The Pressure-Activated Injection Probe allows for either top-down or bottom-up injection of remediation materials when using any Geoprobe® grout or injection machine. The probe allows materials to be injected laterally into the subsurface. Unlike conventional injection methods, this probe ensures accurate placement of the material into the intended injection interval. A key feature of this probe is that it acts as a back-flow preventer, keeping injection material IN the ground and not ON the ground! The probe is available for use with 1.5 in. probe rods (21479) and with 1.25 in. probe rods (18735).