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Old 01-23-2013, 02:22 PM   #1
stevehull
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Default PVC well casing questions

Am here in central Oklahoma and have some questions on PVC well casing.

First, I have a well driller that will drill me a new 250 foot well, but I need to provide PVC casing, submesible pump, etc. Our geology is lots of sand, gravel, some caliche with lots of water bearing gravel 30-60 foot down.

Existing 160 foot well is drawing air with high demand so I need a new well. The new well is for both home use and farm use (irrigation, watering livestock etc).

A few questions . . .

Is four inch schedule 40 (20 foot with bell ends) OK? Or is schedule 40 overkill in erms of thickness?

Rather than using a screen, I have seen drillers cut narrow slits in PVC wall with hacksaw in the first couple of pipes to allow water into casing. I know to backfill the borehole after liner is in with fine gravel. Is this slit techniqe OK?

Thanks in advance,

Steve



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Old 01-24-2013, 12:21 PM   #2
Valveman
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I have my doubts about a well driller who doesn’t supply his own casing? I would use 5”, 160# or better. The slots should be sized to the size of the gravel pack being used. I use .035” slotted with very small gravel, almost like sand. This is your filter to keep the sand out of the pump. If the slot size and gravel pack size is not correct, you will always be pumping sand.



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Old 01-24-2013, 12:41 PM   #3
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Valveman, thank you! The well driller is a guy that normally drills geothermal bore holes for closed loop heating/cooling systems.

Good point on a five inch casing but I don't understand the reference to "160#". What does that mean?

I do know that you need to size the gravel packing to the screen or slit size.

Would you put a cap on the bottom of the PVC casing - or leave it open?

Thanks,

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Old 01-24-2013, 01:58 PM   #4
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160# means 160 pound rating, which is about one size thinner than sch 40. Geo wells are completely different than water wells. The right size slots along with the right size gravel, being installed correctly, and developing the well are very important in getting a sand-free well. If I have a hard clay bottom I don’t use a cap. If there is not a good bottom to drive the casing into, I would use a cap to keep the gravel out. But you have to be careful not to break the cap off with a bailer or the pump.

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Old 01-24-2013, 05:37 PM   #5
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Valveman. Thanks for the info on the 160 lb wall rating. In terms of drilling the well with a rotary rig (8 inch diameter) is there something I need to be aware of in terms of well construction differences (water well vs geo)?

Around here all water and geo drillers use rotary drills, no mud slurry while drilling and the borehole stays open long enough to lower the casing into place.

I was planning on placing the submersible pump about 20 feet off the bottom to allow room for sand debris to settle down there.

In a prior message you spoke about a 0.035 inch slot (hacksaw blade width?) in the sasing and a casing surround sand slightly larger than that width that to prevent sand in the well. Should the casing surround sand be crushed sand (rough irregular shape) or sand from a river bottom (round sand). We have both available here. Shape can make a big difference.

Thanks again.

Steve

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Old 01-24-2013, 09:05 PM   #6
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8 ¼” bit for 5” casing should be good. That gives you about 1 ½” of space for gravel all the way around the casing. I don’t know about not using mud? Mud is what keeps the hole open until you get the casing and gravel down. You need a consistent layer or thickness of gravel all the way around the perforated casing. If you push perf casing into sand, you will always pump sand.

Air rotary rigs don’t usually drill very well when they encounter a water-bearing zone, because it caves in without mud to hold it open.

Silica sand comes in bags and can be sized to filter the particular size particles in the well. Smaller water-bearing sands need smaller gravel as a filter. And smaller gravel needs smaller slot size to keep the gravel out of the well. Skill saw perforations are very large, requiring large gravel, and then you get a sandy well.

The 20’ of rat hole to catch the sand is not a good idea.

Here is a link to some Silica sand I found.
http://www.targetproducts.com/UserContent/SpecSheets/fltwatwlAB.pdf

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Old 01-24-2013, 11:22 PM   #7
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Valveman,

I was not going to use a Skillsaw to cut perforation slots in the PVC casing as I agree that those slots are too wide (letting in sand). I can order perforated 160# pipe and a woven sock to cover it for the first 100 feet or so.

Or would cutting narrower slots in the lower 100 feet (every 6-8 inches) with a hacksaw work OK.

Many of the wells here are cut with hacksaw with no sock over the PVC casing. Casing just backfilled with coarse sand . . .

What is problem with having pump up 20 feet with room for sand debris below that?

Thanks for all your help, you have been GREAT.

Steve

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Old 01-25-2013, 12:14 AM   #8
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First off, your driller is not a well driller.
That will make a big difference when it come time to develop the well.
There is a lot more to a well than just digging a hole in the ground.
Drilling in large sand and gravel needs to be done with a mud rotary rig. Without mixing mud to hold the sand and gravel back, it is like trying to keep a hole open in a box of marbles.

((This is to help me understand a little about what you have.
Why is the well you have 160' deep? Has it always pulled in air or is this a recent occurrence?
And why would the driller go 250' if there is water at 40 to 60 feet
?))

If your well will be in sand or gravel, you would need to buy a screen that is sized for the sand where the water is going to come from.
The size of the screen can go from .006 and up to ? it just depends on the sand the well is being developed in. Most of ours around here are in the .010 to .014 range (that is in thousandths)
Unless you are trying to keep out small kids and animals, cutting them with a saw is never a good idea.

The casing you use should be (well casing) not just sch40 pipe. There is a difference.

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Old 01-25-2013, 01:28 AM   #9
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Water well dude,

Thanks for the reply. Let me restate the situation.

The well driller is a person that regularly drills 250 foot deep, 8 inch boreholes for closed loop geothermal applications in our area. The geology here in central Oklahoma is that the bore holes will not quickly collapse. There is time to put in the vertical loop or, in the case of a water well, to place a PVC casing and then pour sand/gravel around the casing. I called him again tonight and sometimes he does use mud, but many (most) times he can drill the borehole without it.

The existing 160 foot well (40-50 years old) is recently (last summer) drawing air and due to the extreme drout, the water table is lowering. In the summer, when I irrigate around the house, the water looks "milky" indicating that the pump is cavitating due to the water table being unable to provide enough water to replenish the water being pumped out of the well. Last fall I had to replace the 1 HP pump, lower the pump placement by an additional 10 feet (to within 15 feet of the well bottom) and this winter the project is to put in a deeper well.

Yes, there is some water at 40-60 feet down, but the quantity of water is low and getting lower due to long term low rainfall. In this area, most all water well drillers now recommend putting in a 250 foot well as there are more water bearing gravels down lower. The geology is that there are water sands and then shale/clay layers and then more water sands with more and more available water the deeper you go.

I also understand that the casing should be 160 # PVC and I can get 5 inch diameter locally for about $2.50 a foot (20 foot lengths with bell ends). I can also get some sections of perforated 5 inch, 160 # PVC casing pipe and a woven "sock" to slide on the perforated casing to prevent sand intrusion into the casing (all from local pump water well supply house in Oklahoma City).

Many local drillers do not use perforated pipe, but cut it themselves in the PVC casing with a hacksaw blade so the cut is only about 20% around the casing diameter and many cuts being put in. The well drillers do not use a sock, but just washed gravel/stone outside the liner. The well supply house thinks this is "old school" and strongly prefer the pre made perforated pipe for the lower 40-80 feet and the use of the sock. Obviously, sections of the casing higher up are solid casing and not perforated.

The cost of the borehole digger is $1000 to dig a 250 foot deep 8 inch borehole. I would have the PVC casing on hand and he helps me put it in borehole, glue up the joints, fill the cavity space with sand/gravel and grout the upper twenty feet with cement. Well drillers that do all the above, are charging $20 or more a foot to drill and place PVC casing. Then the extra expense for electric submersible pump . . .

Once I have a PVC cased well, I can install a 1 HP submersible pump.

Does the above explain the situation?

Thanks for your help.

Steve

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Old 01-25-2013, 01:22 PM   #10
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If you have to set the pump 20’ up to have a place for the sand to fall, then you don’t have a good sand-free well. What are you going to do when that 20’ is full? As WWD says, use the right size slots, the right size gravel, no sock, and use centralizers on the casing. Then set the pump close to the bottom to keep any sand from filling in the well. Making a sand-free well will save you thousands in pump maintenance in the long run. There is a reason well drillers charge $20 a foot and posthole diggers only charge $4 per foot.



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