Research on loess-paleosol engineering geology features from the borehole cores of a typical section in Baoji area, Shaanxi province, China

Despite the vast amount of research regarding loess-paleosol engineering geology features on the Loess Plateau in Northwest China, a completely systematic study from Holocene series (Q4) to Lower Pleistocene series (Q1) is absent. Investigation into this yet unexplored problem was done firstly by sampling undisturbed cores in a typical loess-paleosol section at Jiacun Terrace in Baoji area, Shaanxi province, China. Then physical–mechanical parameters and material formation of the cores were systematically tested in the office. The results show that there is an inversely proportional correlation between density and void ratio, and a directionally proportional correlation between liquid limit and plastic index. The results also show an inversely proportional correlation between compression modulus and compressible factor, and an inverse relationship between internal cohesion and internal friction angle. The depth of collapsible soils in the borehole is about 7.8 m, which is more accurate than published data. The analysis on grain sizes and carbonate content shows that there is a directionally proportional correlation between 0.075–0.010 mm grain content and carbonate content, both of which have inversely proportional correlations with internal cohesion and are directly proportional to internal friction angle. Testing on major clay minerals has proven that there are directly proportional relationships between <0.002 mm colloid grain content with major clay minerals and internal cohesion, as well as an inversely proportional relationship with internal friction angle. In addition, testing also shows that the dominant clay minerals in loess-paleosol series are the mixed-layer illite/montmorillonite (I/S), and are not single illite or montmorillonite or kaolinite. This testing and analyzing of undisturbed core samples from a typical loess-paleosol section is important to the exploration of landslide-triggering mechanisms and engineering construction safety.