Update of the CPT-based liquefaction triggering procedureBoulanger & Idriss 2014

23/10/2014

2

Existing Liquefaction Analysis Methods

Since 1970s several liquefaction analysis

methods have been developed

Laboratory and Field Methods

Numerical Methods

Empirical Methods

Simplified Empirical Methods:

Seed and Idriss (1971) was first

Youd et al. (2001) (NCEER)

Cetin et al. (2004)

Idriss and Boulanger (2006, 2008,2014)

3

Overview of the simplified empirical methods

Liquefaction is usually evaluated with the estimation of factor of safety against liquefaction,

FoS or FSL

4

Idriss & Boulanger 2008 Boulanger & Idriss 2014

5

Boulanger & Idriss 2014 or B&I 2014

B&I 2014

Presented a revised CPT-based liquefaction triggering procedure

New relationships for the Magnitude Scale Factor (MSF)

Modified MSF expression to be a function of magnitude, soil type and density of the soil layer at depth

Update of CPT-based approach

Changed qc1N as a function of FC

Changed CRR versus qc1Ncs

Examination of the CPT-based case histories in terms of soil behaviour type (SBT) index, Ic

These revisions were based on findings from experimental data

(eg cyclic triaxial testing), analytical (eg analyses of ground

motion records) and case history studies (including 50

Christchurch case histories 25 CPTs summarized in Green

et al 2014).

6

CPT-BASED CASE HISTORY DATABASE

Limited to cases with Ic

7

CPT-BASED CASE HISTORY DATABASE

8

Magnitude Scale Factor (MSF)

The value of Cyclic Stress Ratio (CSR) loading is calculated using the following

History of the development of MSF

Seed et al. (1975): proposed a relationship between "Number of Equivalent Uniform Cycles, Neq" &

Magnitude, M, which led to the initial development of "MSF Values".

Seed & Idriss (1982): Tabulated "MSF Values" as a function of M; used shaking table test results by

DeAlba et al. (1976) relating CRR to "Number of Uniform Cycles".

Idriss (1999): Used results of cyclic tests on frozen samples by Yoshimi et al. (1984) to derive a

revised relationship between Neq & M, which resulted in a revised MSF relationship.

Boulanger & Idriss (2014): Modified MSF expression to be a function of magnitude, soil type, and

density (void ratio) of the soil.

MSF=f(earthquake & ground motion characteristics; DR; FC)

and is used to account for duration effects on the triggering of liquefaction

9

Magnitude Scale Factor (MSF)

10

Magnitude Scale Factor (MSF)

Effects of density void ratio on slope b

looser

denser

11

Magnitude Scale Factor (MSF)

looser

denser

The revised MSF relationship shown in Figure 2.6 is believed to provide an improved accounting

of how this relationship should vary with soil characteristics compared to currently available MSF

relationships that provide a single relationship for all cohesionless soils.

MSF is picking up the Mw vs cycles; the link between CRR and Nc (density dependent) less cycles required to liquefy a looser soil than a denser one

Loose relatively flat (need lower no of Nc to develop liquefaction)

12

LOADING - Cyclic Stress Ratio

RESISTANCE - Cyclic Resistance Ratio

Update of CPT-based approach, B&I 2014

The deterministic version of the CPT-based

correlation in terms of qc1N for different values of FC,

rather than in terms of the equivalent clean sand

penetration resistance (qc1Ncs)

13

Update of CPT-based approach, B&I 2014

No change

The terms Q and F are used in soil behavior type

classification charts, such as the chart shown in

Figure (Robertson 1990, SBT).

The exponent n varies from 0.5 in sands to 1.0 in

clays (Robertson and Wride 1998).

The term Ic represents the radial distance between

any point on this chart and the point defined by Q =

2951 and F = 0.06026%.

Circular arcs defined by constant Ic values are used

to approximate the boundaries between different soil

behavior types in this chart

14

Update of CPT-based approach, B&I 2014

Large scatter

Ic=2.6 ranging from 15% to 100% FC

Correlation of lab estimation with

representative CPT depth

Inherent limitations in using Ic to

predict FC

Influence of fines - plasticity

Large uncertainty in the Ic vs FC due

to:

15

Update of CPT-based approach, B&I 2014

CFC

CFC is a fitting parameter

based on specific data or in

the absence set equal to

0.0 (mean) and check for

SD=0.29

Positive CFC for larger

estimate of FC

Negative CFC for lower

estimate of FC

CFC = 0.07 is approximately

equal to the relationship

developed by Robinson et

al (2013) for liquefiable

soils along the Avon River

16

Update of CPT-based approach, B&I 2014

Equivalent clean sand

adjustment (qc1Ncs) which are

considered appropriate for non-

plastic to low-plasticity silty

fines M.. due to changes in the

updated case history dataset

More FC correction in B&I

2014

5.4

16.0 0.01 0.01

I&B 2008B&I 2014

17

B&I 2014 and MBIE Clarifications and updates

Mw SLS ILS ULS

PGA

7.5 0.13 0.20 0.35

6.0 0.19 0.30 0.48

CFC=0.07

B&I 2014 [A]

B&I 2014 [B]

Lower Mw higher PGA is more

critical for B&I 2014 triggering

method

Mw=7.5 is critical for I&B 2008

Mw=6.0 is critical for B&I 2014

18

Update of CPT-based approach, B&I 2014

For sites outside of the Canterbury earthquake region, it would only be appropriate to adopt the 2014

methodology where a seismic hazard de-aggradation plot or other data has been examined to determine

appropriate representative design events

Implementation of 2014 liquefaction triggering procedure for mapping of expected damages at SLS, ILS or

ULS levels of seismic loading REQUIRE use of representative PGA-M values consistent with de-

aggregation of seismic hazard

Highlights the importance of PGAs on either back or forth estimation of liquefaction triggering and the use

of any of existing liquefaction vulnerability parameters (Sv1D, LPI, LSN, LPIc etc)

Deterministic towards a Performance Based Liquefaction Analysis (Kramer et al, 2008 Performance-

Based Liquefaction Potential Evaluation), which is another way to characterize liquefaction potential in

terms of the liquefaction resistance (CRR q or N) required to produce a desired level of performance

Prudent to perform parametric analyses

Using CFC=-0.29, 0.0, 0.29 to evaluate the sensitivity to FC estimates

Ic cut-off shall be repeated for 2.4, 2.6 and 2.8 to evaluate sensitivity to this parameter

Results can then be used to illustrate the importance of site-specific sampling and testing for a given

project, while recognizing that some amount of sampling and testing should always be required for

high risk / high consequence projects

19

Examples

20

Examples CBD, Bealey Avenue

80% clay-like 20% sand-like

21

Examples Cashmere

60% clay-like 40% silt-like

22

Examples Woolston

80% sand-like 20% silt-like

23

Examples Beckenham

20% sand-like 80% silt & clay-like

24

Examples New Brighton

100% sand-like

25

Examples Aranui

100% sand-like

26

Examples Parklands

25% clay-like 75% sand-like

27

Examples CBD, Bealey Avenue

80% clay-like 20% sand-like

2008, 7.5

0.13 0.20 0.350.19 0.30 0.48

2014, 6.0

28

Examples CBD, Bealey Avenue

80% clay-like 20% sand-like

2008, 7.5

0.13 0.19

2008, 6.0

2014, 7.5 2014, 6.0

29

2008, 7.5

0.13 0.20 0.350.19 0.30 0.48

2014, 6.0

Examples New Brighton

100% sand-like

30

2008, 7.5

0.13 0.19

2008, 6.0

2014, 7.5 2014, 6.0

Examples New Brighton

100% sand-like

31

Elements that affect site response in order of importance:

Input motion (Mw PGA)

Soil profile

Soil material properties

Method of analysis