It’s improved water quality on many wells and lowered water fouling and consumer costs.
By Gary Shawver, MGWC
Grouting polyvinyl chloride (PVC) with neat cement grout has been something that, for the most part, the industry has shied away from. The concern was that the heat of hydration may cause the casing to fail or to distort severely, and these concerns are valid.
Therefore, we’ll discuss these concerns and how I recommend one approach using neat cement grout with PVC casing.
The Backstory
First, I want to get into a little history of why I opted to investigate neat cement grout for PVC casing.
In much of our area, we finished some deeper wells into a sandstone formation with casing depths ranging from 300 feet to more than 500 feet. Some local county requirements required we case and pressure grout to the top of the sandstone formation to preclude upper aquifers from allowing water to intermingle with the underlying sandstone formations.
For many years, we would drill an 8.75-inch borehole to the top of the sandstone formation, install 5-inch ID × 59/16-inch OD casing, and pressure grout the entire length of the casing. This worked fine, but due to the high static water level in many of these wells, over time this caused a high iron problem in the water.
As with any new procedure, things are learned after the first few times, and you can get your procedure and routine streamlined and working smoothly.
We always used neat cement grout with the 5-inch steel casing in lieu of a high solids bentonite grout due to the deep depths the grout was required to be placed. Neat cement is more fluid than a high solids bentonite and typically allows more time before it sets, thus making is easier to place the grout. In most cases, we chose a ready-mix truck to bring the grout to the site, so it also eliminated the labor-intensive task of mixing 94 pounds bags of Portland cement.
Additionally, these sandstone wells were not screened as most of the sandstone we came upon was cemented well enough (with natural calcium deposits) that it stood open after being drilled. However, the upper parts of the formation sometimes were poorly cemented and thus it was imperative we used a neat cement grout to ensure the casing, once placed, stayed in place and did not settle any farther into the formation.
Using neat cement also assured the grout stayed in place and did not migrate into the water-bearing formation, potentially causing a major failure in the well. Since a high solids bentonite grout does not have the structural capacity of neat cement, hence, neat cement was the grout of choice.
Neat Cement Grout with PVC Casing
As consumers became fussier about the quality of their water (and rightfully so), we looked at whether it was feasible to install PVC casing and use neat cement.
I asked a national firm supplying bentonite grouts to the drilling industry if it had an interest in doing a seminar for our company on using neat cement grout and PVC casing. The firm responded positively, and a seminar was conducted at which a PVC casing was grouted with neat cement grout with its assistance.
The project well entailed setting approximately 350 feet of 6.9-inch OD SDR 17 PVC casing inside a 10-inch borehole. The depth to bedrock was about 20 feet, and 20 feet of 10-inch ID casing was set temporarily in a 12.25-inch hole (and subsequently removed upon the completion of the well).
A 10-inch hole was then drilled to the approximate casing depth of 350 feet with high pressure air. Some upper water was encountered, but the flow as I recall was less than 5 gallons per minute. Upon completion of the 10-inch hole, we removed the drilling tools and commenced with setting the 6.9-inch PVC casing.
On this project, we put a cement plug inside a 2-foot piece of PVC casing. This plug was fabricated at our shop and we attached it to the bottom of the first joint of PVC casing. As we set the PVC casing, we continued to fill the casing with clean water. We did this to add weight to the casing so it wouldn’t float and to help balance the pressure differential between the inside and outside of the casing.
When the casing was to casing depth, we installed a 6.9-inch PVC coupling and left the casing elevators under the coupling, with the casing hanging about 6 inches off the bottom of the 10-inch borehole. We then installed a second set of elevators just above the coupling and used that set to tie the 6.9-inch casing to the 10-inch casing to prevent the 6.9-inch casing from floating out of the 10-inch hole during placement of the neat cement grout.
Neat cement grout mixed at 6 gallons of water per 94-pound bag of Portland cement weighs about 15 pounds per gallon, much heavier than the 8.4 pounds per gallon that the water weighs inside the casing. This differential can and will cause the casing to lift out of the borehole during placement of the grout due to its buoyancy.
This also applies to steel casing as well. While steel casing has a heavier weight per foot, the same can still occur. The buoyancy factor is an issue one needs to always consider when grouting a long string of casing with any type of grout.
Calculations Needed
Upon completion of the casing installation, we then installed a ¾-inch steel tremie along the outside of the 6.9-inch PVC casing and to the bottom of the annulus. Freshwater was circulated until circulation was achieved.
We then did the calculations of the collapse pressure of the casing while using neat cement grout. The Resistance to Hydraulic Collapse Pressure (RHCP) for SDR 17 PVC casing is 224 PSI at room temperature (or about 70°F). That is the pressure required to collapse the casing if nothing is on the inside of the casing to offset the pressure from the outside.
So, here’s how we did our calculations using the neat cement grout, figuring to bring the grout to 25 feet of the surface:
- Collapsible PSI needs to have a safety factor as close to a 2-to-1 ratio as possible
- 325 feet of 15 pounds per gallon neat cement equals 253.5 PSI
- 325 feet of 8.4 pounds per gallon inside of pipe equals 141.1 PSI
- Subtract the two and you have a differential pressure of 112.4 PSI
- SDR 17 RHCP is 224 PSI, divided by 112.4 PSI, equals your safety factor of 1.99
* For every 10° increase in temperature due to hydration, take 6 PSI off of the RHCP.
The grouting phase consisted of pumping 85 sacks of neat cement grout (one sack would fill nearly 4 feet of annulus) and pumping it through a ¾-inch steel tremie down the annulus. The tremie was pulled to about half the depth of the annulus after almost one half of the grout was pumped.
We then set up a stock tank full of water, installed the ¾-inch tremie down the center of the casing, and installed a small submersible pump in the stock tank. The water was pumped down the center of the casing, with the water returning to the stock tank. This was done to allow cooler water to be cycled down the casing to offset the heat of hydration. During the pumping of the water, we continued to monitor the temperature of the water as it returned to the tank. After four hours of pumping, we did not have more than 2° of rise in the water temperature.
We then terminated the water circulation and left the grout to set overnight. The next morning, we found the grout at roughly 40 feet below grade. We circulated water from inside the casing for about one hour and found little rise in the water temperature.
We then completed the drilling of the underlying sandstone and completed the drilling portion of the project. When the well was completely developed and the tooling removed, we mixed a high solids bentonite grout from 40 feet back to the surface and removed the 10-inch temporary surface casing. The well was now complete.
Final Thoughts
While I do not know how high the temperature of the neat cement can get to during curing, I know it can generate significant heat. Since this project, we have never circulated water again when we used neat cement with PVC casing. I believe if clean water is inside the casing of this size, it will dissipate the heat. As an FYI, our ground temperature below 6 feet in our area runs about 50° to 52°F.
If you should have a washout in the annulus or a large crevice where the cement can go back in, you may have more of a problem with the heat of hydration affecting the PVC casing. This is where you may wish to circulate water to keep the heat of hydration from affecting the casing.
Once the grout has cured initially, if you wish to fill the annulus with neat cement on a longer string of casing, say 500 feet for example, you can continue to grout in phases. Simply keep the pressures offset inside and outside the casing and do your calculations. The downside is it may take you 48 hours to grout a long string of casing. I would recommend the cement set for a 12-hour period before doing a second grouting phase.
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As with any new procedure, things are learned after the first few times, and you can get your procedure and routine streamlined and working smoothly. The ability to use neat cement to grout deep strings of PVC casing has enhanced our ability to bring PVC casing into a geologic area where we previously had not installed PVC. This has improved the water quality on many wells and lowered both the fouling of the wells and the cost for the consumer in not having to spend extra dollars on water conditioning.
My own 600-foot well, which is cased and pressure grouted with neat cement to a depth of 550 feet, has dramatically lowered an otherwise potentially high-iron content well. My water treatment specialist, who has done work in my area for years, stated he had never seen iron content so low in a well of this depth—even on a new well. A simple water softener is all that I need.
Gary Shawver, MGWC, is vice president of Shawver Well Co. Inc. in Fredericksburg, Iowa. He has been in the water well industry for more than 40 years and is a Master Groundwater Contractor. He served on the NGWA Board of Directors. Shawver is semi-retired, having sold his business to his employees. He can be reached at grs@shawverwell.com.