Unconfined Aquifer - Permeability of soil - Field test

Summary

This video explains the concept of soil permeability and its significance in engineering, particularly for groundwater flow through aquifers. It covers the formation of soil mass with voids between particles and the permeability property that allows liquids to pass through. The video focuses on unconfined aquifers and the pumping test method to measure permeability in the field. It also discusses hydraulic gradients, Darcy’s Law, and the assumptions made during the test. The pumping test involves creating a well, measuring water levels, and observing the steady state of flow to determine soil permeability.

Takeaways

• 😀 Soil is composed of small solid particles, known as soil grains, which leave voids between them that allow water to flow through.
• 😀 Permeability is the ability of soil to let water or liquids pass through its voids, and it is an important engineering property.
• 😀 Laboratory tests measure soil permeability, but field tests are often required due to natural variations in soil structure like non-homogeneity and stratification.
• 😀 Coarse-grained soils are challenging to sample undisturbed, making field tests like the pumping-out method necessary for accurate permeability measurements.
• 😀 Aquifers are underground layers of permeable rock or unconsolidated material, such as gravel or sand, that store and transmit groundwater.
• 😀 There are two types of aquifers: unconfined aquifers (without a confining layer) and confined aquifers (bounded by impermeable layers).
• 😀 Water in unconfined aquifers flows directly between the surface and the saturated zone, while confined aquifers are bounded by impermeable layers above and below.
• 😀 The pumping-out test for unconfined aquifers involves creating a well that reaches the aquifer and pumping it to lower the water table, forming a cone of depression.
• 😀 Observation wells placed at different distances from the pumping well are used to monitor the change in water levels and assess permeability.
• 😀 Darcy's Law is applied to calculate permeability, where discharge is related to the soil's permeability, the area of water flow, and the hydraulic gradient caused by the flow.

Q & A

What is permeability in soil?

Permeability is the property of soil that allows water or any liquid to flow through its voids, which are the empty spaces between the soil grains.

Why is it important to measure the permeability of soil in engineering projects?

Permeability is important because it influences water movement through the soil, which can affect the stability and performance of foundations, drainage systems, and other civil engineering structures.

What are the challenges in measuring soil permeability in the field?

The main challenges include the heterogeneity and stratification of natural soil deposits, making it difficult to obtain a representative sample for laboratory testing, especially for coarse-grained soils where permeability values are significant.

What are the two main types of aquifers encountered in nature?

The two main types of aquifers are unconfined aquifers, which are directly connected to the surface, and confined aquifers, which are bounded by impermeable layers of soil or rock.

What is the key difference between an unconfined and a confined aquifer?

An unconfined aquifer has no confining layer above it, allowing water to flow freely between the surface and the aquifer, while a confined aquifer is sandwiched between impermeable layers, restricting vertical water flow.

How does the pumping-out test work in measuring permeability?

The pumping-out test involves creating a well that reaches the aquifer and pumping out water. The drawdown of the water table is measured, and based on the changes in water levels at observation wells, permeability is calculated using Darcy's law.

What is the cone of depression in a pumping-out test?

The cone of depression refers to the shape formed in the water table when water is pumped out from the well, resulting in a lower water level at the well and a surrounding area where the water table is depressed.

What is Darcy's law, and how is it applied in permeability calculations?

Darcy's law states that the discharge of water from a well is equal to the permeability of the soil, multiplied by the area through which water percolates, and the hydraulic gradient causing the flow. It is applied by measuring the water level changes and calculating permeability.

What assumptions are made when calculating the permeability of soil using the pumping-out test?

The assumptions include that the flow is laminar, the soil mass is homogeneous and isotropic, the test well penetrates the entire thickness of the aquifer, and the water flow towards the well is radial and horizontal.

What is the steady state of flow in a pumping-out test?

The steady state of flow is reached when the amount of water being pumped out of the well equals the amount of water refilling the well, resulting in a constant water level in the well and observation wells.