PVC casing must always be anchored when grouting. Photo courtesy of Silver-Line Plastics Corp.
This month, I'd like to touch on some procedures and methods of grouting I've picked up over the years, but first, the reasons for grouting casing: The primary reason to grout casing is to seal the annular space between the casing and the hole. This prevents communication between the surface and the producing formation. This communication can be in either direction. In most cases, grout prevents surface contamination, such as coliform bacteria, pesticides, insecticides and other less-than-pleasant things, from reaching the aquifer.
A good grout job also can prevent a high-pressure aquifer from flowing to the surface. I know of a 6-inch well in Camden Co., Ga., that was set into a pretty high-pressure aquifer and left ungrouted. The well was way out in the back of a nursery, and nobody went out there very often. Sometime after the well was put into production, it started to leak around the casing. Pretty soon, the leak turned into a flood. Eventually, the owner noticed a new, small stream flowing across the back of his property. Upon investigation, he found that the well was sitting in the middle of a “spring” that was about 10 feet across and 500 feet deep! At the time, it was flowing between 2,000 gallons and 3,000 gallons per minute. The owner called the state, which called the driller. The driller said he couldn't do anything about it, so the state pulled his bond and license.
The state called me to go see what I could do. I was told the state had the driller's $20,000 bond money to plug the well. I looked at the site and did some figuring. It didn't look to me like the $20,000 would cover the cement - let alone the logistics - to pump into what had turned into a swamp nearly 60 feet across. I gave my findings to the state and that was the last I ever heard. I think it's still flowing. What a waste of a resource for the lack of a decent grout job.
There are some significant differences between grouting PVC casing and steel casing. Set up for lowering 2-inch PVC casing into the hole. Photo courtesy of Robertson GeoConsultants Inc.
PVC casing is light compared to steel, and will try to come out of the hole due to the difference in hydrostatic pressure inside the casing and the hydrostatic pressure exerted by the grout column on the outside. For this reason, it always must be anchored. If not, it probably will come out of the hole, leaving a huge mess and a ruined well. Under some circumstances, steel casing also can float. I've seen enough upthrust on some strings of casing to pick up the back of the rig! A simple upthrust calculation during the planning stage of your cement job will warn you.
Another concern is heat of hydration. When cement sets - or hydrates - the chemical reaction produces heat. PVC loses strength quickly when temperatures rise above the manufacturer's recommendations. When cementing deep strings of casing with deep water levels, it is important to equalize the pressure between the inside and outside of the casing to prevent collapse. If the casing is cemented inside, through a tremie line, it is important to seal the top of the casing so that the pump pressure will equalize the pressure between the casing and the annulus. This is called “the Halliburton method” and is, by far, the best and most efficient way to cement casing in the water well industry.
Another way to cement casing is by the “outside tremie” method. This is done by setting the casing on the bottom of the hole and running a tremie line on the outside of the casing to pump the cement. This can be done only if you have drilled a hole large enough to get the casing and the tremie in the hole. This method can lead to two problems. First, since you have drilled a larger hole, it requires more cement, which equals more money and more heat of hydration. The thicker the cement, the more heat is generated.
Second, if the casing is not completely full of water or mud, and sealed, tremendous differential pressure will be exerted on the outside diameter of the casing during the curing process. This heat can easily weaken the casing beyond its collapse resistance. If you cement casing this way, it is a good idea to fill the casing to the top, seal it, and provide a method to maintain internal pressure to prevent collapse.
I hope this helps alleviate problems in cementing PVC casing. If you have problems, give me a call - advice is free, and usually worth what you pay for it!
Since I know that a lot of you read my articles just to see what I've done lately to my bride, Lottie, I thought I'd give you the latest chapter. I made a road trip up north a couple weeks ago. When I got ready to leave, Lottie said, “Don't forget to bring me something ….” I usually get her a peace offering when I'm on the road, and I guess she's gotten spoiled by the practice. I only figured to be gone a couple days, so I didn't take too many clothes with me. The job ran on a little, and then I had to make a delivery a couple states away. By that time, I was out of clean clothes, so I stopped and bought a couple shirts and some jeans. I was thinking about what to send Lottie when I happened to look around the truck at the laundry piled up, just as I passed a UPS store. Perfect! I made a u-turn, pulled in and sent my laundry home, freight collect! Lottie got the package, paid the $12, and instead of a gift from her Prince Charming, found my laundry - I couldn't resist. The “little voices” told me to do it!
When I got home, I gave back her $12. It was worth it! Took her out for a steak dinner later and all was forgiven ….
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