18E.100.040 – Appendix C Geological Assessment – Mine Hazard Geotechnical Report
A. A geotechnical report shall include the following:
1. The first page of the document shall be labeled identifying the submittal as a "Mine Hazard Geotechnical Report."
2. The date when the geological assessment was performed and the date when the report was prepared.
3. The parcel number(s) of the site.
4. Site address, if one has been assigned by the County.
5. A brief description of the project (including the proposed land use) and a description of the area to be developed.
6. A description of the surface and subsurface geology, hydrology, soils, and vegetation on the site and a list of the mine hazard indicators, as set forth in PCC 18E.100.020 B., that were found on or in the vicinity of the site.
7. A description of the analytical tools and processes that were used to develop the report.
8. Surface exploration data such as borings, drill holes, test pits, wells, geologic reports, and other relevant reports or site investigations that may be useful in making conclusions or recommendations about the site under investigation.
9. A description of historical data and information used in the evaluation, together with sources. Such data and information shall include:
a. Topographic maps at a scale and contour interval of sufficient detail to assess the site. The site boundaries and proposed site development shall be overlain with the mine plan view map.
b. Copies of illustrative mine maps showing remnant mine conditions, if available.
c. Aerial photography, as appropriate.
d. Geological data including geologic cross-sections and other illustrative data as appropriate.
e. Available historic mine records indicating:
(1) The dates of operation.
(2) The date of cessation of active mining.
(3) The number of years since abandonment.
(4) Mining methods used and shoring and timbering information.
(5) The strength of the overlying rock strata.
(6) The extracted seam thickness.
(7) The dip or inclination of the strata, workings, and surface.
(8) The projected surface location of the seam outcrop or subcrop.
(9) The estimated depth of the seam outcrop or subcrop, if covered by glacial outwash, glacial till, or other materials at depth.
(10) Total material tonnage produced, estimated mine by-product material produced, and the estimated extraction ratio.
10. A summary of the results, conclusions, and recommendations resulting from the geological assessment of the mine hazards on or in the vicinity of the site. This summary shall address all of the information required in PCC 18E.100.030 B.
11. An accurate site plan drawn at a scale of 1" = 20', 1" = 30', 1" = 50' (or other scale deemed appropriate by the Department) is required. The Department may require that the site plan information listed below be based on a field survey by a licensed surveyor. The site plan shall include:
a. The limits/location of the mine hazard area(s), as set forth in PCC 18E.100.020 A. The mine plan view map shall be reproduced at the same scale as the topographic map, showing the location of the mine, the extent of the mining, the proposed site development, if applicable, and any remnant abandoned mine surface features. At a minimum this shall include:
(1) The layout of the underground mine.
(2) The location of any mine entries, portals, adits, mine shafts, air shafts, timber shafts, and other significant mine features.
(3) The location of any known sinkholes, significant surface depressions, trough subsidence features, coal mine spoil piles, and other mine-related surface features.
(4) The location of any prior site improvements that have been carried out to mitigate abandoned mine features.
(5) Zones showing varying overburden-cover-to-seam-thickness ratios, when appropriate.
(6) Cross-sections of the estimated depth of any mine workings.
b. The location of any existing and proposed structures, utilities, on-site septic systems, wells, and stormwater management facilities.
c. The full geographical limits of the proposed project area (area to be developed).
d. The location and unique identifier of geotechnical borings, CPT soundings, or other survey or explorations used to characterize subsurface conditions.
e. Extent of cross-section(s) used to evaluate the three-dimensional subsurface geologic and groundwater conditions at the site.
f. Extent of cross-section(s) used in the evaluation of subsurface instability.
g. Dimension the closest distance between the identified mine hazard area boundary and the project area.
h. Existing topography on the site presented in 2-foot contours.
i. Property lines for the site.
j. North arrow and plan scale.
12. A statement as to the relative degree of accuracy and completeness of the maps and information reviewed, especially regarding historic mine map accuracy, and reasons why such sources are considered reliable for the purposes of this report.
13. A detailed description of any prior grading activity, soil instability, or ground failure.
14. Analysis and recommendations, if any, of the potential for future trough subsidence and special mitigation.
15. Assessments and conclusions regarding ground stability for both the existing and developed conditions shall be presented and documented. These assessments and conclusions shall include:
a. Evaluation of the potential types of ground failure mechanisms that may affect the site.
b. Quantitative stability evaluation of conditions of the various failure mechanisms using state-of-the-practice modeling techniques. Limiting equilibrium methods of analysis shall state the stability conditions as a factor of safety. The most unstable failure geometry(ies) shall be presented in the form of a cross-section(s), with the least stable failure geometry for each failure mechanism clearly indicated. The stability evaluation shall also consider dynamic (earthquake) loading, and shall use a minimum horizontal acceleration as established by the current version of the Washington State Building Code.
16. Mitigation recommendations using engineered measures to protect the structure(s) and any adjacent structures, infrastructure, or adjacent wetlands or critical fish and wildlife habitat from damage or destruction as a result of proposed construction activities shall be designed by a professional engineer. The geotechnical report shall contain:
a. Design plans and associated design calculations for engineered structures or drainage systems (e.g., structural foundation requirements, retaining wall design, etc.).
b. Recommendations and requirements pertaining to the handling of surface and subsurface runoff in the developed condition.
c. Identification of necessary geotechnical inspections to assure conformance with the report mitigation and recommendations.
d. Proposed angles of cut and fill slopes, site grading requirements, final site topography (shown as 2' contours), and the location of any proposed structures, on-site septic systems, wells, and stormwater management features or facilities associated with the development detailed within the body of the report and shown on a site map at the same scale as that required in Section A-8 of this Appendix.
e. Soil compaction criteria and compaction inspection requirements.
17. A list of references utilized in preparation of the report.
B. The geotechnical report shall be prepared by an engineer with documentable experience in coal mine hazard investigation and mitigation and shall be co-written by a geotechnical professional where geological interpretations are necessary or prudent in the mitigation of the mine hazard. The geotechnical report shall be prepared under the responsible charge of an appropriately licensed geotechnical professional(s) and be signed, sealed and dated by the geotechnical professional(s).
C. The geotechnical professional(s) who prepared the geotechnical report shall stamp the report with his or her license stamp/seal and provide a statement of qualifications.
D. The Department may request a geotechnical professional to provide additional information in the geotechnical report based upon existing conditions, changed conditions, or unique circumstances occurring on a case by case basis.