Nansemond Extensometer Schematic
This schematic shows the Nansemond borehole extensometer (USGS 365337076251606 59D 39) in cutaway profile. A 12 inch borehole extends down 2000.5 feet, with the last 41.5 feet being a cement base for the 2 inch steel extensometer pipe. Surrounding the steel 2-inch extensometer pipe is a 7 inch steel casing with 4 slip joints, and surrounding that casing is bentonite grout. The borehole widens at 250 ft. depth to 20 inches and adds a second, 14 inch steel casing. The borehole widens again at 20 ft. depth to 30 inches and adds a third, 24-inch steel casing.
The 2 inch steel pipe extends several feet above the ground surface and is attached to a weighted fulcrum arm, which offsets the weight of the pipe. A static reference table is affixed to the above-ground portion of the 2 inch pipe.
The above-ground instrument table sits above the extensometer and is set into the concrete pad with 4 inch steel pipes so that it moves with the land surface. The instrument table is attached to the static reference surface by a linear potentiometer, which records the changes between the static reference table and the instrument table.