Manual for Abandoned Underground Mine Inventory and Risk Assessment
Section 8: Priority Site Recommendations
8.1 GENERAL DISCUSSION
The purpose of this section of the manual is to establish guidelines for making Priority Site Recommendations. These recommendations should be formulated by considering the site evaluation criteria discussed in this section of the manual when reviewing the summarized information produced as the result of the Section 7: Priority Site Investigations. All Surface Deformation Group or Mine Opening Group sites should be remediated unless they can be clearly documented to not be a threat to the roadway.
8.2 PRIORITY SITE EVALUATION CRITERIA
The overburden in a priority site area is in effect functioning as a "bridging" structure oafs and bedrock over subsurface voids related to abandoned underground mines. The coordinating District engineer in formulating Priority Site Recommendations should determine whether there is a current requirement to repair or replace this existing bridging structure of soil and bedrock.
The following criteria are guidelines for formulating Priority Site Recommendations. Most of these criteria would not singlely determine if remediation is warranted for a given site. Several are too interrelated to be considered separately. Of course, in extreme cases such as detection of voids at extremely shallow depths, a single criterion such as "Minimum Overburden Thickness" would alone be a deciding factor for a recommendation to remediate a given site. However, for most sites, all the listed criteria should be considered as a whole to determine the overall risk to the traveling public which the site represents.
The following is a discussion of the individual priority site recommendation criteria:
8.2.1 Minimum Overburden Thickness:
This criterion may be utilized as a means to evaluate the configuration of the existing bridging structure of overburden materials. It is a measure of the minimum vertical soil and bedrock interval between the roadway surface and the top of detected voids. This interval in site areas not exhibiting mine subsidence will be the vertical distance from the roadway surface down to top of the originally mined mineral seam.
A site is known "worst case" areas for this criterion should commonly be those where the top of the apparently failed bedrock structure or void(s) related to mine subsidence has progressed upward toward the roadway surface. The configuration of the existing bridging structure of overburden materials which remains supporting the roadway is reduced in these areas. This depth of intact overburden may be significantly less than the overburden depth to the original mine voids.
An abandoned mine opening could be a less common example of a "worst case" area for this criterion. An abandoned mine opening in its originally constructed form was, in effect, an excavated void extending from existing grade to the mineral seam to be extracted. These mine openings were horizontal, vertical or sloping as originally constructed. Methods of abandonment were highly variable depending on the age of the mine. A given priority site may be found to be generally safe for the traveling public, while having a smaller portion of the site considered high risk due to the presence of an abandoned mine opening(s).
8.2.2 Rock Quality Designation (RQD) of Minimum Overburden Overlying the Mine Void, Top of Apparent Failed Bedrock Structure, or Void(s) Related to Mine Subsidence:
This criterion may be utilized to evaluate the structural integrity of the existing bridging structure of overburden materials. It is determined for a given borehole location through studies performed on the recovered core samples. The RQD value is the percentage of the length of core run which is made up of continuous pieces of core sample which are four (4) inches in length or greater.
8.2.3 Ratio of Unconsolidated Materials to Bedrock in The Overburden Interval:
This criterion may be utilized to evaluate the load versus available supporting structure which the overburden, taken as a whole, represents. The ratio is determined by dividing the total depth of unconsolidated materials above the bedrock topographic surface by the thickness of intact bedrock in the overburden. This thickness of bedrock should be from that bedrock topographic surface down to the top of apparent failed bedrock structure or voids related to mine subsidence.
This ratio is a good indicator of the relative amount of the vertical overburden interval between the roadway surface and the top of the apparent mine void which is comprised of bedrock having structural value. The unconsolidated materials should be considered as a dead load on the structure represented by the underlying intact bedrock. Locations of "worst case" overburden conditions would typically be those where the thickness of unconsolidated materials is equal to , or greater than the thickness of intact bedrock between the bedrock topographic surface and the shallowest failed bedrock structure, or voids related to mine subsidence.
8.2.4 Type of Bedrock in Overburden:
This criterion may be utilized to evaluate the structural ability of the bedrock to span voids or unconsolidated materials potentially underlying the roadway. This structural ability is related to the strengths of the various lithologic types of bedrock present.
8.2.5 Unconfined Compressive Strength of Bedrock in Overburden:
This criterion may be utilized as a laboratory measurement of the inherent material strength of the different types of bedrock in the overburden interval. This information, when combined with the bedding and fracture information, may be utilized to physically characterize the supporting bedrock "beam" of overburden spanning potential subsidence areas.
8.2.6 Bedding and Fractures of Bedrock in Overburden:
This criterion may be utilized to evaluate the structural integrity of the bedrock overburden. The vertical thickness and horizontal dimension of intact rock occurring within each of the types of bedrock present in the overburden interval should be reviewed. In the case of progressive structural failure and resulting subsidence, each intact portion of bedrock spanning the subsiding area is called upon to function as a beam spanning the subsiding area. The length, width and depth of this beam, as represented by the portions of intact bedrock in the overburden directly reflect the likelihood of subsidence progressing ultimately to the roadway surface.
Stress fractures in the recovered bedrock core samples may be observed. These fractures are characteristically oriented approximately 45 degrees to the vertical axis of bedrock core material. If these fractures are of a greater age, their surfaces are usually moderately to heavily stained as the result of long term groundwater flow. If stress fractures which are not heavily stained by groundwater are present in the recovered core samples, they are a good indication of relatively recent structural failure of the affected bedrock. Such recent structural failure may be attributable to the subsidence of an abandoned underground mine and associated overburden. It is a general indication of the loss of supporting structural bedrock integrity at the borehole location.
8.2.7 Detection of Surface Subsidence Activity / Mine Opening(s):
This criterion may be utilized to evaluate how active subsidence mechanisms are in the site area. Information developed through Priority Site Investigation may or may not document evidence of surface deformation. If no surface deformation activity is detected, this would be one positive factor in the consideration to defer remediation. Another factor which might further strengthen the argument for non-remediation of a site evidencing no surface deformation activity would be the applicability and availability of Time Domain Reflectometry (TDR) and/or other acceptable forms of site monitoring. Such monitoring could be placed on the site as a condition of Priority Site Recommendations.
If surface deformation is detected and cannot be attributed to other factors such as failed drainage structures, etc., then it should be interpreted as documenting the occurrence of surface subsidence related to the abandoned underground mine(s) underlying the roadway. If such information is developed for a priority site, the coordinating District engineer should realize that surface and subsurface subsidence is active in the priority site area. The question then to be answered is whether or not the subsidence activity has affected, or will affect in the near future, the roadway and thereby the safety of the traveling public. It is intended that if surface deformation has occurred, or a mine opening(s) exists on the site, remediation should be required.
Correlation of data anomalies between the various forms of Priority Site Investigation information should be considered a strong indicator of subsidence activity. Some of the forms of this information which might independently contain or contribute to a combined form of documentation of surface deformation may include: 1) ground survey data; 2) profilometer data; 3) FWD data; 4) visual observations; 5) measurement of features data; 6) ground and aerial photography, and; 7) borehole camera observations.
8.2.8 Hydrology and Piping Potential:
Various forms of information gathered during the Detailed Site Evaluation and Priority Site Investigation should be considered in evaluating the hydrology and related soil piping potential of the priority site. Some of the forms of this information which should be considered in evaluating the hydrology and related soil piping potential include:1) Number of Subsidence Events; 2) Recent Dewatering; 3) Ratio of Unconsolidated Materials to Bedrock in the Overburden Interval; 4) Minimum Overburden Thickness (Approx.), and 5) Problems Reported During Active Mining.
The existence of localized, high volume groundwater flows within abandoned underground mines should be verified. If such flows exist, remediation techniques should, if possible, allow for the continuation of such groundwater movements. This should prevent the interruption of the stable state of the groundwater regime beneath the site. If interruption of such groundwater flows is required for the chosen form(s) of remediation, the coordinating District engineer should analyze the possible resulting impacts on the groundwater regime beneath the roadway, as well as in up gradient and down gradient locations adjacent to the roadway.
8.2.9 Risk to the Traveling Public:
Various forms of information gathered during the Detailed Site Evaluation and Priority Site Investigation should be considered in evaluating the risk to the traveling public which the priority site represents. Some of the forms of this information which should be considered in evaluating the risk to the traveling public include: 1) Average Daily Traffic (ADT); 2) Classification of Roadway; 3) Average Daily Truck Traffic (ADTT); 4) Traffic Speed; 5) Type of Pavement; 6) Availability of Reasonable Detour Routes, and; 7) Structures in Roadway.
8.3 RECOMMENDATIONS
Priority Site Recommendations should either specify remedial construction or deferral of remediation. Recommended remediation may involve the entire site, or only a portion (or portions) of the site . All sites will be unique and therefore requiring site-specific considerations. The coordinating District engineer should exercise his or her best judgement to formulate recommendations based on the following criteria and other site-specific considerations.
8.3.1 Factors Affecting the Selection of Remedial Techniques:
If remediation is recommended, selection of the most appropriate alternative technique(s) is critical. A number of factors should be considered in evaluating which remediation technique, or combination of techniques, would be most appropriate for the conditions anticipated to be encountered during construction. These factors should include:
8.3.1.1 Hydrogeologic Setting:
Some mined mineral seams, and particularly coal seams, are commonly found to also be confined aquifers. Some seams are more water-bearing than others. The amount of groundwater associated with the mined mineral seam at a particular site will depend on the occurrence and location of local and regional aquifers, the local strike and dip of the bedrock, interconnection of the mine(s) with other adjacent mines, and the seam's elevation at the site relative to the local and regional aquifers.
8.3.1.2 Geometry, Size and Condition of Mine Voids Requiring Remediation:
The geometry of the mine voids will be a direct function of the mining method(s) utilized in the layout and operation of the mine. The dimensions of the mine voids will be the direct result of the height of the extracted mineral(s), the nature and condition of bedrock forming the mine roof and floor, and the method of roof support.
Present conditions within an abandoned underground mine may be highly variable. Varying degrees of roof rock failure, roof support pillar/post punching into the mine floor, mine floor heave, support pillar crushing, etc. may commonly be encountered. Other abandoned underground mines may be found to be in about the same condition as they were at abandonment. If a large amount of roof fall has occurred in the mine, random compartmentalization of the mine may have resulted.
All the above-described factors may greatly influence the effectiveness of the different forms of remediation. This situation may be especially true for remedial techniques employing the remote placement of stable backfill materials in mine voids.
8.3.1.3 Type and Condition of Overburden:
The type of bedrock, its bedding and fracture patterns and its compressive strength should greatly affect the selection of remedial work to be undertaken. The coordinating District engineer should work closely with the project design staff to incorporate information about the existing structural strength of overburden materials into the design considerations for remedial construction.
This information should be integrated into the preliminary design investigations undertaken to determine the preferred method of site remediation. This decision making should typically consider how to: 1) incorporate the natural strength of the bedrock into remedial techniques which do not radically disturb the overburden; 2) physically increase the existing strength of the bedrock in the overburden, or; 3) physically remove and replace the overburden.
8.3.1.4 Site Constraints - Surface and Subsurface:
Some site constraints which may affect the selection of a remedial technique could include: above and below ground utilities, above and below ground structures, right of way width, etc. These types of constraints may dictate which alternative form of remediation is the most practical to construct.
8.3.1.5 Type of Roadway:
The type of roadway being remediated will have a large influence on the method of remediation to be undertaken. In the case of multi-lane highways with full width shoulders, the possibility of limited lane closures while maintaining traffic allows for increased flexibility in the chosen construction methods and sequencing of operations. The roadway as a structure also has a varying ability to bridge unconsolidated subgrade conditions. For example, an original slab pavement with asphalt overlay has much more structural value than rubblized pavement with an asphalt overlay.
8.3.1.6 Type and Volume of Traffic:
The type and volume of traffic being serviced by the roadway may directly affect the choice of a remedial construction technique. This statement does not mean that a different level of safety is desired for different types of roadways. It means that some roadways carrying greater volumes of heavier vehicles must be made structurally stronger to ensure the same level of safety that may be achieved through less extensive efforts on roadways carrying lower volumes of lighter weight vehicles.
8.3.1.7 Presence of Traffic in or Nearby Remedial Construction Project Area:
If an acceptable detour is available for the proposed remedial work area, traffic can be eliminated from the remedial construction area. In this case, almost any form of remedial construction operations which can be set up within the site can be performed.
If there is not an acceptable detour around the proposed remedial work area, then traffic must be maintained during remediation. If for a given site this is the case, the ongoing remedial construction operations must be conducted at all times well away from the traffic in a manner that will not create dangerous driving conditions for the traveling public.
8.3.1.8 Adjacent Land Use and Groundwater Utilization:
These factors need to come under serious consideration when remedial options being considered may include affecting the groundwater table(s) associated with the mined mineral seam and the overlying overburden. The possible forms of remediation that could affect the local or regional groundwater may include remote placement of stable backfill materials into mine voids or undercutting excavation extending down through the mined mineral seam. The remote placement of stable backfill materials into subsurface voids is regulated by the Ohio EPA for the protection of the public drinking water resources. Roadway excavation may potentially result in a catastrophic dewatering event. A dewatering event within the right of way may induce new mine subsidence within or beyond the right of way, and/or well dewatering on adjacent lands beyond the right of way.
Please refer to the warnings and discussions provided in Section 7.4.2.2 Intrusive Investigations ( including all subsections).
8.3.2 Remediation Alternatives:
8.3.2.1 Emergency Action / Road Closure:
This alternative should be recommended for consideration by the District Deputy Director if conditions as revealed during the priority site investigations are considered by the coordinating District engineer to pose an imminent danger to the safety of the traveling public. See Section 10: Emergency Action / Road Closure for a detailed discussion of this subject.
8.3.2.2 Excavation and Controlled Backfill:
This alternative is the most desirable because it unquestionably eliminates any voids within the remedial work area. Excavation of the roadway is performed down to the base of the mined interval. The positive aspect of this form of remediation is that the abandoned mine beneath the roadway is totally eliminated. The negative aspect of this form of remediation is that the existing roadway has been eliminated and must be completely replaced.
The groundwater hydrology of the work area must be fully evaluated before recommending this alternative. Groundwater associated with the mined interval, and other upper lying aquifers may flood incised excavation areas. Excavation may result in mine dewatering. Intact barriers of the mined mineral sufficient to resist any potential hydraulic "blowout" from adjacent abandoned underground mines should be maintained between remedial construction excavations and any adjacent mine voids. Dangerous gases may also be released during excavation.
8.3.2.2.1 Mine Opening Stabilization: This alternative should be considered when a mine opening is found to be open or containing debris which can be removed by equipment. Back fill materials should typically be 601.07 dumped rock fill. All stabilized mine openings should be marked with a permanent monument located over the center of the shaft. These monuments should be located by ground survey and become a permanent record of soils information for the roadway site.
8.3.2.2.1.1 Shaft and Slope Entries: All initial operations should be performed with equipment and on-site storage of materials being located beyond the pre-determined Shaft Danger Zone (See Figure 7.2). Shaft and slope entries which are to be backfilled should be cleared of existing debris through excavation by a crane equipped with a clamshell bucket or other equipment. If shaft or slope entries cannot be completely cleared, the crane should then repeatedly drop a demolition ball on any debris remaining in the mine opening until compaction of debris has been achieved prior to commencing backfilling operations. Cleared shaft and slope entries should be backfilled with 601.07 dumped rock fill placed by conveyor. Initially placed dumped rock backfill materials should be typically 601.07 Type C.. Later backfill materials can grade into 601.07 Type D and then #1 and #2 aggregate. Filter fabric should be placed over backfilled shaft and slope entries to prevent soil piping downward through the dumped rock backfill.
8.3.2.2.1.2 Drift Entries: These horizontal mine openings can also be cleared of debris by excavation equipment and then similarly backfilled to a limited horizontal distance with 601.07 dumped rock backfill, followed by materials grading to #1 and #2 aggregate. Filter fabric should be placed over backfilled drift entries to prevent soil piping downward through the dumped rock backfill. A mine drain should be considered if the potential for the impounding of mine drainage behind placed materials is a possibility. A mine vent may also be appropriate if there is a chance of gas accumulating in the mine due to the placement of backfill materials. Pneumatic stowing can often be specified if backfill materials need to be placed at a greater horizontal distance back into the mine from its opening at grade (See 8.3.2.5.3 Pneumatic Stowing).
Please refer to the warnings and discussions provided in Section 7.4.2.2 Intrusive Investigations (including all subsections).
8.3.2.3 Dynamic Compaction:
This alternative employs the dropping of a heavy weigh onto the surface in an effort to induce collapse of subsurface voids and/or consolidation of the subgrade. The weigh is dropped though the use of a crane. Site constraints may dictate whether this form of remediation can be considered for a given location. This form of remediation has not been performed or evaluated by ODOT at this time.
8.3.2.4 Implosion of Mine Voids Through The Use of Explosives:
This alternative eliminates underground voids by inducing the collapse of the abandoned underground mine voids through the use of explosives. Explosives must be placed by an experienced blaster. Proper timing delays and explosive placement within the abandoned underground mine and associated overburden are critical to the successful use of this remedial alternative. A pre-blast survey should be performed, including a groundwater survey as required. Dynamic compaction and/or controlled backfill may also be used in association with this form of remediation depending on site characteristics. This form of remediation has not been performed or evaluated by ODOT at this time.
8.3.2.5 Remote Placement of Stable Backfill Materials in Voids and Unstable Subgrade Areas:
8.3.2.5.1 Notes:
All placed materials must be pre-approved by the Ohio EPA (OEPA).
All drilling and grouting work must conform to the OEPA waste injection well permit requirements. If industrial waste products such as flyash comprise 50% or more of the grout components by weight, an OEPA waste injection well permit must be applied for and received before construction. An OEPA permit fee will be required.
8.3.2.5.2 Drilling and Grouting Program:
This alternative eliminates voids beneath the roadway through placement of cement grout in abandoned underground mine voids.
Low slump grout is first placed in barrier boreholes. The barrier boreholes are located externally to the roadway area to be stabilized. This method of barrier borehole location is performed so as to eliminate any roadway areas from remaining in the angle of draw area of influence from non-remediated mine voids beyond the roadway (See Figure 8.1). Barrier borehole drilling and grouting should effectively create an isolated portion of the abandoned mine works which should be stabilized so as to effectively guarantee the future stability of the roadway site.
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Figure 8.1 : Barrier Borehole Location
N.T.S.
Higher slump grout is then placed in production boreholes located internally to the completed barrier borehole drilling and grouting. This higher slump grout placed in production boreholes acts to flood the abandoned mine voids in the area defined by the barrier boreholes. Production grouting should proceed from the geological "down-dip" end to the "up-dip"end of the isolated mine workings so as to "squeeze" any existing groundwater out of this portion of the void system , ahead of the production grout placement.
8.3.2.5.2.1 Shaft Stabilization:
Some shafts will be found to contain unconsolidated random backfill materials. Such shafts can be stabilized by drilling and grouting operations which can fill voids in the uncontrolled, existing backfill. Shafts which are to be stabilized by drilling and grouting an unclassified fill should be angle drilled and grouted by equipment located beyond the Shaft Danger Zone.
Confirmation drilling atop the drilled and grouted shaft would be the final phase of such remedial construction. All stabilized shafts should be marked with a permanent monument located over the center of the shaft. These markers should be located by ground survey and become a permanent record of soils information for the roadway site.
8.3.2.5.3 Pneumatic Stowing:
This alternative eliminates voids beneath the roadway by filling them with aggregate materials. These materials are placed by pneumatic transport down cased boreholes which intercept voids to be eliminated.
8.3.2.5.3.1 Drift Entry Stabilization:
This alternative can be utilized to eliminate voids related to horizontal mine openings found to be extending beneath the roadway. Such drift mine entry voids can be filled with aggregate materials pneumatically placed horizontally through the mine entry located downslope of the roadway.
8.3.2.6 Bridging of Roadway over Subsidence Risk Area ( Land Bridge):
The maximum length of potential subgrade subsidence which a land bridge would be expected to span for the site under consideration should first be determined when considering this remediation alternative. Span lengths may be estimated utilizing available historic maps and other collected records regarding methods of mining. The accuracy of the original abandonment maps and records filed by the miner or mining company can be highly variable.
Some forms of land bridging structures to be considered may include:
8.3.2.6.1 Continuous Reinforced Concrete Pavement (CRCP):
This alternative utilizes concrete pavement continuously reinforced in only the direction of travel as a structure to span potentially minor subsidence-related settlements.
8.3.2.6.2 Double Reinforced Pavement:
This alternative utilizes concrete pavement continuously reinforced in both directions as a structure to span potential subsidence-related settlements. This structure is, in effect, acting as a bridge deck founded on the subgrade.
8.3.2.6.3 Pre-Cast Concrete Spans:
This alternative utilizes pre-cast concrete sections such as box beams or bridge spans to form a structure to span isolated mine haulage ways acting as major conveyances of groundwater flow beneath the roadway. A fairly accurate location for placement of these box beams or bridge spans is required for this remediation effort to be effective. Precast sections should be founded on bedrock.
8.3.2.6.3.1 Mine Shafts:
This alternative utilizes pre-cast concrete box beams or bridge spans to form a capping structure over a mine shaft (vertical) entry. The existing lateral support ("cribbing") within the mine shaft should be evaluated and found to be in a durable, stable condition before considering this form of remediation. In the case of shafts which cannot be backfilled or otherwise stabilized, soils should be removed to bedrock. Box beams or bridge spans should then be founded on footers formed on the exposed bedrock. Vertically cast end walls should be specified for end sections of pre-cast bridge spans. This specification would ensure that an enclosed shaft cap is created when all precast bridge spans are placed.
All capped shafts should be marked with a permanent monument located over the center of the shaft. These monuments should be located by ground survey and become a permanent record of soils information for the roadway site.
8.3.2.7 Deferral of Remediation and Performance of Normal Post-Investigations Site Monitoring:
This alternative should be chosen when information developed through Priority Site Investigations has documented that conditions related to the abandoned underground mine beneath the roadway do not currently appear to pose a threat to the safety of the traveling public.
8.3.2.8 Deferral of Remediation and Specification of Additional Studies or Site Monitoring:
This alternative should be chosen when information developed through Priority Site Investigations has documented no apparent threat to the safety of the traveling public, but certain site areas or conditions warrant further long-term studies or specific forms of monitoring.
8.3.2.9 Combination of Techniques:
A variety of conditions requiring remediation may be found on a given site. In such instances, a combination of the above-described techniques may be appropriate.
8.3.2.10 Other Site-Specific Alternatives:
The above-described alternatives do not include all possible forms of remediation. They are some of the more commonly considered methods of remediation. The best alternative for remediation of a given site may be unique due to above and below ground constraints, condition and nature of soils and bedrock, hydrogeologic setting, etc. The coordinating District engineer should use his or her best judgement when recommending a unique remediation alternative.
8.4 SITE SPECIFIC MONITORING REQUIREMENTS
Monitoring requirements will be determined by the coordinating District engineer. This monitoring effort will be based on conditions documented during Priority Site Investigations.
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