Nonparametric Monitoring for Geotechnical Structures Subject to Long-Term Environmental Change

Figure 6

The procedures of the single sensor analysis using the empirical mode decomposition (EMD) and the Hilbert-Huang transform (HHT) compared with the precipitation records that are separately measured at a weather station near the retaining wall site. Note that the precipitation data were not used in the time-frequency analysis. (a) The one-month-long raw tilt time history shows the dominant daily trend mixed with various nonlinear, non-stationary events due to unknown factors over time. The material properties of reinforced concrete and backfilled soil were unknown. The raw sensor data are too complicated to understand what happens to the wall. (b) The daily trend was disentangled from the complex raw signal using the EMD. (c) The disentangled daily trend was processed using the HHT to obtain instantaneous frequencies over time. The baseline frequency remains at one per day, but some peaks were observed occasionally. (d) Precipitation records measured separately at a weather station near the wall site were compared in the same time scale. Concurrence was observed between the peaks in the instantaneous frequencies and precipitation records. The peak of the instantaneous frequency increased when precipitation began and decreased when precipitation stopped, which implies that the drainage system is performing satisfactorily.

(a) Time history of the raw slope signal

(b) Time history of disentangled daily trend using EMD

(c) Instantaneous frequency of the daily trend using HHT

(d) Precipitation at weather station wall near site