ENGINEERING GEOLOGICAL CHARACTERISTICS OF DEEP OVERBURDEN IN AN AIRPORT FIELD ON SOUTHERN MARGIN OF QINGHAI-TIBET PLATEAU

The Quaternary deep overburden is developed in a proposed airport site on the southern margin of the Qinghai-Tibet Plateau, and induces a great restriction on the construction of the project. Drilling data shows that the deep overburden is generally distributed in the entire field area with a thickness of more than 30m, and the maximum hole depth of 105m has not been exposed. Vertically, the deep overburden can be divided into four layers from top to bottom: Holocene debris flow accumulation layer(Q₄^sef), Holocene alluvial accumulation layer(Q₄^al+pl), Holocene lacustrine accumulation layer(Q₄^l) and Late Pleistocene Alluvial accumulation layer(Q₃^al+pl). Among them, the debris flow is mainly composed of breccia and gravel, with a maximum thickness of about 13m, distributed on the west side of the research site; the Holocene alluvial strata is composed of complex materials, mostly silty fine sand and round gravel, without obvious layering rules, with a maximum thickness of about 7m; the lacustrine sedimentary silty clay is plastic-flow plastic, with sand in some parts, and the maximum thickness is about 43m; the late Pleistocene alluvial strata are dominated by fine sand and medium-coarse sand, with pebbles and gravels locally, and the thickness is greater than 42m. The analysis shows that the cause of the deep overburden is closely related to the rapid uplift of the Himalayas since the Late Pleistocene and the interglacial climate, while the surface overburden is closely related to the warming and the short-term alternate climate of cold and warm of the Qinghai-Tibet Plateau climate since the Holocene. The test results reveal that the physical and mechanical properties of the deep overburden are obviously different, and there are major engineering geological problems including uneven settlement, seepage failure, slope stability, and foundation damage induced by freezing and thawing.