CLIFFS launch meeting26 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

Diameter at failure point

Bark

Core

F

Clamp

lf

lf1

dc

d

dfc

df

Diameter at clamp

Ground Surface

Reference Surface

e

Failure Points

dfc1, df1

Root

Pull out test in progress

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

0 50 100 150 200

Displacement (mm)

Fo

rce (

KN

)

Deeper Slip

Less influence

c´v

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.

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

∝∑

+∑

sin

'tan''

W

Nc φl

Forces associated with each slice

soil 1

γ1 c′1φ′1

soil 2

γ2 c′2 φ′2

α

U1 S

W

τ

U2

ul

X2′

E2′

X1′

E1′

N′

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)

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

'tansincos' 12

W

UUuWcF

φll

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

'tansintansincos' 12

W

ubWαKUUuWc φll

Additional Forces due to Vegetation, Reinforcement and

Hydrological changes

soil 1

γ1 c′1φ′1

soil 2

γ2 c′2 φ′2

+ c′v

α

U1 +∆U2 S

W

τ

U2 +∆U2

ul +∆uvl

X2′

E2′

X1′

E1′

N′

Dw

Wv

β

X2′

Tθ

∆hw

The General equation is adapted for inclusion of vegetation effects,

reinforcement and hydrological changes as follows:-

F =

( )( )[ ]]cos)cos(sin)[(

'tansin)sin(sin)()()(cos)()'( 1122

θβαφθβα

TDWW

TDUUUUuuWWcc

wv

wvvvvv

−−+∝+∑

+−−∝∆+−∆+−∆+−∝++′+∑ ll

( )( )[ ]∝∑

∝−−−∝+∑=sin

'tansincos' 12

W

UUuWcF

φll

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

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

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

Reinforcement, vegetation and hydrological effects may be

added (Sheet 2)

Root Force

Root direction

Additional cohesion Change in water table

Mass of Vegetation

Wind force

Wind 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 -0.1 -0.05 0 0 0

2 5 45 -0.1 -0.1 -0.1

3 0.6 45 -0.1 -0.05

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

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

override)

• 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

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.

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

Example deep slip – importance of correct water forces at toe

of 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

Example deep slip – importance of correct water forces at toe

of 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

Example deep slip – importance of correct water forces at toe

of 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 = γγγγwhw22 / 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

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.

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.

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?

Trained roots in Bali

Response of Slope Stability to Vegetation changes due to

Climate Change

Thanks to all colleagues involved in supporting this

work.

John Greenwood

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)