Study On Settlement Characteristics Of High Fill Ground In Loess Hilly And Gully Regions Based On Creep Test

The terrain of the Loess Plateau in northwest China is mainly mountains and hills.Cities and towns are mostly distributed along flat areas of gully-valley.With the rapid advancement of urbanization process,the terrain restriction on urban expansion has become increasingly prominent.Cutting hills to backfill ditchs has become an effective way to solve the shortage of construction land,which results in the emergence of many high fill projects.The deformation of loess high fill ground is significant and spatiotemporal distribution is complex,the settlement problem is the main technical challenge faced by engineering construction.The high fill ground in loess hilly and gully regions was taken as the research object in the paper.Based on the study of loess creep characteristics and creep mechanisms,the key issues such as the shear characteristics and shear mechanisms of fill interface,the spatiotemporal distribution and influencing factors of fill settlement,the in-situ testing method of fill properties were systematically studied.The main research works and conclusions are as follows.1.Long-term one-dimensional creep tests were designed to study the creep characteristics of compacted loess.The analyzing was conducted from two perspectives of standard compression method in national code and creep method.The results show that when the standard compression method is used for analysis,the greater the compactness,the lower the moisture content,the greater the structure yield strength of compacted loess,and the lower the compressibility.The change of compressibility with compactness and moisture content under high pressure is obviously different from that under low and medium pressure.For compression deformation calculation,the secant modulus method model combining linear function and power function is proposed in the paper.This model can make stress-strain function pattern relate to the essence of structural yield strength,and explain the reasons of differences in various existing secant modulus method models.Based on these,the fill settlement calculation formulas considering the change of compactness and moisture content are derived.When creep method is used for analysis,the primary consolidation deformation of compacted loess is completed within two hours,and the secondary consolidation deformation lasts for a long time.The secondary consolidation coefficient isn’t constant over time.For creep calculation,it is proposed to use the Bugers model when there is an amount of laboratory test data,which has high accuracy and the change in model parameters with influence factors is consistent with the creep micro principles and macro laws.When there is less laboratory test data,the modified A-m-n empirical model is proposed in the paper,which can achieve the correlation between model function form and structural yield strength,and has a certain accuracy.Comparing the standard compression method with the creep method,the standard compression deformation has a larger deviation to long-term creep deformation.It is more reasonable to evaluate the long-term post construction settlement of loess high fill from the perspective of creep,and the standard compression method is suitable for evaluating construction period settlement and short-term post construction settlement.2.The multi-scale joint tests about soil microstructure evolution during the creep process were conducted to study the creep mechanisms of compacted loess.The pore size distribution was obtained through mercury intrusion tests,the pore morphology complexity,pore roundness,and pore directionality were obtained through scanning electron microscope tests,the three-dimensional connectivity of pore was obtained through CT scanning tests.The results indicate that the primary consolidation stage of compacted loess is characterized by large and medium pore compressing and reducing obviously,pore shape flattening,morphology complexity reducing,directionality enhancing and three-dimensional connectivity reducing.The morphology complexity,directionality,and three-dimensional connectivity change of pore during the secondary consolidation stage is similar to that of the primary consolidation stage.The differences are that this stage is dominated by compressing and disappearing of small and microporous pore,and pore shape tends to be a stable structure of circular shape.This is also the reason of the secondary consolidation deformation rate is low but occurs for a long time.The change of compactness and pressure affects the mainly compressing pore type in creep process,while the change of moisture content affects the mainly compressing pore type,the pore morphology complexity variation and pore roundness variation in creep process.3.The creep characteristics and creep mechanisms of undisturbed loess were studied.The results show that the secant modulus of undisturbed loess doesn’t increase monotonously with the increase of pressure,the secant modulus method is no longer applicable and compression deformation needs to been calculated by e-p curve.The creep deformation amount and secondary consolidation coefficient of undisturbed loess are greater than those of compacted loess,the reason is that the diameter of mainly compressing pore in secondary consolidation stage is larger and is dominated by medium pore compressing.The creep calculation method for compacted loess is also applicable to undisturbed loess.4.Composite sample simulating the interface contact between fill and undisturbed loess was designed to study the shear characteristics of interface,scanning electron microscope tests on the interface micro morphology after shear were conducted at the same time to study the interface shear mechanisms.The results indicate that there is a phenomenon of strength degradation in interface shear.The shear plane occurs within interface when the compactness of fill is low.The shear plane occurs on the undisturbed loess side when the compactness is high or moisture content is low.The position of shear plane is random when moisture content is high.The microscopic mode of interface failure after shear is that the dense distribution zone of tensile pore appears on compacted loess side,the pore on undisturbed loess side opens and the soil particle scatters and falls off.The higher the compactness,moisture content,and pressure,the more obvious the microstructure change on both sides of interface after shear.5.Based on the constitutive model and parameters proposed by the creep study,the numerical model of high fill in loess hilly and gully regions was established.After the numerical model was validated by in-situ monitoring data,the construction period settlement and post construction settlement of fill were simulated.The results show that both the construction period and post construction settlement are very significant,and it must take several years for post construction settlement to stabilize.The terrain of gully leads to significant differential settlement of fill appearing within the slope range during post construction stage,the horizontal displacement towards to the center of gully is significant.There is “soil arching effect” in fill,and the interlayer settlement in shallow fill is smaller in middle of gully than both sides of gully,while it is opposite in deep fill.The post construction settlement value is linearly related to the compactness,fill height and interface slope,and a power function relationship with the moisture content.The importance order of various factors for post construction settlement control is compactness,fill height,moisture content,interface slope.The creep of undisturbed loess under fill also makes a significant contribution to the post construction settlement of high fill.6.The standard penetration test,dynamic penetration test and cone penetration test were used to conduct in-situ testing of fill properties,and combined with exploratory well sampling tests to study the in-situ evaluation method of loess high fill properties.The results indicate that the cone penetration test can effectively test the statistical laws and spatial non?uniformity characteristics of fill properties,with good testing correlation and high testing sensitivity.The testing curve can identify layered filling information and fill depth boundary.It is the preferred method for in-situ evaluation of loess high fill properties.Through the studies in the paper,the compression creep properties of loess can been further understood,the long-term occurrence reason of loess high fill ground settlement is revealed,the special boundary effect of gully terrain to fill and its influence on the settlement spatial distribution are clarified,the settlement control measures are proposed from the aspects of fill design,construction,and testing in post construction.The conclusions can provide technical support for implementing efficiently and safely of high fill projects in loess hilly and gully regions,and has good practical guidance significance and application value.