Desiccation And Cracking On The Surface Of Ancient Earthen Architectures

Geotechnical engineering in the 21st century is characterized by a new trend, the research on consolidation and preservation techniques for conserving the cultural relics built with geometrials. As an important component part of cultural relics, the ancient earthen architecture provides very precious information for politics, economy, art, architecture, science and technology in ancient China. The earthen architectural heritage is not only the witness for the bright history of China with a civilization five thousand years old, but also the precious heritage of the world. In northwest China, there are a large number of ancient earthen architectures left along the Silk Road, such as the Great Wall, Beacons built in Qin or Han dynasty, and the Ancient City of Loulan, Jiaohe and Gaochang, which is included in the "Transnational Joint Application Plan For Cultural Heritage in Silk Road" in China. In the last years, the State Administration of Cultural Heritage and Ministry of Science and Technology of China jointly organized and implemented a series of key scientific research projects on earthen heritage conservation for the practical consolidation and protection of the ancient earthen architectures.Exposed in the open environment, the cultural relics suffered a series of physical and chemical weathering induced by natural agency, and underwent a degradation in properties and function of construction materials. To understand the degradation process, the process of degenerate, which is the essence of the cultural relics protection, the weathering mechanism of construction materials should be firstly studied. Based on the scientific understanding for the weathering mechanism, corresponding measures can be draw up and implemented to protect different types of cultural relics.This work is part of National Key Project of Scientific and Technical Supporting Programs Funded by Ministry of Science & Technology of China (NO.2006BAK30B02). This thesis focused on the shrinkage and cracking mechanism of surface soils in the ancient city of Jiaohe, involved in the rain erosion by field investigation and laboratory test from the geotechnical engineering viewpoint. Soil shrinkage and water retention curves of earthen materials in ancient city of Jiaohe were tested, and the desiccation cracking phenomena was discussed based on the physics theories soil water interaction mechanism. Continuous monitoring system and digital image processing methods were used in laboratory test and verified so to establish a quantitative relationship between the surface cracks'pattern and their influencing factors. The generation and detachment mechanism of surface crust on ancient earthen architectures was firstly revealed. These results provide new research direction for study of weathering mechanism, and establish a theoretical basis for the large-scale protection and reinforcement projects on earthen heritage. The main contents included in this thesis are as follows:1. Soil is a deformable multiphase porous medium. The desiccation of soil is a set of processes that include drying, shrinkage and cracking. Research indicated that there were two modes of liquid transfer at the boundary of the soil sample and inside the porous space during desiccation of soil. One was purely liquid transfer induced by boundary evaporation, and the other one was liquid transfer induced by evaporation inside the body and vapor diffusion through the body. The basic process behind the shrinkage of a soil was a decrease in liquid pressure and more generally an increase in suction, caused by the evaporation at the level of menisci, generating an additional attractive force between the components of the matrix. Therefore, an increasing internal compression and the matrix shrink of the sample occurred.2. An improved method for volume measurement, determining soil shrinkage characteristic curves (SSCC), was proposed. Through comparison of test and model fitting results, the paraffin oil method, tending to narrow the measurement error, is a better alternative to describe the SSCC of structure-less clay paste than the caliper method. The observation of geometry factor indicated that, at the beginning of desiccation, the clay paste was controlled only by a vertical downward movement of soil particles due to gravity. With further drying, the soil became stable and the horizontal shrinkage component increases, resulting in a three dimensional shrinkage. Using the improved volume measurement, the soil shrinkage characteristic curves of different soils were measured and determined. The results showed that, due to the presence of some non-connected pores which were not filled by free water, a shift of the shrinkage curve was found. Results of the investigation also indicated that the different soil samples showed different shrinkage properties according to clay type and clay content. The shrinking capacity of the specimen volume increased with the content of clay, but there was an inverse proportionality between the clay content and the void ratio at shrinkage limit. Test results also indicated that the shrinkage limits decreased with the content of sand in soils, and that the rates of changes in void ratio and degree of saturation of the soils were highly affected by the evaporation rates. However, the shrinkage curves of the soils, was not affected by the drying rates. An identical shrinkage curve was obtained for the same soil under different drying rates. Similarly, the relationship between void ratio and matrix suction for the same soil under different drying rates was also identical.3. Experimental model of cracking showed that desiccation macro-cracks in soil were the consequence of constrained drying shrinkage and tensile stress generation that exceeds soil tensile strength. Typically, three kinds of constraints responsible for the desiccation can be identified:(1) a frictional or any other traction or displacement boundary conditions; (2) any eigen-stress concentrations within the soil sample; (3) intrinsic factors, such as soil texture and soil structure. Research indicated that the Griffith's criterion (defined in terms of macroscopic tensile strength) was useful for modelling the strength of a volume of an elementary representative of soil submitted to tensile states. Griffith's criterion provided a fundamental basis for such a sample that has randomly distributed flaws in case that considering the hypothesis that the number, shape, and orientation of flaws should not change with the sample size. Tensile strength, depended on the degree of saturation and on suction, was also related to the most critical flaw in the Griffith sense. Besides, from a microscopic point of view, the study emphasized the important role of soil structure (such as the presence of defects), degree of saturation and suction.4. The computer image processing technique was applied to quantitatively analyze and describe the structure and geometric characteristics of crack. Some factors influencing the shrinkage, such as temperature, sample thickness and composition of soil, were taken into account. The test results showed that with the increase of sample thickness, the space between soil cracks were reduced, while the length and width of cracks were increased. The crack length, width, aggregate area and their most probable value were related to the probability density functions, which tend to increase with temperature increase. With a thicker soil layer, the average crack length, width, aggregate area and crack intensity factor increased. It was also observed that the effect of soil types on the crack pattern should not be neglected. In addition, it was observed that cracking occurred at three stages and the water loss rate increased after the shrinkage cracks had appeared on the surface of soil samples.5. The formation of soil surface crust mainly depends on soil permanent properties, such as soil texture, mineralogy, organic and inorganic polymers, composition of exchangeable cations, aggregate stability, antecedent water content and wetting rate. The aggregate stability and clay content have substantial effects on formation of surface crust. Research shows that soils with 20-30% clay were the most susceptible to crust formation. With clay content above 40%, soil structure became more stable, and crust formation was restrained. In addition, smectite soils, low levels of exchangeable sodium percentage and electrolyte concentration were all beneficial to the formation of soil surface crust. Wetting rate, antecedent moisture content, also significantly affect soil susceptibility to crusting, but this was often neglected by many researchers. Fast wetting of dry soil caused aggregate slaking and crusting whereas high antecedent moisture content decreased aggregate disintegration.6. Field survey, systematic sampling and laboratory tests were conducted to study the weathering mechanism of earthen architecture taking the Jiaohe Relics site, Xinjiang, China as an example. Particle size analysis, X-ray diffraction and chemical analysis of soluble salts illustrate that surface crusts are characterized by finer particles and lower soluble salts contents compared with the host soil. Temperature monitoring by IR thermography camera and thermal sensors shows that thin crusted layers response greatly and quickly to the environmental temperature change. It is concluded that the local saturation and slurry film generation due to rainfall are mainly responsible for the formation of surface crusts; and the crust will be subsequently detached by wind erosion and expansion-contraction induced by the change of temperature. Surface crusted layer prevents the wall from weathering to some degree at its earlier formation stage but tend to accelerate the deterioration of the earthen architectures in the long term considering that Jiaohe site suffer from strong sandstorm and strict weather condition.