Abstract: The claystones of the upper Tertiary Luoyang Formation has widespread distribution at the foundation of the Xixiayuan Project hydroelectric power station on the Yellow River. The field surveys reveal that the claystones have specific engineering geological characteristics, mainly found to exhibit weather and disintegration within a short period of time when exposed to the atmosphere or moistened, and the rock properties often change to properties with more characteristics of soil during excavation. In order to gain better understanding about the claystone of Luoyang Formation, a series of experimental laboratory studies were carried out involving in-situ large scale shear test. For some samples tested in this study, the clay mineral component of the claystone is high with maximum 53.7 % which have a mount of smectite with a range of 28 %－38 %, the swelling ratio 0.6 %－4.9%, the axial compressive strength 0.11－1.04 MPa, the shear strength (kPa). The claystone of the Luoyang Formation has properties of hard soil-soft rock.

Key words: Xixiayuan project, claystone, engineering geological property, in-situ large scale shear test