Landslides and Road Deactivation Terry Rollerson, Mike Wise, Denis Collins, Wilson Muir, Denis Collins, Wilson Muir, Russ Wong, Tom Millard

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

Topics of Discussion n Study approach n Types of road deactivation landslides n Contributing factors n Prevention n Real Life Examples n Conclusions

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

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

Landslide Data - “on-site” n type of deactivation n type of landslide n slope position n slope gradient n terrain n soil type n slope morphology n down slope environmental effects n contributing factors n preventable / not preventable n signs of incipient failure (slumping, tensions cracks)

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

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

Prevention n Deactivate the entire road system n Full pull back (reconturing) on slopes >60% n Outsloping rather than x-ditching where slopes below the road are >50% to avoid concentration of road drainage n Trenching of x-ditches to native ground when draining seepage sites or surface stream channels (remove all residual fill)

Real Life - 1 n Situation: u Lower roads deactivated but not the back end n Contributing factors u Short-term planning / decisions u Oversteepening, overloading and drainage n Prevention u Proper planning / assessment and deactivation from the back end out

Real Life - 2 n Situation: u Partial pull back n Contributing factors u Oversteepening u Overloading n Prevention u Full pull back where safety is not compromised u Blasting in isolated locations may be feasible

Real Life - 3 n Situation: u Partial pull back with x-ditches n Contributing factors u Oversteepening and overloading u Additional water n Prevention u Full pull back where safety permits u Trench x-ditches to native ground

Real Life - 4 n Situation: u X-ditches only, landslides on 50-60%+ slopes below roads n Contributing factors u Excess water diverted onto slope n Prevention u Outsloping or very very frequent x-ditches u Partial pull back &/or decompaction

Real Life - 5 n Situation: u Cut slope failure above partial pull back u Same as with no deactivation - fairly rare n Contributing factors u loss of toe support n Prevention u Leveling top of partial pull back u In most cases not preventable

Real Life - 6 n Full pull back n No landslides but may get minor sediment redistribution

Lesson learned: The right technique in the right place n Always deactivate the back end, even if the costs seem high n Full pull back on slopes > 60% (50%?) n Trench to native ground at seepage sites and surface drainage channels on slopes > 50%? n Disperse water when slopes below > 50%

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

Contact us at: n Vancouver Forest Region u 2100 Labieux Road u Nanaimo, B.C., V9T 6E9 n fax n u u

Conceptual frequency plot of deactivation landslide types

Road / Slope Geometry

Road deactivation landslide report