Sustainable Land Development Today - July/August 2008 - (Page 31) 2 to 30 ft. FIGURE 3 TYPICAL PIT-TYPE SUBSIDENCE CONFIGURATION (MAHAR AND MARINO, 1981) vertical displacement can be unnoticeable to the naked eye in hilly ground or where maximum settlement is small. Typically, sags over room-and-pillar mines have a maximum settlement of less than four feet. More massive pillar or floor failures result in sag expressions which can be quite large and are typically less than 2,000-feet wide. An idealized subsidence profile over a collapsed area of abandoned room-and-pillar workings is shown in Figure 3. PIT PLAN CRACK SURFACE EDGE OF PIT COLLAPSED SOIL SECTION Mine subsidence and damage potential In areas where underground coal mines exist, developers are faced with the possibility of the future collapse of the mines and the associated damage of any proposed structure. Although many may focus on the potential of subsidence of the ground surface, it is ultimately the potential damage which is the most important factor. Whether or not subsidence occurred on the project site would not be a controlling issue if the damage level determined by the subsidence engineer was nominal or acceptable. In other words, more important than the risk and severity of the subsidence is the subsidence damage potential. Subsidence damage potential can be summarized in categories such as: none, slight, moderate, severe, and very severe with detailed descriptions of each damage intensity provided. These various levels of damage depend upon the severity of the subsidence and the building element affected. When evaluating the overall subsidence concerns, the acceptable level of damage must be identified. For example, when within the designated time span, the damage tolerance level is greater than the expected severity, then the risk be- comes subservient to the severity. In other words, the risk (mine stability) will not matter even with the possibility of subsidence. Some times the potential subsidence damage can be cost effectively identified in a Phase 1 Subsidence Engineering Study. This level of investigation does not require site specific deep drilling and testing. Phase 1 information is gathered from a variety of sources on the: I Site I Project I Geologic I Mining conditions I Subsidence history and characteristics Where the potential subsidence damage to the proposed structures is unacceptable to the developer, it is important to establish the potential of subsidence at the project site. The risk of the subsidence at the project site now becomes a controlling factor in determining if and what mitigation measures can be taken to reduce the subsidence potential. THE COMPANY YOU CAN BUILD ON.> ® AND DRIVE ON. AND GROW ON. AND COUNT ON. Tensar International Corporation improves property values by providing technology-driven site solutions for common earthwork problems such as grade changes requiring retaining walls, and poor soil conditions affecting the cost of roadways, parking lots and building structures. Engineered around patented Tensar® Geogrids, these systems offer cost-effective, reliable solutions. Expect proven performance from innovative systems such as: • Spectra® Roadway Improvement System • Mesa® Retaining Wall Systems • Sierra® Slope Retention System For the competitive edge, depend on Tensar International Corporation to help you reduce costs and increase property value on your next project. Call 866-504-7950 for a no-obligation review of your site plans or visit www.tensarcorp.com/t12 today. ©2008, Tensar International Corporation. 1/2PH-TLDT4C08. Circle 149 • or www.SLDTonline.com/webcard www.SLDTonline.com 31 http://www.tensarcorp.com/t12 http://www.tensarcorp.com/t12 http://www.SLDTonline.com/webcard http://www.SLDTonline.com
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