Soil Dynamics - Local Site Effects - Ground response - Geotechnical Earthquake Engineering

Soil Dynamics
6. Local Site Eﬀects
Cristian Soriano Camelo1
1Federal University of Rio de Janeiro
Geotechnical Engineering
September 12th, 2017
Cristian Soriano Camelo (UFRJ) Soil Dynamics September 12th, 2017 1 / 29

Outline
1 Introduction
Local Site Effects
2 Effects of local site conditions on ground motion
Theoretical ground response, measured amplification functions
3 Ground Response Analysis (cont.)
Examples

Outline
1 Introduction
Local Site Eﬀects
Examples

Introduction
Local Site Effects:
- Local site effects play an important role in earthquake resistant
design and must be accounted for a case by case basis ⇒ development
of one or more design ground motions.
-The development of site specific design ground motions involves
concepts presented previously and is one of the most important aspects
of geotechnical earthquake engineering.
-From surface observations of site effects it can be seen: amplification
of the ground motion, increment of the signal duration, ground motion
variability.

Introduction
Local Site Effects - The extent of the influence of the local
conditions depends on the inherent stiffness, density, and damping
characteristics of the soil deposit.
Schematic representation of different topographic, geological and soil conditions.

Outline
1 Introduction
Local Site Eﬀects
Examples

Effects of local site conditions on ground motion
Local site conditions can profoundly influence all of the important
characteristics: amplitude, frequency content and duration of
strong ground motion.
Theoretical evidence: Consider, two soil deposits with the same
geometry, unit weight, damping ratio, and different stiffness (different
shear wave velocities).
Site A Site B

Theoretical evidence: Assuming a linear elastic soil behavior and a
rigid bedrock, the following are the transfer functions.
Site A will amplify low-frequency bedrock motions more than stiff site (Site B). In a
more realistic situation, any description of local site conditions should include
density and stiffness of the bedrock. Harder bedrocks are expected to produce
higher amplification factors than softer bedrocks.

Evidence from Measured Ampliﬁcation Functions: computation
of spectral ratio of the Gilroy No.1 (Rock) and Gilroy No.2 (Soil)
motions.

Evidence from Measured Ampliﬁcation Functions: From
measured data, low-frequency components of Gilroy 1 (rock) motion
were ampliﬁed by the soils underlying the Gilroy 2(soil) station.
In addition, the high frequency components of Gilroy No.1 (rock) were
attenuated - Spectral Ratios below 1.

Evidence from Measured Surface Motions: San Francisco Bay
Area, 1989.
Bay area divided into three zones: Bay Mud (normally consolidated silty clay),
Alluvium and Rock/Shallow residual soil zone (After Seed et al., 1990).

Evidence from Measured Surface Motions: Yerba Buena Island
is a rock outcrop and Treasure Island a 400-acre (1,6 Km2) man made
hydraulic fill.
Response spectra evidenced that the presence of soft soils at Treasure Island site
caused significant amplification of the underlying bedrock motion.

Eﬀects of surface topography: Observed acceleration time histories
during the 1985 Chiba-Toho-Oki earthquake on a small hill
Kurita et al., (2005)

Effects of surface topography: Ratio of Fourier spectra of
acceleration records at Station No.3 at the top and Station No.5 at the
foot Kurita et al., (2005) .
It can be seen an increase with shaking frequency. Analysis of topographic
irregularities is a complex problem that involves different patterns of amplification
and deamplification.

Effects of surface topography: Topographic effects can be
estimated from exact solutions to idealized problems (Aki, 1988). For a
triangular infinite wedge subjected to vertically propagating SH-waves,
apex displacements are amplified by a factor 2π/φ, where φ is the
vertex angle of the wedge Faccioli, (1991) .

Effects of surface topography
Vertically propagating shear waves with a displacement amplitude of 1cm at great
depth approach the ground surface illustrated below. Estimate displacement
amplitudes at points A, B, C and D assuming the infinite wedge solution.
Point Angle, φ Amplification Factor* Amplitude
A 120 = 2π/3 3,0 3,0 cm
B 264=1, 4π 1,4 1,4 cm
C 135=3π/4 2,7 2,7 cm
D 180=π 2,0 2,0 cm
*Ampl. Factor = 2π/φ

Outline
1 Introduction
Local Site Eﬀects
Examples

Examples of Ground Response Analysis
Example 1: Undamped Elastic Layer on Rigid Rock
Natural Period (Ts)= 4H
vs
= 4∗21,4m
457,2m/s = 0, 19s
Natural Frequency (ω0)= 1/Ts = 5, 3Hz

Input motion: Kobe, 1995 M=6,9

Transfer Function and Response spectra

Example 2: Undamped Elastic Layer on Elastic Rock (same input
motion, Kobe - 1995, M=6,9
The elastic bedrock absorbs a signiﬁcant amount of energy compared to the rigid
bedrock and results in lower resonance.

Example 3: Damped Elastic Layer on Elastic Rock (same input
motion, Kobe - 1995, M=6,9
Damping aﬀects the response at higher frequencies more than at lower frequencies.

Example 4: Equivalent Linear Analysis: Single Layer on Elastic Rock
(same input motion, Kobe - 1995, M=6,9
Approximation to the non-linear soil behavior.

Shear modulus and damping ratio

Time history plot

For Further Reading I
Steven L. Kramer.
Geotechnical Earthquake Engineering.
Prentice Hall, 1996.
CEEN 545 - Lecture 21 - Nonlinear Site Response. Link
DEEPSOIL, 1D equivalent linear and nonlinear site response analysis platform.
Link to DEEPSOIL

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