Study of historic rammed earth structuresin Spain and India

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Rowen Travel Award winning entry 2006 Presented 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 India Synopsis The 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.

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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 world’s 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 Jaquin MEng University of Durham

Keywords: Earth structures, Historic structures, Spain, India, Cracking, Repairing, Earthquakes Š Paul Jaquin

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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.

Water The 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-

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Table 1: Sites mentioned in text and Fig 1 and Fig 2 Number

Site

1

Alcala de Guadaira, Andalucia, Spain

2

Ambel, Aragon, Spain

3

Banos de la Encina, Andalucia

4

Talamantes, Aragon, Spain

5

Elche, Valencia, Spain

6

Carmona, Andalucia, Spain

7

Tabernas, Andalucia, Spain

8

Basgo, Jammu and Kashmir, India

9

Leh, Jammu and Kashmir, India

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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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

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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.

Cracking The 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.

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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 building’s 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.

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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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Crack stitching The 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 measures Rammed 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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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.

Conclusions Earthen 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

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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

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.

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 material’s 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 • •

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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

Kenneth Severn Award 2008 The natural world is full of examples of structure and design which have influenced the development of structural engineering; however while nature and evolution’s 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 kennethsevernaward@istructe.org 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