With the increasing construction of ultra-long tunnels, complex and highly variable geological conditions along tunnel alignments pose major challenges to geological investigation, design, and construction. Conventional vertical drilling mainly provides discrete vertical geological profiles and is limited in its ability to continuously characterize lithological variations, fracture zones, and groundwater conditions along the tunnel axis. To overcome this limitation, this study proposes an integrated investigation approach based on horizontal directional drilling (HDD) for continuous along-axis geological exploration. Using the Tianshan Shengli Tunnel as the geological setting, the technical advantages of HDD for tunnel investigation—including ultra-long reach, ultra-high accuracy, high penetration rate, and strong adaptability—are first summarized. An integrated investigation method is then developed by combining HDD with targeted borehole coring, hydraulic fracturing, comprehensive borehole logging, and borehole TV imaging. A 2271 m long investigation borehole was drilled along the tunnel axis from the portal section. As a result, precise directional control limited the maximum deviation between the HDD borehole trajectory and the tunnel axis to only 6.32 m. Meanwhile, the lag distance between cuttings was determined through theoretical calculations to reconstruct the true borehole positions corresponding to the collected cuttings. Based on XRD mineralogical analysis, macroscopic observations, and preliminary investigation results, the lithology of the tunnel surrounding rock was delineated with high resolution. In addition, daily borehole inflow was monitored, and tunnel inflow during construction was predicted using the groundwater dynamics method and an empirical railway relationship, yielding an expected normal inflow of 4016.6 m3/d and a maximum inflow of 12,049.8 m3/d; furthermore, borehole TV footage was used to accurately locate inflow points and intervals with well-developed joints and fractures within the surrounding rock. Highlights This study proposes an HDD–downhole geophysics method for tunnel investigation, classifies surrounding-rock lithology from cuttings and cores, and predicts tunnel construction inflow from HDD borehole inflow monitoring data.
Zhao et al. (Tue,) studied this question.