Bamboo Housing in Pabal

7/29/2019 Bamboo Housing in Pabal

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EWB-UK Research Conference 2009

Hosted by The Royal Academy of Engineering

February 20

Community of Practice: Habitat

Author: Jaspreet Grewal

Institution: University of Southampton

Bamboo Housing in Pabal

Jaspreet Grewal

Engineers without Borders (EwB) have proposed that in order to overcome theissue of affordable housing within rural areas of India, the option of usingBamboo as a permanent structural component should be explored.

Investigations into the material strength, structural properties andconstructability of Bamboo will be used to provide architectural solutions. This isrequired to fit into the context of the rural town of Pabal, India.

This district was identified by EwB, as part of on-going research within the areathrough educational organisations such as engINdia and Vigyan Ashram(VA).

Pabal:

The village of Pabal was chosen as the location to implement my research for anumber of reasons. Its proximity to the urban centre of Pune and Mumbaiprovides easy access to the Indian Institute of Technology (IIT), whereinformational and laboratory resources can be used. The natural climate of Pabalis not severely susceptible to the monsoons compared to the rest of India,therefore implementing a Bamboo structural solution within this region would

decrease the risk of rotting.

The total population living within the two kilometre radius of the town equals toapproximately nine-thousand people. The town centre consists of mainlyconcrete dwellings, compared to surrounding areas where there are anabundance of wooden houses owned by the more underprivileged. It is withinthe hamlets surrounding the town centre where the research and implications ofBamboo as a structural element will be most applicable.

Pabal is an agricultural community, where one farmer typically owns two-acresof land. Therefore a reliable water supply for irrigation systems is critical,

however the availability of water sources in Pabal is not consistent. Furthermorethe lack of a wastewater management system has led to common illnesses dueto the waterborne microorganisms.


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PROJECT AIMS:

To identify the advantages and disadvantages of Bamboo as a structuralmaterial;

To categorise the ideal species for construction available withinIndia, and more specifically, Pabal.

To bring together research into the strength of Bamboo, soaccurate assumptions regarding the strength can be made for the

species specified.

To investigate material properties of the Bamboo specie specified,such as characterising the stiffness, deflections and Youngs

Modulus.

To report other issues affecting the suitability of Bamboo as abuilding material, such as its resistances to fire, moisture and

insects, as well as reporting its use within a seismic zone and

lifespan.

To coordinate and collate all data and information regarding Bamboo into asuccessful architectural solution;

To determine the function of the structures to be constructed usingBamboo.

To use precedent studies of similar projects and initiativesworldwide and within India to provide a basis for design.

To incorporate design schemes and construction methods usedtraditionally across India.

To employ material characteristics of Bamboo to overcomeconstraints within the architectural layout.

To explore various options relating to the detailing of the building,such as establishing low labour jointing details.

Location of

Pabal

within

Pune


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THE ANATOMY OF BAMBOO:

Background:

Bamboo: one of the only materials in the world with such a vast range of uses

and functions; from its use as a material within construction as well as withinfurniture design, to its use as a culinary delicacy and a decorative plant in someparts of the world. Research into using Bamboo fibres as a reinforcing elementwithin concrete structures is also being explored.

Bamboo is one of the fastest growing plants on the planet, with some reportingrates of one-metre of growth per day. It belongs to the Gramineae family, andtherefore it is not classed as a tree, but as a grass. There have beenapproximately 1100-1500 different species of Bamboo identified and classed intoa range of over 60 genres. Bamboo can be classed into two distinct typescategorised by its use as a delicacy and its industrial use; herbaceous and woodybamboos.

The environmental benefits of Bamboo are significant as the shoots producegreater biomass, 30% more oxygen and consume twelve tonnes of carbondioxide more than a hardwood forest of comparable size. Its extreme growthrate and ability to regenerate without annual replanting make Bamboo anattractive renewable resource.

The advantages of cultivating Bamboo are due to its widespread root systemwhich prevents soil erosion and also has a high level of nitrogen consumptionhelping to mitigate water pollution.

Anatomy:

The hollow cylindrical stem of the Bamboo shoot is referred to as a culm. Thestructure of the culm has a significant influence on the properties of the Bambooand therefore its use as a structural element. Although the culm has thepotential to reach its full height and diameter within a short space of time, thestructural strength of Bamboo comes from the thickness of the culm, whichgradually hardens and matures over a maximum of sixty years.

The culm can be split into nodes and internodes which run perpendicular andparallel to the culm respectively. The nodes have a great influence on themechanical strength of the bamboo as they create a diaphragm across the cavityof the culm, providing stability along the height. The walls of the culm arelayered, where each layer is of varying thicknesses and are made up of acombination of fibres and vascular bundles (containing phloem and xylem).Within the nodes, the vascular bundles intersect, whereas within the internodesthe bundles run parallel. The internodes surround a large cavity called a

lacuna, where the thickness of the internodes tends to decrease with the culmheight. The build up of internode layers become smaller and denser towards theoutside of the culm, making the outer layer the strongest.


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STRUCTURAL:

Species:

Bamboo grows worldwide, where China and India are the richest countries inBamboo resources, collectively growing half the worlds population of Bamboo.There are an estimated 1500 different species which have been identified, eachwith its own properties and consequent uses. India is home to 130 of these

species, categorised into 18 genera.

The National Mission on Bamboo Applications (NMBA) have concluded that ofthe 130 species present within India, only 16 have significant commercial valueand therefore has conducted research into each specie type, aiming to identifyits properties. From this list, identification of the most suitable species forconstruction purposes have begun and a table of the names and properties arebelow:

Specie NameWhere

grown?Uses: Culm: Flowering:

Bambusa

balcooa

- North

East India

- House

construction

- 30m

height - Gregarious

- West

Bengal - Scaffolding

- dark green

colour

- Flowering

cycle = 35 - 45

yrs

- Bihar

- Ladder

construction - thick walls

-

Jharkhand

-

At the beginning of its lifecycle, Bamboo develops as

an underground network of rhizomes, made up of

tough stems producing roots and new shoots.Rhizomes can occur in two forms where they are

classed as either running Bamboo which produces

long, fast-spreading rhizomes and can often lead to

an unwanted amount of Bamboo. Or can be classed

as clumping Bamboo where the rhizomes occur in

short, slow-spreading clumps. Rhizomes can last for

up to twenty-years, where the hollow, segmented

structure provides them with the strength and

lightness to remain anchored within the soil.

Internode

Node

Lacuna


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Uttaranchal

Bambusa

Bambos

-

throughout

India - thatching

- 30m

height

- Flowering

cycle = 40 - 60

yrs

- Roofing- dark greencolour

- strong ,

hollow shoot

Bambusa

Nutans

- North

East India

- House

construction

- 20m

height - Gregarious

- Orissa - Basketry

- dark green

colour

- Flowering

cycle = 35 yrs

- Bengal - Craft

- smooth

outer texture

Bambusa

Polymorpha

- Arunachal

Pradesh

- House

construction

- 25m

height

- Gregarious

and Sporadic

-

Meghalaya - Pulping

- light

green/grey

colour

- Flowering

cycle = 55 - 60

yrs

- Tripura

- North

east India

Dendrocalam

us Brandisii

- Manipur

- House

construction

- 20m

height

- Gregarious

and Sporadic

-

Karnataka - Basketry

- green/grey

colour

Dendrocalam

us Giganteus

- North

East India

-

Construction

- 30m

height - Sporadic

- West

Bengal - Boat masts

- dull green

colour

- Flowering

cycle = 40 yrs

- Bihar

- waxy

appearance

Dendrocalam

us Hamiltonii

- NorthEast India

-Construction

- 30mheight

- periodicgrowth cycle

- Himachal

Pradesh - Roofing

- dull green

colour

- Flowering

cycle = 30 - 40

yrs

- culinary

delicacies

- waxy

appearance

Oxytenanther

a Stocksii

- Southern

India

-

Construction

- 10m

height - Sporadic

- Furniture

-

yellow/green


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colour

Melocanna

Bambusoides

- North

East India

-

Construction - Gregarious

- wovenproducts

- Flowering

cycle = 40-44yrs

Table notes:

Yellow = Species within close proximity of Pabal, Maharastra state Blue = Species with the largest growth rates within India Gregarious = plant growth in clusters Sporadic = single plant growth, spread across a wide rangeFrom the table, two species have been selected to further investigate in relationto the close proximity of growth to Pabal. However there is proving to be limitedresearch on these species. Therefore assumptions using the informationconcerning popular species which have been studied in more depth, will need tobe made.

Strength & Material Properties:

Tensile Properties:Due to the strong fibres within the shoot, Bamboo has a tensile strength equal tomild steel. However it should be noted that the tensile strength parallel to theshoot is far greater than perpendicular to the grain direction. This has importantimplications for the design of connections to transfer loads.

Compressive Properties:The compressive strength of the shoot comes from the hollow cylindrical shapeof the culm, where the strongest fibres are along the outer edge, with stiff nodeswhich act as diaphragms occurring at intervals along the length.

If the Bamboo shoots were to grow perfectly straight then they would possessrelatively high axial compression, however in reality this is not the case whereBamboo grows along a straight axis. The lack of straightness results in culmsbuckling before reaching their compressive strength and therefore research canbe conducted within this area.

Bending Strength:Bamboo is most vulnerable to excessive bending along its axial length due to thefailure of fibres which run along this direction. This causes the shoot to lose itscircular form and hence its strength.

Tests conducted within the past have confirmed that there are a number ofparameters which greatly affect the bending strength of Bamboo. Parametersinclude; age, moisture content, position in the culm, skin surface and thedistribution of node.

A paper looking into the bending strength of Guadua Bamboo (one of the mainLatin American Bamboo species used within construction) has investigated the

differences in bending strengths of various Guadua Bamboo specimen, using 3and 4-point bending tests. Specimens included short, round, long, and split


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Guadua. The modulus of rupture and Modulus of elasticity were found to begreatly different within each specimen. It was found that by determining thedensity of the outer layer of the culm, the modulus of rupture and elasticityvalues could be approximated. The report also highlights the importance ofwhere the specimen was taken along the culm height, as due to difference in

wall thickness and density, the modulus of rupture and elasticity can vary alongthe culm height. These conclusions can be broadly applied to other specieshowever individual data sets will need to be obtained.

In comparison to mild steel, concrete and wood, Bamboo tends to have a lowmodulus of elasticity, and therefore methods to utilise its comparable flexibilitywithin seismic zones is being investigated. Bending tests conducted on Bambooshoots have all shown the culm to regain its original shape, due to its strongfibres which remain intact.

Shear strength:The low shear strength of Bamboo is one of the biggest problems of its materialproperties. The relatively thin walls of the shoot contribute greatly towards thecracking and splitting of the culm.

Buckling failures:Due to the slender nature of the Bamboo shoots, determining the bucklingstrength is critical and therefore should be considered within the design ofBamboo columns. Some studies have approached solutions to overcome this bycreating up to eight-culm columns, where eight Bamboo shoots are tied togetherto create the column. As in any case, necessary bracing will be required to resistflexural movements along the shoot length.

Although studies using finite element models to predict the buckling strength ofBamboo shoots (grown predominantly within China) produce conservativevalues, the results of these studies can still be used effectively.

Density:

For species grown within Ghana (which also occur within Asian and Americancontinents), it has been found that the density of Bamboo is greater than timber.Values for density average at 700-800 kg/m, although each species has its ownindividual value. There is a positive correlation between the density and strengthof Bamboo, where the higher the density is, the stronger the shoot.

Tables adapted from a range of experimental sources have been included toillustrate the range of values for the material properties of Bamboo:


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Other Issues:

Treatment Methods:In its natural state, Bamboo is a non durable material due to its susceptibility tomoisture, insects and fire. Industries which rely on the manufacture of Bambooproducts have developed various treatment methods to combat these materialdisadvantages.

In general, methods to combat the growth of fungi and degradation of the shootwhen placed into the ground, techniques using tar-oil are used. The base of theBamboo column is usually treated using a hot and cold method, where the culmis submerged in the heated preservative and then cooled. Alternative installationmethods of injecting tar-oil along the internodes are also used.

New methods which are widespread across industry include the Boucherieprocess which replaces all the sap within the culm with preservative pumpedfrom underneath the shoot within the ground. Dip-diffusion methods involvesoaking the shoot in preservatives for a period of time.


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With occasional maintenance and replacement of certain degraded culms,Bamboo structures have been known to last up to 100 years in regions withinColombia using Guadua specie types.

Seismic Resistance:

The low modulus of elasticity combined with the high strength to weight ratio ofculms has made Bamboo structures react well to earthquakes. The lightness ofBamboo structures in comparison to steel/concrete results in relatively smallinertial forces from earthquakes. The flexibility of Bamboo structures creates areduced response to ground movements, due to longer natural frequencyperiods. However it is to be noted that like timber, Bamboo has a brittle failuremode, which is an important factor to consider within seismic design. The designof ductile connections can greatly reduce the effects of this.

Comparison to Timber:

Comparisons to timber further illustrate the advantages of Bamboo as a

structural material;BAMBOO TIMBER

Vascular bundles of culm containphloem, xylem and fibres all in one

Tubes are stronger towards theoutside of the culm (outside layeris denser and smaller)

No bark. Relies on hard, shinycortex to protect it, has nocambium

Culm is born of the diameter it willhave through its lifetime

Develops as a linked undergroundnetwork of roots, branching off toproduce roots and stems

Higher density

Separate structures within Timber Strongest wood towards centre =

heartwood

Has cambium and bark (cambium= thin layer of cells making thetree broader over time)

Tree trunk grows broader with timedue to cambium

Each tree = independent organism

Lower densityARCHITECTURAL:

Functionality:

One of the primary aims given by EwB for this project was to use theimplications of my research to contribute towards the development of affordablehousing.

What remains to be determined is whether the design outlined within this projectwill propose to construct individual dwellings per family or whether the designcan use the structural information gathered to plan something of a larger scalesuch as a set of dwellings in a terrace style, or community area such as a villagehall, or on a larger scale, a school.

The opinions by the people of Pabal should determine the decision. From the

research and appraisals carried out in Pabal by engINdia, structural solutionsusing Bamboo should be targeted towards the outer, poorer areas of Pabal.


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Hence the focus towards a dwelling or set-of-dwellings type structure will beapproached.

Precedents:

Several precedents using Bamboo structures across the world and India havebeen identified, within an affordable housing context. It has been found thatthere are many opportunities for pre-fabrication using Bamboo. Many differentapproaches to jointing and connections have been found, however the mostpromising is the flexible rubber joint shown within the One Year House. Oncethe structural constraints of Bamboo are identified, design can begin.

Simon Velez:

METI school, Bangladesh:

Cross Waters Ecolodge, China:

Another example of the possibilities with Bamboo,

designed by Velez:

(Image Source: Second International Conference on

Modern Bamboo structures)

The low-cost House:

Velezs set up a prototype affordable house enablingColombian locals to grow and build their own houses

using Bamboo. Although within a Latin American

context, several parallels can be drawn from this

project.

Designed by German architects, the

school consists of 6 classrooms, with a

combination of traditional earth materials

and Bamboo used for the load bearingstructure.

(Image Source: Pawalitschko, R. 2007:

DETAIL Magazine Analogue Construction

Usin Local Resources

ZERI Pavilion, Germany:

A great example which highlights Bamboo

to its full potential is the ZERI Pavilion,

designed by the Bamboo specialistarchitect; Simon Velez

(Image Source: Website: www.zeri.org)


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Asif Khan & Julia King; The One year House

International Network for Bamboo and Rattan (INBAR): Bamboo

Housing Development Programme:

CONCLUSION:This paper highlights and summarises research carried out so far into theapplicability of Bamboo as a structural material and therefore there are still

factors where research needs to be collated and experimented on further.However the primary message is clear; Bamboo has numerous advantages as astructural, economical and sustainable material. However more awareness of thematerials advantages needs to be made through the introduction of buildingcodes, more inclusion of Bamboo within building works globally and further

research to establish the structural properties of various Bamboo species

The One Year House was developed in response to the

growing lack of aid and shelter within Thailand. Aflexible jointing detail as shown below, was developed

by the architects. The inclusion of this connection

method within my designs will be further looked into.

(Image Source: RIBA McAslan Bursary 2007)

Model Bamboo House, Ecuador:

Extensive research by INBAR found that the principle

differences in building with Bamboo worldwide were

due to wall system construction methods. This house

was built in order to draw direct comparisons of various

wall systems

(Image Source: INBAR Housing Programme)

Pre-fabricated Bamboo Housing: The Eco-Housing

Project, Nepal and Viviendas Hogar de Cristo

(VHC) Housing Project;

Low cost housing methods using pre-fabricated

Bamboo wall panels have been used enabling speedy

construction.

(Image Source: INBAR Housing Programme)


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Author: Paul Jaquin

Institution: Ramboll Whitbybird (formerly Durham University)

Previously published: (Geotechnique, February 2009)

How mud bricks work using unsaturated soil mechanics principles to explain

the material properties of earth buildings.

Paul Jaquin

Across the world, soil is piled , shaped, formed into bricks or compacted betweenforms, to make solid walls. One third of the worlds population live in buildingsconstructed from soil. Although many other building material are now available,soil remains one of the most ubiquitous construction materials on the planet.While often seen as the preserve of the developing world (and many view themove away from earth buildings as a step forward in development), earth isbecoming increasingly popular as a modern, chic, green building material.

The scientific understanding of earth buildings is relatively low when comparedto steel or concrete. Traditional construction uses heuristic and locally developed

techniques. While these methods work well locally, they are dependent on thelocal soil and construction techniques which makes technology transfer betweendifferent regions difficult.

In the last 30 years there has been a drive to better understand earthenconstruction. Practitioners such as Gernot Minke, Hugo Houben and SatpremMaini have performed many experiments to improve construction techniques andrefine raw material to improve the performance of earth buildings. Theseinitiatives are relatively poorly documented and more rigorous scientific methodhas only been only recently introduced. Work undertaken at Durham Universityhas used the principles of unsaturated soil mechanics to better understand the

engineering properties of earth buildings. Unsaturated soil mechanics considersthe behaviour of soil where the pores are filled with both air and water. Inaddition to friction and interlock, water is held in tension between the soilparticles in the form of liquid bridges. These liquid bridges act as a bond givingthe soil additional strength and stiffness over that of a fully saturated soil.

This paper will describe some aspects of research carried out as part of a PhD atDurham University. Some of the principles of unsaturated soil mechanics, suchas surface tension, relative humidity and the attractive force between particlesare outlined. Arguments for using the principles of unsaturated soil mechanicsto describe earth buildings are discussed. A series of experiments to test thisnotion were devised. These experiments are described and the results arediscussed. The wider implications of considering earth buildings as highlyunsaturated soils are then presented and conclusions drawn as to how tounderstand their engineering specification, construction and behaviour.

Unsaturated soil mechanics principles

In an unsaturated soil, water is held between soil particles by surface tension.Surface tension exists in all fluids. At a vapour to liquid interface there is adifference in pressure between the liquid and the vapour. As the vapour pressureis greater than the liquid pressure, there is an unsymmetrical force balance atthe interface. The interface (meniscus) must compensate for the difference in

the two pressures and behaves as a membrane acting in uniform tension. Water


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Author: Paul Jaquin



held between two soil particles within such menisci and is known as a liquidbridge. The combined tension of this membrane and the lower pressure of thewater provides an attractive force between the soil particles (Fisher 1926) asdescribed by Figure 1 and Equation 1.

mensicus pressureF F F= +

( )2neck T neck a wF r r u u = +

EQUATION 1 MAGNITUDE OF THE ATTRACTIVE FORCE BETWEEN SOILPARTICLES AS A RESULT OF THE LIQUID BRIDGE

Where mensicusF is the attractive force due to the meniscus and pressureF is the

attractive force caused by the difference in pressure between the air and the

water, neckr is the radius of the neck of the liquid bridge, T is the surface tension

and au and wu are the air and water pressure respectively.

Relative humidity is the ratio between the actual vapour pressure in the air andthe maximum possible vapour pressure. There is a unique relationship betweenwater tension and relative humidity which was first described by Lord Kelvin(Thomson 1871).

The origins of strength in earthen materials have long been the cause of muchdebate. An authoritative textbook on the subject (Houben and Guillaud 1994)and state of the art review (Avrami and Guillaud 2008) describes the strength as

being a result of electrostatic forces, cementation, capillarity and friction. It hasbeen argued (Jaquin 2008) that although electrostatic forces can be used todescribe the attraction between clay platelets, the magnitude of attractive forcesbetween larger particles must be a result of the liquid bridges between theparticles, in addition to the interparticle friction and interlock. It was proposedthat the number and strength of these bridges determines the overall samplesstrength and stiffness. The number of liquid bridges depends on the number ofpores in the soil across which they can act. At a high negative pore waterpressure, liquid bridges act across the majority of pores in a soil sample,resulting in a highly bonded, therefore stronger sample. At low negative porewater pressures, there are fewer liquid bridges which leads to lower sample

strength.

Experimentation

A small series of simple tests were proposed to investigate the link between thestrength of earth buildings and the pore water pressure. The experiments usedprobes to measure the magnitude of the negative pore water pressure (tension)called tensiometers (Loureno, Gallipoli et al. 2008). Soil cylinders wereconstructed using methods based on the light Proctor method developed at theUniversity (Horncastle 2006). Soil was oven dried at 105C, before being mixedwith a known volume of water to create a mix of known water content. The

mixture was bagged and left for 7 days to allow the moisture content to


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equalise. To make a sample this mixture was sequentially compacted in fivelayers of equal mass, with each layer receiving 25 blows from the Proctorhammer. The final layer was a fine screed of particles smaller than 425m. Thislayer provided a flat surface against which a tensiometer was placed and whichwas used for loading. The cylinders were then placed on a mass balance, and

the mass noted while the cylinders air dried. On reaching a specified mass, andtherefore water content, the cylinders were wrapped in a rubber triaixal sleeveand sealed to prevent further evaporation. The cylinders were left for at least 7days prior to testing to allow water to distribute evenly within the samples. Thesamples were tested in unconfined compression, in a rig shown in Figure 16.Tensiometers were placed against the top surface of the cylinder, through theloading platen to measure the change in pore water pressure during the loading.The axial strain and load were measured. Following failure the whole sample wasweighed and placed in the oven to determine the final water content.

Results

The sample cylinders were prepared at a known water content of 12%, anddifferent samples allowed to air dry to different water contents. Figure 17 showsthe results of unconfined compression tests carried out on samples at watercontents ranging from 10.2 to 5.5%. Samples with a high initial water contenthave a low negative pore water pressure, whereas samples with a lower initialwater content have a higher negative pore water pressure. On loading of thesamples (increasing deviator stress) the three highest water content samples(10.2, 9.4 and 8.4%) show an increase in pore water pressure, whereas thelower water content samples show a reduction in pore water pressure. The peakdeviator stress (sample strength) increases from 170kPa to 600kPa with thewater content reducing from 10.2 to 5.5%. Lower water content samples weretested, but these proved to have a negative pore water pressure beyond therange of the tensiometers (-1500kPa).

Figure 18 shows the peak deviator stress versus water content. It comprisesthose samples shown in Figure 17 and further experimentation on samples oflower water content, where the negative pore water pressure was beyond therange of the tensiometers. This shows a linear relationship between watercontent and sample strength. showing the strength reducing from a peak of1100kPa at a water content of 2% to a minimum of 100kPa at water contentsclose to those of compaction (12%).

Interpretation

The change in suction on loading of unsaturated soil samples has been observedby a number of other researchers (for example Cunningham, Ridley et al. 2003;Tarantino 2007). To explain this behaviour the dilatancy of the sample must beconsidered, as must the fact that water considered as incompressible comparedto air. If unconfined loading of a fully saturated soil sample is considered, porewater pressure would reduce (become more negative) if the sample were todilate. The dilatant behaviour of the whole soil sample causes an increase in thetotal pore volume. As the sample is not drained, no water is lost, so the samevolume of water occupies a larger pore volume so its pressure is reduced. Thisbehaviour may be observed for the high water content samples (10.2 and


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9.4%). The lower water content samples (7.2 to 5.5%) show an increase inpore water pressure, which may be attributed to a mean contractant behaviourof the sample. This is caused by a reduction in volume of the air filled pores. Thewater filled pores are effectively rigid when compared to the much morecompressible air filled pores. Increased loading of the water filled pores causes

an increase in the pore water pressure which is measure by the tensiometers asa reduction in negative pore water pressure. This concept is shown diagrammaticin Figure 19. It has been argued (Jaquin 2008) that the mean pore size,described by the Air Entry Value (AEV) of the soil is the point about which thischange occurs. At this point all of the pore are water filled but the pore waterpressure of the whole sample is still negative.

Implications

Water in earthen structures

There is a small but finite volume of water present in the form of liquid bridges

between soil particles in earth structures when air dry. This water is undertension and the magnitude of the pore water pressure (suction) is related to therelative humidity of the surrounding air. Evaporation of water from unsaturatedsoils has been shown to be proportional to the suction (Wilson, Barbour et al.1995). Jaquin 2008 argued that this evaporation will continue from the earthstructure until the relative humidity of the pore air is equal to the humidity ofthe surrounding air. The strength and stiffness of earth structures, and thereforetheir inherent viability is a function of the mean relative humidity of a region. Itcan be shown that the international distribution of earth buildings maps verywell to regions where the mean surface relative humidity is low.

Earth structures are not renowned for their resistance to water, and especially inthe UK this is seen as a reason to not use them. However, improvedunderstanding of their behaviour in the presence of water may be this to changein the future. Rainfall again the surface of an earth wall will not lead todegradation of the face for the wall or to the collapse of the structure. If oneconsiders a sheet of water against the face of a wall for example where a drainis leaking water against the face, water will be drawn into the wall throughcapillary action. The volume and speed of infiltration by capillary action can bedescribed by Washburns equation (Washburn 1921) if the diameter of thecapillary is known. The diameter of the capillary can be considered as the meanpore size for the soil. The rate of advancement of water though a capillary tube

of diameter 0.002mm (the size of a the smallest silt particle) is around 12mmper hour. Ahead of the wetting from in the capillary tube there will be a region ofincreased relative humidity, where the liquid bridges will increase in size andreduce in strength, causing a gradual loss of strength and stiffness. Where waterdoes advance into an earth wall through capillary action, the soil particlessurround the pore through which the water is advancing lose the attractive forcebetween them and s oil is locally saturated and will slump to its internal frictionangle. If the walls are well protected to prevent rainwater ponding on the topssurface, and there is a sufficient overhang to stop rainwater draining from theroof and impacting the wall, then the integrity of the wall should be maintained.


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Cement stabilisation

In many parts of the developing world, cement is added to earth bricks toimprove their durability and strength (for example Maini 2007). The majority ofresearch into cement stabilisation has been heuristic and the reasons for

successful or unsuccessful experiments have not been effectively probed. Anunderstanding of the behaviour of water in earth structures allows a bettercomprehension of how the cementing reaction continues within earth bricks.Many studies (for example as outlined in Minke 2007, Houben and Guillaud1994) have shown that the strength of mud brick buildings increases withincreasing cement content up to a critical cement content, beyond which thestrength reduces with increasing cement content. The reasons for this peakcement content have previously been unclear. (Jaquin 2008) argues that thereare two aspects competing for the water within the earth structure. These arethe cement reaction, which requires water to form the cementing products, andthe formation of liquid bridges which are a result of the relative humidity of the

surrounding air. As a result of evaporation of water from the cement stabilisedsample, there is insufficient water to form the cementing products, leavingunreacted cement powder within the bricks, which do not contribute to strength.Any increase in volume of cement within a brick will not lead to an increase instrength because there is insufficient water with which to form a cementingmatrix.

Conclusions

This paper has shown that one of the main reasons for the mechanical behaviourof earthen building materials is the presence of liquid bridges between the soilparticles. The unconfined compressive strength has been investigated and it has

been argued that the mechanical properties are a result of the number andstrength of liquid bridges within a sample. Further investigation (detailed inJaquin, Augarde et al. 2007; Jaquin 2008; Jaquin, Augarde et al. 2008) showsthat other mechanical properties such as the stiffness are also related the porewater pressure. It has been shown that the behaviour of water within an earthenstructure may be better explained by considering the material as a highlyunsaturated soil and that this approach may also be useful in explaining themechanical behaviour of cement stabilised earth bricks.


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FIGURE 15 ATTRACTIVE FORCE BETWEEN SOIL PARTICLES CAUSED BYSURFACE TENSION


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FIGURE 16 UNCONFINED COMPRESSION TESTING RIG


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600

700

-800-700-600-500-400-300-200-1000

Pore water pressure (kPa)

Deviatorstress(kPa)

5.5

7.27.1

8.4

8.6

9.4

10.2

FIGURE 17 DEVIATOR STRESS - PORE WATER PRESSURE RESULTS

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0 200 400 600 800 1000 1200

Deviator stress at failure (kPa)

Watercontent(m3/m3)

FIGURE 18 WATER CONTENT - STRENGTH RELATIONSHIP


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Suction

All pores water filledAir filled pores compressing

Water filled pores effectively rigid

AEV

Acting as a saturatedsample

All pores water filled,

mensici at sample surface

Failure of sample

Increase in pore water pressure

Reduction in pore water

pressure

Sample dilating

Initial suction

FIGURE 19 IDEALISED STRENGTH - WATER PRESSURE BEHAVIOUR

References

Avrami, E. and Guillaud, H. 2008. Terra Literature Review: An Overview ofResearch in Earthen Architecture. Los Angeles, The Getty ConservationInstitute.

Cunningham, M. R., Ridley, A. M., Dineen, K. and Burland, J. B. 2003. Themechanical behaviour of a reconstituted unsaturated silty clay.Geotechnique 53(2): 183-194.

Fisher, R. A. 1926. On the capillary forces in an ideal soil: Correction ofFormulae given by W.B.Haines. Journal of Agricultural Science 16: 492-505.

Horncastle, T. 2006 Rammed earth construction. School of Engineering. Durham.MEng.

Houben, H. and Guillaud, H. 1994. Earthen Architecture: A comprehensive guide.London, UK, Intermediate Technology Development Group.

Jaquin, P. 2008 Analysis of historic rammed earth construction. School ofEngineering. University of Durham. PhD.

Jaquin, P., Augarde, C. and Legrande, L. 2007. Unsaturated characteristics oframmed earth. E-UNSAT. First European Conference on Unsaturated Soils,Durham, UK, In press.

Jaquin, P., Augarde, C., Toll, D. G. and Gallipoli, D. 2008. The strength oframmed earth materials. Geotechnique Under review.


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Loureno, S. D. N., Gallipoli, D., Augarde, C. E., Toll, D. G. and F., E. 2008.Calibrations of a high suction tensiometer. Geotechnique 58(8): 659-668.

Maini, S. 2007. Earthen Architecture and Stabilised Earth Techniques inAuroville, India. International Symposium on Earthen Structures,Bangalore, Interline Publishing.

Minke, G. 2007. Building with earth - 30 years of research and development atthe University of Kassel. International Symposium on Earthen Structures,Bangalore, Interline Publishing.

Tarantino, A. 2007. A possible critical state framework for unsaturatedcompacted soils. Geotechnique 57(4): 385-389.

Thomson, W. T. 1871. On the Equilibrium of Vapour at a Curved Surface of aLiquid. Phil. Mag. 42(282).

Washburn, E. W. 1921. The Dynamics of Capillary Flow. Physical Review 17(3):273.

Wilson, G. W., Barbour, S. L. and Fredlund, D. G. 1995. The prediction ofevaportative fluxes from unsaturated soil surfaces. Unsaturated Soils/Soil

Non Satures, Paris, France, A. A. Balkema, Rotterdam.

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