Soil Mechanics

“Soil Mechanics is the application of laws of mechanics to the Engineering Problems deals with soils e.g sediments or other unconsolidated accumulation of soil particles produce by the mechanical & chemical disintegration of rocks”

Since Soil is generally a three phase material consists of Soil Solids, Water & Air. So that’s why it exhibits different properties and behave differently under same load conditions. So the following is some of the basics properties and parameters that involves while studying Soil Mechanics and Geotechnical Engineering.

General Parameters

1. Va=Volume of Air
2. V=Total Volume of soil mass
3. Vw=Volume of Water
4. Vs=Volume of Solids
5. Vv=Volume of voids
6. W=Weight of Soil Mass
7. Ww=Weight of Water content
8. Ws=Weight of Solids
9. Weight of Soil Mass=W=Ws+Ww,(Air weight neglected)
10. Volume of Voids=Vv=Va+Vw
11. Wsat=Weight of fully Saturated Soil
12. (Ws)sub=Submerged Weight or Buoyant weight of soil below water surface or under ground water table.

Index Properties of Soil

1. Water Content(W)=Ww / Ws
2. Void Ratio(e)=Vv / Vs
3. Porosity (n )=Vv / V
4. Degree of Saturation(Sr)=Vw / Vv
5. Air Content(ac)=Va / Vv
6. Percentage of Air Void( a)=Va / V

Densities

Definition: It is the Weight of any material per unit Volume of that material and Units of measurement is SI is Kg/m3 or g/cm3 .

1. Bulk Density or Bulk Unit Weight of Soil mass (r)=W / V
2. Dry Density or Dry Unit Weight of Soil(rd)=Ws / V
3. Desity of Soilds (rs)=Ws / Vs
4. Saturated Density of Soil Mass(rsat)=Wsat / V
5. Submerged Density of Soil Mass(rsub)=(Ws)sub / V

Specific Gravities

Definition: It is the ratio of density of any material to the density of water and since it is the ratio, so this is the unitless quantity .

1. Specific gravity of Soil Solids(G)=rs / rw
2. Bulk or mass Specific gravity of Soil Solids(Gm)=r / rw

Watch the Video, these parameters have been discussed briefly.

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Underpinning

Methods And Construction Techniques

The Rehabilitation of an existing building, mostly 40 to 50 years old, motivated by a change of use or structural damage, which may be a consequence of insufficient Soil Bearing Capacity, may require an underpinning project.

This type of work requires skilled labour, not only constructors, but also in the planning stage, since there is not an universal solution applicable to all cases. In fact, the underpinning solution depends on many factors, among which are the mechanical properties of the support stratum of soil, the conservation conditions of the foundation elements and, above all, the restrictions imposed during this operation.

Micropiles

Micropiles are presented as a variant of deep foundations, and consist of piles of small diameter between 75mm and 350mm, cast in situ, vertical or executed with an angle. These elements, when compressed, transmit their forces to the ground primarily by lateral friction (floating piles), although there is a small contribution from the bearing resistance.. In general, the execution of micropiles is divided into the following stages.

• ·         Drilling to the specified depth.
• ·         Placement of the reinforcement.
• ·         Gravity fill injection of grout.
• ·         Pressure postgrouting injection, when applicable.

Pre-Stressed Connection

The use of this GEWI type systems results, firstly in the installation of a certain normal stress at the interface between the beam and the underpinned element. Moreover, the load transfer to the micropiles produces, according to the strut and tie method, tensions that can be absorbed at the expense of the resistance of these steel bars.

Jet grouting

Jet grouting The physical process of jet grouting technique can be summarized in the following steps

•  Soil fracture: the initial structure of the soil is broken and the soil particles or fragments are dispersed by the action of one or more horizontal jets.
•  Mixing and partial replacement: a part of the particles or fragments of soil is replaced and the other part is mixed with the injected grout.
•  Cementation: the remaining soil particles are bonded together as the grout sets and hardens, forming a single body.

This technique can be applied to both incoherent and cohesive soils, as a result of the conversion of the potential energy, obtained from pumping the grout, into kinetic energy.

Underpinning Tips

Normally, this process needs to be designed or lead by a structural engineer for better results, but here are a few tips that will help you during the underpinning process.
The underpinning process must be started from the corners and the working inwards.

• Do not underpin below non-load bearing walls.
• Start underpinning under a strip of footing. It is recommended to start with at least 3 feet long, two feet wide and two feet in depth.
• After the excavation has been completed, add concrete to the cavity. Concrete should be mixed using one part cement, three parts sand, and six parts aggregates.
• Remember to use formwork on the edges.
• Allowed concrete placed to set for at least two days.
• Use a rod bar ensuring that the cavity under the existing foundation is filled up.
• Once the concrete has gained sufficient strength, break off the projecting footing.
• Cut the concrete with the mass of concrete surface.
• Backfill and compact. If you are having problems achieving the required consolidation, use a hose to add water to the soil.

Definition

Liquefaction refers to a phenomenon where saturated, loose, cohesionless soils lose strength due to earthquake ground motion or other sudden change in stress condition, in which material that is ordinarily a solid behaves like a liquid.

When soil becomes saturated with water, it enters a state known as liquefaction where it stops acting like a solid and starts behaving like a liquid.

Factors Affecting the Liquefaction of Soil

Fundamental factors that influence liquefaction susceptibility of saturated sands are considered, from the background of comprehensive experimental evidence from test results on reconstituted specimens. It is shown that at identical initial void ratio-effective stress state, undrained (constant volume) behaviour is profoundly affected by the fabric that ensues upon sample reconstitution. Water pluviation simulates in-situ behaviour closely. Very loose moist tamped states are unlikely to be accessible to in-situ sands. The susceptibility to liquefaction, both static and cyclic, depends not only on the initial state variables, but is also strongly affected by the effective stress path during undrained shear. On post cyclic static loading, the virgin strain softening sand is strain softening no more, but deforms with a continually increasing stiffness if the cyclic loading terminates with a residual zero effective stress. Very small expansive volumetric strains due to pore pressure gradients during short duration loading, or after its cessation could transform a sand into a strain softening type, which otherwise would be dilative if completely undrained.

How to Avoid Liquefaction?

If a structure is new construction, you should check liquefaction susceptibility before you build. However, if a structure already exists, there are measures you can take to reduce the damage caused by earthquake-related liquefaction. Structures can be retrofitted and reinforced to reduce the impact of violent shaking, and the soil under and around them can also be densified, solidified, reinforced, drained and/or dewatered.

All buildings in earthquake-prone areas can be strengthened through bracing, reinforcing masonry, sheer plating (such as adding plates of plywood to stud walls), and bolting walls to foundations. In the interior, it’s always a good idea to strap water heaters to the wall and secure heavy objects like bookshelves and mirrors to prevent them from falling when the building shakes.

Liquefaction of Soil  Tank

Foundation: Types, Use, Objectives

Foundations

Definition & Introduction

Foundation are structural elements, which transfer loads to the soil from columns, walls or lateral loads from earth retaining structures.

A structure essentially consists of two parts, namely the super structure which is above the plinth level and the substructure which is below the plinth level. Substructure is otherwise known as the foundation and this forms the base for any structure. Generally about 30% of the total construction cost is spent on the foundation.The soil on which the foundation rests is called the “foundation soil”.Shallow FootingDeep Foundation

Types of Foundation

The two main types of foundation are :

• Shallow foundation/Footings
• Deep foundation

Shallow Foundation

Shallow Foundation are usually located no more than 6 ft below the lowest finished floor OR Depth (D) of foundation is less than or equal to its width (B). When the soil bearing capacity of soil upto low depth is sufficient to take the structure load then it is provided.

Shallow Footing

Types of Shallow foundation

2. Combined footing
3. Cantilever or strap footings
4. Wall footings
5. Raft or Mat foundation

Use of Shallow Foundation

A shallow foundation system generally used when

1.   The soil close the ground surface has sufficient bearing capacity
2.   Underlying weaker strata do not result in undue settlement. The shallow foundations are commonly used most economical foundation systems.

Deep Foundations

The shallow foundations may not be economical or even possible when the soil bearing capacity near the surface is too low. In those cases deep foundations are used to transfer loads to a stronger layer, which may be located at a significant depth below the ground surface. The load is transferred through skin friction and end bearing.Deep Foundation

Types of Deep foundation

1. Pile foundation
2. Pier foundation
3. Types of Pile foundation :
4. Friction pile

Objectives of a foundation

• To distribute the total load coming on the structure on a larger area.
• To support the structures.
• To give enough stability to the structures against various disturbing
• forces, such as wind and rain.
• To prepare a level surface for concreting and masonry work.

Machine Foundations

Foundations provided for machines are called machine foundations.

These foundations have to be specially designed taking into account the impact and vibration characteristics of the load and the properties of soil under dynamic conditions. Thus the design of foundations of Turbines, Motors, Generators, Compressors, Forge hammer and other machines having a rythmic application of unbalanced forces require special knowledge of theory of harmonic vibrations.

All the above consideration are made in the design of machine foundations because inertial forces of rotating elements of machine contribute dynamic loads in addition to their static loads. Moreover, the machinery vibration influences adversely the foundation supporting soil by densifying it which may result differential settlement of the foundation.

Design Requirements

Machine foundations must fulfil the following design requirements.

1. A machine foundation should be safe against shear failure.
2. It should not settle excessively under static loads.
3. There should be no resonance due to dynamic force i.e the natural frequency of the foundation soil system should not be coincide with the operating frequency of the machine.
4. The amplitude at operational frequency of the foundation system must be within telerable limits.
5. The vibrations of the foundation soil system must not annoying to the workers working in that area.
6. It should not create bad effect on the other precision machines and instruments.

Types of Machines Foundations

Machine foundations are broadly classified into the following three types, depending upon the type of machines for which they are provided:

1. Reciprocating type Machines Foundations.
2. Centrifugal type Machines Foundations.
3. Impact Type Machines Foundations.

Soil Penetrometer Test

Penetrometer is a fantastic little invention which geotechnical engineers and technologists find very handy. It is a small handheld gauge which contains a telescoping rod which can be pushed into the soil. The distance the rod goes into the soil corresponds to a compressive strength on the dial.

Measurement of Soil

The pocket penetrometer measures the compressive strength of the soil. Most penetrometers available today contain units of tons/ft2 or kg/cm2, and the compressive strength is read directly from the gauge. Some common conversions are:

1 ton/ft2= 2000 psf = 13.9 psi

1 kg/cm2= 98.1 kPa

Limitations

A pocket penetrometer is a primative instrument that is subject to many errors such as non-uniform soil. As a minimum, you should take a series of measurements in one area and average them. The penetrometer should not replace laboratory testing or field analysis, or be used to produce foundation design data.

Factor of Safety (FoS) and Margin of Safety (MoS)

In this blog you will learn about the Factor of Safety (FoS) and Margin of Safety (MoS) that is used in the design of Civil Engineering Structures e.g Buildings, Bridges, Over-head Tanks etc.

Factor of Safety (FoS)

It is the load carrying capacity of any structure or system beyond the expected or actual loads.

Factor of Safety (FoS) used in the design of any civil engineering structures e.g Buildings, Bridges, Over-Head tanks and other RCC, Steel, Wood and composite structures.

The Term Ultimate Stress and Allowable Stress is can be expressed as:

Purpose of FoS

The main purpose of FoS to take some extra loads or to resist some accidental loads by the structure that may occur during the life of that structure. So the structure designer intentionally built the structure stronger then the allowable load. The accidental loads may be due to Earthquakes, Wind, Snow or any other dead or live load that is unexpected in the structure.

Mathematical Representation of FoS

Factor of Safety (FoS) = (Ultimate Stress)/(Allowable Stress)

FoS= ðus/ðas Eq (1)

Example

ðus= 600 N/mm2, ðas= 300 N/mm2

Put the values of ðus and ðas in the above Eq (1)

FoS = (600)/(300) = 2

FoS = 2 , mean that the structure can still take load twice of Allowable Load.

You can derive any value from Eq (1) if there is any two of them are known.

Margin of Safety (MoS)

MoS can be simply defined as it is the margin/gap provided from the allowable stress to ultimate stress. Margin of Safety (MoS) totally depends onFactor of Safety (FoS).

Purpose of MoS

The main purpose of MoS to add some extra margin to adjust or resist an accidental loads by the structure that may occur due to any reason in the structure.Through Margin of Safety (MoS) the Structural designer store some reserve capacity in the structure to take extra loads beyond the allowable load. Those Structures whose margin of safety (MoS) is equal or greater than the value one are more resistant to accidental loads. The MoS value adds the margin to structure to take some additional loads.

Mathematical Representation of MoS

Mos = FoS – 1

if FoS = 2

MoS = 2-1 = 1

So MoS = 1 , mean that the structure has still reserve the load carrying capacity upto 100% more than the allowable load.

Types of Soil & Suitable Soil for Foundations Bed

In this article you will learn about different types of Soil and Suitable soil for any structure’s foundation bed.

Soil

Soil is sediments or other unconsolidated accumulations of solid particles produced by the chemical and physical disintegration of rocks which may or may not contain organic matter.

Types of Soil

The soil on which the foundations of various types of structure is to be constructed are classified into:

(1) Sand

The grain size of Sand varies between 0.075mm to 2mm. The shape of the grains may be angular, rounded or irregular and silica is the major constituents of sand.

Properties of Sand

• It doesn’t shrink when dry.
• It doesn’t swell when wet.
• It is cohesionless.
• It doesn’t affected by the action of frost.
• It doesn’t allow water to rise up by capillary action.

Suitability

Coarse sand provides a good foundation bed. Since it is angular shape that’s why it prevent the structures from slipping and also from escaping from under surface of the foundation concrete. Fine and saturated sand are not suitable for foundation use.

(2) Gravel and Shingle

This type of soil consists of mostly big size particles of coarse material resulting from the disintegration of rocks and often transported by water from their original source. Size of the particles varies from 3 mm to 200 mm. The stone particles having size more than 200 mm are termed as boulders.

Properties of Gravel and Shingle

• It is not affected by freezing of water.
• It doesn’t swell when wet.
• It doesn’t shrink when dry.
• It has great power and strength of load bearing.
• It doesn’t settle over the load.

Suitability

Gravel and Shingle provides a good foundation bed and is suitable for foundation of almost all types of structures.

(3) Clay

It consists of particles having grain less than 0.002. It is composed of microscopic and sub-microscopic particles of weathered rocks. It consists of particle having grain size less than 0.002 mm.

Properties of Clay

• Clay can retain it shape vertically when hard but flows down when wet and exert pressures.
• It consolidates under load and may cause settlement of the structure.
• It is cohesive type of soil.
• It shrinks and cracks when dry.
• It swells and heaves when wet.

Suitability

It is suitable for foundation of ordinary and light structures. But when heavy structure is to be constructed then must check through various laboratory tests because it is can settle down when saturated. It is also difficult to excavate when dry or when heavily saturated.

(4) Silt

It is finer variety of soil having grain size of 0.002 mm to 0.6 mm.

Properties of Silt

• It has slight tendency towards swelling and shrinkage.
• It is relatively impervious.
• It is not as superior is sand.
• It is generally found in beds of river, canals and reservoirs.

Suitability

It is not considered as a good foundation material.

(5) Alluvial Soil

This soil is transported by water forces and mixed with soils of different origin. When velocity of water reduced then large size particles are start settling down. On further reduction of the velocity of water, still smaller fraction separates out. Thus the alluvial soils are deposited according to the grains sizes.

Properties of Alluvial Soil

• It is a cohesive soil.
• It is plastic but consolidate under load.
• It Cracks on Drying.

Suitability

This type of soil are suitable for light Structures.

(6) Black Cotton Soil

This type of soil is inorganic in nature. It is also called peat and bungum.

Properties of Black Cotton Soil

• It swell when confined between walls.
• It can withstand a high pressure in dry state.
• It becomes so soft during rains that even mam cannot walk through it.
• It is dark, grey or black in color.

Suitability

It is the most unreliable soil for foundation bed.

(7) Reclaimed soil

This is also known as made-up soil. This types of soil consist of ballast or brick bats, ashes, old iron pieces etc used for filling the low lying areas or back filling

Properties of Reclaimed Soil

• Its bearing strength is very low.
• It is usually porous in nature.

Suitability

This type of soil is not suitable for laying or constructing structure over it.