CLIFFS launch meeting 26 October 2005, Holywell Park, Loughborough - PowerPoint PPT Presentation

CLIFFS launch meeting 26 October 2005, Holywell Park, Loughborough University Response of Slope Stability to Vegetation changes due to Climate Change John Greenwood

Vegetation • Recent research and demonstration projects • Stability analysis to take account of vegetation and hydrological effects • Influences of Climate change

Signs of assistance from the vegetation ? - Water Lane, Kent

Grasses on dunes (The Wash)

Dune Grasses – Deep roots

Shallow Slips - M69 - Vegetation probably plays a part

Slips on M11 _ Can vegetation help prevent them?

CIRIA Bioengineering Demonstration site set up on M20 View to West (1994)

M20 - View to West (1998)

M20 Vegetation Trials, Conclusions over the 5 year trial period • Significant root growth to 1.2m or more • Roots often follow fissures and discontinuities • Moisture changes due to roots masked by seasonal changes • Window sampling too destructive to vegetation • Standpipe levels dominated by seasonal changes • Tensiometers appropriate for monitoring seasonal changes and storm events (detail in Ciria RP81) • Vegetation maintenance regime important

EU ECOSLOPES PROJECT Testing with the NTU shear box / pull out apparatus

EU project - Ecoslopes • Characterising contribution of vegetation • Characterising plant/root architecture • Characterising loading on vegetation • Resistance to tree overturning • Effect of fires on vegetation, erosion, and slope stability • Forest stand stability • Root architecture and tree stability modelling • Slope stability modelling (Limit equilibrium, energy approach, numerical modelling, etc) • Project database • Slope Decision Support System • www.ecoslopes.com

Root Clamping for pull-out

Root pull-out notation/terminology F Reference Surface Diameter at clamp Ground Surface e Clamp Bark Core � f Diameter at failure point d c Root d � f1 d fc d f d fc1 , d f1 Failure Points

Pull out test in progress

Actual pull-out result on Hawthorne root, 21.9 mm Dia 3.50 3.00 2.50 Force (KN) 2.00 1.50 1.00 0.50 0.00 0 50 100 150 200 Displacement (mm)

c´ v Deeper Slip Less influence

Slope stability analysis • Traditional methods of limit equilibrium stability analysis –Bishop, Janbu, Fellenius (Swedish) etc. • Methods are prone to error particularly for submerged slopes and deep slip surfaces with high ‘ ∝ ’ values. • Problems because water forces not taken fully into account.

The stability equation solution based on effective interslice forces Many of the problems associated with the conventional stability analysis equations are overcome if the equilibrium of the soil slice is considered in terms of effective interslice forces to derive the stability equations (Greenwood 1987, 1989, 1989b) The basic stability equation for the factor of safety, equation (1), is accepted as correct. ( ) ∑ + φ � c ' N ' tan ' F = ...... (1) ∝ ∑ W sin

Forces associated with each slice soil 1 γ 1 c ′ 1 X 2 ′ φ′ 1 W X 1 ′ E 2 ′ E 1 ′ U 2 U 1 soil 2 S γ 2 c ′ 2 φ′ 2 τ u � α N ′

Figure. Forces acting on a slice of the stability analysis b – Revised approach using effective a –conventional approach using total interslice forces and interslice interslice forces water forces (Greenwood (Barnes 1995) 1987,1989)

The Greenwood General slope stability equation is derived by taking account of all the water forces acting on the slice and assuming the resultant of interslice forces is parallel to the slip surface :- [ ] ( ( ) ) ∑ + ∝ − − − ∝ φ � � c ' W cos u U U sin tan ' = 2 1 F ∑ ∝ W sin

By appropriate assumptions, the General equation may be adapted to include an estimation of the horizontal interslice force based on the coefficient of horizontal earth pressure, ‘K’ :- [ ] ( ( ) ( ) ) ∑ + ∝ − − − ∝ + − ∝ φ α � � c ' W cos u U U sin K tan W ub sin tan ' F = 2 1 ∑ ∝ W sin

Additional Forces due to Vegetation, Reinforcement and Hydrological changes D w β W v soil 1 γ 1 c ′ 1 X 2 ′ ∆ h w φ′ 1 X 2 ′ W X 1 ′ E 2 ′ E 1 ′ U 2 + ∆ U 2 U 1 + ∆ U 2 soil 2 S γ 2 c ′ 2 φ′ 2 θ T τ + c ′ v u � + ∆ u v � α N ′

[ ] ( ( ) ) ∑ + ∝ − − − ∝ φ � � c ' W cos u U U sin tan ' = 2 1 F ∑ ∝ W sin The General equation is adapted for inclusion of vegetation effects, reinforcement and hydrological changes as follows:- F = [ ] ( ( ) ) ′ ∑ + + + ∝ − + ∆ − + ∆ − + ∆ ∝ − α − β + θ φ � � ( c ' c ) ( W W ) cos ( u u ) ( U U ) ( U U ) sin D sin( ) T sin tan ' v v v 2 2 v 1 1 v w ∑ + ∝ + α − β − θ [( ) sin cos( ) cos ] W W D T v w

Stability Spreadsheet SLIP4EX - SLOPE STABILITY ANALYSIS (NTU Oct 2002) Sheet 1 - Comparison of Methods (See sheet 2, for effects of reinforcement, vegetation and hydrological changes) PROJECT NTU DESCRIPTION OF ANALYSIS: reinforced example Date: Oct-02 Enter slice Data Height 1 Unit wt 1 Height 2 Unit wt 2 Height 3 Unit wt 3 Breadth Alpha Cohesion* Phi' hw1 hw2 hw K Slice Nr m kN/m^3 m kN/m^3 m kN/m^3 m degrees kN/m^2 degrees m m m 1 1.2 20 4.2 -20 8 25 0 1.44 0.72 0.2 2 5.4 20 4.8 -3 8 25 1.44 5.9 3.67 0.2 3 8.1 20 4.8 16 8 25 5.9 4 4.95 0.2 4 9 20 4.8 36 8 25 4 5.9 4.95 0.5 5 4.8 20 4 57 8 25 5.9 0 2.95 0.5 6 0 0 0 0 0 0 0 0 0 0 7 0 0 0 0 0 0 0 0 0 0 0 8 0 0 0 0 0 0 0 0 0 0 0 9 0 0 0 0 0 0 0 0 0 0 0 10 0 0 0 0 0 0 0 0 0 0 0 11 0 0 0 0 0 0 0 0 0 0 0 12 0 0 0 0 0 0 0 0 0 0 0 13 0 0 0 0 0 0 0 0 0 0 0 14 0 0 0 0 0 0 0 0 0 0 0 15 0 0 0 0 0 0 0 0 0 0 0 Calculated forces on slices Total Resistance - Moment equilibrium Total Resistance - Horizontal force equilibrium General General Simple Simple Swedish Bishop General General Simple Simple Swedish W U1 U2 u Dist force cohesive res K' K' K ' K' slice kN kN kN kN/m2 kN kN kN kN kN kN kN kN kN kN kN kN kN 1 100.80 0.00 10.37 7.20 -34.48 35.76 66.57 67.39 66.67 67.49 64.92 84.26 70.85 71.72 70.95 71.83 69.09 2 518.40 10.37 174.05 36.70 -27.13 38.45 201.59 201.68 197.82 197.91 197.60 202.94 201.87 201.96 198.09 198.18 197.87 3 777.60 174.05 80.00 49.50 214.34 39.95 285.33 289.31 282.00 285.98 273.24 268.08 296.83 300.97 293.36 297.50 284.25 4 864.00 80.00 174.05 49.50 507.85 47.47 210.68 273.05 283.77 346.14 236.46 309.57 260.42 337.51 350.77 427.86 292.28 5 384.00 174.05 0.00 29.50 322.05 58.75 123.32 203.41 126.31 206.40 55.25 170.80 226.42 373.48 231.92 378.97 101.44 6 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 7 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 8 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 9 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 10 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 11 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 12 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 13 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 14 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 15 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 total 982.62 220.38 887.49 1034.85 956.58 1103.93 827.47 1035.66 1056.38 1285.63 1145.09 1374.34 944.93 Factors of Safety (no reinforcement or vegetation) Moment equilibrium Force equilibrium F m F f Greenwood General 0.90 0.77 Greenwood General (K as input) 1.05 0.93 0.97 0.83 Greenwood Simple Greenwood Simple (K as input) 1.12 1.00 Swedish 0.84 0.69 Bishop 1.05 Janbu (fo =1.05) 0.95 Bishop iteration Janbu Iteration F initial F input F calc F input F calc 1 1.06 1.05 0.95 0.95

Factors of Safety (no reinforcement or vegetation) Moment equilibrium Force equilibrium F m F f Greenwood General 0.90 0.77 Greenwood General (K as input) 1.05 0.93 Greenwood Simple 0.97 0.83 Greenwood Simple (K as input) 1.12 1.00 Swedish 0.84 0.69 Bishop 1.05 Janbu (fo =1.05) 0.95 Bishop iteration Janbu Iteration F initial F input F calc F input F calc 1 1.06 1.05 0.95 0.95

Reinforcement, vegetation and hydrological effects may be added (Sheet 2) Root Root Additional Mass of W ind W ind Force direction cohesion Change in water table Vegetation force direction delta delta T Theta c'v hw1 hw2 delta hw Wv D Beta kN (/m) deg kN/m2 m m m kN (/m) kN (/m) deg. slice 1 0.95 45 0 -0.1 -0.05 0 0 0 2 5 45 -0.1 -0.1 -0.1 3 0.6 45 -0.1 -0.05