calibration of safety factor for micropile in transmission tower foundations based on relative reliability approach

This paper determines the design safety factor of micropiles utilized in the foundation of electric power transmission towers against the geotechnical failure due to the compressive force (failure of micropile-soil cohesion) by using the relative reliability approach. on the basis of this approach, the design processes are conducted in a way so that the reliability of foundation would be greater than that of tower. in other words, the failure of tower structure should occur prior to that of foundation. in order to calculate the safety factors in terms of specific reliability level of foundation with respect to tower, reliability analyses were adopted using “monte carlo sampling” method. furthermore, the strength statistical characteristics of transmission towers and micropiles have been extracted based on the reports of previous tests. the results of which revealed that, considering the target value of relative reliability factor (rrf)- the ratio between failure probabilities of tower to foundation- as 12, the values of safety factors attained in the current research are equal to 2.0 and 2.20 respectively for lattice and pole transmission towers. it should be remarked that these safety factors are only valid in cases of micropile design in dense sandy (sp-sm, sp&gw-gm) and clay-silt (sc, sm&sc-sm) soils.