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Geotechnical Engineering

 

Manual for Abandoned Underground Mine Inventory and Risk Assessment
Section 7: Priority Site Investigation

7.1 GENERAL DISCUSSION

The purpose of the Priority Site Investigation is to develop a comprehensive body of information to which the evaluation criteria in Section 8 should be applied to develop Priority Site Recommendations. This body of information should include gathered existing information, information from the site visit and site monitoring, and information gathered from newly conducted site investigations as described in this manual section. The coordinating District engineer should be aware that unmapped portions of the subject mine(s) and/or other unmapped mines may be present in the immediate vicinity. These mines may be more prone to failure than the identified, mapped mine(s). The Priority Site Investigation should include limited field work in adjacent, suspect areas to determine the existence, condition and extent of any adjacent unmapped mines.

Priority Site Investigations should be performed in the order of the prioritized listing of the sites for each of the Detailed Site Evaluation risk assessment site groups. All sites within the particular Detailed Site Evaluation risk assessment group should be individually investigated before this work proceeds to sites in the next lower Detailed Site Evaluation group.

For an individual site to be the subject of this step in the risk assessment portion of the process, it should be the uninvestigated site with the highest risk assessment rating in the District. This site priority should have been documented by the prioritized site listing for the Detailed Site Evaluation Group in which the site is included.

7.2 NOTIFICATIONS

The coordinating District engineer should notify the appropriate highway authority staff and outside agencies of the intent to conduct investigations on the priority site. The appropriate staff to be notified should include: 1) the District Deputy Director; 2) the District Highway Management Administrator; 3) the District Public Information Officer; 4) the County Manager; 5) the Administrator of the Office of Highway Management in the Central Office; 6) the Ohio Highway Patrol, and; 7) the Ohio Department of Natural Resources, Division of Mining and Reclamation and the Division of Geological Survey.

The coordinating District engineer should remain in close communication with the County Manager throughout the site investigation. The engineer should review past maintenance records for the site with the County Manager. The coordinating District engineer should keep the County Manager informed of new site information as it becomes available as the result of the site investigations. This channel of communication is very important.

WARNING:

The District and the county need to be prepared to manage the worst case conditions that might be revealed during Priority Site Investigations. Investigations may indicate the need for immediate road closure and/or emergency remediation. The availability and identification of reasonable detour routes, as documented in the Detailed Site Evaluation, should be reviewed with the District Roadway Services Manager and the County Manager. If Emergency Action/ Road Closure is considered, refer to Section 10.

7.3 REVIEW OF ALL AVAILABLE INFORMATION

All available information should be acquired and reviewed before on-site investigations are planned and initiated in the field. Additional information which should be acquired at this time includes: 1) roadway construction diaries; 2) historic and new aerial photography (conventional and infrared if available or needing to be ordered), 3) satellite imagery (if available), and ; 4) ODNR, DGS Quaternary Geology Maps. This information should be added to the site information gathered during the inventory and risk assessment process through Detailed Site Evaluation. (See Appendix C).

Most of the available geological and historic mining information will be acquired from the ODNR, Division of Geological Survey. Other valuable ODNR information may also be acquired by contacting the ODNR, Division of Mining and Reclamation District Abandoned Mine Land (AML) Project Officer. The coordinating District engineer should contact the appropriate ODNR, DMR Project Officer to inform them of the impending ODOT priority site investigation and to request local AML information and/or assistance in site investigations.

The District should develop a composite plan view of the priority site by overlaying the best available roadway drawings and the acquired ODNR, DGS mine map(s). This plan view should be digitally compiled, if possible, for the best accuracy. Accuracy of this overlaying process is critical for the best interpretation of other existing information and the information which will result from the impending site investigations.

A copy, or copies, of the above-described composite plan view should be utilized to indicate points on the site where existing information has been acquired and reviewed. The actual information for these points should also be added to the composite plan view if practical.

This work should allow the District to be as informed as possible before beginning the decision making process of planning and initiating site investigations in the field. This informational review is important because the District will be committing its limited inventory and risk assessment resources to this one particular site. The more efficient such a field operation can be, the more sites the District will be able to afford to investigate in the interest of the public safety.

7.4 FIELD INVESTIGATIONS

7.4.1 Area of Investigations:

The purpose of these site investigations is to determine if the existing roadway, including shoulders, is supported by stable subgrade materials. The minimum width of the investigation area should eliminate any roadway areas from remaining in the surface area which could be influenced by the subsidence of mines directly beneath, or adjacent to, the roadway. Investigations data may verify the existence of stable conditions or provide information in support of the recommendation to stabilize a portion, or portions, of the known abandoned underground mine(s) so as to effectively guarantee the future stability of the roadway site.

The coordinating District engineer should designate an investigations area including the roadway and adjacent areas. This work should be done prior to the layout of the ground survey baseline and initiation of field investigations. The width of this investigations area perpendicular to the roadway centerline should be defined by the area from the outside edge of paved shoulder to outside edge of paved shoulder, plus an additional distance beyond the edge of each paved shoulder to allow for surface subsidence related to adjacent mines. This additional distance beyond the edge of the paved shoulder should be equal to the depth of the overburden at the edge of the paved shoulder times the trigonometric tangent value for the angle of draw. If a specific angle of draw is not known for the particular site geology, an angle of draw value of 35 degrees should be considered for use as a rough rule of thumb ( Figure 7.1).

7.4.2 Forms of Investigative Techniques:

Based on the earlier performed review of all available information, the District should select the forms of investigative techniques to be conducted on the priority site. These investigative techniques should either be of a non-intrusive or intrusive nature.

Non-intrusive investigations are defined as those techniques which are conducted on the roadway without any resulting physical affect or damage. Intrusive investigations are defined as those techniques which affect the roadway site at, and possibly below, grade. Non-intrusive investigations should be conducted first so that the data acquired can be utilized to direct the intrusive investigations for maximum efficiency, with minimum roadway damage

Schematic: Minimum Width of Priority Site Investigation Area

Figure 7.1 : Minimum Width of Priority Site Investigation Area
N.T.S.

The two forms of investigations are discussed in the following text. The District should choose which of these investigative techniques is applicable for the priority site. Site location, anticipated soil and rock conditions in the overburden, groundwater conditions and chemical constituents, and above and below ground constraints should define which investigative techniques are appropriate for the priority site.

As priority site investigations are conducted, copies of the earlier described composite plan view should serve as working drawings to display new information and/or data. All information resulting from non-intrusive and intrusive investigations should be referenced to the established, ground surveyed baseline.

A detailed discussion of the nature, applications, and limitations of the individual forms of investigations is presented in Section 5: Site Monitoring. The following text discusses the use of the available techniques for Priority Site Investigations.

7.4.2.1 Non-Intrusive Investigations:

7.4.2.1.1 Ground Survey Techniques:

The very first on-the-ground activity for a priority site investigation should be the establishment of a surveyed, stationed baseline throughout the site. This stationing will allow for the different forms of site data to be commonly indexed for comparative analysis.

All point elevations to be monitored, as they are known at this time, should be identified and their elevation accurately recorded through ground survey.

7.4.2.1.2 Aerial Photography:

The District should request, from the Office of Aerial Engineering , a listing of the available historic aerial photography for the site. The District should review this list, determine the aerial photography which would be of interest to the site investigation, and order that photography from the Office of Aerial Engineering. Depending on the date and type of film exposed for the most recent aerial photography, new aerial photography, if needed, should be requested from the Office of Aerial Engineering.

Older historic photography may show above-ground abandoned underground mine structures, previous roadway alignments, and/or original stream bed locations which may also appear on the individual abandoned underground mine map(s).

Conventional aerial photography should be useful in detecting particular features, including subsidence features, drainage irregularities, and abandoned mine-related structures.

The particular advantage of infrared photography over conventional photography is that in some cases it can detect localized variations in ground vegetation or areas of differential surface temperatures. These forms of information may be key indicators of either surface subsidence, irregular drainage features, near-surface voids or unconsolidated conditions not otherwise notable during ground reconnaissance.

Aerial photography should be reviewed in the office, and particular features to be verified or investigated on the site should be marked with a grease pencil.

7.4.2.1.3 Visual Observations:

The District should perform a walking reconnaissance of the entire roadway site. Particular features grease penciled on aerial photography should be investigated during this work. This reconnaissance should be conducted as the first on-the-ground activity by the District and should involve a visual inspection of the full width of the right-of-way. The County Manager, or their chosen representative should be invited to attend this reconnaissance for the purpose of identifying any areas where unusual maintenance problems may have occurred on the site.

7.4.2.1.4 Measurements of Particular Features:

Features previously noted on original site data sheets and/or noted during succeeding site monitoring should be physically measured. New measurements should be compared to any previous ones. This comparison of current and past measurements of the same features may provide information on the occurrence and severity of subsidence activity in the site area.

7.4.2.1.5 Falling Weight Deflectometer (FWD):

If applicable for the nature of the priority site, the FWD should be run in all lanes and all shoulders where possible. FWD data should be analyzed for points of outlying data as an indicator of areas having shallow, anomalous subsurface conditions. FWD data will be in the form of numerical values indexed to each of the seven FWD sensors. These sensors are designated as DF1 through DF7. The DF1 sensor is the closest geophone to the falling weight, and therefore collects the shallowest subgrade data. The DF7 sensor is the farthest geophone from the falling weight, and therefore collects the deepest subgrade data.

The following are general rules of thumb for analyzing FWD raw data:

  1. High DF1's and average DF7's probably indicate weak pavement conditions.

  2. Average DF1's and high DF7's probably indicate weak subgrade conditions.

  3. High DF1's and high DF7's could indicate a problem area.
  4. Significantly different values for DF1 through DF7 other than what appears to be the average range of values typical for the site may indicate areas of subsurface anomalies.

7.4.2.1.6 Heavy Weight Deflectometer (HWD):

If applicable for the nature of the priority site, the HWD should be run in all lanes and all shoulders where possible. HWD data should be analyzed for points of outlying data as an indicator of areas having shallow, anomalous subsurface conditions. The District should exercise caution in choosing the amount of weight to be dropped by the HWD. Excessive weight dropped on the pavement may damage some travel lanes and/or shoulders. If this damage occurs, the HWD becomes an intrusive, rather than non-intrusive, form of site testing.

7.4.2.1.7 Profilometer:

The profilometer should be run in all driving lanes and shoulders where possible for the purpose of detecting settlements (troughing) or heave of the pavement. Any troughing locations detected within a site should be further investigated as possible points where pavement could be bridging shallow, anomalous subsurface conditions or voids. Areas of heave may be detected adjacent to areas of subsidence.

7.4.2.1.8 Dynaflect:

If applicable for the nature of the priority site, the Dynaflect should be run in all lanes and all shoulders where possible. Dynaflect data should be analyzed for points of outlying data as an indicator of areas having shallow, anomalous subsurface conditions.

7.4.2.1.9 Surface Ground Penetrating Radar (GPR):

The GPR should be run in all lanes to look for anomalies and/or voids in the subgrade immediately below the reinforced pavement. GPR may detect pavement bridging over unconsolidated soil conditions and/or voids. GPR data collected in non-reinforced, paved shoulder areas may detect deeper subsurface soils anomalies. The depth of GPR penetration is highly site-specific, based on soil, bedrock, and groundwater characteristics.

7.4.2.1.10 Microgravity:

This form of testing is very time consuming and requires all traffic removed from the roadway. Resulting data may show reduced gravity values in the presence of voids or disturbed subsurface materials. This testing technique should be utilized as a secondary non-intrusive form of investigation. Its best application may be for smaller areas where other forms of non-intrusive testing, such as FWD, profilometer, etc., have recorded anomalous data.

7.4.2.1.11 Surface Seismic Studies:

Surface seismic studies may be conducted to create a profile of the below-grade bedrock surface. This information can be utilized to determine the soil-to-rock ratio of the overburden lying between the roadway grade and the top of the abandoned underground mine voids. Surface seismic studies may also be utilized to record the bedrock structure below the roadway. These studies may be able to detect unconsolidated bedrock conditions indicating lower lying mine voids or subsidence activity.

7.4.2.1.12 Resistivity:

Resistivity studies may provide data indicating the subsurface location of soil anomalies and/or voids. Success of this investigative technique is site-specific, based on soil, bedrock, and groundwater characteristics.

7.4.2.1.13 Electromagnetic Scanning:

This technique should be considered as a possible means to find mine haulage way rails and concentrations of roof bolts, etc. However, it is sensitive to adjacent traffic and/or other metallic objects. Electromagnetic scanning should still be considered on sites where the equipment can be operated well away from passing vehicles. One example of such a location would be at the edge of the right-of- way in rural settings.

7.4.2.1.14 Comparative Analysis of Non-Intrusive Data:

Past ODOT experience in utilizing several forms of non-intrusive data on study areas has not revealed one particular form of geophysical testing that will alone provide well-defined locational information for evidence of subsurface soils anomalies and/or voids. Correlation between areas of anomalous data recorded by several different non-intrusive types of investigations has been found on some past sites. These areas were then targeted for further investigated during later intrusive site investigations.

The coordinating District engineer should perform a comparative analysis of the data resulting from the various chosen non-intrusive forms of investigations. This effort should basically involve a unified compilation of the of anomalous data locations detected by all the chosen non-intrusive forms of investigations. This compilation should be performed on a copy of the composite plan view.

Once this data compilation has been performed, the resulting plan view displaying the information should be studied for: 1) concentrations (common areas) of data anomaly occurrence, and; 2) patterns of data anomalies which might suggest verification of, or adjustment needing to be made to, the abandoned underground mine location as displayed on the composite plan view.

This new information, and the previously collected existing information, should be considered, as a whole, in specifying and directing forms of intrusive investigations to be performed as a next phase of priority site investigations.

7.4.2.2 Intrusive Investigations:

7.4.2.2.1 WARNINGS:

  1. BE CERTAIN NOT TO CREATE A MINE DEWATERING EVENT.

  2. CALL THE OHIO UTILITY PROTECTION SERVICE (OUPS) 48 HRS BEFORE DRILLING OR EXCAVATING.

7.4.2.2.2 Drilling Program:

The coordinating District engineer should formulate a subsurface investigations program. This program should be tailored to the individual site and should only involve drilling enough boreholes to define the extent of the abandoned underground mine and the conditions in and above that mine in areas of higher concern. Examples of areas of higher concern would include those locations where surface subsidence has been detected, or where various forms of non-intrusive investigations have commonly recorded anomalous data. Other areas of higher concern would include locations where a main haulage way, main mine entry, large mine room, intersecting haulage ways, or areas of low cover or changed cover are suspected of being located beneath the roadway.

Maps for several different abandoned underground mines may exist for the priority site being investigated. A gap between the available underground mine maps , or a lack of information within or along the edge of, one of the available maps, may exist for portions of the site. These roadway areas should be the subject of limited subsurface investigations for the purpose of determining if unmapped underground mines do exist in these areas, and if so, what the conditions are within the mine. Boreholes executed within these areas should be limited and staggered at uneven intervals. Once mines are encountered in these areas and the conditions within these mines are determined, drilling of the unmapped area should be terminated and the subsurface explorations should move on to the next area.

The potential danger of vertical or horizontal surface subsidence or collapse is possible in and around abandoned mine openings. This subsidence may occur as the result of settlement related to existing, unstable mine opening backfill material. Another cause of such subsidence may be the failure of existing mine opening cribbing. Any work proposed and performed in the vicinity of such a potential mine opening site should address the necessary safety considerations for such potential occurrences of collapse. Such events of collapse or surface subsidence may be induced by the physical disturbance or vibration induced by surface activities. The surcharge of surface loading, such as the weight of equipment and any materials stockpiled on the site, may also induce a subsurface failure of mine cribbing.

Initial intrusive investigations at potential shaft locations should involve angle drilling to define the shaft location and condition(s). The coordinating District engineer should designate a danger zone surrounding the potential shaft location prior to any investigations. This danger zone should be defined by the expected shaft dimensions plus an additional distance away from the shaft equivalent to the overburden depth at the mine shaft multiplied by the trigonometric tangent value for the angle of draw. If a specific angle of draw is not known for the particular site geology, an angle of draw value of 35 degrees should be considered for use as a rough rule of thumb ( Figure 7.2). Initial drilling locations should lie outside of this shaft danger zone.

The drilling program should specify: 1) number, location and angle (if applicable) of proposed boreholes; 2) recovery of boxed cores and/or soil samples as required; 3) receipt of written borehole logs, and; 4) the type and frequency of testing to be conducted on recovered cores and/or samples. Boreholes should be located in shoulder and berm areas when possible. All boreholes should be sealed in accordance with the current ODOT Specifications for Subsurface Investigations.

All boreholes should be cased upon completion if the borehole location permits the temporary extension of the casing above grade. These cased holes can serve at a minimum as groundwater observation wells during investigations. They would provide an opportunity for groundwater sampling and flow monitoring. They may also be utilized for remedial construction purposes in the case where subsurface conditions require immediate placement of stable materials in encountered voids beneath the roadway. Boreholes which cannot be temporarily cased should be sealed.

At the conclusion of priority site investigations or any resulting remedial construction, some cased boreholes may either be modified to remain as permanent piezometers or observation wells utilized for groundwater monitoring. The casings should be removed from all other remaining boreholes and the boreholes should be sealed by tremie grouting. The use of tremie grouting is important to construct continuous grout columns in boreholes through consolidated materials, and to increase the likelihood for lateral placement of grout in mine voids encountered by some boreholes.

schematic: Shaft Danger Zone

Figure 7.2 : Shaft Danger Zone

N.T.S.

Even if the conditions within a mine at the location of one of these boreholes has been judged to be reasonably stable, it is still important to consider tremie grouting the mine void(s) at these borehole locations. The reason for this is that the drilling of the borehole itself may have created a weakened point in the mine roof by shattering the structure of the bedrock. Lateral placement of grout at these borehole locations would construct a pillow of grout which would provide support to the mine roof.

Boreholes which are to be sealed by tremie grouting may also be considered for their potential value as post-investigations time domain reflectometry (TDR) monitoring locations.

Analysis of rock cores should, as a minimum, include determination of Rock Quality Designation (RQD) for each borehole location. Testing of rock cores for unconfined compressive strength should also be considered depending on availability of laboratory services and time required for such core testing.

Soil samples, as a rule of thumb, should be taken at five foot intervals. Samples should be tested for Standard Penetration, gradation, Atterburg limits, and moisture content. ODNR, DGS Quaternary Geology maps should be consulted when recording the nature of the different unconsolidated materials recovered in the soil samples.

7.4.2.2.3 Borehole Monitoring:

As mentioned briefly above, a borehole has utility beyond providing physical samples and a log of the in-situ soils and bedrock. There are a number of forms of site monitoring which should be considered for utilization if a subsurface investigations program is to be performed.

7.4.2.2.3.1 Borehole Ground Penetrating Radar (GPR):

Borehole GPR data collected between closely spaced boreholes may provide information related to areas of subsurface anomalies and/or voids. This subsurface investigative technique will be more applicable if other forms of intrusive testing have already isolated a relatively small area in which the borehole GPR might then be utilized.

7.4.2.2.3.2 Time Domain Reflectometery (TDR):

Some boreholes which are to be sealed by tremie grouting may also be considered for their potential value as post- investigations time domain reflectometry (TDR) monitoring locations. TDR coaxial cables properly tremie grouted into boreholes can provide ongoing, low cost earth movement monitoring information or can alternately be installed to function as a piezometer or observation well.

7.4.2.2.3.3 Slope Inclinometer:

A slope inclinometer may provide subsurface, lateral earth movement ("sidedraw") data in areas adjacent to subsidence areas. However, TDR monitoring points may provide the needed information for a lesser cost.

7.4.2.2.3.4 Borehole Seismic Studies:

Borehole seismic studies may provide very good location information for subsurface soil anomalies and/or voids. Pairs of boreholes, one on each side of the area to be analyzed, are required to conduct this work.

7.4.2.2.3.5 Borehole Video Camera:

This form of borehole monitoring can provide real time viewing of subsurface conditions in the borehole. This information can help with interpretation of drilling results immediately following removal of the drill tools from the borehole. Borehole camera viewing of the entire section of overburden requires a stable, uncased borehole. If the overburden is unstable, the camera can be lowered through hollow stemmed augers or casing. The video tape of this information can immediately be available for presentation to management in the case of high risk conditions being encountered.

7.4.2.2.3.6 Piezometers:

Piezometers can provide valuable short-term and long-term data. This information is valuable in assessing the affects that ground water may play in the hydrogeological environment of which the abandoned underground mine is a part. Piezometers should first be installed into the abandoned underground mine well upslope of any areas where future excavations into the coal mine or coal seam are anticipated. These piezometers will provide information on the static head of any mine pool existing in the portion of the mine to be studied.

The ground elevation of initially installed piezometers should be greater than the maximum head possible within the mined mineral seam. This elevation can be estimated by first determining the highest elevation of the base of the mineral seam within the mine. This work can be accomplished usually by: 1) finding the highest mine floor elevations displayed on the furthest up-dip extent of the available abandoned underground mine map, or; 2) finding the highest base elevations of the mined mineral seam for the furthest up-dip extent of the mined mineral utilizing structural geological information obtained from the ODNR, Division of Geological Survey (DGS). The suggested approximate elevation of initially installed piezometers would then be calculated by adding the maximum reported mined mineral seam thickness to the maximum, up-dip elevation of the base of the mineral seam within the mine. If geologic units above and below the mined mineral seam are hydraulically "connected" with the mined unit, then the above-described simplistic calculation of hydostatic head in the abandoned mine may not apply.

Particular attention should be given to the possibility of an interconnection between the mine(s) under the roadway and other adjacent mines. All available maps should be reviewed in light of this possibility. It is possible that an abandoned underground mine beneath a roadway may be interconnected to one or more similar mines having a slightly higher elevation and lying beyond the right- of -way If such an interconnection may exist, the highest mine floor elevation found in the interconnected mine(s) should be used for the above calculation of the initial piezometer elevation.

7.4.2.2.3.7 Observation Wells:

Observation wells provide valuable information particularly if remediation may follow priority site investigations. A loosely cased borehole with no annular sealing most likely represents the type of casing that would be utilized if drilling and grouting operations were performed. Groundwater levels recorded in these wells will provide a good indication of the conditions which would need to be overcome by any proposed remediation. The elevation of initially installed piezometers should be determined as described above for piezometers.

7.4.2.2.4 Exploratory Excavation:

7.4.2.2.4.1 WARNINGS:

  1. MINE POOL CONDITIONS SHOULD BE DETERMINED BEFORE EXCAVATION IS INITIATED.
  2. MINES WILL "BREATH" AS THE RESULT OF CHANGING BAROMETRIC PRESSURE. OUTSIDE AIR WILL BE DRAWN INTO THE MINE VOID(S) ON DAYS WHEN THE BAROMETRIC PRESSURE IS RISING. AIR, INCLUDING ANY EXISTING GASES, WILL BE EXPELLED FROM THE MINE VOID(S) ON DAYS WHEN THE BAROMETRIC PRESSURE IS DROPPING. EXPELLED GASES MAY BE OF AN EXPLOSIVE AND/OR LETHAL NATURE.
  3. GAS MONITORING EQUIPMENT SHOULD BE ON-SITE AND IN OPERATION DURING ANY EXCAVATIONS INTO POSSIBLE MINE VOIDS. THIS EQUIPMENT SHOULD BE USED TO DETECT ANY GASES WHICH MAY BE EXPELLED FROM ENCOUNTERED MINE VOIDS.
  4. UNDER NO CIRCUMSTANCES SHOULD ANYONE ENTER ANY EXPOSED MINE WORKS.
  5. MINE WORKS EXPOSED DURING EXCAVATIONS SHOULD BE SEALED BEFORE THE SITE IS LEFT UNATTENDED. THIS SEALING WILL PREVENT PERSONS FROM ACCESSING THE SITE AND ENTERING DANGEROUS MINE WORKS.

7.4.2.2.4.2 General Discussion:

Exploratory excavation is a tempting option when surface features suggest limited excavation required to reveal conditions in a suspected mine subsidence feature or collapsed mine entry. Though it is true that in some cases such excavation might be accomplished relatively quickly, the consequences could be serious and substantial. A sudden release (hydraulic "blow-out") of a mine pool or the release of gases from within an excavated mine could instantaneously be upon the personnel performing the excavations, as well as the general public traveling the roadway and living in the surrounding area. A hydraulic "blow out" may occur when excavation removes the stabilizing barrier of the mined mineral seam which is resisting the hydraulic head within an adjacent, flooded mine (Figure 7.3).

An example of this situation would be if a priority site investigations coordinating engineer decided to order a backhoe to excavate along an apparent coal seam cropline where an iron/sulfate bearing seepage zone suggested the possibility of an entry into an abandoned underground mine. If excavation operations were to encounter a water-filled void, a rapid uncontrollable release of mine water would occur.

Potential Hydraulic "Blowout" Location

Figure 7.3 : Potential Hydraulic "Blowout" Location
Source : U.S.D.I., Office of Surface Mining
N.T.S.

The dewatering of an abandoned underground mine void can have a destabilizing effect on conditions within the particular mine intercepted. Such a dewatering event can also cause the dewatering and destabilizing of adjacent, interconnected mines. Surface subsidence activity in areas overlying such dewatered mines may be induced or accelerated by dewatering.

The coordinating District engineer should review elevations shown on the available abandoned underground mine map(s) and be certain there is no possibility of mine pool dewatering before considering excavation. Piezometers and/or observation wells should be established to provide information on the static hydraulic head of any existing mine pool(s) in the area to be excavated. Monitoring equipment for methane, carbon monoxide, hydrogen sulfide and oxygen should be utilized in areas of excavation. This monitoring is particularly critical in low-lying, or below-grade, areas of excavation.

Any excavations having a vertical aspect (a downward or partially downward direction) may induce further collapse of the materials upon which the excavation equipment and personnel are located. Small surface subsidence features can sometimes be associated with much larger voids below grade.

7.4.2.2.4.3 Surface Features in Shallow Overburden Areas:

Excavation of such features can still have the danger of sudden mine dewatering, releases of deadly and/or explosive mine gases, and the possibility of induced further collapse. As mentioned above, such features can often be associated with a much larger existing void below grade.

7.4.2.2.4.4 Mine Openings:

Excavation of these features can have all the dangers described in the early excavation discussions. Prior to any excavation, shaft and slope entry location and conditions should be determined through angle drilling from surface locations outside of the potential shaft danger zone. Please refer to the related discussions in the earlier7.4.2.2.2. Drilling Program text and to Figure 7.2.

As discussed in the above-referenced text, the potential danger of vertical or horizontal surface subsidence or collapse is possible in and around abandoned mine openings. This subsidence may occur as the result of settlement related to existing, unstable mine opening backfill material. Another cause of such subsidence may be the failure of existing mine opening cribbing. Any work proposed and performed in the vicinity of such a potential mine opening site should address the necessary safety considerations for such potential occurrences of collapse. Such events of collapse or surface subsidence may be induced by the physical disturbance or vibration induced by surface activities. The surcharge of surface loading, such as the weight of equipment and any materials stockpiled on the site, may also induce a subsurface failure of mine cribbing.

Excavation of such sites is often most safely performed utilizing a crane with clam-shell bucket. The crane should be located outside of the shaft danger zone.

7.4.2.2.5 Compilation of Intrusive Investigations Data:

Data from the different forms of intrusive investigations should be compiled on a copy of the composite plan view so as to create a comprehensive view of this data. This compilation should include detailed notations of actual information if practical.

7.4.2.2.5.1 Cross-sectional Plotting of Data:

A cross section of all the developed data should be plotted to display, in profile, the existing mine and conditions, as determined by the various investigative techniques. Data should be displayed at the appropriate stations, with left or right of centerline off-set notations. Separate profiles, both left and right, should be alternately developed for divided roadways.

If left and right cross sectional profiles are developed, information for a common mine panel (room), haulage way, etc. may appear at approximately the same elevation on the left and right profiles. However, these same features may appear at different horizontal stations on left and right profiles unless the common feature runs exactly perpendicular to the roadway centerline.

7.4.2.2.5.2 Comparative Analysis:

As with the non-intrusive data, the resulting intrusive data compilation should be studied for: 1) concentrations (common areas) of anomalous data or void detections, and; 2) patterns of anomalous data or void detections which might suggest verification of, or adjustment needing to be made to, the abandoned underground mine map location(s) as displayed on the existing composite plan view. Adjustments of the composite plan should be made where judged appropriate by the coordinating District engineer. Comparative analysis may reveal data from different intrusive studies that clearly document, or strongly suggest, similar or different subgrade conditions.

7.5 COMPREHENSIVE REVIEW AND INTERPRETATIVE SUMMARY OF ALL SITE INFORMATION

The coordinating District engineer should collect all investigations data and all previously gathered information. This information can be used to synthesize the best evaluation of the site conditions as related to the abandoned underground mine lying beneath or nearby the roadway. This effort should include an effort to develop correlations between data anomalies, void detections, surface deformations, etc. in the site investigations information. It should also define locations, areas and/or zones of particular subsurface conditions in relation to the roadway.

This work should provide clearly defined information to which the evaluation criteria in Section 8 can be applied to develop Priority Site Recommendations.

7.6 SITE REEVALUATION

The site may be reevaluated for possible placement in a different risk assessment site group as the result of site information gathered for, or developed during, the Priority Site Investigation. This reevaluation should be conducted as described at the conclusion of Section 5: Site Monitoring.

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Updated: 04/07/2011

FHWA
United States Department of Transportation - Federal Highway Administration