A mining company proposed underground extraction of some coal reserves. Unfortunately, several significant pipelines traversed over the proposed mined-out area. Because of the potential for unplanned ground subsidence, the pipeline companies became concerned about possible subsidence damage and pipeline safety. In order to have an independent evaluation to determine if sufficient mine support would be left after coal extraction; a number of the pipeline companies retained the geotechnical engineering services of MEA, Inc., a UES Company. Based on review of the available information and the proposed mine design, MEA determined that the risk for subsidence was too great. The mining company said sufficient permanent support would be provided by the planned pillars. However, concern for pipeline damage from short or long-term mine subsidence after the mining was complete still existed. As a result of the concern for pipeline safety from the proposed subjacent mining, a consortium of pipeline companies brought legal action. The court determined that mediation was appropriate.
Pursuant to this order, MEA met with the mining company. A number of “one on one” technical discussions took place. As a result, considerably wider pillars were left to support the pipeline. The improved support can be seen by comparing Figure 1 to Figure 2. Figure 1 depicts the mine plan approved by the state regulatory agency while Figure 2 is the layout used after discussions with MEA. Note that the supported pillars were made about twice their intended width.
The added support provides significantly greater methodology used and recognized by the State. The concern was summarized in a report and pertains to the added support which provides significantly greater protection for the pipelines involved. MEA had a further concern, however, regarding the design methodology used and recognized by the state. The concern was summarized in a report and pertains to the softening of the mine floor (especially from pooling water). Based on computer modeling results, MEA found the effect of floor softening to be substantial, and recommended it be accounted for in the design.
Despite MEA’s warning, sludge (a water suspension of coal processing waste) was injected into the mine. The injection of sludge was followed by mine collapse and subsidence (see Figure 3). MEA believes the mine floor was softened by the water in the sludge. The now softened floor couldn’t hold up the coal pillars, resulting in a massive collapse of the mine. The subsidence propagated up to the ground surface with ground cracking towards its limit. As can be seen in Figure 3, the added support has suppressed the subsidence from progressing under the pipeline.
In summary, from the “one on one” discussions with the mine design engineer, MEA was able to provide the coal company an engineering understanding of the concerns for mine stability. The result was the added protection to the pipeline from increasing the width of support pillars. Ultimately, these wider support pillars provided enough resistance to suppress an adjacent massive mine collapse from propagating into the Zone of Influence of the pipeline.