###### Abstract  
Granite residual soil has distinctive engineering characteristics due to its unique properties, and the resulting slopes are less stable and less resistant to rain erosion. The granite residual soil was improved by the addition of 5%, 10%, 15% and 20% fly ash, and the effects of fly ash on the intensity index and penetration of granite residual soil were investigated by triaxial strength tests and permeability tests. In combination with scanning electron microscopy measurements, a study of the stability of fly ash-modified granite residual soil slopes by modeling rainfall using the finite element software ABAQUS revealed the following: (1) the permeability coefficients of the residual granitic soils decreased by one order of magnitude when fly ash was added; (2) the improvement in the triaxial strength index of the improved soil was most pronounced when the dosage of fly ash was 15%, so that a dosage of 15% was considered optimal; and (3) numerical simulations concluded that the stability of the slope formed by 15% fly ash-improved soil fill improved significantly relative to the original slope, with the coefficient of safety increasing from 1.06 to 1.42, and the resistance to water seepage also significantly improved. granite residual soil; fly ash; triaxial shear test; permeability coefficient; slope stability Article Research on the Improvement of Granite Residual Soil Caused by Fly Ash and Its Slope Stability under Rainfall Conditions  
[PERSON], [PERSON], [PERSON], [PERSON], G. Research on the Improvement of Granite Residual Soil Caused by Fly Ash and Its Slope Stability under Rainfall Conditions  
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Granite residual soil is a special soil with favorable structural properties that is widely distributed in southeastern China and is a common soil in southern coastal areas [1]. Granite residual soil is easily disturbed, losing structural strength and easily disintegrating in contact with water [2], which leads to multiple engineering problems, such as slope instability and foundation collapse. Numerous researchers have studied the mechanical and hydraulic properties and slope aspects of granite residual soils. [PERSON] et al. [3] studied the pattern of the microstructural evolution of granite residual soils in Shenzhen and found a phenomenon similar to overconsolidation in granite residual soils. [PERSON] et al. [4] determined the foundation-bearing capacity of granite residual soils based on static load tests, side compression tests and indoor micro load tests based on the available information. [PERSON] et al. [5] conducted field tests and indoor experiments on granite residual soils and found that slope destabilization in granite residual soils is mainly induced by rainfall. [PERSON] et al. [6] analyzed the reliability of granite residual soil slopes in Fujian and concluded that the slope reliability is related to the coefficient of variation and correlation of its soil parameters, and an instability may occur even when the slopes are considered to be stable based on the traditional factor of safety method. Using energy-dispersive spectroscopy, computer tomography, stereomicroscopy and scanning electron microscopy (SEM), [PERSON] et al. [7] discovered a relationship between the horizontal fissure and the strength anisotropy of the material. [PERSON] et al. [8] proposed the use of pressure plates and filter paper methods to determine the water-soil characteristic curve (SWCC) for granitic residual soils, corrected several equations based on the experimental data and discussed the results. A database of 237 small strain stiffness measurements made on 35 distinct residual soils by [PERSON] et al. [9] is given. The characteristics determining the way saturated residual soils react to tiny strains are discovered and recommended based on a thorough and well-justified investigation. The protooliths of granite residual soil are composed mainly of quartz, mica, hornblende, feldspar and other fine-grained materials [10].