Study on Needle Penetration Test for Estimating Mechanical Parameters of Sandy Dolomite

Sandy dolomite is a special engineering geological phenomenon in Yuxi section of Central Yunnan Water Diversion Project. According to the preliminary statistics, it is mainly the completely sandy dolomite and the severely sandy dolomite in the study area, with broken structure, high weathering degree and developed joints. Due to the special engineering characteristics of sandy dolomite, many disasters have occurred in the construction, which seriously hindered the progress of the project. However, there are few references related to the mechanical properties of sandy dolomite. In this paper, we selected the typical sandy dolomite in the Xiaopu tunnel and nearby field outcrops in Yuxi section as the study object, and needle penetration tests were carried out on completely and severely sandy dolomite. The mechanical parameters of sandy dolomite were estimated indirectly by needle penetration index (NPI), and the correlation was compared and analysed. The results show that there was a good correlation between needle penetration index (NPI) and uniaxial compressive strength (UCS). According to the estimation of uniaxial compressive strength (UCS) of needle penetration ratio, the completely and severely sandy dolomite belong to soft rock. The cohesion (c) and internal friction angle (φ) are estimated based on the needle penetration index (NPI), and the value is close to the value recommended by the design institute. Needle penetration test can estimate the mechanical parameters of sandy dolomite simply, quickly and effectively.