Measuring Vertical Soil Movements and Moisture At a Single Bore-Hole.

Vertical movements from shrinkage and swelling of individual layers within a soil profile are often inferred from the changes in the elevation of subsurface anchors that are physically connected to a measurement site at the surface by a rod or rods reaching the surface through an access tube.  To relate vertical movements to changes in soil water content, water content may be measured in nearby soil through a separate access tube.  A major limitation of this method is the introduction of spatial variability in vertical soil movement measurements, as each measurement is influence by a separate volume of soil which may have different shrink-swell properties.  An additional bore-hole is required for measuring water content, adding the potential for further errors.  To avoid these potential sources of error, we propose a method utilizing a single bore-hole for all vertical soil movements and moisture measurements.  To achieve this, rare earth magnets were inserted perpendicular to the surface at desired depths in a single bore-hole. The relative position of each magnet can be measured using commercial Hall-effect sensors in combination with a Neutron Moisture Meter for water content.  The instrument includes a position sensor capable of resolving vertical soil movements at the millimeter scale.  This method allows for greater resolution of vertical soil movements with depth and the reduction of potential errors associated with spatial heterogeneity.   Results are pending.