Research On Group Anchor Effect Of Wooden Bolts In Rammed Earthen Sites

Cultural heritage carries the splendid civilization,inherits the historical culture,and maintains the national spirit,which is of great significance for inheriting and carrying forward the excellent traditional culture of the Chinese nation.The earthen sites are one of the important and immovable cultural heritages,conserved especially in the arid and semi-arid natural environment of the ancient Silk Road in China.Due to the influence of natural factors and human activities,the rammed earthen sites are widely suffered structural diseases,such as unloading deformation cracks,tectonic-induced cracks,and construction joints,etc.,which lead to the instability and collapse of the earthen sties.In the context of a large number of earthen sites in urgent need of conservation and reinforcement,the earthen ruins in China are still in the "emergent reinforcement stage",and the mechanical stability control of rammed earthen sites is a top priority.The full-length bonded anchorage technology has less construction disturbance and can effectively control the development of fractures,so that it has become the most effective method to ensure the stability of the earthen sites.As a type of traditional building material in the ancient Chinese architecture,wooden bolts fully exhibit their suitability for the reinforcement of earthen sites in terms of their characteristics,structural properties and cultural backgrounds.Previous researches have focused on the development of anchorage materials and the mechanical properties of single anchor for rammed earthen sites.However,the interaction among bolts and the effect of anchor group have rarely been mentioned.Based on the national "One Belt,One Road" initiative and the urgent scientific needs in the field of cultural relics protection,the research on the anchoring effect of the wooden anchor group on rammed earth sites conserved in arid and semi-arid environments had been carried out.Through the review of the current literature on earthen sites and rock-soil anchoring research,this thesis scientifically outlined the development in earthen sites and the control index of group anchor effect.The main innovative contents and methods of this paper are as follows:(1)Basic performance tests on anchoring materials were first carried out.In the experiment of optimizing the ratio of the modified grout for the calcined ginger nut,the test method of grout specimens was buried and maintained outdoors,which was more complex to the actual service state.Through the detection of the physical and mechanical properties of the grout within the age of 180 days,the most compatible grout is a mass ratio of calcined ginger nut and quartz sand at 1:1.The physical and mechanical properties of white ash wood bolts,with different water contents and diameters were measured.Parameters such as density,shrinkage and expansion rate,tensile strength,compressive strength and bending strength were provided,and the mechanism of high strength and high toughness of white ash wood bolts was explained from the microscopic scale.(2)Based on the design of orthogonal experiments,pull-out tests on the wooden bolt group anchor in a full-scale physical model were employed.The experimental system of pull-out tests on group anchor for rammed earthen sites was developed to realize the overall pull-out of multiple anchor rods and solve the problem of uneven stress caused by the difference of ultimate pull resistance between bolts.On this basis,the group anchoring effect on the rammed earth with different interval distances,margin distances and embedded lengths under 2,3 and 4 bolts were explored.The load-displacement curves,failure modes of the group anchors,failure characteristics of each component,and distribution and transmission of shear strain on the interface of each bolt.Orthogonal analysis obtained the influence orders of embedment depth,interval distance,and number of bolts for the group anchoring effect,and clarified the force mechanism and influencing factors of the group anchor in the rammed earthen sites.Finally,the physical and mechanical properties of rammed soil are combined to determine the reasons affecting the failure sequence of group anchors.(3)The numerical simulation method of wooden bolt group anchoring system,considering the number of bolts,interval distance,margin distance,embedment length and layered property of rammed earth was established by "contact surface" element in the finite difference simulation calculation software FLAC 3D(version 6.00).The law of stress distribution and transmission on the interface of the anchoring system and in the rammed soil was analyzed,which supplemented and expanded the physical model test results.The anchoring capacity under each influencing factor was evaluated under a multi-variable model.(4)In order to achieve the pull-out detection that could be applied to the anchor system of the actual rammed earthen sites,a large-scale experimental wall was building the traditional rammed technology.The group anchor pull-out experimental system was modified to become a suitable group anchor pull-out device for walls,the results are mutually verified with the indoor model tests.The simulated wall pull-out tests indicated the failure mode of the group anchor system and the strain distribution and transfer characteristics of the bolt interface.The shear-slip model of the full-length bonded wooden bolts was optimized,which improved the theoretical level of the existing research.The research results of this thesis provide technical support for the anchorage design and detection effeciency of anchor group in earthen sites,revealed the failure mechanism of wooden bolt group,providing a theoretical basis for the scientific and standardized anchorage of earthen sites.The research directions including the stability evaluation and anchoring reinforcement technology for the rammed earthen sites have been expanded,and the depth and connotation of conservation of earthen sites have been enriched,thus ensuring the scientificiency and systematicness of conservation in earthen sites.