Subgrade:

• The top soil, also known as the subgrade, is a natural soil layer that has been prepared to withstand loads from the layers above.
• It’s crucial that soil subgrade is never overstressed. The ideal moisture content should be present, and it should be compacted to the desired density.
• Red gravel soil or moorum makes up its composition.

A good subgrade should have these qualities.

• Sufficient compressive strength to bear the imposed wheel load.
• Effective pore space is limited, which limits the amount of water that can enter.
• having the ability to endure abrasive and impact stresses from traffic and weathering

Sub base course:

• The sub base course is the layer of material below the base course, and its main purposes are to maintain the pavement structure structurally, enhance drainage, and lessen the incursion of fines from the subgrade.
• The sub base course with additional fines can act as a filler between the sub grade and the base course if the base course is open graded.
• The usage of a sub base course is not always necessary. For instance, the extra qualities provided by a sub base course may not be necessary for a pavement built over a high-quality, firm subgrade. In certain circumstances, sub base course might not be offered.
• Broken stones and bound or unbound aggregates make up this layer.

Base course:

• The layer of material immediately above the sub base is known as the base course, and it helps to subsurface drainage and additional load distribution.
• It serves as a medium for dispersing stresses to distribute the surface wheel loads.
• Crushed stone, crushed slag and other untreated or stabilised materials could be included.

Binder course:

The binder course typically consists of aggregates having less asphalt and doesn’t require quality as high as the surface course, so replacing a part of the surface course by the binder course results in a more economical design. This layer provides the bulk of the asphalt concrete structure. Its main function is to distribute load to the base course.

Surface course:

• It is the layer directly in contact with the traffic loads and is usually constructed with superior quality materials. They are usually constructed with dense graded bituminous concrete.
• The functions and requirements of this layer are,
  • It provides characteristics such as friction, smoothness, drainage, etc., and it will also prevent the entrance of excessive quantities of surface water into the underlying pavement layers.
  • It must be impervious, firm, strong and skid proof to provide resistance to abrasion, compression, tension, repetitive action of wheel loads and weathering action.
  • It must provide smooth riding surface.
  • It should offer water tight layer against surface infiltration.

Seal coat:

It is a thin surface treatment used to water proof the surface and to provide skid resistance.

Tack coat:

A very thin layer of asphalt emulsion diluted with water is used as a “tack coat” to promote bonding between two layers of binder course. It must be extremely thin, evenly cover the entire area, and dry quickly.

Prime coat:

In order to bond two layers together, a binder layer is placed on an absorbent surface, such as granular bases, and low viscosity cutback bitumen is applied to it. Prime coat, in contrast to tack coat, penetrates into the layer beneath, fills in the gaps, and creates a watertight surface.

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Flexible Pavements

• By transferring wheel load strains from the upper layers to the lower layers through grain-to-grain contact at the granular structure’s sites of contact, flexible pavements can do this.
• A greater area will be affected by the wheel load acting on the pavement, and the stress will decrease as depth increases.
• Flexible pavement typically includes numerous grooves to take use of this stress distribution trait layers.
•  As a result, the idea of a layered structure is used in the design of flexible pavement.
• Using this as a foundation, it is possible to build flexible pavement in several layers, however the top layer must be highest quality to withstand the greatest compressive loads as well as wear and tear.
• Less high-quality material can be utilised because the lower layers will be under less stress.
• Bituminous materials are used to build flexible pavements.
• These can take the shape of asphalt concrete surface courses, which are typically used on high-traffic roads like national highways, or surface treatments like bituminous surface treatments, which are typically found on low-volume roads.
• Flexible pavement layers reflect surface layer deformation from the bottom layers.
• When designing flexible pavement, the total performance is taken into consideration, and the stresses generated should be kept considerably below the allowed stresses of each pavement layer.

Rigid Pavements

• The flexural strength of rigid pavements is sufficient to distribute the stresses caused by wheel loads to a larger area below. Figure below depicts a typical cross section of stiff pavement.
• Since there is just one layer of material between the concrete and the sub-grade, this layer can be called the base or sub-base course.
• In contrast to flexible pavement, rigid pavements are installed either directly on the prepared subgrade or on a single layer of granular or stabilised material.
• Rigid pavement functions as an elastic plate resting on a viscous medium, distributing load by the slab action.
• Since rigid pavements are made of Portland cement concrete (PCC), plate theory should be used to examine them rather than layer theory.
• In a condensed version of layer theory known as plate theory, a concrete slab is viewed as a medium-thick plate that is plane both before and after being loaded.
• Wheel load, temperature changes, and the consequent tensile and flexural stress are what cause the slab to bend.
• Unlike with flexible pavement, the pressures are not transferred from grain to grain.

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HOW IS SOIL REINFORCEMENT DONE?

Tensile components are inserted into the soil as part of soil reinforcement to increase the stability and strength of the soil naturally. In order to accomplish this, surfaces in the aggregate and sub-base of the soil mass are brought into contact with reinforcement components. Tensile loads, which can resist soil movement and give additional support for enhanced strength, are created when pressure on the soil mass results in a strain on the reinforcements. In this manner, a soil-reinforcement system that offers more shear strength than the soil mass alone is produced.

CHALLENGES IN SOIL REINFORCEMENT

• The largest issue for embankments on weaker foundations, such as airports next to sand dunes, is stabilising and strengthening the surrounding soil.
• Geotextile layers are deliberately positioned on the ground to create steep soil slopes. The goal is to make the soil more tensile so that it slides or rotates less.
• For Retaining Walls,Various wall applications, such as on-site infill, are combined with geotextiles to strengthen soil-supporting walls. For retaining walls, geotextile offers an alternative to conventional cast-in-place concrete construction.
• Basement Stabilization,Soil that is soft and organic has a low tensile strength. Traditional methods’ initial cost Land filling might cost up to 50% more than using soil reinforcing techniques geotextiles. To evenly disperse the weight throughout the soil, geotextiles can be employed and reduce the movement of tiny soil particles thus geotextiles are a low-cost option cost-effective substitute for conventional sub-grade displacement, excavation, and ways for replacing and chemically stabilising soil.
•  For Strengthening the Base Course, By making the granular base course material more tensile, soft soil’s ability to support loads can be strengthened. The load-bearing capacity of the soil at the granular base structure is increased by geotextile, improving the tensile strength of the soil. The granular base course is frequently reinforced using a grid system.
• Geotextiles are a practical way to reinforce fragile soils in places like lagoons, sludge ponds, and other similar environments. By offering strong tensile support and anti-deformation capabilities to support building structures and enhance the quality of the soil body, geotextiles can fortify soft soil.

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VARIOUS TYPES OF SOIL NAILING

• Grouted Nail: Following excavation, holes are first drilled in the slope face or wall, and then the nails are inserted into the holes that had already been created. Cement grout is then used to fill the drill hole.
• Driven Nail: During excavation, nails of this type are driven mechanically into the wall. This kind of soil nailing may be installed extremely quickly, but it does not offer a good level of corrosion protection. Typically, this is done as a temporary fix.
• Grouted Nail: Following excavation, holes are first drilled in the slope face or wall, and then the nails are inserted into the holes that had already been created. Cement grout is then used to fill the drill hole.
• Driven Nail: During excavation, nails of this type are driven mechanically into the wall. This kind of soil nailing may be installed extremely quickly, but it does not offer a good level of corrosion protection. Typically, this is done as a temporary fix.

ADVANTAGES

• It is not dependent on heavy equipment.
• It is economical where the geometry of the wall is complex and where space restrictions exist.
• Since nails are of low strength steel, the need for corrosion protection stands reduced
• Construction can be carried out with little disturbance to the environment in terms of noise and vibration.
• Due to the system’s need for some soil displacement in order to mobilise resistance, soil nail walls might not be suitable for applications requiring very precise deformation control for structures and utilities located behind the proposed wall. Although it comes at a higher expense, post tensioning can lessen deflections.
• Soil nail walls are not well suited where significant volumes of groundwater seep into the excavation because it is necessary to maintain a temporarily unsupported excavation face. Existing utilities may impose limits on the position, inclination, and length of soil nails.
• Permanent subterranean easements are needed for soil nail walls.
• Only suited for excavation above groundwater; less suitable for coarse-grained soil and soft clayey soil, which have short self-support times and soils prone to creeping.

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The three phases of soil, or soil solids, water, and air, make up the majority of a typical soil sample. The term “voids” refers to the combination of the sample’s water and air contents.

A sample is referred to as being 100% saturated or purely saturated if the voids are totally filled with water.

A sample is referred to as purely dry if the spaces are entirely filled with air.

On the other hand, a sample is considered wet if some of the spaces are partially filled with water and the rest with air. The graphic illustrates the three-phase system of soil elements in terms of weights and volumes, respectively.

In the figure,

W = weight of the soil sample

WS = Weight of the solids in soil sample, WW = Weight of water in the sample, Wa = Weight of air in the soil sample

V = Total volume of the soil sample, Va = Volume of sir in the sample, Vw = Volume of water in the sample, Vs = Volume of the solids in the sample, VV = Volume of voids in the sample.

Note: –

• In the figure, weight of air (Wa) is assumed to be zero, weight of the voids is equal to theweight of water i.e Ww = WV
• The total weight of the partially and completely saturated soil is W = Ws + Ww whereas,the total weight of completely dry soil is W = Ws
• The total volume of the soil is V = Vv + Vs, where Vv is the volume of voids (Vv = Vw + Va).
• In case of completely saturated and dry soil the volume of voids Vv = Vw and Vv = Va,respectively .
• For (a) Fully Saturated soil (b) For fully dry soil

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Soils left behind by rivers are called alluvial deposits. It contains a lot of organic stuff as well as silt, sand, clay, and gravel. Alluvial deposits can form whenever a river exceeds its banks or where its flow is restricted, but they are often most prevalent in the bottom section of a river’s course.

In the Indo-Gangetic plain, which stretches from Punjab in the west to West Bengal and Assam in the east, as well as in the coastal regions of the northern parts of Gujarat, Narmada, and Tapi valleys, which are formed by sea waves, the majority of alluvial soils are derived from the sediment being deposited by the river Ganga.

According to its age, alluvial soil can be divided into two groups: khaddar and bhangar. Depending on the depth of deposition, the texture of the materials, and the length of time required for maturation, the colour of alluvial soils ranges from light grey to ash grey.

Alluvial soil has a lot of humus and is fertile, which are its main benefits. They are very useful for agriculture and are abundant in potassium.

Black Cotton Soils:

Black cotton soil is made up primarily of clay and can range in hue from light to dark grey. It is a type of soil created in the Indian Deccan by the breakdown of black lava.

By appearance, the black cotton soil appears to be of a dark colour, and a sample test reveals that it exhibits the behaviour of expanding when wet and contracting when dry. Black soils have a high proportion of calcium and magnesium carbonates, are very fine-grained, very argillaceous, and are dark. When wet, they become extremely sticky and moisture tenacious. Large and deep cracks emerge as a result of significant contraction after drying.

As a result, Black Cotton Soil (BC Soil) has a very poor bearing capacity as well as significant features of swelling and shrinkage. It makes a very poor foundation material for roads due to its unusual properties. Black Cotton soils typically have soaked laboratory CBR values in the 2 to 4 percent range.

Although black soil has low nitrogen levels, it is rich in calcium, potassium, and magnesium.

It supports the growth of crops like cotton, tobacco, chiles, oil seeds, jowar, ragi, and maize.

Lateritic Soils:

It is generally accepted that hot, humid tropical regions are where laterite, a type of soil and rock rich in iron and aluminium, developed. Due to their high iron oxide concentration, nearly all laterites are rusty-red in hue. They form as a result of the parent rock’s extensive and continuous weathering. The hilly regions of Orissa and Assam as well as Karnataka, Kerala, Tamil Nadu, and Madhya Pradesh frequently have laterite soils. Due to the presence of iron oxide, soil has a reddish brown tint.

Desert Soil:

In areas with little rainfall, desert soil makes up 90–95 percent of the soil. It is infertile because it contains relatively little nitrogen and organic matter and a lot of calcium carbonate and phosphate. In comparison to the topsoil, the lower layer contains ten times more calcium.

Aridisols are the common name for desert soils (dry soil). However, the soil orders are known as Entisols in extremely arid areas of the Sahara and the Australian outback. New soils called entisols, which resemble sand dunes, are too dry to support significant horizon development.

Hot and dry deserts, semiarid deserts, coastal deserts, and cold deserts are the four basic types of deserts. Arid deserts, usually referred to as hot and dry deserts, have year-round warm temperatures.

Marine Deposits:

Any accumulation of insoluble material, primarily rock and soil particles, that has been carried from land to the ocean by wind, ice, and rivers, as well as any marine animals’ remains, byproducts of undersea volcanism, chemical precipitates from saltwater, and objects from space.

Lithogenous, biogenous, hydrogenous, and cosmogenous are the four different types of marine sediments. Lithogenous are formed on the surface of the earth and are made up of tiny fragments of worn rock and volcanic activity.

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SOIL PROFILE:

Layers (or horizons) that build a soil profile as it ages are called horizons. Topsoil and subsoil are the two main layers that make up the majority of soil profiles. As you descend the soil profile, layers of soil are called soil horizons. There may be easy-to-dig or difficult-to-dig soil horizons in a soil profile distinguish.

Typically, soils have three primary horizons:

A horizon of nutrient-rich topsoil with high levels of organic matter, biological activity, and the majority of plant roots, earthworms, insects, and microorganisms. Because of the organic compounds, the A horizon is typically darker than other horizons.

Clay-rich subsoil is in the B horizon. While holding more moisture than the topsoil, this horizon is frequently less fruitful. Compared to the A horizon, it often has a lighter tint and less biological activity. The texture can also be more dense than the A horizon.

Underlying weathered rock, from which the A and B horizons are formed, is the C horizon. Some soils also have an O horizon mainly consisting of plant litter which has accumulated on the soil surface.The properties of horizons are used to distinguish between soils and determine land-use potential.

FORMATION OF SOIL:

A significant section of the Earth’s crust is covered by soil, which is an unaggregated or uncemented deposit of mineral and/or biological components or pieces. Few microns in diameter to enormous boulders in size make up the gamut of particle sizes. It contains sand, gravel, clay, silt, and other materials.

The geologic cycle, which includes weathering, movement, deposition, and then more weathering, results in the development of soil.

Agencies like regular temperature variations, the impact and splitting action of flowing water, ice, and wind, plants, and animals all contribute to physical weathering.

For instance: – Sand and other cohesion-less soils.

Oxidation, hydration, carbonation, desilication leaching by organic acids, and water all contribute to chemical weathering.

1. Rocks regularly experience oxidation. It works similarly to how steel rusts when it comes into touch with damp air.
2. Carbonation: Carbonic acid, which is created when carbon dioxide and water combine to generate carbon dioxide, can break down minerals that contain iron, calcium, magnesium, sodium, or potassium.
3. Hydration: This widespread phase of rock deterioration involves mixing rock with various soil constituents to create new minerals. Humid regions require you to hydrate more quickly than arid ones do.
4. Desilication: This technique involves leaching away dissolved or colloidal silica that has been liberated during another chemical reaction.
5. Leaching: Leaching is the process by which water-soluble components (Calcium Carbonate) are broken down and removed from the soil by irrigation, subsurface flow, rainfall, or other fluids.

Soil obtained due to weathering are off 2 types namely transported and residual:

These soils continue to cover the parent Rock as residual soils. They are primarily discovered at shallow depths.

Soils that have been picked up, mixed, dissolved, transported, and then redeposited fall under the category of transported soils. They are located at great depths. Depending on how they are transported and deposited, they may be homogenous or heterogeneous, or they may change in particle size, shape, and texture.

Transported soils can be classified further into:

• Soils carried by rivers and streams are referred to as alluvial soils. Sedimentary clay, for instance.
• Aeoline soils are those that are carried by the wind. Instance: Loess
• soil carried by glaciers is referred to as glacial soil. Illustration: Glacial Fill
• Soils deposited in lakes and beds are called locustrine soils. For instance, locustrine clays
• Marine soils are those that have been dumped in the ocean. For instance, marine clays and silts

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Soil Classification Based on Grain Size:

There are several soil classification systems based on grain size of soil, according to which soils have been classified as

• Gravel (>2 mm): These are the particles with little or no fines contributing to cohesion of materials
• Sand (0.1 –2 mm): These are the products of decomposition and weathering of rocks. Visually these are similar to gravel except presence of higher content of fines.
• Silt (0.01 –0.1 mm): These are finer than sand, brighter in colour and exhibit little cohesion
• Clay (< 0.01 mm): These are finer than silts. Clay soils exhibit stickiness, high strength when dry. Black cotton soils and other expansive clays exhibit swelling and shrinkage properties.

The most widely accepted grain size classification system is MIT soil classification system. The Bureau of Indian Standards (BIS) has also adopted the same limits as MIT system for the Indian Standard Classification System for soil grains. The limits of grain size are as follows.

Soil Classification Methods:

The various soil classification systems that have been used in the field of highway engineering are:

• Textural soil classification
• Burmister descriptive classification
• Casagrande soil classification
• Unified soil classification
• U.S Public Roads Administration (PRA) classification
• Highway Research Board (HRB) or revised PRS classification
• Federal Aviation Agency Classification
• Civil Aeronautics Administration classification
• Compaction classification

Highway Research Board (HRB) classification of soil:

The Highway Research Board (HRB) soil classification method is also called the Revised Public Roads Administration (PRA) soil classification system. With just three simple laboratory tests namely sieve analysis, liquid limit and plastic limit, it is possible to classify the soils.

The HRB soil classification system is generally adopted in highway engineering for the classification of subgrade soils. Soils are divided into seven groups A-1 to A-7. A-1, A-2 and A-3 soils are granular soils, percentage fines passing 0.075 mm sieve being less than 35. A-4, A-5, A-6 and A-7, soils are fine grained or silt-clay soils, passing 0.075 mm sieve being greater than 35 percent.

• A-1 soils are well graded mixture of stone fragments, gravel coarse sand, fine sand and non-plastic or slightly plastic soil binder. The soils of this group are subdivided into two subgroups, A-1-a, consisting predominantly of stone fragments or gravel and A-1-b consisting predominantly of coarse sand.
• A-2 group of soils include a wide range of granular soils ranging from A- 1 to A-3 groups, consisting of granular soils and up to 35% fines of A-4, A-5, A-6 or A-7 groups. Based on the fines content, the soils of A-2 groups are subdivided into subgroups A-2-4, A-2-5, A-2-6 and A-2-7.
• A-3 soils consist mainly, uniformly graded medium or fine sand similar to beach sand or desert blown sand. Stream-deposited mixtures of poorly graded fine sand with some coarse sand and gravel are also included in this group.
• A-4 soils are generally silty soils, non-plastic or moderately plastic in nature with liquid limit and plasticity index values less than 40 and 10 respectively
• A-5 soils are also silty soils with plasticity index less than 10%, but with liquid limit values exceeding 40%. These include highly elastic or compressible, soils usually of diatomaceous of micaceous character.
• A-6 group of soils are plastic clays, having high values of plasticity index exceeding 10% and low values of liquid limit below 40%; they have high volume change properties with variation in moisture content.
• A-7 soils are also clayey soils as Aindex, (LL greater than 40% and P1 greater than 10%). These soils have low permeability and high volume change properties with changes in moisture.

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