Desalination and Water Treatment 110 (2018) 298–307 www.deswater.com April doi: 10.5004/dwt.2018.22329 Influence of irrigation method on the infiltration in loess: field study in the Loess Plateau Jiading Wanga, Ping Lia,*, Yan Maa, Tonglu Lib aState Key Laboratory of Continental Dynamics/Department of Geology, Northwest University, 710069 Xi’an, China, email: [email protected] bDepartment of Geological Engineering, Chang’an University, 710054 Xi’an, China Received 7 November 2017; Accepted 4 February 2018 abstract It is well known that loess soils are collapsible upon wetting, and subsidence or cracking or failure of structures induced by loess collapsing poses serious threat to human being. Wetting is the most important prerequisite for loess collapsing; however, how the irrigation water, both man-made and natural, infiltrates and flows in loess is not well known. For this reason, a field soaking test simulating flooding irrigation method and a rainfall infiltration test simulating dripping irrigation method were conducted in instrumented sites in the Loess Plateau. This paper presents the results from soil water meters to reveal the infiltration process in loess based on the soil water content variations. The results highlight the significance of the preferential flows when a large amount of water is irrigated to the soils (flooding irrigation). Owning to the presence of preferential paths, the water infiltrates from both shallower and greater depths to the intermediate depths, as a result, bell-shaped zone of wetting and saturated zone are developed in the soils. However, the influence of environmental factors is of dom- inance when the amount of irrigation water is very small (precipitation). The rain water, pore water, and water vapor transform from one to another depending on the rates of precipitation and evapora- tion as well as soil properties. For this reason, the maximum depth of the wetting front measured in a drought year was less than 3 m. The test results provide valuable information for interpreting the infiltration of water in loess with respect to varying irrigation method, soil heterogeneity, and envi- ronmental factors. However, such information is required for modeling the wetting-induced collapse of loess and analyzing the stability of structures built on loess. Keywords: Loess; Irrigation method; Field soaking test; Rainfall infiltration test; Preferential paths; Water content variation; Collapse settlement; Environmental factors 1. Introduction contacts decreases, leading to grain slippage and distortion. Barden et al. [7] proposed the conditions for soil collapsing Loess soils are widely distributed in arid and semi-arid including: (1) an open, partially saturated, and potentially regions around the world, including countries such as China, metastable structure; such an open structure is characterized Russia, and the United States [1–4]. Loess soils are typically by a random particle arrangement, high porosity and signifi- a kind of clayey silt with an open structure, generally in a cant amount of macro pores; (2) a high enough applied stress; state of unsaturated condition and collapsible upon wetting. (3) a high enough matric suction or other bonding agents to Holtz and Hilf [5] described the collapse as the result of cap- stabilize the structure when dry; and (4) the addition of water illary pressure approaching zero and the degree of satura- to reduce the matric suction or slake the bonding between tion increasing to 100%. Burland [6] suggested that due to particles. Alonso et al. [8] described the collapse as the result wetting, the negative pore water pressure at the inter-particle of stress state reaching the yielding surface due to wetting or loading or both. * Corresponding author. 1944-3994/1944-3986 © 2018 The Author(s). Published by Desalination Publications. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creative commons.org/ licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. J. Wang et al. / Desalination and Water Treatment 110 (2018) 298–307 299 Fredlund and Gan [9] described the collapse as the failure rainfall), infiltrates and flows in loess with varying irrigation of soil structure as the result of loss of shear strength due to method, soil heterogeneity and environmental factors is not matric suction decrease. Wang and Gu [10] and Wang et al. well known. The infiltration and soil–atmosphere interaction [11] submitted a mechanism for self-load collapsibility that in various types of soils were investigated commonly using saturated loess liquefaction caused by Earth micro-tremors artificial rainfall test or column test or numerical modeling and studied the influence factors of loess collapsibility. Wen [23–26]. Li et al. [27] revealed the rainfall infiltration process and Yan [12] stated that unlike compacted soils in which mat- based on a full-scale field test in a saprolite slope. The vari- ric suction contributes much to maintain the structural sta- ation of volumetric water contents in the soil indicates that bility when dry, in naturally deposited loess soils clays and the maximum wetting front during the rainy reason was lim- paurocrystalline carbonates exist as cementing materials for ited to the top 3 m, and a transient perched water table could large-grained particles, these cementations are water sensi- develop in the soil during a round of very heavy rain. A full- tive and their failure upon wetting thus leads to grain slip- scale field test was conducted by Zhan et al. [28] in a grassed page and rearrangement. expansive soil slope. The results indicate that the presence of Not only ground subsidence induced by collapse but grass significantly increased the infiltration rate and reduced collapse-associated geological problems such as cracking of the surface runoff; the cracks and fissures developed in the ground floor slabs, failure and erosion of slopes can bring soil played an important role in the hydrological process. The intolerable damages to the structures built on or in collaps- results of an artificial rainfall test in the Loess Plateau show ible loess especially with the recent infrastructure develop- that the influenced depth of rainfall infiltration was 0–2 m in ment in arid and semi-arid regions accompanied by the use a drought year and 0–3 m in a rainy year (with the annual pre- of large amounts of water. The sources of water can be either cipitation of 460 and 850 mm, respectively) [29,30]. The study natural, such as rainfall and fluctuation in groundwater by Gvirtzman et al. [31] suggests that the advancement of the table, or man-made, such as excessive irrigation (i.e., con- wetting front was hampered due to the alternating silty-sand stant flooding method) and leakages of water or sewer pipe- and sandy-clay loess layers, and above-hydrostatic pressure lines [13–15]. The collapse of soils due to wetting may result is developed within intermediate saturated layers, which in ground subsidence as much as 2%–6% of their thickness enhances the wetting front advancement. The results of a full- [16,17]. In Heifangtai loess platform of China, great ground scale field test conducted in a loess cut slope by Tu et al. [32] settlements, cracks, depressions, sinkholes, and caves have show that the depth of wetting front during the rainy season been commonly seen since the settlement of several thou- was within the top 2 m, while it could reach about 3 m under sands of immigrants there and the start of irrigation projects very high rainfall intensity, that is, 120 mm/d. The results of a by lifting the Yellow River water in 1960s [18,19], see Fig. 1. field test in a cover which consists of a 0.9 m-thick compacted As per the previous investigation by the authors, the ground loess underlain by a gravel layer conducted by Zhan et al. [33] settlement as the result of loess collapsing in the irrigation suggest that the preferential flows took place in compacted region extending an area of about 14 km2 could reach as large loess during the rainfall; as a result, the maximum water as 5 m for 30 years since the irrigation projects were used, and storage capacity was not reached at the onset of percolation. one more meter to the maximum ground settlement in the Vegetation had insignificant influence on the water storage following 10 years [20,21]. The problem of loess collapsing capacity of the soil. In summary, few studies differentiated due to irrigation was also met in Jingyang loess platform of the infiltration in loess in terms of irrigation method also paid China [22], as serious as that in Heifangtai platform. attention to the influence of environmental factors on the flow In summary, the infiltration of water is the most import- behavior of loess [34–37]. In this study, the results of two field ant prerequisite for loess collapsing. However, how the irri- tests representing different irrigation methods (i.e., flooding gation water, both man-made and natural (i.e., flooding and and dripping) are presented, that is, field soaking test and Fig. 1. (a) Flooding irrigation in summer; (b) sinkholes and (c) caves induced by loess collapsing in Heifangtai loess platform. 300 J. Wang et al. / Desalination and Water Treatment 110 (2018) 298–307 rainfall test. In both tests, soil water meters were used to mea- at a depth more than 120 m, and the annual precipitation of sure the soil water content changes, upon which the infiltra- Qingcheng County is about 500 mm. tion in loess was interpreted. The collapse settlements both on the ground and in the deep were also measured in the soaking 2.2. Test design test. The influence of environmental factors was considered in the rainfall infiltration test because there was a very little A test pit of 20 m in diameter which is greater than the amount of irrigation water (precipitation).