# Directional Core Drilling as an Approach to Reduce Uncertainty in Tunneling Construction
## 1 Introduction  
The construction of any tunnel or underground work, whether in soil or rock, is associated with a high degree of uncertainty regarding the geomechanical characteristics and stability conditions throughout all of the stages of a tunnel project [1, 2]. In this sense, qualitative and quantitative assessment of tunnels has attracted a great deal of interest in recent decades [3], with the idea of integrating the ground conditions into a holistic analysis system for the construction phase of a tunnel. Many case studies have been assessed to define the costs and characteristics of the construction phase of a tunnel [4, 5, 6, 7, 8, 9, 10, 11, 12], finding the ground characteristics to be the most relevant factor. Despite the amount of research completed to date, each case has its particularities related to the results and conclusions extracted, due to the uncertainty associated with subsurface excavations, making it difficult to obtain a generalized approach. One of the existing techniques that can provide the most relevant information, and reduce the uncertainty, is to create boreholes that intercept the tunnel route which are used to determine the degree of fracturing, the characteristics of these fractures, the presence of water, and the intact rock features [13]. The information obtained from the subsurface has inspired several authors. For instance, [PERSON] and [PERSON] [14] presented an approach for determining the cost of a tunnel based on the GSI, while [PERSON] et al. [2] made an interesting adaptation of the study by [PERSON] [6] to estimate the costs generated by the excavation and support processes as a function of the ground conditions and excavation diameter. On the other hand, [PERSON] et al. [12] presented an empirical approach but with an important dispersion due to the geological uncertainty faced in tunneling. [PERSON] et al. [15, 16] indicated that the geotechnical independent parameters that define a tunnel's support requirements,and therefore the associated costs, comprise the GSI, rock mass deformation, depth, and cross-section excavated. However, the traditional approach to determining the relevant parameters for vertical or inclined boreholes only provides knowledge on specific parts of a future tunnel. Such approaches are unable to obtain information on the entire route of a tunnel without incurring excessive costs and impacting a project's viability. This fact creates an important remaining uncertainty that can influence the total cost of a tunnel and the finishing deadline due to low excavation performance [17] or TBM entrapments [18], among other setbacks. It also leads to potential problems in terms of the physical stability of the construction and environmental and safety issues [8; 19; 20; 21; 22; 23]. Therefore, it is crucial to define a general and flexible system for analyzing costs in the construction phase of a tunnel and, therefore, reduce the uncertainty of the project whenever possible [24]. Directional core drilling (DCD) is a technique that can help to improve the subsurface knowledge in a tunnel construction project. It combines two pre-existing and well-known techniques: drilling with core recovery and directional drilling [25; 26; 27]. These techniques have been applied successfully in mine exploration in recent decades [28; 29], as well as in recent tunneling projects [30; 31; 32]. The techniques permit drilling up to a few kilometers with directional changes [33] and are especially relevant in fault areas and the portals of tunnels. On the other hand, horizontal core drilling (HCD) has also been used for tunnel characterization, and it requires the machine to be placed at the same level of the tunnel as an advanced or parallel tunnel [34; 35; 36]; therefore, it requires a previous excavation stage. However, the DCD system allows for testing along the planned tunnel trajectory from the surface or another non-parallel placement, and it is able to characterize the geological and geomechanical properties of the rock mass to be traversed.