ROCK STRENGTH FROM FAILURE CASES: POWERHOUSE SLOPE STABILITY STUDY FORT PECK DAM, MONTANA

THE SLOPE ADJACENT TO THE POWERHOUSES AT FORT PECK DAM HAS BEEN UNSTABLE SINCE 1934 WHEN ITS TOE WAS EXCAVATED DURING CONSTRUCTION OF THE RESERVOIR OUTLET WORKS. THE 1934 EXCAVATION INITIATED PROGRESSIVE SLIDES IN BEARPAW SHALE COLLUVIUM. DEBRIS FROM THESE SLIDES HAS CONTINUED TO MOVE UP TO THE PRESENT TIME. SLIDE ACTIVITY IN THE 1950'S WAS STUDIED BY THE GARRISON DISTRICT, CORPS OF ENGINEERS, DURING DESIGN OF THE SECOND POWERHOUSE AT FORT PECK. RELATIVELY MINOR EXCAVATION AT THE TOE OF THE POWERHOUSE SLOPE IN THE LATE SUMMER AND AUTUMN OF 1970 ACCELERATED MOVEMENT OF THE EXISTING SLIDE DEBRIS AND INITIATED NEW SLIDES DOWNSTREAM FROM THE POWERHOUSES. REFERENCE POINTS IN THE SURFACE OF THE SLIDE DEBRIS ADJACENT TO THE POWERHOUSES MOVED DOWNSLOPE AT AVERAGE RATES OF 0.1 TO 2.1 FT/YR FROM 1953 TO SEPT. 1970 AND AT AVERAGE RATES OF 0.1 TO 2.4 FT/YR FROM SEPT. TO DEC. 1970. REVIEW OF PUBLISHING MOVEMENT DATA ON CLAY SHALE SLOPES INDICATED THAT THESE MOVEMENT RATES ARE TYPICAL FOR LONG TERM CREEP OF OLD SLIDE DEBRIS. MOVEMENT OF SLIDE DEBRIS SLOWED SIGNIFICANTLY WHILE THE GROUND WAS FROZEN DURING THE WINTER OF 1970-71 BUT ACCELERATED DURING THE SPRING THAW OF 1971. AFTER THE SPRING THAW, MOVEMENT AGAIN SLOWED SIGNIFICANTLY. AS LONG AS NO ADDITIONAL EXCAVATION IS DONE IN THE LOWER PART OF THE POWERHOUSE SLOPE, THE PROBABILITY OF CATASTROPHIC MOVEMENT IS CONSIDERED VERY LOW. THE SLIDE DEBRIS ADJACENT TO THE POWERHOUSES IS, HOWEVER, EXPECTED TO CONTINUE CREEPING DOWNSLOPE AT AVERAGE LONG TERM RATES OF 0.2 TO 2.0 FT/YR WITH SEASONAL ACCELERATIONS TO SHORT TERM MOVEMENT RATES OF 2 TO 10 FT/YR. EFFECTIVE STRESS MOHR-COULOMB SHEAR STRENGTH PARAMETERS REQUIRED FOR LIMITING EQUILIBRIUM OF THE 1934 AND 1950'S SLIDE MASSES WERE CALCULATED WITH THE MORGENSTERN-PRICE METHOD OF STABILITY ANALYSIS. THE 1934 SLIDES MOBILIZED AN AVERAGE SHEAR STRENGTH CHARACTERIZED BY THE COHESION INTERCEPT IN TERMS OF EFFECTIVE STRESSES EQUAL TO 0 AND THE ANGLE OF INTERNAL FRICTION IN TERMS OF EFFECTIVE STRESSES EQUAL TO 11.5 TO 14 DEGREES. THE 1950'S SLIDES MOBILIZED AN AVERAGE SHEAR STRENGTH CHARACTERIZED BY THE COHESION INTERCEPT IN TERMS OF EFFECTIVE STRESSES EQUAL TO 0 AND THE ANGLE OF INTERNAL FRICTION IN TERMS OF EFFECTIVE STRESSES EQUAL TO 11.5 DEGREES. THIS IS CONSIDERED THE IN-SITU RESIDUAL STRENGTH OF THE BEARPAW SHALE IN THE POWERHOUSE SLOPE. STABILIZATION MEASURES FOR THE POWERHOUSE SLOPE WERE INVESTIGATED. STABILIZATION MEASURES OTHER THAN FLATTENING THE SLOPE BY EXCAVATION ARE FAVORED BECAUSE OF POSSIBLE DISTURBANCE TO THE SHALE IN THE SLOPE BY EXCAVATION AND RELATED EFFECTS. SUBSURFACE DRAINAGE OFFERS SOME POTENTIAL FOR STABILIZING THE SLIDE DEBRIS ON THE SLOPE AND SHOULD BE INVESTIGATED FURTHER WITH A PILOT DRAINAGE SYSTEM. FLATTENING THE SLOPE TO A UNIFORM 1 ON 6 INCLINATION IS RECOMMENDED IF EXCAVATION IS CHOSEN AS A STABILIZATION MEASURE. A GRADING PLAN WITH A 1 ON 6 SLOPE WAS DEVELOPED FOR THE SLIDE AREA ADJACENT TO THE POWERHOUSES; THE ESTIMATED QUANTITY OF EXCAVATION WAS 1.8 MILLION CU YD. THIS GRADING PLAN DID NOT INCLUDE THE NEW SLIDE AREAS DOWNSTREAM FROM THE POWERHOUSES. MORE INFORMATION MUST BE OBTAINED ON THESE LATTER SLIDE AREAS AND ON POSSIBLE FUTURE POWER PLANT EXPANSION BEFORE STABILIZATION MEASURES CAN BE PLANNED IN DETAIL AND EVALUATED. /AUTHOR/

  • Supplemental Notes:
    • Contract DACW45-70-D-0071 Rept No MRD-1-73 5 APP
  • Corporate Authors:

    Army Corps of Engineers

    Missouri River Division
    Omaha, NE  United States 
  • Authors:
    • Hamel, J V
  • Publication Date: 1972-4

Media Info

  • Features: Figures; References; Tables;
  • Pagination: 160 p.

Subject/Index Terms

Filing Info

  • Accession Number: 00236682
  • Record Type: Publication
  • Report/Paper Numbers: Final Rpt
  • Files: TRIS
  • Created Date: Jun 26 1974 12:00AM