Sand will not hold a parapet
“The destructive effect of the wind on our works greatly increases the labor of the siege. Strong parapets, built of fine sand, the only available material, are half destroyed in a week, and the trenches are correspondingly filled up; hence a force must be constantly kept on repairs.” Brooks had seen the same thing at Fort Pulaski.
Major Brooks’ journal, OR I, 28, Pt. 1