SOIL, GEOTECHNICAL ENGINEERING AND FOUNDATION ENGINEERING

SOIL :

Natural aggregates of mineral grains, loose or moderately cohesive inorganic or organic in nature that have the capacity of being separated by means of simple mechanical processes.

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Structures are built with soil

Dams , embankment

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Structures are built in soil

Structural foundations – footings, piles, rafts, tunnels

Geotechnical engineering

 A unique combination of science, experience, judgment and a passion for understanding the uniqueness and variability of ground conditions resulting from the forces of nature.

 It is the art of determining the properties of unseen and variable materials to provide a facility that perform as expected at acceptable level of risk and at an optional cost.

 Geotechnical engineering involves investigation and engineering evaluation of earth materials including soil, rock, ground water and man-made materials and their systems, structural foundations and other civil engineering works.

 The practice involves applications of the principles of the soil mechanics and knowledge of engineering principles, formulas, construction techniques and performance evaluation of civil engineering work influenced by earth materials.

 The base up on which knowledge structure is built in Geotechnical Engineering is a through comprehension of the elements of geologic environment.

Foundation Engineering

 In a broad sense, foundation engineering is a art of selecting, designing and constructing the elements that transfer the weight of structure to the underlying soil or rock.  The role of engineer is to select the type of foundation, its design and supervision of construction.

Before the engineer can design a foundation intelligently, he must have a reasonably accurate conception of the physical properties and the arrangement of the underlying materials. This requires detailed soil explorations.

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General Observation

Soil does not posses a unique or linear stress-strain relationship.

Soil behavior depends up on the pressure, time and environment.

Soil at every location is essentially different Nearly in all the cases, the mass of soil involved is underground and cannot be seen entirely, but must be evaluated on the basis of small size samples, obtained from isolated locations.

Most soils are very sensitive to disturbance from sampling and thus the behavior measured by a lab test may be unlike that of in situ soil.

     The foundation engineer should posses the following information Knowledge of soil mechanics and background of theoretical analysis Composition of actual soil strata in the field.

Necessary experience-precedents-what designs have worked well under what designs have worked well under what conditions-economic aspects Engineering judgment or intuition - to find solutions to the problems.

Definition of foundation

The lowest part of a structure is generally referred to as foundation.

Function of foundation

To transfer load of the superstructure to the soil on which it is resting.

Requirements (Functional)

A properly designed foundation is one that transfers the structural load throughout the soil without overstressing of soil which can result in either excessive settlement or shear failure, both of which can damage the structure.

Classification of Foundations

 Shallow foundations  Deep Foundations Shallow foundations located just below the lowest part of the superstructure they support; deep foundations extend considerably deeper in to earth.

PLAN

Shallow Foundations

Concentrated Load Distributed Load ELEVATION PLAN ELEVATION

Combined Rectangular Footing

Shallow Foundations

Combined Trapezoidal Footing

PLAN ELEVATION

Wall Footing

ISOMETRIC VIEW

Shallow Foundations

Raft Foundation

Loads on foundation

Dead Load

: Refers to the overall weight of the structure. Includes weight of the materials permanently attached to the structure (such as flooring) and fixed service equipment (such as air conditioning)

Live load

: Refers to the weight of the applied bodied that are not permanent parts of the structure. Applied to the structure during part of its useful life (e.g. people, warehouse goods). Specified by code.

Wind loads

: Acts on all exposed parts of the structure. Calculated using building codes.

Earthquake Forces

: Building code is consulted.

Depth and location of foundation

Depth and location of foundation depends on 1. Zone of significant volume changes in soil.

2. Adjacent structures and property lines.

3. Ground water 4. Underground defects

Depth and location of foundation

Zone of significant volume changes in soil :

Clays having high plasticity shrink and swell considerably up on drying and wetting respectively.

Volume change is greatest near ground. Decreases with increasing depth. Volume changes usually insignificant below a depth from 1.5-3.0 m and does not occur below volume changes.

Depth and location of foundation

Adjacent structures and property lines.

Structures may be damaged by the construction Part extending property line Property line of new foundations, as a result of vibrations, undermining by excavation or lowering of the water table. After new foundations have been constructed, the (new) loads they place on the soil may cause settlement of previously existing structures as a result of new stress pattern in the surrounding soil.

New Footing In general, deeper the foundations and closer to the old structure, greater will be the potential for damage to old structures.

45 0 Limit for bottom of deeper Footing Existing Footing

Depth and location of foundation

Ground water

Presence of water reduces soil bearing capacity, larger footing size more cost. During construction pumping is necessary – adds to the cost of construction.

Underground defects

Footing location affected by underground defects Faults, caves, mines, sewer lines , underground cables and utilities.

Bearing Capacity : Modes of Failure

Strip footing in dense soil Load q (kN/m 2 ) Sudden appearance of a clearly defined distinct failure shape

General shear Failure

Bearing Capacity : Modes of Failure

Strip footing in Relatively loose soil Load q (kN/m 2 ) q u (1) q u (2)

Local shear Failure

When Load reaches q u(1) At q = q u(1) further settlement takes place with jerks Not so distinct failure surface develops; does not reach ground surface At q = q u(2) Failure surface finally reaches ground surface; not distinct Settlement are more in this case as compared to earlier.