Setting a gate post in soft soil

[onewheat]

I'm trying to set some 8' 6x6 posts for a gate. I had holes augered in about 3' down - the first one I tamped in with gravel/soil and it was rock solid with about 1/3 of the hole filled like it should be. I tamped everything in for about 2/3 of the hole and pretty much filled the rest and slightly tamped it down. It's very solid.

At the second hole, the dirt seems to be much looser. I did the same thing I always do but I'm 2/3 of the way up the hole and the post is still loose - I'm using the same dirt/gravel but it's just not tightening up. I'm thinking I have to dig it back out and start over or possibly go with concrete but the looseness of the soil on this side of the driveway has me a bit concerned that even with concrete, the surrounding soil will still be somewhat loose and not support it sufficiently.

I'm looking for ideas on how to stabilize the post and/or soil here.

 

[jack stand]

Can you hang the gate on the "good" post?
It's a good practice to brace a gate post as you would build a corner using high tensile corner design for loose soils.

 

[Firebrick43]

Even if you use concrete, a single post is not sufficient for a gate, you need a second post and a brace as @jack stand linked video linked.

Second, add a little water to the soil if it’s not tamping correctly. You are using a ~1-1/2” Diameter steel bar, old disc axle, or an actual tampering bar? A 2x4 doesn’t hack it.

Third, 3’ deep is not sufficient. A good post hole is 42” deep or even better 48” deep with a 8.5’ post to have decent strength to being pushed or pulled on like a gate will. 6” doesn’t sound like much but it makes a world of difference.

 

[onewheat]

My posts are 20' apart, so each will have a 10' farm-type gate. I am using a ~70" tamping bar on the soil (my shoulders and triceps are sure ). I'm thinking I can probably dig the second post out and hopefully dig deeper, but may have to go with braces to get more stability.

 

[NUTTSGT]

How big are the holes augered out to ?

What is your frost depth?

 

[onewheat]

The hole is probably 15" or so - I think mainly because that hole has such loose dirt around it. The other hole was a very tight 12" but the dirt was not loose at all. I'm in TN, so the frost depth is 12".

 

[The Cobbler]

any gate with that much cantilever will pull the posts over unless very well secured or braced somehow .

 

[mike93lx]

Keep excavating loose soil and replace it with something compactable

 

[PopcornSutton]

The bigger the hole, the more resistance it will give when filled with concrete. Bracing the post is fine, until you swing the gate 90 degrees unless you brace for that too.

 

[Firebrick43]

The bigger the hole, the more resistance it will give when filled with concrete. Bracing the post is fine, until you swing the gate 90 degrees unless you brace for that too.

There is a big difference between a constant cantilevered load (gate closed) and one that is infrequent while you drive thru.

 

[kwb]

Mass is your friend.
Bigger/ deeper hole and more concrete. 4ft min depth 24in dia min if just an ag type tube gate 10ft.
You are basically installing a jib crane base.

15in is a fence post hole.

 

[Joemctag]

Pretty much all the gate’s I’ve built, I had a post on each side going up 7 or 8 ft or even more at a farm , and connected the posts together with a crossmember. Only time any sagged or needed adjustment was when a tree fell on them. Some of mine had no concrete at all; only tamped in well. Just an idea.

 

[whateg01]

There is a big difference between a constant cantilevered load (gate closed) and one that is infrequent while you drive thru.

Depends on how many kids are jumping on it when it's open!

 

[rockinacummins]

I'd never hang a gate off of a single post, they need to be braced to offset the force of the gate. I've set post just by tamping the soil if it was pretty high in clay content. Most of ours around here, however, are in pretty sandy soil so we have to set them in concrete or drive them directly into the ground without digging a hole. For concreted posts we usually try to go at least 3' deep and for direct drive we try to go 4-5' (or until we hit rock).

 

[Firebrick43]

Depends on how many kids are jumping on it when it's open!

Sounds like a discipline problem not a gate problem

 

[MarcSeattle]

I've used a method of putting a treated 4x4 under the soil between the posts, running horizontally. Cut so it fits snug and attach securely to the upright posts. This helps resist the weight of the gate that will make the posts tilt inward. That works really well on normal house garden gates, and I don't know whether that would work something so big. 20 foot span with heavy gates is a lot of torque. Even if the base was perfectly solid I wonder how much a 6x6 would flex with that kind of torque.

 

[PopcornSutton]

I've got a pair of 12' farm gates across my driveway. The posts are 8" round treated. The hole was dug with a 15" auger and we have a lot of broken shale, about 3' deep. The holes were cleaned out of loose material posts set and filled with sakrete. They have been there over 10 years and no sagging. It all depends on the ground.

At that time, I had a 753 Bobcat and I rented an auger. Of all the holes I drilled with it, about 1/3 I couldn't get down. A Bobcat is very light in front, not much down pressure at all. I guy down the road was having a fence put in and that guy had a large tractor set up so he had hydraulic down. I paid him to come down and drill the holes I couldn't.

 

[Hank11]

A concrete footing extending past both post holes will give you posts that won’t move. If doing this by hand its a whole lotta work. A little easier alternative is to make each hole oversize and square. The flat side of the hole offers a lot of resistance to movement when the post is loaded. Maybe a 2x2 foot square hole, 3 or more feet deep.

 

[LOW1]

If you look at a standard metal t fence post it will have a plate that helps prevent the post from being pulled sideways. You can add a similar plate to a wooden post by using a 2 x 8 etc (or metal) placed so that it resists the sideways pull of the gate. Of course you have to dig a bigger hole.

We would occasionally put a second post six or so feet behind the gate post and make x bracing out of twisted number 9 wire. Never used concrete.

The posts would be at least 36 inches deep

 

[Firebrick43]

Concrete just rots off the post faster and makes the inevitable replacement a major chore to dig up and dispose of.

And its not necessary at all if the post/strainer system is constructed right. Variations of the H brace has been in use with gates for 100's of years. And the older wood gates were much heavier than the now common pipe gates.

 

[kwb]

Concrete just rots off the post faster and makes the inevitable replacement a major chore to dig up and dispose of.

And its not necessary at all if the post/strainer system is constructed right. Variations of the H brace has been in use with gates for 100's of years. And the older wood gates were much heavier than the now common pipe gates.

Concrete can do that if not done properly (going a bit above grade, doming the top, bit of gravel for drainage in the bottom) but why anyone would use wood for a gate post for anything spanning more than 5' is beyond me.

 

[Firebrick43]

Concrete can do that if not done properly (going a bit above grade, doming the top, bit of gravel for drainage in the bottom) but why anyone would use wood for a gate post for anything spanning more than 5' is beyond me.

Concrete will always cause the wood to rot faster no matter how well done it is.

Why use wood? Because it can last 30 plus years if done right, 50 plus years if you use Hedge or black locust post. I don't know what 5' has to do with it? A 10" post with H brace will handle a 16' gate just fine if prepared correctly. Hedge will last longer than Steel pipe set in concrete. Even railroad rail post set in concrete eventually rust off. The ones my grandfather set are starting to fail.

 

[nadogail]

I used a 10’ section 4X6” I beam that I drove 4’ into the soil under a 36” deep hole with a BFH, I then filled the rest of the hole with concrete.

My neighbors joked that I built a Nuclear Surviveable fence.

 

[racecougar]

There is a big difference between a constant cantilevered load (gate closed) and one that is infrequent while you drive thru.

That assumes the use-case is typically closed, intermittently open.

I know in my case, my gate is only closed to prevent idiotic Amazon drivers from coming down my ice-covered road and getting stuck; so rare occasions. It's open 99.99% of the time.

 

[Firebrick43]

That assumes the use-case is typically closed, intermittently open.

I know in my case, my gate is only closed to prevent idiotic Amazon drivers from coming down my ice-covered road and getting stuck; so rare occasions. It's open 99.99% of the time.

If it’s open all the time you place a “landing post” aka “crutch post to take the weight of the gate while open.

It’s also not a bad idea to add a gate holdback to the landing post.

 

[racecougar]

Meh, it's lashed to a pole to make sure it stays open.

I get that it could be fancier, but it's worked well enough for the past 33 years.

 

[Snapped-off]

I pulled up 8 or so pine fence posts this weekend that were set with concrete. Age is around 35 years. I didn't see any signs of rotting on them and they were all coated with something below grade.

I'm going to add on a few hundred feet of fence next year and 30-40 years is good enough for me. I'll probably coat the bottoms and set in concrete. I'm thinking the double gate will get 6x6 posts. 12-16 feet wide, still undecided.