Geotechnical Engineering I: Water in Soil - Permeability, Essays (university) of Engineering Science and Technology

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Geotechnical Engineering I CE- 242

1

Water in Soil: Permeability

Introduction

• Soil is a porous media. (It has solid grains and

pores/voids in between). Voids are interconnected.

• Under saturated conditions voids are filled with water

and allow water to pass through when subjected to

differential head.

• “Permeability is a measure of the ease with which

water flows through soils and/or rocks”.

• No soil is absolutely impermeable but some are

relatively more impervious while others are pervious.

Scope of study

• It is important for Geo technical engineers when

dealing with flowing water and/or coming in contact

with water.

• Analysis of stability of foundations and foundation

excavations coming in contact with flowing and/or

GW.

• Analysis of seepage through dams or embankments
• Design of drainage systems.
• Estimation of wells yield and design of tube well. etc

Flow of water through soils

• The structure of the solid particles in any soil will always have voids. These voids provide the water and air with continuous paths of flow.
• The flow will affect both the structure and stability of the soil mass. For example, the flow of water through an earth dam will eventually create larger and larger paths, much like pipes, that could eventually lead to the collapse of the entire structure. water Loose soil Dense soil

Hydrological cycle Hydrology is the study of water movements across the earth. It includes assessments of rainfall intensities, stream flows, and lake water levels, known as surface water hydrology , as well as studies of ground water, known as groundwater hydrology. The various movements are part of grand process called hydrologic cycle.

Groundwater

• Subsurface water may be divided into two sections:
  • The portion below the groundwater table is called the phreatic zone. This water is subjected to positive pressure as a result of the weight of the overlying water. Most subsurface water is in the phreatic zone.
  • The portion above the groundwater table is called the vadose zone. This water has a negative pressure, and is held in place by capillary action and other forces present in the soil.

Artesian and surficial springs Artesian springs/wells are wells that flow under their own pressure. These require a sloping permeable layer of rock (Aquifer) with a recharge zone higher than the well.

Confined aquifer Clay, silt Water level in aquifer standpipe x

Confined aquifer : A water bearing layer, overlain and

underlain by far less permeable soils.

Head loss Head loss between two points in a pipe

Bernoulli’s Equation

u

w

2 g

v

h    Z

Pressure head, hp Total head, h Velocity head, hv Elevation head, hz Bernoulli’s equation was named after the Swiss mathematician Danial Bernoulli (1700- 1782).

Expressing energy in unit of length:

Total head =

Velocity head

Pressure head

Elevation head

fluid particle Datum Z

Bernoulli’s Equation (Application to soil and rock)

For flow through soils, velocity (and

thus velocity head) is very small.

Therefore,

Total head =

Velocity head

Pressure head

Elevation head

fluid particle Datum Z

Total head = Pressure head + Elevation head

Bernoulli’s Equation (Application to soil and rock)

length AB, along the stream line

• At any point within the flow regime:
  • Pressure head = pore water pressure/w
  • Elevation head = height above the selected datum
• Hydraulic gradient, i between A and B is the total

head loss per unit length.

i  TH A  TH B l AB A B Hydraulic gradient Water

Hydraulic gradient A B

Hydraulic

gradient, i

Piezometric heads