Chapter 1: Introduction to Soil (DBT244/BGN254) /Chapter 2: Soil Mechanics (DCT400)

Background

• “Soil” refers to the loose layer of earth that covers the surface of the planet.
• An estimated 70 percent of the earth’s surface is covered with water, while the remaining 30 per cent constitutes land.
• Soil is the part of the earth’s surface - includes disintegrated rock, humus, inorganic and organic materials.
• For soil to form from rocks, it takes an average of 500 years or more. Soil is formed when rocks break up into their constituent parts.
• When a range of different forces act on the rocks, they break into smaller parts to form the soil. These forces also include the impact of wind, water, and salts’ reaction.
• There are three stages of soil:

1) Solid soil

2) Soil with air in the pores

3) Soil with water in the pores

• Various types of soil undergo diverse environmental pressures.
• Soil is mainly classified by its texture, proportions and different forms of organic and mineral compositions.
• The layer of the earth that is composed of soil and is influenced by the process of soil formation is called pedosphere (Uppermost layer of Earth surface, composed of soil and subject to change due to erosion by water and wind).
• To an engineer, soil is a material that can be:

1) built on: foundations to buildings, bridges.

2) built in: tunnels, culverts, basements.

3) built with: roads, runways, embankments, dams.

4) supported: retaining walls, quays (jetty).

• But what exactly is soil and what is soil made of?

Definition

What is Soil?

Technically, the soil is a mixture that contains minerals, organic matter, and living organisms. But, soil can refer to any loose sediment. There are many types of soil that are distributed around the world and these are classified into the following:

1. Clay Soil
2. Sandy soil
3. Silt Soil

Typically, the soil consists of 45% minerals, 50% empty spaces or voids and 5% organic matter.

Soil performs many important functions such as:

1) Providing a growth medium for the plants.

2) Acts a modifier of the earth’s atmosphere.

3) One of the most crucial components of the biosphere (Part of earth where life exist-all ecosystem).

4) Provides habitat for organisms.

Soil Layers

Soil is arranged in layers or horizons during its formation. These layers or horizons are known as the soil profile. The layers can easily be identified by the soil colour and size of soil particles. The different layers of soil are:

1) Topsoil

2) Subsoil

3) Parent rock

Soil Formation

How is Soil Formed?

Soil is formed by weathering (proses luluhawa) of rocks. Solid rock can weather away in one of the three ways into the soil, namely:

1. Mechanical Weathering
2. Chemical Weathering
3. Biological Weathering

Mechanical Weathering

This is commonly observed near the surface of the earth. Also called physical weathering, as this process is influenced by physical forces such as wind, water and temperature.

Chemical Weathering

Chemical weathering occurs when rocks are broken down by chemical reactions. Often, such types of weathering can change the chemical composition of the soil.

Biological Weathering

Though not an actual weathering process, living organisms weaken and subsequently disintegrate rocks, often by initiating mechanical or chemical weathering. For instance, tree roots can grow into cracks in the rock, prying them apart and causing mechanical fractures. Microorganisms can secrete chemicals that can increase the rock’s susceptibility to weathering.

Composition of Soil

The soil is composed of different components: 5% organic matter, 45% minerals, 20-30% different gases and 20-30% water. Therefore, the soil is known as a heterogeneous (Diverse in ingredients or constituents) body.

Given below is the composition of soil in detail:

1) Organic Matter

Organic substances are found in very small amounts in the soil. Plants and animals are the main sources of organic matter. Depending upon the decomposition stage, the organic matter is of the following three types:

• Completely decomposed organic matter
• Partially decomposed organic matter
• Undecomposed organic matter

2) Minerals

Minerals are an important element of the soil. These are solid components composed of atoms. These occur naturally and have a fixed chemical composition. In most soils, feldspars, micas, and quartz are the main primary mineral constituents, and pyroxenes and hornblendes are present in smaller amounts.

3) Gaseous Components

The air-filled pores of the soil contain gaseous components. Nitrogen and oxygen present in the pores is generally the atmospheric air fixed by the microorganisms. However, the composition of carbon dioxide is higher due to the gas produced by microorganisms present in the soil.

4) Water

The soil dissolves the minerals and nutrients in the water and transports it to different parts of the plants. These are essential for the growth and development of the plant.

Physical Properties of Soil

Cohesive soils are a type of soil that sticks to each other. Cohesive soils are silts and clays, or fine-grained soils. A cohesionless soil (non-cohesive) is soil that does not adhere to each other and relies on friction. These soils are sand and gravel, or coarse-grained soils.

Engineers are primarily interested in a soil's mechanical properties: 

1) Strength.

2) Stiffness.

3) Permeability.

These depend primarily on the nature of the soil grains, the current stress, the water content and unit weight.

Use of Soil in Construction

Soil is directly used to make building materials, such as cement and brick, as well as indirectly used to grow the plants used to make building materials such as wood boards and insulation fibers. Historically, many homes and other structures were made from soil or soil that was caked and dried into blocks.

Ground Water

Groundwater is water that exists underground in saturated zones beneath the land surface. The upper surface of the saturated zone is called the water table. Groundwater is also often withdrawn for agricultural, municipal, and industrial use by constructing and operating extraction wells. Groundwater is often cheaper, more convenient and less vulnerable to pollution than surface water. Therefore, it is commonly used for public water supplies.

Soil Classification

Characteristic of Soil

Importance of Soil

Soil is an important element essential for the survival of living organisms. The importance of soil is mentioned below:

1. The fertile soil helps in the growth and development of the plants. The plants thus produced are healthy and provide food, clothing, furniture, and medicines.
2. It supports many life forms including bacteria, fungi, algae, etc. These microbes, in turn, maintain environmental balance by retaining the moisture and decaying the dead organisms.
3. The topsoil supports certain life activities such as reproduction, hatching, nesting, breeding, etc. of a few organisms.
4. The organic matter present in the soil increases the fertility of the soil which is responsible for the growth of the plants. It also contains certain minerals and elements that are necessary for the plants to carry out their cellular activities.
5. Soil is used for making cups, utensils, tiles, etc. The contents in the soil such as gravel, clay and sand are used in the construction of homes, roads, buildings, etc.
6. Useful mineral medicines such as calcium, iron, and other substances such as petroleum jelly for cosmetics are extracted from the soil.
7. The soil absorbs the rainwater. This water is evaporated and released into the air during sunny days, making the atmosphere cooler.

Why are soils important?

Soil is our life support system. Soils anchor roots, hold water and store nutrients. Soils are home to earthworms, termites and a myriad of micro-organisms that fix nitrogen and decompose organic matter. We build on soil as well as with it.

Soil plays a vital role in the Earth’s ecosystem and without soil, human life would be very difficult.

Soil provides plants a foothold for their roots and holds the necessary nutrients for plants to grow. Soil filters the rainwater and regulates the discharge of excess rainwater, preventing flooding. It also buffers against pollutants, thus protecting groundwater quality.

Soil is capable of storing large amounts of organic carbon. It is the largest terrestrial store of carbon. On average, the soil contains about three times more organic carbon than the vegetation and about twice as much carbon than is present in the atmosphere. This is of particular importance in efforts to mitigate climate change. Carbon can come out of the atmosphere and be stored in the soil, helping to re-balance the global carbon budget.

Soil provides people with some essential construction and manufacturing materials: we build our houses with bricks made from clay and we drink coffee from mugs that are essentially baked soil (clay). Water is served in a glass made from sand (silicon dioxide).

Rocks and minerals come to mind as the basis of soil material, however the soil also hosts a great deal of living organisms. The biodiversity of visible and microscopic life which uses the soil as their home is vast. The soil is one of the planet’s great reservoirs of undiscovered microorganisms and therefore genetic material which can become the basis of other scientific research such as developing new medicines.

Soil is also an archive. It presents a record of past environmental conditions by storing natural artifacts from past ecosystems like pollen. Many artifacts from human history are also stored underground, which archeologists carefully uncover and use to understand how civilizations have evolved.

Soil functions are general soil capabilities that are important for many areas of life including agriculture, environmental management, nature protection, landscape architecture and urban applications. Six key soil functions are:

1. Food and other biomass production
2. Environmental Interaction: storage, filtering, and transformation
3. Biological habitat
4. Source of raw materials
5. Physical and cultural heritage (artifacts)
6. Platform for man-made structures: buildings, highways

Soil Affect Construction

Soils are the foundations on which we build our houses, our factories, our shopping centers, our stadiums, our skyscrapers - all of the structures of our daily lives.

Some impervious (kedap air) clay soils serve as building materials to line landfills, seal ponds and dams, or insulate buried pipelines.

Soil can make or break construction projects. Engineers and soil scientists measure soil strength to see how easily a soil changes shape or shifts, to see whether it will bear the weight of structures.

Watch out for soil texture and type of clay. Building on the wrong soil, or without footings on unstable clay or sand, leads to foundations that crack, landfills that leak, dams that break, and sports fields that flood.

All soil is vulnerable to erosion.

During the construction process, soil becomes loose and loses its stability due to activities. Soil erosion happens when construction equipment begins to kill vegetation. Vegetation provides root systems, which hold dirt in place and prevent erosion. When builders grade the site, dirt can be displaced by both wind and water erosion. Provide temporary support where necessary.

Types of soil vary, and soil types are an important variable when considering where to build.

Types of soil including granular and fine. Fine soil is made of silt and clay. affected by water—so, a soil that changes when wet wouldn’t be the best foundation for a construction project.

The other is granular soil, which looks like small stones but is a blend of gravel, sand and particles. is the stronger of the two. 

The soil resistance must also be considered for construction projects

The soil should be studied and its components—namely, the size of its particles, the plasticity and the amount of water—need to be known in order to decide how the to-be building would survive on that ground. Is construction plausible and smart to create on that soil?

For a building to exist long-term, the soil of a site is a leading component in the decision process and a hinge to long-term success. Failure of structures is normally due to the load applied or soil supporting the structures.

How to control soil erosion on construction sites?

Soil erosion is one of the major concerns at construction sites but is often neglected. On top of that, construction works churn up the soil making it more vulnerable to erosion. A heavy runoff due to rain or any other construction activity results in the washing away of soil, which though is a slow process, but if not rectified initially, can result in project delay.

The control of soil erosion helps preserve and protect the construction site and the structure, thus promoting sustainable construction.

Measures to Control Soil Erosion on Construction Sites

Some of the important construction practices to control soil erosion are:

1) Minimize disturbed area on the construction site

it is advised to disturb only the area required for the project. The rest of the area should be left undisturbed to conserve the natural vegetation and the topsoil.

2) Divide the project into sections

While working on sites prone to erosion, divide the working land into different sections to control the erosion and sedimentation in a phased manner. The erosion control for each section should then be undertaken in an orderly way, one after the other.

3) Soil stabilization

Soil stabilization can be achieved either temporarily or permanently, depending on the requirement of the project. The use of mulch, blankets, and wood binders can act as a temporary measure. The permanent methods include planting, seeding, green buffer, and channel stabilization.

4) Slope protection 

The slopes can be protected through several methods in order to control soil erosion. The method chosen depends on the slope of the land. Various methods include:

• Geo-textiles
• Turf blankets
• Mats
• Silt fencing
• Fiber rolls

5) Runoff water control methods

Some of the methods used to control the runoff water within and around the construction sites are:

• Sediment traps or ditches
• Soil nails
• Articulated concrete blocks
• Riprap
• Geotextiles

6) Dewatering 

The groundwater or accumulated rainwater can be removed from the construction site by using dewatering methods. For massive projects, special permits are required to undertake dewatering techniques.

7) Sediment control traps for construction site

Runoff water on construction sites can be reduced by employing sediment control traps or basins. These units help the sediments to settle before the water is discharged. The sediment basins are constructed with a capacity to store at least two years of stormwater.

8) Construction entrances

The sediments can get carried away by the movement of construction vehicles at a site. Two stabilized construction entrances mainly made of crushed stone help reduce the amount of sediments getting carried away. This entrance requires regular maintenance, and the crushed stones are required to be replaced with new stones every time.

If this is not possible, it is recommended to wash off the tires of the vehicles that come in and out of the project site.

9) Site inspections

Regular inspections of the project site can control the sediment and erosion problems to a large extent. After every storm or runoff, the site must be inspected for any issues. Scheduled inspections can ensure that the implemented erosion control system is working correctly and facilitates the early detection of potential problems.

Safe bearing capacity of soil

The capacity of the soil to allow the loads coming from the foundation is called the bearing capacity of the soil.

It is essential to know the bearing capacity of the soil before designing the foundation. Different field tests can determine it. However, standard safe bearing capacity values are available for different soil types, and these values can be assumed where test data is limited or speedy construction is required.