1) The crack area density (Dc) is the ratio of the crack area (A) to the
total surface area of the soil specimen (At), which is considered as
one of the most important indicators of the extent of cracking
(Xiong et al., 2008).
Dc ¼ A=At ð1Þ
2) The averaged width of cracks (AW) is defined as:
AW ¼ A=Lt; ð2Þ
where Lt is total length of cracks,which are assumed to be in slender
rectangle shape.
3) The crack connectivity index (CI) is defined as (Liu et al., 2008):
CI ¼ Nj=Nj þ Ne ð3Þ
where Nj is the number of junctions and Ne is the number of end
points of the cracks. The range of CI is between 0 and 1 in which 0
means no intersected cracks and 1 means that all the cracks are
intersected without any end points.
4) The fractal dimension (FD) indicates the complexity of crack shape
(Liu et al., 2008),which ranges from 1 to 2, with larger value indicating
more complex cracks. The box-counting algorithm was used to
estimate fractal dimension of the cracks:
FD ¼ −logðbox countÞ= logðbox sizeÞ ð4Þ
where the box count is the number of box that is counted.