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EARTH BUILDINGHistory, science and conservation

Paul Jaquin and Charles Augarde

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Zhenchen Lou. Hakka Tolou, Fuijan Province,

China. Constructed 1912

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Paul Jaquin and Charles Augarde

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iv EARTH BUILDING – History, science and conservation

Published by IHS BRE Press

IHS BRE Press publications are available fromwww.brebookshop.com orIHS BRE Press

Willoughby RoadBracknell RG12 8FBTel: 01344 328038Fax: 01344 328005Email: [email protected]

Printed on paper sourced from responsibly managedforests

Requests to copy any part of this publication shouldbe made to the publisher:

IHS BRE PressGarston, Watford WD25 9XXTel: 01923 664761Email: [email protected]

The authors and publisher accept no responsibility,nor liability, in any manner whatsoever for anyerror or omission, nor any loss, damage, injury, oradverse outcome of any kind incurred as a resultof the use of the information contained in thisbook or reliance upon it. Readers are advised to

seek specific professional advice relating to theirparticular construction project and circumstancesbefore embarking on any building work.

Reasonable care has been taken to ensure theaccuracy of the information in the book at thetime of printing. Drawings and technical details areindicative and typical only and final detailing for anyproject remains the responsibility of the designer.

The publisher accepts no responsibility for thepersistence or accuracy of URLs referred to inthis publication, and does not guarantee that anycontent on such websites is, or will remain, accurateor appropriate.

EP 101

© Copyright Paul Jaquin and Charles Augarde 2012

First published 2012

ISBN 978-1-84806-192-7

Front cover images:

Left, Rammed earth, Kasbah Caid Ali, Asslim,Morocco Top right, Adobe bricks drying, Aït Ben Haddou,Morocco Middle right, Cob toilet block, Melon car park,

Eden Project, Cornwall, UK. Courtesy of Jackie Abeyand Jill Smallcombe, Abey Smallcombe cob builders Bottom right, Rammed earth barn, Villafeliche,Spain

Back cover image:

Lime-rendered cob house, Devon, UK

Index compiled by Paul Nash

http:///reader/full/www.brebookshop.commailto:[email protected]:[email protected]:///reader/full/www.brebookshop.commailto:[email protected]:[email protected]

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PREFACE v

PREFACEThis book is the result of research carried outat Durham University, and subsequent workat historic earthen sites around the world.Paul Jaquin undertook a PhD supervised byDr Charles Augarde, and the research shed light onthe important mechanisms behind the mechanicalbehaviour of earthen construction. We believe

this research is the first to view earth buildingsin the framework of unsaturated soil mechanics,and we show that, by doing this, many aspects ofthe behaviour of earth buildings can be explainedbetter. By appreciating the unsaturated natureof earth buildings, it is possible to understandthe changes in behaviour of earth buildings andthe causes of damage, and therefore to developsuccessful restoration strategies.

We have focused specifically on theconservation of historic earth buildings, but the

principles outlined are equally applicable tomodern earthen construction. We have includeda section on the history of earth building to allowreaders to place buildings within a historical andgeographical context.

Although much of the research focused onthe conservation of historic rammed earth, theunderlying principles are common to all typesof earth building, and while the damage andconservation techniques are also inclined towardsrammed earth, many of the damage and repair

strategies are independent of the type of earthenconstruction.

This book is written for engineers, conservationprofessionals, and those interested in earth buildings.

As earth is such a varied material, and many aspectsof earth building are based on experience, thebook does not generally provide specific values (for

example for strength or stiffness), or rules of thumbfor earthen construction and restoration.There is still a great deal of work to be

undertaken before understanding of earthbuilding reaches the levels of more conventionalbuilding materials such as steel and concrete.Further research is required into earth buildingsat all scales, from investigation of the interparticlecontacts to the development of codes of practiceand standards. Of particular interest are thebehaviour of earth buildings in earthquakes (which

is not covered in this book), the exposition ofarchaeological sites, and the thermal behaviour ofearthen buildings. Earth is becoming increasinglypopular as a sustainable construction material, andit is hoped that this book will lead to improvedscientific and engineering understanding of earth asa construction material.

Paul Jaquin

Charles Augarde

November 2011

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vi EARTH BUILDING – History, science and conservation

ACKNOWLEDGEMENTS �

Thanks must go first to Professor Chris Gerrardof the Department of Archaeology at DurhamUniversity, who first wondered whether the largecracks in a building he was studying in Spainwere a problem, and who really set this line ofresearch going. Thanks also to Professor DavidToll of the School of Engineering and Computing

Sciences at Durham University, who on viewingan early experimental rammed earth wall in thecivil engineering laboratory, noted that it ‘looks likesuction’. We also thank Nick Clarke, Publisher atIHS BRE Press, whose vision for the publication ofthis material has finally come to fruition.

Many students at Durham University havebeen involved in earth-building research, and havehelped us understand these materials throughtheir work, specifically Chris Beckett, CynthiaHendy, Tom Horncastle, Tom Howard, Steven

Perkins, Jenny Durie, Lucie Le Grand and JacintoCanivell. Thanks to those with whom we worked ondeveloping this field at Durham, namely Dr SergioLourenço (now at Cardiff University) in the field ofunsaturated soil mechanics and Dr Cathy Clarke(now at Stellenbosch University) for help with thechemistry.

Paul was privileged to undertake a Research Associate role at the University of Bath during thesummer of 2008, and thanks go to Professor PeterWalker, Dr Andrew Heath and Dr Enrico Fodde atUniversity of Bath for their discussions and supportduring this period of research. Thanks also go toManfred Fahnert, organiser of the Lehm Express in

Morocco, who taught Paul to clay plaster.Parts of the research overseas have beenundertaken with the aid of travel grants from theInstitution of Structural Engineers and Engineerswithout Borders for visits to India and Bhutan.

Thanks go to members of the ICOMOSInternational Committee on the Conservationof Earthen Architecture for their expertise andsupport, and to Paul’s colleagues both in the UKand in Sweden, for their support and interest inthis non-conventional building material. Thanks

also go to those who have supplied photographswhich has enabled us to show a much wider rangeof earth building.

Finally, for help, support and enthusiasm,and for being dragged on earth-building-related‘holidays’ for many years, thanks go to Paul’sgirlfriend, Eleanor Trueman.

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CONTENTS vii

CONTENTSPREFACE � v �

ACKNOWLEDGEMENTS �

vi �

NOMENCLATURE � viii �

INTRODUCTION �

1 �

1 �

TYPES OF EARTHEN CONSTRUCTION 3 �

1.1 Introduction 3

1.2 Earthen construction principles 3

1.3 Monolithic earth walls 5

1.4 Unit construction 9

1.5 Conclusions 12

2 � HISTORY OF EARTH BUILDING 13 �2.1 Introduction 13

2.2 Eastern Asia 14

2.3 Central Asia and the Indus valley 15

2.4 Asia 16

2.5

Africa 17

2.6 Europe 20

2.7 North America 21

2.8 South America 23

2.9 Australasia 25

2.10 Conclusions 26

3 � FUNDAMENTAL BEHAVIOUR OF EARTHEN 27CONSTRUCTION MATERIALS �

3.1 Introduction 27

3.2 Soil mechanics 27

3.3 Soil strength 28

3.4

Effective stress 28

3.5 Unsaturated soil mechanics 29

3.6 Fundamentals 29

3.7 Relative humidity 30

3.8 The soil water retention curve 31 3.9 Compaction 32

3.10 The role of clay 33

3.11 The role of stabilisers 34

3.12 The effect of water content on the 34

mechanical behaviour of earth structures

3.13 Current research 35 3.14 Conclusions 36

4 DAMAGE TO EARTH BUILDINGS 37 �

4.1 Introduction 37

4.2 Structural 37

4.3 Water 43

4.4 Render 48

4.5 Organic matter

50 4.6 Abrasion 51

4.7 Concluding remarks 52

5 CONSERVATION STRATEGIES 53 �5.1 Introduction 53

5.2 Conservation principles 53

5.3 Earth-building analysis and repair strategy 54

5.4 Foundation issues 56

5.5 Cracks 57

5.6 Wall lean 60

5.7 Water 64

5.8

Face repair 69 5.9 Repair to the wall using fallen or similar 71

material

5.10 Whole building reconstruction 75

6 � CONCLUDING REMARKS 77 �

REFERENCES � 78 �

BIBLIOGRAPHY � 81 �

INDEX � 83 �

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c

viii EARTH BUILDING – History, science and conservation

NOMENCLATURE �

CEB Compressed earth blocksCSH Calcium silicon hydrateOWC Optimum water contentRH Relative humiditySWRC Soil water retention curve

Apparent cohesionF Force

Attractive force due to surfaceF tension tensionF pressure Attractive force due to pressure

difference g Acceleration due to gravityh Relative humidity, heightN Normal force

p0 Pressure of saturated water vapour pv Pressure of water vapourR Universal gas constantr Radius

rneck Radius of the neck of a liquid bridgerx , ry Radius of curvature of meniscusSr Degree of saturation

s SuctionT TemperatureT s Surface tensionu Pressure, pore water pressureua Air pressureuw Water pressurevw Molar volume of waterθ

Contact angleμ Coefficient of friction ρ d Dry densityσ Stress, total stressσ ′ Effective stressτ Shear strengthφ

Macroscopic friction angleφ ′ Effective macroscopic friction angle

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1INTRODUCTION

INTRODUCTION �

Earth buildings are perceived by many as simple‘mud huts’, liable to damage, and earth as atthe bottom of the list of desirable constructionmaterials. While earth buildings are more liable todamage by water than those constructed from otherconstruction materials, earth is one of the simplestand most sustainable construction materials, and

many of the oldest structures in the world areconstructed from this material. Around 30% of theUNESCO World Heritage Sites are constructed fromearth.

In common with buildings made from allother types of construction materials, historic earthbuildings are liable to damage, through lack ofmaintenance to protect against the weather, changesto the local environment, or damage caused byexternal factors. This book aims to present thereasons for the occurrence of such damage, and

to provide strategies that may be useful in theconservation of historic earth structures. We do thisby providing a scientific rationale for the behaviourof earth buildings. By viewing earth buildings in theframework of unsaturated soil mechanics, we areable to better understand their behaviour, and thusthe damage that earth buildings may suffer.

The book begins with an introduction into thedifferent types of earth building, and we argue thatalthough the construction techniques are markedlydifferent, there are definite common themes thatapply to every type of construction material. InChapter 2 we briefly describe the history of earthbuilding, using some of the main archaeological andarchitectural sites worldwide. This serves to allowthe reader to place any structure under investigationinto an international and chronological context. Theprinciples of general and unsaturated soil mechanicsare outlined in Chapter 3. Relatively recent research

findings allow for an improved understanding ofthe mechanical behaviour of earth buildings, andin Chapter 4 we show how this relates to observeddamage mechanisms in earth buildings. Finally, inChapter 5 we present mitigation and repair solutionsthat may prove useful in the conservation of historicearthen sites.

This book is not a practical guide on earthenconstruction, nor is it an engineering textbook.Techniques for different types of earthenconstruction can be learned from numerous sources,both through practical courses and through guides.

A history of earth building is provided that focuseson some main sites, although many are not includedfor brevity, and the history of specific earthen sitesis not explored. The principles of unsaturated soilmechanics are explained, but there are many logicalsteps that are not included, and reference should be

made to our journal papers and to other engineeringtextbooks. As earthen construction is so varied, andas there is a much smaller pool of test data than forother, more conventional construction techniques,specific values for soil testing and mechanicalproperties are not given. We do not provideinformation on field or laboratory techniques forthe testing of earthen materials. Engineering analysismethods for structures are not explained, and theadvice of a competent engineer should always besought when considering the conservation of historicearth buildings.

Finally, the behaviour of earthen buildingsin earthquakes is not specifically discussed. Thisbehaviour is complex, and although the damagemechanisms are similar to those described inthis book, we do not deal with these, or with theretrofitting or reconstruction of earth buildings afterseismic events.

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31 TYPES OF EARTHEN CONSTRUCTION

CHAPTER 1

TYPES OF EARTHEN CONSTRUCTION �

1.1 INTRODUCTIONEarth is one of the simplest construction materials,and all earth buildings have the same basicconstituent parts: soil from the ground, and water.These are mixed together and formed into shapes,and then water evaporates to leave an earthenstructure. Other materials can be added to the

mixture, either to help with production, or toimprove the mechanical properties of the finishedproduct. Soil types, construction techniques andbuilding traditions vary greatly across the world, andmany techniques are variations and combinationsof those described below. There are many ways toform earth into structures, but this book deals withtechniques where earth is the major constituentmaterial. The techniques are split into two classes:those in which independent units are constructed,dried, then transported and laid to form a structure

– namely adobe and compressed earth blocks;and those where homogeneous monolithicconstructions are produced in situ, such as cob andrammed earth. This book does not discuss buildingtechniques where earth is only a secondary material,and therefore wattle and daub, rammed tyres andearth bag construction, or stone and turf buildingsare not discussed.

1.2 EARTHEN CONSTRUCTION PRINCIPLES All earth construction follows the same basicprinciples. Soil is taken from the ground andmixed with sufficient water to allow moulding andplacement. Most of this water then evaporates,leaving the soil in a new state. In wholly unstabilisedearthen structures (from sandcastles to the GreatWall of China) this water forms into bridges betweenparticles in equilibrium with the surrounding air, andit is these bridges that provide a major component

of the additional strength compared with completelysaturated or completely dry soil.

In many forms of earthen construction, othermaterials (called stabilisers) are included in the earthmixture. These combine physically or chemicallywith the soil, and are included to improve themechanical properties of the structure (for example,

straw or hair to improve tensile capacity and reduceshrinkage cracking, or cement to improve the shearstrength), or to improve the workability of the earthmixture (for example urine), or to provide waterrepellency (for example bitumen or silicone).

All forms of earthen construction follow similarinitial steps:1. Soil is first dug from the ground. This soil should

be taken from below the topsoil, and ideallyfrom above the water table. Where earthenconstruction is common, there may be well-

known deposits of suitable soil that can beutilised (Figure 1.1).

Figure 1.1: Taking suitable soil from next to the site of awall. Jomsom, Nepal

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4 EARTH BUILDING – History, science and conservation

2. The correct proportion of water to be added tothe soil depends on the construction method,particle size distribution, and compactive effortto be applied. If the soil dug is too wet, it maybe spread on the ground to allow pore waterto evaporate. If the particle size distribution

is not correct for the construction type, thenthe soil may be sieved to remove particlesthat are considered too large. This sievingcan be done by manually removing largerstones, or by passing the soil through a meshof fixed aperture. This is best achieved usinga frame, as shown in Figure 1.2. In order tomanufacture the correct soil mix, different soilsare combined.

3. Water and any additives are then mixed intothe soil in defined proportions. Traditionally,

the soil is piled into a cone, with water pouredinto the centre. The soil is then mixed in withthe water by people or animals walking aroundon it (Figure 1.3), or by shovelling the earthinto the water until all the water is mixed in.

Alternatively a rotary mixer (Figure 1.4) can beused. When conserving earth buildings, thetype and function of additives placed in theoriginal structure should be well understoodbefore further additives are used.

When the soil, water and additives have beencombined to a homogeneous mixture, it shouldimmediately be formed using the techniquesdescribed below.

Figure 1.2: Sieving soil using a mesh frame. Asslim,Morocco

Figure 1.3: Mixing earth with straw and water by

walking. Asslim, Morocco

Figure 1.4: A rotating drum mixer that can be usedto mix earth and water. Civil Engineering Laboratory,Durham University. Courtesy of Chris Beckett

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This is the arabic for ‘al taub’

MS word, and windows, need to have

Arabic translation enabled, or they just put

the individual letters right to left.

see http://office.microsoft.com/en-gb/word-help/install-system-support-for-multiple-

languages-HP005258876.aspx?CTT=5 origin=HP003089535


1.3 MONOLITHIC EARTH WALLS

1.3.1 Rammed earthThe term ‘monolithic’ refers to an earth wall thatis constructed in situ, and which functions as ahomogeneous wall. There are two basic types: walls

formed using formwork (rammed earth) and wallsformed without formwork (cob).

Rammed earth uses a mixture of clay and sandysoil mixed with water and then compacted withinformwork to form a monolithic earth wall. Earth iscompacted between the formwork in layers untilthe formwork is filled. The forms are then moved toanother location so that another section of wall canbe constructed.

Rammed earth is known as:• tapial in Spanish, and tapia in Portuguese, which

are corruptions of the Arabic al taub ( );• pisé (or pisé du terre) in French, a corruption ofthe Latin verb pinsere (to ram or pound);

• hāngtŭ ( ) in Chinese.

Soil suitable for rammed earth is shown inFigure 1.5. If the rammed earth is not stabilised,then the soil mixture should contain more clay andsilt [1], and if the mixture is to be stabilised, a silty,sandy soil should be used[2].

P e r c e n

t a g e

p a s s i n g

100

80

60

40

20

Clay Silt Sand Gravel

0.001 0.01 0.5 1 2 10

Particle size (mm) (logarithmic scale)

Rammed earth Cob Adobe

Figure 1.5: Particle size distribution for different typesof earthen construction

To make a rammed earth wall, a formwork box isconstructed to the width of the required wall. Thisbox is filled with soil, which is then compacted byphysical or mechanical means. Traditional formwork(Figures 1.6 and 1.7) can be moved along the lengthof the wall and vertically, so that when one formworkbox is filled with earth, it can be dismantled andmoved to allow further compaction to take place.Modern formwork (Figure 1.8) is similar to that used

Figure 1.6: Master mason Malem with rammed earth formwork. Asslim, Morocco

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Figure 1.7: Rammed earth wall under construction. Jomsom, Nepal

for concrete, and may be modular, allowing for large Modern rammed earth tends to use pneumatic orsections to be built in one go. electric hammers fitted with flat feet to provide

Soil is poured into the formwork, and usually compaction, whereas traditional rammers tend tocompacted using a rammer in layers of around be timber poles with shaped ends, or sometimes100 mm. The rammer may take a number of forms, incorporating stones at the end of the rammer.with a size and shape dependent on the culture. When the formwork box is filled, it is removed

Figure 1.8: Modern rammed earth formwork. Margaret River, Australia

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7

and moved to another location, giving rise tocharacteristic ‘lifts’ in rammed earth construction.In some rammed earth traditions the formwork issupported on timbers placed on the wall, and theseleave characteristic holes denoting the position ofeach lift (Figure 1.9)

Openings in rammed earth are formed usingblanks, or by the placement of lintels between tworammed earth wall sections. Lintels may be curved(Figure 1.10), allowing feature openings to becreated. Traditionally, lintels were made from timberor stone, but modern rammed earth constructionoften features precast concrete or steel lintels.


Decoration of rammed earth can take the formof additional materials such as fired bricks betweenlifts (Figure 1.11), or the insertion of coloured layersof soil or relief patterns into a rammed earth wall.

Figure 1.10: Precast concrete lintel in modern rammedearth. St Thomas More Church, Perth, Australia

Figure 1.9: Density banding, lifts between formworkboxes, and holes through the wall. Villena, Spain

Figure 1.11: Rammed earth with decorative fired brick between the lifts. Villafeliche, Spain

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1.3.2 CobCob construction technique involves the directplacement of a mix of moist soil and straw toform a wall (Figure 1.12)[3].The word cob isderived from the Old English word for loaf ,and similar techniques are known by different

names in different cultures, such as chineh (inFarsi) and pakhsa (in Uzbek). A similar layeredtechnique has been described in Oman and Iran(Figure 1.13)[4].

The particle size distribution of soil used for cobis shown in Figure 1.5. The soil tends to be moresandy than that used for adobe construction. Cobalmost always contains short straw or grass added tothe earth mixture to provide resistance to shrinkagecracking, and improved strength.

Cob is usually constructed by a team of

two people, one working on the wall, and theother at the base, shovelling the cob mix to thehead of the wall. A cob wall is constructed inlayers 400–600 mm high; wet cob is forked intoposition and compacted at the head of the wall.

Figure 1.13: The layered technique (see wall, left).Decorative adobes between each lift. Isfahan, Iran.Courtesy of Armin Yavari

Figure 1.12: Cob barn, Cumbria, UK

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9

Compaction takes place either by treading themixture, or by using long-handled, flat-footedtampers[5]. Where the compacted cob falls outsidethe line of the wall, it may be shaved from theface to provide a vertical face. Constructionprogresses with each layer of cob allowed to dry

slightly before the wall can be stood on and thenext layer placed. Walls are typically between0.5 m and 1 m thick, and may taper as the wallrises.

Openings are formed either by adding ‘blankformers’ to the wall, around which cob is placed,or by adding lintels at the correct height duringconstruction and then cutting the openings outafter the cob wall is finished.

Because of the free-form nature of cob, it isvery simple to construct structures that curve both

on plan and on elevation (Figure 1.14). For thisreason, cob is often used as an artistic materialfor sculpture or temporary structures, and is verysimply decorated either by inserting objects intothe wall or by creating relief patterns.


1.4 UNIT CONSTRUCTION

1.4.1 AdobeIn contrast to in situ monolithic constructionmethods, many forms of earthen construction arebased on units. Here the term unit is defined as a

block constructed from earth and allowed to air-drybefore being used in the construction of a structure.(Fired bricks are manufactured using specific typesof clay, which are heated to high temperaturesso that the clay is vitrified, and cinder, breeze orbesser blocks are manufactured using cement, andare quite different products.) Two types of unitconstruction are discussed here: adobe, used todescribe units that are constructed without recourseto mechanical advantage, and compressed earthblocks, constructed using a machine to provide

increased compactive effort.The term adobe has a range of meanings indifferent cultures. Here it is used to describe a unitthat may be formed by hand or in a frame, butwhich is not compacted. Units are made of similar

Figure 1.14: Modern cob building. Melon car park, Eden Project, UK. Courtesy of Jackie Abey and Jill Smallcombe, Abey Smallcombe cob builders

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This is the arabic for ‘al taub’

MS word, and windows, need to have

Arabic translation enabled, or they just put

the individual letters right to left.

see http://office.microsoft.com/en-gb/word-help/install-system-support-for-multiple-

languages-HP005258876.aspx?CTT=5 origin=HP003089535


size to each other to allow them to be laid togetherin a mortar to form a wall.

The term ‘adobe; is also likely to be taken fromthe Arabic for brick, al taub ( ), where it hasbeen corrupted into the Spanish adobe. In Turkiclanguages this type of construction is known as

kerpiç.The soil used for adobe construction tends to

be richer in clay and silt than the soil mixtures usedfor rammed earth or cob construction (Figure 1.5).These soil types may be more susceptible toshrinkage, and so additives such as straw or grass areoften included to prevent cracking.

Hand-formed units are made by roughlyshaping the wet soil to the required dimensions.This method allows any shape to be formed (forexample into spheres or cuboids). More common

is the use of a standardised mould; to increaseproductivity, moulds can be made to produce morethan one brick at a time (Figures 1.15 and 1.16).

Figure 1.15: Adobe mould. Drâa Valley, Morocco

Figure 1.16: Adobes in moulds. Earth-building

course, UK

To make blocks, wet soil is placed into themould and allowed to dry slightly; then the mouldis removed and the unit is allowed to dry in theair (Figure 1.17). To assist drying, the bricks can berotated onto their header face after a time, to allowa larger surface area to be exposed. The bricks areusually left to dry for a period of weeks before beinglaid to form a structure.

Figure 1.17: Adobes drying. Aït Ben Haddou, Morocco

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1 TYPES OF EARTHEN CONSTRUCTION 11

Adobe walls are constructed in the same manneras fired masonry, with similar bond patterns andmortar bed joints. The mortar should be composedof a similar material to that of the brick. To lay bricks,each brick is first dipped or coated in water, and thenlaid into the mortar bed. Adobe can be formed into

arches (Figure 1.18), and barrel vaults and domesare popular in many parts of the world. Decorativepatterns may be formed in adobe walls by using adifferent bond pattern, by laying the adobes on end,or by manufacturing insets or protrusions from thewall (Figure 1.19).Many adobe walls are renderedafter construction. This both protects the face andallows a clean face to be presented, reducing thechance of water penetration into the mortar joints.

1.4.2 Compressed earth blocks

Compressed earth blocks (CEB) are made in apress, which allows a large compressive force to beimparted to the brick. This means that a lower watercontent can be used than for adobe (see Section 3.9),because a great compactive effort is applied, and thusa higher dry density brick is obtained. Moulds can beinserted to allow the brick to be faceted around theedges, and frogs and raised sections can be added toimprove interlocking (Figure 1.20).

In some cultures, cement or other bindersare added to the earth block mixture to improve

the mechanical properties, and in these cases it isimportant to ensure that sufficient water is added toallow the full cementing reaction to take place.

A large amount of research has been undertakenby non-governmental organisations into theimprovement of earth brick presses for use indeveloping countries. The CINVA ram was developedin the 1950s, and many agencies have contributed tothe subsequent development of the presses, includingthe BREPAK machine[6, 7] and the Auram 3000 pressdeveloped and manufactured by the Auroville EarthInstitute[8].

Cement-stabilised earth blocks must be allowedto cure for around 28 days before they are used ina structure. A mortar similar to that of the brick isused for their construction. Compressed earth blocksoffer a viable alternative to fired brick or cementblock construction (Figure 1.21). Buildings are usuallypainted or rendered using a cement-based render,and as a result do not appear any different from otherconstruction types.

Figure 1.18: Adobe arches, Isfahan, Iran. Courtesy of Armin Yavari

Figure 1.19: Decorative adobes. Aït Ben Haddou,Morocco

Figure 1.20: Cement-stabilised compressed earthblocks. Bangalore, India

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Figure 1.21: Cement-stabilised compressed earth blockhouse. Bangalore, India

The type of construction is usually definedby the availability of suitable soils. Where clayand silty soils are found close to the surface, thenadobe construction tends to dominate. Wheremore silty and sandy soils are found, rammedearth and cob construction are more common.

There is a wide variety within the techniquesdescribed. The dimensions and details of rammedearth formwork and rammers, the techniquesfor cob construction, and the shapes and sizes ofadobe and compressed earth block differ greatlyaround the world.

The selection, grading and mixing ofsoils are requirements for all types of earthenconstruction. Mixing can take place usingpeople or animals, or more mechanised meanssuch as rotary mixers. Increased mechanisation

has also led to improvements in compactedearthen construction, such as rammed earth andcompressed earth block. In modern rammedearth construction, compaction using pneumaticor electric rammers is used rather than manualcompaction, and compressed earth blocks uselevers and mechanical advantage to impartcompaction energy.

Chemical stabilisation has been practisedfor a very long time, either to improve theworkability of the mixture, or to improve its

mechanical properties. Historically, thesechemicals have been bitumen or lime, butmore recently cement has been added both tocompressed earth blocks and to rammed earth, toproduce what is perceived to be a more modernconstruction material. Physical stabilisationgenerally uses materials that are placed to act intension within the earth, examples are materialssuch as straw, grasses or plastic fibres.

Earth building has recently been promoted as1.5 CONCLUSIONS a sustainable construction technique, because ofThis chapter has outlined earthen construction the low transport costs if materials are excavatedtechniques where earth is the main constituent. close to the construction site, and the low energyDistinct types of monolithic and unit construction input compared with more common constructionhave been identified, and the initial steps common techniques such as fired brick, Portland cementto all types of earth building have been described. or steel. The development, for example, ofIndependent of the construction type, earth cement-stabilised rammed earth and extrudedstructures are formed by shaping wet soil into the unfired clay bricks means that earthen buildingrequired shape, followed by some drying to form a may in the future take its place among more‘solid’ structure. conventional building materials.

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2 HISTORY OF EARTH BUILDING 13

CHAPTER 2

HISTORY OF EARTH BUILDING2.1 INTRODUCTIONBuilding using earth is one of the oldest constructiontechniques; it provides simple shelter using a freelyavailable material. Buildings made from earth arefound in many parts of the world, and in differentforms, sometimes mixed with other traditionalconstruction materials such as timber or stone, and

sometimes with more modern inventions such ascement and steel. As described in Chapter 1, moistsoil is formed either as a monolithic wall that is thenallowed to dry, or into independent units, such asbricks or blocks, which are allowed to dry beforebeing placed as a wall.

It is probable that earth-building techniquesdeveloped independently in different parts of theworld, and spread with the movement of peoples.Early people were constantly moving, followinghunting and gathering patterns dictated by the

surroundings. The earliest shelters utilised natural

features such as caves, and the first earth buildingsmay have been extensions to natural features, suchas mounds of earth at cave entrances or pits duginto the ground.

The development of settled agriculture allowedthe first permanent shelters, because more timeand effort could be expended in their construction.

Agriculture first developed in fertile river valleys, andhere the silt and clays provided excellent buildingmaterials for earth construction. The first earthenbuilding technique to develop is likely to have beenwattle and daub: construction of a façade or roofusing timber or grasses, which is then covered in earth.Later a rammed earth type of technique may havedeveloped, with earth placed against or betweenwalls made from timber and compacted into place,forming a thicker wall. The development of unitconstruction could have developed later; initially units

were formed by hand, and later more cuboid blocks

Figure 2.1: Spread of earth-building techniques around the world

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were made using formwork. When dry, these could betransported, allowing the production of the materialsto be separated from the location of the building,so that, for example, suitable earth could be takenfrom a river valley liable to flooding and used for theconstruction of buildings at a higher level.

Agriculture[9] and earth construction developedindependently in the main cradles of civilisation.The development of agriculture beside major riversled to people gathering together in towns for the firsttime. These fertile river valley civilisations had accessto the right types of soil for earth construction, andthere is evidence for the development of earthbuilding independently in the valleys of the Tigrisand Euphrates, Nile, Indus, Jordan, Murghab and

Yellow Rivers. These cultures remained independentfrom each other, yet appear to have developed very

similar earth-building techniques. As civilisationand trade developed, techniques were refined andimproved. This is difficult to chart, because earth-building techniques can vary from settlement tosettlement and from year to year, but some patternsdo emerge. It would appear that the transitionfrom hand-moulded to cuboid bricks occurred inMesopotamia around 5000BC, and that rammedearth was not found in South America prior to itsintroduction by Europeans.

Earth is generally used in combination with

other building materials when these are available:for instance, wattle and daub houses combineearth construction with timber, and are found in

Japan and northern Europe. Where stone is readilyavailable, earth is used as a plaster or a mortar, asin Malton in the north of England, or turf is used forroof or wall construction, as in the Western Isles ofScotland. There are also instances of monumentalarchitecture in one construction type and vernacularconstruction in another, such as the stone cathedralsof England contrasting with the surrounding wattleand daub vernacular houses.

2.2 EASTERN ASIAThe Euphrates and Tigris river valleys were home topreviously nomadic civilisations that first developedsettled agriculture and buildings around 9000BC.These civilisations used hand-moulded oval bricks toform circular structures, found at sites of Djade al-

Mughara in Syria and Tappeh Ozbaki in Iran. Theseoval bricks appear to have been used until around6000BC, at sites such as Jericho and Netiv Hagdud.From around 6000BC onwards, square bricks arefound at the Tell Hassuna[10] site in Iraq, and at

Jericho the buildings change from being circular on

plan to rectangular.Settlements along the Euphrates and Tigris

rivers gradually grew in size and complexity. By3500BC the city of Uruk was the largest in theregion, with rammed earth buildings and adobetemples. Closer to the Arabian Gulf, the Assyriancities of Ebla and Mari vied for influence, andexcavations at these sites show that both siteshad earthen city walls and adobe palaces[11].Technology developed such that when the Zigguratof Ur was constructed, around 2100BC, it was

built with an adobe brick core and faced withfired brick set into bitumen[12]. Such zigguratsmay have earlier been constructed in adobe, andhave not withstood the ravages of time, or remainunidentified to the present day.

The settlement of Çatalhöyük developedindependently on the alluvial plains of theÇarşamba river in central Turkey. This settlementis still being excavated, but may have beenthe largest in the world at the time, with 5000inhabitants at its peak between 7300BC and

6800BC[13]

. The city was built from adobe, withirregular plan buildings packed so tightly thataccess was via the roofs.

Many settlements in this region feature acore of earthen buildings that have been renewedand rebuilt over the centuries. In Iran, thecities of Yazd and Isfahan contain many historicadobe buildings (Figure 2.2). The city of Tousis surrounded by rammed earth walls, and thecitadel of Bam, which dates from around theseventh century AD, was probably the largestadobe building in the world before its collapse inan earthquake in 2003[14].In Yemen, the city ofShibam is renowned for its particularly tall adobebuildings. From around 1700, the residents beganto build upwards, and currently around 500adobe ‘skyscrapers’ reach up to 30 m high. Closeby is the town of Tarim, home to the Al-MuhdharMosque. The adobe minaret of this mosque,completed in 1914, is probably the tallest earthenstructure in the world, at 53 m.

http:///reader/full/2003[14].Inhttp:///reader/full/2003[14].Inhttp:///reader/full/2003[14].In

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Figure 2.2: Cob and adobe walls at Ghaleh Yavar, acastle outside Isfahan, Iran. Courtesy of Armin Yavari

2.3 � CENTRAL ASIA AND THE INDUS

VALLEYSettled civilisation developed in the Indus valleyaround 7000BC. The small adobe settlement ofMehrgarh in modern Pakistan was a forerunnerof the much larger Indus valley civilisation thatdeveloped around 3000BC. The civilisation spreadalong the Indus river, with the two large settlementsof Harappa and Mohenjo-daro emerging around2600BC. Both settlements were laid out in a gridpattern, with adobe houses and individual streets.

Settled agriculture and the first buildings incentral Asia are related to the Bactria-Margiana

Archaeological Complex, comprising around300 discrete fortified adobe brick enclosures atsites such as Namazga-, Altyn- and Gonur-Depethat have been dated to between 2200BC and1700BC. Although the peoples of central Asiawere largely nomadic, in western Uzbekistan

there are several forts or qalas built in adobe, andthese are thought to date from around 300BC.Few of these settlements have remained to thepresent day because of shifting trading patterns,conflict, and meandering rivers. Settlements thatdid survive grew to become major trading centres

such as Balkh and Merv.Balkh (Bactria) in modern Afghanistan is called

Umm Al-Belaad (Mother of Cities) because of itsantiquity. Although the city dates from 2000BC,the oldest standing structures are the rammed earthTakht-e Rostamand Top-Rustam dated AD 300–500and attributed to the Buddhist or Zoroastrianreligions. Balkh became a pre-eminent city in theregion, and a centre of trade and commerce[15].When Muslim traveller Ibn Hawqal visited the cityaround AD 950 he described it as ‘built of clay with

ramparts and six gates’.The city of Merv in Turkmenistan is relativelyunique among archaeological sites, with severaldifferent settlements constructed adjacent to ratherthan on top of each other, allowing archaeologiststo uncover earlier structures without destructionof those built later. Almost all of the structures inMerv are built in earth, with the earliest settlementsdated to around 2000BC. The city was almostcontinually inhabited until its abandonmentand destruction in 1787, and is now a major

archaeological site[16]

.Both Merv and Balkh lay on importanttrade routes that crossed central Asia, and manyother earthen settlements grew up on what hasbecome known as the Silk Road. The adobecity of Panjakent in western Tajikistan is firstmentioned around 500BC, and was probably thehighlight of the Silk Road before its decline in theeighth century[17].The site has been extensivelyexcavated since the 1940s, and is now a touristattraction. Further east, the Uyghur empirecapital and Silk Road city of Ordu-Baliq (KharBalgas) in modern Mongolia featured rammedearth defensive walls and buildings. The citywas established in AD 745, but was abandonedin AD 840. The armies of Alexander the Greataround 330BC, the Muslims around AD 720,and Genghis Khan in 1220 each ransacked ordestroyed many earthen settlements, and as aresult many of the sites in central Asia are mereshadows of their former selves.

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2.4 ASIASettled civilisation in China first developed around2300BC when nomadic peoples settled on thealluvial plains of the Yellow River, beginning with theLungshan civilisation. The soft soils here could becut to form pit houses and heaped to form rammed

earth mound walls. This allowed the developmentof defensive settlements such as those found atLianyungang, Jiangsu, Taosi, Erlitou and Longwan[18].Evidence of formwork boards and rammingimplements have also been found at Pingliantai[19].

During the Warring States Period (475–221BC)rammed earth was used for the construction ofmore elaborate walls at larger settlements such asLangya, Anyang, Linzi and Xiadu. The Qin dynasty(221–206BC) was the first to construct rammedearth defensive walls along their northern frontiers

in western China. These walls were repaired andextended by the Han (206BC–AD 202) and Jin(AD 265–420) dynasties. The Tang dynasty (AD618–907) expanded Chinese borders and trade,but was harassed by tribes to the north, and as aresult built fortified settlements in north-westernChina along the eastern part of the Silk Road.These settlements, such as Jiaohe, Gaochang andXi’an, are all encircled with large rammed earthwalls, and the city of Kashgar in western China isbuilt from adobe, whereas the fortress of Baishui,

at the western end of the Great Wall, is constructedwholly in rammed earth[20].The Tang dynasty collapsed around AD 907,

which led to a period of major upheaval in China.The next dynasty to produce major monumentalearth architecture was the Ming dynasty (1368–1644), which pursued a policy of aggressiveexpansion. The walls of the Ming capital Xi’an,originally of rammed earth, were faced in stone,and along the Silk Road and to the northern bordersthe Great Wall was repaired and upgraded, andnew forts were constructed in adobe at Jiayuguan(Figure 2.3) and Hexibao.

In the Fuijan province of central China, the roundhouses of the Hakka people have recently been givenWorld Heritage site status. These large rammed earthbuildings, called Tulou (literally ‘earth structures’) aredefensive homes to many families, and can be up to60 m across and four storeys tall. The oldest of thesebuildings was built in 1308, and their constructioncontinued well into the 20th century.

Figure 2.3: Part of the early Ming dynasty Great Wall, Jiayuguan Fort, China. Courtesy of Kate Clarke

There are interesting parallels between the

Chinese characters and the ideas they represent.The character for rammed earth hāngtŭ( )is composed of the word shăng ( ), meaning‘to ram’, and tŭ( ), meaning ‘earth’. The word

shăng is itself made up of the radicals dà( ),meaning ‘big’, and lì( ), meaning ‘strength’, thusimparting the idea that rammed earth walls arebig and strong. Earth ( ) is also a character in thewords for city walls, chéng ( ), and internal walls,qiăng ( ), indicating that earth was used for wallconstruction.

Rammed earth and adobe are found on theTibetan plateau and in parts of the Himalayasas both monumental and vernacular buildingtechniques. At the west end of the Himalayasrammed earth is found in the north Indian state ofLadakh, in palaces such as those at Shey and Leh(Figure 2.4), and in a fort at Basgo. In the Nepalikingdom of Mustang, much of the capital city ofLo Manthang is constructed from rammed earth,and a defensive wall, dating from 1380, surrounds

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Figure 2.4: Leh Palace, Ladakh, India

the city[20]. The country of Bhutan, at the east

end of the Himalayas, continues to promotetraditional building materials, with many homesand monumental architecture built in rammedearth (Figure 2.5).

Figure 2.5: Rammed earth section of private house nearKyichu Lhakhang monastery, Bhutan


2.5 AFRICA Although Africa is known as the cradle of mankind,archaeology has not yet revealed a great historyof earth building in Africa. The African mud hut,constructed from woven reeds or timber with anearth plaster, may have remained unchanged for

millennia[21]. The earliest woven reed and branchearth-covered sites have been dated to 5000BCat sites in the Nile Delta, such as Mermide andFayum. The Egyptian dynasties appeared in the Nilevalley around 2900BC, and the clay river silt mixedwith desert sand and straw from cultivated grainsallowed hand-made adobe brick manufacture. Thelarge independent adobe structures at Shunetel-Zebib and Nekhen, dating to 2750BC, and adobepyramids found at Tanis[22] show that adobe wasused as a monumental construction technique

before the better-known stone edifices were built. Adobe continued to be used as a vernacularconstruction material in Egypt. The settlement ofDeir el Medina (1550–1080BC), which was hometo the masons of the Valley of the Kings, comprisessquare, single-room adobe houses laid out in a gridpattern. A relief and frescoes at the tombs of QueenHatshepsut (d. 1458BC) and an official duringher reign, Rekhmire, both describe the processfor making rectangular adobe bricks in formwork.The city of Tel el-Amarna was a new capital city

built by the pharaoh Akhenaten around 1353BCbut abandoned soon afterwards. This city featuressingle-storey rectangular adobe buildings withexternal stairs leading to a flat roof.

Rameses II (1279–1213BC) embarked on manybuilding projects, and adobe bricks from his majorconstruction projects were stamped with his seal[23].Egypt, however, remained relatively isolated fromthe rest of Africa, and as a result had little influenceon the building techniques found throughout therest of the continent.

In north Africa, the Phoenician civilisationspread from the eastern Mediterranean, foundingsettlements along the north coast. Their capitalat Carthage (in modern Tunisia) was founded in814BC, and excavation reveals rammed earth wallsused in homes there[18]. The famous Carthaginiangeneral Hannibal crossed into Europe in 218BC,and the Roman author Pliny the Elder describesthe rammed earth towers in Africa attributed toHannibal[24].

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“Moreover, are there not in Africa and Spain

walls made of earth that are called framed walls,

because they are made by packing a frame

enclosed between two boards, one on each side,

and so are stuffed rather than built, and do they

not last for ages, undamaged by rain, wind and

fire, and stronger than quarry stone? Spain still sees the watchtowers of Hannibal and turrets of

earth placed on mountain ridges.”

Around AD 700 Islam spread through north Africa, and the valleys of the Drâa and Dadès riversin modern Morocco are filled with hundreds oframmed earth kasbahs (Figure 2.6), such as AïtBen Haddou and Tamnougalt, the earliest dated toaround AD 1000. The city walls of both Marrakechand Fes are built in rammed earth, and it appearsextensively in monumental Muslim architecture,

such as at the El Badi Palace in Marrakech, built in1578. Muslim rule in Egypt promoted the use ofadobe brick, with the 10th-century Fatimid tombsbuilt in adobe[25].

Moses Maimonides, a Jewish writer andphilosopher, born in Cordoba in 1135, but residingin Morocco, Egypt and Israel, wrote of rammedearth[16]:

“The builders take two boards, about six

cubits long and two cubits high, and place them

parallel to each other on their edges, as far apartas the thickness of the wall they wish to build;

they steady these boards with pieces of wood

fastened with cords. The space between the

boards is then filled with earth, which is beaten

down firmly with hammers or stampers; this is

continued until the wall reaches the requisite

height and the boards are withdrawn.”

Although complex societies have been presentin west Africa since around 1500BC, the firstdocumented is the Ghanaian empire, ruling a large

part of west Africa from around AD 830. Whilemuch of the monumental architecture is stone, itis assumed that current earth-building practicesfound in Ghana (Figure 2.7), such as adobe and cob

Figure 2.6: Kasbah in Asslim, Drâa Valley, Morocco

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Figure 2.7: Vernacular mud brick at Yikpabongo village,Ghana. Courtesy of Claire Jaquin

Figure 2.8: Great Mosque of Djenné, Mali. Courtesy ofCarolina Castellanos

construction, were used in vernacular architecturein antiquity. The demise of the Ghanaian empirearound 1235 precipitated the development ofthe Mali Empire, with its famous earthen cities ofDjenné and Timbuktu.

The original great mosque of Djenné wasprobably first built around 1200, but fell intodisrepair before being reconstructed in 1907[26].The characteristic style is similar to other sites inwest Africa, such as the Sankore and DijinguereBer mosques in Timbuktu in Mali, built around1320, and the Grand Mosque of Agadez in Niger,built around 1515. These buildings are unique,being decorated with bundles of palm stalks thatproject from the wall and serve as a scaffold forannual replastering of the buildings (Figures 2.8and 2.9)[27].

Earth buildings are found as far east asCameroon, where the homes of the Musgumpeople are inverted catenary dome structuresbuilt in earth[28]. Monumental earth buildingsare not found in the forested and more humidregions of central and southern Africa, but earthconstruction in various forms continues to beused across Africa.

Figure 2.9: Dijinguere Ber Mosque, Timbuktu, Mali.Courtesy of Carolina Castellanos

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2.6 EUROPEEarth building takes many forms in Europe, withadobe and rammed earth found in southernEurope, whereas in northern Europe earth is usedin conjunction with timber in wattle and dauband half-timbered techniques. The earliest use of

adobe in Europe can be dated to around 5300BCat the settlement of Sesklo in Greece, with smallhomes built on stone foundations[29]. The use ofearth with timber in northern Europe means thatmany archaeological sites have decayed, and onlyfoundations remain, making it difficult to assess thebuilding materials. Further east, in Hattuša, centralTurkey, remains of adobe buildings dated to around1600BC have been found.

Rammed earth may have been brought toEurope by the Phoenicians, who spread from the

eastern Mediterranean and founded settlements inSpain, such as Morro de Mesquita[18]. The Romanarchitect Vitruvius describes rammed earth usedin the French city of Marseille, and adobe beingused to construct the Greek city of Athens[30].The Latin verb pinsere, meaning ‘to pound’, haspassed into French as pisé, meaning rammed earth.

Although much Greek and Roman monumentalarchitecture was built in stone, earthen construction

continued to be used in vernacular constructionthroughout Europe. St Isidore, the Catholic bishopof Seville, described the rammed earth technique,paraphrasing Pliny in his work Etymologiae, writtenbefore AD 636[31].

Islam came to southern Europe in AD 711,

bringing with it building technologies from north Africa. Conflict at this time led to the constructionof many rammed earth and adobe fortifications.Excavations of the fortifications of Calatayud inSpain have been dated to AD 884, and the Muslimdefensive walls of historic cities of Cordoba, Sevilleand Granada are built in rammed earth. The WorldHeritage site of the Alhambra Palace in Granada(Figure 2.10) was constructed from rammed eartharound 1238.

Though earth continued to be used as a

building material, its use declined with theincreasing penetration of fired brick from the16th century onwards. In northern Europe, wattleand daub techniques developed as vernacularstructures. Cob structures dated to around 1400have been found in parts of the UK [32], and thisbuilding technique continued to be used as avernacular technique until well into the 19thcentury.

Figure 2.10: The rammed earth Alcazaba at the Alhambra of Granada, Spain

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At the end of the 18th century the politicalclimate in Europe was turning towards freedomfor the common man and revolution against theruling classes. In this climate, rammed earth was‘rediscovered’ and championed by FrenchmanFrançois Cointeraux. Cointeraux published a series

of leaflets on rammed earth in Lyon in 1791[33].These were translated into English[34, 35], German[36]

and Italian[37], allowing the technique to spreadacross Europe (Figure 2.11) and to the UnitedStates[38].

Earth building again declined with the adventof the Industrial Revolution in Europe, because firedbrick became more easily available, but was againrediscovered following the world wars. After theFirst World War, trials of rammed earth and rammedchalk buildings were undertaken in the UK [39],

championed by architect Clough Williams-Ellis[40],and following the Second World War rammedearth was used in East Germany, leading to thedevelopment of the first German building standardfor the material.

Earth building in Europe is now seeing arevival, with national associations in many Europeancountries. Traditional earth building conservationhas been studied, and best practice guides have

Figure 2.11: Haus Rath, Weilburg, Germany.Constructed 1828


been produced[41, 42]. The number of modernearth buildings is increasing, led by specialistearth-building practitioners such as Martin Rauchand Gernot Minke. Research and developmentinto earth building is being undertaken by researchgroups at CRA Terre in France, and at several

institutions in the UK, including the universities ofDurham and Bath.

2.7 NORTH AMERICAEarth was used as a construction material by Native

Americans in modern Mexico and the southernUnited States. The Aztec civilisation in Mexicoconstructed major monumental architecture incut stone, but vernacular buildings are thought

to have been adobe. The Hohokam culture ofsouthern Arizona constructed adobe homes withslightly sunken floors cut into the alluvial soils, andremains of adobe Hohokam structures at the CasaGrande National Monument have been dated toaround AD 600[43]. The Pueblo peoples of modernNew Mexico built adobe structures several storeyshigh that were home to numerous families. Themost famous is the Taos Pueblo, which is dated toaround AD 1000[44].

Europeans coming to North America continued

to use adobe for the construction of many missionsand frontier forts, such as the adobe Tamacacori,Guevavi and Calabazas Jesuit missions in Arizonabuilt in 1691. In Albuquerque, the governor’s housewas built in adobe in 1706. As European settlementmoved westwards, forts were established to protectthe settlers from Native American raids. Remains ofthe adobe Fort Union (1851) and Fort Selden (1865)are testament to the US army using the availablematerials to construct defences.

Many cities on the west coast, such as San Jose and Los Angeles, may have originally beenconstructed in adobe, but continual expansion andrebuilding mean that little remains of these originalstructures. A single adobe wall remains in SantaClara University in San Jose, built in 1822 and partof the original lodges around which the universitywas founded. Casa de Estudillo in the Old Townof San Diego was built using adobe in 1829, andhas recently been restored as a historic monument.Lured by gold mining, Chinese immigration to

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Figure 2.12: Church of the Holy Cross, Stateburg, SouthCarolina, USA. Courtesy of David Grey

the west coast of the United States brought withit construction techniques such as rammed earth,which was previously unknown in the region. InPalo Alto, California, a businesswoman named

Juana Briones built a rammed earth and cobhouse in 1845, and Chinese immigrants built

a rammed earth herb shop in 1855 (the ChewKee Store in Fiddletown, California[45]). LaterEuropean immigrants are probably responsible forroughly 150 rammed earth buildings clustered inthe San Antonio Valley in Monterey County, builtaround 1896[46].

German immigrants to the east coast of theUnited States brought the rammed earth techniquefrom Europe. Hilltop House in Washington, DC,was built in 1773 in rammed earth. BushrodWashington (nephew of George Washington)

built rammed earth lodges on his estate at MountVernon in Alexandria, Virginia, in 1812. FutureUS president and architect Thomas Jefferson wasaware of the technique[38], but it is unlikely that hepersonally constructed any buildings in rammedearth, although slave quarters at the BremoPlantation in Virginia, designed by Jefferson, werebuilt in this material by his friend General JohnHartwell Cocke around 1819.

A rammed earth house was built in Trenton,New Jersey, by S W Johnson, drawing on the work

of François Cointeraux in Europe. Johnson hopedto provide a model to newly arrived Europeansto settle farm land, and published a pamphlet in1806 detailing rammed earth construction[35]. Thisnew construction technique was championed by

John Stuart Skinner, editor of The American Farmermagazine, who published many articles on rammedearth in the early 19th century. Others began toexperiment in rammed earth, and there are manyarticles in periodicals from the time referring torammed earth. South Carolina academic William

Anderson was a key proponent of rammed earth,and in 1850 built the Church of the Holy Cross nearStateburg, South Carolina (Figure 2.12).

Experimentation with new building techniquesin New Orleans led to the construction of the newMarine hospital in 1867[47]. This building was tobe iron framed, with rammed earth infill panels.Construction began, but the building was vastlyover budget, never completed, and eventuallydemolished. The use of rammed earth extended

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into Ontario, Canada, where St Thomas Churchin Shanty Bay was built in 1838, and homes inGreensville in 1868. The expansion of the railroadsat the end of the 19th century meant that it becamemuch easier to transport heavy constructionmaterials around the country, and the use of locally

sourced building materials such as rammed earthand adobe declined.

Rammed earth in North America sawanother revival in the 1920s, following theinterest generated in Europe by the well-knownEnglish architect Clough Williams-Ellis. A bookwas published in 1924 by Karl Ellington[48],and in 1926 an official from the United StatesDepartment of Agriculture published Bulletin No1500 detailing rammed earth construction. Thedepression and New Deal programme in the early

1930s saw several deliberately labour-intensiveconstruction techniques tested, with ThomasHibben building seven rammed earth houses atGardendale, Alabama, in 1935. This period alsosaw the first academic research, with Dr RalphPatty and others publishing the results of erosiontesting at South Dakota Community Collegethrough the 1930s, leading to the publicationby the Federal Government of technicaldocumentation for rammed earth construction[49].

Rammed earth advocate David Miller built

his rammed earth home in Greely, Colorado,around 1940, set up the Rammed Earth InstituteInternational, and inspired a new generation ofmodern earth builders such as David Easton, PaulGraham McHenry and Bruce King.

Modern adobe construction is continuingapace in North America, with established abodemanufacturers in New Mexico. Stabilised rammedearth construction has become an establishedconstruction technique on the west coast of North

America, with experts such as David Easton andMeror Krayenhoff continuing to produce a largenumber of structures. Cob building is developingin Oregon, led by the Cob Cottage Company witha small number of buildings complete, and a widerange of training courses offered.

2.8 SOUTH AMERICA Archaeological evidence of earth building inSouth America is scant, with the richest area beingthe coastal regions of northern Peru. A recentlydiscovered temple at the Ventarrón site in northernPeru appears to be constructed from blocks cut

directly from river sediment, and has been datedto around 2000BC[50].The earliest recorded earthbricks relate to the Moche culture, which flourishedin northern Peru between AD 100 and AD 800.The centre of this civilisation was the city of CerroBlanco, with two pyramids, dedicated to the sunand the moon. Huaca del Sol and Huaca de laLuna are adobe core pyramids around 50 m tall.Distinctive marks on each adobe brick suggest thatmany different communities were involved in theconstruction of these structures.

Contemporary to the Moche culture was theLima culture (AD 100–650) of central coastal Peru.This culture also built adobe pyramids, such asthe Huaca Pucllana (Figure 2.13) and the Huaca

Juliana, the latter being 25 m tall and formed usingadobes stacked vertically. In the south of Peru theNazca civilisation, most famous for the Nazca Lines,built its capital at Cahuachi in adobe, which isstill being excavated today. Although there is littlearchaeological evidence of vernacular architecture,it is likely that both monumental and vernacular

constructions used adobe bricks.

Figure 2.13: Huaca Pucllana, adobe pyramid in Lima,Peru. Courtesy of Louise Davies

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The collapse of the Moche culture around AD 750 led to the development of the Lambayequeculture, who continued to build adobe pyramids atsites such as Batán Grande, Túcume and Apurlec.The largest civilisation to develop following thedecline of the Moche was the Chimu, who emerged

around AD 900 and built their capital of Chan Chanclose to the modern city of Trujillo in northern Peru.Chan Chan was probably the largest city on thecontinent at that time, home to up to 26 000 peopleand surrounded by adobe walls around 15 m high.Ten ‘royal’ enclosures are surrounded by 9 m talladobe walls covered in relief patterns (Figure 2.14).The Chimu civilisation was conquered by the Incas,whose monumental architecture utilised cut stone,although it is likely that the vernacular buildingcontinued in adobe.

The arrival of European settlers brought newbuilding techniques from Europe, to develop

missions and settlements. In 1549 a Jesuit missionarysent a request to Europe to send ‘artisans able tohandle soil, and carpenters, for the construction ofa rammed earth wall’ for the construction of theColégio da Campanhia in São Paulo. São Paulobecame a focus of rammed earth building, with

many monumental and vernacular buildings. Therammed earth cathedral of Taubaté was built in1645, and the Church of Our Lady of the Rosary in1720. Architectural styles followed those in southernEurope, and the rammed earth House of theChamber was built in 1776 in a style similar to thatfound in Portugal around the same time. In 1850major flooding in São Paulo made many buildingsunsafe, precipitating a public campaign against earthbuildings, which led to the demolition of muchof the historic earthen architecture. Building with

adobe continues to be popular in many Andeanparts of South America[20].

Figure 2.14: Chan Chan reliefs, Trujillo, Peru. Courtesy of Louise Davies

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2.9 AUSTRALASIAEarth building is not used by the nomadic aboriginalpeople native to Australia, but European settlersexperimented with a wide range of buildingtechniques from their home countries. An earlyreference to rammed earth in Tasmania is given in

the Hobart town gazette of May 1823:“Resolved that the mode of building in

Pise, or rammed earth, appearing to this Society

to be both economical and expeditious, the

Society earnestly recommend its adoption in Van

Diemen’s Land”.

In 1839 the South Australian newspaperreported on 30 rammed earth houses beingconstructed, and rammed earth was often usedas a quick construction technique in gold rushand frontier towns such as Penrith in New South

Wales and Rushworth in Victoria. Europeansettlers of New Zealand tried many forms ofconstruction, including rammed earth and adobe,

but earthquakes in 1846 and 1855 meant thatall forms of masonry fell out of favour. The bestknown historic earth monument in New Zealandis Pompallier House in Russell, built in 1841(Figure 2.15)[20].

Rammed earth building in Australia was

rediscovered by an English-trained architect,G F Middleton, who was employed by theCommonwealth Experimental Building Station.Middleton conducted numerous tests that werewritten into the famous Bulletin No. 5 in 1953,which until recently was the accepted standardreference in Australia and New Zealand.

Earth building has recently developed in both Australia and New Zealand, with active nationalbodies for the promotion of earth building.

Architects such as Graeme North in New Zealand

and rammed earth contractors Stephen Dobson andRick Lindsay in Australia have allowed earth buildingto flourish.

Figure 2.15: Pompallier House, Russell, New Zealand. Courtesy of Robert McClean

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2.10 CONCLUSIONSIn Chapter 1, we identified two aspects of earthenconstruction: monolithic and unit construction. Unitconstruction requires soils that are particularly richin clay and silt: these are generally found in rivervalleys, and are usually combined with a binder

material such as straw to produce small units. Theseunits can be dried and carried short distancesaway from the production site. As a result, theearliest large settlements using earthen constructionmaterials, such as along the Tigris and Euphratesrivers, the settlement of Catalhöyük, and those ofHarappa and Mohenjdaro along the Indus river,seem to have developed with settled agriculture,using the river valley soils combined with thecultivated crops now available. Conversely, wheresuitable soils were ubiquitous (such as softer loess

soils), they can easily be excavated to produce piledand then rammed earth type structures. In western Asia the Lungshan culture and in North Americathe Hokokam peoples developed piled and earthshelter building techniques.

As civilisation developed, the buildingmaterial became of secondary importance tothe architecture, and thus we find angled adobebricks, such as at Huaca Pucllana in Lima or in theDrâa valleys of Morocco, or patterns cut into earthrenders such as at Chan Chan in Peru. Rammed

earth became decorated by the inclusion ofdecorative brick lines between each lift.Earth has served as the construction material

for many types of construction. Its primary use isusually vernacular construction, and the earliestsettlements, such as those in the Indus valley, andaround the Tigris and Euphrates rivers were able togrow because of the ubiquity of the constructionmaterial. Earth was used for the construction oflarge religious and memorial monuments such asthe ziggurats in western Asia and the pyramidsof the Sun and Moon in modern Peru, and those

in ancient Egypt before the stone construction.Earth is particularly notable for its use in defensiveconstructions, particularly city walls. In China thesewalls are perhaps the largest, with the city walls ofXi’an and Beijing being around 20 m thick at thebase. In the Himalayas, the walls surrounding Lo

Manthang in Mustang are built in rammed earth,and in north Africa and southern Europe rammedearth was used for the city walls of many Islamiccites. The walls of Seville and Cordoba in Spainand Marrakech in Morocco are all constructed inrammed earth.

Although earth continued to be used as both avernacular and a monumental construction material,its use declined over time as other constructionmaterials become available, both through improvedproduction processes (first for timber and stone, and

later for steel and concrete) and through improvedtransportation methods. These, coupled with theimproved mechanical properties that other materialsexhibit, mean that earth began to fall out of use insome parts of the world.

By the 18th century, the industrial revolutionssweeping Europe and North America meant thatother construction techniques could provide aviable alternative to earth building. Although earthbuilding proved popular in the mid-west UnitedStates, the coming of the railways allowed more

efficient construction techniques to develop.Similarly, the development of Portland cement in1824 and the use of iron and steel in constructionpushed earth building away from mainstreamconstruction.

Earth building in the developed world hasrecently seen a resurgence as a sustainableconstruction material. The virtues that made itso viable to early builders, namely low transportdistances, simple construction processes andeasy availability, make earth a potential ultimatesustainable construction material.

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3 FUNDAMENTAL BEHAVIOUR OF EARTHEN CONSTRUCTION MATERIALS 27

CHAPTER 3FUNDAMENTAL BEHAVIOUR OF EARTHEN

CONSTRUCTION MATERIALS3.1 INTRODUCTION

All earth buildings have the same ingredients: agraded mixture of subsoil, water and, in somecases – stabilisers such as Portland cement, lime,bitumen or straw. Wall construction takes placeusing a mixture with an optimum water content toallow good compaction, from which evaporation

occurs and the wall gains strength: it is thereforeclear that the water in the mix has an importantrole. But the mechanisms by which strengthdevelops and changes over time in these materialshave rarely been rigorously examined, and effectiveconservation would seem to be handicappedwithout this understanding.

This chapter covers the physical basis for thedevelopment of strength in earthen constructionmaterials, and begins with some basic soilmechanics, followed by an examination of the

behaviour of earthen construction materials atthe particle level. The fundamental and mostimportant source of strength in unstabilised earthenconstruction materials is shown to be due to thesmall amounts of water held in the earth mixtureat particle contacts, and in clay bridges betweenlarger particles. The striking example is that ofa sandcastle, where too much water makes thecastle flow away and too little makes it blow away.Improved understanding of the role of water inearthen construction materials at the particle levelis used in later chapters to inform conservationstrategies for heritage structures.

3.2 SOIL MECHANICSCivil engineers refer to the subsoil (below the topsoil,the zone in which plants grow) or earth used forbuilding as soil, and to the study of its behaviour as soilmechanics. This encompasses methods of determiningthe strengths and stiffness of soils: strength fordetermining the stability and safety of structures built

in or on soil, and stiffness for determining movementsduring use (of tunnels, foundations, retaining wallsetc.). Topsoil is not used for earth building, and will notbe considered further here.

Soil is an accumulation of mineral particlesformed by the weathering of rocks. The type of soildepends on the rock from which it originates, and onthe processes it has undergone since weathering. Asoil is often described by the distribution of particlesizes it contains, and names given to ranges of particlesizes are shown in Table 3.1. (Different conventions

are used in different parts of the world; the tableshows the classifications used in the UK.) Particlessmaller than 0.002 mm are termed clay , and areoften considered to be different from larger particlesbecause of the relative importance of the electrostaticcharges held on the surfaces of clay particles. In fact,this feature of clays has little effect on their strength.

Table 3.1: Particle sizes to BS 1377[51]

Name Maximum dimension (mm)

Clay 0.002

Silt 0.06Sand 2

Gravel 60

Cobbles 200

Boulders > 200

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In its natural state soil consists of interlockedsolid particles with voids (or pores) between. Thesevoids are filled with fluid: this is usually air or water,but other fluids (such as oil) can be present. Whereall the pores in the soil are filled with water, the soil isdescribed as saturated; where air is present in some

of the pores, the soil is described as unsaturated.Most earthen construction materials (which can beregarded as manufactured soils) are unsaturated.However for most conventional construction projectscivil engineers usually assume soils to be saturated,and the established theories of soil strength havebeen based on this assumption. This is the naturalstarting point for an explanation of soil mechanics.

3.3 SOIL STRENGTHSoil strength usually refers to soil shear strength, as soilshave little or no tensile strength; they usually fail inshear. We refer in this chapter mainly to uncementedsoils, but for cemented soils (stabilised rammed earth,cement-stabilised block, or rocks such as sandstone)tensile failure is possible. It is now widely accepted thatthe source of a soil’s strength (clays included) lies inthe friction between particles[52]. Friction is a conceptfamiliar from school physics, usually demonstrated by asimple block on a plane, as shown in Figure 3.1.

Rφ

N

F

Figure 3.1: Simple model of friction, showing frictionangle φ

When the block is just about to slide, a force F isresisted by friction between the block and the planebeneath. The magnitude of the resisting force isdetermined by the value of the normal force N andthe coefficient of friction μbetween the block and theplane. The resultant force R is oriented at an angle φ

tothe vertical, where μ= tan φ . The angle φ

provides a

measure of the shear strength of the assembly throughthe following equation:

F = μR = R tan φ (3.1)

For soils we measure shear strength via a

macroscopic angle of friction φ (rather than lookingat individual contacts), which is associated with theangle of repose (slope) of a pile of soil. For sandsand gravels φ can rise to 40°, but for most soils it liesbetween 15° and 30°. Engineers determining thepossibility of soil failure use the following equationfor points in the soil mass:

τ = σ ′ tan φ ′ + c (3.2)

where τ is the shear strength, i.e. the shear stress

on a plane just at the point of failure, and φ ’ is theeffective angle of friction. The dashed superscriptsindicate that these are ‘effective’ values (see below).This deals with the frictional strength of soils. Ifthere is cementation between the soil particles(for instance in stabilised rammed earth) then inaddition to the frictional strength there will be acomponent of apparent cohesive strength (c) whichdoes not vary with the applied normal stressesunless the cementitious bonds are broken.

3.4 EFFECTIVE STRESSSaturated soils contain water in the interparticlevoids (the pore water), and one of their mostimportant features is that failure can occur bothfrom changes in the applied load and from changesin the pressure in the pore water. The theory ofeffective stress states that the behaviour of soils isgoverned by a single stress (the effective stress),which is defined as:

σ′ = σ – u (3.3)

where σ is the total stress (the stress due to theapplied loads) and u is the pore water pressure.So if σ rises or u falls, then the effective stress rises.The effective stress is the normal stress that controlsfrictional behaviour, and so in either of these casesthe shear strength increases. Alternatively, a rise inpore water pressure (i.e. a reduction in effective

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3 FUNDAMENTAL BEHAVIOUR OF EARTHEN CONSTRUCTION MATERIALS 29

stress) can cause a failure with no change in appliedloads. Examples of such behaviour are subsidencedue to a rising water table, or landslides induced byrainfall. Therefore, unlike many other constructionmaterials, the presence of water in soils makes theirbehaviour more complex.

3.5 UNSATURATED SOIL MECHANICSThe soil mechanics routinely used by engineersassumes full saturation and effective stressparameters, as described above. However, manyconditions in the field are unsaturated. That is,the voids between soil particles are not entirelyfilled with water; air is present too. This seeminglyinnocuous change has a major influence on the

mechanical and hydraulic behaviour of the soils.Earthen construction materials are never completelydry, but nor are they saturated (if they were, onecould not add more water to them). Materials suchas rammed earth, cob and adobe are effectivelymanufactured unsaturated soils. In what followswe examine some of the theories that attemptto explain observed macroscopic unsaturatedbehaviour by looking at the particle level.

3.6 FUNDAMENTALS A highly simplified model of an unsaturated soilcomprises spherical particles linked by water held in‘bridges’ between particles, as shown in Figure 3.2.This allows us to make assumptions about thedistances and interactions between particles, andallows us to undertake much simpler modelling.We can then apply the properties of this simplifiedmodel to real soils.

The shape and size of the water bridges aredetermined by various physical properties andeffects. The most fundamental one is the contactangle. This is the angle that a water/air interfacemakes at a solid surface. If we imagine a drop ofwater on a smooth solid surface, we can clearly seethe contact angle θ (Figure 3.3). For hydrophilicmaterials the contact angle is between zero and90°. For hydrophobic materials the contact angle isbetween 90° and 180°.

Water

Soil particles

Air

Figure 3.2: Simple model of an unsaturated soil

Water droplet

Figure 3.3: Contact angle of a droplet of water.Courtesy of Richard Iles

The second fundamental property of the waterheld at bridges is surface tension, which exists atany water/air interface. Surface tension arises fromthe different forces on water molecules close to theinterface compared with those in the body of thewater. A molecule located within the body of thewater is subject to equal attraction in all directions,whereas at the surface the absence of equal attractionin all directions leads to a net attractive force in theplane of the surface, known as the surface tension.For a large body of water the water surface will beflat, and only at the extreme edges will the surfacecurve to follow the contact angle. For a narrow porecontaining water the two solid surfaces are closetogether, and the water surface will then be curved,forming the menisci seen in Figure 3.2.

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In this situation, and because of surfacetension, equilibrium can occur only if there is anet pressure difference between the water andthe air. The curvature of a meniscus is linked tothis pressure difference by the Young–Laplaceequation:

^ua ± uwh=T sc1 + 1m (3.4)r x r y

Where ua is the air pressure, uw is the water pressure,T s is the surface tension, and r x and r y are the radiiof curvature of the meniscus. Considering the caseof the idealised unsaturated soil, if the air pressure inthe pores is atmospheric, Equation 3.4 implies thatthe water pressure is negative, so that both sides ofthe equation yield positive values, and this is indeedthe case. This negative pressure is often referred to

as a positive ‘suction’ (denoted by s, and defined asthe difference between the air and water pressures,ua – uw). Therefore the presence of air in the poresmeans that water is held between the particlesin menisci with curvature. These cannot exist inequilibrium without there being a pressure differenceacross the air/water interface. With the air pressureat atmospheric, equilibrium can be reached only ifthere is a negative pressure in the water. This suctionthen provides an additional force pulling particlestogether (see Figure 3.4) and, crucially, an additional

normal force between the soil particles that increasesthe macroscopic shear strength of a sample. Thepresence of suction then can be seen to strengthenan unsaturated soil as compared with its saturatedstate, and these ‘liquid bridges’ contribute to thestrength of earthen materials.

Additional normalforce between particles

Suction in water

Figure 3.4: The role of suction between two particles

3.7 RELATIVE HUMIDITY All air contains water in vapour form, so the airheld in pores will itself contain water vapour.Relative humidity is a measure of the amount ofwater vapour in the air. A body of water containswater molecules in constant random motion;

the magnitude of this motion is determined bytemperature. At the surface of the water body somemolecules have sufficient momentum to escapefrom it, and join the water molecules present asvapour in the air surrounding the water. Moleculesas water vapour are also in random thermal motion,sometimes losing energy and returning to liquidwater. When the number of molecules leaving thewater equals the number of molecules arriving,an equilibrium state is reached. When moremolecules are leaving than arriving, the liquid water

is evaporating; when more molecules are arrivingthan leaving, the water vapour is condensing intoliquid water. The different gases in air (e.g. watervapour, oxygen and carbon dioxide) exert different

partial pressures on their surroundings as a result oftheir different molecular kinetic energies. The partialpressure associated with the water vapour is referredto as the water vapour pressure. Relative humidity(RH) is a measurement of how much water vapouris present in the air as a proportion of the maximumamount there could be (which is determined by

temperature, and is termed ‘saturated’1

). It isdefined as the ratio of the water vapour pressure pvto the vapour pressure when the air is saturated, p0:

(3.5)

There is a relationship between the suction in themenisci between particles and the relative humidityknown as the Kelvin equation:

(3.6)

Where R is the universal gas constant, T istemperature (in degrees K), and vw is the molarvolume of water. By combining Equations (3.4) to(3.6) we can remove the suction term, to give:

(3.7)

1

Do not confuse this use of ‘satura

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