Abstract
This paper presents a model for estimating the moisture of loess from an image grayscale value. A series of well-controlled air-dry tests were performed on saturated Malan loess, and the moisture content of the loess sample during the desiccation process was automatically recorded while the soil images were continually captured using a photogrammetric device equipped with a CMOS image sensor. By converting the red, green, and blue (RGB) image into a grayscale one, the relationship between the water content and grayscale value, referred to as the water content–gray value characteristic curve (WGCC), was obtained; the impacts of dry density, particle size distribution, and illuminance on WGCC were investigated. It is shown that the grayscale value increases as the water content decreases; based on the rate of increase of grayscale value, the WGCC can be segmented into three stages: slow-rise, rapid-rise, and asymptotically stable stages. The influences that dry density and particle size distribution have on WGCC are dependent on light reflection and transmission, and this dependence is closely related to soil water types and their relative proportion. Besides, the WGCC for a given soil sample is unique if normalized with illuminance. The mechanism behind the three stages of WGCC is discussed in terms of visible light reflection. A mathematical model was proposed to describe WGCC, and the physical meaning of the model parameters was interpreted. The proposed model is validated independently using another six different types of loess samples and is shown to match well the experimental data. The results of this study can provide a reference for the development of non-contact soil moisture monitoring methods as well as relevant sensors and instruments.
Highlights
Loess is a yellowish, predominantly silt-sized sediment that is rich in calcium carbonate and most abundant in semi-arid regions of inner Eurasia [1]
Within the theoretical framework of unsaturated soil mechanics, it is recognized that the variation of moisture content in unsaturated loess leads to the redistribution of matric suction and suction stress, and is the key factor affecting the strength and deformation behaviors such as collapsibility [5,6,7,8,9,10,11]
6, where the light reflection status at each stage of is schematically that the grayscale value increases as the water content decreases
Summary
Predominantly silt-sized sediment that is rich in calcium carbonate and most abundant in semi-arid regions of inner Eurasia [1]. Within the theoretical framework of unsaturated soil mechanics, it is recognized that the variation of moisture content in unsaturated loess leads to the redistribution of matric suction and suction stress, and is the key factor affecting the strength and deformation behaviors such as collapsibility [5,6,7,8,9,10,11]. The increase in soil moisture content due to rainfall and other hydrologic processes is the main cause that account for many geotechnical problems and disasters in loess areas, such as ground subsidence and collapse, abrupt and excessive settlement of foundation, and landslides, leading to great economic losses and casualties [12,13,14]. The rapid and accurate assessment of the moisture content of loess is of importance to engineering construction practice in loess-covered regions
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