On the Use of Pickets and Flukes as Snow Anchors ART FORTINI Sierra Madre Search & Rescue International Technical Rescue Symposium Denver, CO November 2002
Pickets Placements Traditional vertical Horizontal Sierra picket Behavior Strong snow Powdery Snow
Flukes Rake angle Behavior Strong snow Powdery Snow
Multi-Tool Anchors Parallel anchors Series anchors Crossed tools Equalization methods
Effects of snowpack Mechanical properties Compressive strength Tensile strength Shear strength Layering Weak layers Icy layers Disturbance and settling time
Anchor Failure System breaks at the weakest link Software failure Snow failure Hardware failure rope webbing σ c A c σ s A s biner hardware compression shear
Shear vs Compressive Failure σ c > σ s Determined by snowpack shear A s A c << A s A c = LW A s = f(d,w) Failure shear if σ c A c > σ s A s comp if σ c A c < σ s A s compression A c W
CAUTION! Building a man-rated system out of snow Material is extraordinarily weak σ s,t < 1 psi (snow) σ c < 10 psi (snow) σ t > 100,000 psi (steel) Material properties vary tremendously Material properties typically unknown when building the anchor Fortunately, the applied loads are typically small
Compressive Failure Seen only when compressive stress is higher than compressive strength and shear stress is less than shear strength Dominant effect in traditional picket placements in any snow all anchors in weak, powdery snow
Shear Failure Surface area of stress cone Much larger than area of tool Stress cone extends horizontally at ~45 o upward at ~30 o Dominant effect in strong snow flukes, mid-clipped pickets top view side view
Stress Cone Area of stress cone area of fracture surface
Traditional Vertical Picket Applied load shear Resultant Forces compression
Traditional Vertical Picket Greatest force is applied to snow near the surface F max = σ c WL/2 Compressive strength of surface snow has strong effect on anchor strength Wide top beneficial Local compressive failure of the snow is common Picket rotates forward & is pulled from snow
Traditional Vertical Picket: Failure Modes Picket cuts through the snow Ice wedge forms in flange Most pronounced at top of picket
Traditional Vertical Picket: Failure Modes Surface snow failure followed by rotation and pullout Zone of compressive failure Typically happens very suddenly Same behavior if fully buried, but at higher load
Traditional Vertical Picket: Failure Modes Picket buckling followed by rotation and pullout Small shock load from buckling Typically happens very suddenly Same behavior if fully buried, but less likely
Traditional Vertical Picket: Failure Modes Ice axes can buckle too
Traditional Vertical Picket: Observations Snow disturbance is minimal Snow typically fails in compression near surface Compressive failure zone progresses downward Picket rotates forward & is pulled axially from the snowpack
Traditional Vertical Picket: Recommendations Use the firmest snow possible Disturb and allow to settle, esp. surface Use wide pickets Use long pickets Completely bury top of picket or axe Place top of picket just above strong layer Put maximum stress at strongest part of snowpack Beware of sudden, catastrophic failure
Horizontal Picket Theoretically twice as strong as fully buried traditional vertical pickets Load is uniformly distributed over entire length Less sensitive to strength of surface snow
Horizontal Picket Proper placement Top view Side view
Horizontal Picket: Failure Modes Weak, powdery snow (compressive failure): F max = σ c A c (> F max = σ c A c /2) Can occur within a weak layer Strong, monolithic snow (shear failure): F max = σ s A s Transition from compressive failure (slow) to shear failure (sudden) when σ c A c > σ s A s
Horizontal Picket: Failure Modes Compressive failure (cutting) LOW STRENGTH Graceful Weak layer can be catastrophic Use wide, long pickets Shear failure (uplift) HIGH STRENGTH Can be sudden Bury pickets as deep as possible Side view
Horizontal Picket: Failure Modes Shear failure at weak layer Can be low strength Sudden failure Probe the snow! Weak interface Side view
Horizontal Picket: Failure Modes Rotation Caused by Not attaching load to center strong or weak area near one end of picket Off-center loading Icy zone Top view
Horizontal Picket: Failure Modes Rotation Gradual start Results in loss of frontal area Can have very sudden results Prevention Use center hole Use symmetrically swaged cables Probe for icy zones and weak areas
Horizontal Picket: Recommendations Use the firmest snow possible Disturb and allow to settle Make trench wide enough to backfill properly Use pickets with reinforced cross section Bury as deeply as possible Maximize area of shear cone, A s Beware of sudden failure, esp. in strong snow
Sierra Picket: Vertical placement, midclip Top view Side view
Sierra Picket: Vertical placement, midclip
Sierra Picket: Vertical placement, midclip Less sensitive to weak layers than horizontal picket Easier to place than horizontal picket requires one trench instead of two Observed to be 23% stronger than horizontal picket (n=4+3)
Sierra Picket: Vertical placement, midclip 23% stronger than horizontal picket of same size Why? F max = σ s A s Snow has same properties A s must be larger Can we estimate A s?
Sierra Picket: Vertical placement, midclip Estimation of A s d θ side view w top view φ A s = wd / sinθ + d 2 tanφ / [tanθ sinθ] + d 2 / [cosφ tanθ]
Sierra Picket: Estimation of A s Field observations 45 o radiation from sides of hardware ~30 o up angle from bottom of hardware Horizontal picket 16 deep w=24, d=16, A s = 2282 in 2 Sierra picket 0 deep w=2, d=24, A s = 3502 in 2 53% increase in A s
Pickets: Estimated Shear Cone Areas 7000 2" x 24" picket Estimated area of shear cone (in2) 6000 5000 4000 3000 2000 1000 Sierra picket horizontal picket 0 10 12 14 16 18 20 22 Clip depth (in)
Sierra Picket: Failure Modes in Strong Snow Failure along shear cone Some warning but sudden and explosive Large amounts of snow ejected Beware of flying hardware
Sierra Picket: Failure Modes in Strong Snow Forward rotation Caused by poor backfilling of trench Weak snow in front of top half of picket Results in loss of frontal area Rotation is gradual Rotation is visible: STOP IF YOU SEE IT!
Sierra Picket: Failure Modes in Strong Snow Failure of carabiner hole Requires strong snow 2115 ± na lb f for 2-ft picket 1 Newer generation hardware Thick edge web present 1060 ± 42 lb f for 3-ft pickets 1 Old generation hardware Thick edge web absent 1 Tests performed on single pickets at Mt. Rainer in high density snow on 6/21-22/02
Sierra Picket: Failure Modes in Strong Snow Failure of picket web Common if girth hitched More likely in strong snow Causes bending of picket Loss of frontal area (A c ) Decrease in effective length A s decreases 1 Tests performed on single pickets at Mt. Rainer in high density snow on 6/21-22/02
Sierra Picket: Failure Modes in Strong Snow Failure of carabiner hole 2115 ± na lb f for 2-ft picket 1060 ± 42 lb f for 3-ft pickets Higher bending moment? Not likely 1 Tests performed on single pickets at Mt. Rainer in high density snow on 6/21-22/02
Picket Strengths: Observations in Strong Snow 3-ft pickets do not appear to be much stronger than 2-ft pickets 1 avg avg % increase % increase Length 2 ft 3 ft (actual) (theory) horizontal midclip 17552 2010 15 15 vertical midclip 2210 10602-52 120 vertical top clip na na na 50 3 1 Tests performed at Mt. Rainer in high density snow on 6/20/01 & 6/21-22/02. 2 Carabiner hole failed in one 2-foot and both 3-foot pickets. 3 Compressive failure expected.
Sierra Picket: Failure Modes in Weak Snow No testing done in weak, powdery snow Expect compressive failure Low strength F max = σ c A c Slow, controlled travel of picket through snowpack
Sierra Picket: Ice Axe Version 85 cm axe Girth hitched axe 12 below head Buried head 0 below snow surface Failure mode Axe rotated forward & pulled out 2030
Sierra Picket: Recommendations Use the firmest snow possible Disturb and allow to settle Make trench wide enough to backfill properly Use pickets with reinforced cross section Bury as deeply as possible Maximize area of shear cone, A s Beware of sudden failure, esp. in strong snow
Fluke: Resultant Forces shear Rake angle, β Applied load Resultant Forces compression Side view Top view
Fluke: General Behavior Compressive force is uniformly applied to frontal area Downward component of force on fluke Digs fluke deeper Deeper snow may be weaker Ice layer may deflect fluke Rake angle determines upward component on snowpack Tends to lift out large area of snow Decreases area of shear cone Side view
Fluke: General Behavior Weak, powdery snow Compressive failure F max = σ c A c Large area is beneficial Less sensitive to weak layer than horizontal picket Strong snow Shear failure F max = σ s A s Greater depth is beneficial
Fluke: Failure Modes Compressive failure (cutting) LOW STRENGTH Probably stronger than horizontal picket of equal area No ice wedge Side view Graceful failure Can undergo significant travel with minimal loss of strength Use large flukes to increase A c and increase strength Holes may decrease effective A c
Fluke: Failure Modes Shear failure (uplift) HIGH STRENGTH Can be sudden and explosive Increase strength by increasing A s Bury fluke as deep as possible Use small rake angle
Fluke: Failure Modes Effects of rake angle, β As β increases: θ increases, A s decreases, and F max decreases If we assume θ = 30 + β, then... θ β Side view
Fluke: Effect of Rake Angle Estimated shear cone area (in2) 4,000 3,500 3,000 2,500 2,000 1,500 1,000 500 0 50 in2, 8" burial depth Commonly available 0 10 20 30 40 50 Rake angle (degrees)
Fluke: Effect of Rake Angle Unable to test multiple rake angles Destroyed each fluke on first test with β = o
Fluke: Failure Modes Bending of fluke Results in decrease in A s due to decrease in width Reduces effective strength Wedge shape may result in transition to compressive failure Fluke cuts through snowpack Requires very strong snow Wire rope tearing through fluke May result in change in rake angle Requires very strong snow Failure of wire rope Very sudden Requires very strong snow
Fluke: Failure Modes Impact with ice layer Can be sudden Fluke lies down on ice surface Minimal frontal area Fluke moves easily through snow Impact more likely with large rake angle Side view Icy layer
Fluke Recommendations Probe below for icy layers Disturb snow and allow settling Large frontal area desirable, esp. in soft snow Bury deeply to increase A s Shallow rake angle desirable 83 in 2 @ 45 O weaker than 42 in 2 @ 25 O 228 vs 390 lb f in same snowpack Arrange multiple flukes side by side, not in a row Prevents rear flukes from traveling in path weakened by lead fluke
Multi-Tool Anchors Individual anchors will typically move When initially loaded (>2 ) At higher loads (amount highly variable) Strengths of individual, identical anchors can vary greatly Individual points should be: Equalized, but not self equalizing Beyond shear zone of other anchor points
Multi-Tool Anchors Equalized, but not self equalizing 50 100 50 50 self equalizing 100 50 100 50 100 equalized 150
Multiple Vertical Pickets: Arrangement Case 1 F max = σ c A c Q 1 Q 1 = n/2 n Q1 Q2 α 1 0.50 0.50 1.00 2 1.00 1.12 0.62 3 1.50 1.76 0.74 4 2.00 2.57 0.83 Case 2 F max = σ c A c Q 2 Q 2 = α (1-3/2α+α 2 )/(1-α) α L
Multi-Tool Anchors Crossed tools Bury head of vertical tool Pro Sometimes stronger than single vertical axe Fast and easy Con Inefficient use of tools This horizontal tool is shallow and weak Single, deep horizontal axe would be stronger 860-1615 (1 tool) 500-1500 (2 tools)
Multi-Tool Anchors Vertical axe + horizontal picket Buried head of vertical tool Buried horizontal tool 12 Pro Stronger than single vertical axe Con Inefficient use of tools Marginally stronger than h-picket alone 1960 (2 tools) 860-1615 (1 axe) 1300-2115 (1 picket)
Multi-Tool Anchors Tandem Sierra pickets Keep lead picket outside of rear stress cone Should be >2 x rear picket depth Avoid upward force on lead picket Rear picket should be deeper than front 3040 1345
Knot failed at 3300 lb f 50 o 1.7d Multi-Tool Anchors Tandem Sierra pickets (2 x 2 ) Keep pickets well separated greater than 3d Avoid intersection of shear cones
Multi-Tool Anchors Observations Best results when each tool is independent Don t get fancy Interactions among nearby tools unpredictable Placing tools nearby each other does not appreciably increase the size of the shear cone Inefficient use of tools
Anchor Strengths 1 : Horizontal Picket [2 x 2 ] Horizontal placement 12-16 deep, mid clip 1755 + 416 lb f [1300-1850] Sierra Picket [2 x 2 ] Vertical placement, mid clip 2210 + 400 lb f [1730-2670] Fluke [5.8 x 8.6, 0 o ] 12 deep placement 1 Tests performed 2500 at Mt. lbrainer f in high density snow on 6/20/01 & 6/21-22/02. Your strengths WILL vary!
Summary: Pickets & Flukes Flukes and pickets show large scatter in strength Use multiple, equalized flukes / pickets Pickets & ice axes: Traditional pickets should have head completely buried Horizontal pickets stronger than traditional pickets Vertical pickets with mid clip strongest Failure usually very sudden, esp. in strong snow Flukes Arrange in parallel, not series Beware of icy layers & large rake angles Failure can be graceful or sudden
Summary Strength of any snow anchor is strongly influenced by the properties of snowpack Strength Compressive Shear Layering Weak layers Icy layers Disturbance and settling Snow pack properties are highly variable Use redundant anchors
Trade-offs Strength Bollards & deadmen > P-pickets > flukes & h-pickets > t-pickets & axes Time to set up anchor Bollards & deadmen > h-pickets > flukes > P-pickets > t-pickets & axes Gracefulness of failure flukes, h-pickets & P-pickets > t-pickets & axes Sensitivity to snowpack layering h-pickets > flukes > t-pickets & axes > P-pickets > deadman
Recommendations Know your snowpack Strong or powdery? Layers? Compact and allow to settle (σ c & σ s ) Strong snow Bury hardware deep (A s ) Weak, powdery snow Use hardware with large frontal area (A c )
Acknowledgements The Sierra Madre Search & Rescue Team Seattle Mountain Rescue for providing travel assistance Gordon Smith & Vera Wellner of SMR for organizing testing opportunities Everyone who sacrificed personal hardware, pulled on a rope or buried an anchor while the testing was being conducted