Talk Outline

1. 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 UngermanDepartment of Earth Science, Utah Valley University michael.bunds@uvu.edu

2. Talk Outline Overview of landslide Objectives & methods Results Interpretations and Implications

3. Sherwood Hills Landslide Location 500 m N Wasatch fault

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

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

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

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

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

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

10. Water Table and Displacement Rate • Displacement rates are minimums for time periods between surveys • Water table heights are average of measurements over 1 month prior to survey • Best-fit model is exponential, • DR = 1.49 *e(1.05*WT) + 19.6 • R2 = 0.78 • Implies maximum DR ~ 7 m/yr if WT were to reach ground surface, but this is sensitive to data and model fit • DR = 1.49*e(1.05*WT) + 19.66 • R2 = 0.78 Minimum Displacement Rate, mm/yr ground surface Water Table Height, m water table Slump Displacement (dm) Water Table Height (m) slump displacement Date

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

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

13. Rheological Model Derivation • 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 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 WT • Shear stress, ss • Magnitude in absence of Pfcalc’d using r=2000kg/m3, 8.2m depth, glide plane dip, fundamental eqn’s of stress, assume shorizontal=0 • 57.4 & 27.6 kPa for 22.7 & 10o dips • Effect of WT change (‘effective shear stress’) • Calculate change inssequivalent to change in effectivesn caused by WT rise using,ss=m*(sn – Pf) + Co:Dss= m*Pf • For 1 m rise in WT Pf increases ~9.8kPa,Dss= 0.227*9.8kPa = 2.1 kPa

14. Rheological Model Results • Like WT vs DR, highly nonlinear; exponentials provide best-fit curves • Closer to brittle behavior than more linear response (e.g., power-law) • 22.7o dip: de/dt= 6.5E-23*e(0.48*ss) + 1.25E-9 • 10o dip: de/dt = 1.3E-16*e(0.48*ss) + 1.25E-9 • R2= 0.78 Strain rate, 1/s 10o dip 22.7o dip Effective shear stress, kPa

15. Conclusions • Slump 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 years • Slump boundaries are complicated/diffuse • Slump 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 fit • Strain rate in glide plane as a function of effective stress is well modeled by exponential function

17. EXTRAS

18. Some Student Participants in this Work Ryan Mower Robert White Paul Gardner Victoria Sailer Adam Sealey Jessica Oxford Ben Erickson Ashley Elliot Phil Goble Colton Norman Brittany Ungerman Patrick Lowe

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

20. Landslide Displacements

21. House damage near head scarp

22. Slump Motion

23. GPS base set-up

24. RTK rover with bipod and Zephyr Geodetic antenna

25. Survey marker drilled in road curb

26. 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.

27. Near headscarp north end toe