Borehole extensometers are deeply anchored benchmarks. To construct each, a hole is drilled to a depth at which the strata are stable. The hole is then lined with a steel casing with slip-joints to prevent crumpling as subsidence occurs. An inner pipe rests on a concrete plug at the bottom of the borehole and extends to the top. This inner pipe then transfers the stable elevation below to the surface. A measurement of the distance from the inner pipe to the surrounding land surface provides the Harris-Galveston Subsidence District (HGSD) with the amount of compaction that has occurred.
Although the accuracy of this measurement method is impressive, there is one drawback. The high cost to construct and install the equipment prohibits their use in sufficient numbers, resulting in a lack of adequate information for the entire Harris-Galveston and Fort Bend areas. And the sufficient amount of and wide distribution of data is extremely important. Over time, as technologies have evolved, HGSD moved toward more cost-efficient and equally accurate forms of measurement…but borehole extensometers are playing an important role in this new era. Three of HGSD’s existing extensometers have been outfitted with GPS (Global Positioning System) antennas, and are now the only stable GPS points within the greater Houston area.