TECHNICAL PAPER

Examining the carbon footprint andreducing the environmental impact ofslope engineering optionsNick O 'Riordan , Duncan Nicholson , Lynsay Hughesand Alan Phear (Arup Group)

Considerable progress has beenmade in the assessment, managementand maintenance of existingearthworks and in the selection ofmethods of construction of newearthworks. Techniques involvingmulti-criteria analysis, carbon-footprinting and life cycle assessmentof competing schemes providetransparent methodologies for use inenvironmental impact assessment.

This review paper providesexamples of use of the varioustechniques, and in particularconcentrates on areas of innovationincluding:

use of carbon calculators tochoose between competing slope orearthwork stabilisation solutions

methods of ground treatment tominimise earthwork impact (eginsitu soil strengthening, soilnailing, or green walls)

combining performance-baseddesign, under environmental loadcases such as earthquake and flood,with carbon footprint and costanalyses.

The EU Sustainable DevelopmentStrategy directive, renewed in 2006,has a three-part approach, settingthe following objectives by 2020:

to reduce greenhouse gas

emissions by 20%to reduce primary energy usage

by 20%to increase use of renewable

forms of energy to 20% of the

energy mix.A recent Sustainable

Development Commission report- Prosperity without Growth - statesthat the world produces 770g ofCO2 per $ of income and that thisfigure must reduce to 6g per $ by2050, if all 9 billion of us are toenjoy European income levels in2050 and simultaneously stabiliseatmospheric CO, at 450ppm.

It is generally agreed thatconstruction activity contributesabout 25% to global carbonemissions, and so it will be essentialto reduce emissions in construction

to pursue sustainable development.Economic growth relies on the

confidence of society in all its formsand the predictability of outcome ofa certain course of action. There isthus a strong driver to put groundengineering activities into a contextwhich can show improvementsin these indicators of sustainabledevelopment. Relevant standards areISO 14064 (2006) and 14065(2006).

This paper provides thebuilding blocks for embodiedenergy and equivalent carbondioxide calculations associatedwith the most common forms ofslope engineering. It places thesecalculations into the wider contextof transport and resource planning.

Conventional gasoline sedanConventional gasoline SUV

Conventional Gasoline pickupUrban diesel bus (off peak)

Urban diesel bus (peak) 11

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In the UK, the Environment Agencyhas produced a carbon calculator forconstruction activities and this hasbeen extended specifically forhighway-related projects by theHighways Agency. Thesedocuments provide a basicframework from which an inventoryof emissions in terms of CO,equivalents can be prepared forsignificant movements of earthworkand other materials. Suchcalculation systems enable broaddecisions to be made, but are notsufficient to allow differentmethodologies to be compared.

O'Riordan and Phear (2009)

observed that as confidence in

Commuter rail (SFBA Caltrin ) C 1101`Light rail ISF Muni)

Light rail (Boston Green Line)Small aircraft I

Midsize aircraftLarge aircraft

Conventional gasoline sedanConventional gasoline SUV

Conventional Gasoline pickupUrban diesel bus (off peak)

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1 2

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Energy consumption ( MJ/PKT)

4 5 63

7ff:_:::=^^ 111-1

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Urban diesel bus (peak) EI- $1Commuter rail (SFBA Caltrin) T-1L-IL

Light rail ISF Muni) (-7^ALight rail ( Boston Green Line) I 1 11 J11111111111111"

Small aircraftMidsize aircraft

Large aircraft

0 50 100 150

Greenhouse gas emissions ( g C02e/PKT)

200 250 300 350 400 450

q Vehicle active operation '_ Vehicle inactive operation n Vehicle manufacturing q Vehicle maintenanceW1 Vehicle insurance q Infrastructure construction n Infrastructure operation LI Infrastructure maintenancen Infrastructure parking q Infrastructure insurance q Fuel production

Figure 1: Energy consumption and equivalent carbon dioxide emissions , expressed in terms of PKT(passenger-km travelled ) after Chester and Horvath (2010). Infrastructure components are shown inshades of red and orange, fuel production and consumption for a given vehicle use is shown in greenand grey respectively and vehicle components are in blue.

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carbon calculators grows, so suchcalculations can be incorporatedinto design, optioneering and valueengineering activities. Infrastructureclients, designers, and contractorsare all finding carbon accountingto be a useful tool, but in differentways. Designers are increasinglyusing this method in combinationwith traditional cost comparisons todecide between different schemes.Contractors are increasinglyapplying carbon accounting to theirconstruction management systems.

This is because it has beenrealised that carbon reductionis a useful way to combineenvironmental management and"lean construction" methods. There

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is great legal and ethical pressure oncontractors to manage and minimisethe environmental impact of theirwork. There is also always greatpressure to increase operationalefficiency, reduce costs and makebest use of resources. Both topicsare concerned with controllingwaste. Environmental impact fromconstruction plant is closely linkedto the efficient use, or otherwise, ofthat plant and carbon accounting isa good way to show this.

Good construction practicewill also reduce both waste andenvironmental impact. Examples ofthis are co-ordinated planning of thedifferent workstreams, for exampleby workspace booking, and bymaking good quality workmanshipa key objective: poor workmanshipincreases waste, costs and thecarbon footprint.

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Embodied energy is defined as thetotal energy (in Joules) that can beattributed to the use of an item orcomponent. For the constructionindustry, embodied energy includesthe energy used in extraction of theraw materials from the earth; theprocessing of that raw material intofinished products; transportation tosuppliers and then on to site; theconstruction process; the demolitionand recycling; and the constructionand maintenance of any associatedtemporary works.

Quantifying embodied energy isimportant because it encompassesassociated environmental impactssuch as resource depletion andgreenhouse gas emission. Researchinto the relationship betweenembodied energy and carbondioxide shows a high correlation:1GJ of embodied energy produces0.098t of CO2 (CSIRO, 2007).The Inventory of Carbon and Energy(Hammond and Jones, 2008)provides detailed sources ofembodied energy coefficient datafor most materials encounteredin slope engineering, drawn froma wide range of publications. Toprovide greater accuracy to theassociated carbon dioxide emissioncoefficients, the authors have carriedout their own estimations ratherthan apply a common conversionfactor across the whole dataset.As far as practicable, coefficientssimilar or identical to those providedby Hammond and Jones are used incalculations in this paper.

There is thus an emergingknowledge base that can be usedand extended more generally intoground engineering activities.By establishing energy usage

GROUND ENGINEERING FEBRUARY 2011

Transport

Extraction of raw materials

Processing into materials

Transport

Production of building products

F(steel pipe, concrete etc

Transport

Earth work

Recycle

Transport

Production of constructionmachinery

Transport

Construction of foundation

Temporary staging

Waste dumping

Service/maintenance

Installation

Structural failure &rebuild of embankment :Target of this research

Figure 2. Flow chart of the process boundary (Chau & Soga, 2007)

5. DJM 50%

4. Geotextile

3. High quality embankment

2. SCP 20%

1. SCP 30%

0 1

ITFF

FTF1

0 Material - soil & sandMaterial - geotextile

• Material - cementTransport

[]Installation

3 4 5 6 7

Embodied energy ( GJ) (x104)

Figure 3 . Construction Embodied Energy of the embankmentdesigns (Chau & Soga, 2007)

and associated greenhouse gasequivalents for geotechnicalprocesses, a common vocabulary canemerge that will facilitate dialogueacross disciplines. For example,the CO, emissions consequencesof choosing to provide a new road,on a particular route, to providebetter mobility around a congestedtown can be compared with theconstnlction and maintenanceemissions associated with the roaddesign and construction (O'Riordanand Phear, 2009). The consequencesin terms of emissions of shorter orlonger construction times, and theassociated processes to achievethem, can be explicitly provided.

Similarly, maintenance decisionsand processes can be examined.Using multi-criteria analysis(MCA) techniques, in whichquantifiable parameters are usedexplicitly, geotechnical processescan be put into the context ofsustainable design, constructionand maintenance practice that upuntil now has concentrated onthe end-user's behaviour insidebuildings. Therefore, although there

are no direct environmental impactsassociated with embodied energy,this link to carbon dioxide suggestsa context for interpreting embodiedenergy data.

Fundamentally, there is littleabsolute knowledge of the energyfootprint of current groundengineering operations. Reid andClark (2000) developed a whole lifecost (WLC) model for earthworksslopes based upon available failureinformation. The implicationsof this work were that slopes asshallow as 1 in 5 could be justifiedon economic grounds. Differentconclusions could be reached usingMCA in which emissions and/orenergy consumption parameters areintroduced into the assessment.

More recently, Workman andSoga (2004) evaluated the embodiedenergy associated with the tunnelconstruction between the Stratfordand the St Pancras terminus of theChannel Tunnel Rail Link project.This work has been extended intoactivities such as retaining walldesign and construction, railwaytrackbed design and maintenance,

and embankment construction onsoft ground (Chau et al, 2007; andO'Riordan, 2007).

Hughes et al (in press) describea "bottom-up" approach to thecarbon accounting of earthworksusing detailed plant movements,and soil treatment options that canbe aligned with bills of quantities.

Examination of current practicecan produce counter-intuitiveresults. For example Harbottle etal (2008) examine the performanceof five remediation projects andconclude that on an emissionsbasis, the "best" overall solution isto provide a cover system, ratherthan use geotechnical processessuch as soil stabilisation or washing,bioremediation or transport tolandfill.

Recent work in the US (forexample, Chester and Horvath,2009) highlights the importance ofan holistic approach to the provisionof new transport infrastructure.Figure 1 puts into context therelative contribution, in carbon andenergy terms, of the constructionof the infrastructure on which thetransport operates. The calculationsfor fuel consumption are based uponlifetime figures. As an example, for agiven road alignment, an optimisedvertical profile which seeks tominimise changes in gradient andassociated fuel consumption wouldyield substantial environmentalbenefits beyond the constructionchoices embedded in theconstruction of the road itself.

O'Riordan and Phear (2009)examine the background ofperformance-based design in thewider context of earthworks at largeand the same principles can beapplied to slope engineering. Theuse of probabilistic methods toestablish the adequacy of a designenables more efficient and economicsolutions to be developed.Embankment and cutting stabilitycan conveniently be treated in thisway (see, for example, Reid andClarke, 2000). Chau and Soga(2007) report on a Japanese study inwhich various ground treatmentmethods were considered forhighway embankments in the Tokyoarea that have to accommodate highseismic forces. The parametersstudied are shown in Figure 2 andthe outcome of EE calculationsshown in Figure 3. SCP denotessand column piles and DIM is aform of deep soil mixing using the"wet" process (Hanson et al, 2001).

By combining these figures withthe probability of failure of theselected treatment system under thedesign earthquake Chau and Sogareport the relationship in Figure

29

TECHNICAL NOTE4, from which can be seen that

the geotextile reinforcedembankment provides the mostenergy-efficient solution: little is tobe gained in strengthening the soilto resist the design earthquake usingsoil mixing, in this case.

Carbon footprinting

It can be seen from Figure 3 that thechoice of treatment, in terms ofmovement of natural soil slopes inembankments and cuttings involveslittle variation in total embodiedenergy. However, with reference toFigure 1 and the demonstrabledominance of vehicular fuel usage,flatter gradients for a road or railwaythan would arise from a conventionalbalanced cut and fill are likely toalter the total energy and carbonbalance of a particular piece ofinfrastructure.

In urban and developedlandscapes, there are landacquisition constraints (HighwaysAgency, 2001). Flatter gradientsrequire deeper cuttings, henceengineered slopes such as thoseinvolving soil nailing, reinforced soiland similar measures, are employedto minimise land-take.

Figure 5 is taken from acomparative study of a simple4.57m high engineered slopeusing reinforced soil (MSE) andan equivalent mass concrete wallreported by Rafalko et al. (2010).

Not surprisingly, the dominantcomponent, 85% of the total, liesin the manufactured elements,including imported selected backfillfor the MSE wall. The wall, locatedat Washington Dulles Airport, is

MSE well

TieStrip

E

Finished grade

Concrete facing panels

Select backhlf

•1.SCP30%0 2. SCP 20%• High quality embankment

Geotextileo DJM 50%

• Construction EE• Lowerbound life cycle EEn Upperbound life cycle EE

o n IN3 t 2-

23 2.5 2

height, is associated with the soilnail solution, substantially lowerthan for reinforced soil and anequivalent gravity wall. Similarcalculations can be repeated for themanufactured elements of crib andgabion walling solutions.

Figure 4. Life cycle Embodied Energy for different aseismic groundtreatment designs (Chau & Soga, 2007)

40m long and we find that the energyassociated with manufacturingamounts to 21.5 GJ/m or4.8 GJ/mz retained material for theMSE wall and 55 GJ/m or 12.2 GJ/M2 retained material for the concretegravity wall.

By way of comparison witha soil nailing solution for themanufactured elements, we canconsider a similar geometry tothat of Figure 5, and assume thatthe stand-up time for the naturalground is sufficiently long for sucha solution to be used. The leadingparameters are:

70 degree, Sm high retained slopein stiff clay

20mm diameter, 6m longgalvanised steel nails at 1.5m c/c

nail grouted into a 150mmdiameter drilled hole

150mm thick sprayed concretefacing with mesh reinforcement(A142 or similar).

Using Hammond andJones, 2008) we find an energyrequirement of 6.9 to 9.6 GJ/m, or1.2 to 1.6 GJ/m2 retained material

Randombackfill

3.66mConcrete1 Reinforcing strip (typical)

levelling padl

ENVIRONMENTAL INDICATORS FOR MSE WALL

Inputs Output air emissions

Photo-Energy Abiotic oxidant Acidification Climate

consumption depletion formation (kg SO, eq.) changeStage : IGJ) (Kg Sb eq .) (kg CZHa eq.) Ikg CO0 eq.)

Manufacturing 860 700 69 540 99 000Tansportation 129 34 20 100 11 100Construction 18.8 9.7 1.6 9.4 1600Dismantling 4.6 2.3 0.6 2.8 450

Sustainable practice in slopeengineering, design and constructioncan play a significant part in reducingenergy use and carbon emissions.Tools for the calculation of carbonand embodied energy are readilyaccessible and these enable designand construction choices to becompared and evaluated.

Performance-based designmethods, in which probability offailure/increased maintenanceduring the design life can beestablished, are well suited tocarbon and energy accountingmethods presented herein. Choicescan be made transparent and readilycommunicated across themultiple disciplines involved,and the public at large.

Gravity wall

Concretegravity

wall

2.59m j

1.5 1 0.5

Annual probablity of failure

Finishedgrade

RandomSacktll

COMPARISON OF MANUFACTURING MATERIAL FOR MSE AND GRAVITY WALL

Stage:

Inputs Output air emissions

Photo-Energy Abiotic oxidant Acidification Climate

consumption depletion formation (kg SO, eq.) change(GJ) (Kg Sb eq .) (kg C2H4 eq.) (kg CO, eq.)

MSE wall 860 700 69 540 99 000

Gravity all 2200 910 147 1310 420 000

Figure 5 . Comparison of Embodied Energy and other indicators for MSE and concrete gravity wall(Rafalko et al., 2010)

References

Chau, C and Soga, K (2007),Technical environmentalconsideration chapter for theInternational familiarisation of ISOCode for Geotechnical EarthquakeDesign. Pers. comm.Chau, C, Nicholson, D, O'Riordan, Nand Soga, K (2007), "EmbodiedEnergy as an environmental impactindicator for geotechnicalinfrastructures ". Proceedings ofGeocongress, New Orleans,November 2007, ASCE.Chester, M V and Horvath, A(2009), "Environmental assessmentof passenger transportation shouldinclude infrastructure and supplychains". Environmental ResearchLetters Vol. 4.CSIRO (2007), www.cmmt.csiro.au.Environment Agency (2007),Carbon Calculator for ConstructionActivitiesHammond, G P and Jones, C I,Inventory of Carbon and Energy(ICE) Version 1.6a. SustainableEnergy Research Team, Universityof Bath, 2008.Harbottle, M J, Al-Tabbaa, A andEvans , C W (2007), "Sustainabilityof land remediation part 1: overallanalysis". ICE Proceedings,Geotechnical Engineering 161.Hanson , T, Parry, L, Graham, M,Troughton, V and Eriksson, H(2001), "Limix a dry deep mixingsystem used at Channel Tunnel RailLink Contract 440". UndergroundConstruction conference, London.Highways Agency (2001) RoadImprovement Within Limited LandTake, Highways Agency AdviceNote HA 85/01.Highways Agency (2008) HighwaysAgency Carbon Calculation andReporting Requirements, InterimAdvice Note 114/08.Hughes, L, Phear, A, Nicholson, D,Pantelidou, H, Kidd, A and Fraser, N(in press), "Assessment ofembodied carbon of earthworks - abottom-up approach". ICEProceedings, London.

O'Riordan, N (2007), "Technicalsustainability of construction onvery soft ground". Soft GroundEngineering (Long M, Jennings Pand Rutty P) Engineers Ireland.O'Riordan, N and Phear, A (2009)"Measuring and mitigating theenvironmental impact of earthworksand other geotechnical processes".Earthworks in Europe 2ndInternational Seminar, London.Rafalko, S D, Sankey, J E andFreitag, N (2010), "Sustainabilitymeasures for MSE walls andbaseline environmental impactevaluations". Earth RetentionConference No. 3, ASCE.Reid, J M and Clark, G T (2000)'Whole life cost model for earthworksslopes". Report TRL 430, TransportResearch Laboratory, UK.

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