Laserfiche WebLink
initial pillar dimensions were 30 by 80 ft, and the pillars <br />were retreat mined from west to cast. The area was tra- <br />versed by a City of Palisade waterline, service roads, util. <br />ity lines, and Cottonwood Creek, Where the stream tra- <br />vcrscd the section, the sircambcd was approximately 25 ft <br />wide. To prc%vnt vertical deformation near Cottonwood <br />Creek, a 10• angle of draw and bamer pillars were used to <br />isolate the sircambed from damage. Several rows of <br />pillars were left in place directly below the stream to pro- <br />tect the stability of the stream, The depth of cover for the <br />section ranged from 350 to 700 ft. The minimum ovcrbur- <br />den occurred in the northeast corner. This section in- <br />cluded the major geological characteristics of both prcvi- <br />cus sections, but the amount of topographical relief in the <br />arca wan somewhat greater. <br />SUBSIDENCE -MONITORING PROGRAM <br />Figures S, 6, and 7 show the subsidence -monitoring net. <br />works located over the three study sites. The network for <br />each site was designed to measure the maximum subsid. <br />cnce, the longitudinal subsidence profiles, and the rate that <br />subsidence progressed over the sections. Major factors <br />that affected collected subsidence data included survey <br />accuracy and frequency, monument spacing, monument <br />constrrktion, and surveying instrumentation. The fre- <br />qucncy and accuracy of surveys were affected by climatic <br />and geographic conditions at the site, respectively. <br />MONUMENT SPACING AND CONSTRUCTION <br />The locations of the subsidence monuments were estab- <br />hshcd on the basis of coordinates and mine maps supplied <br />by Roadside Mine personnel. Both the underground and <br />surfacc surveys were tied to the Colorado State Pune <br />coordinate system, which allowed direct correlation be- <br />tuccn surface and underground positions. <br />The network layouts for the northwest and southwest <br />sections were designed with monuments spaced at 100 -ft <br />intervals. Topography, vcgclation, and localized soil ,:on• <br />ditions dictated the actual location of each monument, <br />thus, actual spacing ranged from 90 to 100 ft. When sur- <br />scycd, this spacing yi�_ided a representative subsidence <br />profile of these two sections. The third -west section was <br />designed with monuments spaced at 25 -ft intervals. The <br />clover Spacing was implemented to yield more detailed <br />tnfarmatian over the Cottonwood Creek and to improve <br />the ci aluation of the angle of draw. <br />The survey monuments were 1 -in -diem, 4 to 6 -11 -long <br />steel rods with machined points driven into the ground. <br />Monuments were installed to a depth of 3 to 5 ft using a <br />gas-po,vcred pneumatic hammer r r a sledge hammer. Ap- <br />proximately 6 in of each rod was left above the ground to <br />acci,mmodate the target used in the horizontal position <br />sur%cys Minimizing the height of the monuments above <br />the ground decreased the error in the horizontal position <br />that might have resulted from monuments tilting during <br />subsidence. The 1-in-diam steel rods were easy to store, <br />transport, and install, and provided extremely stable moni- <br />toring stations. <br />MONUMENT LOCATIONS <br />Nonhwest Section <br />Sal cover in this area ranged from a few feet to a <br />depth of approxdmatcly 10 ft. Akhough the soil was mod- <br />erately rocky, k was of adequate depth to permit the mon- <br />uments to be install cd with few problems, The only areas <br />where monuments were difficult to install were along out- <br />crops at the north end of this study arca, where some <br />monuments were moved a few feet to facilitate installation. <br />in all cases, the monuments were driven to refusal, or until <br />approximately 6 in of the rod remained above the surface. <br />In no case was a monument omitted from a planned loca- <br />tion because of the inability to drive it into rocky soil. <br />The network layout used for this site is shown in fig- <br />ure S. It consisted of two north -south 0ncs (A and C), <br />and a diagonal B -fine. This type of monitoring layout was <br />used to give an overall picture of subsidence progression <br />during mining. The diagonal line traversed the arca with <br />the least amount of overburden, with both cock of the line <br />stopping near or on a rock outcrop. The rock outcrops, <br />because of the extreme changes in their elevation and <br />location, affected the ability to obtain angles of draw at <br />this particular site. <br />Southwest Section <br />This section bad similar topographic and geologic char- <br />actcristica to the northwest section-, however, it lay in a <br />drainage basin. The monument locations arc shown in frg- <br />ute 6. Rock outcrops were not a factor in the installation <br />of these monuments. Two lines were installed over this <br />section to monitor subsidence near a 24-in-diam waterline <br />and the main road to the Ulc Water facility. <br />