Soil Block Home Construction - Universal Design Consortium

Soil Block Home Construction - Universal Design Consortium

Soil Block Home Construction
BTEC SustainableBuildings III Conference
SantaFe, New Mexico
OctoberI7-I8,200I
CharlesW. Graham,Ph.D.,AIA, and RichardBurt, Ph.D,MCIOB.I
Abstract
Affordable housingis neededin almostevery country of the world. In the United States,
the needfor affordablehousingis especiallycritical in the Coloniasalongthe TexasMexico border. Among the many altemativesavailablefor low-cost housingproduction
in areaslike the Colonias,compressedsoil block and straw bale consfuction are two
altemativesbeing studiedfor their suitability. A researchproject involving a review of
the literatureon soil block home constuction, comparisonof compressedsoil block
machines,and analysisof proceduresfor constructingcode-complianthousingwas
completed. The most common field testprocedurefor soils, thejar or sedimentationtes!
was studiedto seehow much variancetherewas betweentests. Six samplesof the same
soil were testedten times to measurethe amountofvariance betweenthe samples. The
findings were that fte jar testsgaveacceptableresultsif the analystfollowed standard
practices. This paperpresentsa summaryof the major categoriesof earthhome
constuction, a review of soil block making machines,and concludeswith a summaryof
the findings of the soil testprocedures.
Introduction
Affordable housingis neededin almostevery county of the world. kr the United States,
the needfor affordablehousingis especiallycritical in the Coloniasalongthe TexasMexico border. Residentsin the Coloniasneighborhoodsmake an averagehousehold
incomeof only about$11,000annually(Salinas1988). Studieshavealsoshownthat
Coloniasresidentslike to build their own homesto savemoney. At least60% of the
300,000residentswho live in approximately1,500communitiesalong the Texas-Mexico
border provide sweatequity through their own labor and use scavengedrecycled,and
low-cost materialsto eitherbuild their homesentirely, or add major additionsonto them
to savecosts(Roach 1993;Ward and Macoloo t992).
'
CharlesGraham is an architect and the Mitchell EndowedProfessorof Housing Researchin the
Departmentof Construction Scienceat Texas A&M University. Richard Burt is an Assistant Professorin
the Department of Construction Scienceat Texas A&M University. Before coming to the United States
from England,Dr. Burt was a CharteredBuilding Surveyor. Dr. Grahamcan be reachedat Dept. of
ConstructionScience,College of Architecture, College Station,Texas77843-3137,979-U5-0216(phone),
(email). Dr. Burt can be reachedat Dept. of Construction Science,College
[email protected]
(phone),[email protected]
ofArchitecture, College Station,Texas77843-3137,979-U5-0994
(email).
Among the many altemativesavailablefor low-costhousingproductionin areaslike the
Colonias,compressedsoil block and staw bale constuction are currently being studied
for their suitability. Coloniasresidentsare often from Mexico and otherLatin American
counfriesand so they readily identify with most forms of concreteor masonry
constuctiorl which is popularin thesecounties. Many have alsohad experiencewith
adobebricks and rammedearthconstruction,but they view them asbeing inferior to
manufacturedmasonryunits suchas fired clay bricks and concreteblocks. Few,
however,have actually seencompressedsoil blocks usedin code-complianthome
constuction.
The trend in Texasis for housingmadeof compressedsoil block to be usedfor higher
incomehome owners,but studiesareunderwayto determinethe efficacy of using
compressedsoil blocks in home constuction for low-incomepeople. Demonstation
projects around the world have shownthat this approachis feasible. Preliminary studies
by architects,engineers,anthropologists,sociologies,and public policy expertshave
found that compressedsoil blocks for the constuction of walls are affordable,the
materialsarereadily available,and when homesaredesignedproperly,residentswill
acceptthem. What is needed,however,are demonstation projectsin the Coloniasalong
the Texas-Mexico border to show the residentsthere how to build atftactive,codecomplianthousingtltat will allow the homeownersto contibute their time and resources
in the selflassistedhousingconstuction approachesthat they are familiar wittr. This
paperreviews the various methodsof earthenconstuction, discussescompressedsoil
block constuction techniques,and reportsthe findingsof a study in which a simple field
test for the compositionof the soil from which the blocla aremadewas evaluatedfor its
accuracy.
Earth Construction Techniques
Houbain (1994) identified threeways in which unbakedearthis usedas a building
material:
o Unbakedearttrin monolithic load-bearingform;
o Unbakedearthin the form of load-bearingmasonry;and,
o Unbakedearthin conjunctionwith a load-bearingstucture.
Cob walling, suchas usedin England,is an exampleofunbaked earthusedin monolithic
load-bearingform. Figure 1 showsexamplesof cob wall constuction in England.
Another monolithic load-bearingform of earthconstuction is rammedearth. This is one
of the oldest methods,dating back to the medievalages. Figure 2 showsrammedearttr
constructionin Arizona. Figure 3 showshow unbakedearthis formed to createa
monolithicload-bearinewa[.
Figure1. CobWallingConstruction
in
England
Figure2. RammedEarthConsffuction
Aiznm
@hotocourtesyof RammedEarttr
Development
Inc. at
http//www.rammedearth.
com/)
Figure 3. Building a Cob Wall
Unbaked earthmay also be formedinto masonryurie suchasbricks or blocks using
varioustechniques.A taditional methodusedin the SouthwestUnited Statesis adobe.
Adobe bricks are often hand formed in molds and dried in the sun. They areplentifi.rl,
they are inexpensive,ard when usedproperly, they can yield good resultsfor home
consftuctionbecauseof the durabilrtyin serviceand flexibility in supportof different
home designs. Figures4 and5 show examplesof adobeconsffuction.
Figure 4. Adobe Chwch in New Mexico
Figure5. AdobeHouseinFort Davis,
Texas
Anotherapproach
to houseconstuctionusingearthenmaterialsis in conjunctionwith a
supportstucture.Wattleanddaubis anexampleof ttrisform. A slightlyhigherlevelof
sophistication
to theconstuctionprocessis requiredbecause
theearthenmaterialsmust
be incorporated
with carefullyplacedwoodenmembersto createthewallsfor stuctures.
Oneof thebenefitsof thistypeof constuction,however,is thatthinnerwallsare
possible,therebytakingup lessfloor space.Figures6 and7showexamplesof wattleand
daubconstuction.
Figure6. WattleandDaubWall
Construction
Figure7. WattleandDaubWall
Construction
Compressed
Soil Block Construction
In contemporary
timesmachines
havemadeit possibleto producehigherquatlfybricks
or blocksusingsoil asthebasicingredient.Sundried,uncompressed
adobebrickscanbe
improvedgeatly by,compressing
thesoilto higherdensities.In manycases,compressed
soilblockscomeout of themachinereadyto lay in thewallsin their"green"condition,
withoutadditionaldryingor baking.Thisis because
thesoil is compressed
to veryhigh
densities.Further,themachinesusedto compress
thesoilblocksarecapableof making
manybricksin a shortperiodof timethatareruriformin density,shapeandoverall
dimensions.Machinesin usetodayincludebottrmanuallyoperafed
andmechanically
methodsto compress
thesoil intobricks. Oneof themajorlimitationsof the
operated
is ttrattheyareslowandoneis limitedin howmuchforcecanbe
nranualmachines
apphedto thebricks. Figures8 and9 showexamples
of two manuallyoperated
machines.
Figure8. CinvaRamManualMachine
Figure9. AuramPressManuallvlachine
Addinga hydraulicramto compress
thesoil andautomated
conveyorsto deliverbricks
from themachineto thework areaprovidesa highlevelof productioncapacityand
quahtyto theprocess.As manyas300-320bricla per hourcanbe producedfrom these
machines.Compressed
soilbricksmayhavecompression
stengthsof 1,200-1,400
p.s.i.,suitablefor load-bearing
constuctionundertheright conditions.As noted
previously,bricksfromttresemachines
areconsistent
in stengthanddimensiorq
aslong
procedures
arefollowedfor qualitycontol (e.g.soil mixeshaveto havethe
asstandard
correctamountof clayandsan{ moisturehasto beverycloseto beingthesamein all
unitsproduce4andhandlingandplacement
techniques
haveto bllow accepted
procedures).TheAdvancedEarthenConstuctionTechnologies
(AECT)machines,
producedin SanAntonio,Texas,aregoodexamples
of qualitymechanically
operated
machines.Thesemachinesareavailablein threedifferentsizes:theImpact2001Series,
the3000Series,andthe4000Series.
Figurel0 showstheImpact2001Seriesmachine.It is a small,tailer-mountedmachine
thatcomeswith eithera 6.5h.p.gasolineor 7.0h.p.dieselengine,andeithermanually
operated
mold. Thismachinecanproduce230- 300
operatedmold or automatically
blocksperhourin a varietyof dimensions.For example
, 2 ll2 " - 4" (5.0cm - 4.5 cm)
thiclg5.5" (14 cm) wide,and12" (30.5cm) longarecommon.Eachblockweights
between9-18pounds(4.1Kg to 8.1Kg) depending
onthesoilandblockthickness.
Blocksarebondedtogetherusingthewet thin soil slurryor otherconventional
methods.
Thesoil slurryis madewith waterandscreened
soil.Blockscanalsobeplacedin the
wall usingthetaditionalthickmudmortarmethod.
TheImpact2001Seriesmachineusesa widevarietyof soilswith prepared
naturalsoil
in therangeof 4-12percentTypically,themachinerequiressoilwith a
moistures
combinedclay (15-20percent)andsilt (powder)contentof approximately
25-40percent
(by volnrne),anda sharpsandcontentof approximately
40-70percent(by volume).The
machinedoesnot requireanyaggegateGock) to makea strongsoil blockfor most
Theblock compressive
applications.
stengthsrangefrom 600p.s.i.(42Kglcrr2) to 1,200
p.s.i.(70 Kglcrrf) dependingon thesoil.A forceof approximately
72,000lbs.is usedto
produceblockswith 1,091p.s.i.compressive
stengthon 5.5in. x 12in. x 2.0-4.5in.
placedon theblockduringblockproduction
at lesspressure
block This machineoperates
andthrs it canwork acrossa wettersoilrangethanthelargerAECT machines.
Thenexthigherproductioncapacityis providedby the3000Seriesmachine.It hasa
dieselengneanda largeenoughhopperto holdsoil for dozensof blocksto beproduced
at a time. Thismachineis capableof producing300blocksperhourandis suitablefor
themediumapacity contactor.An exampleof thismachineis shownin Figure11.
Figure12givesanexampleofthe largestmachine
availablefrom AECT,the5000Series.
Thismachinehasa four cylinderdieselengneandanevenlargerhopperfor soil storage.
It utilizesa tumtablettnt hasfourmoldsin it Eachtimethemachinemakesa
compressed
soil block,thetumtablerotates90". In thefirst stagethe soilsaredropped
into themold"in thesecondstagethesoil is compressdin thethird stagethebrick is
raisedup outof themol4 andin thefourthstagethebricksexit ontotheconveyor.
Bricls comeout of themachineat therateof approximately
800bricksperhorn. Up to
230,000lbs.of force/pressure
is appliedto thesoilto producebricksof 1,643p.s.i.
compressive
stengthon 10'kl4" block Themanufacturer
claimsthatit takessix or
sevenworkersto keepup with themachinewhenremovingbricksandstackingthemnear
the work areas.
Figrre 10. AECT Impact2001Compressed
Soil
BlockMachine
@hotocourtesyof AECT at
h@://www.webspace4me.net/-fwehman)
Figure11. AECT3000Series
Compressed
Soil Block Machine
(Photocourtesyof AECT at
h@://www.webspace4me.neV-fwehman)
Figure11. AECT 5000SeriesCompressed
Soil
BlockMachine
(Photocourtesyof AECT at
htp //www.webspace4me.net/-fwehman)
Oualitv Control
productioncapacityof themachines,
To achievemrudmum
whethermanuallyor
mechanically
operated,
theusermustfollow certainprocedwesthathavebeendeveloped
fromhistoricalexperience
andernpiricalanalysis.Essentially,
therearetwo basicareas
of qualitycontrol: suitabilityof soil,andsuihbilityofmasonryunis.
Suiubilityof Soil
A reviewof theliteraturefoundthattherearethreecharacteristics
thatgreatlyaffect
suitabilityof soil. Theseincludethecomposition
ofthe soil,themoisturecontentof the
sorl,andtheplasticrtyof thesoil. An idealsoilwouldbecomposed
of soilwith a
(15-20
(powder)
percent)andsilt
combinedclay
contentof approximately
25-40percent
(by volume),anda sharpsandcontentof approximately
40-70percent(by volume).The
(rocla)to makea stong soilblockfor
mechanical
machinesdo not requireanyaggregatn
t/i'
however,fine aggregates
mostapplications,
up to diarreterandnot morethan5-10
percentof thevolumearesometimes
allowed.
Soilmoisturecontentcanftmgefrom4-l2o/oby weight depending
uponthesoilmix (e.g.
sandandclaypercentages).
As notedpreviously,theImpact2001Seriesmachineby
AECT c:n useslightlywettersoilsthanthelargermachines.Theplasticityof thesoilis
pnrnarilya fi.nctionof theclay. Thehighertheplasticityindexof thesoilthegreaterits
shrinkandswellcharacteristics
at differentmoisturecontents.Moremoisturecauses
the
theclayto shrink Claywith plasticity
clayto expandovertime,anddryingcauses
indexesof up to 25 or 30wouldbe acceptable
for mostapplications.Theplasticityindex
of themixedsoil,includingclay,silt,andsand/gravel,
shouldnot exceed12-15(the
differencebetweentheUpperandLowerAtterburgLimits,asdetermined
by laboratory
goal
testing).Theconstuctor's is to useminimummoisturein a mixtureof clay,silt and
thathasa plasticityindexshownthroughhistoricaluseof thesoilsto produce
sand/gravel
blocksthatcanbe laidin thewallswithoutunduedryrngtimes. Excellentreferences
for
theseprocedures
maybe foundin McHenry(1984),Easton(1996),andMinke (2000).
Suitabilityof MasonryUnits
Oncea suitablesoilmixturedesignandoptimalmoisturecontentaredefined,blocksmay
be produced.However,theirsuitabilityfor consftrctionmustbe examined
carefully,
procedures
onceagainfollowingestablished
for analysis.Thebuildingcodes(e.g.see
theNew MexicoAdobeandRammedEarthBuildingCode,at
http://www.earttrbuildins.com/nm-adobe-code.htnl)
requirethatthemodulusof rupture,
compressive
stength,andabsorption
rateof thebrickcomplywith at leastminimum
standards.Thesewill not be discussed
in greatdetailhere,but anotherexcellentexample
is Boulder,Colorado'sAltemativeBuildingMaterialsCode,
ofthe coderequirements
whichcanbefoundat http://www.azstamet.com/-dcat/Boulder.htn
Chapter97,
EarthenMasonryUnits,givestherequirements
for code-compliant
earttrenconstuction.
Test
Jaror Sedimentation
reviewof theliteratureconducted
An extensive
by ttreauthorsfoundthatoneof themost
for soilmix designis thejar or sedimentation
commonqualitycontol procedures
test
(sometimes
alsoreferredto astheshakerjar test). Thisis oneof themostcommontests
foundin theliteranreon earthbuilding. Thistestmeasures
theproportions
of clay,silt,
andsand/gravel.
Thejar testconsistsof the followingsteps:
jar up to 1/3of ib volumewith dry soil;
Fillinga quartcanning
Addingcleanwaterup thesecond-third
of thejar's height;
Addinga pinchof saltto thewater;
Mixing thesoil,waterandsaltwith a paddleor otherdevice;
Wittr ttrelid onttrejar, shakingthejar vigorouslyuntil thesoilparticlesarein
suspension;
6. Lettingthejar sit for onehour;
7. Again,with fte lid onthejar, shakingit vigorously,andallowingit to sit for
oneminute;
8. After oneminute,madcingtheheightofthe finegravelandsand"whichwill
readilysettleto thebottomofthejar, asTl;
1.
2.
3.
4.
5.
9. After 30 minutes,adda secondmaft to thepointwherethefine gravel,sand
andsilt havesettledout of thewater,asT2;
10.After another24 hours,addinga markat thehighestlevelof thefinegravel,
sand,silt, andclay,just wherethewaterandsoil contentshaveseparated
visually,asT3; and"
I l. Calculating
ttrepercentages
of theingredienbof thesoilby followingttre
equations
whereT1 : depthof sandT3-T2: depthof clay,T2-Tl: depthof
silt andwhereeachdepthis dividedby T3 andthenmultipliedby 100.
Reliabilityof JarTests
Statistical
An analysisof ttrestatistical
reliabilityof drejar testfoundthatit is reliableif oertain
procedures
arefollowed. To studythereliabilityof thejar test,six samplesof soilswere
tested10timesin a laboratorysetting.Thegoalwasto comparetheresultsof the
measurernents
recordedwith eachotherto seehowmuchvariancetherewasbetweenthe
readings.Thesamplereadingswererecordedindependentb
by two graduatestudents
wittr civil engineering
undergraduate
degrees
in theDepartrnent
of Construction
Science
at TexasA&M Universityoverthe swnmerof 2001. Figure12showsttrejar afterthe
soilhasfinallvsettled
Figure12. Jaror Sedimentation
TestPerformedat Texas
A&M Universiw
-lar 'l
sitf
Sand
Jar 2
Clav
Sand
sitr
Clav
Mean
44.0
24.8
31.1Mean
40.9
17.9
41.2
StDev
3 6
270
//-2n.
399
443
386
26
195
163
214
1?9
422
403
429
Vin
3 StDev
33 1 qst tel
29 1 q s t e l
3 4 4 Max
264 Min
1 7
41 q
Mav
1 6
450
430
47r|
5 7
R 6
37
160
Clav
344
q 5 lt c l
cst cl
Ranoe
Jar t
5 0
Sand
Mean
456
StDev
95tJCl
951Cl
1 :
465
448
Mar
47R
Min
439
3 9
Ranoe
Jar 5
Mean
SlDev
95UCl
95LCl
Max
Min
Rannc
Sand
431
2n
443
418
47 1
408
6 3
??6
30n
190
110
sitf
229
5
261
197
303
143
160
sih
249
3 F
272
226
333
21A
11q.
8r
Clav
Ranne
-lar A
silr
Sand
1 5
3ql
3 1 q [\.,lcan
495
4 F : StDev
348 95tlcl
2 9 0 qst cl
51 4
179
?52
477
142
336
3 R 6 [\rlav
24
Min
3.C
543
451
-lar 6
3 2 0 Mean
? 3 StDev
3 4 1 95UCl
3 0 0 9 5 1C l
35'2 Mav
) 4 6 Min
1 n 6 Ranoe
1.2
2 6
21 1
1 1 7
9A
sitf
25
3 3
47.5
27.2
30.7
443
5,n,7
231
275
42A
323
224
8 :
9 9
31 8
246
7 t
1 4 3 Ranoe
Clav
3.0
I
Sand
45 9
364
324
A O
Clav
291
? F
Table 1. StatisticalResultsof Jar or SedimentationTest
Table 1 showsthe resultsof the testson the six jan. The resultsshow that thereis some
variation betweenthe resultsrecordedfor eachjar. The silt result for Jar 3 is of particular
interestas the minimum value was I4.3% andthe maximum value was 30.30 , a ftmgeof
16%. This would appearto indicate a largeamountof variation for ttris test. However,
the rangebetweenthe upper and lower 95% confidenceintervalsis only 6.40/o,suggesting
that ttris testhasgreaterprecisionif more testsare carriedout. This experimentonly
evaluatedthe variancewithin eachsampleand did not t€stthe soil compositionagainst
other methodsof testing. The next phasein this researchis to comparethe accuracyof
thejar testwith a laboratorymethodfor soil analysis,suchasparticle size analysisby
sedimentation/sieving.
The New Mexico Adobe and RammedEarth Building Code statesthat "Each of the tests
prescribedin the code shall be appliedto sampleunits selectedat random at a ratio of 5
units/25,000bricks to be usedor at the discretionof the building official"
(http://www.earthbuildine.corn/nm-adobe-code.htnl).
This test is to establishthe
jar
produced.
suitability of the blocks
The testwill help to make surethat a good soil
mixf,re, with a high probability of meetingthe finish block requirements,canbe
produced. The experimentwith thejar test indicatesthat the more samplesone tests,the
closerone will get tci finding an accurat€representationof the soil being used. The
authorswould recommendthat at least5 randomsoil samplesfrom variouslocationsin
10
the mixed batch of soil, as it would go into the hopperof the machine,shouldbe tested.
More sampleswould be better,but 5 shouldbe the minimum. Following the code
requirementto test brick samplesat a ratio of 5 uni*/25,000 bricks, one would then
samplethe soil at a minimum of 5 locationsin the samequantityof soil necessaryto
produce25,000bricks. Again, theseareminimum requirementsfor soils analysis. Other
testssuchas plasticity, sieveanalysis,drop test moisturecontentand slump needto be
incorporatedinto the processof determiningthe suitability of the soil for making codecompliantbricks.
Recommendations for Future Research.
The quality conffol of soil block manufacnreis a fruitfrrl areafor research.Researchat
TexasA&M University will be focusedinitially on the urccuftrcy
andreliability of field
testsfor the compositionof the soil usedin the manufactureof the soil blocks. Future
researchwill also focus on the soil block units themselves.The New Mexico Adobe and
RammedEarth Building Code requiresan averagecompressivestength of 300 pounds
per sqwre inch and a modulus of rupture of 50 poundsper squareinch for compressed
soil block units. Researchwill be conductedto identiff the compositionof soils in Texas
that will produceblocks to meettheserequirements.The researchwill alsoinvestigateif
the compressivestength or modulus of ruphre of the compressedsoil blocks can be
predictedby a nurnberof independentvariablessuchas the compositionof the soil.
References
Easton,David. 1996.The RammedEarttrHouse. White River Junction,Vermont Chelsea
GreenPublishing.
Houben,Hugo. (1994)Earthen Architecture and Modernity. Proceedingsof the 'Out of
Earth'Conference 1994,Cente for EarthenArchitecture,University of Plymouth,
England.
Roach,Katherine. 1993. "Investigation of ColoniasResidents'Potentialfor Self-Help Housing
Consfuction" (unpublishedthesis). Deparffnentof ConstructionScience,TexasA&M
University.
McHenry, Paul Graham. 1984. Adobe and RammedEarlh Buildines. New York JohnWiley &
Sons.
Minke, Gemot. 2000. Earth ConstructionHandbook.Southampton,U.K.: WIT Press.
Salinas,E. (1988). The ColoniasFactbook A survevof livine conditionsin rural areasof South
Texas and West TexasBorder Counties. Surveyforthe TexasDepartrnentof Human
Services.Austin, Texas:Office of Stategic ManagementResearch,and Development.June.
Ward, Peter;and Macoloo, G. Chris. 1992."Articulation Theory and Self-Help Housing
Practicesin the 1990s." International Journal of Urban and Regional Research(March),
vol.16,no.1.
11