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TRANSCRIPT
H. JAMES LAW SCS ENGINEERS
USA
MAJOR PARAMETERS THAT AFFECT OUTCOME OF LANDFILL SLOPE STABILITY MODELING
INTRODUCTION • EXAMPLES OF PAST LANDFILL FAILURES • STABILITY MODELING
-‐ TWO MAJOR PARAMETERS -‐ SENSITIVITY STUDIES
• CASE STUDY RESULTS • CONCLUSION
SLOPE STABILITY MODELING
We can build almost anything to last, sound, and STABLE structure... -‐ Bridges, sky-‐scrapers, or something fun such as New Mossley Bonfire, Belfast
What about sanitary landfills? -‐ Stable, yet protect health & the environment!
EXAMPLES OF PAST LANDFILL FAILURES USA Landfill, Ohio; Mar 1996
• Ref. Stark, 2008 – Waste was placed 25m above then
exisYng waste mass in less than 8 months; quick loading; high liquid level
– ExcavaYon at the toe; frozen toe over the winter; liquid back-‐up (recirculaYon)
– Failed on March 11, 1996; waste moved 300m with >1M m3
EXAMPLE OF WASTE MASS SLIDE Hiriya Landfill (unlined), Israel; 1997
• Ref. Stark, 2008 – 1:1.3 to 1:1.6; ave. 1:1.4(H) – 43 to 64 m ; ave. 56 m waste height – Operated from 1952 – 1998 – Winter of 1997: a major slide occurred – Leachate level: 7 to 23 m below plateau surface
EXAMPLE OF WASTE MASS SLIDE Payatas Landfill, Philippines, 2000
• Ref. Stark, 2008 – Unlined, operated since 1973 – Waste was pushed over the brink of landfill to create more space
for landfilling but also ended up with very steep slope – 1:1.5 – 30 m waste height prior to failure – Heavy rain from two typhoons – 68 cm – Failure in July 10, 2000; 1.2M m3 slide and over 250 fataliYes
EXAMPLE OF LANDFILL FAILURE USA Landfill, 2011; aher heavy rain events
Source: Tomlin, 2011
EXAMPLE OF FINAL COVER SLIDE Chrin Landfill, Kentucky; Apr 2013
www trbimg com
• Veneer failure due to steep slope, cover soil saturaYon, and presence of geosyntheYc layers
• Can trigger a more massive deep-‐seated waste mass failure below
EXAMPLE OF WASTE MASS SLIDE Big Run Landfill, Kentucky Sep 2013
• hjps://kydep.wordpress.com/2014/04/24/ – An 3.5ha waste mass slide on 4/9/2013 – 800,000 tons MSW (about 1 yr of waste
disposal) moved 120m – Depth of slide is about 6m – No liner damage except about 0.1ha at
toe
STABILITY MODELING – MAJOR PARAMETERS • Bojom liner system interface with lowest shear
strength • Landfill leachate level above bojom liner system • Final landfill slope configuraYon (slope steepness &
slope height) • Waste shear strength (changes with decomposiYon);
in-‐situ waste density (varies by compacYon effort) • FoundaYon soils & groundwater level below landfill • Landfill gas management pracYces • Landfill operaYons & filling sequences • Surface run-‐on & run-‐off controls
SENSITIVITY STUDIES
• Design Phase – IdenYfy the Bojom liner system’s weakest interface with lowest shear strength; material selecYons
• OperaYon Phase – Measure landfill leachate level above bojom liner system; from gas wells, cleanouts, leachate sumps
SLOPE STABILITY INDICATOR
• Landfill Slope Stability – Expresses by calculaYng its Factor of Safety (FS) against sliding
• FS = RaYo of resisYng forces over driving forces. FS = 1.5 meaning 50% more strength than required for equilibrium. FS = 1.2 meaning 20% more strength than required for equilibrium
• Mode of Failures – Shallow/veneer transiYonal; deep-‐seated, global circular within waste or block-‐type along weakest liner system interface
PUBLISHED INTERFACE SHEAR STRENGTHS
TYPICAL SLOPE SECTION PROFILE
• Max. Waste Height = 70 m • Perimeter Berm Height = 3 m
SENSITIVITY STUDY -‐ INTERFACE SHEAR STRENGTH vs FS
• Leachate level – assumed zero
SENSITIVITY STUDY – LENGTH OF TEXTURED LINER vs FS
• Interface shear strength – smooth = 11˚ • Interface shear strength – textured = 18˚
SENSITIVITY STUDY – LEACHATE LEVEL vs FS
CASE STUDY RESULTS – LEACHATE LEVELS ON FS
CONCLUSION Ø When performing site-‐specific slope stability modeling –
§ A need to idenYfy bojom liner system Weakest interface. Use of textured instead of smooth geomembrane will increase FS against slope instability.
§ A need to expect potenYal leachate head on liner, based on landfill operaYons plan, SWM plan, and weather condiYons. An elevated leachate head on liner will decrease FS to a point triggering instability of the landfill mass.
§ Take into account actual slopes, material properYes, liquid levels and other factors during design phase.
CONCLUSION (cont’d)
Ø Landfill operator should monitor the performance of the leachate collecYon system to confirm that the condiYons assumed in the stability analyses are sYll valid.
Ø addiYonal monitoring and controls should be implemented judiciously, such as landfill surface posiYve drainage and applicaYon of daily cover soil, to verify that landfill operaYons are not having an adverse impact on slope stability.
