Table
1.
Mechanical composition and porosity for packed porous media
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| Citation: | Huifang Wang, Mingyu Wang. Infiltration Experiments in Layered Structures of Upper Porous and Lower Fractured Media. Journal of Earth Science, 2013, 24(5): 843-853. doi: 10.1007/s12583-013-0378-2
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Unsaturated zones often consist of both uncon solidated sediments and fractured bedrocks. Precipitation andother forms of water applied on the surface such as irrigation can infiltrate into the unsaturated zones through various porous media and fractured media, and then reach to the underlying saturated zones (Pirastru and Niedda, 2010). It is understandable that investigation of infiltration through an unsaturated zone which consists of both porous media and fractured media is important for comprehensively understanding water circulation and effectively managing groundwater resources and contamination control. Previous studies on infiltration through unsaturated zone have usually focused on top porous medium layers such as tillage soil or soil-gravel layers in their upper profile (Mastrocicco et al., 2010; Wang et al., 2008). For soil, the unsaturated hydraulic parameters could be predicted based on the capillary theory and liquid surface tension (van Genuchten, 1980; Mualem, 1976). Richards equation, the function of water movements in unsaturated porous medium, was developed to describe the unstable flow in the structured soil, such as preferential flow (Wang et al., 2009; Weiler and Naef, 2003), representative with dual-porosity model, dualpermeability model etc. (Vogel et al., 2000). The numerical methods were used to calculate and predict soil water distribution (Gerke et al., 2009).
The cubic law for saturated flow was well established for parallel-plate fractures theoretically and experimentally (Bear et al., 1992), while unsaturated water movement in fractured media has no reliable model (Wang and Bodvarsson, 2003; Liu and Bodvarsson, 2001), because they presented various complex flow forms (Or and Tuller, 2003; Doughty, 1999; Nicholl et al., 1994). Furthermore, the studies on water flow in the unsaturated fractured rock generally focused on geometry and hydraulic properties of fractures while neglecting recharge processes related to the upper boundary (Zhou et al., 2006; Xu et al., 2003; Liu et al., 2002). Additionally, the studies on infiltration through the layered structures with porous fractured media were few (Pirastru and Niedda, 2010), while there were a lot of reports about that through the layered structures with various porous media. As special heterogeneous structures, the textural interface would make water flux transfer and lead to lateral flow. Usually, for the layered structures with upper fine-lower coarse porous media, a capillary barrier develops and water cannot enter the coarse porous media till the water entry pressure heads on the interface is larger than that of coarse porous media (Ross, 1990). For the layered structures with upper coarse-lower fine porous media, a hydraulic barrier is formed and decreases water movement in layered structure (Corradini et al., 2000). When porous media was underlain by fractured rock, the effects on water movement from porous-fractured interface would differ with that from interface of fine-coarse or coarse-fine porous media, due to significantly hetero-geneous hydraulic behaviors of fractured rocks.
Thus, although the unsaturated zone with the layered structure of upper porous media and lower fractured media was widespread, studies on infiltration through these structures were almost none. In addition, research on water flow in the fractured rock seldom involved water recharge from upper porous media. Therefore, the infiltration process and mechanism through the layered structures with upper porous lower fractured media, involving the interaction of hydraulic behaviors between upper porous media and fractured media and water flow near the interface between them are quite unclear. In order to reveal the effects of the structure of porous-fractures media on water flow, the infiltration experiments of three kinds of layered structures with upper porous media and lower fractured media were conducted under constant simulative rainfall. The investigation about water flows in those structures should assist on understanding and quantifying water and solute transfer in the unsaturated zone with this type of layered structures.
The layered structures with fractured rocks (fr) and porous media were developed. The upper porous media in layered structures were sand-rock (sr) fragment mixtures and sand-soil (ss) mixtures or one of them. Sand and soil were sampled from the flood bed in Juma River and the farmland of the Fangshan District, Beijing, respectively. Rock fragments were bought from building material market, where the rock fragments are pebble in majority with size of 2–40 mm.Sand and rock fragments mixed according to their mass ratio of 1 : 2.5, and sand and soil mixed according to their mass ratio of 1 : 1. Particle size distribution of sand and soil was determined using SEDIMAT4-12 (Umwelt-Geräte-Technik GmbH, Germany) in Beijing Academy of Agriculture and Forestry Sciences (see Table 1).
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The lower fractured rocks in layered structures were simulated by cement concrete blocks. The cement concrete blocks with various shapes were assembled in the pmma cabinet to construct a cube sample with three parallel-plate fractures penetrated non-horizontally this cube and three horizontal fractures (Fig. 1a). Dip angles for three parallel-plate fractures of aa′c′c, bb′d′d, and ee′f′f in sketch diagram were 75°, 75°, and 68.3°, respectively (Fig. 1b). The fractured density and fracture rate were 0.069 4 cm-1 and 1.93% for this fractured rock with height of 61.5 cm and bottom area of 13 040.2 (115.4×113) cm2, re-spectively. Aperture and permeability of each parallel-plate fracture are listed in Table 2. Equivalent per-meability of fractured rock was calculated according to cubic law and the principle of water amount bal-ance by following formula
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(1) |
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where libi is pertinent area of each fracture vertical to infiltration direction, which is product of aperture for each fracture parallel to infiltration direction (bi) (cm) and pertinent length of those fracture extending in horizontal surface (li) (cm); ki is permeability of each fractures parallel to infiltration direction (cm·h-1); S and D are horizontal surface area (cm2) and thickness (cm) of the fractured rock respectively; Li is infiltration path of each fractures parallel to infiltration direction (cm); ρ is water density (g·cm-3); and μ is coefficient of dynamic viscosity (kg·cm-1·h-1).
Three different layered structures were designed in this study (Table 3). They were double layered porous media (28.5 cm sand-soil mixtures and 19.5 cm sand-rock fragment mixtures) with fractured rock (ss30+sr20), single sand-rock fragment mixtures with the fractured rock (sr30), and single sand-soil mixtures with the fractured rock (ss30) (Table 3). Different kinds of porous media were packed on the same fractured rock with depth of 61.5 cm layer by layer in pmma cabinet to keep homogeneity. And the packed bulk density of air-drying sr and ss were 1.92 to 1.97 g·cm-3 and 1.53 to 1.55 g·cm-3, respectively. At the same time, soil moisture sensors (SDI12-TDT, Acclima Inc., USA) were buried at various depth of porous media to measure volumetric water contents (θ) of porous medium and on the interface of porousfractured media, and the data were gathered and saved by data logger (CR1000, Campbell Sci. Inc., USA). Then the TDT data for ss and sr were calibrated by the volumetric water content of samples measured using dry oven respectively. The depths and locations of TDT in the porous media are presented in Fig. 2 and Table 3. Drainage parts of this infiltration system were three grooves with slopes on the bottom of pmma cabinet, which were same as the fractures strikes, and the positions of outflow lets were showed as a′, e′, and d′ in Fig. 1b. We applied to artificial simulation rainfall experiments for three times with rain intensity of 4.86 cm·h-1 for ss30+sr20, sr30, and ss30 in turn. The fractured rock was firstly dried for ss30+sr20 before infiltration experiment. Rainfall time was 6, 5, and 5 h for ss30+sr20, sr30, and ss30, respectively. The test equipments for infiltration experiments are presented in Fig. 3. It should be noted that there are spaces between fractured rock and four walls of pmma cabinet. In order to avoid leakage from those spaces, the spaces near the interface of porous-fractured media were filled by foam boards, and then were caulked with cement and silicon sealant (see Fig. 3b). The outside of those spaces (i.e., four sidewalls of pmma cabinet) was impervious planes.
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For ss30 and ss30+sr20 with the same upper po rous media of ss, the infiltration processes were dif-ferent from each other due to various time-varying permeability values of porous media which were affected by different interfaces. Comparing infiltration porous media and fractured rock would account explicitly the effects of fractured rock on water flow in upper porous media.
The values of rain intensity (r) for ss30 and ss30+sr20 were the same during the rainfall infiltration experiments. It was noticed that the start time of surface ponding (tp) and depth of surface ponding (h1) changing with time during experiments. By neglecting evaporation losses during the simulative precipitation, infiltration rate (i) for layered structures can be calcu-lated by the following formula
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(2) |
The infiltration rates for ss30 and ss30+sr20 were obtained using Equation (2), and they are drawn in Fig.4.The observed data showed that the start time of ponding in the surface of two layered structures had little difference, and they were 0.63 and 0.85 h, respectively for ss30 and ss30+sr20, while there still were considerable differences for varying rate of ponding depth (infiltration rate) between them. The time of wetting front reached the bottom of surface layer was taken as the division point of infiltration period for two layered structures. During the earlier stage of infiltration (0–1.5 h), the infiltration processes was almost same for the two layer structures due to the same overlying porous media. However, during the later stage of infiltration, i for ss30 decreased with time more quickly than that of ss30+sr20, and i for ss30+sr20 was 2–3 times larger than that for ss30 at the end of infiltration. Wetting front changing with time in porous media can be analyzed based on the observation data for the place of water moving in the profile of drying porous media, and mean velocity of water movement in the fractured rock for ss30 was calculated according to the thickness of fracture rock and the time difference between water reaching interface of pore-fractured media interface and seepage starting. From Fig. 4, during the earlier infiltration stage, wetting fronts in upper layer of both structures were almost same and increased with time in power functions, when water reached the interface of layered structures, the velocity of wetting front moving in creased considerably, and wetting front in the coarse porous media for ss30+sr20 increasing with time turned to a linear relation. Water movement from interface of fine-coarse porous media to the bottom of coarse porous media (20 cm) spend about 0.79 h, while it spend only about 0.39 h from the porousfractured interface to the bottom of fractured rock (61.5 cm). Obviously, the velocity of wetting front moving in FR was greater considerably than that in SR.
From the experimental data, lower coarse porous media and fractured rock would not affect infiltration in upper porous media until water got to interface of layered structures. For ss30+sr20, ponding water on the surface infiltrated into lower coarse porous media and would accumulate in this layer if drainage from lower boundary of coarse porous layer was not well. Thus, besides the effects in which there was water accumulation on the interface of fine-coarse porous media to get the water-entry pressure of coarse porous media, there was no effect on whole infiltration of the layered structures. On the other hand, the interface of porous-fractured media decreased infiltration rate considerably, which was similar to the reports that air entrapment in the profile between surface saturated layer and bottom water table decreased infiltration rate of porous media during ponding water infiltration experiment (Starr et al., 1978). The rock should prevent air in the upper porous media vented, consequently decreased effective flow cross-section, and reduced its permeability. While the fractures provided the vent of air discharge, that the amount of entrapped-air was varied made infiltration rate changed with time.
The calibrated volumetric water content (θ) changed with time in the various depths of porous media for ss30 and ss30+sr20 were obtained from TDT data, and curves of water content versus time in the profile were drawn respectively according to the time whether water content got relative stable in Fig. 5. The differences between curves of water content versus time for two layered structures changed with profile depths before water content got stable (Fig. 5). Both the start time and trend of increasing for θ of ss in the upper part (0–10 cm) of profiles were similar for ss30 and ss30+sr20. The start time of increasing for θ of ss in the middle part (10–20 cm) of profile for ss30+sr20 was earlier slightly than that for ss30 due to the variation of infiltration rate for two layered structures, and the trend of θ changing was similar. While θ of ss near the interface of layered structures were much different between ss30 and ss30+sr20, the time of start increasing and getting stable for θ of ss in the lower profile (such as 23.15 cm) for ss30 were later about 0.5 h than that for ss30+sr20. The TDT values showed the considerable differences for θ of ss on the interfaces between ss30 and ss30+sr20, θ of the former was obviously larger than that of the later (Fig. 5), which showed the interface of ss30 got relative saturation but that of ss30+sr20 had not been saturated. After water content got relative stable, θ of SS at various depths for ss30 remained increase slowly with time, while that for ss30+sr20 was kept at about 0.38 cm3·cm-3. It also can be seen from Fig. 5 that θ on the interface for ss30 was higher than that in adjacent upper measuring point, and the θ on the interface of fine-coarse porous media for ss30+sr20 got to one peak then reverted to the stable value. There showed two points for the effects of fractured rock on the water change in the profile of ss. The first one was that the start time of water content increasing on the interface of porous-fractured media was earlier than that on the interface of fine-coarse porous media, and water amount on the former was larger than that on the later. The second was that water contents at porous profile for ss30 remained increasing slowly even in the later infiltration period. Although i for ss30 was lower greatly than that of ss30+sr20 at the later stage of infiltration, the start time of increasing for θ on the interface for ss30 was earlier than that of ss30+sr20. The reasons were probably that fractured rock on the lower boundary of ss prevented water movement downward and water accumulated on the interface for ss30. The fractured rock with large equivalent permeability (4 826.2 cm·h-1) and little fracture rate (1.93%) perched water on the rock and provided path of water flow from fractures. The "perched water" on the interface of porous-fractured media decreased the head difference during infiltration, which made water potential gradient in the ss for ss30 decreased after ss became relative saturation. Consequently, it extended balance time of water distribution in the profile. In addition, the entrapped air in the porous media would vent from fractures slowly increasing water content of ss.On the interface of porous-fractured media, that the amount of "perched water" changed with time which made water content on the interface varied.
According to the former analyses about the effects of fractured rock on infiltration and water distribution of porous media, the flow in the upper porous media was determined by the hydraulic properties of upper porous media and rainfall intensity, before water flow got the interface of porous-fractured media. After that, the fractured rock caused variation of permeability horizontally and prevented water flow vertically of upper porous media. Thus, various kinds of porous media in the upper layers with different hydraulic properties would affect seepage in the lower layer of fractured rock.
The start of seepage for layered structure in this study was determined by conductivities and thickness for porous media and fracture media respectively. For saturated hydraulic conductivities of upper porous media, they were estimated according to infiltration capacity. Figure 4 showed the infiltration rate for ss30 and ss30+sr20 were less than rain intensity, while the infiltration rate of sr30 was equal rain intensity. When equivalent permeability of underlying fractured rock was much greater than the saturated hydraulic conductivity of the upper porous media, the variation of start of seepage for various layered structure with the same fractured rock (ss30, sr30, and ss30+sr20) was mainly determined by conductivities and thickness for porous media. Seepage for sr30, ss30, and ss30+sr20 began at 0.75, 1.97, and 2.28 h after rainfall, respectively. For the i layered structure, by letting mean velocity of flow in the j layer with thickness of Lij be vij, start time (Ti) of seepage for those layered structures can be calculated by followed formula
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(3) |
Here, we hypothesized the type of water flow in the porous media for ss30 and ss30+sr20 was the plug flow. The mean velocity of flow in ss, sr, and fr would be calculated according to the observation data Ti, the time of wetting front reaching the interface of layered structure, and Lij by Equation (3). While the water flow in sr30 was uneven in one horizon, mean velocity of flow in fr could not be calculated from the observed data. The calculated mean velocity of flow in each layer for three layered structures is listed in Table 4. From Table 4, both the mean velocities of flow in ss and fr for ss30 were less than those for ss30+sr20, which reflected fr more preventing infiltration in the ss and, inversely, ss more restraining water flow in fr for ss30 than ss30+sr20. Meanwhile, the mean flow velocity in sr for ss30+sr20 was less than that for sr30, which reflected ss for ss30+sr20 decreasing the water flow in sr.
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The curves of outflow rate varied with time in whole experimental and infiltration period for ss30, sr30 and ss30+sr20 are drawn respectively in Figs. 6a and 6b. From Fig. 6a, the changing trend of outflow rate for three layered structures were all increasing quickly to one peak value and kept it for certain period of time, then deceased to near zero and kept the lower value till seepage ending. Nevertheless, there were different peak value of outflow rate for the three different layered structures, and its descending order was sr30, then ss30+sr20, and lastly ss30. Moreover, the times that kept in the peak values were also different, they were about 2.8 and 3.8 h, respectively, for ss30 and sr20 during the 5 h rainfall, while it was 5.5 h for ss30+sr20 during the 6 h rainfall. When infiltration ending, the rate of outflow decreased quickly for sr30 and ss30+sr20, they spend0.58 and 0.82 h, respectively, to reduce the outflow rate to less 0.5 cm·h-1, while the outflow rate for ss30 began to decrease obviously after 6 h of infiltration, then it decreasedslowly till infiltration ending. Figure 6b showed rapidity of change for outflow rate of the layered structures at the infiltration stage, and they were same as the order of the peak values for outflow rates. The increasing speed of outflow rate for sr30 was fastest and the its change with time fitted power function, while the curve of outflow varied with time for ss30+sr20 and the curve for ss30 seemed better to fit the "S curve" function. At the frontend of "S curve" (i.e., outflow rate increasing gently) for ss30+sr20 and ss30, the fractured rock may become moist under the limited rate of supply water due to the small hydraulic capacity of ss on the surface layer.
Since the rainfall, seepage, and water content in porous media had been measured for the layered structure, water in fractured rock can be calculated according to the principle of water volume balance. The formula is as follows
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(4) |
where R is rainfall for each infiltration experiment (cm), P is the depth of surface ponding (cm), O is seepage (cm), θ is volumetric water content of porous media, D is the depth of porous media measured by TDT sensors, hF is water amount in fractured rock and ρf is fractures "porosity" also called fractures rate in fractured media, which determined using fractured volume divided by the total cube volume (cm3·cm-3). The subscript i is the number of porous layers.
The change of water amount in the fractured rock with time for three kinds of the layered structures is drawn in Fig. 7. It showed that the trends of water amounts with time were disorder at the early stage, while the water amounts increased with time in a linear function later for all the layered structures. Additionally, the descending order for start time of water increasing and increasing rate of water amount in the fractured rock was sr30, then ss30+sr20, and finally ss30 (Fig. 7), which were decided by thickness of upper porous media and their conductivities. The infiltration experiments for ss30+sr20, ss30, and sr30 were conducted in turn. Thus, except for the ss30+sr20 experiment, the fractured rock for the other layered structures in the experiments had been moist. At the early stage of infiltration, the water amount in the fracture rock for ss30+sr20 firstly increased then decreased and this trend was repeated for twice, the reason could be that drying fracture walls may absorb water repeatedly to get balance between viscous force and gravity for water film, while the water amount directly increased for sr30. For ss30, water amount firstly increased then decreased to a small value and kept it for 6 h. Near the infiltration ending, the hF for ss30+sr20 and sr30 were higher than the thickness of fractured rock (61.5 cm), which provided possibility for the saturation of layered structures and changing of water recharge process. On the other hand, the hF for ss30 was 30 cm about after infiltration of 22 h, which lower than the rock thickness.
During the infiltration experiments, water amount was accumulated gradually in the lower layer of frac-tured rock because the drainage capacity of three grooves on the bottom of pmma cabinet was less than the least hydraulic capacity of upper porous media. Since the equivalent permeability of fractured rock was larger greatly than that of porous media, the grooves also limited the hydraulic capacity of frac-tured rock. After water amounts in the fractures be-came balanced, the increase of water amounts in frac-tures was determined by saturation and hydraulic ca-pacity of upper porous media. It was same as the order of outflow rate for three layered structures, the water amount in the fractures for sr30 increased fastest, that for ss30+sr20 was second, and that for ss30 was slowest because the infiltration rate in ss was decreased by the fractured rock. For ss30+sr20 and sr30, the water amounts in the fractured rock were more than the volume of fractures, which caused that the extra water moved upward to the upper porous media.
From the infiltration experiments for three kinds of the layered structures with upper porous-lower fractured media, the difference of infiltration process were observed between the layered structures with and without underlying the fractured rock. The specific findings are given as follows.
(1) From the experiment results achieved in this investigation about the infiltration rate and wetting front in the layered structures overlying the same fine porous media, it is noticed that the fractured rock reduced considerably the infiltration rate of upper porous media, while underlying coarse porous media, as a "buffer" to store the water from upper fine porous media during infiltration, had little effect on the infiltration rate of the upper fine porous media. Before water reached to the interface of the layered structures, the infiltration rate and wetting front were almost the same for the same upper porous media of the two layered structures. On the other hand, after water arrived to the interface, wetting front or velocity of flow in the fractured rock with a small fracture rate was larger greatly than that in coarse porous media. Moreover, the water was still perched on the rock due to the small fracture rate, which made the TDT values relative stable and larger than those at the adjacent upper points. Meanwhile, it was observed that the fractured rock extended the time of water balancing and water content in the profile of the upper porous media increased slowly in the later infiltration process. The interface of fine-coarse porous media had not been saturated, and the water content in the porous media kept stable after they got to relative stable. It seemed that the prevention of vertical water movement from the underlying fractured rock was greater than that from the underlying fine-coarse porous media.
(2) From the experiment results achieved in this investigation about the infiltration in the layered structures with the upper various porous media underlying the same fractured rocks, it was founded that the hydraulic conductivity of upper porous media noticeably affected the mean velocity of flow in the fractured rock. Normally, the greater the hydraulic conductivity of porous media was, the earlier outflow from fractured rock, the faster their outflow rate got to peak, and the larger the peak value was. Furthermore, it seemed that the degree of moisture for the fractured rock determined the time of its saturation and the process of water amount increase in the fractured rock. When the discharge capacity of the fractured rock was limited while hydraulic conductivity of upper porous media was great, the fractured rock was saturated easily and then extra water would move upward to the upper layer of porous media. Consequently, the infiltration process through upper porous media would change correspondingly.
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Huifang Wang, Mingyu Wang. Infiltration Experiments in Layered Structures of Upper Porous and Lower Fractured Media. Journal of Earth Science, 2013, 24(5): 843-853. doi: 10.1007/s12583-013-0378-2
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