Full-scale testing to assess climate effects on embankments

Proceedings of the Institution ofCivil EngineersEngineering Sustainability 000Month 2009 Issue ES000Pages 1–13doi:

Paper 800022Received 11/12/2008Accepted 26/02/2009

Keywords:embankments/field testing &monitoring/research & development

Paul N. HughesGeotechnical ScientificOfficer, School of CivilEngineering and Geosciences,Newcastle University,Newcastle upon Tyne, UK

David G. TollSenior Lecturer, Schoolof Engineering, DurhamUniversity, UK

Stephanie GlendinningReader in EnvironmentalGeotechnics, School of CivilEngineering and Geosciences,Newcastle University,Newcastle upon Tyne, UK

Domenico GallipoliSenior Lecturer, Departmentof Civil Engineering,University of Glasgow, UK

Joao MendesPostgraduate ResearchStudent, School ofEngineering, DurhamUniversity, UK

Pauline E. MillerResearch Associate inGeomatics, School of CivilEngineering and Geosciences,Newcastle University,Newcastle upon Tyne, UK

Geoff ParkinSenior Lecturer in Hydrologyand Water Resources, Schoolof Civil Engineering andGeosciences, NewcastleUniversity, Newcastle uponTyne, UK

Full-scale testing to assess climate effects on embankments

P. N. Hughes MSc, PhD, S. Glendinning PhD, J. Mendes LEG, G. Parkin PhD, D. G. Toll PhD, CEng, MICE,D. Gallipoli PhD and P. E. Miller PhD

A unique facility for engineering and biological research

has been established with the aim of improving funda-

mental understanding of the effects of climate change on

slopes. This paper describes the building and monitoring

of a full-scale embankment representative of UK infra-

structure, the planting and monitoring of representative

vegetation, and the construction of a system of sprinklers

and covers to control climate. A summary of the results

of the first experiments simulating predicted the future

UK climate and the response of the embankment is also

presented. The information that has begun to be

gathered is providing data related to the failure modes

anticipated as a result of climate change and hence on the

sustainability of UK infrastructure slopes.

1. INTRODUCTION/JUSTIFICATION OF FULL-

SCALE TESTING

The strength of the materials from which infrastructure slopes

are created and the status of the water held within the pore

spaces are key factors controlling the stability and hence the

engineering sustainability of slopes.1 Plants respond to small

changes in environment; a temperature difference of only 1–

2 C, or drought, will alter the composition of the plant

community, its water use and rooting characteristics. Future UK

climate change scenarios published by the UK climate impacts

programme (UKCIP)2 anticipate consistent and significant

increases in temperature of up to 3 C on average in the south

east of the UK over the next 50 years. Changes in rainfall are less

consistent, but key aspects are: little change or a small increase

in annual rainfall; a general increase in winter rainfall and a

decrease in summer rainfall; a more marked increase in winter

rainfall in the north west of the UK (up to 30%); a more marked

decrease of summer rainfall in the south east of the UK (up to

25%). This is discussed in more detail by Kilsby et al. in this

issue.3 It is therefore a reasonable assumption that the water

regime within infrastructure slopes will change as a result of

climate change, and so will the vegetation growing on those

slopes. Given that it is anticipated that existing and new-build

infrastructure embankments will be in use for at least 50 years,

questions remain about how the water regime will change in

these embankments and how this will affect their stability. In

this paper, in general, the term sustainability has been used

interchangeably with durability; it is felt that a durable slope

that requires minimum maintenance and repair will have a

lower life cost and cause less disruption to members of the

public and hence be considered sustainable.

Several potential modes of failure for infrastructure slopes have

been postulated, and all are inter-related. When considering

how climate change may affect these modes of failure, the

following should be noted (further information on these modes

of failure can be found in the literature.4,5

(a) Shrink–swell action. Changes to the water content of clay

materials present in slopes created from or in clay soils

(common in the UK) cause volume changes—increases in

water content will cause an increase in volume and vice

versa. Such changes in water content are brought about by

seasonal variation in rainfall combined with changing

demands from vegetation, both spatially and seasonally.

Volume changes, particularly for railway embankments,

cause serviceability problems as tracks have to be kept

within very exacting line and level tolerances. Volume

change is exacerbated by the presence of high-plasticity

materials and deciduous trees. If temperature and seasonal

variation of temperature are to increase and rainfall patterns

are to change, then it is reasonable to assume that shrink–

swell cycles will also change, with the potential to increase

in magnitude. Patterns of vegetation growth are also likely

to change, with the potential to cause volume change to

greater depths because of plant rooting to greater depths

during periods of drought.

(b) Progressive failure. Successive cycles of shrink and swell

induced by water content changes can lead to strain

softening and areas of reduced strength within the clay

slope. These can form shear planes and cause failure to

occur progressively, beginning (probably) at the toe of the

slope and eventually leading to an ultimate limit state

failure. With an increased magnitude of shrink–swell cycles,

climate change has the potential to lead to reduced times to

reach ultimate limit state failure.

(c) Pore water pressure equilibration (re-wetting).

Embankments created from, and cuttings within, over-

consolidated clays prevalent throughout the UK experience

high negative pore water pressures that last for approxi-

mately 10–15 years on average after construction, providing

apparent stability to the slope. Over time, precipitation will

cause pore water pressures to increase, leading to potential

instability. With increased rainfall in winter, the time taken

Proceedings of the Institution of Civil Engineers esu800022.3d 23/3/09 15:12:11

Engineering Sustainability 000 Issue ES000 Full-scale testing to assess climate effects on embankments Hughes et al. 1

PROOF

to reach unstable levels of pore water pressure could be

reduced; alternatively, increased summer temperatures may

lead to increased suctions and hence improve stability.

(d) Surface creep. The action of drying and shrinkage can lead

to surface cracking in dry periods. Cracks can allow water

into an embankment and so accelerate the re-wetting

process and reduce the number of cycles required to reach

failure by progressive means. Additionally, when surface

cracks re-close due to winter wetting, the closure will tend

to be in the downslope direction, due to gravitational forces.

Over a number of successive cycles, this leads to a mass

surface movement downslope, potentially leading to shal-

low, surface failure. If increased summer temperatures lead

to deeper cracks, the downward movement may penetrate

deeper into the soil and hence lead to more significant

failures.

In order to establish whether these scenarios are likely to occur

as a result of climate change, a combination of numerical and

physical modelling and testing is necessary. Physical modelling

using a centrifuge has advantages in terms of the ability to

simulate long time-series events. However, this has limitations;

in particular, not being able to simulate the complex interac-

tions between soil and vegetation, and also limitations on the

small-scale instrumentation that can be used. Data from

instrumentation on ‘real’ embankments can be extremely useful,

but in a study of climatic effects there are a number of problems,

including lack of control over the weather and the potential for

undiscovered sub-surface features to affect the results, making

data interpretation either very difficult or impossible. However,

the construction of a full-scale trial embankment negates these

problems, allowing long-term experiments to be conducted to

establish durability, and hence sustainability, in a controlled

environment and timeframe.

The aim therefore was to establish a facility for engineering and

biological research to improve the fundamental understanding

of the effects of climate change on slopes. Specific objectives

included

(a) the building and monitoring of an embankment represen-

tative of UK infrastructure

(b) planting and monitoring representative vegetation

(c) creating a controlled climate

(d) running experiments to simulate future UK climate and

monitoring the response of the embankment.

Through improvised compaction methods it was hoped to be

able to produce properties similar to those of older embank-

ments, although artificial ageing of earth structures is very

difficult to quantify and reproduce. This paper summarises the

outputs against these objectives.

2. SPECIFICATION, CONSTRUCTION, MONITORING

AND TESTING OF THE BIONICS EMBANKMENT

The full-scale testing component of the Bionics programme was

fulfilled by the construction of a purpose-built embankment

representative of UK transport infrastructure (for both highways

and railways). The embankment was richly monitored with both

industry-standard and novel instrumentation, with a climate

control system capable of subjecting half of the embankment to

predicted future climates.

2.1. Specification of embankment/fill

A full-scale earth structure was required to provide data on the

interaction between hydrological, biological and geotechnical

processes. An embankment (as opposed to a cutting) was chosen

so that its constituent material could be closely controlled,

monitored and tested both during and after construction.

Published information6,7 and discussions with stakeholders were

used to establish the general design principles for the embank-

ment shown in Figure 1. The geometry of the embankment

(90 m long, 29 m wide, 6 m high with a 5 m crest) was based

on a report by Perry et al.7 1:2 slide slopes were considered

representative of UK infrastructure, and the length, total width

and crest width were based on cost limitations.

In terms of construction, a compromise had to be reached

between the characteristics of older, heterogeneous rail

embankments (compaction was not controlled, drainage is poor

and pore pressures tend towards positive at the centre and

negative at the surface) and the more uniform, better engineered

highway slopes with diametrically opposing pore water pressure

profiles. The embankment was thus split into four main

engineering testing sections, two compacted to current specifi-

cations for highway works8 and two compacted to a lower

specification more representative of Victorian railway

embankments. The methodology for construction was developed

using compaction trials carried out on site immediately prior to

the main build. Each test panel was to be hydraulically separated

by a double layer of heavy-duty plastic (VisqueenTM). A 0?5 m

free-draining capping layer was placed on the crest to prevent

desiccation cracking and the formation of unrealistic boundary

conditions. A 200 mm layer of top soil was placed on the side-

slopes of the embankment to facilitate vegetation growth and

simulate current infrastructure embankments. A drainage

blanket under the embankment was also considered, but the

stakeholders consulted when drawing up the specification

advised that this would be unrepresentative of real UK

embankments.

In order to be able to study the modes of failure described in

Section 1, the design specified fill of moderate to high plasticity

that was to represent as closely as possible the properties of

London clay (London clay having been identified by stake-

holders as being of particular interest and having well-

researched material properties). After a literature review and

field surveys, a seed mix representative of real embankments

was selected to plant on the embankment.

2.2. Construction

Nafferton Farm, Stocksfield, Northumberland (Ordnance Survey

grid reference NZ 064 657) was selected for the site of the

Bionics embankment. A site investigation demonstrated that the

underlying conditions were stiff–hard glacial till to a depth in

excess of 16 m underlain by carboniferous limestone. A search

for suitable fill material in the local area found that the available

material most closely matching the design specification within

reasonable distance of the site was Durham lower boulder clay,

sourced from an industrial development on the east of Durham

city. Testing of the material showed it to be of intermediate

plasticity (plastic limit 23?2%, liquid limit 41?7%), with a

moderately high shear strength (peak w9 27?5 , c9 4 kPa as

opposed to London clay with typical peak values of w9 20˚and c9

12 kPa) ;. Whilst the properties of this material did not match


2 Engineering Sustainability 000 Issue ES000 Full-scale testing to assess climate effects on embankments Hughes et al.

PROOF

those of London clay perfectly, it was considered that an

acceptable compromise had been reached within the constraints

of time and budget demanded by the project as a whole.

Additionally, boulder clay of this type is typical of over 60% of

the British Isles and hence reasonably representative of UK

infrastructure.

Construction commenced in the late summer of 2005. The well-

compacted sections were constructed in 0?3 m lifts, each

receiving 18 passes of a 7300 kg self-propelled smooth drum

vibrating roller. The ‘poorly’ compacted sections were con-

structed in 1?3 m lifts with minimum tracking by a tracked

excavator (22 000 kg) rather than by compaction plant. Core

cutter density measurements were taken during construction of

the separate panels (Table 19). These results demonstrate that a

measurable difference in density was achieved using the two

different methods, although it is difficult to achieve ‘poor’

compaction of soil when using modern, heavy compaction plant

on this type of soil.

Installation of double layers of VisqueenTM between each test

panel proved challenging because it was difficult to compact fill

right up to the edge of the membranes and also to maintain

verticality during the construction of each lift. These problems

were overcome using an improvised system of straw bale

formwork whilst working around the membranes, as shown in

Figure 2.

Measurements of soil matrix suction were also taken when

sampling for density tests using equipment developed at


90 m

4 m

18 m

29 m

N

Reinforced earth ends

Reinforced earth ends

Area of sprinklers Tensiometer/piezo strings

Tensiometer/piezo strings(discussed in Section 4.1)

Position of soil cores(discussed in Section 6.4)Area of covers

5 m wide crest, 0.5 m granular fill capping layer

Biological test plot

Biological test plot

Poor-compaction test plot

Poor-compaction test plot

Idealised cross-section (Figure 4(a))

Highways Agency specificationtest plots

Impermeable membranesbetween each test plot

18 m

18 m

18 m

4 m

KEY

Y SC

SC SC

D

C

B

A

Y

X

SCSC

SCSC

SCSC

X

Figure 1. Plan of the Bionics embankment

Compac-tion

Bulkdensity:Mg/M3

Watercontent:

%

Drydensity:Mg/M3

Airvoids: %

Degree ofsaturation:

%

‘Poor’ 1?93 20?7 1?6 6?0 85?3Good 2?01 20?1 1?7 3?2 91?4

Table 1. Summary of core cutter density results9Figure 2. Construction of impermeable membranes


PROOF

Durham University. Results from the tests are shown in

Figure 3.9 The plots clearly show the difference in construction-

generated soil suction between the well-compacted and poorly

compacted panels. As testing was conducted on the open surface

of a core sample (i.e. exposed to air), it was important to

eliminate suctions generated by surface drying effects when

interpreting these data. Once these effects have been ignored, it

was possible to deduce that the well-compacted samples

produced relatively high recorded suctions of up to 2150 kPa,

whilst the poorly compacted panels produced relatively low

recorded suctions of between 240 and 280 kPa. These are

indicators of relative amounts of stress relief, where the

difference in stress is due to the different levels of

compaction.

2.3. Instrumentation and monitoring

After construction of the embankment, an extensive array of

monitoring equipment was installed. Installing monitoring

equipment during construction had been considered in the

planning stages, but the difficulty of achieving consistent

compaction close to the instrumentation combined with the

inevitability of damage to instrumentation during construction

was considered too great a risk. Instrumentation was therefore

installed in boreholes created by a Dando Terrier 130 tracked

mini drilling rig.

A range of instrumentation types was installed on the

embankment, representing industry-standard geotechnical

equipment and hydrological, agricultural, biological and

metrological sensors in addition to experimental systems being

developed by Dixon and Spriggs10 and Mendes et al.11 The

ability to measure positive and negative pore pressures and

shrink–swell deformations in the slope were viewed as

particularly critical in this project. Bearing this in mind, the pore

pressure monitoring equipment selected was chosen for its

capacity to read positive and negative pressures and was

combined with electrical methods of soil water content

measurement within the root zone where pore pressures can be

inferred indirectly.

A combination of in-place and probe inclinometers was selected

to combine the benefits of continuous measurement with the


00

_50

_100

_150

_200

_250

_300

_350

_400

50 100 150 200 250 300 350

Suct

ion:

kPa

Time: days

(a)

00

_50

_100

_150

_200

_250

_300

_350

_400

50 100 150 200 250 300 350

Suct

ion:

kPa

Time: days

Time: m

Time: m(b)

Panel A, Layer 8Panel A, Layer 9Panel D, Layer 5Panel D, Layer 8Panel D, Layer 9

Panel B, Layer 9Panel B, Layer 15Panel C, Layer7Panel C, Layer 11Panel C, Layer 16Panel C, Layer 18

Figure 3. Soil suctions recorded in (a) samples from ‘poorly’ compacted sections of theembankment (panels A and D (see Figure 1)) and (b) samples from the well-compacted <sections of the embankment (panels C and D)


PROOF

number of locations that can be monitored. The instrumentation

layout was designed to provide information on all potential

modes of failure, and was developed using initial numerical

simulations of the embankment behaviour including construc-

tion stresses and subsequent equilibration of pore water

pressures.

The selected instrumentation enables monitoring of pore

pressure, water content, deformation and weather conditions in

both poorly and well-compacted sections of the embankment,

on north and south aspects and within the climate-controlled

and non-climate-controlled zones. This information can then be

used to analyse slope behaviour and calibrate numerical models

of embankment behaviour. The data can also help to establish

whether any one mode of failure is critical, and thus establish

the future performance of infrastructure slopes subjected to

climate change. The combination of commonly used instru-

mentation with experimental systems is intended to promote the

development of new technologies within the field of slope

stability. The instruments installed are listed in Table 2, which

also shows the parameters measured by each instrument and the

failure mode (as discussed in Section 1) for which it provides

information. The distribution of instrumentation within an

idealised cross-section of panel B (see Figure 1) of the

embankment is shown in Figure 4(a) and in plan view in

Figure 4(b).

2.4. Testing

During installation of the instrumentation on the Bionics site,

undisturbed samples were recovered from instrumentation

boreholes. These samples were characterised using a range of

tests in accordance with BS 1377.12 A summary of the initial

characterisation testing data is given in Table 3.

2.5. Vegetation

A seed mixture of grasses and wildflowers was selected to

represent both a typical Highways Agency seed mixture and

species typical of UK grassland. However, because of seeds

already present in the top soil and colonisation by local species,

it is inevitable that, over time, the slopes of the embankment will

most closely represent a north east England grassland. The

embankment was sown in July 2006 to study how vegetation

may affect slope failure mechanisms and also to see how aspect

and soil compaction may influence species composition and

diversity of the grassland mixture. A survey of plant species

growing on the slopes of the embankment was undertaken in

July 2007. In summary, it found that both species richness and

diversity of species were higher on the south-facing slopes than

on the north-facing slopes. The effects of soil compaction were

more complex. There was a highly significant interaction

between aspect and soil compaction. The south-facing well-

compacted plots had significantly lower wildflower species

richness than elsewhere, and species composition was signifi-

cantly different.


Parameter measured Typical usagePrimary information on

failure mode(s)

Flushable piezometers Positive and negative pore pressure,continuous measurement

Geotechnical monitoring Shrink–swell, re-wetting

Durham tensiometers High values of positive and negativepore pressure continuousmeasurement

Experimental, underdevelopment by DurhamUniversity

Shrink–swell, re-wetting

GEO observations,tensiometers

High values of negative porepressure, point measurement


Standpipe piezometers Piezometric surface, pointmeasurement


In-place inclinometers Slope movement, continuousmonitoring

Geotechnical monitoring Progressive failure

Probe inclinometers Slope movement, point measurement Geotechnical monitoring Progressive failure

Magnetic extensometers Settlement/heave, point measurement Geotechnical monitoring Shrink–swell, surfacecreep

Acoustic waveguides Slope movement, continuousmeasurement

Experimental, underdevelopment byLoughborough University

Progressive failure

Theta probes Volumetric soil water content,continuous measurement

Agriculture irrigation controland geotechnical monitoring.


Water content profile probes Volumetric water content, pointmeasurement

Agriculture irrigation control Shrink–swell, re-wetting

Temperature probes Soil temperature, continuous measure-ment

Agriculture, biology,hydrology, meteorology


Weather stations Rainfall, wind speed/direction,pressure, humidity, air temperature,sunlight (inputs to potentialevapo-transpiration calculation;indirect, requires calculation)

Meteorology, agriculture,hydrology, biology


Remote methods: Lidar, terres-trial laser scanner, GPS

Movement, condition of vegetation Remote ground surfacemonitoring

Progressive failure,surface creep

Table 2. Summary of embankment instrumentation


PROOF

Both engineering condition and climate therefore affect species

composition on engineered slopes. It is therefore reasonable to

assume that the changing engineering condition of an

engineered slope as a response to climate change and the

changing climate will both have an effect on vegetation and

hence the slope’s engineering and environmental sustainability.

The issue of slope vegetation is dealt with in more detail by

Glendinning et al.13 in this issue.

3. CLIMATE CONTROL SYSTEM

In order to study the long-term behaviour of infrastructure

slopes with reference to future climate scenarios for the UK

(as summarised in Section 1 and detailed by Kilsby et al.3), it

was necessary to heat and cover sections of the embankment,

to control rainfall and to provide a section to represent

present climatic conditions. A controlled climate needed to

be provided to half of the test plots (and ambient weather


Southern slope12.5 m

(a)

(b)

GPS aerial

6 m

18 m

2 m

5 m

Tensiometer

Soil moisture probe

Piezometer

Theta probes

Tensiometer

Soil moisture probe

Piezometer

Theta probes

Inclinometer

Magnetic extensiometer

GPS aerial

Micro weather station

Temperature probes

Magnetic extensiometer

Inclinometer

Micro weather station

Temperature probes

Northern slope12.5 m

Southern slope12 m

Northern slope12 m

5 m

Figure 4. (a) Idealised cross-section of embankment instrumentation (position ofcross-section shown on Figure 1). (b) Plan view of embankment instrumentation inwell-compacted panel B

Compaction Bulk density: Mg/m3 Dry density: Mg/m3

2?5 kg 2?11 at 15?5% WC* 1?82 at 15?5% OMC{4?5 kg 2?25 at 12?9% WC* 2?00 at 12?9% OMC{

Atterberg limits Average liquidlimit: %

Average plasticlimit: %

Average plasticindex: %

Average naturalwater content: %

Soil is clay of intermediateplasticity (BS 1377)

41?7 23?2 21?6 19?4

Laboratory permeability ‘Poorly’ compacted panels 1?6610210 m/sWell-compacted panels 8?8610211 m/s

Shear box tests of reconstituted material:WC 23?3%; bulk density 2?03 Mg/m3;as-placed water content 20%

Q9p 5 27?5˚c9p 5 3?7 kPa

*WC, water content{OMC, optimum water content for compactionQ9p, c9p, peak parameters measured at relatively small strain

Table 3. Soils testing data summary


PROOF

monitored throughout) so that the effects of post-construction

equilibration could be separated from changes induced by

climate. It is appreciated that there will be an effect of post-

construction equilibration of construction-generated pore water

pressures. However, with time and increasing numbers of

experiments, it may be possible to quantify these effects.

The installed system consists of an array of 48 rotating sprinkler

heads mounted on 1?1 m poles covering half of the embankment

(Figure 5). A programmable automated control system allows

operators to control the timing and intensity of simulated

rainfall events and isolate different test sections for different

simulations. The system can simulate continuous rainfall

intensities of 2–11 mm/h in its current configuration. However,

the system has been designed so that, with minimum

modification, rainfall intensities of up to 25 mm/h can be

generated for short periods.

A retractable cover system to allow artificial drying and heating

covers a total of 250 m2 of the embankment. The cover material

used is transparent and in the ‘rest’ condition, the cover system

stows the fabric at the crest of the embankment where its

shadow has minimum impact on vegetation as the crest is

composed of rockfill. It is estimated that by covering the

embankment at night, a temperature difference of 1–2 C can be

achieved without impacting on ultraviolet or infrared light

reaching the vegetation.

Climate and environmental monitoring form a vital part of the

climate control system. Surface water run-off is measured by

two collection systems. Coarse high-volume measurement is

made via collecting drains running 18 m across the toe of the

slope on two of the test sections; the run-off volume is measured

as it flows into a collecting tank where sediment is also collected

for surface erosion studies. Finer, smaller volume run-off is

measured using 1 m drains.

An ambient weather station installed to Royal Meteorological

Society guidelines14 is situated 300 m from the embankment

site. A further two micro weather stations are positioned on the

north and south slopes of the embankment; these are sited at

much lower heights to monitor the different climatic conditions

experienced close to surface due to the different aspects. There is

evidence that there are significant differences in wind speed and

temperature between the two sides of the embankment, which

will have a significant impact on evapo-transpiration and hence

infiltration, runoff and soil water condition.15 A log is kept of

artificial rainfall (sprinklers) and artificial drying (covers). This

information is then integrated with climate monitoring data to

give an accurate ‘weather’ log for the climate-controlled areas.

4. MONITORING RESULTS

The results presented in this section cover the commissioning

phase of the embankment monitoring systems, prior to

installation of the climate control systems (the results of which

are detailed in Section 6).

4.1. Pore water pressures

The system to measure pore water pressure was installed in the

Bionics embankment in April 2007, i.e. later than the other

geotechnical instrumentation systems; the delay was due to

development work on the high-capacity tensiometers that

formed part of the system. The continuous measurements

presented here relate to the period November 2007 to June 2008

for both well-compacted and poorly compacted sections.

Figure 6 shows that the pore water pressures of the well-

compacted panel dropped during November 2007 (during a

period with little rain) and suctions were recorded near the

surface. However, with increased rainfall in late November/

December, pore water pressures become positive throughout,

and this was maintained throughout the following wet January.

In the drier conditions of February to May, pore water pressures

continue to drop and small suctions were recorded again near

the surface. It must be stated that in some cases there is a more

rapid response to rainfall events at depth than would be

anticipated from measured permeabilities; this may be due to

some preferential flow of water down the measurement borehole

although it is also believed that the mass permeability of the soil

is somewhat higher than the measured values as will be

discussed later in this section.

Figure 7 shows the pore water pressure data plotted as vertical

profiles for each month of measurement (position of tensi-

ometers/piezometers shown on Figure 1). Data from a nearby

flushable piezometer at a depth of 4?5 m are also included. The

values show a progressive increase with depth down to 3 m, but

then drop back at 4?5 m as recorded by the flushable piezometer.

The profiles generally show that, in December, pore water

pressures near the surface rise considerably, exceeding the

hydrostatic condition near the surface on 01/01/2008. Pore

water pressures near the surface (,2 m depth) then reduce over

subsequent months, but values at 4?5 m depth show an increase.

This suggests that water that infiltrated during the wetter

months of November–January is draining down through the

embankment and causing a rise in values some two to three

months later. This is consistent with a soil permeability of

approximately 561027 m/s. This value of permeability is in

excess of values recorded in the laboratory (see Table 3),

indicating that the mass permeability of the fill is somewhat

greater than that measured in a triaxial cell. This is supported by

observations made when digging excavations for parts of the

embankment climate control system where water-bearing

cracks, parallel to the ground surface, were found at a depth of

approximately 700 mm. Where pore pressures in excess of

hydrostatic have been recorded, this can be attributed to the

gravel capping layer present on the embankment crest.

Although the material is, in principle, free draining, the presence

of some fines within the gravel may lead to some temporary

retention of water.


Figure 5. Rainfall sprinkler component of climate controlsystem


PROOF

Pore water pressure values show more variation for the poorly

compacted panel (Figure 8) than the well-compacted panel,

probably due to greater and more heterogeneous permeability. It

was observed that the tensiometers reacted quite rapidly to

weather conditions, showing quite rapid increases in pore water

pressure when precipitation occurred (more observed in heavier

rainfall) and vice versa for drier periods.

Vertical profiles of pore water pressure for the poorly compacted

panel are shown in Figure 9 (position of tensiometers/piezometers

shown on Figure 1) along with the results from flushable

piezometers at depths of 4?5 and 5?5 m. There is a general increase

of pore water pressure with depth down to 2 m. The pore water

pressure then tends to decrease, reaching a minimum at around 4?5

m before increasing again at the base of the embankment (below

5?5 m). In the well-compacted panel, pre water pressure values

reached a maximum around 3 m before dropping back at 4?5 m

depth, whereas in the poorly compacted panel the maximum

occurs around 2 m and reduces at 3 m and 4?5 m. Again, pore

pressures in excess of hydrostatic can be attributed to the gravel

capping layer. Differences between the pore pressure profiles of

well- and poorly compacted panels can be explained by differences

in permeability caused by the compaction methods used.

4.2. Deformation

Surface settlement has been monitored using magnetic extens-

ometers since May 2006. Plots of the settlement in panels A and

B are shown in Figures 10 and 11 respectively. Thus far, the

well-compacted panels have settled between 12 mm (recorded in

panel C) and 16 mm (panel B), though settlement still appears to

be occurring. In the poorly compacted panels, the measured

settlements are between 26 mm (panel A) and 33 mm (panel D),

though the rate of settlement had reduced to almost zero by the

summer of 2007. Maximum values of settlement were recorded

in magnets closest to the surface (1?3 m below ground level).

Analysis of readings from the extensometers in the well-compacted

panels has also shown that, after periods of intense rainfall,


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Figure 6. Pore water pressure records for the well-compacted panel suction (SSindicates suction station at different depths). Vertical spikes show daily rainfall andvertical drop lines show reset stages

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01/09/2007 12:0201/10/2007 12:0201/11/2007 12:0201/12/2007 12:0201/01/2008 12:0201/02/2008 12:0201/03/2008 12:1501/04/2008 12:45Hydrostatic line

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PROOF

swelling occurred (particularly in the fill close to the surface).

Longer datasets are required, but this finding may indicate that

there is potential for progressive failure within the Bionics slope).

Internal slope movement has been monitored using inclin-

ometers since August 2006; thus far no significant movement

has been recorded.

4.3. Summary

Overall, the project has fulfilled the aim of establishing a

research facility for engineering and biological research. It has

also met the objectives of constructing and monitoring an

embankment representative of UK infrastructure, planting and

monitoring representative vegetation and creating a controlled

climate. The information that has begun to be gathered is

providing data related to the failure modes summarised in

Section 1 and hence on the sustainability of infrastructure

slopes. The next stage of the work was simulation of a future UK

climate and monitoring the embankment’s response.

5. RAINFALL EXPERIMENTS

The aim of the experiments was to subject a section of the

embankment to a controlled rainfall event representative of a

future UK climate scenario. Of particular interest was the effect

of a prolonged wetting event and a shorter, more intense rainfall

event as these were considered significant in the context of the

failure scenarios outlined in Section 1. The aim was also to

compare the response of the embankment sections to extreme

events with ambient conditions occurring naturally at the site. It

has been observed that a prolonged wet period followed by a

major rainfall event is more likely to cause failure than when

antecedent conditions are dry; this is discussed further by

Glendinning et al.16 in this issue.

5.1. Methodology

A series of three experiments using the rainfall sprinkler system

was run between May and July 2008. These tests were initially

intended to provide a commissioning test of the system and then

to simulate the effects of persistent moderate rainfall and a


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Figure 8. Pore water pressure records for the poorly compacted panel suction (SSindicates suction station at different depths). Vertical spikes show daily rainfall andvertical drop lines show reset stages

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Figure 9. Pore water pressure profiles of poorly compacted panel


PROOF

period of dry weather followed by a short storm event (Table 4).

The way in which the magnitude of the rainfall events was

calculated and its relationship with current and future climate

events for different parts of the UK is discussed in more detail by

Kilsby et al.3 in this issue. The storm event was the same

intensity as the inundation experiment but three days shorter;

the significance of this being that the storm experiment was to

follow a prolonged wet period after a short dry spell and hence

investigate the effect of antecedent conditions.

Instrumentation on the embankment, particularly the soil

moisture probes, piezometers and movement sensors, was


Test Name Description

1 Commissioning Shake-down tests of rainfall system, control computer, pumps valves plus rainfallspatial distribution and rainfall rates

2 Inundation Simulation of persistent moderate rainfall: 33 mm/day for five days, followed bytwo days without rain, repeated three times

3 Storm Simulation of two-day storm event, 33 mm/day for two days

Table 4. Summary of rainfall tests May–July 2008

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Figure 10. Settlement of poorly compacted panel A17

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Figure 11. Settlement of well-compacted panel B17


PROOF

monitored more intensely during the rainfall simulations. In

addition to instrumentation readings, plant surveys were

undertaken before and after the series of tests, and 16 cores were

recovered before and after for analysis of gravimetric water

content, root depth/density, shear strength properties and

mineralogical analysis.

6. OVERVIEW OF RESULTS

The series of experiments conducted has generated a significant

quantity of data. This paper gives a broad summary of the

findings in relation to the failure mechanisms identified in

Section 1.

6.1. Pore pressure

The reaction of pore water pressures to ambient weather

conditions prior to the rainfall experiments can be seen from the

vertical profiles of pore water pressure in Figures 7 and 9. In the

well-compacted panel, values exceeding the hydrostatic condi-

tion were recorded on 01/01/2008 within 2 m of the surface in

response to prolonged intermittent heavy rainfall during

November and December 2007 followed by intense rainfall in

January 2008. Pore pressures near the surface then dropped

back in subsequent months when there was significantly less

rainfall. Generally, both poorly and well-compacted sections

present near-hydrostatic values at shallower depths, even

exceeding hydrostatic conditions after heavy rainfall, while

deeper inversion occurs to give negative values of pore water

pressure at around 4?5 m depth.

Measurements taken during the rainfall tests showed that the

initial period of simulated rainfall did not cause a significant rise

in pore water pressure. However, when the large rainfall event

was combined with significant antecedent rainfall, pore

pressures were observed to increase; these results are of more

relevance to winter rather than summer conditions. This is a

similar situation to that occurring in January 2008, and shows

that antecedent rainfall is important in the generation of high

pore pressures at depth. It should be noted that high antecedent

rainfall is one of the likely scenarios predicted by UKCIP.2

6.2. Water content

Water content monitoring equipment was installed in April

2008 and therefore a long record of soil water content prior to

the beginning of the rainfall experiments is not available.

However, soil water content was measured before, during and

after the rainfall tests. Figure 12 shows a summary of the data

collected during the inundation test.

Figures 12(A) and (b) clearly show a ‘wetting up’ process

occurring in the soil down to 600 mm in the panels subjected to

artificial rainfall (panels A and B). This contrasts with plots of

soil water content in panels C and D, which show no wetting up

consistent with ambient weather conditions at the time. This

pattern is as could be anticipated from applying water to one

half of the embankment and not the other.

The plots also show that, in all cases, below 600 mm there was

little or no change in soil water content throughout the

experiment. This is consistent with a lack of pore pressure

change during the experiments (see Figures 6 and 8). This has

been attributed to the permeability of the soil: in the upper 600

mm, macro-scale permeability (due to the presence of cracks

and discontinuities) dominates the hydraulic conductivity of the

soil; below 600 mm, there are far fewer cracks and the

permeability is much lower. The presence of cracks may also

explain the rapid response of pore pressures at depth recorded


Volumetric water content: %(a) (b)

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Figure 12. Soil water content changes: (a) panel A, south lower; (b) panel B, south lower; (c) panel C, south lower; (d) panel D, southlower. (B), (D) and (A) after the date on each plot represent before, during and after inundation test respectively


PROOF

by the tensiometers (Figures 6 and 8). Observations of crack

depths and their response to rainfall by Mason17 support this

argument, as do the horizontal cracks at 700 mm depth found in

trial excavations conducted during installation of the cover

system. There is also the possibility that vegetation may be

producing an effect in the upper 300 mm of soil where the

majority of the change in water content is observed. This will be

discussed further in Section 6.4. Water content measurements

showed a similar reaction to the shorter storm simulation

experiment. Future experiments will attempt to create more

intensive rainfall over the same period to see if this causes more

rapid wetting up of the slope.

6.3. Deformation

Extensometer readings were taken once a week during the

rainfall experiments. Preliminary analysis shows that very small

amounts (1–2 mm) of heave occurred in the top 3 m of the

embankment during the tests (Figures 10 and 11).17 This

supports the argument that rainfall induces surface heave, thus

potentially increasing the rate of shrink–swell associated failure

mechanisms and surface creep effects. During the rainfall tests,

inclinometer readings indicated some small downslope move-

ments, but these were within the potential error range of the

equipment used (1 mm). Evidence from remote methods

deployed on the embankment indicated that localised areas of

shallow soil creep occurred in the top soil.18 However, as this

makes up only the top 200–300 mm of the slope, the

inclinometer systems cannot detect this movement. Again, this

may be an early indication of the increase in surface creep as a

result of increased rainfall.

6.4. Vegetation

Sixteen 1 m long soil cores were taken from different test

sections (shown on Figure 1) prior to and after the rainfall

experiments to investigate the effects of soil density, total

rainfall and aspect on plant rooting. Cores taken on 7 May 2008

and two months later (10 July) were tested in the laboratory to

determine root quantity and distribution.

The level of compaction, and hence soil density, was observed to

have an effect on the total root mass within the different

sections of the embankment. In sections that were well

compacted, plant roots did not penetrate as deeply as in poorly

compacted sections, consistent with studies conducted by

Whalley et al.19 The interface between the top soil and

engineered fill appears to concentrate roots in the top 300 mm

of the soil. This supports the Highways Agency requirement that

slopes are scarified prior to the placement of top soil to make a

more gradual change in density, rather than creating a barrier to

root growth.

Measurements taken after the rainfall experiments showed

differences in root patterns between the areas of the embank-

ment exposed to artificial rainfall and those that were not. Root

mass increased in the top 200–300 mm of soil significantly (by

15–20%) in the sections receiving additional artificial rainfall.

This change occurred in both well- and poorly compacted zones

and on both north and south aspects. Although only a few

results are available, they do suggest that climate conditions

influence root mass and depth. It can therefore be inferred that

climate change has the potential to influence rooting properties

and hence overall slope stability.

There was a significant difference in root mass between the

north and south aspects of the embankment (up to 40% in

places). Temperature probes in the embankment showed a 1–2 C

(warmer on the south-facing side of the embankment) difference

in temperature between the two aspects of the embankment and

for the purposes of this experiment the two sides can be

considered as experiencing two different climates. The results

recorded so far demonstrate the significant effects that a slight

change in temperature and prevailing wind speed and direction

can have on root densities on infrastructure embankments. Even

when not considering potential future climate change, aspect

would appear to be a significant factor in embankment slope

stability and has already been demonstrated to be a factor in the

stability of natural and man-made slopes.7,20

7. CONCLUSIONS

The experiments conducted using the climate system and the

Bionics facility have demonstrated a capability to provide useful

information about failure mechanisms both in terms of

engineering and vegetation responses to climatic events. It has

thus been demonstrated that the facility and the accompanying

research can provide data concerning the long-term sustain-

ability of slopes, providing an invaluable resource to support

other research. Once completed, the results of this research will

be used by engineers and infrastructure asset managers to

design and maintain sustainable engineered infrastructure

slopes through the selection of appropriate vegetation, drainage

techniques and management systems.

Initial findings support the idea that climate change will affect

the engineering behaviour of slopes and the vegetation growing

on them, and that there is an interaction between the two

systems that will influence their mutual long-term sustain-

ability. As with much research, more questions have been asked

than have been answered. It seems that near-surface processes

are important to the long-term engineering and biological

systems and are poorly understood. The Bionics facility is in a

unique position to provide researchers with a means of studying

these processes as they require an understanding of the

interaction of soil and vegetation that can only be achieved

using a full-scale physical model.

ACKNOWLEDGEMENTS

The authors wish to acknowledge funding for the project from

the Engineering and Physical Sciences Research Council

(Bionics GR/S87430/01), UKCIP, Newcastle University and the

Railway Safety and Standards Board. They would also like to

acknowledge the continued and vital support from all the

stakeholders, all of whom are listed on the project website.21

Additional thanks are extended to researchers Richard Holland,

Jack Mason, Bruce Stephenson and Marc Protheroe for their

assistance during the climate experiments and to Peter Helm of

Newcastle University for his assistance in the submission of this

paper.

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Full-scale testing to assess climate effects on embankments

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