Best backfill material for the annulus after the filter pack and seal?

• A. Sand
• B. Cement and/or bentonite grout materials
• C. Gravel
• D. Fine clay

Answer: (B)

The space between the outside of the screened interval and the borehole wall must be sealed with a material that forms a durable, low-permeability barrier and supports the casing. Cementitious grout and/or bentonite grout materials are ideal because they cure into a solid, dense seal that prevents vertical water movement and contaminant migration along the annulus, while also stabilizing the casing. Mixing cement with bentonite combines strength with very low permeability, making the seal robust under groundwater pressures. Sand or gravel are too permeable and are used for the filter pack around the screen, not as annulus backfill. Fine clay alone can swell or crack and doesn’t provide the same long-lasting structural seal. So cement and/or bentonite grout is the best backfill for the annulus.