Abstract

In August 2021, landslides occurred on the slope of the Cortinas sector over an area of more than three kilometers, which affected electrical towers, a national vehicle highway, an oil pipeline and a gas pipeline. Currently, these infrastructures have limitations in their operation, generating significant economic losses and therefore, in the case of the gas pipeline, the construction of a HDD of two thousand meters in length at depths of up to one hundred meters is proposed, with the purpose of being able to bypass the zones unstable and restore transportation to normal conditions. The design and construction of said work constitutes a challenge, since the project area has very special geological conditions, since the hillside deposits are very susceptible to failure and there is apparently a stress tensor of an active fault that directly affects the stability of the area. The projected HDD crosses a ravine and a slope with steep topography with difficult access, as well as different layers of sedimentary rock with intercalations, which are folded and highly fractured and saturated with water. These special conditions generated difficulties and opposed the completion of several attempts by other HDDs, but taking into account that this alternative constitutes basically the only solution from a technical point of view, it was necessary to carry out some borehole and multiple seismic geophysical tests and geoelectrical that would allow defining a detailed stratigraphic profile to be able to analyze the constructive feasibility and, in such case, the most appropriate method, as well as the geomechanically characterization of the rocks, since according to the numerical modeling they indicate that the stability of the drilling may be affected due to plasticization at its limits, with detachments of rock fragments and jamming of the tools necessary for its construction.

Abstract

Using CPTu profiles for subsoil characterisation, transformation equations must be used to obtain the hydro-mechanical properties for structures and infrastructure designing. Additionally, the uncertainty and the spatial variability of measured parameters must be taken into account for a reliable geotechnical design. In this work, we used a Stochastic Simulation approach to define reliable 3D models of two geotechnical designing variables for granular soils (friction angle–’ and the Darcy permeability coefficient–k) from tip resistance (qc), sleeve friction (fs), and pore pressure (u2) profiles. The selected method – the Sequential Gaussian Co-Simulation (SGCS) – provided reliable optimized 3D models of the spatial distribution of the variables of interest and allowed quantifying the propagation of the estimation uncertainty associated with the raw measurement models through the transformation equations. Overestimation (OE) and Underestimation (UE) percentages for a confidence interval of 68% were calculated throughout the 3D model: granular soils showed a larger uncertainty than fine soils concerning the measured variables (qc, fs, and u2). In granular soils, the measured variable uncertainty varies up to 100% but the derived variables show different behavior: ’ shows UE and OE less than 25% while k reaches 100%. These differences in the propagated uncertainties depend on the transformation equations and the measured variable dependence.