Prestressed concrete cylinder pipes (PCCPs), which are composite pipes, are widely used in cross-basin water transfer projects and urban underground pipe network construction owing to their low cost, long life, high pressure-bearing capacity, and ease of construction and installation. However, in the course of long-term service, PCCPs may undergo damage and fail to different degrees due to the combined effect of external loads, ion erosion in the soil, and uneven settlement of the foundation. Hence, long-term monitoring is essential for the safety evaluation and risk assessment of pipelines. In this study, a prototype centralized filament-breaking damage test was performed on a large-diameter embedded PCCP with an inner diameter of 3.4 m and a length of 5 m, revealing a correlation between the number (proportion) of broken filaments and the extent of PCCP damage. The results showed that the maximum wire breakage ratio of the PCCP specimen was approximately 20% under the design internal pressure. Meanwhile, a safety monitoring method for PCCPs was established based on distributed acoustic sensing, which helps monitor destructive events, such as wire breakage and retraction, and cracking of the mortar protective coating and core concrete during PCCP operation. Moreover, the vibration signal pattern was analyzed, and its characteristics were deciphered, providing a new means of monitoring the long-term operational safety of PCCPs and providing early warning.

Distributed acoustic sensing (DAS) technology applied to telecommunication optical fiber networks offers new possibilities for structural health monitoring. The dynamic responses of five bridges are extracted along a 24-km long optical fiber crossing the Lyon metropolitan area in France. From their characteristics signals, three physical parameters informing on the health of structures have been determined: vibration frequencies, damping and modal shapes. The fiber measurements are in agreement with velocimetric data serving as reference. The telecom optical fiber records the dynamic response of bridges in several directions and thus allows the reconstruction of 3D deformation modes using their orthogonality properties. Time tracking of frequencies, commonly used to assess structural integrity, shows that the average values of natural frequencies vary cyclically between day and night. The increase in frequencies during the night does not exceed 2% and probably reflects an overall stiffening of the structures due to the drop in temperature. The telecom fiber allows to obtain deformation and damping identity of structures, highlighting soil-structure coupling between the bridge and underlying soil. This study shows that it is possible to assess the spatial and temporal variability of bridge dynamic response from DAS data using existing fiber networks.

Shallow sediments can respond non-linearly to large dynamic strains and undergo a subsequent healing phase as the material gradually recovers following the passing of seismic waves. This study focuses on the physical changes in the subsurface caused by the shaking from a buried chemical explosion detonated in a borehole in Nevada, USA, as a part of the Source Physics Experiment Phase II. The explosion damaged the shallow subsurface and modified the frequency content recorded by 491 geophones and 2240 Distributed Acoustic Sensing (DAS) channels within 2.5 km from surface ground zero. We observe a gradual shift of resonance frequencies in the 10-25 Hz frequency band in the hours following the explosion and develop a method to characterize the related logarithm-type healing process of the shallow (i.e., upper similar to 25 m) subsurface. We find that stronger levels of ground motion increase the relative degree of damage and duration of the subsurface healing; with the spall region exhibiting the largest degree of damage and longest healing recovery time. We observe coherent spatial patterns of damage with the region located to the southeast of the explosion exhibiting more damage than the southwest region. This study demonstrates that both DAS and co-located geophones capture similar temporal changes associated with the physical processes occurring in the subsurface, with the high-density sampling of DAS measurements enabling a new capability to monitor the fine-scale changes of the Earth's shallow subsurface following the detonation of a buried explosion. Strong seismic waves can damage the soft sediments that compose the shallow layers of the ground. A healing phase of the sediments generally follows the passing of the seismic waves as the medium gradually recovers with time. We study the spatio-temporal response of the subsurface in the vicinity of a large buried chemical explosion that was detonated in a borehole at the Nevada National Security Site, USA. The explosion, which was part of the Source Physics Experiment Phase II, was well instrumented along a surface fiber-optic cable with Distributed Acoustic Sensing (DAS) and hundreds of geophones. We find that the explosion, which generated a spallation of the shallow Earth, primarily damaged the upper similar to 25 m of the subsurface. We characterize the healing of the sediments and find a correlation between the duration of the healing phase and the level of maximum shaking. The high density of sensors also allows us to study spatial variations in the response of the shallow subsurface. This study demonstrates that both DAS and geophone continuous data similarly capture the spatio-temporal variations of the Earth's physical properties following strong ground motions, with DAS enabling meter-scale measurements of the subsurface changes. Shallow subsurface damage and subsequent healing caused by a buried chemical explosion are constrained with DAS and geophone data The explosion caused a relative drop of the average S-wave velocity in the Earth's shallow layers of a few percents The logarithm-type healing process of the subsurface exhibit a longer duration within the spall region

Permafrost degradation is rapidly increasing in response to a warming Arctic climate, altering landscapes and damaging critical infrastructure. Solutions for monitoring permafrost thaw dynamics are essential to understand biogeochemical feedbacks as well as to issue warnings for hazardous geotechnical conditions. We investigate the feasibility of permafrost monitoring using permanently installed fiber-optic seismic networks. We conducted a 2-month seismic monitoring campaign during a controlled thaw experiment using a permanent surface orbital vibrator (SOV) and a 2D-array of distributed acoustic sensing (DAS) cables, and observed significant (15%) shear-wave velocity (V-s) reductions and approximately 2 m depression of the permafrost table beneath the heating zone. These observations were validated by time-lapse horizontal-to-vertical spectral ratio (HVSR) analysis from three co-located broadband seismometers. The combination of SOV and DAS provided unique seismic observations for permafrost monitoring at the field scale, as well as a basis for design and development of early warning systems for permafrost thaw.