With the implementation of national strategies for energy, transportation, and water network development, China""s infrastructure construction has witnessed vigorous growth, establishing the country as the global leader in terms of quantity, scale, and technical complexity of underground engineering projects. Notably, numerous deep engineering projects are located in the mountainous regions of western China characterized by extreme topographical and geological conditions, where construction faces extreme geological environments featuring high altitude, deep burial depths, high stress, high water pressure, high ground temperature, and significant elevation differences. This necessitates the adoption of refined geological investigation methodologies. Existing technical specifications in water resources, power, railway, and transportation sectors prove inadequate for addressing engineering investigation requirements in these complex and hazardous western mountainous areas. Over the past decade, PowerChina Chengdu Engineering Corporation Limited has leveraged its practical experience from major hydropower projects and multiple large-scale pumped storage projects in western China. Through collaborative research with universities and scientific institutions, the corporation has achieved technological breakthroughs in directional drilling for deep engineering applications and wireline coring in thick overburden layers. These innovations have yielded multiple patented technologies, accompanied by the completion of over 20 vertical deep boreholes exceeding 1,000 meters, more than 40 overburden coring boreholes surpassing 300 meters in depth, and 12 ultra-deep directional boreholes. The cumulative drilling workload has surpassed 50,000 meters, providing foundational data for formulating three industry standards: Technical Code for Directional Drilling in Engineering Investigation, Technical Code for Deep Borehole Drilling in Engineering Investigation, and Technical Code for Wireline Core Drilling in Overburden Layers. The establishment and promulgation of these three engineering investigation standards have significantly advanced technical capabilities in deep geological exploration for major infrastructure projects.