particle size up to 20 cm. Hard impurities account for about 20–30%, descending in
the upper 1.3–1.5 m of the layer.
Silty clay: With an average thickness of 1.22 m and a buried depth of 1.00–4.80 m,
this layer looks grayish black due to dust ash pollution. It consists of relatively
uniform soil, of which the partial fine sand layer is 1.20 m thick and 4.00 m deep.
Pebbles: With an average thickness of 3.30–5.10 m and a buried depth of 3.20–
7.50 m, this dark gray-mottled gray layer is mainly comprised of sandstone,
quartzite and granite. Pebbles with a diameter of 20–100 mm represent 55–65%,
and sand and gravel are supplementary, as well as partially fine sand due to alluvial
flooding.
Argillaceous sandstone: The buried depth of the layer ranges from 6.20 to 8.70 m
with an average of 8.19 m. The medium-fine grained sandstones are cemented with
mud-calcium and thick layered. Fissures are not developed, and diagenesis is poor.
In the undisturbed state, the strength of the layer is high, but it rapidly reduces
because the layer is easy to soften and disintegrate when immersed in water or
exposed to sunlight. Given this, such sandstone is considered as soft rock.
According to the local geological data of Lanzhou City, the thickness of this layer is
greater than 100 m.
(h) Hydrogeological conditions
The main object of this study is confined water. The groundwater of the demonstration site is formed by pore water of Quaternary loose rocks in Grade I terrace of
the Yellow River Valley. The water-bearing rock layer is comprised of gravel
pebbles. The aquifer is about 3.5 m thick, and the depth to water table ranges from
1.70 to 2.00 m and descends from south to north. With moderate water abundance,
the single-well water inflow reaches 300–600 m
3 /d. Of the aquifer, the top is a
1.20 m-thick layer of silt or silty clay, and the bottom is comprised of Neogene
argillaceous sandstone, which serves as water-repellent plate.
The groundwater of the demonstration site is mainly recharged by precipitation
via infiltration and by lateral groundwater runoff from high terrace on the south
bank of the valley. The water-table elevation reaches 1534.50–1538.75 m.
Generally, the groundwater flows from south to north, that is from the trailing edge
of the valley to the Yellow River, with hydraulic gradient of 8–10‰.
Classified into the infiltration-runoff type, the groundwater dynamics are mainly
affected by vertical infiltration and runoff conditions. The high water level appears
mostly during September–December while the low water level occurs during
March–June. The variance in the groundwater level is smaller than 0.5–1.0 m,
which indicates stable groundwater level over the years.
Through pumping test in steady-state porous flow model, the permeability
coefficient of the Quaternary loose aquifer in the demonstration site is calculated to
be 75.40 m/d, using the Dupuit formula based on two observation holes. According
to calculations based on unsteady-state water table data of observation holes, the
specific yield of the aquifer water supply is 0.17.
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189
the upper 1.3–1.5 m of the layer.
Silty clay: With an average thickness of 1.22 m and a buried depth of 1.00–4.80 m,
this layer looks grayish black due to dust ash pollution. It consists of relatively
uniform soil, of which the partial fine sand layer is 1.20 m thick and 4.00 m deep.
Pebbles: With an average thickness of 3.30–5.10 m and a buried depth of 3.20–
7.50 m, this dark gray-mottled gray layer is mainly comprised of sandstone,
quartzite and granite. Pebbles with a diameter of 20–100 mm represent 55–65%,
and sand and gravel are supplementary, as well as partially fine sand due to alluvial
flooding.
Argillaceous sandstone: The buried depth of the layer ranges from 6.20 to 8.70 m
with an average of 8.19 m. The medium-fine grained sandstones are cemented with
mud-calcium and thick layered. Fissures are not developed, and diagenesis is poor.
In the undisturbed state, the strength of the layer is high, but it rapidly reduces
because the layer is easy to soften and disintegrate when immersed in water or
exposed to sunlight. Given this, such sandstone is considered as soft rock.
According to the local geological data of Lanzhou City, the thickness of this layer is
greater than 100 m.
(h) Hydrogeological conditions
The main object of this study is confined water. The groundwater of the demonstration site is formed by pore water of Quaternary loose rocks in Grade I terrace of
the Yellow River Valley. The water-bearing rock layer is comprised of gravel
pebbles. The aquifer is about 3.5 m thick, and the depth to water table ranges from
1.70 to 2.00 m and descends from south to north. With moderate water abundance,
the single-well water inflow reaches 300–600 m
3 /d. Of the aquifer, the top is a
1.20 m-thick layer of silt or silty clay, and the bottom is comprised of Neogene
argillaceous sandstone, which serves as water-repellent plate.
The groundwater of the demonstration site is mainly recharged by precipitation
via infiltration and by lateral groundwater runoff from high terrace on the south
bank of the valley. The water-table elevation reaches 1534.50–1538.75 m.
Generally, the groundwater flows from south to north, that is from the trailing edge
of the valley to the Yellow River, with hydraulic gradient of 8–10‰.
Classified into the infiltration-runoff type, the groundwater dynamics are mainly
affected by vertical infiltration and runoff conditions. The high water level appears
mostly during September–December while the low water level occurs during
March–June. The variance in the groundwater level is smaller than 0.5–1.0 m,
which indicates stable groundwater level over the years.
Through pumping test in steady-state porous flow model, the permeability
coefficient of the Quaternary loose aquifer in the demonstration site is calculated to
be 75.40 m/d, using the Dupuit formula based on two observation holes. According
to calculations based on unsteady-state water table data of observation holes, the
specific yield of the aquifer water supply is 0.17.
5.2 Northwest China
189