Properties of an Aquifer (With Diagram) | Geography

Two properties of an aquifer material related to its storage function are its porosity and specific yield. Porosity is the ratio of the volume of voids or pores in a soil mass to its total volume. Shape, size and packing of the grains affect the porosity of granular material. The grain size distribution of the formation or aquifer material can be determined by conducting a mechanical analysis test in a nest of standard sieves with the coarsest on the top and finest at the bottom, covered with lid on the top and bottom pan as receiver.

A representative sample, about 150- 400 gm, is taken in the laboratory by quartering, oven dried and an exact weight poured into the top sieve and covered with lid. The whole nest is shaken through a mechanical sieve shaker (hand operated or electrical) for about 5 min. and the material retained on each sieve and bottom pan is accurately weighed. Percentage of material passing through each sieve gives a point on the grading curve, Fig. 4.1.

Example:

The results of a mechanical analysis test on a soil sample are given below:

Weight of sample taken = 380 gm

Plot the grading curve and report the results qualitatively.

Solution:

Weight of sample taken = 380 g

Table 4.1 Results of mechanical analysis test:

Test Report:

The grading curve is plotted on a semilog paper, Fig. 4.1.

(a) According to IS grain size scale given in the table, below, the soil may be classified as medium to coarse sand with the following percentages:

Coarse sand mixed with some fine gravel (from Fig. 4.1, 18% is finer than 0.6 mm size) = 100 – 18 = 82.0%

Medium sand (between size range 0.6 – 0.2 mm) = 18 – 0 = 18.0%

(b) The effective size D₁₀ is the size corresponding to 10% of the material being finer and 90% coarser (i.e., the size corresponding to p = 10%) and is an index of fineness of the soil. In the given sample-

D₁₀ = 0.45 mm

(c) The uniformity coefficient C_u is the average slope of the grading curve between 10 and 60% sizes and is given by-

C_u = D₆₀/D₁₀ …(4.1)

And gives an idea of the grading or particle size distribution in the material. Lower values of C_u (< 2) indicate more uniform material or poor grading and higher values indicate well graded material. In the given sample-

C_u = D₆₀/D₁₀ = 1.15/0.45 = 2.56

Which represents non-uniform material?

Another term to indicate the effective distribution of grain size is the ‘range of sizes’ C_r defined on the basis of the mean slope of the grain size curve.

C_r= 2 log₁₀ D₁₀₀/D₀ …(4.2)

= 2 log₁₀ 2.8/0.2 = 2.292, say 2.3

A uniformly graded sand has a higher porosity than a less uniform, fine and coarse mixture because in the latter the fines occupy the voids in the coarse material. Wider the range in size, lower is the porosity. The arrangement of grains or the type of packing also affects porosity—in square packing the porosity is as high as 48% while in rhombic packing it is as low as 26%, Fig. 4.2. Regarding the shape of grains, it is seen that angularity tends to increase porosity.