Study Of Temperature Effect Induced By Insolation On Deterioration Of Earthen Monument In Arid Areas

Earthen monuments were remaining constructions built with soil material,which were endued with great historical,cultural and scientific values.They were important component of immovable artifacts.Research on heritage conservation and causes of diseases had become one of key issues in geotechnical engineering.The major factors responsible for damage of earthen monuments and deterioration of earthen materials were generally attributed to natural factors,such as wetting-drying cycles,freezing-thawing cycles,stormy raining,wind-blown sand and activities of animals or human beings.The effect of solar radiation,however,was little discussed.In fact,the outer surface of solids could be greatly heated by the solar radiation,and thus,large temperature gradients exist in the inner part,which may arouse severe thermal stress causing structural breakdown at the near-surface areas.This mechanism has been validated by many researches on weathering of rocks or concrete.The mechanical property of loosen soil can be effectively improved by ramming or compressing,which are traditional skills used for building earthen walls.In this way,the binding of soil particles is strengthened,thus the deformation of soil particles induced by thermal expansion or contraction is limited.Breakdown caused by thermal stress seems inevitable for earthen monuments when exposed under strong sunlight.Moreover,this is most likely to be the main factor responsible for deterioration of earthen monuments during the long-term weathering process because the effect of water is rather limited in arid areas.The following studies are on the basis of this hypothesis.The Site of Yar City,which is located at the Turpan city,Xinjiang Uygur Autonomous Region in Northwestern China,was chosen as a typical research model representative for earthen monuments in arid areas.In-situ temperature data were collected to illustrate diurnal variation of temperature and its distribution.On the basis of measuring data,two mechanisms of temperature effect on the physical properties of soil induced by solar radiation were proposed,i.e.thermal shock and thermal fatigue.They were validated by numerical modeling and laboratory thermal cycles tests,respectively.Possibility of thermal shock on the earthen materials was discussed.Variation of peak tensile strength,ultrasonic velocity and thermal properties of compressed soil with different dry density were determined during the laboratory tests.Scanning Electron Microscope(SEM)and Mercury Intrusion Porosimetry(MIP)were employed to study the microstructure evolution of soil during the thermal cycle's tests.The results of this study provide authentic experimental data and basic theoretical support for the scientific research and protection on the earthen monuments in arid areas.In-situ temperature measurements were conducted on artificial Rammed Earth(RE)wall and cob wall,instead of true monuments to avoid any possible damages during the testing procedures.The walls were both constructed following the same traditional skills that were generally adopted in the Site of Yar City.Surface and inner part temperature were measured by infrared imaging system and embedded semiconductor thermometers,respectively.Collecting temperature data showed that surface temperature soared after sunrise and dropped during the cooling night.Distribution of isotherms in the wall body was determined by the geometrical wall section only.Variation of earthen walls'temperature were generally affected by the orientation of wall fa?ades,composition of earthen materials,building skills,intensity of solar radiation,ambient air temperature and wall thickness.During the diurnal variation,averaged temperature of RE wall was higher than the cob wall.Temperature fluctuations were nearly the same on the wall surface,showing little relevance with the position of measuring points.The highest temperature reached 70.2oC,appearing at the western surface of RE wall.As the wall thickness increasing,magnitude of wall temperature fluctuation and Decrement Factor(DF)decreased,Time Lag(TL)increased,of which the eastern part is larger than the western part and the cob wall is larger than the RE wall.The rate of temperature rising was obviously larger than cooling,which hinted possibility of thermal shocks during heating phase.The diffusivity and effusivity of earthen materials,however,were much more moderate compared with other building materials,like stone or concrete.Severe thermal shock damage seemed to be impossible for earthen monuments.Greater temperature gradients were generated at the wall sub-surface,around 10cm thick to the wall surface,which were the major areas greatly influenced by temperature effect induced by solar radiation.Moreover,the temperature gradients reversed twice a day with sunrise and sunset,which would cause breakdown of soil particles through thermal fatigue with cycling expansion and contraction.And this occurs daily.A thermal-solid coupling model was set up on the basis of measuring temperaturedata,heat transfer theory and solar motion physics regarding the thermal stress distribution induced by solar radiation in earthen monuments.Numerical simulation analysis was conducted with COMOSOL Multi-physics software.The simulation results of temperature variation were in accordance with the measuring data.The largest thermal stress was 89.1kPa according to the calculated results for earthen wall with dry density of 1.7g/cm~3 during diurnal thermal regimes,which was under the corresponding ultimate strength of earthen materials.That is,temperature effect induced by solar radiation cannot bring thermal shock failure to earthen monuments.In addition,further analysis showed that when the boundary of heat transfer was steady,temperature of near-surface areas decreased with larger thermal conductivity while temperature of inner-part increased,which would lead to narrowing temperature difference between internal and external wall.The temperature gradients existing in earthen walls were also decreased.The areas influenced by solar radiation,however,were expanded.The extreme temperature values were decreased with larger specific heat capacity during heating phase and temperature gradients were increased while cooling.The thermal stress was mainly affected by Young's modulus and linearly thermal expansion coefficient.It was dramatically increased with increasing Young's modulus and thermal expansion coefficient,especially for the near-surface parts.The influence level of thermal stress induced by solar radiation showed decreasing trend with increasing thermo-physical property,but this effect appeared in a limited extent.Laboratory testing results showed that physical properties of soil sample were greatly influenced by thermal cycles through thermal fatigue effect.The ultimate tensile strength and ultrasonic velocity firstly rose to their peak values and then decreased during the cycling process.However,the ultimate tensile strength was attenuated after 400 cycles while the ultrasonic velocity showed just a little difference compared to the original state.There was no macro-fracture formed during the whole cycling processing,representing that the integrity of soil samples was maintained.At the first stage,amplification of ultimate tensile strength showed positive correlation with dry density of soil samples,which could be ascribed to larger residual humidity contained in the soil samples with bigger dry density.The residual humidity was evaporated by thermal cycles and plasticity of soil decreased,resulting in the improvement of tensile strength.With continuous thermal cycles,the bindings between soil particles were broken by thermal fatigue,which caused changes of soil micro-structures and decreasing of tensile strength.Besides,the attenuation was larger for those soil samples with smaller dry density.Thermo-physical parameters of soil samples were also affected by thermal cycles and their changes could be divided into three stages.Firstly,all of thermo-physical parameters decreased as the evaporation of residual humidity and the attenuation increased with larger dry density.Then,the parameters showed slightly increase because of the damage of soil microstructures.Contacts of soil particles improved due to smaller size of broken aggregates,which benefited heat conduction and diffusion.At last,arrangement of soil aggregates became incompact under cycling thermal stresses.Microstructures changed from aggregate structure to skeletal structure,causing increasement of inter-aggregate pore diameter.For this reason,the thermal conductivity decreased,the thermal diffusivity tended to be stable and the volumetric specific heat became nearly the same.SEM and MIP testing results provided evidences to illustrate the mechanism of thermal fatigue effect from micro-scale.Disintegration of soil aggregates and associated variation of transitional and intra-aggregate pores'accounting were the root causes for the changes of soil physical properties.In detail,debris of soil particles adhere to the aggregates and percentage of inter-aggregate pores increased during the thermal cycles.The total pore volume monotonically also increased and its amplification was much larger at the first stage.The percentage of transitional pores increased during the second stage,which was a typical characterization of soil micro-structure changes.Pore structures were characterized by bi-modal peaks and a more homogeneous pore sizes at the third stage.Changes of micro-pore structures were irreversible and brought about deterioration of earthen materials.They were more obvious for soils with larger dry densities.Fractual dimension of micro-pores could be well characterized the pore structure changes quantitatively.Pore structures were more intricacy and its internal surface was coarser with larger fractual dimensions.Variation of thermal conductivity and thermal diffusivity showed strong correlation with factual dimension changes during thermal cycles.They were both increased with factual dimension increasing,but the amplification was not in the same trend.There was,however,little relevance between fractual dimension and ultimate tensile strength or volumetric specific heat capacity.This may be because they were generally influenced by the binding between soil aggregates and contents of soil particles in unit volume.In conclusion,the thermal fatigue effect on earthen monuments induced by solar radiation was in a weaker intensity compared to other strong weathering mechanism,like wind-blown sand and freezing-thawing cycles.Obvious damages would happen only after accumulation for a certain long time.The damage caused by occasional strong weathering factors,however,would become worse because of the micro-structual breakdown induced by thermal fatigue.In this consideration,much more attention should be paid into the temperature effect induced by solar radiation for the sake of long-term protection of earthen monuments in arid areas.