Agricultural Engineering Exam  >  Agricultural Engineering Notes  >  Soil Mechanics Notes- Agricultural Engineering  >  Definitions - Phase Diagram, Soil Mechanics

Definitions - Phase Diagram, Soil Mechanics | Soil Mechanics Notes- Agricultural Engineering PDF Download

Void Ratio

Void ratio (e) of a soil is the ratio of the volume of voids to the volume of solid. Thus,

\[e={{{V_v}} \over {{V_s}}}\]                    (2.1)

The void ratio is greater than zero (as soil has to contain some voids), but there is no upper limit to the value of void ratio. As this the ratio of two volumes, it is unit less. 


Porosity

The porosity (n) of a soil is the ratio of the volume of voids to the total volume of the soil. Thus,

\[n={{{V_v}} \over V} \times 100\]                      (2.2)

The porosity cannot be greater than 100%. As this the ratio of two volumes, it is unit less.


Water content

The water content or moisture content (w) of a soil is the ratio of weight of water to the weight of solids present in the soil. Thus,

\[w={{{w_w}} \over {{w_s}}} \times 100\]                      (2.3)

The water content is greater than or equal to zero, but there is no upper limit to the value of water content. As this the ratio of two weights, it is unit less.


Degree of saturation

The degree of saturation (S) is the ratio of the volume of water to the volume of voids. Thus,

\[S={{{V_w}} \over {{V_v}}} \times 100\]                           (2.4)

Module 1 Lesson 2 Fig.2.1(a)Module 1 Lesson 2 Fig.2.1(a)

(a) Partially saturated soil (Three-phase system)

Module 1 Lesson 2 Fig.2.1(b)

 (b) Completely saturated soil (Two-phase system)

Module 1 Lesson 2 Fig.2.1(c)

(b) Completely dry soil (Two-phase system)

Figure 2.1. Phase diagrams.

For fully saturated soil, Vv = Vw, Thus, S = 100% or 1. Similarly, for completely dry soil, Vw = 0, Thus, S = 0. For partially saturated soil, S value can be in between 0 to 100%. As this the ratio of two volumes, it is unit less.


Unit weight

Depending upon the state of the soil, the unit weight of the soil also changes. Bulk unit weight ( \[{\gamma _{bulk}}\] or \[{\gamma _t}\] ) of the soil is defined as the total weight of the soil mass per unit of total volume. The bulk unit weight is the unit weight of the soil in its natural condition. In SI unit, it is expressed as ‘kN/m3’. Thus,

\[{\gamma _t}={W \over V}={{{W_s} + {W_w}} \over {{V_s} + {V_w} + {V_a}}}\]                  (2.5)

Saturated unit weight  ( \[{\gamma _{sat}}\] ) of the soil is defined as the total weight of the soil when it is fully saturated per unit of total volume. This is the unit weight of soil under completely saturated condition. Thus,

\[{\gamma _{sat}}={{{W_{sat}}} \over V}\]                     (2.6) 

Dry unit weight ( \[{\gamma _d}\]  ) of the soil is defined as the weight of soil solids per unit of total volume. This is the unit weight of soil under completely dry condition. Thus,

\[{\gamma _d}={{{W_s}} \over V}\]                                 (2.7)

Submerged unit weight ( \[\gamma '\] ) of the soil is equal to the saturated unit weight of soil minus unit weight of the water ( \[{\gamma _w}\] ). The unit weight of water is taken to be 9.8 kN/m3 or approximately 10 kN/m3. Thus,

Unit weight of solids ( \[{\gamma _s}\] ) is the ratio of weight of solid to the volume of solids. Thus,

\[{\gamma _s}={{{W_s}} \over {{V_s}}}\]            (2.9)


Specific gravity

The specific gravity (Gs) of the soil is defined as the ratio of the weight of a given volume of solid to the weight of an equivalent volume of water at 4°C. Thus,

\[{G_s}={{{W_s}} \over {{V_s}{\gamma _w}}}={{{\gamma _s}} \over {{\gamma _w}}}\]                  (2.10)

The specific gravity of the soil varies in between 2.65 to 2.80 (Ranjan and Rao, 2000). Specific gravity is also a unit less quantity.

The document Definitions - Phase Diagram, Soil Mechanics | Soil Mechanics Notes- Agricultural Engineering is a part of the Agricultural Engineering Course Soil Mechanics Notes- Agricultural Engineering.
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FAQs on Definitions - Phase Diagram, Soil Mechanics - Soil Mechanics Notes- Agricultural Engineering

1. What is a phase diagram in soil mechanics?
A phase diagram in soil mechanics is a graphical representation that shows the relationship between different phases of soil at different temperature and pressure conditions. It helps in understanding the behavior of soil and predicting its properties under various environmental conditions.
2. How is a phase diagram useful in agricultural engineering?
A phase diagram in agricultural engineering is useful for understanding the physical and chemical properties of soil and how they affect plant growth. It helps in determining the optimal conditions for plant growth, irrigation requirements, and the selection of suitable crops for different soil types.
3. What are the different phases represented in a phase diagram of soil mechanics?
The different phases represented in a phase diagram of soil mechanics are solid, liquid, and gas. The solid phase represents the soil particles, the liquid phase represents the water content in the soil, and the gas phase represents the air or any other gases present in the soil.
4. How does a phase diagram help in evaluating soil stability?
A phase diagram helps in evaluating soil stability by providing information about the critical points where changes in temperature and pressure can cause phase transitions in the soil. These transitions can lead to changes in soil structure, such as liquefaction or collapse, which can affect the stability of structures built on the soil.
5. Can a phase diagram be used to determine the composition of soil?
No, a phase diagram cannot be used to determine the composition of soil. It only provides information about the different phases present and their behavior under different conditions. Determining the composition of soil requires laboratory testing and analysis techniques such as particle size distribution, soil classification, and chemical analysis.
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