Characterization of the 3‐D fracture setting of an unstable rock mass: From surface and seismic investigations to numerical modeling

Abstract

The characterization of the fracturing state of a potentially unstable rock cliff is a crucial requirement for stability assessments and mitigation purposes. Classical measurements of fracture location and orientation can however be limited by inaccessible rock exposures. The steep topography and high‐rise morphology of these cliffs, together with the widespread presence of fractures, can additionally condition the success of geophysical prospecting on these sites. In order to mitigate these limitations, an innovative approach combining noncontact geomechanical measurements, active and passive seismic surveys, and 3‐D numerical modeling is proposed in this work to characterize the 3‐D fracture setting of an unstable rock mass, located in NW Italian Alps (Madonna del Sasso, VB). The 3‐D fracture geometry was achieved through a combination of field observations and noncontact geomechanical measurements on oriented pictures of the cliff, resulting from a previous laser‐scanning and photogrammetric survey. The estimation of fracture persistence within the rock mass was obtained from surface active seismic surveys. Ambient seismic noise and earthquakes recordings were used to assess the fracture control on the site response. Processing of both data sets highlighted the resonance properties of the unstable rock volume decoupling from the stable massif. A finite element 3‐D model of the site, including all the retrieved fracture information, enabled both validation and interpretation of the field measurements. The integration of these different methodologies, applied for the first time to a complex 3‐D prone‐to‐fall mass, provided consistent information on the internal fracturing conditions, supplying key parameters for future monitoring purposes and mitigation strategies.

Microseismicity of an Unstable Rock Mass: From Field Monitoring to Laboratory Testing

“…No significant changes in the parameters are observed between events occurring in dry or wet conditions. Conversely, a strong air temperature control on the peak frequencies in the Fourier spectra of the recorded events (Figure g) is identified, as already highlighted for ambient noise resonance frequencies in Colombero et al (). Air temperature and frequency fluctuations are almost in phase, with maximum frequency during the hottest hours and lower values in cold periods.…”

“…Past displacement monitoring measurements at the site (Regione, ) highlighted similar reversible modifications in fracture openings due to the seasonal thermal dilation of the rock mass. Colombero et al () detected coherent temperature‐driven seasonal reversible variations in the resonance frequencies of the unstable blocks from ambient seismic noise recordings. The detected MS activity seems therefore emphasized during the periods of rock mass thermal expansion.…”

Low-Cost GNSS Solution for Continuous Monitoring of Slope Instabilities Applied to Madonna Del Sasso Sanctuary (NW Italy)

“…In particular, fracture K4 shows an evident displacement step on the panoramic square located at the top of the cliff. The investigation found that the fracture is open up to a depth of about 16 m [21].…”

“…Lancellotta et al [18,22] made a geomechanical characterization of the site. The detailed investigation carried out also using geophysical methods by Colombero et al [20,21,27] allowed a good improvement of the knowledge of this case. In particular, Colombero et al [21] identified an open and pervasive fractures system delimiting two prone-to-fall sectors (A and B) having estimated volumes of 6000 and 6300 m 3 , respectively ( Figure 3).…”

“…The instability of the courtyard in front of the sanctuary is a factor of risk for people that visit this important cultural heritage site, but also for people that live at the base of the cliff, close to the Orta Lake. Different authors have studied the slope instability that affected the frontal part of the Sanctuary courtyard in recent years [18][19][20][21][22]. The Madonna del Sasso site is also monitored by the Agenzia Regionale per la Protezione Ambiente-ARPA (The regional environmental protection agency of Piemonte region).…”