What size earth auger bit do you use for a 4x4 fence post?

[mo1]

I've seen 3x the size online but nobody seems to do that in real life. What size bit would you use for a 4x4 post? I will use either concrete or the foam stuff, probably the foam, so maybe the hole should be smaller diameter?

I use whatever auger is on the post hole digger, as long as it is at least a few inches larger in diameter than the post so I can tamp the dirt back in around the post. If I was concreting in a post, three times the post diameter would be the correct size. Rarely do posts get concreted here as the soil is heavy clay and holds posts well when compacted.

 

[ctgoldwing]

I planted 100+ 4x4's in the woods for a 'keep the dogs in / keep the deer out' fence. I used a 9" auger down ~36". We have rocky soil so I just backfilled with what came out of the hole.
That was 15 years ago. The only one that failed was the one I snapped off by accidentally hitting it with the loader bucket.

 

[Streetcar]

Streetcar,

Several huge differences in what the power company is probably using for urethane foam and what guys walking into the local hardware are going to be able to purchase. I spent 20+ years in advanced elastomers and there are many grades of urethane for many different applications. I could bore ( pun intended ) your with the details but finished cured urethane can range from glass like to foamy elastic stretchy sponge just by changing the polymerization components/ ratios.
Power company is also sinking that power pole a min of 10 feet and probably closer to 15-18 feet into the ground and they are probably using a very small overbore tolerance which all produces a tight fit and very little foam gets used.

The sika product for homeowners is similar to the utility product. The residential poles are typically eight feet in the ground. Over bore is approximately two inch wider than pole

 

[M59]

The sika product for homeowners is similar to the utility product. The residential poles are typically eight feet in the ground. Over bore is approximately two inch wider than pole

Streetcar,

Sika's web site provides the following guidance on lifespan.

What is the lifespan of Sika® PostFix® Fence Post Mix?

Sika® PostFix® is very durable and does not break down, it will last for several years of service.

retail.usa.sika.com

The "it will last for several years of service" response it not something I find acceptable. "Several" is a number greater than 2 or 3 but not many.
When I take the time to put a post into the ground, I expect it to last decades of years. I suspect ctgoldwing, when he planted a 100+ posts into the ground wanted those posts to stay firmly in place more than "several years" and in fact are still in place after 15 years.

Now if one wants to purchase the commercial grade utility pole setting product from then I might consider running a trial of the foam on a few posts to monitor its performance. Sika does not appear to sell a commercial line of foams for setting utility poles.

Pole Setting Foam - Rainbow Technology

A hydrophobic, two-part polyurethane, expanding foam, designed to rapidly provide intermediate backfill for setting utility poles. It forms a low-density, high-strength foam that can structurally support the pole and maintain its upright positions. Hardware may be strung after 15 minutes. Serves...

rainbowtech.net

 

[sea2summit]

I've seen 3x the size online but nobody seems to do that in real life. What size bit would you use for a 4x4 post? I will use either concrete or the foam stuff, probably the foam, so maybe the hole should be smaller diameter?

a^2+b^2=c^2

4"^2+4"^2=c^2

16"+16"=c^2

32"=c^2

sqrt 32"=c=5.65"

So you need a 5.65" hole minimum. If you're using material to anchor it like concrete or phone you need to determine how much of an ancohor you need for your soil conditions.

 

[M59]

a^2+b^2=c^2

4"^2+4"^2=c^2

16"+16"=c^2

32"=c^2

sqrt 32"=c=5.65"

So you need a 5.65" hole minimum. If you're using material to anchor it like concrete or phone you need to determine how much of an ancohor you need for your soil conditions.

Sea2summit,

You make me feel like I am back in grade school and math teacher saying " You must show your work otherwise I will mark it wrong". Interesting notation.

 

[LD1]

Streetcar,

Several huge differences in what the power company is probably using for urethane foam and what guys walking into the local hardware are going to be able to purchase. I spent 20+ years in advanced elastomers and there are many grades of urethane for many different applications. I could bore ( pun intended ) your with the details but finished cured urethane can range from glass like to foamy elastic stretchy sponge just by changing the polymerization components/ ratios.
Power company is also sinking that power pole a min of 10 feet and probably closer to 15-18 feet into the ground and they are probably using a very small overbore tolerance which all produces a tight fit and very little foam gets used.

Not even close to those depths.

Utility poles are typically 10% of the length + 2ft. So a 40' pole would be set 6', a 60' pole would be 8' in the ground.

a^2+b^2=c^2

4"^2+4"^2=c^2

16"+16"=c^2

32"=c^2

sqrt 32"=c=5.65"

So you need a 5.65" hole minimum. If you're using material to anchor it like concrete or phone you need to determine how much of an ancohor you need for your soil conditions.

A 4x4 post is actually only about 3-5/8 square. So technically a ~5-1/8" auger would be enough

 

[sea2summit]

A 4x4 post is actually only about 3-5/8 square. So technically a ~5-1/8" auger would be enough

I'm sure you're the first person invited to parties

 

[LD1]

I've seen 3x the size online but nobody seems to do that in real life. What size bit would you use for a 4x4 post? I will use either concrete or the foam stuff, probably the foam, so maybe the hole should be smaller diameter?

My first question would be why 4x4 posts? Is this for a deck or shed? Or a fence?

If its for a deck or shed.....it needs concrete UNDER the post. The diameter of the concrete UNDER the post depends on load, soil type, and building design. But generally a 12" diameter hole is most common for decks, and or average size buildings with 8' post spacing.

Now if its for a fence.....Round fence posts are usually cheaper, and if you get the 5"-6" class posts, they are alot stronger than a 4x4.

Now as for hole size and backfill....Most people just set posts and backfill by tamping dirt in. And in order to have enough room to actually tamp dirt (with a 2x4 or the head of a spud bar) a 9" hole works well for a 5"-6" round post. Any smaller and its hard to tamp. Any bigger you are just adding work.

I have drilled ALOT of fence posts....simply because I HAVE an auger and make do with what I have. But a better and easier option is to simply rent a post pounder/driver

 

[LD1]

I'm sure you're the first person invited to parties

Depends on the kind of party