sd lecture01 introduction

8/13/2019 SD Lecture01 Introduction

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Soil Dynamics

Lecture 01

General Concepts


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Most of geotechnical and foundation design is based on the soils behavior under static

loads. Important structures however, require that highly competent engineers know

how to analyze structures under complex dynamic loads.

Examples of these dynamic loads are predominantly earthquakes. However, there is

an increasing interest on the effects from bomb blasts. Other common dynamic loads

are the operation of very heavy or unbalanced machinery, mining, construction (such

as pile driving, deep dynamic compaction, etc), heavy traffic, wind and wave actions.

Ground motions result in increased settlements, and tilting of the foundations, over

and above those calculated via static consolidation theory.

Since the 1960s research has also centered on the damage that ensues from the

liquefaction of the soil during a seismic event. The search for oil in deeper and deeper

oceans has meant designing offshore platforms that are subject to extreme wave and

wind loads. Todays Miami Herald (6 Sept 2006) mentions finding oil at 18,000 feet

depths in the middle of the Gulf of Mexico.

In extreme cases earthquake loads are added to very high wave loadings.

The study of most dynamic loads show patterns that can be used to simplify their

study. Some of these simplifications are shown in the following slides.


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This plot represents the intensity of the load from a low-speed

machine versus time upon its foundation.


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The previous real time plot is typically simplified to this type of

sinusoidal idealization.


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A simplified loading diagram of the single impact of a steel hammer upon a steel plate.


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Contrast the simple hammer plot on the previous slide to this plot that shows the

vertical acceleration of soil particles close to a pile driving hammer when it hits the

pile head-cushion interface.


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This plot is the North-South accelerogram of the El Centro, California earthquake

that took place on 18 May, 1940.


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Acceleration, velocity and displacement and velocity plots from El Centro, CAearthquake, also N-S axis.


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This diagram shows the loading upon the soil below a foundation invert due to a

vibrating machine. Notice the static load offset.


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Similar oscillatory motions occur upon a buildings frame when loaded by steady

wind loads and superimposed gusts.


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A rotating machine that has an unbalanced mass will generate these centrifugal forces.


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Time

The blast wave or shock wave that is caused by the detonation of a conventional

explosive such as TNT or ANFO (ammonium nitrate/fuel oil) results in the rapid

release of a large amount of energy. This is shown by the peak overpressure (pressure

above atmospheric pressure) whose front consists of highly compressed air. The rapid

decrease occurs as the shock wave propagates outward from the center of the

explosion. Clearly, the effect of the shock is not only a function of the amount of energy

from the explosion, but also the distance. The overpressure rapidly decreases behind

the wave front, and thus the pressure can become negative.


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Dynamic loads vary in their magnitude, direction or position with time. It is possible for

more than one type of variation to coexist. Earthquake loads, for example, vary both in

magnitude and direction. Thus, they have three orthogonal directions and their

corresponding rotation components: a total of six component forces and moments which

each vary in magnitude with time.

The figure above could be a wheel load rolling over a bridge deck, and is the instance of

a force that varies in location with time. This is a periodic load, and the era of load

duration is a cycle of motion. The time taken for each cycle is the period. The inverse of

the period is the number of cycles per second, the frequency of the load.


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Why study soil dynamics?

The most common problems that engineers encounter in the field of soil dynamics

include,

-Seismic induced ground movements and wave propagation;

-Foundations for heavy or vibrating machinery;

-The changes of the bearing capacity of foundations under dynamic loads;

-The change in load capacity of deep foundations under dynamic loads;

-The changes of settlement due to dynamic loads;

-Increased lateral earth pressures due to dynamic loads;

-The potential for a soil to liquefy when subjected to dynamic loads;

-The potential for collapse of earth embankments under dynamic loads.

What is the new failure criteria?

How should failure be defined?

What is an acceptable dynamic factor of safety?How is it related to the static factor of safety?

How do soil parameters (, c, , etc) change under dynamic loads?


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South Florida experiences yearly heavy wave forces upon marine structures due to the

high frequency of hurricanes.


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Wave forces are very large dynamic loads upon off-shore platforms.


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The U.S. Geological Survey provided this National Seismic Hazard Map in 1996 for the

Continental United States. The map shows the level of ground shaking with a 10%

probability of being exceeded in 50 years (a 475-year return period). The colors show

PGA (the measure of earthquake shaking) as a percent of the forge of gravity (g).

Frankel, et al, 1997, Geotechnical Fabrics Report, May 2002.


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The Nishinomiya bridge failure was due to very large horizontal displacement of the

soil sub-grade. Notice the 2 meter displacement of the pier head.


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The Hanshin Expressway collapse was due to the very large horizontal components of

the HyogoHyogo--KenKen NambuNambu earthquake, of January 17, 1995earthquake, of January 17, 1995 in Kobe, Japan.


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This Hanshin Expressway pier collapsed due to the very large vertical component ofthe HyogoHyogo--KenKen NambuNambu earthquake, of January 17, 1995earthquake, of January 17, 1995 in Kobe, Japan.

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