3.1 Variation of Water Content with Respect to Time
The variation of water content with respect to time at different depths
under bioretention facilities is presented in Fig. 6.
[Insert Figure 6]
In the process of concentrated infiltration of rainwater through
facilities, the volume water content curve at M and N positions at the
bottom of facilities in the loess site progresses through three stages:
initial stability (I), rapid growth (II), and slow growth (III). The
duration of stage I increases with depth. That is, the duration of stage
I at 2 m is longer than that at 1.5 m. In stage II, the water content
decreases as infiltration depth increases; it takes more than 1.5 m to
reach a stable water content at 2 m depth. In stage III, the maximum
stable water content decreases as depth increases. The stable water
content at 1.5 m, 2 m, and 2.5 m decreases in turn because the porosity
of soil decreases as depth increases, thus the saturated water content
of the soil and the stable water content both reduced.
At the bottom of the roadbed, the soil volume water content fluctuates
only slightly as the water immersion time increases. The soil moisture
content was basically ± 2% of the initial moisture content throughout
the experiment. Under the test conditions, the water infiltration of
facilities exerted little influence on the seepage field of the
subgrade.
Further, at the same depth in the retaining wall type facilities,
changes in water content at 0.5 m on the left side of the bottom center
were less intense than at 0.5 m on the right side. There was no
significant difference in the sloping type facilities. It is possible
that the construction of retaining wall facilities significantly
influences the foundation, which then affects the law of water
permeability there.
At the same depth, the rapid growth period of Bioretention I was shorter
than that of Bioretention II. The water infiltration of Bioretention I
affected the foundation water content within a shorter amount of time as
well. When the depth was 2.5 m, the water content of Bioretention I
increased from 17% to 32% over 1.5 hours, while that of Bioretention
II risen from 17% to 31% over 3.5 hours. The time also gradually
increased with the infiltration depth, which can be attributed to the
structure of the facility. Bioretention I stores more water than
Bioretention II, so it has a larger seepage height. The water content of
the site also changed more rapidly in Bioretention I than in
Bioretention II.
As water infiltrated the site, the soil became saturated once the
saturation of soil exceeded 85%. According to the relationship between
saturation and volumetric water content (Wu et al., 2018b), the average
saturated volumetric water content of the site is 46%. After 24 h of
infiltration time, the maximum water content of the soil was 39.8%
< 46%. The centralized infiltration process of the facility
in the loess field is unsaturated. Huang et al. (2015) obtained similar
experimental results.