MASS WASTING

A. Introduction

• downslope movement of material due to gravity

• classification based on type and speed of movement
  

B. Types of Mass Movement

1. Creep  

a. seasonal creep: movement of soil by heave

• caused by expansion & contraction due to:

  • freeze-thaw

  • wetting & drying

• operates in upper few feet of soil

b.continuous creep: strain response to stress generated by weight of overburden

• driven by gravity alone, not heave

• may affect consolidated rock

• functions at levels well below the surface

2. Falls: abrupt free fall of loosened blocks or boulders of solid rock

a. bedding, jointing & fracturing of bedrock important

b. failure may be initiated by:

• weathering

• water pressure in joints

• undercutting   

3. Slides

• failure of material at depth & movement along a well defined planar surface

• material moves as a cohesive block or series of blocks bounded by several slip surfaces

a. translational slides: failure along a nearly planar surface 

b. rotational slides: failure along a concave surface     

4. Flows: deformation of an entire mass that flows downslope as a viscous fluid

a. earth flows photo credits: http://landslides.usgs.gov/learningeducation/images/slumgullion/slumcolor1.jpg
http://landslides.usgs.gov/learningeducation/images/slumgullion/slumcolor2.jpg

b. debris or mud flows   

C. Predicting Slope Failure
map credit: USGS 

1. Identifying old failures

a. air photos & field surveys

b. features to look for:

• joint & fracture patterns

• semi-circular scarps

• hummocky, irregular surfaces

• vegetation differences

• moisture differences

• leaning trees, utility poles

• disturbed cultural features (e.g. cracked roads)

2. Predicting future failures  

• Factor of Safety = ratio of resisting forces to driving forces

D. Summary

• gravity is the driving forces, not flowing water

• classification by type of movement & speed: creep, falls, slides, flows

• where failures have occurred in the past, they can occur in the future, so recognizing old failures is important, as well as understanding their causes