A Modified Polyvinyl Alcohol Additive for Improving Borehole Cleanliness and Stability in Deep‐Buried Tunnel Investigation

ABSTRACT Horizontal directional investigation (HDI) is a novel method for assessing the engineering geological characteristics of deep‐buried tunnels in mountainous areas. However, HDI technology faces challenges in cuttings removal and borehole wall stability, which limit its widespread use. A multifunctional additive PH‐VA was synthesized by modifying polyvinyl alcohol (PVA) with a polyacrylamide derivative (PH‐7) to cope with these challenges. The effects of PH‐VA on the rheological and wall‐fixing properties of the solid‐free flushing fluids were comprehensively evaluated by rheological behavior analysis, rheological model fitting, rheological parameter analysis, mud ball immersion test, linear expansion test, and compressive strength test. The results showed that PH‐VA could act as both the rheology modifier and wall‐fixing agent. PH‐VA significantly improved the rheological properties of the flushing fluid, such as shear thinning, low shear rate viscosity, thixotropy, etc., while maintaining good wall‐fixing properties of the flushing fluid. In addition, the structural features and modification mechanism of PH‐VA were analyzed using Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), X‐ray diffraction (XRD), and environmental scanning electron microscopy (ESEM). The analysis results indicated that an ester exchange reaction occurred between PVA and PH‐7, and the molecular chains of PVA and PH‐7 formed a network structure by entangling with each other. The results of this study provide a new solution to the problems of low borehole cleanliness and poor borehole wall stability during HDI drilling, which is conducive to promoting the application of HDI technology.