ECS Lessons Learned
OBSERVATIONS AND LESSONS FROM THE SCHOOL OF EXPERIENCE
After more than 30 years in the industry, we’ve amassed some great takeaways pertaining to the work we do. We felt this key information was important to share, so in 1996, Lessons Learned was created as a mailed piece sent out to our peers. Now, we are pleased to offer Lessons Learned electronically once a month, accessible on this page or delivered directly to your inbox when you sign up.
Abandoned Mine Subsidence
In large areas of the eastern half of the US, underground coal mines have been operated for 150+ years. Because of the age of these mines, coupled with adverse geologic/ground water/stress conditions and high extraction ratios, wide-spread instability of the old mine workings has resulted. Mine subsidence is the lateral or vertical ground movement caused by a failure initiated at the mine level of underground man-made mine workings including but not limited to coal mines, clay mines, limestone mines, etc. that directly damages residences, (infrastructure) or commercial buildings.
An underground mine collapse can express itself at the ground surface over time. Settlement/subsidence can cause significant structural damage to supported buildings and infrastructure. When the immediate roof or pillars of a mine collapse it causes the ground surface above the mine opening to sink or subside. Mining engineers generally agree that at some time, most underground mines will ultimately collapse. Currently there is no way of knowing when or where mine subsidence will occur.
Mine subsidence typically manifests itself in two (2) separate forms; either as a ‘pit’, or as a ‘sag’ (or ‘trough’).
In general, when a ‘pit-type’ subsidence feature appears, it typically measures about 6 to 8 feet in depth (which is about the same as the height of the coal seam extracted in the underlying mine), and about 5 to 35 feet wide. In order for a ‘pit-type’ subsidence feature to form, the underlying mine must be shallow (usually less than about 100 feet) and the weak sedimentary bedrock over the mine roof is about 50 feet or less thick. Pit-type subsidence occurs very quickly due to the collapse of the shallow mine.
Alternatively, a ‘sag’ or trough-type subsidence feature appears at the ground surface as a gentle depression which can be several inches up to a few feet in maximum depth, and may encompass several acres. The initial signs typically occur over a period of days with gradual (additional) settlement occurring over many months or years. This type of mine subsidence is sometimes (wrongly) attributed to ‘normal wear and tear’ since the damage to structures can be subtle. Sag-type subsidence can develop over abandoned mines of any depth and is generally the result of coal-pillar collapse or failure of the immediate roof over the abandoned mine.
As discussed above, structural damage can appear suddenly or gradually over a period of years. Damage can include cracked or damaged foundations, cracks in basement walls, ‘popping and cracking’ sounds as the structure settles over time, floor slabs which appear tilted or unlevel, or ruptured underground utility lines. In severe cases, the subsidence-damaged structures must be demolished. There are instances in several States where high schools, apartment/office/commercial buildings, and infrastructure had to be demolished.
In areas of previous coal mining, as geotechnical engineers we evaluate not only the near-surface soil and bedrock conditions, but also the potential for mine subsidence. Thankfully, an extensive library of old mine maps are available through various State–level agencies. We research these maps to determine if mine abandoned mine subsidence is a threat to the site, thereby indicating the need for an expanded subsurface exploration program and/or computer modelling of the conditions.
We hope this Lessons Learned has been informative. For more information, contact the ECS office nearest you.
ECS Group of Companies
The ECS Group of Companies (ECS) is an employee-owned engineering consulting firm with more than 2,000 employees providing geotechnical, construction materials, environmental and facilities consulting services. ECS has grown to over 65 locations and five subsidiaries spread across the Mid-Atlantic, Midwest, Southeast and Southwest. The firm is ranked 69 in Engineering News-Record’s Top 500 Design Firms (April 2020) and 52 in Zweig Group’s 2020 Hot Firms (June 2020).