1

Embankment and cut slope monitoring and

analysisDr Joel Smethurst

Introduction

A number of instrumented sites have been used to:

• Monitor seasonal changes in soil moisture and pore water pressure/suction

• Understand vegetation effects

• Assess impact of natural variation in climate

• Consider performance under climate change

2

Climate of the South-East

Soil moisture deficit (SMD) = amount of water required to recharge the profile

Rainfall and potential evapotranspiration, South-East England (Southampton)

020406080

100120140160180

SMD

(mm

)

Jan

Feb

Mar

Apr

May Jun

Jul

Aug

Sep

Oct

Nov

Dec

0

20

40

60

80

100

120

Jan

Feb

Mar

Apr

May Jun

Jul

Aug

Sep

Oct

Nov

Dec

Mon

thly

rain

fall

and

PET

(mm

/mon

th)

RainfallPET

Track quality

m/c

depth

summer

winter

3

Vertical extensometer displacements

-20

-10

0

10

20

30

40

50

01-A

pr-0

6

01-J

un-0

6

01-A

ug-0

6

01-O

ct-0

6

01-D

ec-0

6

31-J

an-0

7

02-A

pr-0

7

02-J

un-0

7

02-A

ug-0

7

02-O

ct-0

7

02-D

ec-0

7

Dis

plac

emen

t (m

m)

North side CREST - magnet 1.1m depth

North side MIDSLOPE - magnet 0.62m

Magnolia Road, SouthendNorth side

Trees removed from crest

Data courtesy of Network Rail/ Geo-observations

Slope instability

To link togetherClimate > vegetation > changes in soil moisture > vertical and lateral displacements

Shallow instability/creep caused by elevated surface pwp’s

Cracking over drying zone at the surface

Positive pwps

Elevated surface winter pwps

Summer dry condition

Downslope creep

Strain softening of clays and deep seated failure

Development of plastic shear forms deep seated mechanism

Taken from Ellis and O’Brien (2007)

4

Long term continuous monitoring of soil moisture, climate and pore water pressures in a London Clay cutting

Newbury site• Cut slope in London Clay

(constructed 1997)• Uniform geology• Shallow slope angle ~16°• Grass/small shrubs vegetation

cover

Measurement of:• Pore pressure• Climate• Soil moisture content

Newbury - instrumentation

Instrumentation installed in four groups

Curb ofroad

A

B

C

D

Weatheredclay

London Clay

Instrumented section of

slope

5

Newbury – measured moisture contents in weathered clay (Group A)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

01-A

pr-0

3

30-S

ep-0

3

31-M

ar-0

4

30-S

ep-0

4

31-M

ar-0

5

30-S

ep-0

5

01-A

pr-0

6

30-S

ep-0

6

01-A

pr-0

7

01-O

ct-0

7

Volu

met

ric m

oist

ure

cont

ent

TDR10.3 m

TDR20.6 m

TDR30.9 m

TDR41.5 mWinterSummer

Moisture content measured by TDR ThetaProbes

0

0.5

1

1.5

2

2.5

3

3.5

4

20 30 40 50 60

Volumetric water content %

Dep

th fr

om g

roun

d su

rfac

e (m

)

12 Aug 03

14 Aug 03

22 Aug 03

03 Sept 03

30 Sept 03

17 Oct 03

04 Nov 03

28 Nov 03

15 Dec 03

Newbury – changes in volumetric moisture content – in weathered clay

Seasonal change in moisture content over top 1.25m depth of soil

Neutron probe site A : 2003

End of summer

Winter

Neutron Probe

6

Newbury – soil water balance

Rainfall and ET

Assume that the major moisture

changes occur only in the

rooting zone

Draw up/re-infiltration of water from below the major rooting zone

At Newbury= 0.8 m to 1.2m depth

Runoff

Simple 1-D soil moisture balance model

(Runoff only occurs at field capacity)

Newbury – estimated soil moisture deficit 2003

Rainfall

Modelled Soil Moisture Deficit

AE < PE when SMD > 58mm

0

20

40

60

80

100

120

140

160

01-J

an

31-J

an

02-M

ar

01-A

pr

02-M

ay

01-J

un

02-J

ul

01-A

ug

01-S

ep

01-O

ct

01-N

ov

01-D

ec

01-J

an

SMD

(mm

)

Actual ET < Potential ET

Total available water (TAW) = 144 mm

Readily available water (RAW) = 58 mm

0

5

10

15

20

25

30

35

40

Daily rainfall (mm/day)

Rai

nfal

l (m

m)

7

Newbury – estimated SMD and measured drying

0

20

40

60

80

100

120

140

160

01-J

an-0

3

31-J

an-0

3

02-M

ar-0

3

02-A

pr-0

3

02-M

ay-0

3

02-J

un-0

3

02-J

ul-0

3

01-A

ug-0

3

01-S

ep-0

3

01-O

ct-0

3

01-N

ov-0

3

01-D

ec-0

3

31-D

ec-0

3

SMD

(mm

)

SMD calculated fromclimate parameters

Group A neutron probereadings

Group C neutron probereadings

Group A TDR data

Acknowledgements

Co-investigators: Prof. William Powrie, Dr Derek Clarke

Funded by:

Collaborators:

8

Magnolia Road, Southend

Pound Green, Reading

Old railway embankments suffering serviceability (shrink/swell) problems

Magnolia Road/Pound Green• Old (> 100 yrs) poorly compacted

London Clay embankments• Ground conditions mixed – ash and

ballast materials on the top of the embankments/old counterfortdrains

• Large mature tree cover and permanent moisture deficit

Measurement of:• Lateral and vertical displacement• Pore water pressures• Soil moisture content• Rainfall

Site locations

Magnolia Road site

Newbury sitePound Green site

9

Newbury site – Vegetation changes between summer-winter

09 May 03 09 July 03 10 Sept 03 24 Oct 03

Newbury – changes in volumetric moisture content – in grey clay

0

0.5

1

1.5

2

2.5

3

3.5

20 30 40 50 60

Volumetric moisture content %

Dep

th fr

om g

roun

d su

rfac

e (m

)

22 Aug 03

03 Sept 03

30 Sept 03

17 Oct 03

04 Nov 03

15 Dec 03

Seasonal change in moisture content over top 1.0 m depth of soil

Neutron probe site C : 2003

SMD calculated from area between moisture curves

10

Newbury – soil water balance

RAW = 58mm

Infiltration and ET

TAW = 144mm

Total depth of available water in the rooting zone = 144

mm

Plants can readily access the initial ~40% of the water in profileActual ET = full Potential ET

The plants become stressed and find it harder to remove the remaining water:Actual ET = Potential ET x f(SMD)

Draw up/re-infiltration of water from below the major rooting zone

Max. draw up from measured summer suction gradient at Newbury = 0.04 mm/day = negligible

Newbury – estimated potential evapotranspiration (Penman – Montieth)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

01-J

an-0

3

31-J

an-0

3

03-M

ar-0

3

02-A

pr-0

3

03-M

ay-0

3

02-J

un-0

3

03-J

ul-0

3

02-A

ug-0

3

02-S

ep-0

3

02-O

ct-0

3

02-N

ov-0

3

02-D

ec-0

3

02-J

an-0

4

ETo

mm

/day

Daily ETo mm/d A34 solarimeter

Monthly ETo mm/d XLS solarimeter

Monthly ETo mm/d Soton 1963-90

11

Modelling London Clay slopes (grass cover) with future climate data

Assumptions• Maximum SMD = 180 mm• Rough grass/herbs cover• No climate effect on vegetation

We have used the BETWIXT (Watts et al, 2004) future climate data sets to simulate SMD’s from 2010 – 2100 (based on Hadley 2002 model)

Data for London near Heathrow Airport

Grass cover typically returns to SMD = 0 in winter, so more informative about summer drying

0

20

40

60

80

100

120

140

160

180

Jan

Feb

Mar

Apr

May Ju

n

Jul

Aug

Sep Oct

Nov

Dec

Rai

nfal

l and

PE

T (m

m)

Rainfall 1961-1990 PET 1961-1990Rainfall 2020's PET 2020'sRainfall 2050's PET 2050'sRainfall 2080's PET 2080's

London Heathrow, medium-high emissions

Simulated daily soil moisture deficits 1960-2100 (London Heathrow Apt)

0

20

40

60

80

100

120

140

160

180

200

1960 1980 2000 2020 2040 2060 2080 2100

SMD

(mm

)

Met Office data, 1960-2005Betwixt medium high scenario, 2010-2100Grass cover, TAW = 180 mm, RAW = 90 mm

Maximum SMD = 180 mm

12

Maximum and average annual Soil Moisture Deficit

Grass cover, TAW = 180 mm, RAW = 90 mm Betwixt medium high scenario, London Heathrow

0

20

40

60

80

100

120

140

160

180

200

1950 1970 1990 2010 2030 2050 2070 2090 2110

SM

D (m

m)

Maximum annual SMD

Average annual SMD

Excess rainfall that becomes Runoff

0

50

100

150

200

250

300

350

400

450

500

1950 1970 1990 2010 2030 2050 2070 2090 2110

Run

off (

mm

)

Annual runoff

13

Different emissions scenarios

0

20

40

60

80

100

120

140

1950 1970 1990 2010 2030 2050 2070 2090 2110

SMD

(mm

)

Average annual SMD - actual dataAverage annual SMD - low emissions scenarioAverage annual SMD - high emissions scenario

Future climate modelling

Pattern shows:• Increase in maximum SMD• Increase in summer drought (higher average SMD)• Most winters the soil still re-wets (although at slightly lower frequency)

Implications:• Greater clay shrinkage for longer periods• Increase in the magnitude of shrink-swell cycles• Vegetation will become stressed (die back) earlier in the year and more often• Possible reduction in vegetation cover (leading to erosion problems)

Limitations:• Climate models• No modelled change in vegetation type, or the influence of this on soil drying

14

Conclusions

• Seasonal moisture and pore water pressure changes cause a number of embankment serviceability and failure problems

• Measured changes in soil moisture content can be closely correlated with a simple water balance (SMD) model

• The SMD model and climate scenarios can be used to estimate the likely future cycles of soil moisture

Newbury – measured pore water pressures in grey clay

-80

-60

-40

-20

0

20

01-A

pr-0

3

30-S

ep-0

3

31-M

ar-0

4

30-S

ep-0

4

31-M

ar-0

5

30-S

ep-0

5

01-A

pr-0

6

30-S

ep-0

6

01-A

pr-0

7

01-O

ct-0

7

Pore

wat

er p

ress

ure

(kPa

)

C 1.0 m C 1.5 m C 2.0 m C 2.5 mWinterSummer

Pore water pressure measured by vibrating wire piezometers

15

-10

0

10

20

30

40

50

60

70

80

90

01-A

pr-0

3

30-S

ep-0

3

31-M

ar-0

4

30-S

ep-0

4

31-M

ar-0

5

30-S

ep-0

5

01-A

pr-0

6

30-S

ep-0

6

01-A

pr-0

7

01-O

ct-0

7

Suct

ion

(kPa

)

C 0.3 m C 0.6 m

Newbury – surface suctions in grey clay

Winter Summer

Suctions measured by tensiometers