Study on permeability and collapsibility characteristics of sandy loess in northern Loess Plateau, China

•Two conditions of activating the collapsibility of sandy loess fully were proposed.•A new method of evaluating the collapsibility in sandy loess areas was suggested.•Special changes of sandy loess layer properties caused by infiltration were found. To explore the permeability and collapsibility characteristics of sandy loess in northern Loess Plateau of China, the in-situ tests were carried out by monitoring information of moisture content and collapse deformation in the soil layers. The laws in conditions of simulated rainfall infiltration and saturated infiltration were compared and analyzed. The main results are as follows. (1) The water infiltrates with funnel-shaped. The moisture content decreases with the increase of depth under simulated rainfall infiltration. All sandy loess layers can be nearly saturated under saturated infiltration. (2) The water infiltrating in sandy loess site is fast. The infiltration rate decreases with depth as a power function. (3) The proportion of water lateral migration in sandy loess site is small. For simulated rainfall infiltration, the development of cracks experienced three stages of appearing, expanding and keeping. For saturated infiltration, the development of cracks experienced the process of appearing, expanding, local small cracks cutting the main cracks and the later cracks developing makes the early cracks have trend of bending and closing. (4) The differential collapse deformation is most obvious near the test pit boundary and the maximum collapse deformation occurs in the test pit center. The collapse deformation caused by rainfall infiltration is only 11.6% of saturated infiltration. (5) Two conditions must be satisfied at the same time to fully activate the collapsibility of sandy loess. Firstly, the moisture content variation reaches the initial collapse threshold of moisture content increment. Secondly, the moisture state reaching the initial collapse moisture content increment must be kept for enough time. (6) The evaluation of site collapsibility type and the calculation of ground collapsibility value can be carried out according to the formula Δs=∑i=1nαβδsihi, the parameter β and α is selected according to the evaluation type, application condition and depth. (7) The seepage force drives the silt components moving and changes the original structure of sandy loess, which makes the water holding capacity in infiltration stage and water loss capacity in dissipation stage shows in layered distribution