The morphological characteristics of coarse grains of residual granite soils have a significant impact on their engineering mechanical properties. However

accurately and quantitatively characterizing the real and complex shapes of soil particles one by one has always been one of the main difficulties in the study of constitutive models and numerical models. To find statistical laws from disordered and irregular geometric characteristics of soil particles seemingly based on statistical mechanics

the maximum projection profile of extensive coarse particles in four grain groups was obtained by means of optical microscopy and digital image processing

and then using Fast Fourier Transform (FFT) method to obtain Fourier descriptors

D

n

which contains all the morphological information of particle contour. The particle profile can be reconstructed accurately and randomly with amplitude

Cn

and phase φ

n

. On the basis of analyzing the statistical mechanics law of particle geometry

the geometrical morphology of coarse grains

including mean elongation

angularity and surface texture

was quantitatively described for residual granite soils in South China in four particle size intervals. Finally

the difference of grain morphological parameters between undisturbed residual granite soil

alluvial river sands and offshore alluvial quartz sands was discussed. The results show that

D

2

of 0.5-1.0 mm coarse grains is the largest and

D

2

-

D

8

5-10 mm coarse grains is the smallest

which means that the larger the coarse particle size of residual soil or alluvial soil is

the higher the regularity of the particles is. Compared with the coarse grains in alluvial river sands and sea sands

the values of

D

n

(n

>

8) reflecting meso-and micro0scopic characteristics of grain surface roughness in each grain size range of granite residual soil are basically similar

for having not experienced longterm transportation. The larger coarse grains in alluvial sands with low values of

D

n

(n

>

8) are mostly rock debris

and the mineral components are mainly amphibole and epidote. This indicates that they are rounded in the process of transportation

which reduces the degree of surface roughness. However

the value of

D

n

(n

>

8) of 0.5-1.0 mm granular group is basically the same as that of each granular group of granite residual soil

and the surface roughness is higher

which is related to the higher quartz content in the coarse grains of the two groups. This indicates that the fracture and destruction of quartz particles will occur in the process of transportation. The particles are broken into small ones

but their surface will not be ground round. And the geometric characteristics of soil particles are affected by the mineral composition of particles. A linear relationship between log (

D

n

) and log (n) is found in double logarithmic coordinates. The model parameters of different kinds of soils are calculated to characterize the geometrical shape of coarse particles in different types of soils quantitatively

which lays a foundation for the subsequent discrete element modeling of coarse particles.