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.