EWB-UK Research Conference 2009Hosted by The Royal Academy of EngineeringFebruary 20Community of Practice: HabitatAuthor: Paul JaquinInstitution: Ramboll Whitbybird (formerly Durham University)Previously published: (Geotechnique, February 2009)
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.
F F F
neck T neck a w
F r r u u
= + −
EQUATION 1 MAGNITUDE OF THE ATTRACTIVE FORCE BETWEEN SOILPARTICLES AS A RESULT OF THE LIQUID BRIDGE
is the attractive force due to the meniscus and
is theattractive force caused by the difference in pressure between the air and thewater,
is the radius of the neck of the liquid bridge,
is the surface tensionand
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 asbeing 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 of pores 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 samplestrength.
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 (Lourenço, Gallipoli
2008). Soil cylinders wereconstructed using methods based on the light Proctor method developed at theUniversity (Horncastle 2006). Soil was oven dried at 105°C, before being mixedwith a known volume of water to create a mix of known water content. Themixture was bagged and left for 7 days to allow the moisture content to