What is the minimum length of grout seal extending from the bottom of the casing to grade?

• A. 10 ft
• B. 25 ft
• C. 50 ft
• D. 100 ft

Answer: (C)

In drilled wells, the grout seal fills the space between the outside of the casing and the borehole to block vertical water movement and protect the aquifer from surface contaminants. Extending this grout from the bottom of the casing all the way up to grade creates a continuous hydraulic barrier that helps prevent contaminants at the surface from moving down along the casing.

The minimum length commonly required is 50 feet. That depth ensures the seal reaches well below shallow infiltration zones, making it much harder for surface water or contaminants to bypass the barrier along the casing. Shorter lengths (such as 10 or 25 feet) wouldn’t reliably block that potential pathway, while a much longer seal (like 100 feet) goes beyond the minimum typically needed.