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Improved Hazard Assessment, Effect of Water Table Height on Displacement Rate, and Failure Plane Rheology from Nine Years of Monitoring of the Sherwood Hills Slump, Provo, Utah Michael P. Bunds, Daniel Horns, Brittany Ungerman Department of Earth Science, Utah Valley University michael.bunds@uvu.edu

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Overview of landslideOverview of landslide Objectives & methodsObjectives & methods ResultsResults Interpretations and ImplicationsInterpretations and Implications Talk Outline

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Sherwood Hills Landslide Location N 500 m Wasatch fault

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Overview of Landslide Rotational slump; Manning Cyn. Shale on failure planeRotational slump; Manning Cyn. Shale on failure plane Suburban setting, one house destroyed, several significantly damaged,Suburban setting, one house destroyed, several significantly damaged, Became evident in ~1998 after movement following wet winterBecame evident in ~1998 after movement following wet winter Scarp Shortening in toe Locally evident headscarp

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Undergraduate student-driven researchUndergraduate student-driven research Long term (since 2004): track movement of the slumpLong term (since 2004): track movement of the slump Delineate extent of slumpDelineate extent of slump Compare sensitivity of measured displacement to other movement indicatorsCompare sensitivity of measured displacement to other movement indicators Track WT height, relate it to displacementsTrack WT height, relate it to displacements Study Objectives

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Methods Displacements measured using RTK GPS; referenced to off- slump stable markersDisplacements measured using RTK GPS; referenced to off- slump stable markers ~1 cm precision on bulk slump movement referenced to stable off-slump markers~1 cm precision on bulk slump movement referenced to stable off-slump markers ~2 to 5 surveys per year since 2004~2 to 5 surveys per year since 2004 Water table heights measured by UGS and usWater table heights measured by UGS and us

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Landslide Displacements Max 65.8 cm motion since March 2004Max 65.8 cm motion since March 2004 Yellow/green vectors are total accumulated displacement from March 2004 to May 2013Yellow/green vectors are total accumulated displacement from March 2004 to May 2013 Red vectors are displacement from September 2006 to May 2013Red vectors are displacement from September 2006 to May 2013 Well SH1 ~X-section line Modified from Ashland, 2006

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Slump Displacement and Water Table Height Displacement increases during wet, high WT yearsDisplacement increases during wet, high WT years Slump moves during dry (& wet) yearsSlump moves during dry (& wet) years Slow movement during dry years does not produce features that record movementSlow movement during dry years does not produce features that record movement Slump may have been active when decision to develop was madeSlump may have been active when decision to develop was made Slump Displacement (dm) Water Table Height (m) Date slump displacement water table Well SH1

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Implications: Landslide Boundaries Larger & more consistent movement to north than expectedLarger & more consistent movement to north than expected Diffuse and/or complicated boundariesDiffuse and/or complicated boundaries Where is the toe?Where is the toe? ? ? Modified from Ashland, 2006

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Water Table and Displacement Rate Minimum Displacement Rate, mm/yr Water Table Height, m Displacement rates are minimums for time periods between surveys Displacement rates are minimums for time periods between surveys Water table heights are average of measurements over 1 month prior to survey Water table heights are average of measurements over 1 month prior to survey Best-fit model is exponential, Best-fit model is exponential, DR = 1.49 *e (1.05*WT) + 19.6 DR = 1.49 *e (1.05*WT) + 19.6 R 2 = 0.78 R 2 = 0.78 Implies maximum DR ~ 7 m/yr if WT were to reach ground surface, but this is sensitive to data and model fit Implies maximum DR ~ 7 m/yr if WT were to reach ground surface, but this is sensitive to data and model fit ground surface DR = 1.49*e (1.05*WT) + 19.66 R 2 = 0.78 Slump Displacement (dm) Water Table Height (m) Date slump displacement water table

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Rheological Model Assumes Mohr-Coulomb rheology Assumes Mohr-Coulomb rheology Equate effect frictional state at elevated WT (& Pf) to an equivalent effective shear stress (at 0 Pf) Equate effect frictional state at elevated WT (& Pf) to an equivalent effective shear stress (at 0 Pf) 0.21 friction coefficient (φ = 12 o ) [Derived from method of slices FoS=1.01] 0.21 friction coefficient (φ = 12 o ) [Derived from method of slices FoS=1.01] Two geometries Two geometries initial state of stress Effect of WT on state of stress = 0.21 state of stress at elevated WT effect of P f (WT) = 0.21 Equivalent shear stress initial state of stress shear stress equivalent to P f (WT) change Minimum Displacement Rate, mm/yr Water Table Height, m Strain rate, 1/s Effective shear stress, kPa

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Rheological Model Geometries 22.7 o Glide Plane 10 o Glide Plane Modified from Ashland, 2006

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Rheological Model Derivation Relates ‘effective shear stress’ on glide plane to shear strain rate (de/dt)Relates ‘effective shear stress’ on glide plane to shear strain rate (de/dt) Strain rate derived directly from displacement rate: de/dt = DR/(glide plane thickness) = DR/0.5 mStrain rate derived directly from displacement rate: de/dt = DR/(glide plane thickness) = DR/0.5 m Effective shear stress is change in shear stress that produces same change in frictional stability as increase in pore fluid pressure from a rise in WTEffective shear stress is change in shear stress that produces same change in frictional stability as increase in pore fluid pressure from a rise in WT Shear stress, sShear stress, s Magnitude in absence of P f calc’d using =2000kg/m 3, 8.2m depth, glide plane dip, fundamental eqn’s of stress, assume horizontal =0Magnitude in absence of P f calc’d using =2000kg/m 3, 8.2m depth, glide plane dip, fundamental eqn’s of stress, assume horizontal =0 57.4 & 27.6 kPa for 22.7 & 10 o dips57.4 & 27.6 kPa for 22.7 & 10 o dips Effect of WT change (‘effective shear stress’)Effect of WT change (‘effective shear stress’) Calculate change in s equivalent to change in effective n caused by WT rise using, s = * n – P f ) + C o : s = * P fCalculate change in s equivalent to change in effective n caused by WT rise using, s = * n – P f ) + C o : s = * P f For 1 m rise in WT P f increases ~9.8kPa, s = 0.227*9.8kPa = 2.1 kPaFor 1 m rise in WT P f increases ~9.8kPa, s = 0.227*9.8kPa = 2.1 kPa

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Effective shear stress, kPa Rheological Model Results Like WT vs DR, highly nonlinear; exponentials provide best-fit curvesLike WT vs DR, highly nonlinear; exponentials provide best-fit curves Closer to brittle behavior than more linear response (e.g., power-law)Closer to brittle behavior than more linear response (e.g., power-law) 22.7 o dip: de/dt = 6.5E-23+ 1.25E-922.7 o dip: de/dt = 6.5E-23 *e (0.48* s) + 1.25E-9 10 o dip: de/dt = 1.3E-16* e (0.48* s) + 1.25E-910 o dip: de/dt = 1.3E-16* e (0.48* s) + 1.25E-9 R 2 = 0.78R 2 = 0.78 Strain rate, 1/s 22.7 o dip 10 o dip

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Conclusions Slump most likely was active prior when decision to develop neighborhood was madeSlump most likely was active prior when decision to develop neighborhood was made Slump moves every year but only produces obvious geomorphic evidence during wet, large displacement yearsSlump moves every year but only produces obvious geomorphic evidence during wet, large displacement years Slump boundaries are complicated/diffuseSlump boundaries are complicated/diffuse Slump displacement is highly dependent on water tableSlump displacement is highly dependent on water table Data suggest ~7m/yr displacement rate is possible result in the unlikely event WT were to reach ground surface, but this is sensitive to data and model fitData suggest ~7m/yr displacement rate is possible result in the unlikely event WT were to reach ground surface, but this is sensitive to data and model fit Strain rate in glide plane as a function of effective stress is well modeled by exponential functionStrain rate in glide plane as a function of effective stress is well modeled by exponential function

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EXTRAS

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Some Student Participants in this Work Ryan MowerRyan Mower Robert WhiteRobert White Paul GardnerPaul Gardner Victoria SailerVictoria Sailer Adam SealeyAdam Sealey Jessica OxfordJessica Oxford Ben EricksonBen Erickson Ashley ElliotAshley Elliot Phil GoblePhil Goble Colton NormanColton Norman Brittany UngermanBrittany Ungerman Patrick LowePatrick Lowe

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Survey Reference Point Stability Position since 2006 determined using NGS OPUS system – regional frameworkPosition since 2006 determined using NGS OPUS system – regional framework Within error, reference point has not movedWithin error, reference point has not moved

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Landslide Displacements Motion (cm) Survey markerTotalazimuthplunge mhd1365.1229.923.1 mhd1565.8230.224.8 mhd2055.0234.220.1 Average62.0231.422.7average

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House damage near head scarp

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Slump Motion

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GPS base set-up

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RTK rover with bipod and Zephyr Geodetic antenna

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Survey marker drilled in road curb

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Horizontal Error RTK Rapid Point Method using bipod set-up, Zephyr Geodetic antenna Single measurement + 1.33 cm, 95% c.i. Four measurements + 0.55 cm, 90% c.i.

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36.7 37.6 31.9 32.3 33.4 40.5 40.8 38.3 36.7 34.1 29.1 13.3 9.2 9.4 11.6 13.3 11.0 39.9 36.5 24.0 15.5 6.1 0.8 2.2 0.7 Near headscarp north end toe

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