SCHEDULE

OUTLINE In Class Exam 2 Landsliding Types Mechanics Sensitivity Probability

MIDTERM 2a Mean: 14.4 Range: Class performance is now where previous classes were by finals

MIDTERM 2a Which is most difficult? 1/4 description 1/4 coverages 1/4 functions 1/4 code

Landslide Type: Deep-Seated Large Gradual Lithologic Control Groundwater Control

Landslide Type: Shallow Rapid Small Rapid Root Control Saturation Control

Landslide Mechanics Factor of Safety: Ratio of Forces FS=resisting/initiating FS<1: landslide FS>1: no landslide

Landslide Mechanics q o is the surcharging weight of the vegetation  is the angle of the slope  is the friction angle of the soil C is the soil cohesion C r is the root reinforcement h is the soil thickness z is the saturated thickness g w is the unit weight of water g s is the unit weight of saturated soil

Stability Calculation FS_grid q0_grid slope_grid phi_grid Cr_grid C_grid z gw gs_grid h_grid

Computational Process FS_grid Stand cover Soil cover dem Grids of Slope, Cohesion, friction... literature

Example Using q 0 =10-40 psf C=0 psf C r = psf phi=35-40 deg g s =150 pcf g w =72 pcf

Problem 1: Equation usually doesn’t predict cells that slid, but predicts sliding in cells that didn’t. Reasons: uncertainty in parameters uncertainty in process Solution: Pseudo probability: FS is only correlated with landsliding

Problem 2: Parameters are uncertain/variable Solution: Model uncertainty in each parameter, i.e. Cohesion might be within range (3-10 kPa) 1. Choose a value for each parameter 2. Calculate FS 3. Repeat 4. Calculate fraction of times FS<1

Problem 3: How do you model soil saturation? Solution: Assume steady state rainfall Q in =Q out q*w*d=P*fa P c =d c *(qw)/fa where d c is the critical value of z that makes FS=1 z>h :never slides z<0 :always slides

Problem 4: Isn’t this all too complicated to use? Solution: Map hazard ratings (low/moderate/high)

Problem 5: What are the consequences of management? Solution: Observe fraction cells that slid with each mgt Assume similar fraction for future mgt Map/calculate resulting landsliding

Example If we map the topography as high/mod/low And map management as road/cut/none We can make a 3x3 table of hazard/mgt We can calculate fraction of cells in each zone that have slid We can ‘what if’ on other road & harvest plans