paper: jaquin
Rowen Travel Award winning entry 2006Presented on 8 November
2007 at the IStructE, 11 Upper Belgrave Street, London SW1X 8BH at
18:00h.
Study of historic rammed earth structures in Spain and
IndiaSynopsisThe Rowen Travel Award was granted for the study of
historic rammed earth structures in Spain and India. Eight
locations in northern Spain and three in northern India were
visited during October and November 2006. The object of the visits
was to gain a greater understanding of historic rammed earth.
Methods of construction, modes of failure and repair techniques
were investigated. Use of rammed earth as a modern building
material is increasing, and the study of historic structures can
inform development of the technique today. A number of examples
which are considered to be of interest to practicing engineers are
presented. The examples deal with the presence of water in earthen
structures, cracking and methods of crack repair, the facing of
rammed earth with a less permeable material, and medieval seismic
protection measures. This work forms part of a PhD looking into the
analysis and conservation of historic rammed earth structures, with
field visits being a major aspect of the study. Rammed earth is an
ancient construction technique, developed independently in parts of
China, the Middle East and north Africa. Soil is taken from the
ground and compacted between vertical formwork boards, which are
then removed leaving a mass soil wall. The technique is widespread
in regions where the soil is not sufficient to make sun dried clay
bricks, or where lack of timber makes its use for building
uneconomic. The desert section of the Great Wall of China and parts
of the Potala Palace in Lhasa are made from rammed earth. In north
Africa and Spain, Berber Muslims used rammed earth to build
fortification during the Islamic Caliphate. Rammed earth continued
to be used in Spain under Christian rule, and was exported to the
New World in the early 16th century. In Europe rammed earth was
used as a vernacular construction technique in the late Middle Ages
and continues to be used in north Africa today. The recyclable
nature and low transport costs associated with using soil dug in
situ, means that rammed earth has found a new niche as a
sustainable construction material. Improved thermal performance and
inherent relative humidity control mean that rammed earth
structures have a pleasant internal environment, and a number of
projects in the UK are testament to its growing popularity. At the
Eden Project, St Austell, Cornwall, rammed earth is used as the
primary element in the visitor centre. Pines Calyx, a conference
and training centre in Kent, is constructed wholly from rammed
earth. A new lecture theatre under construction at the Centre for
Alternative Technology in Machynlleth, Wales, boasts the tallest
rammed earth walls in the UK. In North America, the Desert Living
Centre, outside Las Vegas, has been constructed from rammed earth
and aims to provide Nevada residents with information on
sustainable living. Rammed earth has become a popular construction
technique in New Mexico and Arizona, and is gaining popularity on
the western seaboard as far north as Vancouver. In Australia and
New Zealand rammed earth has become an accepted alternative
construction technique, pioneered by those looking for a
sustainable building, but now becoming mainstream. Current
construction guidelines for earthen architecture and rammed earth
in particular are extremely simple, being mainly based on masonry
design. Understanding of the behaviour of earth structures is
extremely limited when compared to that of steel or concrete. One
third of the worlds population reside in earthen buildings, the
majority being small, non-engineered structures. This lack of
understanding means that descriptions of the behaviour of the
material are very limited. The growth in popularity of earth
building in the developed world should act as an incentive to
improve understanding of earth as a building material such that
structures can be better designed in the developing world. Work
undertaken at Durham University looked at rammed earth as a soil,
applying geotechnical engineering principles to a structural
material. Rammed earth may be treated as a highly unsaturated soil,
an area of soil mechanics which is highly active and not yet fully
understood. We have shown that the additional strength present in
earthen structures is due to the phenomenon of suction, and that
close to sample saturation, the strength is proportional to the
suction. Suction is observed in any non-saturated soil sample.
Where air and water coexist in soil pores, a difference in pressure
between them causes the interface of the air and water to curve,
commonly seen as a meniscus. The interface acts as a sheet in
bi-axial tension, curving to accommodate the pressure difference,
known as surface tension. The combined action of the surface
tension and the pressure difference add together to provide an
attractive force across the pore, and it is this attractive force
which provides strength in addition to interlock in soils. The
effect can most easily be seen in the example of a sandcastle.
Where dry grains are used, the mixture will only rest at the
frictional angle of repose of the grains, and when a saturated
sample is made, the same angle of repose is observed. However if a
small amount of water is used, an impressive sandcastle can be
constructed. There are a limited number of modern rammed earth
buildings which are available for study, and only a small body of
work being undertaken in laboratory testing of rammed earth as a
structural material. Studying historic sites, in many different
states of repair offers an insight into historic construction
techniques, any specific failure mechanisms which may occur, and
repair methods which have been implemented. This both allows
informed discussion on the preservation of historic sites and may
provide direction for modern rammed earth construction. Field
visits were undertaken to observe historic rammed earth structures
in Spain and in India, looking specifically at construction
techniques, failure mechanism and repair methods. Spain was chosen
for its high density of historic rammed earth buildings and
excellent transport links. A further study of historic rammed earth
in northern India was also undertaken whilst attending a conference
in the region. Around 60 sites were visited in Spain, at 24
locations, which are shown in Fig 1. Five sites were visited in
northern India, at three locations shown in Fig 2. Visits to sites
in southern Spain were undertaken in January 2006 and to northern
Spain and India in October and November 2006. The Rowen Travel
Award was used to fund the travel in October and November 2006.
Paul JaquinMEngUniversity of Durham
Keywords: Earth structures, Historic structures, Spain, India,
Cracking, Repairing, Earthquakes Paul Jaquin
26|The Structural Engineer 22 January 2008
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2
Previously work had been undertaken to classify historic rammed
earth construction techniques, and to understand the historic and
geographic distribution of rammed earth (Jaquin et al. 2007). This
allowed the rammed earth sites visited to be placed into a wider
context, and will in the future allow comparison between historic
rammed earth constructions. The classification enabled comparisons
to be drawn between early Muslim rammed earth in Spain, Morocco and
India, and between rammed earth found in the New World and that of
16th century Spain and Portugal. Recorded failures in other types
of earthen structure were studied to allow comparisons between
rammed earth and other types of earthen architecture. Considering
failure mechanisms of historic structures means research can be
best directed into manners of rectifying these problems and
preventing their occurrence in new constructions. Unfortunately in
studying historical structures, we are looking at those which have
either survived, or have failed to an extent which is practicable
neither for repair nor complete destruction, and thus a great
number of rammed earth buildings may have existed which have not
lasted until the present day. When observing historic structures,
the reasons for failure must be ascertained and it should be
remembered that buildings abandoned following one failure may have
suffered subsequent failures, or their abandonment may not be
linked to structural failure. A number of issues found during the
visits are presented, these are considered to be of most relevance
to practising structural engineers and those with an interest in
earthen architecture. The majority of sites discussed are those
visited during travel funded by the Rowen Travel Award, and some
reference is made to sites visited during a previous field trip.
The issue of water in earthen structures is explored and it is
shown that water is perhaps not as big an issue for earthen
structures as would be imagined. However, where excess water is
found, major problems can occur. An example of major cracking of a
rammed earth structure is given, and likely causes are discussed.
Crack stitching observed in India is then presented. The facing of
rammed earth with a less permeable material is then explored. The
practice of facing rammed earth with masonry to prevent artillery
damage is shown at one site, along with the damage this has caused.
The repair of weathered rammed earth using concrete is then shown,
and parallels are drawn between the two practices. Finally medieval
seismic protection measures are shown, indicating that medieval
engineers may have had an excellent grasp of seismic structural
behaviour.
Table 1: Sites mentioned in text and Fig 1 and Fig 2Number1 2 3
4 5 6 7 8 9
SiteAlcala de Guadaira, Andalucia, Spain Ambel, Aragon, Spain
Banos de la Encina, Andalucia Talamantes, Aragon, Spain Elche,
Valencia, Spain Carmona, Andalucia, Spain Tabernas, Andalucia,
Spain Basgo, Jammu and Kashmir, India Leh, Jammu and Kashmir,
India
WaterThe small amount of water present in the rammed earth
provides extra strength to the material through suction as
explained earlier. However, excess water causes the rammed earth to
become saturated, losing first strength then integrity, eventually
leading to complete destruction of a building. Rammed earth is
generally found in dry climates, around the Mediterranean rim,
through central Asia and in parts of China and Himalayan regions.
While not accepting of excessive humidity, rammed earth is able to
tolerate heavy rainfall events. Fig 3 shows a rammed earth
retain-
ing wall during a severe rainstorm, and demonstrates that rammed
earth is able to withstand the flow of water over its surface. In
the Himalayas, rammed earth buildings are often snow bound during
the winter months. Whilst rammed earth may not survive in a more
humid climate, short duration, high intensity rainfall is
acceptable, as is frozen snow resting against walls during the
winter months in the Himalayas. When water penetrates a structure
and does not evaporate sufficiently thus is able to persist within
the structure, the water content of the rammed earth increases, and
initially the surface structure is lost. Evidence of this may be
seen as a dried slurry trail (Fig 4), which was observed at many
sites. This process was often observed where a structure was not
maintained, and lack of maintenance meant that water was able to
enter the structure. Where a roof was open, rainwater was
channelled onto the top of a wall. While this is not immediately
problematic, a slurry does indicate the movement of material
downwards, and over time will lead to complete erosion of the
structure. Where water is allowed to flow through a rammed earth
wall, the free surface of the wall is seen to degrade. The castle
at Banos de la Encina (Fig 5 and Fig 6) was constructed in 967AD
using a lime rich rammed earth mix. It was used as a Muslim
fortification until surrendered to the Christians in 1225 (Ramos
Vazquez 2003), and continued to be used until 1626. The structure
lay abandoned and was used as the village graveyard from the mid
19th century until 1928. As a result the internal ground level is
over 5m higher than the external. The walls are founded on bedrock
and all the material inside the walls has been transported there
from the surrounding area. The building appears structurally sound
and whilst large cracks are evident, these do not appear to be
growing and there are no reports of structural movement. However,
the imposition of 5m of material to the inside of the walls has
caused exfoliation of the external surface of the walls. It is
Fig 1. Sites visited in Spain, January and October 2006, numbers
refer to sites mentioned in text / Fig 2. Sites visited in India,
November 2006, numbers refer to sites mentioned in text 22 January
2008 The Structural Engineer|27
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4
5
7
6
assumed that the cause of the exfoliation is the movement of
water through the wall, and subsequent flow or evaporation from the
external surface. A mechanism by which this might occur is the
dissolution of salts contained within the wall, which then travel
through the wall to the external surface. On reaching the external
surface evaporation of the water takes place, allowing the salt to
precipitate. This precipitate is formed in pores in the surface,
expanding and cracking them, causing removal of the fine grained
surface, which eventually falls off (Hughes 1983, Walls 2003). The
covering of the base of the structure in a skin of less permeable
cement is therefore not recommended, as this leads to the build up
of water behind the impermeable
membrane and movement, rather than solution of the problem. The
dangers of facing a rammed earth wall in a less permeable material
are amply shown when considering the facing of rammed earth with
masonry which occurred following the advent of artillery in the
15th century. While facing with a less permeable material is not
necessarily detrimental, if build up of water is allowed, then
exfoliation or loss of structural integrity due to increased water
content may be expected, Fig 7. The cities of Jaen in Spain and
Xian in China attest to the use of masonry to protect rammed earth
walls from the threat posed by artillery. It is distinctly possible
that there are rammed earth walls hidden behind masonry in many
Fig 3. Rammed earth retaining wall during a severe storm. Alcala
de Guadaira, Spain1 / Fig 4. Slurry formation, Ambel, Spain2 / Fig
5. Wall section, Castle of Banos de la Encina, Spain3 / Fig 6.
Raised ground surface inside Banos de la Encina castle3 / Fig 7.
Wall section diagram, Banos de la Encina, Spain 28|The Structural
Engineer 22 January 2008
paper: jaquin
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parts of the world. A hilltop site at Talamantes (Fig 8 and Fig
9) shows the erosion potential where less permeable masonry is
placed against rammed earth. Here the masonry appears to have
directed moisture towards the rammed earth, causing an increase in
moisture content of the rammed earth. At this higher moisture
content, the suction and strength lessen until the wall is unable
to maintain a vertical slope and failure occurs. The same
phenomenon is observed when previously unsaturated embankments
become saturated during rainfall events, leading to slope failure.
Talamantes provides an excellent example of the dangers of facing
an earth wall with a less permeable skin. The repairing a weathered
rammed earth face with concrete, as was observed in the town of
Elche and at many sites in southern Spain (Fig 10) may lead to the
complete destruction of the original rammed earth structure.
CrackingThe tensile strength of soil is low when compared to
concrete, and the brittle nature of highly unsaturated soil means
cracking in earth structures is commonly observed. A medieval
preceptory of the military orders at Ambel, in north-eastern Spain
has been extensively studied by Dr Chris Gerrard of Durham
University and it has undergone extensive growth and rebuilding
during its lifetime. The nucleus of the building is a 10th century
tower, to which barns and living quarters were added in the 12th
century. Before 1556 large granaries were constructed to store the
produce of the surrounding area. These granaries have suffered a
number of structural failures since construction, indicated by the
reconstruction of internal columns and roof trusses.
Records indicate that masons were employed in 1797 to remove the
top storey of one of the granaries. Crack growth monitoring of this
structure had led to the conclusion that the gable end is moving
away from the rest of the structure at a rate of 0.6mm/year close
to roof level (Fig 11 and Fig 12). The Rowen Travel Award has
allowed the author to visit and extensively survey the building.
Surveys showed that the cracks closest to the gable end are full
wall thickness and that those in the centre of the side wall have
been filled and are not currently moving. Fig 11 shows that the
lower brickwork is leaning outward with respect to the vertical
wall of the white barn in the foreground. It is assumed that the
original granary suffered a major structural failure at some point
in the Middle Ages, likely caused by overloading of the floors with
grain. This failure caused the outward movement of the gable end,
and necessitated the construction of brickwork buttressing on the
lower section and complete reconstruction of the top section. In
the 19th century the top section of wall was known to be moving and
further timber reinforcement was added internally, tying the gable
end to the perpendicular wall. In 1998, alarmed by the crack
monitoring data, the buildings owners added internal steel beams
ties, bolted internally to timber roof trusses, and fixed
externally with H shaped plates butted against the surface. To date
the rate of crack growth has not slowed, indicating either that the
wall has yet to engage the ties or that the tying action has not
been successful. This historic site appears to be in great need of
structural monitoring and repair, but it is not known if invasive
techniques recommended for historic masonry structures may not be
suitable for use in earthen structures.
10
11
12
Fig 8. Masonry faced rammed earth, Talamantes, Spain4 / Fig 9.
Diagram of Talamantes erosion / Fig 10. Facing of weathered rammed
earth in concrete. Elche, Spain5/ Fig 11. Cracked and leaning wall,
Ambel, Spain2 / Fig 12. Diagram of cracked wall, Ambel, Spain 22
January 2008 The Structural Engineer|29
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15
14
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Crack stitchingThe stitching of cracks in earth buildings must
be undertaken with the greatest of care, the brittle nature of
rammed earth means that any stitching must not induce excess
stresses within the structure which would lead to the development
of further cracking. Therefore soft stitching is practiced, where
similar sympathetic materials are used across the crack. In
northern India, the town of Basgo was the capital of Upper Ladakh
and the palace here was likely constructed before 1445. A major
structure on the site showed significant cracking, two large cracks
are immediately evident, one extending almost the full height of
the structure, which appears to have been enlarged by the action of
water flow or freeze thaw weathering, Figs 13, 14. Another crack
extends at 45 from an opening close to the bottom corner of the
structure. In addition, a line of smaller cracks can be observed
running almost parallel to the large vertical crack and are shown
in Fig 15. The presence of the two smaller cracks and the change in
direction of the larger crack indicate that the cracking is caused
by structural movement and not solely through the action of water.
The cracking pattern suggests movement of the base of the wall down
the slope, and opening of the large crack has caused the left side
of the structure to continue its downhill movement, its tie into
the rest of the structure reduced as the length of the crack
increases. Remedial work was carried out in 2004 by John Hurd,
working on behalf on the World Monuments Fund. This placed a
buttress at the base of the slope and introduced soft ties across
the crack to create structural continuity. The method of crack
stitching is outlined by Hurd 2006 and involves the construction of
a mud brick staple across the crack, to half thickness of the wall.
Two staples were cut, the surface material then wetted and hemp
matting placed inside the cut. Mortared sun dried bricks were then
placed within the cut forming a solid staple within the wall. The
Rowen Travel Award allowed the author to visit to the site and
almost 2 years after construction there were few visible signs of
degradation. However, stapling of the crack has occurred only at
the base and the lack of a cap to the
crack means that erosion is still possible, which may in the
long term render the staple ineffective. However the soft stitch
technique, placing a staple of similar material to the original is
likely to provide a similar stiffness stitch which will work well
in the repair of large cracks in earthen structures.
Seismic protection measuresRammed earth is constructed in
horizontal layers, construction progressing horizontally until one
layer is complete. The formwork is then moved vertically and
compaction of the next layer begun. Each layer if known as a lift,
and at many historic rammed earth sites material is placed between
the lift. Lime, straw, stones and brick have all been found placed
between the lifts, and the material is known in Spanish as a male.
Males are not unique to rammed earth, Langenbach 2004 notes their
use in adobe (sun dried clay bricks) walls in Bam, and Hurd 2006
observes males in Turkey, Armenia and Iran. The purpose of this
layer appears to be to prevent building collapse during seismic
movement. Langenbach 2004 argues that the purpose of the layer is
to prevent diagonal shear cracking in homogeneous walls, provide
cracks with a weaker horizontal layer through which to propagate,
or to stop vertical crack growth which would lead to collapse. Hurd
2006 presents the layers acting as ring beams through the body of
the structure tying each layer together, as recommended for modern
seismic resistant design, Two examples of seismically damaged
buildings are presented, one in the Indian Himalaya, the other in
Southern Spain. The Himalayan town of Leh is the capital of Ladakh
and a growing tourist destination. The region is highly seismically
active, as demonstrated by the Kashmir earthquake of October 2005.
A large castle and monastery dominate the town, and watchtowers dot
the horizons. The remains of one such watchtower is shown in Fig
16. The building appears to have suffered major earthquake damage,
but some sections of wall have remained standing. Although a
vertical crack extends through the full height of the structure, a
second diagonal crack can be seen to have been forced to run
horizontally at
Fig 13. Repair of cracked section, Basgo, Ladakh. (Photo: Hurd
2006) / Fig 14. Repair to rammed earth, Basgo, Ladakh / Fig 15.
Diagram of cracks and repairs, Basgo, India / Fig 16. Earthquake
damaged structure, Leh, Ladakh 30|The Structural Engineer 22
January 2008
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19
20
18
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the top of the structure, and has been stopped at mid height
upon reaching a layer of stones Fig 17. A similar situation was
found at the castle of Carmona in southern Spain. A new castle was
built by Christian rulers in rammed earth in 1502, but the whole
site abandoned following and earthquake in 1504. Fig 18 and Fig 19
show vertical cracking turned to horizontal upon reaching a stone
male before again moving vertically to reach the base of the
structure. While it is likely that this cracking occurred during
the earthquake of 1504, it is impossible to tell if other parts of
the structure may have survived the earthquake and have
subsequently been removed, or if the current site similar to that
in 1504. The two examples are able to show the effectiveness of the
horizontal layers between lifts, but without further study it is
difficult to deduce both if the layers were intended to act as ring
beams and if the approach of stacked beams improves the seismic
resistance of a structure. Ring beams within the body of the wall
were found exposed at three sites in southern Spain. Here turned
lengths of timber appear to have been placed during ramming of the
wall, and the careful nature of their placement indi-
cates an understanding of aseismic design. The timbers are first
turned to become perfectly circular, then covered in a lime and
straw mortar, which is then rammed within the wall. These timbers
are placed at 1.6m intervals (every other lift) in a ring through
the building. Fig 20 and Fig 21 show an exposed ring beam at
Tabernas in southern Spain, where part of a wall has been removed
to provide access to the ruined site. Here an unintended
consequence of the ring beams can be seen, that the holes act as
conduits for water, and water is channelled trough the hole in
preference to the surrounding wall, causing degradation of the
timber within.
ConclusionsEarthen structures provide a unique cross over
between the civil engineering disciplines of structural and
geotechnical engineering. To date, this linkage has not been fully
exploited, and the engineering understanding of earthen structures
is distinctly limited when compared to more traditional
construction industry materials. The examples given show that
rammed earth may be considered as both a geotechnical and
structural material, and that the engineering of rammed earth must
be undertaken considering both aspects.
Fig 17. Diagram of earthquake damaged structure, Leh, Ladakh,
India Fig 18. Males arresting crack propagation, Carmona, Spain6/
Fig 19. Diagram of cracked tower, Carmona / Fig 20. Ring beam
embedded within wall, Tabernas, Spain7/ Fig 21. Ring beam hole,
Tabernas, Spain 22 January 2008 The Structural Engineer|31
paper: jaquin
The Rowen Travel Award afforded the author a unique insight into
historic rammed earth construction techniques, and much new
understanding was gained from the experience. The counterintuitive
example of rammed earth survival during a rainstorm was observed,
perhaps providing current construction with the option of external,
unprotected rammed earth walls. Examples of medieval aseismic
design features show that knowledge and understanding of structural
behaviour in the 16th century may be much greater than previously
imagined. The facing of rammed earth with a less permeable skin has
been shown to be detrimental to the survival of the structure, and
whilst this is the current favoured repair technique for weathered
rammed earth in Spain, the lifetime of the structure may be reduced
rather than increased by the repair. Instead sympathetic repairs
such as soft stitching should be employed, using similar material
to the original, with the same thermal, structural and moisture
transfer properties. Repair techniques observed both in Spain and
in India may be applied to earth structures in the UK. Cob cottages
in Devon and Cornwall should be repaired in as sensitive a manner
as used in Himalayan buildings, and earthen structures throughout
the world may benefit from repair techniques derived
from improved understanding gained by the author through these
visits. The option of rammed earth as a modern sustainable building
material should be further explored. Rammed earth structures a 1000
year old are testament to the materials longevity and treatment of
the material as a geotechnical material will allow for more
confident design in the future. Engineering analysis and improved
understanding must be applied to historic earthen buildings to
ensure their structural survival whilst also preserving their
historical and cultural integrity.
REFERENCES Hughes, R.: 1983. Material and structural behaviour
of soil constructed walls. Momentum 175-188 Hurd, J.: 2006.
Observing and Applying Ancient Repair Techniques to Pise and Adobe,
in Seismic regions of Central Asia and TransHimalaya. New Concepts
in Seismic Strengthening of Historic Adobe Structures, Los Angeles,
California Jaquin, P., Augarde, C. and Gerrard, C.: 2007. Historic
rammed earth structures in Spain, construction techniques and a
preliminary classification. Int. Symp. Earthen Structures,
Bangalore, India Langenbach, R.: 2004. Soil dynamics and the
earthquake destruction of the Arg-e Bam. Iranian J. Seismology and
Earthquake Engineering, Tehran, Iran, Special Issue on 26 December
2003 Bam Earthquake 5/4 Ramos Vazquez, I.: 2003. Memoria del
Castillo de Banos de le Encina (Silgo XIII-XVII). Publicaciones de
la Universidad de Jaen: Jaen Walls, A.: 2003. The 300 year old
history of an Arabian Mud brick technology. Terra 2003, 29 November
to 2 December Yzad, Iran, 630 - 654
Paul Jaquin is about to complete a PhD in the Analysis of
Historic Rammed Earth Structures at the University of Durham. Paul
began the PhD directly after completing his MEng degree, also at
Durham. He was sponsored through the course by Whitbybird
Consulting Engineers and by an ICE QUEST Award.
Kenneth Severn Award 2008The natural world is full of examples
of structure and design which have influenced the development of
structural engineering; however while nature and evolutions role is
unseen, so is the role of structural engineers in shaping the
modern world. What would you do to change the perception of
structural engineers, raise their profile and make visible their
contribution to the built environment?To enter the Kenneth Severn
Award, please submit a paper titled Changing the perception of
structural engineers of no more than 4 sides of A4 (including
images) to [email protected] by Friday 22nd February
2008. Points will be awarded for originality, the value of the
paper to the structural engineering profession and clarity of
presentation. The winning paper will be published in The Structural
Engineer (subject to appropriate review), and the author will
receive a prize of 500 and the Kenneth Severn Diploma.
Entrants for the Kenneth Severn Award must be 28 years of age or
under on 1st December 2007. Entry is NOT restricted to membership
of the Institution. For more information visit
www.istructe.org/kennethsevernaward
32|The Structural Engineer 22 January 2008
