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Mitigation Recognition of past subsurface mining activity, Including the study of mine maps, can help Identify potential problem areas so precautions can be taken to prevent or minimize property damage on the surface of the ground. The surest way to avoid structural damage is not to bui ld above underground voids. Also, detai led engineering geologic analyses may show that some areas over an underground mine may be stable because of previous subsidence or because the specific site was not mined out. Sometimes special structural designs can compensate for future ground movement. Backfll ling of the voids before construction can stabilize the surface, but usually at prohibitive cost. Subsidence Is characteristic of some lands underlain by formations containing soluble rocks (for example, the Eagle Valley evaporite, which contains rock salt and gypsum). It Is usually very difficult to accurately predict the exact location or time of any future subsidence from this cause because of the many variables. Land Use Un I ass property damage can be prevented or a II ev I ated, subsidence-prone lands are best used for farming, open space, land fills, open storage areas or surface minerai extraction. Special construction methods can be used for roads and pIpelines spanning subsidence-prone land. Corrective measures after subsidence has occurred are usually very expens i ve. Samet Imes maps of underground ml nes are helpful in relating subsidence-prone areas with proposed surface development. Most problems can be solved using a combination of geologic knowledge and special engineering design and construction methods. Investigations and special engineering designs are costly. In many cases avoidance or selective land use is the only economic and safe solution. Case History One evening in 1974 a Lafayette, Colorado, trailer park res I dent not I cad a two foot ho lei n his front yard. By" morning the hole was 10 feet deep and 10 feet across. The tra II er was moved as the 'ho I e cont i nued to grow unt il it was about 25 feet deep and 25 feet In diameter. The sidewalk, a telephone pol e, a concrete pad and a fence had to be rep I aced after the ho I e was fill ed. Fortunate I y a gas i I ne exposed by subsidence did not rupture. The property owner backfilled the hole, acknowledging the site had previously subsided and had been filled. The site is underlain by an Inclined shaft to an old coal mine. The workings were abandoned more than 50 years ago. Case History Interstate Highway 25 crosses several abandoned coal mines In Weld County. Roadway settlement of more than two feet near Er I e has taken p I ace I n patterns that can be c i ose I y correlated to subsidence over coal mine workings 350 to 400 feet below the surface. Much of the severely damaged road is now below original grade, resulting In a mild roller coaster I ike ride. Estimates for repair of the 3/4 mi Ie section damaged by subs I dence are about $1 m II lion. Case History Fr i day. Apr I I 13. 1 979, was a I ucky day for a group of Colorado Highway Department workers and passersby. I~aintenance crews found a 500 foot deep alrshaft to the abandoned t<.londike coal mine had been reopened by surface subsidence Into the mine. A crater about 20 to 25 feet across opened II ke a funne I I nto the shaft just off the pavement on the northeast corner of the I nterstate-70 Woodmen Park Road interchange near Colorado Springs. The shaft had previously been capped, but the slow deterioration of the surface plug finally caused this reopening. Proper filling and capping of this shaft can prevent future Incidents. 47