1. INTRODUCTION
Waterlogging has become an increasingly common occurrence in recent
years alongside climate-change-induced weather extremes and aggressive
urbanization. Waterlogged structures and roads can severely negatively
impact surrounding communities. Low impact development (LID) facilities
are widely used in the United States, the United Kingdom, China, and
other countries as an effective means of urban stormwater control and
hydrological cycling (Dietz, 2007; Pyke et al., 2011; Ahiablame and
Shakya, 2016; Jia et al., 2016; Hu et al., 2017; Wu et al., 2018a; Li et
al., 2019b). Rainfall distribution is not uniform and evaporation
exceeds rainfall in collapsible loess areas of China, where LID
facilities should be designed not only to reduce the peak rainfall and
minimize rainwater pollutants but also to promote regional rainwater
circulation and effective utilization.
Collapsible loess, a water-sensitive type of soil with a sub-stable
structure, is widely distributed throughout the world (Shroder et al.,
2011; Chen et al., 2018; Lian et al., 2020). Loess soil readily
disintegrates under the action of rainfall and irrigation, which can
cause nearby structures (e.g., buildings, municipal roads) to crack and
even to collapse. Slope instability, landslides, and other geological
disasters are frequently reported in loess areas (Kozubal and Steshenko,
2015; Wu et al., 2017; Yates et al., 2018; Assadi-Langroudi, 2019). It
is crucial to effectively understand the seepage law and moistening
deformation of the infiltration rainwater of LID facilities in loess
areas to ensure the safety of the facilities and surrounding structures.
LID facilities mainly include permeable pavement, green roofs, and
bioretentions. Previous studies on LID facilities have mainly centered
on peak reduction design, non-point source pollution risk reduction, and
portfolio management (Ahiablame and Engel, 2012; Tang et al., 2016;
Eckart et al., 2017; Ma et al., 2017; Winston et al., 2018; Yekkalar et
al., 2018; Chen et al., 2019; Hou et al., 2019a; Sohn et al., 2019).
There has been little research to date on risk prevention or rainwater
detention control measures in loess area LID facilities. Academics and
professionals agree that the LID facilities in collapsible loess areas
must be specially equipped for safety; this may include shallow and
small-scale designs and full paving with impermeable membranes at the
bottom of underground facilities (Zhang, 2016; Si et al., 2018).
Wang et al. (2019) and Deng et al. (2020) studied the impact of
rainwater on loess sites in cases of impervious membrane leakage to
develop a construction optimization scheme. Their work has assisted in
designing LID facilities in collapsible loess areas. However, the
overall anti-seepage measures still have some drawbacks as the factors
that further reduce the rainwater retention time and the total amount of
infiltration and storage in the underlying surface of the facilities can
cause the detention function ineffective. Rainwater infiltration and
soil strength and deformation laws can be used to identify the
controllable critical range of rainwater infiltration in the
construction of LID facilities in loess areas. The numerical simulation
Geo Studio has been used to analyze the seepage field and displacement
field of different bioretention facilities in collapsible loess areas
(Chai et al., 2019; Liang et al., 2020) and ultimately to optimize the
length of underground bioretentions in various loess areas. The function
of LID facilities in the loess area can be ensured using the methods
described above. However, as a unique unsaturated soil, the infiltration
mechanism of collapsible loess is not yet fully understood. The
infiltration of loess is affected by initial water content, temperature,
and pore characteristics (Wang et al., 2010; Haeri et al., 2012; Li and
Li, 2017; Shao et al., 2018; Li et al., 2019a; Zhang et al., 2019).
These characteristics are difficult to simulate numerically because the
parameters and the calculation model of water movement law cannot
effectively reflect water movement through loess; besides, the
infiltration boundary conditions are unrealistic. Compared with the
numerical simulation, the field test study can obtain the actual
infiltration law of water in Loess more accurately.
Many previous researchers have investigated the law of water
infiltration in loess. The depth of water infiltration is limited in
collapsible loess sites if diversion measures are not taken. The
influence of concentrated infiltration in loess areas on water content
is more intense than that in natural rainfall infiltration areas (Tu et
al., 2009; Huang et al., 2011; Min et al., 2017; Wu et al., 2018b; Hou
et al., 2019a, 2020). However, previous studies have centered on the
water infiltration law of single-layer collapsible loess. The structures
of LID facilities vary but are mainly composed of replacement fill and
gravel layers. LID facilities are also often arranged around municipal
roads or buildings, which makes these sites vulnerable to the initial
stress field of the subgrade. The law of water infiltration in a single
loess site does not accurately represent changes in the seepage field
and deformation field of the site after water infiltrates the facility.
A typical LID facility construction project in the loess area of Shaanxi
Province, China, was taken as the engineering background for this study.
Previous optimization results regarding the length of the anti-seepage
membrane at the bottom of a typical bioretention, also a
Q3eol collapsible loess site was used
to selected a full-scale rainwater infiltration test of bioretention
facilities near municipal roads under the worst-case rainfall intensity.
The law of water infiltration was determined accordingly. Consolidation
compression test data was used to calculate the wetting settlement of
the site. The applicability of the modified Green-Ampt model in
predicting the infiltration depth of LID facilities in the loess area
was verified as discussed in detail below.