The effective stresses in saturated soils are crucial for geotechnical engineering, particularly in the ocean environment, but no current transducers can directly measure both vertical and lateral effective stresses. Thus, a novel effective stress transducer based on fiber Bragg grating (FBG) technology is developed to directly measure three-dimensional (3D) effective stress in saturated soils. The design of the transducer ensures that pore water pressures inside and outside the transducer are balanced, allowing the strain to solely reflect the effective stress sustained by the soil skeleton. Two FBG sensing elements of the 3D effective stress transducer are designed to measure the vertical and lateral effective stresses by sensing the strain in the thin plate and the sensing cylindrical shell through the porous disk, respectively. Experimental results indicate that the transducer accurately captures the evolution of effective stress under complex static loads and precisely tracks cyclic stress variations under cyclic loadings. Compared to traditional transducers, the lateral earth pressure coefficient derived from the measurement data of the new effective stress transducer shows advanced accuracy and stability. Moreover, the FBG-based transducer effectively monitors effective stress changes during the excavation, capturing soil stress variations and enabling precise excavation stability assessments. The novel 3D FBG-based effective stress transducer offers a vital method for directly measuring the vertical and lateral effective stresses of saturated soils.

Underground mine pillars provide natural stability to the mine area, allowing safe operations for workers and machinery. Extensive prior research has been conducted to understand pillar failure mechanics and design safe pillar layouts. However, limited studies (mostly based on empirical field observation and small-scale laboratory tests) have considered pillar-support interactions under monotonic loading conditions for the design of pillar-support systems. This study used a series of large-scale laboratory compression tests on porous limestone blocks to analyze rock and support behavior at a sufficiently large scale (specimens with edge length of 0.5 m) for incorporation of actual support elements, with consideration of different w/h ratios. Both unsupported and supported (grouted rebar rockbolt and wire mesh) tests were conducted, and the surface deformations of the specimens were monitored using three-dimensional (3D) digital image correlation (DIC). Rockbolts instrumented with distributed fiber optic strain sensors were used to study rockbolt strain distribution, load mobilization, and localized deformation at different w/h ratios. Both axial and bending strains were observed in the rockbolts, which became more prominent in the post-peak region of the stress-strain curve. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

The mining of deep underground coal seams induces the movement, failure, and collapse of the overlying rock-soil body, and the development of this damaging effect on the surface causes ground fissures and ground subsidence on the surface. To ensure safety throughout the life cycle of the mine, fully distributed, real-time, and continuous sensing and early warning is essential. However, due to mining being a dynamic process with time and space, the overburden movement and collapse induced by mining activities often have a time lag effect. Therefore, how to find a new way to resolve the issue of the existing discontinuous monitoring technology of overburden deformation, obtain the spatiotemporal continuous information of the overlying strata above the coal seam in real time and accurately, and clarify the whole process of deformation in the compression-tensile strain transition zone of overburden has become a key breakthrough in the investigation of overburden deformation mechanism and mining subsidence. On this basis, firstly, the advantages and disadvantages of in situ observation technology of mine rock-soil body were compared and analyzed from the five levels of survey, remote sensing, testing, exploration, and monitoring, and a deformation and failure perception technology based on spatiotemporal continuity was proposed. Secondly, the evolution characteristics and deformation failure mechanism of the compression-tensile strain transition zone of overburden were summarized from three aspects: the typical mode of deformation and collapse of overlying rock-soil body, the key controlling factors of deformation and failure in the overburden compression-tensile strain transition zone, and the stability evaluation of overburden based on reliability theory. Finally, the spatiotemporal continuous perception technology of overburden deformation based on DFOS is introduced in detail, and an integrated coal seam mining overburden safety guarantee system is proposed. The results of the research can provide an important evaluation basis for the design of mining intensity, emergency decisions, and disposal of risks, and they can also give important guidance for the assessment of ground geological and ecological restoration and management caused by underground coal mining.

This study presents a new fully coupled thermal -hydraulic -mechanical (THM) model for variably saturated freezing soil, which examines the freeze-thaw (F -T) actions. The model is derived based on the general form of continuum mechanics for porous media. The mass balance equations cover the conservations of the total water and dry air, where liquid water, ice, and vapor are involved in the total water balance equation. The effective stress law for the unsaturated frozen soil is included in the model to quantify poromechanical behaviors. The pore pressure contains components from pore water pressure, pore air pressure, and ice pressure. A new model for characterizing the unfrozen water content based on temperature and air -water capillary pressure is proposed. The THM formulation is based on multidimensional derivation, thus is versatile to be extended to cases including warm temperature conditions or large deformation behavior. The model was implemented in a 2D finite element package and validated by a set of published laboratory experimental data. The numerical code is also applied to simulate the freeze-thaw actions in highly unsaturated loess located in the northwest of China, where the quasidistributed fiber optic sensing data is collected for field -scale validations. Our simulated thermal -hydromechanical responses match well with in situ monitored results and confirm that freezing -induced heaving is still significant in such highly unsaturated soil.

The deformation of foundation soil caused by freeze-thaw cycles is a typical geological disaster in engineering construction in permafrost areas. Fiber optic sensing technology provides an important technical means for accurate and distributed real-time monitoring of frozen soil deformation. To explore the feasibility of distributed fiber optic strain sensing in monitoring frozen soil deformation, this study utilized a self-developed optical cable-frozen soil interface mechanical characteristics tester to investigate the failure mechanism of the cable-soil interface in frozen soil samples with different dry densities and initial water contents. The experimental results indicate that the fiber optic strain monitoring results accurately reflect the progressive failure characteristics of the cable-soil interface, and the strain softening model can better describe the mechanical properties of the interface. During the freezing process, the liquid water in the soil becomes ice, causing the movement of the freezing front and water migration, and resulting in significant differences in the mechanical properties of the interface. The evolution process of the shear stress at the cable-soil interface at different depths reflects the deformation coordination state with the frozen soil during the cable pullout process, indicating that the measurement range of the cable and the coupling of the interface are closely related to the dry density and initial water content of the soil. This study provides a reference for the application of optical fiber sensing technology in deformation monitoring of frozen soil foundation in cold regions.