Study On The Uplift Bearing Characteristics And Optimization Analysis Of Roots Foundation

With the development of transmission line construction of UHV Tower such as "west to East power transmission,north to South Power Transmission",the traditional excavated foundation has been unable to meet the needs of higher bearing capacity and stability in limited space.How to create a new section form to improve the foundation uplift bearing capacity has become the core of study of excavated foundation development.In recent years,more and more attention has been paid to the root foundation which is characterized by bionics.Its unique section form provides a new way to increase the bearing capacity of the excavated foundation.At present,the research on this type of foundation is mainly focused on compression and horizontal load resistance,and the engineering application is mostly large pier foundation.There are few reports on the engineering application of miniaturized excavated foundation and the study of uplift bearing mechanism.Therefore,it is necessary to carry out the research on the uplift bearing characteristics of root foundation to provide the theory for its application in the construction of UHV tower.In order to fully explore the characteristics of uplift bearing capacity and stress model of root pile,with the combination method involves model test,field test and numerical stimulation,the transfer law of load,the theory of pile-soil interaction and the optimization design problems of pile are discussed.The results show that:(1)Under the test conditions,the multi-layer variable cross-section of the root pile makes the side friction and normal resistance of the foundation develop synchronously,and the proportion of upright resistance increases stably with the increase of load,the load which root can provide reach about 66.6 % under the ultimate load,while the upright resistance of the belled pile develops later than it’s friction resistance.Therefore,the root key pile has a more significant ability to restrain the foundation displacement under the medium and low load.Compared with the same size enlarged bottom pile,the root key pile has a higher ability to resist deformation,so it has a good development prospect.(2)The result of model test show that when the number of layers is small,the stress concentration of root is obvious,and the load diffusion ability of root to the displacement field of soil is weak.With the increase of layers,the load diffusion effect of root is also significantly improved.Therefore the potential of bearing of multi-layer root pile is better than belled pile;With the increase of distance,the vertical displacement discipline of soil near the surface of root pile presents a special rule of "saddle type",that is,the vertical displacement of soil at its surface will appear twice "peak value" within a certain distance,and the trend of vertical displacement of soil along the horizontal direction can be divided into three areas successively: plastic shear failure area,normal attenuation area,displacement strengthening area(the second peak value).Thus,if taking the plastic zone as a minimum distinction of two piles,the roots pile’s distinction should extend to the displacement strengthening area.(3)Optimization problem of the foundation is discussed by numerical simulation results based on typical soil parameters,the longitudinal distance of the root between two layer should be 4L at better(L is length of the root).Under the premise of ensuring economy and rationality,the value range of the root density should be0.1 < α < 0.3(α is the ratio of the cross-sectional area of the root to the area of the circle with the same diameter).The angle of the root among ±20°has no significant effect on the bearing capacity of the foundation.Considering the safety and convenience of process of engineering,it is recommended to install the root horizontally or clockwise at a certain angle;(4)the calculation model of Foundation’s anti pulling deformation well reflects the deformation law of foundation even if different combinations of roots.For single-layer root foundation,the maximum error between calculation value and simulation value is less than 30 %,while the maximum error of multi-layer root foundation is less than 20 %.The accuracy of prediction of the model on multi-layer root key is more prominent,and its accuracy is largely affected by the load distribution law of multi-layer roots.The results meet the error range requirements of engineering practice,and can provide guidance for the application of root foundation.