These guidelines were prepared by a subcommittee of the Geoscience Committee on Seismic Hazard Issues at the request of the Nevada Earthquake Safety Council, which is affiliated with the Nevada Division of Emergency Management, Department of Motor Vehicles and Public Safety, and Division of Special Services.
Significant seismic hazards are present in
Nevada. With the increase in
population, the evaluation of liquefaction is becoming more important for land
use planning and development. The
intent of these guidelines is to provide a standardized minimum level of
investigation for liquefaction in Nevada.
They were prepared using established guidelines for liquefaction
evaluation in California, and the current standard of practice in the greater
metro Las Vegas, Reno, Sparks and Carson City areas.
These guidelines were prepared by The Association of Engineering Geologists, Great Basin Section in Reno, Nevada and the Southwestern Section in Las Vegas, Nevada in conjunction with the Nevada Bureau of Mines and Geology, the University of Nevada, Reno, the University of Nevada Las Vegas, other Nevada professional geological/geotechnical engineering organizations, and the private geological/geotechnical engineering consulting community.
This document provides general guidelines for evaluating, mitigating,
and reporting of liquefaction hazards in Nevada. It is intended as a guide for
performing liquefaction investigations and analyses, not as a prescriptive
“standard”. Liquefaction hazard
assessment requires considerable engineering and professional judgment. This
document, therefore, should only be treated as a general guide. It is the consensus of the authors that the
use of new or innovative practices should be encouraged and not be limited by
this document.
For specific details on undertaking the liquefaction evaluation the readers are advised to refer to a recent publication entitled “Recommended Procedures for Implementation of DMG Special Publication 117 – Guidelines for Analyzing and Mitigating Liquefaction in California” (Martin et al., 1999- Ref. 3). This publication is available through Southern California Earthquake Center, University of Southern California.
The investigation of sites for potential
liquefaction shall be included in geotechnical investigations, when any one or
more of the following factors apply: (1) where there is potential for
liquefaction, or (3) where required by the governing agency, or (2) when
requested by the client.
III. Screening
Investigations for Liquefaction Potential
A. Introduction
The purpose of screening investigations is to determine whether a given site has obvious indicators of a low potential for liquefaction failure (e.g., bedrock near the surface or deep ground water without perched water zones), or whether a more comprehensive field investigation is necessary to determine the potential for damaging ground displacements during earthquakes.
B. Screening
Investigations for Liquefaction Hazards should address the Following Basic
Questions:
1. Are potentially liquefiable soil types present?
The vast majority of liquefaction hazards are associated with
saturated sandy and silty soils of low plasticity and density. Cohesive soils with clayey content (particle
size < 0.005 mm) greater than 15% are generally not considered susceptible
to soil liquefaction. Liquefaction
typically occurs in cohesionless sands, silt, and fine-grained gravel deposits
of Holocene to late Pleistocene age in areas where the ground water is shallow
than about 50 feet. Some gravelly soils
are vulnerable to liquefaction if encapsulation by impervious soils prevents
rapid dissipation of seismically induced pore pressure.
2. If present, are the potentially liquefiable soils saturated or
might they become saturated?
In order to be susceptible to liquefaction, potentially
liquefiable soils must be saturated or nearly saturated. Preliminary analysis of hydrologic
conditions such as current, historical and potential future depth(s) to
subsurface water should be undertaken.
Current groundwater level data, including perched water tables, may be
obtained from permanent wells, driller's logs, and exploratory borings. Historical groundwater data can be found in
reports by various government agencies, although such reports often provide
information only on water from production zones and ignore shallower water.
3.
Are
the potentially liquefiable soils relatively shallow?
In general, liquefaction hazards are most severe in the upper
50 feet of the surface, but on a slope near a free face or where deep
foundations go beyond that depth, liquefaction potential should be considered
at greater depths. (Note that for site response characterization, the shear
wave velocity of a potentially liquefiable deposit is characterized to a
greater depth.)
4. Does the geometry of potentially liquefiable soils pose
significant risks that require further investigation?
Thick deposits of liquefiable soils require further
investigation. Additionally, relatively
thin seams of liquefiable soils, if laterally continuous over sufficient area,
can represent potentially hazardous planes of weakness and sliding, and may
thus pose a hazard with respect to lateral spreading and related ground
displacements.
IV. Evaluation
of Liquefaction Resistance
Liquefaction investigations are best performed as part of a
comprehensive investigation as outlined below. These Guidelines are to promote
uniform evaluation of the resistance of soil to liquefaction.
A. Detailed Field Investigation
1. Engineering Geologic Investigations
The engineering geologic investigations should include relative
age, soil classification (percentage of fines passing the #200 sieve and
Plastic Index), three-dimensional distribution, and general nature of exposures
of earth materials within the area.
Surficial deposits should be described in terms of their general
characteristics (including environment of deposition) and their relationship to
present topography and drainage. Due
care should be exercised in interpolating or extrapolating subsurface
conditions. Engineering geologic
investigations should determine:
a. The presence, soil type, gradation, and distribution (including
depth) of unconsolidated deposits;
b. The age of unconsolidated deposits, especially for Quaternary
Period units (both Pleistocene and Holocene Epochs);
c. Zones of flooding or historic liquefaction; and,
d. The groundwater level to be used in the liquefaction analysis
based on data from well logs, boreholes, monitoring wells, geophysical
investigations, or available maps.
2. Geotechnical Field Investigation
The vast majority of liquefaction hazards are associated with
sandy and/or silty soils. For such soil
types, there are currently two widely accepted approaches available for
quantitative evaluation of the soil's resistance to liquefaction. These are: (a) correlation and analyses
based on in-situ Standard Penetration Test (SPT) (ASTM D1586-92) data (see Ref.
3 for details), and (b) correlation and analyses based on in-situ Cone
Penetration Test (CPT) (ASTM D3441-94) data.
Both methods have relative advantages and disadvantages (see Table 1
below). Although either method will
suffice for certain site conditions, there is considerable advantage to using
them jointly. Another valid approach is
the shear wave velocity based liquefaction hazard evaluation (Youd and Idriss,
1997; Andrus, et al. 1999).
3. Geotechnical Laboratory Testing
Laboratory testing is recommended for determining grain size
distribution (particularly the fines content [percent passing the #200 sieve]),
plasticity, unit weight, and moisture content of potentially liquefiable
layers. Note that the moisture content
of a sample taken below the water table can be used to assess the in-situ void
ratio and thereby density.
Table 1:
Relative Merits of SPT and CPT
|
SPT ADVANTAGES |
CPT ADVANTAGES |
|
A sample is retrieved. This permits
identification of soil type with certainty, and permits evaluation of fines
content (which influences liquefaction resistance). |
Continuous penetration resistance data is
obtained and so it is less likely to "miss" thin lenses and seams
of liquefiable material. |
|
Liquefaction resistance correlation is
based primarily on field case histories, and the vast majority of the field
case history database is for in-situ SPT data. |
The CPT takes less time than the SPT
since no borehole is required. |
|
MAJOR DISADVANTAGE |
MAJOR DISADVANTAGE |
|
The SPT provides only averaged data over
discrete increments. It does not
distinguish data particular to thin inclusions (seams and lenses). |
The CPT provides poor resolution with
respect to soil classification, and so usually requires some complementary
borings with samples to more reliably define soil types and stratigraphy. |
For most common structures built using the Uniform Building Code (UBC),
as a minimum a probabilistically derived peak ground acceleration with a 10%
probability of exceedance in 50 years (i.e. 475-year return period) should be
used when site-specific analyses are performed. The factor of safety for level
ground liquefaction resistance has been defined as FS = CSRliq / CSReq where
CSReq is the cyclic stress ratio generated by the anticipated earthquake ground
motions at the site, and CSRliq is the cyclic stress ratio required to generate
liquefaction (Seed and Idriss, 1982). A
factor of safety in the range of about 1.1 is generally acceptable for single
family dwellings, while a higher value in the range of 1.3 is appropriate for
more critical structures. Furthermore,
consequences of different liquefaction hazards vary. For example, hazards stemming from flow failure are often more
disastrous than hazards from differential settlement. Table 2 provides general guidelines for selecting a factor of
safety. This factor of safety assumes
that high quality, site-specific penetration resistance and geotechnical
laboratory data were collected, and that appropriate ground-motion data were
used in the analyses. If lower factors
of safety are calculated for some soil zones, then an evaluation of the level
(or severity) of the hazard associated with potential liquefaction of these
soils should be made.
Table
2: Factors of Safety for Liquefaction Hazard
Assessment*
|
|
|
Factor
of Safety |
|
|
Consequence
of Liquefaction |
(N1)60Clean
Sand |
Non
Critical Structure |
Critical
Structure |
|
|
|
|
|
|
Settlement |
£
15 |
1.1 |
1.3 |
|
|
£
30 |
1.0 |
1.2 |
|
|
|
|
|
|
Surface Manifestation |
£
15 |
1.2 |
1.4 |
|
|
£
30 |
1.0 |
1.2 |
|
|
|
|
|
|
Lateral Spread |
£
15 |
1.3 |
1.5 |
|
|
£
30 |
1.0 |
1.2 |
*
Developed based on guidelines given in Ref. 3
Such hazard assessment requires considerable engineering and
professional judgment. The following
is, therefore, only a guide. The
assessment of potential liquefaction of soil deposits at a site must consider
two basic types of hazard:
1. Translational site instability (sliding, edge failure, lateral
spreading, flow failure, etc.) that may potentially affect all or large
portions of the site; and
2. A more localized hazard at and immediately adjacent to the
structures and/or facilities of concern (e.g., bearing failure, settlement,
localized lateral movements).
As Bartlett and Youd (1995) have stated: "Two general questions
must be answered when evaluating the liquefaction hazards for a given site:
1. 'Are the sediments susceptible to liquefaction?'; and
2. 'If liquefaction does occur, what will be the ensuing amount of
ground deformation'?"
Mitigation
should provide suitable levels of protection with regard to the two general
types of liquefaction hazards previously discussed. The scope and type(s) of mitigation required depend on the site
conditions present and the nature of the proposed project. Individual
mitigation techniques may be used, but the most appropriate solution may
involve using them in combination. For
more details on the effectiveness of various mitigation techniques see Ref. 3.
Reports that
address liquefaction hazards may also need to include the following:
A. If methods other than Standard Penetration Test (SPT; ASTM
D1586-92) and Cone Penetration Test (CPT; ASTM 3441-94) are used, description
of pertinent equipment and procedural details of field measurements of
penetration resistance (borehole type, hammer type and drop mechanism, sampler
type and dimensions, etc.).
B. Boring logs showing raw (unmodified) N-values if SPT's are
performed; CPT probe logs showing raw qc-values and plots of raw
sleeve friction if CPT's are performed.
C. Explanation of the basis of the methods used to convert raw
SPT, CPT or non-standard data to "corrected" and
"standardized" values.
D. Tabulation and/or plots of corrected values used for analyses.
E. Explanation of methods used to develop estimates of field loading
equivalent uniform cyclic stress ratios (CSReq) used to represent the
anticipated field earthquake excitation (cyclic loading).
F. Explanation of the basis for evaluation of the equivalent
uniform cyclic stress ratio necessary to cause liquefaction (CSRliq) at the
number of equivalent uniform loading cycles considered representative of the
design earthquake.
G. Factors of safety against liquefaction at various depths and/or
within various potentially liquefiable soil units.
H. Conclusions regarding the potential for liquefaction and likely
deformation and its likely impact on the proposed project.
I. Discussion of proposed mitigation measures, if any, necessary
to reduce potential damage caused by liquefaction to an acceptable level of
risk.
J.
Criteria
for SPT-based, CPT-based, or other types of acceptance testing, if any, that
will be used to demonstrate satisfactory remediation.
VII. Definitions
ASTM American Society for
Testing and Materials
CPT Cone Penetration Test
(ASTM D3441-94).
CSR Cyclic stress ratio — a normalized measure of cyclic stress
severity, expressed as equivalent uniform cyclic shear stress divided by some
measure of initial effective overburden or confining stress.
CSReq The equivalent uniform cyclic stress ratio representative of the
dynamic loading imposed by an earthquake.
CSRliq The equivalent uniform cyclic stress ratio
required to induce liquefaction within a given number of loading cycles [that
number of cycles considered representative of the earthquake under consideration].
FS Factor of safety —
the ratio of the forces available to resist failure divided by the driving
forces.
Ground
Loss Localized ground
subsidence.
Lique-
faction Significant loss of
soil strength due to pore pressure increase.
N Penetration
resistance measured in SPT tests (blows/ft).
N1 Normalized SPT N-value
(blows/ft); corrected for overburden stress effects to the N-value which would
occur if the effective overburden stress was 1.0 tons/ft2.
(N1)60 Standardized, normalized
SPT-value; corrected for both overburden stress effects and equipment and procedural effects (blows/ft).
PI Plasticity Index; the difference between the Atterberg
Liquid Limit (LL) and the Atterberg Plastic Limit (PL) for a cohesive
soil. [PI(%) = LL(%) - PL(%)].
qc Tip
resistance measured by CPT probe (force/length2).
qc,1 Normalized CPT tip resistance
(force/length2); corrected for overburden stress effects to the qc
value which would occur if the effective overburden stress was 1.0 tons/ft2.
SPT Standard Penetration Test (ASTM D1586-92).
UBC The Uniform Building Code, published by the International
Conference of Building Officials (ICBO, 1997), periodically updated.
(1)
Andrus,
R., Stokoe, K.L., and Chung, R.M. (1999).
“Draft Guidelines for Evaluating of Liquefaction Resistance using Shear
Wave Velocity Measurements and Simplified Procedures,” Report No. NIST IR-6277,
National Institute of Standards and Testing, Gaithersburg, MD.
(2)
Bartlett,
S.F., and Youd., T.L. (1995).
“Empirical Prediction of Liquefaction-Induced Lateral Spread,” Journal
of Geotechnical Engineering, Vol. 121(4), pp. 316-329.
(3)
Martin
G.R., and Lew M. (Editors) (1999).
“Recommended
Procedures for Implementation of DMG Special Publication 117 – Guidelines for
Analyzing and Mitigating Liquefaction in California,” Southern California
Earthquake Center, University of Southern California, March.
(4)
National
Research Council (NRC) (1985).
“Liquefaction of Soils During Earthquakes,” Committee on Earthquake
Engineering, National Research Council, Report No. CETS-EE-001.
(5)
Seed,
H.B., and Idriss, I.M. (1982). “Ground
Motions and Soil Liquefaction During Earthquakes,” Earthquake Engineering
Research Institute (EERI) Monograph.
(6)
Youd,
T.L., and Idriss, I.M. (Editors) (1997).
“Proceedings of the NCEER Workshop on Evaluation of Liquefaction
Resistance of Soils,” Salt Lake City, NCEER Technical Report NCEER-97-0022,
Buffalo, NY.
Accepted on 18 February 2000