Sealing Neutron Probe Access-Tubes in Rocky Soils using Expandable Polyurethane Foam

Installation of neutron probe access tubes for measuring soil water content requires the use of a water-tight seal between the tube and the soil to prevent preferential flow of water along the tube-soil interface. We tested expandable polyurethane foam as a sealant. The foam generating kit consisted of two liquid components that when mixed expanded up to 15 times the original volume and hardened within minutes. The foam density was 0.19 g cm(-3) when hardened and was not wettable. Count ratios (CRs) in moist sand, with foam and air in the void space between the tube and the sand, differed by <1.5% for the small hole diameters (9.0 and 11.5 cm), but in a larger 20-cm diam. hole in saturated sand, they were 8% higher with foam in the void space than with air, evidence of some thermalization of the fast neutrons by the foam. Count ratios decreased as hole diameter increased, but the loss of sensitivity was due mainly to changes in the zone of influence of the void space, not the foam. In rocky soil, insertion of tubes in bore holes to a depth of 1.6 m and sealing with foam took < 2 min per hole. We found no evidence of preferential flow between the tubes and the soil, confirming the foam created water-tight seals. The foam is an acceptable material for sealing neutron probe access tubes. It is not a significant source of the thermalized neutrons at hole diameters typical of field installations, is easy to use, and allows access tubes to be quickly inserted and sealed.