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I would like to dig some 6-12 inch diameter dry wells (any diameter in that range will work). The holes need to be at least 6 feet deep to reach the permeable layer of the soil. However, the equipment the local rental agency has will only dig 4-foot deep holes.

What is the right equipment to dig 6-foot holes?

The soil is clay with some sand. There are no rocks. It digs very easy.

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    On a side note... Not saying you haven't already, but make sure you call the utility locating company so you don't cut through a buried cable, water line, or worse... Buried electric. Can be deadly. It's probably in the high price range for a small project, but there are machines called hydro vacs which dig holes with vacuum power and high pressure water. It's actually what I do for a living. – BigLake Apr 10 at 15:18
  • If these are just dry wells then they don't need to be round at all. I'm also not sure why you think you need more than one, even if you are trying to keep a field drained. Just run your drainage pipes to one large dry well that can be dug by hand or with a small backhoe. – Robert Cline Apr 10 at 21:32
  • Around here you'd need a power auger, with extension, and at least one, if not two, fairly hefty friends. – Hot Licks Apr 11 at 2:27
  • @BigLake you can do far, far worse than buried electric when digging... – Doktor J Apr 11 at 20:10
  • I didn't say you couldn't. And You're not going to dig up a high pressure gas main in your back yard, especially if you call utility services as you're supposed to a service line carries as little as a quarter of a pound of pressure. Not saying it's a good thing, but those line strikes that make headlines are almost always 4-6" gas mains on streets. I've been in the industry almost a decade. – BigLake Apr 11 at 22:56
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Depends a lot on the ground. The advice to call for utility locating ahead of time is spot-on, no matter what you do to dig them.

In my ground, an auger is generally useless, since it will find a rock that it can't shift just about every time, so an excavator or backhoe with a bucket is the only tool (short of a well drilling rig that can drill through the rocks) that's going to work, and the well-drilling rig is far too expensive to set up for such piddly holes.

Well, actually, a pick and a shovel and a clamshell post-hole digger might work, if you want to go cheap and physical, and speed is not of the essence.

In nicer ground than mine, you'd dig some 2 ft deep large holes and then use your 4 ft auger, if cheap and physical and not very fast were your guiding principles, but you preferred renting the auger to a clamshell. In my ground (if not using a backhoe/excavator) I'd go prospecting with the clamshell and then dig a large hole to move the bigger rocks aside and provide room to go deeper with the clamshell.

Beware of climbing into a hole deeper than waist deep that could collapse around you, since that can kill you, which could be regarded as "not cheap."

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    I agree with this. If we didn't want to pay more for an extended bit - two rental shops in my town will have a 6 foot bit... I would dig two feet down and start from there. The top part of you cast is fine to have loose dirt, in fact some people recommend a mushroom pour. – DMoore Apr 10 at 16:03
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My personal favorite is vacuum excavation. It's nothing fancy nor expensive; a respectably large wet-dry vac does the job nicely. By "respectably large" I'm suggesting something with a 2.5" suction hose and a 14 gallon or larger capacity.

The process is simple. Alternate between a tool that loosens the soil and the vacuum to remove soil from the growing hole. With two people working in a moderately sized hole the steps can be done concurrently. My tool of choice for loosening soil is a "digging bar." It's a straight steel bar about 6 feet long with a flat on one end and a point on the other. Occasionally a rock or clump of soil will block the hose and it'll have to be cleared, but progress is relatively quick and I find that it's much less effort than the traditional clamshell post hole digger.

Whereas your holes are to be 6 feet deep, and this is about the same length as the digging bar and also the pipe included with most wet-dry vacs, it may be necessary to extend these. Purely by way of example, Home Depot (US) offers an "extension wand accessory" for their Rigid brand vacuums; one or two of these may be enough to reach the bottom. The digging bar could be extended by welding or otherwise attaching a length of steel pipe at the end.

By the way: don't forget to empty the vacuum drum regularly. It turns out that 14+ gallons of soil is heavy!

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    does this cause excessive wear on your shop vac? – bowl0stu Apr 11 at 15:08
  • It should not cause vac wear&tear.. you keep the filter in place.. – HerrBag Apr 11 at 20:04
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I dug some deck footers a couple years ago and my rental shop had 10" auger bits with a two-man auger (4 handles and a motor in the middle) for about $100 a day, made quick work of several holes like you're talking about (I dug 7). The bit itself was only about 3' long, but they'd also rent an extension bar if you wanted to go deeper (I'm sure there's some max depth but it went as deep as I wanted.

Some of the other options presented are not cheap (hiring a backhoe or tractor-mounted auger) or very physically straining (clamshell, hand auger) so depending on your budget, physical fitness and local availability of tools, these two-man motorized augers might be a good option.

  • Ok, so basically I can use a standard two-man auger, it's just that my rental shop didn't have that? If the spirals only go for 3-feet, how does the dirt get out of the hole? (Obviously the extension bar will not lift dirt) – Tyler Durden Apr 10 at 21:53
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    @TylerDurden Once you get to the point where the bit can't pull the dirt all the way out you have to go with a lift out technique, where on the count of 3 both guys lift the bit (full of dirt) towards the top of the hole while it's still spinning and it deposits the dirt outside the hole. It's slower going, but it works – Dan Apr 10 at 21:59
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Depends on your budget, and the value of your time.

1) Hand auger

These things are backbreaking - but if you have unlimited time they will eventually do the job. Notice the extensions? You can drill until you run out of extensions or the ground is too hard to cut. Performance is improved by using a sharp cutting edge, just like a sharp spade cuts better than a blunt one.

Note any rocks stop these things dead, so you have to use a crowbar to dislodge the rock, or dig another hole and hope to miss them all.

Also, a 4" / 100mm hole is half the work of a 6" / 150mm hole and is about 1/10th the work of a 12" / 300mm hole.

On the plus side, this tool is character-building, and great work for students on workdays :)

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2) Tractor Mounted Auger / Post Hole Digger

This one is front-mounted

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These are also found mounted on the rear PTO

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For deeper holes, you use a longer auger

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Naturally you would hire this tool rather than buy it. If you have no tractor then its better to just get a contractor to do it - probably drill all your holes in a morning session if you pre-clear access and clearly mark the locations before-hand.

  • It likely depends on the soil but, in my experience, with no down pressure, those 3-point hitch augers approach uselessness. – canadianer Apr 10 at 20:21
  • "Character-building", indeed. – bishop Apr 11 at 2:45
  • @canadianer erm - the tractor's pushing it down, so ~hundred kilos of pressure minimum ? – Criggie Apr 11 at 4:54
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    @Criggie most 3-point hitches don’t actually push down; they rely on the weight of the implement to lower. – canadianer Apr 11 at 17:13
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Just call a daylighting company. For around 400 bucks they'll suck the dirt out of the ground in the shape of cylindrical holes in about an hour for all of them. Using clamshells deeper then four feet sucks. Hand-held augers suck, unless you hate your back. Your time is worth money. 6' holes are tough.

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