deposits of the river valley, which forms a water-repellent plate and also the
basement of the Lanzhou Yellow River valley basin.
Quaternary (Q) stratum: The Holocene (Q4al) stratum is comprised of alluvial
deposits of Grade I terraces of the Yellow River Valley. It has a binary structure.
The horizontal layer of silty clay on the top looks light yellow, brownish yellow or
yellowish brown. Generally 3–5 m thick, the silty clay is loose and porous, and
contains calcium mixed with roots, humus, root holes and wormholes. The pebble
layer on the bottom is blue gray and gray black. The diameter of pebbles ranges
from 2 to 150 mm and numbers 20–50 mm for 55–60% of the pebbles. The pebbles
mainly contain granite, meta- sandstone and quartzite. They are well rounded,
mostly sub-circular and easy to differentiate. Fine sand is also filled, which is
relatively loose and generally 3–5 m thick.
(f) Geological features
From the geological perspective, the site is situated in the eastern part of the early
Caledonian orogenic belt in the middle Qilian Mountain which is classified into the
Qilianshan fold belt. The early Caledonian fold belt extends eastward from Akesai
and Subei to the east of Lanzhou through the Danghe River, the upper Sherteng
River, Shule Nanshan, Datong Nanshan, and northwest Xining. Folds were first
raised after a set of pre-Sinian metamorphic rock formations were deposited. At the
end of the Sinian system, the entire Qilian Mountains were raised with folds.
During the Cambrian, Ordovician, Silurian and Devonian periods, this fold belt is
generally uplift without subsidence. During the Carboniferous, Permian, Triassic
and Jurassic periods, the fold belt partially subsided and became subject to sedimentation. In the eastern part During late Jurassic period, large-scale subsidence
occurred in the eastern part of the fold belt. This gave rise to some larger basins that
received thick early Cretaceous red deposits, represented by the Lanzhou-Hekou
Basin. As a result, the underlying early Cretaceous layer in this area is widely
exposed. After the Cretaceous, the Hetou area was partially uplifted and subject to
denudation under the impact of the late orogenic movement, and only some of the
depressions accepted the Tertiary red deposits. Therefore, the underlying upper
Cretaceous layer is missing in the site, while the underlying Tertiary layer is found
the in some areas. After the Tertiary period, due to intense and frequent new
tectonic movements, multi-step valley terraces were formed in this area, and
Quaternary loose sediments were widely deposited. For example, loess widely
covers the underlying layers of the Tertiary and previous periods.
(g) Engineering-geological conditions
The strata in the site are roughly divided into four layers:
Miscellaneous fill: With an average thickness of 1.30 m, this layer consists of
different soils, which includes predominantly pebbles and fly ash and a small
amount of silt. Concrete construction debris are also observed, with the maximum
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5 Application Cases
basement of the Lanzhou Yellow River valley basin.
Quaternary (Q) stratum: The Holocene (Q4al) stratum is comprised of alluvial
deposits of Grade I terraces of the Yellow River Valley. It has a binary structure.
The horizontal layer of silty clay on the top looks light yellow, brownish yellow or
yellowish brown. Generally 3–5 m thick, the silty clay is loose and porous, and
contains calcium mixed with roots, humus, root holes and wormholes. The pebble
layer on the bottom is blue gray and gray black. The diameter of pebbles ranges
from 2 to 150 mm and numbers 20–50 mm for 55–60% of the pebbles. The pebbles
mainly contain granite, meta- sandstone and quartzite. They are well rounded,
mostly sub-circular and easy to differentiate. Fine sand is also filled, which is
relatively loose and generally 3–5 m thick.
(f) Geological features
From the geological perspective, the site is situated in the eastern part of the early
Caledonian orogenic belt in the middle Qilian Mountain which is classified into the
Qilianshan fold belt. The early Caledonian fold belt extends eastward from Akesai
and Subei to the east of Lanzhou through the Danghe River, the upper Sherteng
River, Shule Nanshan, Datong Nanshan, and northwest Xining. Folds were first
raised after a set of pre-Sinian metamorphic rock formations were deposited. At the
end of the Sinian system, the entire Qilian Mountains were raised with folds.
During the Cambrian, Ordovician, Silurian and Devonian periods, this fold belt is
generally uplift without subsidence. During the Carboniferous, Permian, Triassic
and Jurassic periods, the fold belt partially subsided and became subject to sedimentation. In the eastern part During late Jurassic period, large-scale subsidence
occurred in the eastern part of the fold belt. This gave rise to some larger basins that
received thick early Cretaceous red deposits, represented by the Lanzhou-Hekou
Basin. As a result, the underlying early Cretaceous layer in this area is widely
exposed. After the Cretaceous, the Hetou area was partially uplifted and subject to
denudation under the impact of the late orogenic movement, and only some of the
depressions accepted the Tertiary red deposits. Therefore, the underlying upper
Cretaceous layer is missing in the site, while the underlying Tertiary layer is found
the in some areas. After the Tertiary period, due to intense and frequent new
tectonic movements, multi-step valley terraces were formed in this area, and
Quaternary loose sediments were widely deposited. For example, loess widely
covers the underlying layers of the Tertiary and previous periods.
(g) Engineering-geological conditions
The strata in the site are roughly divided into four layers:
Miscellaneous fill: With an average thickness of 1.30 m, this layer consists of
different soils, which includes predominantly pebbles and fly ash and a small
amount of silt. Concrete construction debris are also observed, with the maximum
188
5 Application Cases