Pole barn project: Two questions

[rbstern]

Getting ready to start my pole barn in earnest. 36x30 with 6x6 posts.

Question 1: Proximity of trees to retailing wall and rear post hole.

In the pic below, the fence post in the foreground represents the back right (if viewed from front of barn) pole. The fence post on the far left of the pic is the back left post. I plan to build a railroad tie retaining wall about two to three feet behind these, spanning across and beyond the wide of what will be the back of the barn. Wall will be approximately 3' high.

The two groups of hardwood trees are within about 4' and 6' of the fence post. My plan was to cut down the closer group of trees, and leave the oak on the right.

Any concerns, relative to the wall or the post hole drilling? I could move the entire barn structure and retaining wall location about 5' forward, without a signficant change in my plan. In fact, it will involve less fill. That's strictly an aesthetic consideration relative to the rest of our yard.

View attachment 684369

Question 2: Digging post holes. I have three tools for digging hole on hand. 1) Backhoe on my tractor (bucket width is 16"). 2) HF Predator powered auger with 6" bit, max digging depth about 24". 3) Shovel.

I need four foot deep holes, large enough to accomodate 6x6 posts. Obviously, the Predator auger is too limited. I could start with it, and hand dig the holes. I did that recently with a fence project. Could supplement with a manual post hole digger. But my back starts to hurt when I think about it.

Worried that I will remove too much dirt with the backhoe, even if skillfully applied.

I could splurge on an auger with 12" bit for my tractor, but I don't anticipate needing it often. I have no other pole buildings in mind that would require 6x6 posts set 4' deep. It would be an $800 piece of gear that would sit around 99.99% of it's life. Suppose I could sell it when done.

I could rent an appropriate piece of equipment (two man auger?).

I could pay someone to come with the right equipment and dig the hole.

Thoughts?

 

[i7win7]

I'd hire it out or get Saturday rental of skid steer or mini skid steer w/auger - no charge if used on Sunday also.

 

[Peter 315]

You could dig them with the backhoe and use sonotubes then backfill...

 

[rbstern]

Might have a good solution to question 1: Checking with a Sunbelt rental about 25 miles from me that lists Cat 1 post hole auger for $67/day.

 

[BoylermanCT]

I'd hire it out or get Saturday rental of skid steer or mini skid steer w/auger - no charge if used on Sunday also.

You must have different rental places by you. Around here 1 day rental is 8 hours on the machine's hour meter. And you can pick up at the end of the day, but it has to be returned by closing on Saturday or they charge you two days.

We dug my pole barn holes with a tractor backhoe until the tractor broke its frame after 9 holes. We rented a Kubota excavator to dig the remaining 5 holes. The excavator was much better at digging than the backhoe. Drilling was not a good option for us as CT is one big rock pit. Why do you think we have all those beautiful stone walls everywhere?

 

[s219]

For wind resistance, you really want the post holes to be neat and accurate, and in most cases in southern clay soil, you will need to backfill with dry concrete mix, as the clay spoils cannot be used for backfill. I think an auger is the best bet. If you dig the holes with a backhoe, they are going to be too large to practically backfill with concrete mix unless you have $$$.

I use a 12" auger and it's OK for 4x6 posts. For 6x6 posts it's tight. I usually drill the holes, then go back with a string and measuring tape to make sure the holes are perfectly centered. If not, I adjust with a handheld post hole digger.

The next thing to factor in is the required footing size. It will depend on the structure load and snow load, and the bearing pressure of the soil. You have to work the loads down to each post, then divide by the area of the footing. The pressure needs to be less than the soil bearing pressure. I normally design for 2000 PSF soil bearing pressure in our tidewater Virginia clay. Sometimes I can't meet that limit with a 12" diameter footing. I either need to widen the bottom of the hole (pain in the butt when it's a deep hole) or add more posts to spread the load out.

 

[rbstern]

The next thing to factor in is the required footing size. It will depend on the structure load and snow load, and the bearing pressure of the soil. You have to work the loads down to each post, then divide by the area of the footing. The pressure needs to be less than the soil bearing pressure. I normally design for 2000 PSF soil bearing pressure in our tidewater Virginia clay. Sometimes I can't meet that limit with a 12" diameter footing. I either need to widen the bottom of the hole (pain in the butt when it's a deep hole) or add more posts to spread the load out.

The soil is very hard packed red clay. I've augered holes nearby, and it's hard. I think 12" should be okay, but if the auger rental works out, and they've got a 14 or 16" bit, I'll opt for it.

 

[ning]

Strong second to renting a bobcat with an auger. I have an auger for my tractor but last summer I had 12 five foot deep footings to dig in summer-hard clay and rock. The bobcat can provide down-pressure and the oomph to dig through most of it. Here we can also rent over the weekend for the daily rate (as long as you don't go over 8hrs on the clock) which was useful as I ran into a rock layer in a few holes that I had to use a digging rod and sledge hammer to get through (and the bobcat was off the clock during that). I think it was $250, but it saved my back.

 

[jaxs]

If you inquire around you will likely find one or more people that dig holes flat rate depending on size,depth and soil. They generally have a skidsteer for conventional use then invest in an auger.

 

[s219]

The soil is very hard packed red clay. I've augered holes nearby, and it's hard. I think 12" should be okay, but if the auger rental works out, and they've got a 14 or 16" bit, I'll opt for it.

I don't think I have ever seen bigger than a 12" auger on typical PTO based augers (which lack downforce on a typical tractor), but skid steer units (which are powered by hydraulics and have downforce capability) come in 16" and larger sizes, and can dig pretty well. I think we used 16" pre-cast cookies on my barn and those holes were dug with a 20" skid steer unit, which made short work of it.

Surprisingly, the bearing capability of soil doesn't directly translate to the hardness. Our clay is almost impossible to dig by hand and is packed pretty hard down low, but tests at 1900-2200 PSF bearing capacity. In other parts of my property you can bust through the clay into soft sand, which is easy to dig. It tests much higher at 3000-4000 PSF bearing capacity. I think the bearing capacity is really based on the ability of the soil to easily compact and stabilize. The sand is pretty much self-compacting. Clay is difficult/impossible to compact, so it can't support as much load.