INTRODUCTION

Transmissivity (or hydraulic conductivity) refers to the ease with which water can flow through a soil. This property is essential for the calculation of seepage through earth dams and beneath sheet pile walls.

Transmissivity is also important in solving problems related to the yield of water-bearing strata, seepage through earthen dams, stability of earthen dams, and the performance of canal bank embankments affected by seepage and settlement.

The falling head method for determining transmissivity is generally used for soils with low discharge, whereas the constant head permeability test is suitable for coarse-grained soils that allow a reasonable discharge within a given time. For very fine-grained soils, a capillary permeability test is recommended.

In general, the transmissivity of soils is determined by two methods:

1. Constant head method
2. Falling Head method

As we are dealing with soils in and around aquifers, where the soil is usually highly permeable, we will perform constant head test.

Darcy's Law

Darcy's law, formulated by Henry Darcy, describes the laminar flow of fluid through a homogeneous soil medium and states that the velocity of flow (v) is directly proportional to the hydraulic gradient (i). Thus,

v=ki

Where, k is a constant known as the coefficient of permeability. The velocity v is also referred to as the superficial velocity or discharge velocity.

If the discharge velocity (v) is known, the discharge (q) can be expressed as:

q=vA=kiA

where, A is the cross-sectional area of the soil, including both solids and voids.

When the hydraulic gradient is equal to unity (i=1), the coefficient of permeability becomes equal to the velocity, i.e.,

k=v

Hence, the coefficient of permeability can be defined as the velocity of flow through soil under a unit hydraulic gradient. The coefficient of permeability is commonly expressed in units such as mm/s, cm/s, or m/day.

Formulas Used

1. K = Q*L*t*A*∆H Where,
   Q=amount of water collected in graduated cylinder,
   L = Distance between base of manometers,
   t = fixed time interval for collection of water,
   A = Area of the mould,
   ∆H = Difference in energy between both manometer levels
   
   
2. K²⁷ = KT(μ^T/μ²⁷) Where,
   μ^T = coefficient of viscosity at T degree Celsius,
   μ²⁷ = coefficient of viscosity at 27 degree Celsius
   
   
3. T = KD Where,
   K = coefficient of permeability,
   D = Aquifer thickness
   

Aquifers

Formations that contain groundwater and are sufficiently permeable to transmit and yield water in usable quantities are known as aquifers. These formations may be recharged directly from above through precipitation and infiltration, or from recharge areas located elsewhere on the Earth’s surface.

The quantity of water stored in an aquifer depends on the porosity of the formation, whereas its ability to yield water depends on its permeability.

Types of Aquifers

1. Unconfined Aquifer

   An aquifer that possesses a water table is referred to as an unconfined aquifer. In this case, the water table forms the upper boundary of the zone of saturation, while a relatively impermeable or less permeable layer constitutes the lower boundary. Unconfined aquifers are also known as free aquifers, phreatic aquifers, water table aquifers, or non-artesian aquifers. The water table in such aquifers is generally irregular and varies in slope depending on factors such as recharge and discharge areas, pumping from wells, and the permeability of the soil.

2. Confined Aquifer

   An aquifer that is bounded above and below by layers of significantly lower permeability is termed a confined aquifer. It is also known as a pressure aquifer or an artesian aquifer. Confined aquifers are completely saturated and do not possess a free water table. Their recharge typically occurs at specific locations on the Earth’s surface where the aquifer is exposed.

3. Leaky Aquifer

   Leaky aquifers are partially confined aquifers in which water leakage occurs through the overlying or underlying semi-permeable layers.

Purpose of the Experiment

The primary objective of determining the transmissivity of an aquifer is to evaluate how effectively groundwater can move through its saturated thickness. Transmissivity (T) is defined as the product of hydraulic conductivity (K) and the saturated thickness (b) of the aquifer:

T=K*b

This parameter is commonly determined through pumping tests or slug tests, where changes in water levels are observed in response to the extraction or injection of water.

Benefits of the Experiment

1. Water Resource Management

   Helps in estimating the quantity of water that can be sustainably extracted from an aquifer. It also aids in the design of wells, including appropriate spacing and depth, to prevent overexploitation.

2. Groundwater Supply Planning

   Supports reliable planning for municipal, agricultural, and industrial water supply. It is essential for evaluating recharge potential and sustainable yield of aquifers.

3. Aquifer Characterization

   Provides information about key physical properties of the aquifer, such as thickness, permeability, and uniformity. It also assists in distinguishing between confined and unconfined aquifer behavior.

4. Contaminant Transport Studies

   Plays a crucial role in predicting the movement of contaminants within the aquifer. This aids in environmental protection through improved risk assessment and remediation planning.

5. Engineering and Infrastructure Development

   Assists in the design of engineering structures such as dams, tunnels, and landfills by identifying groundwater flow patterns. This helps in avoiding potential construction issues caused by unexpected groundwater movement.