Feasibility of bare fiber Bragg grating sensing technology in the measurement of inner wall axial stress along open‐ended pipe pile in sand

Abstract

Larger-diameter open-ended pipe piles are widely used as foundation for offshore structures. The lack of high-quality inner wall axial stress measurement results in difficulties in loading analysis of open-ended pipe piles during penetration and service period due to the effect of soil plugging. In this paper, two groups of model tests to assess the feasibility of bare fiber Bragg grating (FBG) sensing technology in monitoring the inner wall axial stress profile of open-ended aluminous pipe piles during penetration and horizontal loading are presented. One open-ended aluminous pipe pile was instrumented with bare FBG sensors and then penetrated into the sand bed with the rate of 20 and 30 mm/min, respectively. Two horizontal static loading tests were also conducted after 24-h penetration. The installation process of bare FBG sensors along open-ended aluminous pipe pile is introduced. The distribution profiles of axial stress and unit shaft resistance on inner wall and outer wall along model pile is discussed in detail. Model test results indicated that the bare FBG sensing technology was feasible to measure the axial stress of inner wall along open-ended pipe piles during penetration and horizontal loading in sand. During horizontal loading, the average values of q so /q si are 0.717 and 0.727, respectively, for model pile with penetration rate of 20 and 30 mm/min. The axial stress difference of inner wall and outer wall along model pile in penetration and horizontal loading is the result of a combination of the inside and outside soil friction as well as of the pipe in-wall shear stiffness. K E Y W O R D S bare FBG sensing technology, inner wall axial stress, open-ended pipe pile, outer wall axial stress, sand bed | INTRODUCTIONLarge-diameter pipe piles (Figure 1) are often used for the foundation in offshore engineering, due to their capacity to transfer loads of superstructures to competent soil layers. Estimation of bearing capacity of these piles requires account for not only the interaction between the pipe pile and the outer soil, but also the interplay with the inner soil. [1][2][3][4][5][6][7] The soil layer inside the pipe pile usually does not conform to the displacement of soil outside the pile in pile penetration,