Study of salt migration on the upper part of the Great Wall under the rainfall-radiation cycle

Erosion diseases often appear on the upper part of the Great Wall of China. Low precipitation, intense solar radiation, and severe soil salinization occur in northwest China, and the migration of salt and damage can be described with respect to the rainfall-radiation cycle. Rainfall and radiation conditions were simulated in the laboratory, and reproduced samples were subjected to cyclic experiments to determine salt migration and the damage progress. Surface hardness and ultrasonic velocity were used to measure the surface damage, and the microstructure damage was determined by computed tomography and scanning electronic microscopy. The results show that under the rainfall-radiation cycle, most of the salt in the upper part, including sulfate, migrates to an area 2–3 cm from, while a small part of the salt, including chloride, moves down into the lower half of the sample. In areas where salt is concentrated, the soil density decreases, cracks develop, and a vulnerable zone appears, reducing the surface hardness and ultrasonic velocity. The concentration of salt causing destruction of the soil structure is the main factor leading to erosion of the upper walls of earthen sites. Infiltration of rainwater is required for the migration of soluble sulfate and the damage to soil on the upper wall. Increasing radiation intensity would greatly accelerate the migration of soluble sulfate and damage to the soil structure.