- Slides: 20
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. Structures are built with soil Dams , embankment v. Structures are built in soil Structural foundations – footings, piles, rafts, tunnels v
Geotechnical engineering v v 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.
v v v 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. v The role of engineer is to select the type of foundation, its design and supervision of construction. v 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.
General Observation 1. 2. 3. 4. 5. 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.
v § § 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 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 v Deep Foundations v Shallow foundations located just below the lowest part of the superstructure they support; deep foundations extend considerably deeper in to earth.
Shallow Foundations Concentrated Load PLAN ELEVATION Distributed Load PLAN ELEVATION Combined Rectangular Footing
Shallow Foundations PLAN ISOMETRIC VIEW Combined Trapezoidal Footing ELEVATION Wall Footing
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 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. Part extending property line Structures may be damaged by the construction 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 Property line 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. 450 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 Load q (k. N/m 2) Settlement (mm) dense soil Sudden appearance of a clearly defined distinct failure shape General shear Failure
Bearing Capacity : Modes of Failure Strip footing in Local shear Failure Settlement (mm) Relatively loose soil Load q (k. N/m 2) qu (1) qu (2) When Load reaches qu(1) further settlement takes place with jerks At q = qu(1) Not so distinct failure surface develops; does not reach ground surface At q = qu(2) Failure surface finally reaches ground surface; not distinct Settlement are more in this case as compared to earlier.