| Bidirectional Reinforced Composite Foundation was born in the 1970 s.It has been widely used in highways,railways,airport,housing construction and other fields until today.Compared with the traditional vertical reinforcement composite foundation,the horizontal reinforcement is added to Bidirectional Reinforced Composite Foundation bearing system.The interaction between lateral reinforcement,pile and soil leads to the complicated mechanism of the force and deformation in Bidirectional Reinforced Composite Foundation.So theoretical research lags behind engineering practice in the field of Bidirectional Reinforced Composite Foundation.Geogrid is a common lateral reinforcement in Bidirectional Reinforced Composite Foundation.It chemical nature-high polymer is prone to unidirectional,unrecoverable chemical changes,and causes the strength of the material to decrease,which has a adverse effects to Bidirectional Reinforced Composite Foundation.Under the background of large-scale application ofBidirectional Reinforced Composite Foundation,studying adverse influence of aging geogrid to bidirectional composite foundation properties(including deformation and stress)is an inevitable requirement for a century-old project,which is also the first core content of this paper.Finding a way to improve the stress state of Bidirectional Reinforced Composite Foundation can reduce the amount of materials,increase the safety factor,increase the life of the project,and have good social and economic benefits.Relevant regulations stipulate that the geogrid should be straightened when using.The author suspect that prestressing geogrid can slow down the aging speed of the geogrid,Improve the stress state of Bidirectional Reinforced Composite Foundation.Verifying the above conjecture is the second core content in this paper.The indoor model test is a commonly study method in geotechnical engineering field.It has a benefit of controllable variable,low time cost,and small investment.Therefore,the author decided to use it to study the two core contents proposed in the previous paragraph.For the first study content(adverse influence of aging geogrid to Bidirectional Reinforced Composite Foundation properties),establish a static internal scale model firstly,and use the aging machine to age the geogrid to different levels(aging 100 h,200h,400h)rapidly,then embedding aging geogrid to composite foundation as a lateral reinforcement,lastly embedding instrument to obtain data,which including deformation and stress in composite foundation.The adverse influence of the aging geogrid on the above contents is studied by controlling the aging time ofgeogrid.For the second study content(to verify whether the prestressed geogrid can Improve the stress state of Bidirectional Reinforced Composite Foundation),apply appropriate prestress to the aging geogrid and embed the Bidirectional Reinforced Composite Foundation.The difference between prestressed aged geogrid and non-prestressed aged geogrid is analyzed in two aspects:deformation and stress.If the model experiment finds that prestressed Bidirectional Reinforced Composite Foundation with aging geogrid has smaller deformation,then the prestress can slow down the aging rate of Bidirectional Reinforced Composite Foundation.In this paper,first of all,Bidirectional Reinforced Composite Foundation and geogrid aging is described.It briefly introduces the experts’ research trends at domestic and abroad,then elucidate the blaze new trails and research objectives of this topic.Then the indoor model experiment is discussed.It includes similar conditions for model experiment,selection of test materials and equipment,development of experiment protocols,and acquisition and processing of test data.Finally,results of model experiment are discussed from perspective of settlement and stress,and they are core content of this paper.Conclusions of this paper are as follows:1)In aged geogrid Bidirectional Reinforced Composite Foundation,its components(cushion layer,pile reinforcement area and bearing layer)settlement increase with the increase of the upper load.Among them,the settlement speed of pile reinforcement area is the fastest,and its value accounts for the largestpercentage of total settlement.This means bearing capacity of Bidirectional Reinforced Composite Foundation decreases with the increase of the geogrid aging time,Also observed in model experiment that attenuation rate is first fast and then slow.2)Geogrid Strain is positively correlated with the magnitude of upper load.The strain of the geogrid is significantly worse than that of the slightly aged geogrid under the same load.3)From a perspective strain,prestressed geogrid can Improve the stress state of Bidirectional Reinforced Composite Foundation to a certain extent,which is mainly manifested that prestressed aging geogrid composite foundation has smaller settlement and greater bearing capacity,.prestressed aging geogrid composite foundation also has a smaller strain.Model experiment also found that it’s more effective to a small load or slight aging geogrid composite foundation by the method of prestressed geogrid.4)The stress around the geogrid increases with the increase of upper load.Stress below the geogrid is bigger than that above the geogrid;pile-soil stress ratio below the geogrid is bigger than that above the geogrid,they increases with the increase of upper load when upper load is small,they no longer increases or decreases slightly when upper load reaches a certain threshold.Pile-soil stress ratio and stress above the geogrid have nothing to do with aging geogrid degree;The maximum value of the pile-soil stress ratio below the geogrid reduces with the increase of geogrid aging time.5)In Bidirectional Reinforced Composite Foundation,There’s negative frictional resistance on the upside of pile,and positive frictional resistance on the underpart of pile.Axial force and side friction resistance increase with the upper load increase.Positive friction resistance decrease with geogrid aging time.6)From a perspective of the stress,the prestressing aging geogrid can improve the stress state of Bidirectional Reinforced Composite Foundation to a certain extent,prestressed aging geogrid composite foundation has smaller soil stress,greater pile stress,and pile-soil stress ratio under the geogrid than non-prestressed aging geogrid of that.Prestressed aging geogrid composite foundation also has smaller pile negative frictional resistance and bigger positive frictional resistance than non-prestressed aging geogrid of that.But there is no obvious law on the influence of prestressing to pile axial force. |
