Sand Permeability: Sorting and Roundness Effects
Permeability measures how easily a fluid can move through a material. For sand, permeability is significantly influenced by the characteristics of the individual sand grains, specifically their sorting and roundness.
Understanding Grain Properties
- Sorting: Refers to the uniformity of grain sizes within the sand sample. Well sorted sand contains grains mostly of the same size, while poorly sorted sand has a wide range of grain sizes.
- Roundness: Describes the shape of the grains. Well rounded grains have smooth, curved edges, whereas poorly rounded (angular) grains have sharp edges.
Impact on Permeability
- Sorting: Uniformly sized grains (well sorted) pack efficiently, creating larger and more interconnected pore spaces between them. Poorly sorted sand has smaller grains filling the gaps between larger ones, reducing the size and continuity of pore spaces, thus lowering permeability.
- Roundness: Well-rounded grains tend to pack more closely, but in the context of uniform grain sizes, they facilitate smoother, less obstructed flow paths. Angular grains can create more tortuous paths and potentially lock together, reducing permeability compared to rounded grains of similar size.
Highest Permeability Combination
The combination leading to the largest and most interconnected pore spaces, and therefore the highest permeability, is:
- Well sorted grains (uniform size)
- Well rounded grains (smooth shape)
This combination maximizes the void space available for fluid flow.
Therefore, the correct option is the one stating well sorted, well rounded sand grains.
Important Questions from Aquifer Properties
- An aquifer has a surface area of 100 km$^2$. With no recharge, assume that pumping out 2 $\times$ 10$^7$ m$^3$ of ground water resulted in a drop of the water table by 6 m. If the aquifer has a specific retention of 15 %, then the porosity of the aquifer is _____ % (rounded off to one decimal place).
- A well penetrates an unconfined aquifer. The water level (i.e., head) in the well prior to pumping is 25 m. After a long period of pumping at a constant rate of 0.05 m$^3$/s, the drawdowns at distances of 50 m and 150 m from the well are observed to be 3 m and 1.2 m, respectively.
The hydraulic conductivity of the unconfined aquifer is ______ $\times 10^{-4}$ m/s (rounded off to two decimal places).
Consider $\pi$ = 3.14. - A confined aquifer with a uniform saturated thickness of 10 m has hydraulic conductivity of $10^{-2}$ cm/s. Considering a steady flow, the transmissivity of the aquifer in $m^2/day$ is _______ (rounded off to one decimal place).
- A stratified confined aquifer consists of three parallel homogeneous and isotropic horizontal layers with thickness of 10 m, 5 m and 5 m. The layers have the same width. The hydraulic conductivities of the strata are 15 m/day, 20 m/day and 30 m/day, respectively. The water flow follows Darcy's law and is parallel to the strata. Considering the same hydraulic gradient for all the layers, the effective hydraulic conductivity of the aquifer is ________ m/day. [in integer]
If $d$ is the depth of an aquifer through which water is flowing, then the relationship between permeability $K$ and transmissibility (also known as transmissivity) $T$ is given by ________.
