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

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

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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

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CLIFFS launch meeting

26 October 2005, Holywell Park, Loughborough University

Response of Slope Stability to Vegetation changes due to Climate Change

John Greenwood

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Vegetation

  • Recent research and demonstration

projects

  • Stability analysis to take account of

vegetation and hydrological effects

  • Influences of Climate change
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SLIDE 3

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

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Grasses on dunes (The Wash)

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Dune Grasses – Deep roots

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Shallow Slips - M69 - Vegetation probably plays a part

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Slips on M11 _ Can vegetation help prevent them?

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SLIDE 8

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

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SLIDE 9

M20 - View to West (1998)

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SLIDE 10

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
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SLIDE 11

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

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SLIDE 12

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
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SLIDE 13

Root Clamping for pull-out

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SLIDE 14

Root pull-out notation/terminology

Diameter at failure point Bark Core F Clamp

f f1

dc d dfc df Diameter at clamp Ground Surface Reference Surface e Failure Points dfc1, df1 Root

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SLIDE 15

Pull out test in progress

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Actual pull-out result on Hawthorne root, 21.9 mm Dia

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 50 100 150 200 Displacement (mm) Force (KN)

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SLIDE 17

Deeper Slip Less influence

c´v

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SLIDE 18

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.

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SLIDE 19

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. F = ...... (1)

( )

∝ ∑ + ∑ sin ' tan ' ' W N c φ

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SLIDE 20

Forces associated with each slice

soil 1 γ1 c′1 φ′1 soil 2 γ2 c′2 φ′2 α U1 S W

τ

U2 u X2′ E2′ X1′ E1′ N′

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SLIDE 21
  • Figure. Forces acting on a slice of the stability analysis

a –conventional approach using total interslice forces (Barnes 1995) b – Revised approach using effective interslice forces and interslice water forces (Greenwood 1987,1989)

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SLIDE 22

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 :-

( ) ( )

[ ]

∝ ∑ ∝ − − − ∝ + ∑ = sin ' tan sin cos '

1 2

W U U u W c F φ

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SLIDE 23

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’ :- F = ( ) ( )

[ ] ( )

∝ ∑ ∝ − + ∝ − − − ∝ + ∑ sin ' tan sin tan sin cos '

1 2

W ub W α K U U u W c φ

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SLIDE 24

Additional Forces due to Vegetation, Reinforcement and Hydrological changes

soil 1 γ1 c′1 φ′1 soil 2 γ2 c′2 φ′2 + c′v α U1 +∆U2 S W

τ

U2 +∆U2 u +∆uv X2′ E2′ X1′ E1′ N′ Dw Wv β X2′ T θ ∆hw

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The General equation is adapted for inclusion of vegetation effects, reinforcement and hydrological changes as follows:- F =

( ) ( ) [ ]

] cos ) cos( sin ) [( ' tan sin ) sin( sin ) ( ) ( ) ( cos ) ( ) ' (

1 1 2 2

θ β α φ θ β α T D W W T D U U U U u u W W c c

w v w v v v v v

− − + ∝ + ∑ + − − ∝ ∆ + − ∆ + − ∆ + − ∝ + + ′ + ∑

  • (

) ( ) [ ]

∝ ∑ ∝ − − − ∝ + ∑ = sin ' tan sin cos '

1 2

W U U u W c F φ

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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 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 2.95 0.5 6 7 8 9 10 11 12 13 14 15 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 Fm Ff 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

Stability Spreadsheet

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Factors of Safety (no reinforcement or vegetation) Moment equilibrium Force equilibrium Fm Ff 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

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Reinforcement, vegetation and hydrological effects may be added (Sheet 2)

Root Force Root direction Additional cohesion Change in water table Mass of Vegetation W ind force W ind direction T Theta c'v delta hw1 delta hw2 delta hw Wv D Beta

slice

kN (/m) deg kN/m2 m m m kN (/m) kN (/m) deg. 1 0.95 45

  • 0.1
  • 0.05

2 5 45

  • 0.1
  • 0.1
  • 0.1

3 0.6 45

  • 0.1
  • 0.05
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Factors of Safety with Reinforcement ,Vegetation and hydraulic changes included Fm Greenwood General No reinforcement/Veg 0.90 with reinf /veg /water as input 1.05 Greenwood General (K as input) No reinforcement/Veg 1.05 With reinf /veg /water as input 1.22 Greenwood Simple No reinforcement/veg 0.97 With reinf/veg/water as input 1.13 Swedish No reinforcement/veg 0.84 With reinf/veg/water as input 0.98

Spreadsheet calculations of change in Factor of Safety due to Vegetation, Reinforcement and Hydraulic changes

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Which vegetation effects have most influence on stability?

  • Mass of vegetation ?- insignificant compared with soil mass
  • Fine roots? – Important for erosion, unlikely to influence cohesion at

depth

  • Wind forces? - Only relates to shallow depth
  • Moisture change/cu change? - possibly some indirect influence at

depth but unlikely below 2 –3m depth (seasonal changes likely to

  • verride)
  • Pore Pressures ? (relate to moisture change) – unlikely to influence in

the longer term, again seasonal and geological effects likely to override

  • Coarse roots? - most likely to influence at shallow depth but few will

penetrate below 1.5 –2m

  • Vegetation effects likely to be most significant at toe
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Stability analysis

2 important points demonstrated by the ‘General’ solution

  • Shape of the critical slip surface governed

by overconsolidation / anisotropy of soils (K values)

  • Calculation of restoring forces at toe

(Where vegetation can have an effect) is very sensitive to hydrological conditions.

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Example deep slip – comparison of circular and wedge type analysis

Factor of Safety Deep circle Wedge Bishop 1.05 1.17 Swedish 0.72 0.86 General 0.83 0.94 General (with K=1.5) 0.96 0.94

slice 1 slice 2 slice 3 slice 4 slice 5 slice 6 c′ = 1.5 kN/m2 φ′ = 22 deg γ = 20 kN/m3 K = 0 K = 1.5

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Example deep slip – importance of correct water forces at toe

  • f deep circle

For Slice 1 of DEEP CIRCLE

Method

Restoring force (kN) Disturbing force (kN) Bishop (?water surface) 54.10

  • 61.09

Swedish (water parallel to slip) 28.88

  • 61.09

Simple (water horizontal) 33.77

  • 61.09

General (actual water surface) 37.70

  • 61.09

General (actual water surface,K=1.5) (50.89)

  • 61.09

Water conditions at toe critical tostability – Vegetation and drainage will help

slice 1 slice 2 slice 3 slice 4 slice 5 slice 6 c′ = 1.5 kN/m2 φ′ = 22 deg γ = 20 kN/m3 K = 0 K = 1.5

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SLIDE 34

Example deep slip – importance of correct water forces at toe

  • f Wedge?

For Slice 1 of Wedge

Method

Restoring force (kN) Disturbing force (kN) Bishop (?water surface) 14.6 1.68 Swedish (water parallel to slip) 14.8 1.68 Simple (water horizontal) 14.8 1.68 General (actual water surface) 14.6 1.68 General (actual water surface,K=1.5) 14.6 1.68 Slice 1 is not sensitive to water conditions because α is very small

slice 1 slice 2 slice 3 slice 4 slice 5 slice 6 c′ = 1.5 kN/m2 φ′ = 22 deg γ = 20 kN/m3 K = 0 K = 1.5

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Example deep slip – importance of correct water forces at toe

  • f Wedge?

For Slice 1 of Wedge Method Restoring force (kN) Disturbing force (kN) Bishop (?water surface) 14.6 1.68 Swedish (water parallel to slip) 14.8 1.68 Simple (water horizontal) 14.8 1.68 General (actual water surface) 14.6 1.68 General (actual water surface,K=1.5) 14.6 1.68 Slice 1 is not sensitive to water conditions because α is very small

slice 1 slice 2 slice 3 slice 4 slice 5 slice 6 c′ = 1.5 kN/m2 φ′ = 22 deg γ = 20 kN/m3 K = 0 K = 1.5

But - Interesting to note that slice 1 could become unstable in its own right due to the water force U2 on the right hand side, ie, U2 = γ γ γ γwhw2

2 / 2 If hw =1.6m, U2 =12.8 kN

Total disturbing force = 14.5 kN (--very close to local failure of slice 1! – could lead to progressive failure)

slice 1 U2

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Benefits/Uncertainties

  • Reducing run-off quantities
  • Roots to bind surface soils and resist erosion
  • Roots to reinforce deeper soils
  • ? Help to control moisture content and pore water
  • pressures. (Dehydration – fissures – vulnerable to

intense rain events) ?

  • ? Will vegetation survive changing climate ?

Concerns re climate change

More severe events – greater risk of instability – vegetation has important role to help moderate the extremes.

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SLIDE 37

Information/research needs

  • Soil Bioengineering – important link between

Engineering and the Environment

  • Gaining of data on effects of the vegetation gives

engineering confidence in the benefits and drawbacks (Ecoslopes)

  • Theoretical Analysis (Correct consideration of water

forces!) of the effects of the vegetation needs to be supported with field observation and measurement (Hydrology at toe most critical!)

  • SI procedures for vegetated slopes being developed.
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Conclusions

  • Slopes more likely to fail under extremes of

climate

  • Vegetation can potentially help to mitigate

the effects of climate extremes

  • Vegetation itself is susceptible to effects of

climate change – less easy to sustain?

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SLIDE 39

Trained roots in Bali

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Response of Slope Stability to Vegetation changes due to Climate Change

Thanks to all colleagues involved in supporting this work. John Greenwood

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References

Greenwood, J.R. (1987). Effective Stress Stability Analysis. Discussion in 9th European Conference on Soil mechanics and Foundations, Dublin Sept 1987. Vol 3, post conference proceedings, Balkema 1989, pp.1082-1083. Morrison, I.M. and Greenwood, J.R. (1989). Assumptions in simplified slope stability analysis by the method of slices. Geotechnique 39, No 3, pp.503-509. Greenwood, J.R., Vickers, A.W., Morgan, R.P.C., Coppin, N.J. and Norris, J.E. (2001). Bioengineering The Longham Wood Cutting field trial. CIRIA Project Report 81, London Greenwood, J.R., Norris, J.E., Wint, J. and Barker, D.H. (2003). Bioengineering and the transportation

  • infrastructure. Proceedings of the Symposium on Transportation Geotechnics, EMGG, Nottingham,

September 2003. Thomas Telford, pp.205-220. Greenwood, J.R., Norris, J.E. and Wint, J. (2004). Assessing the contribution of vegetation to slope

  • stability. Journal of Geotechnical Engineering, Vol. 157, Issue 4 pp 199-208.

Greenwood, J.R. (2004a). SLIP4EX – program for routine slope stability analysis to include the effects of vegetation, reinforcement and hydrological changes. Int. Conf. on Eco-Engineering: “The use of vegetation to improve slope stability”. Thessaloniki, Sept 2004. (Accepted by Geological and Geotechnical Engineering)