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Elementary School Subsidence Mitigation

Hanna, Wyoming

Project At-a-Glance

  • 15,850 cubic yards of grout injected
  • Brierley’s high-mobility grout used with vertical and angle borings
  • Grout confirmation drilling, detection dye, and downhole geophysical surveys used to confirm void fill

Underground coal mining in Hanna, WY operated from 1911 through 1941. During that timeframe over 10,000 tons of coal were produced using conventional room and pillar methods.

Due the appearance of “subsidence-like” features observed within the elementary school football field, Brierley Associates implemented a series of investigations, under our contract with Wyoming DEQ-AML. Surface reconnaissance, geophysical surveys and exploratory drilling were completed to assess public risk for future mine subsidence within the football field and around the school campus and public library. Through our investigative work, it was apparent that a majority of the sand slurry injected during the mid-198’s had been compromised by groundwater and was no longer providing adequate support to prevent subsidence.

Brierley Associates’ AML team originally calculated the volume of void filling grout needed to cover the 3-acre site at 17,000 cubic yards. However, using Brierley’s High-Mobility Grout design along with the use of vertical and angle borings, grout confirmation drilling, detection dye, downhole geophysical surveys downhole cameras and conventional borehole sounding, the volume of grout injected was reduced to about 15,850 cubic yards.

The presence of buildings above the historic underground workings and void space posed a challenge to our AML team to complete the work while making sure that ground movement did not affect the structures. Brierley designed an instrumentation program consisting of crack gauges, laser monitors and visual inspection. During the course of the program no instances of structural damage was detected.