The aim of this research is to increase the understanding of the construction material rammed earth such that structures constructed in rammed earth can be better analysed and conserved. The project arises from the lack of engineering knowledge available to conservators of rammed earth structures and the lack of understanding of the fundamental processes at play in the material of rammed earth.
At Durham University there is a large amount of research being undertaken into modelling of soil. As rammed earth could be considered to simply be a well compacted soil, it is possible to fit rammed earth into a soil mechanics framework. This exciting work probes the mechanisms by which rammed earth and other earthen materials ‘work’. We are hoping to be able to explain the cohesion forces in rammed earth, and to place rammed earth in a geotechnical engineering framework.
A large amount of laboratory testing has been carried out, of both stabilised and unstabilised rammed earth. A detailed method for the compaction, curing and testing of rammed earth cylinders has been perfected. Triaxial and unconfined testing of cylinders has been carried out, varying both the water content and composition of the rammed earth.
Additives to the rammed earth mix, to improve the strength and mechanical characteristics have been tested. Addition of Kaolin and Bentonite clays to improve the strength of rammed earth has been attempted with varying degrees of sucess. Chemical stabilisation using lime and ordinary portland cement has also been tried and experiments in the addition of industrial waste products, rich in cemementing agents have been undertaken, in an attempt to improve the strength of the rammed earth.
A small number of test walls have been constructed in the laboratory. Custom formwork has been constructed, allowing the building of walls 1m x 0.3m and up to 1m high. A novel method has been developed which allows the production of voids beneath samples, meaning beams and cantilevered rammed earth can be tested. Compaction is carried out at a previously determined optimum moisture content, using a Kango electric hammer. The results of these experiments, and subsequent numerical modelling, described below, were presented at the Structural Analysis of Historic Constructions conference in New Delhi in 2006.
The tensile strength of rammed earth samples has been investigated using standard geotechnical and rock mechanics testing methods. Brazillian tests on rammed earth samples have probed the tensile strength of the material. A recent MEng project has looked at jointing in rammed earth structures, the strength of the bond, and ways to improve this strength. Shear box testing of rammed earth samples has compared the strength of jointed rammed earth with that of homogeneous specimens. Further testing investigated the strength of the rammed earth parallel and perpendicular to the compaction plane. Reinforcement of rammed earth using specialist geo-grids has been tested using the shear boxes, with some surprising results. Investigations on the strength of the bond of steel reinforcing bar embedded in rammed earth were have been undertaken, and specialist equipment devised for the testing. Non destructive testing methods using Schmidt hammers have been attempted, but it has been found difficult to correlate the results.
1. Cindy McChlery. MSc 2005
2. Thomas Horncastle. MEng 2006
3. Thomas Howard. MEng 2007
Rammed earth modelling
Numerical modelling of rammed earth is hugely important in understanding and predicting the behaviour of both historic and designed structures. A number of different geotechnical and rock mechanics models have been implemented to capture the behaviour of rammed earth.
A constitutive model for rammed earth based on the Mohr-Coulomb criteria was developed The model captures the different behaviour of the body of rammed earth and the joint layers, and allows for different behaviour along compaction planes. This allows for correct modelling of the behaviour observed during testing of ealls in the laboratory.
Field visits to a large number of historic rammed earth sites have been undertaken. Southern Spain was visited in January 2006, where around 50 historic rammed earth sites were visited. A large number of different construction techniques were observed, and a range of failed and damaged rammed earth structures were visited. Northern Spain was visited in October 2006, and here only 10 sites were looked at, but this time in much greater detail. North India was briefly visited in November 2006 where a small number of rammed earth fortifications were observed.
A 900 year old rammed earth structure near Zaragoza has been studied in particular detail. The building has been extensively studied by Dr Chris Gerrard who is part of the Archaeology Department at the University of Durham. Investigations have been carried out into the structural behaviour of the building. Of particular interest is the movement of the north wall of one tower, which has caused a large crack in the east wall. It can be seen that the north wall is leaning outwards at the base, and thus the top of the wall extends significantly from the base. The crack extends through the full width of the wall, and there are gaps between the north wall and floor at each level.
A finite element model was constructed to allow analysis of the stresses to which the building is subjected, and to identify which parts of the structure are likley to be closest to failure.
Jaquin, P. Augarde, C. E. and Gerrard, C.M. Historic Rammed Earth Structures in Spain International Symposium on Earthen Structures Bangalore August 2007
Jaquin, P. Augarde, C. E. and Gerrard, C.M. Analysis of Historic Rammed Earth Construction. Structural Analysis of Historic Constructions, Delhi November 2006 PDF
Jaquin, P. Augarde, C. E. and Gerrard, C.M. Analysis of Tapial structures for modern use and conservation. Structural Analysis of Historic Constructions November 2004 PDF
Jaquin, P. Analysis of Historic Rammed Earth Structures Presentations by Britain’s Top Younger Scientists and Engineers, Houses of Parliament, London October 2006 JPG
Jaquin, P. Historic Rammed Earth Construction Institution of Structural Engineers Young Researchers Conference JPG
Jaquin, P. Maggaly Ruiz, Earth building in Colombia and the UK May 2007
Jaquin, P. Rammed Earth is for Everyone July 2006 link