1. Seismic Response as Affected by Site Soil
Conditions
Dr. Upul Atukorala, P. Eng.
Principal, Golder Associates Ltd.
Vancouver, BC, Canada
23 July 2015
Seismic Response as Affected by Site Soil
Conditions
Dr. Upul Atukorala, P. Eng.
Principal, Golder Associates Ltd.
Vancouver, BC, Canada
23 July 2015

2. Presentation Outline
1. Introduction
2. Key Aspects of Soil Behaviour
3. Seismic Demand Resulting From Soil Effects
4. Soil Displacements
5. Soil-Foundation Interaction Response
6. Examples – Importance of Soil I/P Parameters
7. Summary
1. Introduction
2. Key Aspects of Soil Behaviour
3. Seismic Demand Resulting From Soil Effects
4. Soil Displacements
5. Soil-Foundation Interaction Response
6. Examples – Importance of Soil I/P Parameters
7. Summary

3.  Local soil/site conditions play a key role in the performance of soil-
structure systems during earthquake loading.
Examples: Damage from the 1964 Niigata EQ, 1971 San Fernando
EQ, 1985 Mexico City EQ, 1989 Loma Prieta EQ, 1995 Kobe EQ,
2011 Christchurch EQ, etc.
Introduction

4.  Incorporating the local soil/site effects in seismic design (in
codes/standards) has been a challenge.
Focus has been to incorporate the effects without overly complicating
the design process.
 For quite some time, seismic analyses were carried out ignoring soil
effects.
Example: Depth of fixity of piles.
Introduction, Contd.

5.  Soils are inherently non-linear, inelastic, stress-level/path dependent,
and undergo volume changes when subjected to cyclic loads.
Soils exhibit a wide range of load-deformation behaviour under
cyclic loading conditions.
- Cyclic Mobility
- Cyclic Liquefaction
Soil Behaviour (is Complex)
SteelSoil

6. Soil Behaviour, Contd.
Cyclic Mobility – Seen From Laboratory Direct Cyclic Simple Shear Tests
Clays and Silts
[Sample shown taken from Fraser Delta, 3 m Depth]

7. Soil Behaviour, Contd.
Cyclic Liquefaction – Seen From Back-Calculated Instrumental Data
Top of Silty Sand [5 m Thick], 3 m Depth
[Wildlife Site, M6.6 Superstition EQ (1987), CA]

8.  The pre-liquefaction and post-liquefaction stress-strain behavior of
soil is very different.
 Primary differences between pre- and post-liquefaction stress-
strain behaviour:
• Reduced stiffness, or moduli of deformation by a factor of 500 to
1000
• S-shaped stress-strain curves
• Reduced shear strength at failure
• Large strains required to mobilize appreciable shear strength
Soil Behaviour, Contd.

9.  The 2014 Canadian Highway Bridge Code (CSA-S6-14) recognizes the
importance of soil behavior and carrying out detailed field investigations, field
test programs, laboratory programs, and engineering analysis to develop
geological/geotechnical models for the site.
 The code permits higher geotechnical resistance factors when it can be
demonstrated that there is a “High Degree of Understanding” on geotechnical
aspects.
Soil Behaviour, Contd.

10.  Current Codes specify the seismic demand in terms of response spectra
for a reference ground conditions with amplification factors to account for
local soil effects.
 Seismic demand can also be established via site-specific ground
response analyses.
 Current Codes specify the seismic demand in terms of response spectra
for a reference ground conditions with amplification factors to account for
local soil effects.
 Seismic demand can also be established via site-specific ground
response analyses.
Seismic Demand Resulting from Soil
Effects

11.  Local site amplification can be understood by studying the response of an
elastic layer resting on bedrock.
 Site parameters that control amplification are:
• thickness;
• shear wave velocity;
• impedance ratio [ρss ● Vss/ρr ● Vr]; and
• critical damping ratio of the surface layer
 Local site amplification can be understood by studying the response of an
elastic layer resting on bedrock.
 Site parameters that control amplification are:
• thickness;
• shear wave velocity;
• impedance ratio [ρss ● Vss/ρr ● Vr]; and
• critical damping ratio of the surface layer
Seismic Demand Resulting from Soil
Effects, Contd.
After Finn & Wightman (2003)

12. Seismic Demand Resulting from Soil
Effects, Contd.
Actual Measurements of Ground
Response, Mexico City EQ, 1985

13.  2005 NBCC incorporated the local soil effects by specifying a Site Class
and Shaking Level dependent short-period and long-period amplification
factors for periods up to 2 Seconds.
 The “upcoming” 2015 NBCC specifies Site Class, Period and Shaking
Level dependent amplification factors up to 10 seconds.
 Some changes have been implemented to the Vs based Site
Classification table, to account for soil effects:
- If there is more than 3m of softer soils between rock and the underside of the
foundation, do not use Site Classes A or B.
- If Vs measurements have been performed, the F(T) values can be adjusted
by the ratio [1500/Vs]1/2
 2005 NBCC incorporated the local soil effects by specifying a Site Class
and Shaking Level dependent short-period and long-period amplification
factors for periods up to 2 Seconds.
 The “upcoming” 2015 NBCC specifies Site Class, Period and Shaking
Level dependent amplification factors up to 10 seconds.
 Some changes have been implemented to the Vs based Site
Classification table, to account for soil effects:
- If there is more than 3m of softer soils between rock and the underside of the
foundation, do not use Site Classes A or B.
- If Vs measurements have been performed, the F(T) values can be adjusted
by the ratio [1500/Vs]1/2
Seismic Demand Resulting from Soil
Effects, Contd.

14. Seismic Demand Resulting from Soil
Effects, Contd.
Table 4.1.8.4B
Values of Fa as a Function of Site Class and Sa (0.2s)
Values of FaSite
Class
Sa(0.2) ≤ 0.25 Sa(0.2) = 0.50 Sa(0.2) = 0.75 Sa(0.2) = 1.00 Sa(0.2) = 1.25
A 0.7 0.7 0.8 0.8 0.8
B 0.8 0.8 0.9 1.0 1.0
C 1.0 1.0 1.0 1.0 1.0
D 1.3 1.2 1.1 1.1 1.0
E 2.1 1.4 1.1 0.9 0.9
F Site Specific Site Specific Site Specific Site Specific Site Specific

15. Seismic Demand Resulting from Soil
Effects, Contd.
 Current Codes incorporate local site effects for free-field conditions.
Soil layers are horizontal; i.e. 1D and homogeneous
 They do not include:
• Effects of site topography
Variations in site topography tend to amplify/attenuate ground motions
i.e. 2D/3D effects
• Kinematic interaction effects of propagating seismic waves with foundations
• Cyclic mobility or liquefaction
 Cyclic mobility and cyclic liquefaction result in permanent ground displacements.
These displacements are difficult to quantify. Guidance is given to the designers
how to estimate the displacements.

16. Seismic Demand Resulting from Soil
Effects, Contd.
Equivalent Linear Method:
Commonly used computer codes such as SHAKE use the equivalent linear method.
The primary input parameters for these analyses consist of:
- soil stratigraphy and depth to water table
- unit weight of each layer
- small strain shear moduli and soil damping for each distinct soil type/unit
- normalized modulus reduction and damping variations as a function of shear strain

17. Seismic Demand Resulting from Soil
Effects, Contd.
Equivalent Linear Method:
• Not recommended for shaking levels above 0.3 to 0.4 g (because of strong non-
linear effects)
• Watch for shear strain development (caution if strains > 0.1 – 0.2% )
• Many different data bases exist

18. Seismic Demand Resulting from Soil
Effects, Contd.
Non-Linear Analysis:
Commonly used computer codes such as DESRA-2C, D-MOD and FLAC use truly
non-linear analysis methods to compute the site response. The primary input
parameters for these analyses consist of:
- soil stratigraphy and depth to water table
- unit weight of each layer
- stress-strain-strength variations of soils comprising each distinct soil type/unit
Material damping included by following the stress-strain loops incrementally.
Better represents site response of soft soils or deposits subjected to strong shaking.

19. Seismic Demand Resulting from Soil
Effects, Contd.
Non-Linear Analysis:
• Often damping is modelled using Masing or Modified Masing Behaviour.
This approximation leads to higher damping values.

20. Effect of Ground Displacements on Seismic
Response
 Propagating seismic waves cause soil displacements, which induce kinematic
loads on foundations and buried structures.
 In practice, the effects of kinematic loads are incorporated via uncoupled
methods of analysis.
The free-field soil displacements are estimated first. They are thereafter imposed
on the structural units via non-linear foundation springs; e.g. using non-linear p-y
springs for pile foundations in lateral response analysis.

21. Effect of Ground Displacements on Seismic
Response, Contd.
 Cyclic mobility and liquefaction induce large permanent soil displacements. The
displacements are largely influenced by the local or location-specific soil
conditions.
 The foundations and buried structures should either be designed to withstand
these seismic displacements or site soils should be remediated to mitigate the
effects.
The preferred approach has been to mitigate soil effects when feasible.
 Performance-based design (PBD) methodology is increasingly used to design
soil-structure systems. The methodology requires an assessment of both total
and differential soil displacements impacting foundations; i.e. 2014 CHBDC.

22. Effect of Ground Displacements on Seismic
Response, Contd.
 Cyclic liquefaction will not be triggered in all zones at the same instance;
depends on density variations, amplifications/attenuations, etc.
 Sequential softening or liquefaction can significantly alter the foundation response
to seismic loading.
Convention Center Expansion Project
Vancouver

23. Effect of Ground Displacements on Seismic
Response, Contd.
 Coupled dynamic analysis of soil structure systems are time consuming and
require experience in model development, selection of parameters, and
interpretation of results.
 However, assessing realistic deformations is important especially when carrying
out Performance Based Design. Both total and differential soil displacements
impact design of foundations.
 Some of the recent coded have been more prescriptive on total and differential
foundation movements; 2014 CHBDC
 The state-of-practice is to examine the structure capability to tolerate the
computed soil displacements via uncoupled and pseudo-static methods, except
in special cases.

24. Soil-Foundation Interaction Response
 Incorporating the soil effects in the analysis of foundations has the following
benefits:
• increases the period of vibration
• reduces the response due to radiation and material damping
• Incorporates kinematic effects
 Performance-based design involves design for acceptable levels of damage;
requires soil-foundation-interaction response analyses.
 Coupled analysis of these systems is complex, requires engineering
judgment, and Geotechnical and Structural engineers working closely.
May not be required in all cases.

25. Examples
Difference in Ground Surface Response Spectra Computed from
Equivalent Linear and Non-Linear Methods of Wave Propagation Analyses

26. Examples, Contd.
• Impact of Undrained Shear Strength on
Spectra Computed from Non-Linear
Methods
• Lower-bound values can lead to
unconservative demand

27. Examples, Contd.

28. Summary
1. Site soil conditions play a key role in seismic response of soil-structure systems.
2. Soils exhibit a wide range of behaviour under seismic loading; from “cyclic
mobility” to “cyclic liquefaction”, with the latter resulting in large and catastrophic
ground displacements.
3. Effects of local soil conditions on seismic demand are incorporated in the
Codes via Site Classification and Amplification/Foundation factors. They are
applicable for free-field, level ground conditions and do not include effects of soil
liquefaction, or topographic amplifications.
4. Estimates of soil displacements are important in performance-based design.
Recent Codes have been prescriptive on effects of soil displacements on seismic
response of foundations.

29. Summary, Contd.
5. Effects of sequential softening/liquefaction are important when predicting
displacements and associated site response of soil-structure systems.
6. Selection of soil input parameters play a key role when estimating seismic demand
when site-specific analyses are carried out.
7. Lower-bound soil input parameters are not necessarily conservative in seismic
response analysis.

30. Acknowledgements
• Thanks to Drs. Mahmood Seid-Karbasi, Aran Thurairajah, and Roberto Olivera of
Golder Associates’ who provided input during preparation of this presentation.
• Thanks to Drs. Liam Finn and Carlos Ventura for the many discussions that the
author had during the 2014 CHBDC cycle.

31. Thank You For Your Attention !

32. Soil Foundation Interaction Response,
Contd.

33. Effect of Ground Displacements on Seismic
Response, Contd.
 Coupled dynamic analysis of soil structure systems are time consuming and
require experience in model development, selection of parameters, and
interpretation of results.
 The PBD requires an assessment of both total and differential soil displacements
impacting foundations.
 The 2014 CHBDC prescribes that both total and differential soil/foundation
movements be incorporated in the design; differential ½ of the total.
 The state-of-practice is to examine the structure capability to tolerate the
computed soil displacements via pseudo-static methods of analysis; i.e. impose
the computed total and differential displacements on the foundations and
compute resulting forces and bending moments.

34. Effect of Ground Displacements on Seismic
Response, Contd.
 State-of-practice in liquefaction analysis is to use empirical liquefaction
resistance charts for saturated loose cohesionless soils; these soils are difficult to
sample
 The chart is based on single earthquake event data; Magnitude M, Distance R,
and Amax normalized to M7.5. Data converted to other magnitudes using
Magnitude Scaling Factors.
 When Amax is estimated using probabilistic methods, it is the resulting
contribution from many different combinations of M and R.

35. Effect of Ground Displacements on Seismic
Response, Contd.
 Develop ground motions for many pairs of M and R and carry out ground
response analysis and weight response as per de-aggregation results [Kramer,
2008]
 For a given Amax, evaluate a weighted Cyclic Stress Ratio for liquefaction analysis
[Finn & Wightman, 2007]