I am putting in a well field with (7) 400 or 500-ft wells. In researching filling material, I’ve become averse to benonite. A buddy who works for a loop system manufacturer told me to use straight sand instead of bentonite. Through my research I have found that sand has a significantly better TC than bentonite. However, in my quest for knowledge, I have found that the TC is even better with a two-part sand and one-part cement mix. Can someone provide me with the upsides and downsides to doing either an all-sand mix or a 2:1 sand-cement mix? The straight sand mix would be 50/50 coarse and fine. Trying to get the cost of the wells down and performing better by using sand and sand is obviously cheaper than bentonite. The Oklahoma Water Resource Board states that the type of sand must be “water well filter pack sand”, which I don’t even know what that means. Thanks for your help.
Hi and welcome! The industry standard is to use bentonite. If you are concerned about conductivity the use of thermally enhanced grout is advised. Bentonite mixed with sand or graphite. The bentonite is used to protect the earth, not the loops. Eric
Thanks. The aquifer is down 2000ft in Oklahoma. So I’m not sure what else I should be concerned with in this case.
It’s $5250 in added cost because I would have to go an additional 100ft if my TC is 0.4. Well within my cause for concern.
Verticals cost lots of money. Period. If you're talking about 7 loops, that's pretty cheap for the added cost.
Sand thermal conductivity is good if it is saturated with water, but very poor if it is dry. If you drill wells, what is the static water level in the wells? I have three (3) 240' wells through 30' of red clay and the remainder through shale. The static water level in my wells is ~10' from the surface. I used sand backfill up to ~20' from the surface. I used a 1/2" x 6' black iron pipe on the end of a 300' x 3/8" nylon rope to insure the sand was consolidated with no bridges as it was being poured in - basically using the rope and pipe as a tamping rod. Then I poured a couple of 5 gallon buckets (50# each) of bentonite pellets on top of the sand to form a ~15' water tite plug on top of the sand to keep surface water out. PEL-Plug TR30 is what I used: https://pdscoinc.com/water-well-environmental/ My well driller was from Oklahoma.
We grout everything with 1.6 graphite/betonite grout, you can save significant costs by shortening the boreholes by using higher conductive grout. Only when saltwater/gas is in the mix, we use cementacious grout, which is usually around 1.0 - 1.2 conductivity. But if you lessen the number of boreholes, you must watch the pressure drop. Kind of antifreeze and pipe diameter come into play.
