“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.
Va=Volume of Air
V=Total Volume of soil mass
Vw=Volume of Water
Vs=Volume of Solids
Vv=Volume of voids
W=Weight of Soil Mass
Ww=Weight of Water content
Ws=Weight of Solids
Weight of Soil Mass=W=Ws+Ww,(Air weight neglected)
Volume of Voids=Vv=Va+Vw
Wsat=Weight of fully Saturated Soil
(Ws)sub=Submerged Weight or Buoyant weight of soil below water surface or under ground water table.
Index Properties of Soil
Water Content(W)=Ww / Ws
Void Ratio(e)=Vv / Vs
Porosity (n )=Vv / V
Degree of Saturation(Sr)=Vw / Vv
Air Content(ac)=Va / Vv
Percentage of Air Void( a)=Va / V
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 .
Bulk Density or Bulk Unit Weight of Soil mass (r)=W / V
Dry Density or Dry Unit Weight of Soil(rd)=Ws / V
Desity of Soilds (rs)=Ws / Vs
Saturated Density of Soil Mass(rsat)=Wsat / V
Submerged Density of Soil Mass(rsub)=(Ws)sub / V
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 .
Specific gravity of Soil Solids(G)=rs / rw
Bulk or mass Specific gravity of Soil Solids(Gm)=r / rw
Watch the Video, these parameters have been discussed briefly.
A hard foundation laid below ground level to support, strengthen, stabilize and renovate an existing Building or any other Structures is called 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 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.
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.
MICROPILE/EXISTING ELEMENT CONNECTION SCHEME
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.
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. Underpinning must be made only on load-bearing walls.
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.
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.
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 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.
Types of Shallow foundation
Isolated spread footing
Cantilever or strap footings
Raft or Mat foundation
Use of Shallow Foundation
A shallow foundation system generally used when
The soil close the ground surface has sufficient bearing capacity
Underlying weaker strata do not result in undue settlement. The shallow foundations are commonly used most economical foundation systems.
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
Types of Pile foundation :
Load bearing 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.
In this article you will learn about different types of Soil and Suitable soil for any structure’s foundation bed.
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:
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.
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.
Gravel and Shingle provides a good foundation bed and is suitable for foundation of almost all types of structures.
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.
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.
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.
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.
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.
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.
This type of soil is not suitable for laying or constructing structure over it.