3V C Mei. Effect of back - filling material on ground coil performance [ J ] ASHRAE Transactions, 1987, ( 2 ) : 1845 - 1857.
4Elliott Spilker. Ground - couple heat pump loop design using thermal conductioity testing and effect of different back - fill materials on vertical bore length [J]. ASHRAE Transactions, 1998, ( 2 ) :775 - 780.
