Effects Of Water Content And Dry Density On Color Of Expansive Soil Based On Photographic Images

Soil color is a very intuitive feature but easily neglected,which can indirectly reflect the physical and mechanical properties of soil.Soil color is a result of selective absorption of different colors of light,and is closely related to the water content.Expansive soil is a clay with high hydrophilic and swell-shrink characteristic and sensitive to water,its color varies significantly with the change of water content.The tested expansive soil were taken from Nanyang.Firstly,the effects of water content,dry density,and drying or wetting paths on color were studied by the information extracted from the soil digital photos.At the same time,different kinds of expansive soils were tested for comparison.Finally,the mechanism of soil color change with the change of water content was initially discussed based on the measurement of soil standard absorption moisture content and bound water.The main contents and conclusions of the paper are as follows:(1)The test devices and research methods of color study of soil based on photographic images is designed and determined.Considering uniformity and dry density control of the sample,the effect of water content and dry density on the color change of soil was studied using samples with diameter of 61.8 mm and height of 20 mm;considering the consistency of the change of water content and the cracks of expansive soil,The color change rule of tested sample under drying and wetting conditions was studied using samples with diameter of 61.8 mm and height of 5 mm.At the same time,using a standard light box to provide constant illumination conditions,using a triangular frame fix the camera,the camera parameters and camera position are controlled,The entire test device is placed in darkroom to avoid external illumination interference.Gauss’ s fitting method is used to get more accurate color characteristic parameters when the color information of the tested sample image is extracted.(2)The relationships between color of Nanyang expansive soil and water content,dry density,drying and wetting path were explored.The results show that there is a good correspondence between the water content of expansive soil and the R,G,B color information of the photographic images.The R,G,and B values first decrease and then increase with the increase of the water content,and the change curves can be divided into three stages: slow drop,steep drop,and slow rise.Each stage approximates a linear change;dry density effects on color is relatively small,and the degree of influence is related to the water content of the soil sample;the drying and wetting path has different effects on the color change curve of soil,the influence on the slow drop and slow rise stages were relatively small.The drying curve have a clear left shift compared to the wetting curve,the B value is more sensitive to changes of the drying and wetting path.(3)The results of the color contrast test under the dehumidifying conditions of Nanyang expansive soil and two Wuhan expansive soils show that the RGB color value has the same change rule decrease first and then increase with the increase of the water content,however,there are differences between the position of the demarcation point and the slope of the fitted line at different stages;at the same time,the Lab color model of the color and brightness separation is used to count the rule of color change.In the Lab color model,the change of L value is the change of RGB average value,the change of L value is consistent with the change of RGB average value,the change law of the b value is similar to the L value,and the position of inflection point is the same.There are two inflection points appearing in the a-value change curve that firstly increase,then decrease and rise at the last with the increase of the water content.(4)The mechanism of color change caused by change of moisture content is preliminarily discussed based on the results of the expansive soil color test and the measuring of bound water content,It is concluded that the difference of absorption and reflection for light caused by the different forms of bound water in the soil is the main reason for the color change.A simplified model of soil color change was proposed to better explain the three stages of color change of compacted expansive soil with change of water content.