Engineering geological characterization of clayey diatomaceous earth deposits encountered in highway projects in the Tengchong region, Yunnan, China

A type of clayey diatomaceous earth of Pliocene Mangbang Formation (N2m) was encountered in highway projects in the Tengchong region of Yunnan Province, Southwest China. The Tengchong clayey diatomaceous earth is characterized using multiple test methods, such as granulometric analysis, chemical analysis, X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis, shrinkage and swelling tests, unconfined or uniaxial compression test, triaxial test and direct shear tests. The chemical and mineralogical compositions, physical and hydraulic properties and engineering properties of the clayey diatomaceous earth are presented in this paper. It is revealed that the Tengchong clayey diatomaceous earth is an unusual soil or rock which has characteristics of both typical diatomaceous earth and swelling clayey soil or soft clay rock. The diatoms in the earth enhance the connection between micro-structures, and improve the mechanical properties of diatomaceous earth. However, due to the existence of a significant amount of swelling clay minerals, the Tengchong clayey diatomaceous earth becomes swelling soft rock, and is prone to engineering problems or geohazards. Based on the test results, a number of issues regarding classification, discrimination and geohazard control related with the clayey diatomaceous earth are discussed to provide both some basic understanding and new insight into the characteristics of the clayey diatomaceous earth deposits.