SURFACE SUBSIDENCE ASSOCIATED WITH ABANDONED MINE WORKINGS IN THE GOLDENVILLE MINING DISTRICT, NOVA-SCOTIA

Mining in Meguma gold districts has resulted in the development of hun dreds and possibly thousands of underground stopes since gold was firs t discovered in 1861. These stopes are on the order of 1 to 2 m wide a nd extend up to hundreds of metres along strike and tens of metres in height. Many are open or partially filled and capped by surface crown pillars of quartz and phyllite that are as little as 2 m thick vertica lly. The majority of these stopes have been abandoned for more than fi fty years and the long term stability of their surface crown pillars i s uncertain. Rock mass behaviour and surface subsidence have been moni tored in part of the Goldenville mining district since 1991. Surface s ubsidence basins have increased in both number and depth during this p eriod. Stope locations and the nature of crown pillars above the stope s are poorly defined due to lack of accurate mining records and the pr esence of overburden. Furthermore, some of the subsidence features in the area are obviously related to the failure of overburden plugs rath er than bedrock caps. For these reasons, a causal relationship between surface crown pillar failure and the surface subsidence features curr ently existing in the Goldenville district cannot be established at th is time. However, time domain reflectometry and structural data sugges t that the near-surface rock mass between and above abandoned stopes i s undergoing progressive degradation by gravity-induced failure of sla b-like blocks along intersecting and closely-spaced bedding, cleavage and joint surfaces. Uncertainties in factors such as rock mass quality , crown pillar dimensions and water table fluctuations make it impossi ble to predict when and where crown pillars might fail. However, failu re of the weaker crown pillars and consequent surface subsidence are i nevitable if these rock mass movements continue indefinitely.