Abstract: Hydropower projects, pile foundations of bridges and tall buildings in western China are primarily constructed on deep overburden layers within high mountainous valleys. To accurately determine the physical and mechanical parameters of these deep overburden layers, in-situ borehole testing technology has gained significant attention. This paper systematically reviews the current research status and achievements from the aspect of the test instrument, test technology and process, theoretical analysis and data interpretation of various in-situ testing techniques for deep overburden drilling, including static cone penetration test, pressuremeter test, borehole shear test, dynamic penetration test, and standard penetration test. The findings indicate that the main advantages of static cone penetration test are continuous rapid testing and high data accuracy, and its disadvantages include difficulty penetrating gravel soils and an inability to observe soil layers directly. The combined drilling-penetration exploration method and multi-casing penetration technology can effectively obtain cone-tip resistance, side friction resistance, and permeability for dense silt, sand, gravel, pebble and cobble layers in deep overburden. The pressuremeter test can measure in-situ mechanical parameters at different depths, but results are strongly affected by the pore-forming quality, with a lower testing precision in soft soil. Compared to pre-drilling and press-in pressuremeter methods, self-boring pressuremeter minimizes disturbance to the borehole walls, effectively preventing collapse in non-cohesive soils and shrinkage in cohesive soils within deep overburden layers, thereby rapidly and accurately obtaining static lateral pressure, plastic pressure, ultimate pressure, and lateral pressure modulus. The main advantage of the borehole shear test is that the strength parameter is measured under the natural stress state of the overburden, but the main disadvantage is that the shear mechanism and drainage conditions are not easy to control. Borehole shear tests are suitable for saturated fine-grained soils in deep overburden, with results closely approximating consolidated undrained shear strength parameters. The main advantages of dynamic penetration test and standard penetration test are wide application range and identification of sand liquefaction, but the main disadvantage is that the transmission of hammer energy is not easy to determine. In deep overburden at considerable depths, the relationship between dynamic penetration and standard penetration hammer numbers is nonlinear, necessitating appropriate correction of hammer numbers through monitoring hammer energy. Current challenges include a lack of in-situ drilling testing technologies and robust data-interpretation methods for deep overburden characterized by high-stress levels, complex structures, and overconsolidation. It is recommended to develop multifunctional in-situ testing equipment, integrate various in-situ testing technologies, and enhance the multi-source data correlation analysis of test parameters using machine learning. This approach is an effective solution to the problems.

Key words: deep overburden layer, static cone penetration test, self-boring pressuremeter test, borehole shear test, dynamic penetration test, standard penetration test