Abstract
Vertical deformation profiles of subterranean geological formations are conventionally measured by borehole extensometry. Distributed strain sensing (DSS) paired with fiber-optic cables installed in the ground opens up possibilities for acquiring high-resolution static and quasistatic strain profiles of deforming strata, but it is currently limited by reduced data quality due to complicated patterns of interaction between the buried cables and their surroundings, especially in upper soil layers under low confining pressures. Extending recent DSS studies, we present an improved approach using microanchored fiber-optic cables—designed to optimize ground-to-cable coupling at the near surface—for strain determination along entire lengths of vertical boreholes. We proposed a novel criterion for soil–cable coupling evaluation based on the geotechnical bearing capacity theory. We applied this enhanced methodology to monitor groundwater-related vertical motions in both laboratory and field experiments. Corroborating extensometer recordings, acquired simultaneously, validated fiber optically determined displacements, suggesting microanchored DSS as an improved means for detecting and monitoring shallow subsurface strain profiles.
Highlights
For 0–140 and 0–240 m depths that contained the major compression layer (0–20 m), the two trends agreed with each other. These results suggest that microanchored Distributed strain sensing (DSS) could be used for monitoring vertical deformation profiles in a field setting
Pairing DSS with fiber-optic cables installed in vertical boreholes enables the acquisition of spatially continuous strain profiles of subterranean formations
The quality of DSS data is conditioned by ground–cable coupling effects that are difficult to evaluate precisely, especially in near-surface loose sediments under low confining pressures
Summary
Where φ is the soil internal friction angle; k is the lateral earth pressure coefficient; and β is the angle of the rotational failure zone (Fig. 2b). Where Fa is the interaction force between the soil and microanchor; Ff1 is the friction force; Fb is the bearing force; and N1 and N2 are the tensions or compressions provided by the unanchored cable segments, which can be calculated using the measured fiber strain:. A strain of 1% (corresponding to a Fa of 14.9 N under the current parameters) is usually taken as the maximum strain value considering the long-term working performance of the fiber-optic This strain limit can be used for determining the minimum microanchor diameter required, which is instructive for the design of cable anchors (dashed line, Fig. 4b). We will describe two examples of the application of microanchored DSS: (1) a physical model experiment to investigate the strain response of layered soil under drainage and recharge conditions, and (2) a field experiment to monitor stratum compaction in Yancheng (Jiangsu, China). Note that in addition to the experiment described above, an additional experiment having an unanchored cable as the distributed strain sensor was conducted for comparison purposes
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