Bridges & Structures

Manual for Abandoned Underground Mine Inventory and Risk Assessment
Section 4: Initial Site Evaluation

4.1 GENERAL DISCUSSION

The Initial Site Evaluation process is the first level of risk assessment for the established inventory of sites. The Initial Site Evaluation applies established, weighted criteria to gathered existing information and information from one site visit. The Initial Site Evaluation will result in the subdivision of sites into five risk assessment site groups.

The Initial Site Evaluation process is begun with three forms of information being available. The first form of information is the list of all roadway sites apparently overlying abandoned underground mines. The second form is the Site Visit Form and all information gathered in its completion. The third form is the Initial Site Evaluation Criteria provided in this section.

4.2 INITIAL SITE EVALUATION CRITERIA

The Initial Site Evaluation Criteria are utilized to evaluate site conditions and the public's exposure to those conditions

4.2.1 Evidence of Surface Deformation:This criterion is utilized to give an indication of current, or past, subsidence observed in the right-of-way or within view of the right- of-way.

Some examples might include: evidence of fill placement, localized differences in vegetation, puddles and standing water, wet spots, sags in roadway profiles evidenced by oil spot areas in center of lanes, patches in the roadway surface, etc.

Site Condition:	Rating Value:
Yes	Confirmation of this site condition automatically places a given site in the Surface Deformation Site Group, which is the highest priority Detailed Site Evaluation group.
No	A "No" response to this condition will result in the site being placed in one of the site groups other than the Surface Deformation Group.

4.2.2 Presence of Mine Opening(s):This criterion is utilized to report that a mine opening(s) is (are) observed or recorded as being in the right-of-way or within view of the right- of-way. Primary sources of mine opening information at this level of site evaluation will probably be ODNR Abandoned Underground Mine Map Series, individual mine maps, ODOT field reports, roadway drawings, County Managers, local contacts, etc. Some examples of field indicators of such conditions might include: defined, geometric depressions in the existing grade; and remnants of head frame, hoist foundations, mine waste on surface, point source(s) of groundwater expression, etc.

Site Condition:	Rating Value:
Yes	Confirmation of this site condition automatically places a given site in the Mine Opening Site Group, which is the second highest priority Detailed Site Evaluation group after the Surface Deformation Group.
No	A "No" response to this condition will result in the site being placed in one of the site groups other than the Mine Opening Site Group.

4.2.3 Ratio of Unconsolidated Materials to Bedrock in The Overburden Interval: This criterion is utilized as an indicator of subsidence potential. For the use of this criterion, overburden is considered as being composed of only two types of material: 1) unconsolidated material, or; 2) intact bedrock. The subsidence potential will be relatively greater for sites where a larger portion of the overburden is comprised of unconsolidated materials.

Three elevations must be estimated to rate this criterion for a given site. These elevations are for: 1) the roadway surface; 2) the top of the mined interval, and; 3) the bedrock topographic surface within the overburden interval existing between the roadway and the top of the mined interval. The roadway surface elevation may be estimated through review of existing ODOT roadway drawings and U.S.G.S. Quadrangle maps. The top of the mined interval may be estimated through review of several ODNR, DGS forms of information, including individual abandoned underground mine maps and their associated data sheets, bedrock topography maps, geological structure maps, measured geological sections, on-site geological field observations, and other sources of mining/coal information. The elevation of the bedrock topographic surface in the overburden interval may be estimated through review of the ODNR, DGS bedrock topography maps.

The more commonly available forms of surface topographic and bedrock topographic information are larger scaled maps which utilize coarse incremental values. Therefore, the elevations used to calculate the ratio defining the site condition for this criterion will be only approximate. This fact will only allow for an informed "best guess" estimation of the ratio of unconsolidated materials to bedrock in the overburden interval. However, sites which have an overburden interval comprised of a larger portion of unconsolidated materials can be screening for higher risk of subsidence through the use of this criteria. If the apparent unconsolidated material to bedrock estimated ratio appears from available information to be approximately equal to one, the site condition will be rated the same as if the ratio was greater than one.

Site Condition:	Rating Value:
Ratio >= 1	10 Points
Ratio < 1	1 Point

Example of Site Evaluation: Review of the above described information has revealed the following information for a given site: elevation of roadway is 760, elevation of mine floor (from ODNR, DGS mine map) is 725, thickness of mined interval is 5 feet (from ODNR, DGS mine map data sheets, geological structure maps, measured geological sections, and/or on-site geological field observations), and the elevation of the bedrock topographic surface is 740. The site condition for this criterion would be thus determined:

1. Top of Mined interval = mine floor + thickness of mined interval = 725+5 =730

2. Full depth of Overburden = 760-730 = 30 feet

3. Thickness of Unconsolidated Materials in Overburden = 760-740 = 20 feet

4. Thickness of Bedrock in Overburden = 740-730 = 10

5. Site Condition = Ratio of Unconsolidated Materials to Bedrock= 20 feet / 10 feet = 2

6. Rating Value = 10 points

4.2.4 Average Daily Volume of Traffic (ADT): This criterion is utilized as a measure of the traveling public's exposure to the site. The site condition is the total average 24-hour traffic volume of traffic for a given site as determined by the most recent Traffic Survey Report, plus a 3% per year calculated increase to date for the time elapsed since the issuance of the Traffic Survey Report.

Site Condition:	Rating Value:
>30,000 Vehicles	10 Points
20,000 to 30,000 Vehicles	8 Points
10,000 to 20,000 Vehicles	6 Points
5,000 to 10,000 Vehicles	4 Points
<5,000 Vehicles	2 Points

4.2.5 Hydrogeologic Setting: This criterion is utilized to give an indication of current and historic groundwater conditions in the abandoned underground mine within the area of the right-of-way. The general site conditions of "Dewatered", "Flooded" and "Not Flooded" are used to estimate the probable impact of groundwater conditions on the stability of the abandoned underground mine. "Dewatered" indicates the existence of a confirmed report of current, or past, dewatering of the abandoned underground mine voids, either through natural processes or human activities. "Flooded" indicates that the water level in the abandoned underground mine is assumed to be at the mine roof elevation in an upland setting, or assumed to be totally inundated as the result of being at an elevation below the shallowest aquifer on the site. "Not Flooded" indicates the mine may have some minimal amount of groundwater storage and movement within it. If the site has a history of being dewatered and then recharged, then the site should be rated as "Dewatered".

Site Condition:	Rating Value:
Dewatered	10 Points
Flooded	8 Points
Not Flooded	1 Points

4.2.6 Minimum Overburden Thickness (Approx.): This criterion is utilized to give an indication of the potential for subsidence. The site condition is the minimum vertical interval of overburden between the roadway surface elevation and the top of the abandoned underground mine void existing below the roadway. Possible sources of information would include: ODNR, Division Geological Survey information, such as individual abandoned underground mine maps, coal outcrop elevation from topographic information or structure maps, measured geologic sections, etc.; ODNR, Division of Mining and Reclamation information, such as limits of nearby, active surface mine operations ; and contacts with individuals knowledgeable of local, historical mining operations.

Site Condition:	Rating Value:
< 25 Feet	10 Points
25 Feet to 50 Feet	8 Points
50 Feet to 100 Feet	5 Points
>100 Feet	1 Points

4.2.7 Maximum Mined Interval Thickness (Approx.): Thicker seams would be expected to have a higher likelihood of surface deformation. The approximate maximum mined interval (void height) of the abandoned underground mine will probably be determined through review of; 1) individual mine maps; 2) information from the ODNR, Division of Geological Survey and 3) information obtained from local individuals who are, or have been, involved in the mining of the mineral seam.

Site Condition:	Rating Value:
>6 Feet	10 Points
3 Feet to 6 Feet	5 Points
0 Feet to 3 Feet	1 Points

4.2.8 Ratio of Minimum Overburden Thickness to Maximum Mined Interval Thickness (Approx.): This criterion is utilized to give an indication of the potential for highly differential subsidence features at the surface. These subsidence features are also commonly referred to as "sinkholes." The site condition is the estimated ratio of overburden thickness (h) to mining height (m), or "h/m" (Matheson and Eckert-Clift, 1986 ; Peng, 1992).

The overburden thickness (h) is the minimum vertical interval between the roadway surface elevation and the top of the abandoned underground mine void existing below the roadway. The thickness of mined interval (m) is the approximate maximum void height of the abandoned underground mine. Possible sources of information include: ODNR, Division Geological Survey information, such as individual abandoned underground mine maps, coal outcrop elevation from topographic information or structure maps, measured geologic sections, etc.; ODNR, Division of Mining and Reclamation information, such as limits of nearby, active surface mine operations ; and contacts with individuals knowledgeable of local, historical mining operations.

Site Condition:	Rating Value:
Ratio < 5	10 Points
Ratio = 5 to 11	5 Points
Ratio > 11	1 Points

4.2.9 Secondary Mining: This criterion is utilized as an indicator of the probability of the existence of large areas of unsupported mine roof. Evidence of the secondary removal of supporting blocks or pillars left by the original mining operation for roof support may be obtained for some sites. This information will probably be determined primarily through review of: 1) individual mine maps; 2) information from the ODNR, Division of Geological Survey and 3) information obtained from local individuals who are, or have been, involved in the mining of the mineral seam.

Site Condition:	Rating Value:
Yes	10 Points
No	1 Points

4.3 SITE EVALUATION CRITERIA WEIGHTING FACTORS:

The weighting factors for the Initial Site Evaluation criteria are listed below. The weighting factors reflect the importance of the different criteria as related to one another.

4.4 INITIAL SITE EVALUATION FORM

An Initial Site Evaluation Form (Figure 4.1) shall be completed for all inventory sites. This form is structured with a beginning section of site information, an Eliminated Sites Screening section, and then the section listing the Site Evaluation Criteria. A sample completed form is provided as Figure 4.2

4.4.1 Eliminated Sites Screening: The Initial Site Evaluation process will most likely result in the screening of a small percentage of sites into the Eliminated Sites Group. The initial portion of the Initial Site Evaluation form pertains to the documentation of a site being designated as an eliminated site. The most likely reason to eliminate a site from the abandoned underground mine inventory would be conclusive proof that the mine , in fact does not lie beneath, or nearby, the roadway.

One pitfall to avoid in this part of the Initial Site Evaluation process is the elimination of sites without conclusive information indicating that the abandoned underground mine does not pose a threat to the roadway, and therefore the traveling public. If the available information for a given site does not provide conclusive proof that the mine is of no threat to the roadway, and therefore the traveling public, then the site should not be placed in the Eliminated Sites Group.

Two examples of sites which could be eliminated would be those where the mine appeared to be under or nearby the roadway during the initial informational review performed to establish the initial inventory listing of sites, but upon Initial Site Evaluation it was determined that:

Ohio Department of Transportation
Abandoned Underground Mine Inventory and Risk Assessment

Initial Site Evaluation

Figure 4.2: Example of a Completed Initial Site Evaluation Form

1. Excavation for the roadway construction has eliminated ("Daylighted") the mined interval, or;

2. Roadway drawings, mine abandonment maps, field measurements and observations, and /or other data upon careful review reveals that the mine is actually documented to be somewhere other than beneath the roadway.

4.4.2 Surface Deformation and Mine Opening Site Screening: The first two listed Site Evaluation Criteria, Evidence of Surface Deformation and Presence of Mine Opening(s) are utilized to screen certain sites into two site groups. Sites for which there is evidence of surface deformation or the presence of mine openings will be documented as such and automatically be placed in the Surface Deformation Group or the Mine Opening Group for Detailed Site Evaluation. Mine Opening sites should also be evaluated as non-mine opening, general sites.

4.4.3 Rating of General Sites: This form is designed to develop an overall Initial Site Evaluation rating for the balance of the inventory sites which are not screened as Eliminated sites, Surface Deformation Sites, or Mine Opening(s) Sites. The site risk assessment rating is based on the totaling of rating values for all of the criteria. Each of the individual criterion rating values are calculated by multiplying a criterion weighting factor times a rating value which represents a site condition or characteristic. The weighting factors reflect the importance of the different criteria as related to one another.

A numerical site risk assessment rating will thus be established for each non-screened general site by the completion of a Initial Site Evaluation form. This work will provide a risk assessment for each site relative to all other sites within this given site group. These relative risk assessments of all sites not included in the Surface Deformation, Mine Opening, or Eliminated Sites Group will allow for the remaining general sites to be divided into the High Rating and Low Rating Site Groups. This division of sites is accomplished by listing the general sites in order of their overall total risk assessment rating.

4.5 RISK ASSESSMENT GROUPS:

The completion of Initial Site Evaluation forms for all sites will result in the sorting of the entire inventory of sites into five prioritized risk assessment site groups. These subgroupings of sites will be based on five categories of risk determined by the completed Initial Site Evaluation forms. These Initial Site Evaluation risk assessment group categories in order of priority are:

Sites in the Surface Deformation Group, the Mine Opening Group and High Rating Group will proceed to be further studied through the Detailed Site Evaluation process. Initial Site Evaluation information for the sites in the Low Rating Group will be maintained as active permanent files. The highest individually ranked sites in the Low Rating Group will automatically move up into the High Rating Group. This movement will occur as sites initially in the High Rating Group are individually evaluated and site recommendations implemented. All Mine Opening Group sites will also be evaluated and monitored as non-mine opening sites. Therefore, all Mine Opening Group Sites will also appear as sites in either the High Rating Group or the Low Rating Group.

The fifth group of sites determined through the completion of Initial Site Evaluation is the Eliminated Sites group. Although no further investigations will be conducted on these sites, a permanent, inactive record file should be made and maintained of the Initial Site Evaluation for each of these sites. This file should contain copies or documentation of all information considered in the site evaluation.

4.6 SITE MONITORING:

Periodic monitoring of all sites will be conducted in order to detect any changed site conditions. This monitoring may indicate a need to take immediate action or to perform a site reevaluation. A reevaluation of a site with changed conditions may move the site to a risk assessment group having a higher rating.