical inspector. But I'm not sorry -- I probably would have installed a Ufer ground anyway. 
 
There are three ground radial connections, one for each tower leg. Each radial consists of 50' of 1/0 wire. One end of each wire will attach to a tower leg with an aluminum pipe clamp. The wire runs down from the tower leg into the concrete, where it attaches to 3/4" rebar with an aluminum pipe clamp. I used a special copper-based compoud for mating dissimilar metals. The wire exits the slab below grade, where it will be exothermically welded to a ground rod two feet away. The run will continue for 50', with ground rods every 16 feet (four rods per radial, total of 12 rods.)
 
 
Base top form

Base hole top form
click on image to enlarge

This is looking downhill. Note how the uphill edge of the form is almost completely buried. The downhill edge is suspended with plywood, rocks and wood blocks.
 
 
Hole for East (uphill) guy anchor
 

East (uphill) guy anchor hole
click on image to enlarge

When I first staked out this hole, probes with a ground rod revealed something very big and solid under about a third of the hole. I wasn't sure it was ledge -- it could have been a big boulder or even a big root from the apple tree right behind the hole. At the last minute, I moved the location of the hole two feet. Good thing, as the obstruction turned out to be ledge, and we missed most (but not all) of it. You can see how the ledge forms a solid wall on the far side of the hole. It also runs a couple of feet into the bottom of the hole on that side, forming a rock floor four feet down from the surface.
 
This hole was the first one dug by the excavator. As such, it came out sort of like the first pancake you make -- not perfect by any means. Between the ledge and his getting used to the terrain, the hole turned out to be a mess. He broke up some of the ledge with his bucket (good thing it wasn't solid granite.) As he removed very big pieces of ledge, the hole kept getting bigger. At one point he hit a huge boulder. Removing that cause the hole width to increase to 4 feet from the original 3 foot spec, and the upper walls of the hole on the tower side became unstable. Luckily, you only have to pour 18" of concrete, and the bottom of the hole on that side was essentially "undisturbed". By the time it was all said and done, the hole was 9'x4'x4' instead of 6'x3'x4'. As you can see, the hole is quite a bit larger than the rebar, but that's OK. The rebar is sufficiently large to reinforce the area around the base of the anchor. The excess concrete is better than backfilled dirt.
 
Note how the steep uphill grade causes the anchor to stick out of the ground. If this was level land, the bottom edge of the equalizer plate would be just above grade. It looks like the anchor is really far out of the ground, but that's because the front lip of the hole collapsed in that area. After we backfill, it won't look like it's so far above grade.
 
 
Boulders from East guy anchor hole

Boulders from East anchor hole
Click on image to enlarge

Here's a pile of large rocks taken from the East guy anchor hole, with my portable hand drill for size comparison. The bottom rock probably qualifies as a boulder. Glad I contracted with an excavator -- no way I could have budged these with a shovel!
 
 
East anchor hole, ledge side

East anchor hole ledge
click on image to enlarge

Here's a good shot of the ledge side of the hole, which we didn't fill with concrete. It'll be backfilled and tamped.
 
 
East anchor hole after pour

East anchor hole after pour
click on image to enlarge

We're done filling the East anchor hole. There's at least 2 yards of concrete in there, maybe as much as 2.5 yards. The slump was thick enough that we didn't have to fill the last two feet or so on the ledge side of the hole, even though it looks like we did. The concrete formed sort of a hump over there. The excavator will backfill that and we'll tamp it good. With all that concrete in there, I'm not worried about the anchor moving!
 
 
South guy anchor after pour

South anchor hole
Click on image to enlarge

This was the last hole dug by the excavator, and came out the best. The rectangular dimensions were very close and required minimal shovel work to square up. The soil was pretty soft and had almost no rocks. The depth was a bit short of spec on the two downhill sides and some leveling of the floor was required. We put more than the required amount of concrete in this hole to compensate for the short depth on the downhill sides, then added a little more to empty out the cement truck.
 
Like the other uphill anchor, the rod sticks out well above grade. In this case it sticks out less because the change in elevation is only half what it is for the other uphill anchor (4 feet above the base versus 8 feet above the base.)
 
What isn't visible in this photo is a large patch of poison ivy going down the hill towards the tower. In fact, you can see where I removed all the foliage around the anchor hole with Roundup, a weed whacker, and a rake. I got two cases of poison ivy while working in this area -- determining anchor placement, staking the hole, removing the foliage, etc. The second case was pretty bad and I needed steroids (prednisone) for five days. A nasty drug, but it worked. After that, I was very careful to wear plastic dishwashing gloves under my work gloves and a long-sleeved shirt. Immediately after working in this area, I wash with Jewelweed soap, apply Jewelweed salve, wash my clothes in hot water and clean my boots and tools with alcohol. So far, I haven't gotten reinfected.
 
 
South guy anchor misaligned!

South anchor misaligned
Click on image to enlarge

I checked the alignment of all anchor rods during the pour, and thought all of them were aiming directly at the center of the base. After the pour, I discovered this was true for the Northwest and East anchors, but not for the South anchor. At the time, I didn't think the degree of error was all that significant and didn't attempt to move the rod. This illusion was probably due to the hill sloping in two directions, making visual perspective confusing.
 
But the next day I realized the error was considerably more than I had thought. It's on the order of 5-10 degrees. You can see from the picture that the anchor isn't pointing at the center of the base. Heck, it isn't even pointing at the base! What to do?
 
 
Closeup of misaligned South anchor

Closeup of misaligned South anchor
Click on image to enlarge

In this shot, you can see the pier pin in the base. It isn't in the center of the base, but that's deliberate. It's over the center of the rebar cage, which will place the weight of the tower directly over the rebar.
 
Misalignment of the South rod is evident. I'm not exactly sure how this happened. I think we had the rod pretty well aligned before the pour, but somehow it changed during the pour, at some point after I did the last check. There is no evidence of the rod moving -- it's in the same place on the rock and it's laying on the original shallow groove in the lip of the hole. 
 
One possibility is that the excavator operator bumped the rod when he removed a ground rod I was planning on using to wire down the equalizer plate to further stabilize the rod (didn't get to that before the pour started.) He took the rod out because it was interfering with dropping the concrete from the tower side of the hole. But since there's no evidence of the rod moving, I'm not convinced this is what happened.
 
A better theory is that the weight of the excavator on the tower side of the hole shifted all of the ground around the anchor rod. In other words, the rod didn't move, the ground did.
 
Anyway, it was too late to move the rod, so I consulted the gurus on Tower Talk. To my relief, no one expressed any real concern. I wasn't worried about the slab moving or the anchor coming out of it. I also wasn't worried about the rod bending from the guys pulling on it -- no way it would break from that. But I thought if the rod didn't bend, the lower threaded rods in the turnbuckles might bend, making it impossible to adjust the turnbuckles.
 
The overwhelming consensus on Tower Talk was to bend the anchor. I figured the steel rod was so massive that I couldn't do that by hand, and there was nothing nearby on which to hang a come-along. I wondered if maybe I'd need the excavator operator to use his bucket to bend the rod. The impresicion of that scared some people, who suggested I attach a come-along to the bucket instead.
 
Turns out my faith in the stiffness of the anchor rod was totally misplaced. Gerald, K5GW, of Texas Towers provided a little math that showed the rod should bend easily. He convinced me that the rod would automatically be bent into place by the pretension on the guys, and suggested maybe I didn't need to bend it beforehand. I was comfortable with the idea of letting the guys do the work, but I didn't like the idea of the rod moving the soil underground and maybe leaving a gap.
 
So, I decided to try bending the anchor manually.
 
 
South anchor aligned!

South anchor aligned
Click on image to enlarge

I was amazed by how little pressure it took to bend the anchor -- I could have done it with one hand. When I did it, I realized that enough of the rod was sticking out of the concrete that a lot of leverage was possible. 
 
After bending the rod, I got an email from KM1R, a retired tower worker, who said the pros would just bend the rod. But he said I should wait for the concrete to cure to avoid a crack in the surface. Well, too late for that advice. But the small amount of bending required didn't cause any visible movement in the concrete where the anchor enters.
 
I just gave it one fairly gentle tug and, after seeing that the alignment was much better, left well enough alone. It would be very easy to overdo it, and then the rod would have to be bent back the other way.
 
The equializer plate is no longer exactly level, but that's not a problem -- the screw rods in the turnbuckles will compensate for that.
 
 
Panning back to see South anchor aligment

Pan of South anchor
Click on image to enlarge

Looks much better, doesn't it?
 
 
South anchor -- further back

South anchor further back
Click on image to enlarge

If it needs more bending, let the guys do it.
 
 
Base hole after pour

Base hole after pour
click on image to enlarge

We used a bucket loader to shuttle concrete from the cement truck in my driveway to the base and anchor holes on the hill. Nine years ago, I had a cement truck drive down the hill past this very spot to fill the base for the U.S. Tower crankup. But this time the cement company was worried about getting past some trees that had grown up a lot in the intervening years. They were also worried about the truck getting stuck on the hill. We've had a huge amount of rain this year and the ground is very soft. Nine years ago, it was dry as a bone.
 
The bucket was pretty wide. That was great, because we had to shuttle 8 yards of concrete and it only took two hours. It was also great for filling the 6+ foot wide anchor holes. But the bucket was a little wide for the base hole. The operator dumped the mud on the diagonal, minimizing the overflow, but I still had to shovel a lot of excess into the hole.
 
Here we've used a 2"x2" to push off the extra concete and begin the leveling process. A tamping and dragging motion works well.
 
 
Base hole finished

Base hole finished
click on image to enlarge

We've smoothed the top of the base and inserted the pier pin. It looks like only an inch or so is sticking out, but it's really three inches. More than enough.
 
 
Base hole form removed

Base with form removed
Click on image to enlarge

A few days later I removed the base hole form. We built it tough, making removal a little more difficult. It wouldn't budge until I unscrewed the 2"x6"s from each other. Then they came up easily. The short (12"-18") plywood pieces we used for the upper portion of the South and West walls came up after few tugs and whacks with a hand sledge. The long (3') plywood form we used for the North wall was buried too deep and wouldn't budge. Rather than digging it out, I cut it off below grade with a trim saw. I'm not worried about the base moving if and when the plywood rots -- there's another 18" of undisturbed wall underneath it.
 
This shot is looking across the slope, which runs uphill from left to right. Note how the slope makes it look like the base is tilted. But it's really level. The slope creates an optical illusion. From some angles, the tower will look like it's leaning into the hill, even if it's perfectly plumb.
 
Also note how the right side of the base is nearly level with grade and the left side is well above grade. The slope falls about 6" from the uphill side of the base to the downhill side.
 
I said it's level, but actually it's very slightly off. In both directions across the base, the bubble in a level is between the lines, but very slightly to one side. I'd say it's off by about 1/16". I doubt this will make any difference to the plumb of the bottom section, or of the tower itself. However, it turns out the error is in both downhill directions (the hill slopes in two directions away from the base). I'm hoping this will encourage water to flow off the top of the base.