Accurate calibration of effective stress cells using a novel chamber for monitoring effective stresses in three different boundary conditions

Abstract

Accurate monitoring of effective stresses in saturated soils is crucial for reliable safety assessments of marine geotechnical infrastructures. Although recent advance of Fiber Bragg Grating (FBG)-based effective stress cells (ESCs) enables direct measurement of effective stresses, current calibration methods of ESCs overlook boundary condition effects, potentially causing measurement errors under various working scenarios. This study develops a novel calibration chamber for more accurate calibration of ESCs in marine soil under three typical boundary conditions (BCs): (i) ESC placed on a stiff structure surface within saturated Hong Kong Marine Deposits (HKMD) (BC1), (ii) ESC flush-embedded in a stiff structure within saturated HKMD (BC2); and (iii) ESC embedded within saturated HKMD (BC3). FBG output signals of the ESC exhibited good linear correlations with both applied water pressure and effective stress in water environment and saturated HKMD environment, and the calibration factor obtained in water environment and saturated HKMD environment can be named as (Formula presented) and (Formula presented) respectively. (Formula presented) was further validated by comparing direct ESC measurements with those measured indirectly from earth pressure cells (EPCs) and pore-water pressure transducers (PPTs), showing good agreement with both indirect measurements and applied effective stress. The results showed that (Formula presented) under the BC1 was 5 % smaller than (Formula presented), while those under the BC2 and BC3 were 18 % larger, highlighting the impact of different boundary conditions. For the first time, a normalized calibration factor ((Formula presented) ) was proposed to provide the potential to obtain the calibration factor of ESCs in saturated soils quickly in different boundary conditions via calibration factor obtained in water.