Water Well Journal
Issue link: https://read.dmtmag.com/i/557876
"With cement I can get by with a smaller diameter difference like two inches," he says. "A lot of times the extra drilling cost of that extra borehole diameter warrants dropping the size and using cement as a sealant. If it saves me in drilling costs, I go with the cement." When drilling with mud, it may also be good to choose a grout-like cement because drilling mud could affect the bentonite chips, even if the hole is flushed prior to grouting. Schneider also uses a pre-flush additive with cement to help control flash setting and any inter- action with mud residue left in the hole. While smaller diameter differences saves money, going too small can cause bigger problems in the long run by mak- ing it more difficult to grout the well. "Everybody is trying to keep that annular space as small as possible, but sometimes you have trouble by making it so small you can't complete the well satisfactorily," Schneider says. "You're still going to need that diameter to get the pipe that's big enough to pump the grout through. If you're having to drop down to a three-quarter-inch flush thread pipe to pump grout, then you're starting to get a lot of friction loss. So, we'd rather use one-inch or larger tremie pipe if we can." Cement can also lead to casing col- lapse when the collapse strength of the casing isn't properly calculated, espe- cially in larger diameter wells or ex- tremely deep sets. It can be caused not just by the weight of the grout, but also by the pressurization caused by flash setting and friction loss. Good drilling No matter what grout you choose, good drilling practices are critical to grouting success. If you're air drilling, for example, using too much air while drilling could cause grouting problems later on. "While it helps penetration rates and they get the hole to open up relatively easy, they can actually break down some of those fragile formations," Krause says. If you're drilling with fluids and hav- ing trouble keeping drilling fluids in the zone, you may have difficulties grouting the zone because the grouts are going to be much heavier than the drilling fluids. The right equipment also matters. It's wise to choose a grout that works for the equipment you have. In general, piston pumps work fairly well with all types of grout because they don't tend to shear bentonite grout and also create enough pressure to do deeper sets. Progressive cavity pumps are extremely high shear pumps that work well for cement. Because bentonite grouts are sensi- tive to over-shearing, you also want to use a low shear, high turbulence mixer like a vertical shaft mixer that blends the material but doesn't over-shear it. But even when you do everything correctly, problems can still crop up. That's why Dugan and Krause say you should know a manufacturer's rep or supplier you trust. "It's really important to have good support from the manufacturer's reps on these things," Dugan says. "Have a go- to guy who you can call up when you've got a problem. I think that's important when you start using product out in the field." Lastly, when in doubt, always do what's best for the consumer and what's best for the long-term health of the water supply. "Sometimes the state regulations aren't the best for the consumer," Dugan says. "If they don't protect cross con- tamination from getting into the aquifer, then I think contractors need to be con- scientious and go above and beyond the call and do what's right for the project, and ultimately, the consumer." WWJ Jennifer Strawn was the associate editor of Water Well Journal from 2004 to 2007. She is currently in the internal communications department at Nationwide in Columbus, Ohio. She can be reached at strawnj2 @gmail.com. NGWA Provides Resources on Grouting Section 4 of the ANSI/NGWA 01-14 Water Well Construction Standard focuses on grouting, as well as do other resources available in the NGWA Book- store at www.NGWA.org. Click "Water Well Technology" under the Product Categories tab to find resources like Groundwater & Wells. WWJ September 2015 33 Twitter @WaterWellJournl