Skip to contentUnited States Department of Transportation - Federal Highway AdministrationSearch FHWAFeedback

Geotechnical Engineering

 

Manual for Abandoned Underground Mine Inventory and Risk Assessment
Section 5: Site Monitoring

5.1 GENERAL DISCUSSION

The site visit conducted during the Initial Site Evaluation is, in effect, the first site monitoring visit. All sites not eliminated from the inventory as a result of the Initial Site Evaluation process will require continuing periodic monitoring on a permanent basis. The physical site conditions and the exposure of the traveling public to the site will determine the monitoring frequency for each particular site. The nature and frequency of monitoring for a given site may change as the result of changed field conditions either observed during site monitoring, or reported and confirmed by ODOT staff or other persons. Some sites will require reevaluation and possible placement into a different risk assessment site group as the result of these changed conditions detected during site monitoring.

5.2 DIFFERENT STAGES REQUIRING MONITORING

5.2.1 Monitoring Before Priority Site Evaluations:

The forms of monitoring and frequencies of monitoring will depend on the field information and existing information reviewed for the given site at this point in the Abandoned Underground Mine Inventory and Risk Assessment process. Based on this information, the Initial Site Evaluation site group into which the site apparently will be placed will determine the forms and corresponding frequencies of site monitoring which will be appropriate.

Frequencies of the different forms of monitoring should be based on best available information and past field experiences by ODOT.

Suspected Surface Deformation Group Sites should be identified during the initial site visit by the location and documentation of particular features discovered as a result of the site visit. These sites should be assigned the highest level of monitoring. Site specific monitoring, as determined to be appropriate for the observed field conditions, should be undertaken. Detailed Site Evaluations and Priority Site Investigations should be given the highest priority for these sites. The initial monitoring visit should be performed as soon as possible after the initial site visit.

Suspected Mine Opening Group Sites should also be identified during the initial site visit. These sites should be assigned the second highest level of monitoring. Site specific monitoring, as determined to be appropriate for observed field conditions, should also be undertaken.

The balance of the sites visited will form one general group of sites which will be subdivided through the Initial Site Evaluation into the High Rating and Low Rating Groups of sites. These sites should be correspondingly assigned the third and fourth highest level of monitoring. Site specific monitoring as determined to be appropriate for the observed field conditions should be undertaken for these sites.

Once periodic monitoring of sites is initially established, a monitoring file should be established for each site. This file should be reviewed before each succeeding site visit. This effort will familiarize the site monitoring person with particular site features to be observed. Any changed conditions related to previously identified particular features can then be noted.

5.2.2 Monitoring After Priority Site Evaluations:

After Initial and Detailed Site Evaluations, Priority Site Investigations and Recommendations will be undertaken. The Priority Site Investigation should either result in a Priority Site Recommendation to defer remediation or to perform remedial construction. If remedial construction is recommended, separate forms and frequencies of monitoring may be recommended during: 1) construction contract document development; 2) remediation construction and; 3) post remediation construction.

Frequencies of the different forms of monitoring are based on best available information and past field experiences by ODOT.

5.2.2.1 Remediation Deferred:

If the Priority Site Recommendation is to defer remediation, the site still represents a location which at some time in the future may develop conditions which would warrant remediation. Therefore, these sites should remain on the active inventory site listing and should be monitored.

5.2.2.2 During Construction Document Development:

Site conditions may change during the development of construction contract documents. For this reason, site monitoring should be maintained during this period so as to make adjustments to the construction contract documents and to protect the safety of the traveling public in locations where the roadway is remaining open.

5.2.2.3 During Remediation Construction:

Site conditions may change during the remediation construction. This situation may occur due to the progressive nature of the conditions identified for remediation or as the result of conditions induced by the remediation construction itself. In either case, it is essential to be aware of, and vigilant for, the possibility of such changing surface and subsurface site conditions. Such conditions could have a significant impact on the scope of work required to complete remediation construction. They could also present a threat to the safety of persons working in the project area and to the traveling public if traffic is being maintained.

Provisions for site monitoring should be incorporated into remediation construction contract documents. See Section 9: Remediation for further information regarding this subject.

5.2.2.4 Post-Remediation Construction:

Post constructionmonitoring should be conducted to insure that the remediation was successful and that all areas requiring remediation were included in the completed work. This monitoring should typically be initiated with a relatively short time period between individual site monitoring visits. If no problems evidence themselves over time, this time period will continue to lengthen.

5.3 FORMS OF MONITORING

Table 5.1 summarizes the various forms of site monitoring, including applications and limitations. Refer to Appendix E: Forms of Monitoring for a detailed discussion of the forms of monitoring..

TABLE 5.1 FORMS OF MONITORING SUMMARY

Forms of Monitoring Applications Limitations
I. Visual:
Driving All sites None
On the Ground Particular features Utilized to detect physical changes of identifiable site features. Features may include mine-related structures or surface deformation features. None
Ground Photography Utilized to record particular features, such as surface deformation, drainage irregularities, etc. None
II. Non- Intrusive
Ground Survey Techniques Utilized to detect vertical settlements or heave related to underground mine subsidence. Point elevations to be monitored may include mine structures, roadway centerline station hubs, P-K nails driven into surface of driving lanes, etc. This work can be performed by ODOT personnel and equipment Lane closure/ traffic control required
Aerial Photograph Conventional (B/W & Color) Utilized to detect surface subsidence features and drainage irregularities which otherwise might not be noticeable from the ground. This work can be performed by ODOT personnel and equipment. Two optimal times: Late Fall and early Spring.
Infrared (B/W & Color) Utilized to detect subsidence features, drainage irregularities, and possibly near surface voids or unconsolidated conditions not otherwise noticeable from ground reconnaissance. This detection is possible due to the variations in surface temperature, soil moisture and/or related variations in surface vegetation. This work can be performed by ODOT personnel and equipment. Optimal Times: Days when higher solar heating is occurring.
Profilometer Creates a record of horizontal lane profiles through the site. No traffic control is normally required. This work can performed by ODOT personnel and equipment None.
Surface Seismic Methods Refraction Detects depth and thickness of geologic strata and depth to bedrock and water table. Require an extensive array of geophones. If study area is large, reflection may not be practical for general studies. Lane closure/traffic control probably required.
Reflection Detects depth and thickness of geologic strata and possibly voids. Require an extensive array of geophones. If study area is large, reflection may not be practical for general studies. Lane closure/traffic control probably required.
Dynaflect Detects unconsolidated subgrade conditions through the use of a lower energy impulse. This work can be performed by ODOT personnel and equipment. Lane closure/ traffic control required
Falling Weight Deflectometer (FWD) Detects unconsolidated subgrade conditions through the use of a higher energy impulse. This work can be performed by ODOT personnel and equipment. Lane closure/ traffic control required
Heavy Weight Deflectometer (HWD) Detects unconsolidated subgrade conditions through the use of a higher energy impulse. Lane closure/traffic control required. Heavier weights may damage pavements.
Electrical Methods Surface GPR Detects subsurface voids or anomalous conditions. Longer wavelengths (25 to 50 MHZ) are utilized for deeper penetration of the subgrade in shoulders or other areas where no reinforcing steel exists. Shorter wavelengths (100 MHZ to 1 GHz) are utilized to penetrate pavements containing reinforcing steel. These shorter wavelengths are only effective at providing information on subgrade conditions immediately below the pavement. This monitoring currently requires the contracting of external technical services. Success of this technique is highly dependent on the characteristics of the soils, bedrock, and groundwater. Traffic control is required.
Resistivity Studies Produce lateral or vertical electrical profiling of anomalous conditions of subsurface materials. Can detect voids and buried metals. Standard methods require metal stakes driven into ground or pavement. Success depends upon electrical interference on site and size of target.
Electromagnetic Induction (EM) This form of monitoring may be in the form of Frequency- Domain EM or Time-Domain EM. This monitoring is used to obtain horizontal profiles and depth soundings of conductive layers, especially buried metals, such as abandoned rails in mines. This monitoring technique is sensitive to above-ground metallic objects, such as guardrails, traffic, etc.
Spontaneous-Potential (SP) Measures distortions created by local changes in the underlying electrical conductivity of the earth. This technique requires groundwater flow in mines or voids.
Very Low Frequency (VLF) Measures distortions created by local changes in the underlying electrical conductivity of the earth. Data resulting from this technique has a relatively low resolution.
Potential Field Methods These methods may be in the form of microgravity studies or magnetic studies. Near surface anomalies in the strengths of these two fields, in some settings, may be useful in locating subsurface voids. Gravity studies require precise elevation surveys. Magnetic studies are sensitive to ferrous metals.
III. Intrusive (Borings Required)
Electrical Methods  
Borehole GPR This method is utilized to detect subsurface anomalies and/or voids. A radar signal is transmitted from one hole to an adjacent hole. Maximum borehole spacing for this technique may be limited to 10 to 12 feet. May require a larger amount of data processing time.
Time Domain Reflectometer (TDR) Utilized to detect lateral (shear) and vertical (subsidence) movements. This form of monitoring is relatively inexpensive, particularly when drilling is performed for other purposes. Data collection can be performed by one person. Data can be easily interpreted and is usable at the time of collection on the site. Rental of a grout pump, with operator, is required for cable installation. The roadway authority will need to purchase a TDR meter for the data collection from installed cables.
Slope Inclinometer Utilized to detect lateral subsurface movement (shear). This form of monitoring may have application in detection of "side-draw" related to adjacent subsidence activity. Relatively expensive as compared to the costs of TDR monitoring. Requires sophisticated data gathering equipment, time consuming data collection, and extensive data analysis to produce usable information.
Borehole Camera Utilized to view soil and bedrock overburden conditions, also the nature and condition of subsurface voids, etc. Creation of a video record of viewed conditions. This work can be performed by ODOT personnel and equipment. Requires stable overburden conditions. Otherwise, camera will be at risk when lowered in the borehole. Quality of video imagery below the groundwater table is usually of a lesser quality. Traffic control may be required.
Seismic Studies Borehole Seismic Studies Utilized to detect subsurface anomalies, including voids, between adjacent boreholes. Pair of boreholes required. Currently requires contracting of external technical services. May require a larger amount of data processing time.
Groundwater Studies Piezometers Utilized to detect and monitor ground water static head within a particular aquifer. This form of monitoring is relatively inexpensive, particularly if a drilling program is to be performed. Data collection can be performed quickly by one person. Data is easily interpreted and usable at the time of collection on the site. Requires stable overburden condition to allow for installation of well casing, slotted at a particular aquifer's elevation, complete with the appropriate sealing(s) of the annular space, so as to only allow water from one particular aquifer to enter and rise in the casing. Location of piezometers must not interfere with the safety of the traveling public.
Observation Wells Utilized to detect combined static groundwater head for a given borehole location. This form of groundwater monitoring has application in areas where fractured overburden conditions allow for co-mingling of originally separate aquifers. Requires stable overburden condition to allow for the installation of a well casing. Location of observation wells must not interfere with the safety of the traveling public.

5.4 SITE MONITORING GUIDELINES

The Site Monitoring Guidelines for the four active risk assessment site groups resulting from Initial Site Evaluation are provided as Figures 5.1, 5.2, 5.3 and 5.4 on the following pages. These guidelines indicate forms of monitoring applicable to the different stages of site monitoring. They are general monitoring guidelines for each of the site groups. They do not necessarily define the exact monitoring needs for each specific site.

Each site will be found to be unique. Many sites may require adjustment of actual monitoring undertaken depending on site configuration, surface and subsurface constraints, availability of equipment, etc.

5.4.1 Notes:

The lower portion of the Site Monitoring Guidelines includes a "Notes" section. These notations are discussed below:

5.4.1.1 "M" Notation:

The "M" on the Site Monitoring Guidelines indicates that the particular form of monitoring is applicable for a typical site within the given Site Group.

5.4.1.2 "T" Notation:

The "T" refers to sites where traffic is maintained on or adjacent to the roadway during construction. The maintenance of traffic in close proximity to the remedial construction activity adds additional responsibility to protect the safety of the traveling public.

5.4.1.3 "S" Notation:

The "S" on the Site Monitoring Guidelines forms indicates that the particular form of monitoring may be applicable depending on specific site conditions and constraints. Refer to the preceding portions of this section of the manual for descriptions of forms and applications of individual types of monitoring.

Example: Some forms of monitoring may have site-specific applications on Mine Opening Group sites having drift (horizontal) and slope mine entries. In these settings, certain forms of monitoring such as ground penetrating radar (GPR), time domain reflectometery (TDR), and borehole camera observation may be applicable. Determinations of the site-specific need for such forms of monitoring on such a site would be largely based on: 1) the mine opening location relative to the vertical and horizontal alignment of the roadway; 2) the known geometry of the mine opening itself, and: 3) the nature of the soil and rock strata comprising the overburden interval between the roadway and mine opening.

Ohio Department of Transportation
Abandoned Underground Mine Inventory and Risk Assessment


Site Monitoring Guidelines For The Surface Deformation Group

Forms of Monitoring Different Stages of Site Monitoring
Before Priority Site Investigations After Priority Site Investigations
  During Construction Document Development During Remediation Construction Post Remediation Construction
I. Visual
  - Driving M   M S M
  - On the Ground: Particular Features M   M M M
  - On the Ground: Ground Photography M   S S S
II. Non - Intrusive
  - Profilometer M   M S,T M
  - Dynaflect          
  - FWD M   S S,T M
  - HWD          
  - Ground Survey Techniques M   M S M
  - Aerial Photography: Conventional
    (B/W & Color)
M       M
  - Aerial Photography: Infrared (B/W & Color) M       M
  - Surface GPR S   S   S
  - Other **          
III. Intrusive (Borings Required)
  - Borehole GPR       S  
  - TDR     S S S
  - Slope Inclinometer          
  - Borehole Seismic          
  - Borehole Camera     S S  
  - Piezometers     S S S
  - Observation wells     S S S

Notes:

"M" indicates a general monitoring guideline for the site group
"T" indicates where traffic is maintained during construction
"S" indicates determined on a site specific basis
** Other forms of monitoring may be specified on a site-by-site basis as needed.
See Table 5.2 for Listing of "Other forms of Monitoring."

Figure 5.1
Figure 5.2
Figure 5.3
Figure 5.4



TABLE 5.2

Other Forms of Monitoring Guidelines

Form of Monitoring Applications Limitations
Surface Seismic Methods Site-specific. Site-specific.
Resistivity Studies Site-specific. Site-specific.
Electromagnetic Induction (EM) Site-specific. Site-specific.
Spontaneous Potential (SP) Site-specific. Site-specific.
Very Low Frequency (VLF) Site-specific. Site-specific.
Potential Field Methods
        - Gravity Studies
        - Magnetic Studies
Site-specific. Site-specific.

5.5 FREQUENCY OF MONITORING

The exact frequency for the forms of monitoring indicated on the preceding Site Monitoring Guidelines tables will be unique to each individual site. This reflects the fact that no two sites will be identical. Conditions and constraints, at grade and below grade, as well as availability of personnel and equipment, will in many cases dictate the frequency of monitoring. The final decision on monitoring forms and frequencies for each inventory site will be based on the best judgement of the District engineer designated to coordinate the monitoring process. The following text discusses the range of frequencies for the various forms of site monitoring.

5.5.1 Visual Forms of Monitoring:

5.5.1.1 Driving:

The frequency for this form of monitoring can range from daily, in the case of some high risk Surface Deformation Group sites, to two years, in the case of some Low Rating Group sites. This monitoring can often be easily performed by roadway authority personnel traversing the site either to and from work, or during the course of their daily work.

5.5.1.2 On the Ground:

5.5.1.2.1 Particular Features:

The frequency for this form of monitoring can range from daily, in the case of some high risk Surface Deformation Group sites, to two years, in the case of some Low Rating Group sites. This monitoring can often be easily performed by roadway authority personnel traversing the site either to and from work, or during the course of their daily work.

5.5.1.2.2 Ground Photography:

The frequency for this form of monitoring can range from daily, during certain remedial construction operations on some high risk Surface Deformation Group sites or Mine Opening Group sites, to two years, in the case of some Low Rating Group sites. This monitoring can often be easily performed by roadway authority personnel traversing the site either to and from work, or during the course of their daily work.

5.5.2 Non-Intrusive Forms of Monitoring:

5.5.2.1 Ground Survey Techniques:

The frequency for this form of monitoring can range from every week, during some stages of monitoring on Surface Deformation Group sites, Mine Opening Group sites, and High Rating Group sites during certain remedial construction operations if traffic is being maintained, to three years, in the case of some Low Rating Group sites. This form of monitoring can be performed by ODOT personnel and equipment.

5.5.2.2 Aerial Photography:

5.5.2.2.1 Conventional (B/W and Color):

The frequency for this form of monitoring can range from yearly, before priority site investigations and immediately following remediation construction on some high risk Surface Deformation Group sites or Mine Opening Group sites, to three years, in the case of some Low Rating Group sites. This form of monitoring can be performed by ODOT personnel and equipment.

5.5.2.2.2 Infrared (B/W and Color):

The frequency for this form of monitoring can range from yearly, before priority site investigations and immediately following remediation construction on some high risk Surface Deformation Group sites or Mine Opening Group sites, to three years, in the case of some Low Rating Group sites. This form of monitoring can be performed by ODOT personnel and equipment.

5.5.2.3 Profilometer:

The frequency for this form of monitoring can range from every two weeks, on Surface Deformation Group sites, Mine Opening Group sites, and High Rating Sites during certain remedial construction operations if traffic is being maintained, to three years, in the case of some Low Rating Group sites. This form of monitoring can be performed by ODOT personnel and equipment.

5.5.2.4 Surface Seismic Methods:

5.5.2.4.1 Dynaflect

5.5.2.4.2 Falling Weight Deflectometer (FWD):

The frequency for this form of monitoring can range from: 1) every two weeks, on Surface Deformation Group sites, Mine Opening Group sites,, and High Rating Sites during certain remedial construction operations if traffic is being maintained, to; 2) three years, in the case of some Low Rating Group sites. This form of monitoring can be performed by ODOT personnel and equipment.

5.5.2.4.3 Heavy Weight Deflectometer (HWD)

5.5.2.5 Electrical Methods:

5.5.2.5.1 Surface Ground Penetrating Radar (GPR):

The frequency for this form of monitoring may range from: 1) every six months, before priority site investigations and immediately following remediation construction on some high risk Surface Deformation Group sites, Mine Opening Group sites or High Rating sites, to; 2) two years, in the case of these same sites if remediation is deferred or remedial construction took place several years in the past with no apparent occurrence of further mine-related problems.

5.5.2.5.2 Resistivity Studies

5.5.2.5.3 Electromagnetic Induction (EM)

5.5.2.5.4 Spontaneous Potential (SP)

5.5.2.5.5 Very Low Frequency (VLF)

5.5.2.6 Potential Field Methods

5.5.3 Intrusive Forms of Monitoring:

5.5.3.1 Electrical Methods:

5.5.3.1.1 Borehole Ground Penetrating Radar:

The frequency for this form of monitoring can range from weekly to monthly, during certain remedial construction operations on some Surface Deformation Group sites and High Rating Sites during certain remedial construction operations.

5.5.3.1.2 Time Domain Reflectometer:

The frequency for this form of monitoring can range from continuous to three months on Surface Deformation Group sites, Mine Opening Group sites, and High Rating Sites, depending on the particular risk assessment stage being monitored. This form of monitoring can be performed by ODOT personnel and equipment (one TDR meter is currently available).

5.5.3.1.3 Slope Inclinometer:

The frequency of this form of monitoring is site specific.

5.5.3.1.4 Borehole Camera:

The frequency of this form of monitoring is site specific.

5.5.3.2 Borehole Seismic Studies

5.5.3.3 Groundwater Studies:

5.5.3.3.1 Piezometers:

The frequency for this form of monitoring can range from instrumented, continuous (hourly at worst) monitoring, on some Surface Deformation Group sites, Mine Opening Group sites, and High Rating Sites during certain remedial construction operations; to three months, in the case of some High Rating sites where remediation is deferred.

5.5.3.3.2 Observation Wells:

The frequency for this form of monitoring can range from instrumented, continuous (hourly at worst) monitoring, on some Surface Deformation Group sites, Mine Opening Group sites, and High Rating Sites during certain remedial construction operations, to three months, in the case of some High Rating sites where remediation is deferred.

5.6 SITE REEVALUATION

5.6.1 General:

Permanent site monitoring will provide a feedback loop in the process to allow for the detection of changed site conditions. This aspect of the inventory and risk assessment process makes it a dynamic, responsive system. This process feature is necessary because the age of the abandoned underground mines beneath the roadways is continuing to increase. The stability of those mines and the associated overburden strata, at least in many cases, will continue to deteriorate.

5.6.2 Changed Conditions:

Changed conditions may be detected during site monitoring as:

  1. observed changes to previously documented site features, or;

  2. new site features not noted during previous site monitoring. Site reevaluation may also be required as the result of new information developed during Detailed Site Evaluations or Priority Site Investigations.

5.6.2.1 Examples:

Some examples of information which would document the need to reevaluate a site may include:

5.6.2.1.1 New or Changed Surface Deformation Features:  

These types of surface features may be observed on a given site during a routine periodic site monitoring visit, or they may be reported by others and confirmed by inventory personnel

5.6.1.1.2 Changed Site Characteristics Documented Through Development of New Information:

Detailed Site Evaluation and Priority Site Investigations work may reveal a previously unidentified mine shaft location nearby the roadway

5.6.3 Reevaluation Method:

When new changed conditions or information is obtained for a given site, site reevaluation should be performed. This reevaluation should be accomplished by first completing a new Initial Site Evaluation form for the site. The site may be placed in a different risk assessment site group as the result of this reevaluation. Whether or not this occurs, the site should next be further reevaluated by completing a new Detailed Site Evaluation form for the appropriate site group. This work should determine the site's adjusted risk assessment priority in the appropriate site group.

<<Previous TOC Next>>
Updated: 04/07/2011

FHWA
United States Department of Transportation - Federal Highway Administration