Landslides and Road Deactivation

1
Landslides and Road Deactivation

• Terry Rollerson, Mike Wise,
• Denis Collins, Wilson Muir,
• Russ Wong, Tom Millard

2
Introduction

• We have seen a number of landslides associated
  with road deactivation
• We need to identify where this is occurring
• We need to prevent or minimize future occurrences

3
Topics of Discussion

• Study approach
• Types of road deactivation landslides
• Contributing factors
• Prevention
• Real Life Examples
• Conclusions

4
Study Approach

• Compilation of road deactivation landslide
  occurrences
• Site visits to a selected sample of the
  landslides
• Compilation of data and analysis to determine the
  most common relationships between road
  deactivation and landslides

5
Landslide Inventory Approach

• Document as many landslides as possible
• Visit a selected sample of landslides
• Link landslides to terrain types
• Link landslides to deactivation techniques
• Assess contributing factors where feasible
• Assess likelihood of prevention or non-prevention

6
Landslide Data - on-site

• type of deactivation
• type of landslide
• slope position
• slope gradient
• terrain
• soil type
• slope morphology

• down slope environmental effects
• contributing factors
• preventable / not preventable
• signs of incipient failure (slumping, tensions
  cracks)

7
Types of road deactivation related landslides

• Fill slope failures in areas of no deactivation
• Failures below x-ditches (fill / native slope)
• Failures in partial pull back - residual fills
• Failures in partial pull back with x-ditches
• Failures in partial pull back at gullies
• Cut slope failure (all types of deactivation?)

8
Contributing Factors

• Overloading of native slope by residual fills
• Oversteepening of fill materials
• Concentration of water by ditch lines and
  x-ditches
• Delivery of water from cut slope seepage sites or
  gullies by x-ditches to residual fills
• Loss of toe support (cuts in deep materials)

9
Prevention

• Deactivate the entire road system
• Full pull back (reconturing) on slopes gt60
• Outsloping rather than x-ditching where slopes
  below the road are gt50 to avoid concentration of
  road drainage
• Trenching of x-ditches to native ground when
  draining seepage sites or surface stream channels
  (remove all residual fill)

10
Real Life - 1

• Situation
• Lower roads deactivated but not the back end
• Contributing factors
• Short-term planning / decisions
• Oversteepening, overloading and drainage
• Prevention
• Proper planning / assessment and deactivation
  from the back end out

11
Real Life - 2

• Situation
• Partial pull back
• Contributing factors
• Oversteepening
• Overloading
• Prevention
• Full pull back where safety is not compromised
• Blasting in isolated locations may be feasible

12
Real Life - 3

• Situation
• Partial pull back with x-ditches
• Contributing factors
• Oversteepening and overloading
• Additional water
• Prevention
• Full pull back where safety permits
• Trench x-ditches to native ground

13
Real Life - 4

• Situation
• X-ditches only, landslides on 50-60 slopes
  below roads
• Contributing factors
• Excess water diverted onto slope
• Prevention
• Outsloping or very very frequent x-ditches
• Partial pull back /or decompaction

14
Real Life - 5

• Situation
• Cut slope failure above partial pull back
• Same as with no deactivation - fairly rare
• Contributing factors
• loss of toe support
• Prevention
• Leveling top of partial pull back
• In most cases not preventable

15
Real Life - 6

• Full pull back
• No landslides but may get minor sediment
  redistribution

16
Lesson learned The right technique in the right
place

• Always deactivate the back end, even if the costs
  seem high
• Full pull back on slopes gt 60 (50?)
• Trench to native ground at seepage sites and
  surface drainage channels on slopes gt 50?
• Disperse water when slopes below gt 50

17
Next Steps

• If you feel the prescription is not right say so,
  or do more
• If you see new landslides on or immediately below
  deactivated roads take a closer look, let us know
• If you see slumping or tensions cracks on a
  section of deactivated road, do the same
• We will continue investigating these events

18
Contact us at

• Vancouver Forest Region
• 2100 Labieux Road
  
• Nanaimo, B.C. , V9T 6E9
  
• fax - 250-751-7198
  
• e-mail
  
• Denis.Collins_at_gems4.gov.bc.ca
• Tom.Millard_at_gems8.gov.bc.ca

19
Conceptual frequency plot of deactivation
landslide types
20
Road / Slope Geometry
21
Road deactivation landslide report