Research On Aseismic Design Of Electric Power Pylon Pile Foundation In Southwestern Mountainous Areas

The electric power pylons always run and locate at ridge slope site and slope site insouthwestern mountainous areas. It is evident that lots of tower sites instability,infrastructure damage and secondary disasters were found in southwesternmountainous areas in Wenchuan earthquake and Lushan earthquake, which led togreat damage of power grid project in the region. In this paper, we mainly focus onthe research of the pile foundation of transmission lines in southwestern mountainousareas, by using field investigation and data collection, and the numerical analysis areapplied to analyze ground and to analyze ground and pile foundation’s seismic effectof three typical site: ridge slope with thick layer and bedrock, slope with thin layerand bedrock, ridge slope with thin layer and bedrock. Based on these we research thepile foundation seismic design of electric power pylon from following aspect:standard calculation of earthquake action, damping of pile structure, dispose for wasteconstruction-soil, reinforcements for soil around the pile. Following results areobtained:(1) In ridge slope site, earthquake effect of slope toe may increase or decreaserespectively, but absolute earthquake effect can make the local intensity improve2degrees. In slope site, earthquake effect of slope toe present enhancement whichconcerned with elevation and mountain height, while absolute earthquake effect canmake the local intensity improve1degree.(2) When seismic intensity of the area ranges from Ⅵ to Ⅷ, both horizontal andvertical peak acceleration of the pile increases from the bottom to the top, knee of thecurve appears at the interface of bedrock and layer. The difference of top horizontaldisplacement among piles increases with direction towards the invading slope. Thehorizontal displacement ranges from2.0cm to6.5cm while the vertical displacementranges from1.0cm to5.4cm. Shear curve of pile presents in wave shape, and themaximum occurs at the interface of bedrock and layer, the bigger value occurs at thetop and bottom of pile. Bending moment increases from the top to the bottom.Horizontal displacement of the soil around pile changes little from the bottom of thepile to the interface of bedrock and layer, and it increases rapidly from the interface of bedrock and layer to the top of the pile. The result shows that pile foundationtransformation is within the range of allowable value and the result of seismic actionstandard values in standard at present is safe.(3) The analysis shows that the result of seismic action standard values in presentcode can satisfy the safe requirements. But if the pile natural vibration period is0<T<0.1s and seismic intensity is Ⅷ or above Ⅷ, we suggest multiplying horizontalseismic coefficient by amplification coefficient1.0~1.4; if intensity is Ⅶ, multiplyinghorizontal seismic coefficient by amplification coefficient1.0~1.5.(4) When the pile length are same, the pile diameter is inversely proportional tothe horizontal displacement of the pile head, and it is proportional to verticaldisplacement of pile head, internal force of pile body’s and horizontal displacement ofsoil around pile. The changes of pile diameter do not have very obvious effect onvertical displacement of pile soil. When the pile diameter are same, pile horizontalacceleration and displacement increase with the increase of pile length. We suggestthat when intensity is Ⅷ or above Ⅷ, pile diameter should be within1.8m to2.6m,pile length should be within10m to12m.(5) When there is a fort around pile, the pile horizontal peak acceleration, piledisplacement, internal force and displacement of soil around pile at centre of gravityof the pile are bigger. Combined the project’s actual situation, when gradient of slopeis bigger than30°, we’d better not place waste soil around pile and carry away thewaste soil in time, which is good for enhancing anti-seismic property and stability.(6) Reinforcing the soil around pile can reduce the pile peak acceleration, piledisplacement, internal force and displacement of soil around pile. When intensity isⅧ or above Ⅷ, we can enhance the strength of soil around pile in certain rangesduring construction to reduce the pile seismic response.