An engineering geological investigation of ground subsidence above the Huntly East Mine area
Thesis DisciplineEngineering Geology
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Ground subsidence above the Huntly East Mine at the N.Z.E.D. Hostel has affected an area of approximately seven hectares with measured settlements of over 800mm. Extensive damage was suffered by most buildings and services of the hostel complex (a $2M development) with remedial measures being estimated at approximately $450,000. To determine the cause(s) and mechanism(s) of the subsidence, site and laboratory investigations were undertaken. Site investigations included core and wash drilling, geophysical borehole logging, dutch cone penetrometer soundings plus piezometer installation and minotoring. Laboratory investigations included one dimensional consolidation and permeability testing, SEM fabric studies, XRD and chemical tests for clay mineralogy, and determinations of Atterberg Limits and grain size distributions. The mine overburden geology at the site consists of a 35 to 60m thick sequence of mudstones and coal seams of the Te Kuiti Group (Eocene to Oligocene), and overlain by a 50 to 70m thick succession of saturated sands, silts and gravels of the Tauranga Group (Pliocene to Holocene). Within the Tauranga Group three aquifers are present. Drilling, combined with laboratory testing has defined a 4 to 10 metre thick, laterally extensive, highly compressible ignimbritic silt aquitard above the lowest aquifer. The engineering geological model considered most likely to explain the subsidence is mine roof collapse causing void migration to near the top of the Te Kuiti Group sequence resulting in drainage and depressurising of aquifers at the base of the Tauranga Group. Aquifer depressurisation is considered likely to cause consolidation within both the aquifer and aquitards associated with it. Back-analyses of the dewatering consolidation model in terms of both magnitude and rates of settlement are consistent with observed values. A finite difference numerical analysis was developed for estimations of settlement rates. Further field verification of the dewatering consolidation model requires monitoring of piezometers in areas of potential ground movement and inspection of workings under subsided ground.