Case Study: Prediction of Coal Mine Subsidence Below Clubhouse

ABSTRACT: This paper discusses explorations and analyses to predict potential subsidence for design of the $20 million dollar Clubhouse for the Golf Club at Newcastle. Subsurface conditions underlying the three-story Clubhouse included about 75 feet of demolition debris landfill underlain by several levels of abandoned coal mine workings in a moderately to severely weathered sandstone of the Renton Formation. The abandoned coal mines are associated with the Newcastle coal fields where coal mining occurred from about 1880 to 1950. Foundation support for the Clubhouse included a combination of augercast piles bearing on bedrock and shallow spread footings bearing on structural fill where the landfill debris was removed. However, this combined structural system had to be designed to accommodate settlement associated with subsidence of the underlying mine workings. A subsidence hazard assessment was conducted to evaluate subsidence risk and provide the basis for foundation design. The subsidence hazard assessment consisted of a detailed mine map review, borings, index tests on rock core, and subsidence analyses. Both analytical and empirical methods were used to assess potential subsidence. These included: the British National Coal Board Subsidence Engineers Handbook method, empirical standup time and span estimates based on the CSIR geomechanics classification, and two limit equilibrium methods developed by E. Hoek. The design is unusual in the local area since property owners seldom are willing to undertake sufficient explorations and analyses to provide a basis for subsidence calculations. During construction, pneumatic settlement gauges were installed at the top of the rock (base of the structural fill) to monitor short-term settlement predictions.