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Soil Dynamics

Introduction

  • Soil dynamics deals with the engineering behavior of soils subjected to time varying loads and loads applied very  rapidly.
  • Most of geotechnical and foundation design is based on the soil’s behavior under static loads.
  • How soil responds to earthquakes comes in the domain of soil dynamics. 20 to 30m of soil below ground surface significantly influences. The damage patterns of structures during an earthquake. The nature of soils in this zone has a crucial influence on some important parameters of ground shaking that finally travels up to ground surface. This phenomenon is popularly known as soil amplification.
  • Important structures, however, require that highly competent engineers know how to analyze structures under complex dynamic loads.

Nature and type of Dynamic loading on soils

  • Periodic dynamic load: It varies in magnitude with time and repeats itself at regular intervals, for example, operation of a reciprocating or a rotary machine.
  • Non periodic dynamic loads: These do not show any periodicity, for example, wind loading on a building.
  • Deterministic loads: These are those loads that can be specified as definite functions of time, irrespective of whether the time variation is regular or irregular, for example, the harmonic load imposed by unbalanced rotating machinery.
  • Nondeterministic loads: These are those loads that can not be described as definite functions of time because of their inherent uncertainty in their magnitude and form of variation with time, for example, earthquake loads.
  • Cyclic loads: These are those loads which exhibit a degree of regularity both in its magnitude and frequency.

Comparison between soil mechanics & soil Dynamics

  • In soil mechanics, the magnitudes of loads being applied a priori, i.e. loads are known and then analyze and design is carried out. In soil dynamics, sometimes loads may not be known as a priori.
  • In soil mechanics the basic interest is that vertical stresses imposed do not exceed soil capacity and vertical displacement, i.e. settlements are within permissible limits. In soil dynamics, particularly during earthquakes, focus is on lateral movements and attempt is to keep them within limits.
  • In soil mechanics static loads are considered, that do not change with time. Equilibrium equations are used to determine them. In soil dynamics applied loads vary with time.

Similarities between soil mechanics & soil Dynamics

  • There are similarities also in designing. A machine foundation can be designed on similar lines for a prescribed maximum amplitude of vibration.
  • In case of engineering behavior of soils under dynamic loading, special attention needs to be paid to the rate of loading and weather, during shear, the soil dilates or suffers volume reduction. For example, if the rate of loading is increased while conducting an Unconfined Compression Strength test, typically the strength of the soil increases which can be attributed to viscous tests can be negative or positive depending on the density of soil.

Why we study soil dynamics??

  • The most common problems that engineers encounter in the field of soil dynamics include.
  • The change in load capacity of deep foundations.
  • The changes of settlement due to dynamic loads.
  • Seismic induced ground movements and wave propagations.
  • Foundation for heavy or vibrating machinery.

Applications

  • Machine Foundation
  • Geotechnical Earthquake Engineering
  • Construction vibrations
  • Nondestructive characterization of subsurface
  • Offshore structures
  • Traffic and rail induced vibrations

Other problems

  • Vibration isolation and screening so as to reduce or minimize the vibrations transmitted to foundation / building.
  • Seismic protection of buildings,
  • Force transmission so as to reduce the force transmitted to the subsoil.
  • Control and reduction of structural vibrations.

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