Porous Media | Library builder

Roads and embankments are made up by a finite number of layers. They can be considered as porous media that consist of aggregates or granular materials and soils through which fluid can flow. The road layer can appear either unbound or stabilized with bitumen or cement to increase their strength. In roads, most surface layers have very low permeability properties and can often be treated as impervious, at least in roads in good conditions. Usually, all others layers are permeable. The fluid flow behaviour of the different layers is strongly dependent on their particle size distribution and pore space openings.

2.4.1 Grain Size Distribution

The grain size distribution of unbound aggregates or soils is determined by either sieving or by the rate of settlement in an aqueous suspension. Table 2.1 shows the classification of soil particles or aggregates by size according to EN ISO 146881 (CEN, 2002). The distribution of coarse and very coarse soil fractions can be estimated through sieving but the size distribution of the fine soil particles needs to be estimated in a settlement rate test (hydrometer test).

Table 2.1 Classification of soil particles according to the size (CEN, 2002)

Soil fractions	Sub-fractions	Symbols	Particle sizes (mm)
Very coarse soil	Large boulders	LBo	> 630
	Boulders	Bo	>200-630
	Cobble	Co	> 63-200
Coarse soil	Gravel	Gr	> 2.0-63
	Coarse	CGr	> 20-63
	Medium	MGr	> 6.3-20
	Fine	FGr	> 2.0-6.3
	Sand	Sa	> 0.063-2.0
	Coarse	CSa	> 0.63-2.0
	Medium	MSa	> 0.2-0.63
	Fine	FSa	> 0.063-0.2
Fine soil	Silt	Si	> 0.002-0.063
	Coarse	CSi	> 0.02-0.063
	Medium	MSi	> 0.006-0.02
	Fine	FSi	> 0.002-0.006
	Clay	Cl	< 0.002

Figure 2.4 shows then the grain size distribution curve for three soils where the particle size is plotted on the x-axis and the percent mass retained (percent larger than the given size) is plotted on the y-axis.

The uppermost curve in Fig. 2.4 includes the highest proportion of fines (typically defined as silt particles and smaller, i. e. those less than 0.06 mm (60 ^m) in size), about 60%, and the rest are sand sized. The fines content of the middle curve is about 4% and about 54% is sand and what remains is gravel. Finally the lowest curve only has about 1% fines, sand forms about 23% of the material and the rest is gravel.

Porous Media

Particle size, D (mm)

Based on the grain size distribution curve three parameters are frequently determined, that is the effective size D10, the uniformity coefficient Cu and coefficient of gradation Cc. The parameter D10 is the diameter of the largest particle that can be found in the smallest 10% of the particle-size distribution curve. The uniformity coefficient Cu and coefficient of gradation Cc are given as:

where D30 and D60 are the diameter on the particle-size distribution curve, similar to D10 but corresponding to 30% and 60% finest fractions respectively. These three parameters are sometimes used to estimate the saturated coefficient of permeability of some types of soils.

Related Posts: