ERT 349 SOIL AND WATER ENGINEERING "Kita kena sentiasa rasa bagus supaya tindakan kita jadi bagus. Kita kena rasa hebat supaya matlamat kita sentiasa hebat" -Dr HM Tuah

Water Holding Capacity By: Siti Kamariah Binti Md Sa’at School of Bioprocess Engineering

WATER  Water is held in the soil at various degrees of tenacity which varies with the amount of water present.  The tension force of water in unsaturated soils has been described by several expressions such as soil-pull, the force of suction and capillary tension. Suction can be defined as the force per unit area that must be exerted to remove water from the soil. Suction or tension is measured in bars.

WATER  When the soil is saturated or nearly so, the amount of suction is almost zero, but as the soil water depletes, greater amount of energy must be applied to extract water.  The soil can be initially saturated and if it is drained, field capacity can be reached. Some water is still held by surface tension.

Field Capacity  Field Capacity is greatest amount of water the soil can hold under drainage.  For most soils, it is obtained after two days of drainage after the soil was saturated by heavy rain or irrigation.  It is the optimum amount of water needed for agriculture.

Field Capacity  After the drainage has stopped, the large soil pores are filled with both air and water while the smaller pores are still full of water.  At this stage, the soil is said to be at field capacity.  At field capacity, the water and air contents of the soil are considered to be ideal for crop growth

Some soil moisture characteristics

Permanent Wilting Point and Available Water  Below Field capacity, the plant finds it more and more difficult to extract water until the suction or tension reaches 15 atmospheres permanent wilting point is obtained, which is the maximum tension the plant can exert on the soil to extract water.  Available water is the difference between the moisture contents at field capacity and permanent wilting point.  Clay holds more water but the plants exert more tension to extract water more than sand.

Permanent Wilting Point  Little by little, the water stored in the soil is taken up by the plant roots or evaporated from the topsoil into the atmosphere. If no additional water is supplied to the soil, it gradually dries out.  The dryer the soil becomes, the more tightly the remaining water is retained and the more difficult it is for the plant roots to extract it. At a certain stage, the uptake of water is not sufficient to meet the plant's needs. The plant looses freshness and wilts; the leaves change colour from green to yellow. Finally the plant dies.  The soil water content at the stage where the plant dies, is called permanent wilting point. The soil still contains some water, but it is too difficult for the roots to suck it from the soil

Available water content  The soil can be compared to a water reservoir for the plants. When the soil is saturated, the reservoir is full. However, some water drains rapidly below the rootzone before the plant can use it.

The available soil moisture or water content Available water content = water content at field capacity - water content at permanent wilting point

 The available water content depends greatly on the soil texture and structure. A range of values for different types of soil is given in the following table. Soil Available water content in mm water depth per m soil depth (mm/m) sand25 to 100 loam100 to 175 clay175 to 250

 The field capacity, permanent wilting point (PWP) and available water content are called the soil moisture characteristics. They are constant for a given soil, but vary widely from one type of soil to another.

TOPIC 3 Soil Mechanics

Stress and strain in soils  Definition of Key Term

Definition of Key Term  Stress  Intensity of loading - Load per unit area  Ratio of the force ∆ P acting on a plane ∆ S to the area of the plane ∆ S when ∆ S tends to zero. ∆ denotes a small quantity.  Effective stress, σ ’  Stress carried by the soil particles.  The component of the normal stress acting on the soil grains  Unit: kilopascals (kPa) 1 kPa= 1 kN/m 2  Total stress, σ  Stress carried by the soil particles and the liquids and gases in the void

Definition of Key Term  Strain  Intensity of deformation-ratio of the change in dimension to the original dimension or ratio of change in length to the original length  Strain states- a point is a set of strain vectors corresponding to all planes passing through that point. Mohr’s circle is used to graphically represent strain state for two-dimensional bodies.  Pore water pressure, u  Normal stress acting on the pore water.

Definition of Key Term  Elastic Materials  Materials that return to their original configuration on unloading and obey Hooke’s Law.

Stress Concept  The total stress at any point within the soil mass can be written as:  In a dry soil, there is no pore water pressure and the total stress is the same as effective stress.

Stress and displacements due to apply load PzPz PxPx PyPy σzσz σxσx σyσy Z X Y