Download presentation
Presentation is loading. Please wait.
1
Modeling Landscapes - Force Balance
Landslides - brittle failure Debris Flows- plastic materials Rockslide spawns debris flow in British Columbia, GSC
2
FAILURE OCCURS WHEN THE STRESS ON MATERIAL EXCEEDS THE STRENGTH OF THAT MATERIAL.
A deposit at equilibrium can tell you about material strength since you can calculate driving force GRAVITY ~ shear stress ~ shear strength FRICTION AND COHESION
3
Simple Physics……vectors
DRIVING force (SHEARING STRESS) is gravity, RESISTING force is soil/rock strength/FRICTION Steep slopes give greater Driving Force (STRESS) Vectors: length=magnitude
4
FACTOR OF SAFETY Resisting force Driving force >1, slope is stable
<1 slope fails Failure driven by transient event, superimposed on geology
5
LANDSLIDES OCCUR ON A FAILURE SURFACE
LANDSLIDES OCCUR ON A FAILURE SURFACE. ALL STRENGTH ANALYSES ARE MADE IN REFERENCE TO THE FAILURE SURFACE. Translation - flat plane rotation - circular arc
6
SHEAR FORCE h IS THE THICKNESS OF THE SLAB MEASURED
PERPENDICULAR TO THE SURFACE, meters IS THE DENSITY OF THE SLAB MATERIAL, kg m-3 IS THE SLOPE ANGLE OF THE FAILURE SURFACE g IS THE ACCELERATION OF GRAVITY, 9.8 m s-2 SHEAR FORCE
7
GREATER THE NORMAL FORCE THE GREATER THE RESISTING FORCE
FRICTIONAL STRENGTH GREATER THE NORMAL FORCE THE GREATER THE RESISTING FORCE What is normal anyhow? NORMAL FORCE
8
NORMAL FORCE () h IS THE THICKNESS OF THE SLAB MEASURED
PERPENDICULAR TO THE SURFACE, meters IS THE DENSITY OF THE SLAB MATERIAL, kg m-3 IS THE SLOPE ANGLE OF THE FAILURE SURFACE g IS THE ACCELERATION OF GRAVITY, 9.8 m s-2 NORMAL FORCE ()
9
"c" COHESION IS THE TENDENCY OF A MATERIAL TO "STICK" TO ITSELF
"c" COHESION IS THE TENDENCY OF A MATERIAL TO "STICK" TO ITSELF. CLAYS ARE COHESIVE AS ARE ROCKS OR SOILS WHICH HAVE UNDERGONE SOME CEMENTATION. COHESION IS EXPRESSED IN PASCALS (kg m-1 s-2). CLAYS HAVE COHESIONS RANGING FROM TO PASCALS. TILLS HAVE COHESIONS ABOVE PASCALS. OR THE ANGLE OF INTERNAL FRICTION IS A PROPERTY OF THE MATERIAL AND RANGES FROM OVER 50o FOR COARSE GRAVEL TO LESS THAN 30o FOR FINE SAND. IS THE NORMAL FORCE EXCERTED ON THE FAILURE SURFACE BY THE MATERIAL ABOVE IT.. THE NORMAL STRESS CAN BE EXPRESSED IN PASCALS (kg m-1 s-2)
10
How does one find parameters?
Shear stress or strength Normal Force
11
What does each parameter mean?
Hint..this is the equation of a straight line!
12
Soil Strength Parameters (Selby, 1993)
13
Strength or shear force
Normal stress (kN/m2)
14
WATER MATTERS SUBTRACTING VECTORS
FORCE BALANCE MAY TIP BOUYANCY FORCE WET NORMAL FORCE High water table (pore pressure) reduces effective normal force
15
u IS THE PRESSURE OF THE WATER AT THE FAILURE PLAIN WHICH CAN BE EXPRESSED IN PASCALS (kg m-1 s-2) USING THE EQUATION BELOW. h' IS THE ELEVATION OF THE WATER TABLE ABOVE THE FAILURE SURFACE (meters).
16
Factor of Safety Calculating Formula Includes pore pressure
17
Effective Cohesion -- Vegetation Effects
from Selby, 1993 Orgeon slide in clearcut
18
CLEARCUTTING AND LANDSLIDES
Slide scars CLEARCUTTING AND LANDSLIDES
19
100 Till 10 Sand 1 0.1 4 8 12 Factor of Safety
100 Till o (c = 150 kpa, = 33.5 ) 10 Sand Factor of Safety o (c = 1 kpa, = 37 ) 25o slope 1 35o slope saturated 0.75 m slab 0.1 4 8 12 Effective Root Cohesion (kPa)
20
Bishop method of slices
Rotational Failures
21
http://www. uky. edu/AS/Geology/howell/goodies/elearning/module11swf
22
What is the shear strength of debris flow material in Death Valley?
Rise/run 60’/1200’= 0.02 What is the shear strength of debris flow material in Death Valley? 2000 kg m-3 * 9.8 m s-2 * 3 m * sin g h sin 2000 kg m-3 * 9.8 m s-2 * 3 m * 0.02 (h) Measured thickness = 3 m (measured)= 2000 kg m-3 g (given) = 9.8 m s-2 (from map) = tan -1 (0.02) = 1.15o = 1176 m-1 s-2 (pascals) = 1.2 kp
Similar presentations
© 2025 SlidePlayer.com. Inc.
All rights reserved.