WHAT IS SOIL ?
The word soil is derived from Latin word ‘solium’ which means upper layer of earth’s surface. This word ‘soil’ has different meaning for different professions. For an agriculturist, it is the loose surface material of the earth is which plants grow.
For a geologist, it is the material which is produced as a result of disintegration of rocks and which has not been transported from its original position. For an engineer, soils are an incremented accumulation of minerals or organic particles occurring in the zone overlying the rock crust.
Basic Characteristics of Soil
Soil consists of different phases of solid, liquid, and gas and its characteristics depend on the interacting behavior of these phases, and on the stress applied. The solid phase includes clay, non-clay minerals, and organic matter. These elements are categorized by their size as clay, sand, and gravel. The liquid phase is composed of water that contains organic compounds available from chemical spills, wastes, and ground water, while the gas phase is normally air. The size, form, chemical properties, compressibility, .
HOW IS SOIL FORMED?
In the Earth’s surface, rocks extend upto as much as 20 km depth. The major rock types are categorized as igneous, sedimentary, and metamorphic.
Igneous rocks: formed from crystalline bodies of cooled magma.
Sedimentary rocks: formed from layers of cemented sediments.
Metamorphic rocks: formed by the alteration of existing rocks due to heat from igneous intrusions or pressure due to crustal movement.
Soils are formed from materials that have resulted from the disintegration of rocks by various processes of physical and chemical weathering. The nature and structure of a given soil depends on the processes and conditions that formed it:
Breakdown of parent rock: weathering, decomposition, erosion.
Transportation to site of final deposition: gravity, flowing water, ice, wind.
Environment of final deposition: flood plain, river terrace, glacial moraine, lacustrine or marine.
Subsequent conditions of loading and drainage: little or no surcharge, heavy surcharge due to ice or overlying deposits, change from saline to freshwater, leaching, contamination.
TYPES OF SOIL
Soils as they are found in different regions can be classified into two broad categories:
(1) Residual soils
(2) Transported soils
Residual soils are found at the same location where they have been formed. Generally, the depth of residual soils varies from 5 to 20 m.
Chemical weathering rate is greater in warm, humid regions than in cold, dry regions causing a faster breakdown of rocks. Accumulation of residual soils take place as the rate of rock decomposition exceeds the rate of erosion or transportation of the weathered material. In humid regions, the presence of surface vegetation reduces the possibility of soil transportation particle sizes, shapes and composition.
Weathered rock materials can be moved from their original site to new locations by one or more of the transportation agencies to form transported soils. Transported soils are classified based on the mode of transportation and the final deposition environment.
(a) Soils that are carried and deposited by rivers are called alluvial deposits.
(b) Soils that are deposited by flowing water or surface runoff while entering a lake are called lacustrine deposits. Alternate layers are formed in different seasons depending on flow rate.
(c) If the deposits are made by rivers in sea water, they are called marine deposits. Marine deposits contain both particulate material brought from the shore as well as organic remnants of marine life forms.
(d) Melting of a glacier causes the deposition of all the materials scoured by it leading to formation of glacial deposits.
(e) Soil particles carried by wind and subsequently deposited are known as aeoliar deposits.
Methods of determining bearing capacity
Methods of computing the bearing capacity can be listed as follows:
- Presumptive Analysis
- Analytical Methods
- Plate Bearing Test
- Penetration Test
- Modern Testing Methods
- Centrifuge Test
- Increasing depth of foundation.
- Draining the soil.
- Compacting the soil.
- Confining the soil.
- Replacing the poor soil.
- Using grouting material.
- Stabilizing the soil with chemicals
USES OF SOIL IN CONSTRUCTION
Estimates of soil properties generally apply to a depth of about 5 to 6 feet. Interpretations therefore normally do not apply to depths greater than 5 to 6 feet.
- Select potential residential, industrial, commercial, and recreational areas.
- Evaluate alternate routes for roads, highways, pipelines, and underground cables.
- Plan farm drainage systems, irrigation systems, ponds, and other structures for controlling water and conserving soil.
- Correlate performance of structures already built with properties of the kinds of soil on which they are built, for the purpose of predicting performance of structures on the same or similar kinds of soil in other locations.
- Predict the traffic ability of soils for cross-country movement of vehicles and construction equipment.
- Develop preliminary estimates pertinent to construction in a particular area.