Water Well Journal
Issue link: http://read.dmtmag.com/i/715953
G routing of water wells has been pretty much a standard in most areas of the United States for the last 25 years or so. Some areas have had grouting laws and regula- tions in place longer than others, but for the most part, grout- ing of water wells is a routine part of well drilling. While it would be impossible to do an in-depth look at grouting in just this one column, I will attempt to give it a snapshot perspective as I have witnessed it in my career. This column, coupled with the next one in the November 2016 issue of WWJ, will give my overviews and experiences with different grouts and in different settings. Grouting in the water well industry has evolved like most other facets of this industry. I can remember when it was com- mon to use the cuttings from a bailer (cable tool) and let them run down around the casing. Not much annulus was drilled at that time. Today's standards, however, are often a minimum 20 feet of 3- to 4-inch larger borehole size than the outside diameter of the casing. Some states require a minimum of 40 feet. All of the wells our firm has drilled in the last 20 years are grouted the full length of the casing, which in some cases for a domestic water well are up to 500 feet. Like Getting Stitches When one looks at the grouting process and why it's used, it's important to understand proper grouting is vital to ensure safe drinking water. It also helps prevent the migration of water between aquifers should you work in an area where there are multiple aquifers to choose from and where there is a significant difference in water quality between aquifers. I've often made the analogy grouting is like getting stitches after surgery. After the doctor cuts into the skin and does what he needs to do, he sews you back up to prevent infection from getting into your body—and so it is with grouting of water wells. One can drill the straightest hole, put in the best casing to the proper depth, use a good screen and gravel pack, and so forth—but if you do not have a good grout job, you really have nothing in a well. Given the fact most wells are a once- in-a-lifetime expense for a customer, it's vital to use the best grout and the best grouting techniques available. Two Kinds of Grout So what is the best grout? Again, there is not enough time to debate this in this column. However, both neat cement grouts and bentonite-based grouts have their place. Each has their own characteristics which, coupled with a given geologi- cal setting, have their advantages. While many have made the case cement-based grouts shrink and crack over time, I really have second thoughts about that. After any grout has been placed, the earth below the upper 10-20 feet rarely changes temperature. This obvi- ously favors neat cement grout. For many years, only neat ce- ment grouts were used in water wells, especially public water wells. There has been little issue with delivering safe water from wells grouted with neat cement, in my opinion. Using Both Kinds In our operation, we selected both neat cement grout and bentonite-based grouts, depending on well depth and geologi- cal setting. I would estimate we use bentonite-based grouts on nearly 60% to 70% of our domestic wells, and neat cement on 30% to 40%. If we have a deep grout job to do (more than 250 feet), we typically choose neat cement due to its pumpability for the deeper settings. We also look at the formation we are grouting through. If we are grouting through a dry limestone area with a lot of relief, we will use neat cement. We have found since ben- tonite-based grouts hydrate to set and remain set (neat cement dehydrates to set), the lack of moisture in a dry limestone area can cause the bentonite-based grouts to dry out and shrink or completely drop. We actually had a bentonite grout job dry out due to a lack of moisture in the limestone. When the bentonite became dry enough, it dropped, cracking the PVC casing and causing major issues. Conversely in areas where we drill through deep glacial clay areas, we find the bentonite-based grouts are consistent with the surrounding clays and provide a positive seal. We've also found graded bentonite used on top of gravel packs, along with a high-solids bentonite grout, do the best job when GROUTING ISN'T "JUST GROUTING" Choosing which kind of grout to use. DRAWING FROM THE WELL GARY SHAWVER, MGWC In our operation, we selected both neat cement grout and bentonite-based grouts, depending on well depth and geological setting. waterwelljournal.com 46 September 2016 WWJ