77824_05b

the standard penetration test, which obtains data at intervals in the soil deposit.Disadvantages of the cone penetration test are that soil samples cannot be recovered andspecial equipment is required to produce a steady and slow penetration of the cone. Unlikethe SPT, the ability to obtain a steady and slow penetration of the cone is not included aspart of conventional drilling rigs. Because of these factors, in the United States, the CPT isused less frequently than the SPT.

5.5 LABORATORY TESTING

As discussed in Sec. 5.4.2, soil engineering properties that are used in earthquake analyses areusually obtained from field tests (SPT and CPT) or from standard laboratory tests (see Day

SITE INVESTIGATION FOR GEOTECHNICAL EARTHQUAKE ENGINEERING 5.25

FIGURE 5.13 Example of mechanical cone penetrometer tip (Dutch mantle cone). (Reprinted with per-mission from the American Society for Testing and Materials 2000.)

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1999, 2001a). Special laboratory tests used to model the engineering behavior of the soil sub-jected to earthquake loading are typically not performed in practice. For example, in terms ofassessing liquefaction potential, Seed (1987) states: “In developing solutions to practicalproblems involving the possibility of soil liquefaction, it is the writer’s judgment that fieldcase studies and in situ tests provide the most useful and practical tools at the present time.”

Section 5.5.1 discusses the shear strength of the soil, which is an important parameterneeded for earthquake analyses of foundations, slopes, and retaining walls. Section 5.5.2briefly discusses the cyclic triaxial test, which is a valuable laboratory test used for theresearch of the dynamic properties of soil. Appendix A (Glossary 2) presents a list of lab-oratory testing terms and definitions.

5.26 CHAPTER FIVE

FIGURE 5.14 Empirical correlation between cone resistance, vertical effective stress, and friction anglefor clean quartz sand deposits. Note: 1 kg/cm2 approximately equals 1 ton/ft2 (Adapted from Robertson andCampanella 1983; reproduced from Coduto 1994.)

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5.5.1 Shear Strength

The shear strength is an essential soil engineering property that is needed for many typesof earthquake evaluations. There are two basic types of analyses that utilize the shearstrength of the soil: (1) the total stress analysis and (2) the effective stress analysis.

Under no circumstances can a total stress analysis and an effective stress analysis becombined. For example, suppose a slope stability analysis is needed for a slope consistingof alternating sand and clay layers. The factor of safety of the slope must be determined byusing either a total stress analysis or an effective stress analysis, as follows:

1. Total stress analysis� Use total stress shear strength parameters (su or c and �).� Use total unit weight of the soil �t.� Ignore the groundwater table.

2. Effective stress analysis� Use effective stress shear strength parameters (c′ and �′).� Determine the earthquake-induced pore water pressures ue.

Further discussions of the total stress analysis and effective stress analysis are provided next.

Total Stress Analysis. The total stress analysis uses the undrained shear strength of thesoil. The total stress analysis is often performed for cohesive soil, such as silts and clays.Total stress analyses are used for the design of foundations, slopes, and retaining walls thatare subjected to earthquake shaking. The actual analysis is performed for rapid loading orunloading conditions that usually develop during the earthquake. This analysis is ideallysuited for earthquakes, because there is a change in shear stress which occurs quicklyenough that soft cohesive soil does not have time to consolidate; or in the case of heavilyoverconsolidated cohesive soils, the negative pore water pressures do not have time to dis-sipate. The total stress analysis uses the total unit weight �t of the soil, and the location ofthe groundwater table is not considered in the analysis.

To perform a total stress analysis, the undrained shear strength of the soil must be deter-mined. The undrained shear strength su of the cohesive soil is often obtained from uncon-fined compression tests (ASTM D 2166-98, 2000) or from vane shear tests. An alternativeapproach is to use the total stress parameters (c and �) from triaxial tests, such as the uncon-solidated undrained triaxial compression test (ASTM D 2850-95, 2000) or the consolidatedundrained triaxial compression test (ASTM D 4767-95, 2000).

An advantage of the total stress analysis is that the undrained shear strength could beobtained from tests (such as the unconfined compression test or vane shear test) that areeasy to perform. A major disadvantage of this approach is that the accuracy of theundrained shear strength is always in doubt because it depends on the shear-induced porewater pressures (which are not measured), which in turn depend on the many details (i.e.,sample disturbance, strain rate effects, and anisotropy) of the test procedures (Lambe andWhitman 1969).

Effective Stress Analysis. The effective stress analysis uses the drained shear strengthparameters (c′ and �′). Most earthquake analyses of granular soils, such as sands and grav-els, are made using the effective stress analysis (with the possible except of liquefaction-induced flow slides). For cohesionless soil, c′ � 0, and the effective friction angle �′ isoften obtained from drained direct shear tests or from empirical correlations, such as thestandard penetration test (Fig. 5.12) or the cone penetration test (Fig. 5.14).

The effective stress analysis could be used for earthquake-induced loading, provided theearthquake-induced pore water pressures can be estimated. In other words, the effective


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5.28 CHAPTER FIVE

stress generated during the earthquake must be determined. An advantage of the effectivestress analysis is that it more fundamentally models the shear strength of the soil, becauseshear strength is directly related to effective stress. A major disadvantage of the effectivestress analysis is that the pore water pressures must be included in the earthquake analysis.The accuracy of the pore water pressure is often in doubt because of the many factors whichaffect the magnitude of pore water pressure changes, such as the determination of changesin pore water pressure resulting from changes in earthquake loads. For effective stress analy-sis, assumptions are frequently required concerning the pore water pressures that will begenerated by the earthquake.

Cohesionless Soil. These types of soil are nonplastic, and they include such soils asgravels, sands, and nonplastic silt, such as rock flour. A cohesionless soil develops itsshear strength as a result of the frictional and interlocking resistance between the indi-vidual soil particles. A cohesionless soil can be held together only by a confining pres-sure, and it will fall apart when the confining pressure is released. For the earthquakeanalysis of cohesionless soil, it is often easier to perform an effective stress analysis, asdiscussed below:

1. Cohesionless soil above the groundwater table: Often the cohesionless soil abovethe groundwater table will have negative pore water pressures due to capillary tension ofpore water fluid. The capillary tension tends to hold together the soil particles and to pro-vide additional shear strength to the soil. For geotechnical engineering analyses, it is com-mon to assume that the pore water pressures are equal to zero, which ignores the capillarytension. This conservative assumption is also utilized for earthquake analyses. Thus theshear strength of soil above the groundwater table is assumed to be equal to the effectivefriction angle �′ from empirical correlations (such as Figs. 5.12 and 5.14), or it is equal tothe effective friction angle �′ from drained direct shear tests performed on saturated soil(ASTM D 3080-98, 2000).

2. Dense cohesionless soil below the groundwater table: As discussed in Chap. 6,dense cohesionless soil tends to dilate during the earthquake shaking. This causes theexcess pore water pressures to become negative, and the shear strength of the soil is actu-ally momentarily increased. Thus for dense cohesionless soil below the groundwater table,the shear strength is assumed to be equal to the effective friction angle �′ from empiricalcorrelations (such as Figs. 5.12 and 5.14); or it is equal to the effective friction angle �′from drained direct shear tests performed on saturated soil (ASTM D 3080-98, 2000). Inthe effective stress analysis, the negative excess pore water pressures are ignored, and thepore water pressure is assumed to be hydrostatic. Once again, this is a conservativeapproach.

3. Loose cohesionless soil below the groundwater table: As discussed in Chap. 6,loose cohesionless soil tends to contract during the earthquake shaking. This causes thedevelopment of pore water pressures, and the shear strength of the soil is decreased. If liq-uefaction occurs, the shear strength of the soil can be decreased to essentially zero. For anycohesionless soil that is likely to liquefy during the earthquake, one approach is to assumethat �′ is equal to zero (i.e., no shear strength).

For those loose cohesionless soils that have a factor of safety against liquefactiongreater than 1.0, the analysis will usually need to take into account the reduction in shearstrength due to the increase in pore water pressure as the soil contracts. One approach is touse the effective friction angle �′ from empirical correlations (such as Figs. 5.12 and 5.14)or the effective friction angle �′ from drained direct shear tests performed on saturated soil(ASTM D 3080-98, 2000). In addition, the earthquake-induced pore water pressures mustbe used in the effective stress analysis.

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The disadvantage of this approach is that it is very difficult to estimate the pore waterpressures generated by the earthquake-induced contraction of the soil. One option is to useFig. 5.15, which presents a plot of the factor of safety against liquefaction FSL versus porewater pressure ratio ru , defined as ru � u/(�th), where u � pore water pressure, �t � totalunit weight of the soil, and h � depth below the ground surface.

As indicated in Fig. 5.15, at a factor of safety against liquefaction FSL equal to 1.0 (i.e.,liquefied soil), ru � 1.0. Using a value of ru � 1.0, then ru � 1.0 � u/(�th). This meansthat the pore water pressure u must be equal to the total stress (� � �th), and hence theeffective stress �′ is equal to zero (�′ � � � u). For a granular soil, an effective stressequal to zero means that the soil will not possess any shear strength (i.e., it has liquefied).Chapter 6 presents the analyses that are used to determine the factor of safety against liq-uefaction.

4. Flow failures in cohesionless soil: As indicated above, the earthquake analyses forcohesionless soil will often be performed using an effective stress analysis, using �′ andassumptions concerning the earthquake-induced pore water pressure. Flow failures are alsooften analyzed using an effective stress analysis with a value of the pore water pressureratio � 1.0, or by using a shear strength of the liquefied soil equal to zero (that is, �′ � 0and c′ � 0). This is discussed further in Sec. 9.4.

Cohesive Soil. These types of soil are plastic, they include such soils as silts and clays,and have the ability to be rolled and molded (hence they have a plasticity index). For theearthquake analysis of cohesive soil, it is often easier to perform a total stress analysis, asdiscussed below:

1. Cohesive soil above the groundwater table: Often the cohesive soil above thegroundwater table will have negative pore water pressures due to capillary tension of the

FIGURE 5.15 Factor of safety against liquefaction FSL versus the pore water pressure ratio ru for graveland sand. (Developed by Marcuson and Hynes 1990, reproduced from Kramer 1996.)

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5.30 CHAPTER FIVE

pore water fluid. In some cases, the cohesive soil may even be dry and desiccated. The cap-illary tension tends to hold together the soil particles and provide additional shear strengthto the soil. For a total stress analysis, the undrained shear strength su of the cohesive soilcould be determined from unconfined compression tests or vane shear tests. As an alterna-tive, total stress parameters (c and �) could be determined from triaxial tests (e.g., ASTMD 2850-95 and ASTM D 4767-95, 2000).

Because of the negative pore water pressures, a future increase in water content wouldtend to decrease the undrained shear strength su of partially saturated cohesive soil abovethe groundwater table. Thus a possible change in water content in the future should be con-sidered. In addition, a triaxial test performed on a partially saturated cohesive soil often hasa stress-strain curve that exhibits a peak shear strength which then reduces to an ultimatevalue. If there is a significant drop-off in shear strength with strain, it may be prudent to usethe ultimate value in earthquake analyses.

2. Cohesive soil below the groundwater table having low sensitivity: The sensitivitySt of a cohesive soil is defined as the undrained shear strength of an undisturbed soil spec-imen divided by the undrained shear strength of a completely remolded soil specimen. Thesensitivity thus represents the loss of undrained shear strength as a cohesive soil specimenis remolded. An earthquake also tends to shear a cohesive soil back and forth, much as theremolding process does. For cohesive soil having low sensitivity (St 4), the reduction inthe undrained shear strength during the earthquake should be small.

3. Cohesive soil below the groundwater table having a high sensitivity: For highly sen-sitive and quick clays (St 8), there could be a significant shear strength loss during the earth-quake shaking. An example was the Turnagain Heights landslide discussed in Sec. 3.5.2.

The stress-strain curve from a triaxial test performed on a highly sensitive or quick clayoften exhibits a peak shear strength that develops at a low vertical strain, followed by a dra-matic drop-off in strength with continued straining of the soil specimen. The analysis needsto include the estimated reduction in undrained shear strength due to the earthquake shak-ing. In general, the most critical conditions exist when the highly sensitive or quick clay issubjected to a high static shear stress (such as the Turnagain Heights landslide). If, duringthe earthquake, the sum of the static shear stress and the seismic-induced shear stressexceeds the undrained shear strength of the soil, then a significant reduction in shearstrength is expected to occur.

Cohesive soils having a medium sensitivity (4 � St 8) would tend to be an interme-diate case.

4. Drained residual shear strength �′r for cohesive soil: As indicated above, theearthquake analyses for cohesive soil will often be performed using a total stress analysis(that is, su from unconfined compression tests and vane shear tests, or c and � from triax-ial tests).

An exception is cohesive slopes that have been subjected to a significant amount ofshear deformation. For example, the stability analysis of ancient landslides, slopes inoverconsolidated fissured clays, and slopes in fissured shales will often be based on thedrained residual shear strength of the failure surface (Bjerrum 1967, Skempton andHutchinson 1969, Skempton 1985, Hawkins and Privett 1985, Ehlig 1992). When the sta-bility of such a slope is to be evaluated for earthquake shaking, then the drained residualshear strength �′r should be used in the analysis. The drained residual shear strength canbe determined from laboratory tests by using the torsional ring shear or direct shear appa-ratus (Day 2001a).

In order to perform the effective stress analysis, the pore water pressures are usuallyassumed to be unchanged during the earthquake shaking. The slope or landslide mass willalso be subjected to additional destabilizing forces due to the earthquake shaking. These

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destabilizing forces can be included in the effective stress slope stability analysis, and thisapproach is termed the pseudostatic method (see Sec. 9.2.5).

Analysis for Subsoil Profiles Consisting of Cohesionless and Cohesive Soil. Forearthquake analysis where both cohesionless soil and cohesive soil must be considered,either a total stress analysis or an effective stress analysis could be performed. As indi-cated above, usually the effective shear strength parameters are known for the cohesion-less soil. Thus subsoil profiles having layers of sand and clay are often analyzed using aneffective stress analysis (c′ and �′) with an estimation of the earthquake-induced porewater pressures.

If the sand layers will liquefy during the anticipated earthquake, then a total stress analy-sis could be performed using the undrained shear strength su for the clay and assuming theundrained shear strength of the liquefied sand layer is equal to zero (su � 0). Bearing capac-ity or slope stability analyses using total stress parameters can then be performed so that thecircular or planar slip surface passes through or along the liquefied sand layer.

Summary of Shear Strength for Geotechnical Earthquake Engineering. Table 5.4 pre-sents a summary of the soil type versus type of analysis and shear strength that should beused for earthquake analyses.

5.5.2 Cyclic Triaxial Test

The cyclic triaxial test has been used extensively in the study of soil subjected to simulatedearthquake loading. For example, the cyclic triaxial test has been used for research studieson the liquefaction behavior of soil. The laboratory test procedures are as follows (ASTMD 5311-96, 2000):

1. A cylindrical soil specimen is placed in the triaxial apparatus and sealed in a watertightrubber membrane (see Fig. 5.16).

2. A backpressure is used to saturate the soil specimen.

3. An isotropic effective confining pressure is applied to the soil specimen, and the soilspecimen is allowed to equilibrate under this effective stress. Tubing, such as shown inFig. 5.16, allows for the flow of water during saturation and equilibration as well as themeasurement of pore water pressure during the test.

4. Following saturation and equilibration at the effective confining pressure, the valve tothe drainage measurement system is shut, and the soil specimen is subjected to anundrained loading. To simulate the earthquake loading, a constant-amplitude sinu-soidally varying axial load (i.e., cyclic axial load) is applied to the top of the speci-men. The cyclic axial load simulates the change in shear stress induced by theearthquake.

5. During testing, the cyclic axial load, specimen axial deformation, and pore water pres-sure in the soil specimen are recorded. For the testing of loose sand specimens, thecyclic axial loading often causes an increase in the pore water pressure in the soil spec-imen, which results in a decrease in the effective stress and an increase in the axialdeformation.

The cyclic triaxial test is a very complicated test, it requires special laboratory equip-ment, and there are many factors the affect the results (Townsend 1978, Mulilis et al. 1978).Actual laboratory test data from the cyclic triaxial test are presented in Sec. 6.2.

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5.3

2

TABLE 5.4 Soil Type versus Type of Analysis and Shear Strength for Earthquake Engineering

Soil Type oftype analysis Field condition Shear strength

Cohesionless soil Assume pore water pressures are equal to zero, which ignores the capillary tension. Use �′above the from empirical correlations or from laboratory tests such as drained direct shear tests.groundwater table

Dense cohesionless Dense cohesionless soil dilates during the earthquake shaking (hence negative excess pore watersoil below the pressure). Assume earthquake-induced negative excess pore water pressures are zero, andgroundwater table use �′ from empirical correlations or from laboratory tests such as drained direct shear tests.

Loose cohesionless Excess pore water pressures ue generated during the contraction of soil structure. For FSL 1.0, use �′ �soil below the 0 or ru � 1.0. For FSL 1, use ru from Fig. 5.15 and �′ from empirical correlations or fromgroundwater table laboratory tests such as drained direct shear tests.

Flow failures Flow failures are also often analyzed using an effective stress analysis with a value of the pore waterpressure ratio � 1.0, or by using a shear strength of the liquefied soil equal to zero(�′ � 0 and c′ � 0).

Cohesive soil above Determine su from unconfined compression tests or vane shear tests. As an alternative, usethe groundwater table total stress parameters (c and �) from triaxial tests. Consider shear strength decrease due

to increase in water content. For a significant drop-off in strength with strain, considerusing ultimate shear strength for earthquake analysis.

Cohesive soil below Determine su from unconfined compression tests or vane shear tests. As an alternative,the groundwater use total stress parameters (c and �) from triaxial tests.table with St 4

Cohesive soil below Include an estimated reduction in undrained shear strength due to earthquake shaking. Mostthe groundwater significant strength loss occurs when the sum of the static shear stress and the seismic-inducedtable with St 8 shear stress exceeds the undrained shear strength of the soil. Cohesive soils having a medium

sensitivity (4 � St 8) are an intermediate case.

Existing landslides Use an effective stress analysis and the drained residual shear strength (�′r) for the slide plane.Assume pore water pressures are unchanged during earthquake shaking. Include destabilizingearthquake forces in slope stability analyses (pseudostatic method).

Cohesionless Use an soil effective

stress analysis

Use a total Cohesive stress soil analysis

Possible exception

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5.6 PEAK GROUND ACCELERATION

5.6.1 Introduction

As indicated in Fig. 2.14, the ground motion caused by earthquakes is generally characterizedin terms of ground surface displacement, velocity, and acceleration. Geotechnical engineerstraditionally use acceleration, rather than velocity or displacement, because acceleration isdirectly related to the dynamic forces that earthquakes induce on the soil mass. For geotech-nical analyses, the measure of the cyclic ground motion is represented by the maximum hor-izontal acceleration at the ground surface amax. The maximum horizontal acceleration atground surface is also known as the peak horizontal ground acceleration. For most earth-quakes, the horizontal acceleration is greater than the vertical acceleration, and thus the peakhorizontal ground acceleration also turns out to be the peak ground acceleration (PGA).

For earthquake engineering analyses, the peak ground acceleration amax is one of themost difficult parameters to determine. It represents an acceleration that will be inducedsometime in the future by an earthquake. Since it is not possible to predict earthquakes, thevalue of the peak ground acceleration must be based on prior earthquakes and fault studies.

Often attenuation relationships are used in the determination of the peak ground accel-eration. An attenuation relationship is defined as a mathematical relationship that is usedto estimate the peak ground acceleration at a specified distance from the earthquake.Numerous attenuation relationships have been developed. Many attenuation equationsrelate the peak ground acceleration to (1) the earthquake magnitude and (2) the distancebetween the site and the seismic source (the causative fault). The increasingly larger poolof seismic data recorded in the world, and particularly in the western United States, hasallowed researchers to develop reliable empirical attenuation equations that are used tomodel the ground motions generated during an earthquake (Federal EmergencyManagement Agency 1994).


DATAACQUISITION

X-YRECORDER

LD DEF PWPSTRIP CHART

FLUSHING

SPECIMEN

PORE PRESSURETRANSDUCER

PISTON

LOW FRICTIONBRUSHING

CONFININGPRESSURE

CELL

LOADCELL VERTICAL DEFLECTION

TRANSDUCER

VACUUM REGULATOR

BACKPRESSURE REGULATOR

VOLUME CHANGEINDICATOR

FIGURE 5.16 Schematic diagram of the cyclic triaxial test equipment. (Reproduced from ASTM D 5311-96, 2000. Reproduced with permission from the American Society for Testing and Materials.)

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5.6.2 Methods Used to Determine the Peak Ground Acceleration

The engineering geologist is often the best individual to determine the peak ground accelera-tion amax at the site based on fault, seismicity, and attenuation relationships. Some of the morecommonly used methods to determine the peak ground acceleration at a site are as follows:

� Historical earthquake: One approach is to consider the past earthquake history of thesite. For the more recent earthquakes, data from seismographs can be used to determinethe peak ground acceleration. For older earthquakes, the location of the earthquake andits magnitude are based on historical accounts of damage.

Computer programs, such as the EQSEARCH computer program (Blake 2000b), havebeen developed that incorporate past earthquake data. By inputting the location of thesite, the peak ground acceleration amax could be determined. For example, Figs. B.1 toB.11 (App. B) present an example of the determination of amax based on the history ofseismic activity in the southwestern United States and northern Mexico.

The peak horizontal ground acceleration amax should never be based solely on the his-tory of seismic activity in an area. The reason is because the historical time frame ofrecorded earthquakes is usually too small. Thus the value of amax determined from his-torical studies should be compared with the value of amax determined from the other meth-ods described below.

� Code or other regulatory requirements: There may be local building code or other reg-ulatory requirements that specify design values of peak ground acceleration. For exam-ple, by using Fig. 5.17 to determine the seismic zone for a given site, the peak groundacceleration coefficient amax/g can be obtained from Table 5.5. Depending on the distanceto active faults and the underlying subsoil profile, the values in Table 5.5 could underes-timate or overestimate the peak ground acceleration.

� Maximum credible earthquake: The maximum credible earthquake (MCE) is oftenconsidered to be the largest earthquake that can reasonably be expected to occur basedon known geologic and seismologic data. In essence, the maximum credible earthquakeis the maximum earthquake that an active fault can produce, considering the geologicevidence of past movement and recorded seismic history of the area. According toKramer (1996), other terms that have been used to describe similar worst-case levels ofshaking include safe shutdown earthquake (used in the design of nuclear power plants),maximum capable earthquake, maximum design earthquake, contingency level earth-quake, safe level earthquake, credible design earthquake, and contingency design earth-quake. In general, these terms are used to describe the uppermost level of earthquakeforces in the design of essential facilities.

The maximum credible earthquake is determined for particular earthquakes or levelsof ground shaking. As such, the analysis used to determine the maximum credible earth-quake is typically referred to as a deterministic method.

� Maximum probable earthquake: There are many different definitions of the maximumprobable earthquake. The maximum probable earthquake is based on a study of nearbyactive faults. By using attenuation relationships, the maximum probable earthquake mag-nitude and maximum probable peak ground acceleration can be determined.

A commonly used definition of maximum probable earthquake is the largest predictedearthquake that a fault is capable of generating within a specified time period, such as 50or 100 years. Maximum probable earthquakes are most likely to occur within the designlife of the project, and therefore, they have been commonly used in assessing seismic risk(Federal Emergency Management Agency 1994).

Another commonly used definition of a maximum probable earthquake is an earth-quake that will produce a peak ground acceleration amax with a 50 percent probability ofexceedance in 50 years (USCOLD 1985).

5.34 CHAPTER FIVE

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FIGURE 5.17 Seismic zone map of the United States. (Reproduced with permission from the Uniform Building Code 1997.)

5.3

5

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According to Kramer (1996), other terms that have been used to describe earthquakesof similar size are operating basis earthquake, operating level earthquake, probabledesign earthquake, and strength level earthquake.

� USGS earthquake maps: Another method for determining the peak ground accelerationis to determine the value of amax that has a certain probability of exceedance in a specificnumber of years. The design basis ground motion can often be determined by a site-spe-cific hazard analysis, or it may be determined from a hazard map.

An example of a hazard map for California and Nevada is shown Fig. 5.18. This mapwas developed by the USGS (1996) and shows the peak ground acceleration forCalifornia and Nevada. There are similar maps for the entire continental United States,Alaska, and Hawaii. Note that the locations of the highest peak ground acceleration inFig. 5.19 are similar to the locations of the highest seismic zones shown in Fig. 5.17, andvice versa. The USGS (1996) has also prepared maps that show peak ground accelerationwith a 5 percent and 2 percent probability of exceedance in 50 years. These maps are eas-ily accessible on the Internet (see U.S. Geological Survey, National Seismic HazardMapping Project).

The various USGS maps showing peak ground acceleration with a 10, 5, and 2 percentprobability of exceedance in 50 years provide the user with the choice of the appropriatelevel of hazard or risk. Such an approach is termed a probabilistic method, with thechoice of the peak ground acceleration based on the concept of acceptable risk.

A typical ranking of the value of peak ground acceleration amax obtained from the dif-ferent methods described above, from the least to greatest value, is as follows:

1. Maximum probable earthquake (deterministic method)

2. USGS earthquake map: 10 percent probability of exceedance in 50 years (probabilisticmethod)



5. Maximum credible earthquake (deterministic method)

5.6.3 Example of the Determination of Peak Ground Acceleration

This example deals with the proposed W. C. H. Medical Library in La Mesa, California.The different methods used to determine the peak ground acceleration for this project wereas follows:

� Historical earthquake: The purpose of the EQSEARCH (Blake 2000b) computer pro-gram is to perform a historical search of earthquakes. For this computer program, the inputdata are shown in Fig. B.1 (App. B) and include the job number, job name, site coordinatesin terms of latitude and longitude, search parameters, attenuation relationship, and otherearthquake parameters. The output data are shown in Figs. B.2 to B.11. As indicated inFig. B.4, the largest earthquake site acceleration from 1800 to 1999 is amax � 0.189g.

The EQSEARCH computer program also indicates the number of earthquakes of acertain magnitude that have affected the site. For example, from 1800 to 1999, there weretwo earthquakes of magnitude 6.5 or larger that impacted the site (see Fig. B.5).

� Largest maximum earthquake: The EQFAULT computer program (Blake 2000a) wasdeveloped to determine the largest maximum earthquake site acceleration. For this com

5.36 CHAPTER FIVE

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puter program, the input data are shown in Fig. B.12 and include the job number, jobname, site coordinates in terms of latitude and longitude, search radius, attenuation relation-ship, and other earthquake parameters. The output data are shown in Figs. B.13 to B.19. Asindicated in Fig. B.13, the largest maximum earthquake site acceleration amax is 0.4203g.

� Probability analysis: Figures B.20 to B.25 present a probabilistic analysis for the deter-mination of the peak ground acceleration at the site using the FRISKSP computer pro-gram (Blake 2000c). Two probabilistic analyses were performed using differentattenuation relationships. As shown in Figs. B.21 and B.23, the data are plotted in termsof the peak ground acceleration versus probability of exceedance for a specific design lifeof the structure.


FIGURE 5.18 Peak ground acceleration (%g) with a 10 percent probability of exceedance in 50 years forCalifornia and Nevada. (USGS 1996.)

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5.38 CHAPTER FIVE

FIGURE 5.19 Peak ground acceleration (%g) with a 10 percent probability of exceedance in 50 years forthe continental United States. (USGS 1996.)

FIGURE 5.20 Peak ground acceleration (%g) with a 10 percent probability of exceedance in 50 years forAlaska. (USGS 1996.)

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� USGS earthquake maps: Instead of using seismic maps such as shown in Figs. 5.18 to5.21, the USGS enables the Internet user to obtain the peak ground acceleration (PGA)for a specific Zip code location (see Fig. 5.22). In Fig. 5.22, PGA is the peak groundacceleration, PE is the probability of exceedance, and SA is the spectral acceleration.

For this project (i.e., the W. C. H. Medical Library), a summary of the different valuesof peak ground acceleration amax is provided below:

amax � 0.189g (historical earthquakes, see Fig. B.4)

amax � 0.212g (10% probability of exceedance in 50 years, see Fig. 5.22)



amax � 0.40g (seismic zone 4, see Table 5.5)

amax � 0.420g (largest maximum earthquake, see Fig. B.13)


FIGURE 5.21 Peak ground acceleration (%g) with a 10 percent probability of exceedance in 50 years forHawaii. (USGS 1996.)

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There is a considerable variation in values for amax as indicated above, from a low of0.189g to a high of 0.420g. The geotechnical engineer should work with the engineeringgeologist in selecting the most appropriate value of amax. For the above data, based on adesign life of 50 years and recognizing that the library is not an essential facility, an appro-priate range of amax to be used for the earthquake analyses is 0.189g to 0.212g. Using a prob-abilistic approach, a value of 0.21g would seem appropriate.

If the project was an essential facility or had a design life in excess of 50 years, then ahigher peak ground acceleration should be selected. For example, if the project had a 75-year design life and used a 10 percent probability of exceedance, then a peak ground accel-eration amax of about 0.25g should be used in the earthquake analyses (see Fig. B.21). Onthe other hand, if the project was an essential facility that must be able to resist the largest

5.40 CHAPTER FIVE

TABLE 5.5 Seismic Zone Factor Z

Seismic zone Seismic zone factor Z

0 01 0.0752A 0.152B 0.203 0.304 0.40

Notes:1. Data obtained from Table 16-I of the Uniform Building Code (1997).2. See Fig. 5.17 (seismic zone map) for specific locations of the seismic zones 0 through 4.3. Section 1804.5 of the Uniform Building Code (1997) states: “Peak ground acceleration may

be determined based on a site-specific study taking into account soil amplification effects.In the absence of such a study, peak ground acceleration may be assumed equal to the seis-mic zone factor in Table 16-I” (that is, Z � amax/g). In structural analysis, Z is also used incombination with other factors to determine the design seismic load acting on the structure.

FIGURE 5.22 Peak ground acceleration for a specific Zip code location. (USGS 1996.)

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maximum earthquake, then an appropriate value of peak ground acceleration amax would be0.42g. As these examples illustrate, it takes considerable experience and judgment inselecting the value of amax to be used for the earthquake analyses.

5.6.4 Local Soil and Geologic Conditions

For the determination of the peak ground acceleration amax as discussed in the previous sec-tions, local soil and geologic conditions were not included in the analysis. USGS recom-mends that the final step in the determination of amax for a particular site be to adjust thevalue (if needed) for such factors as these:

1. Directivity of ground motion, which can cause stronger shaking in certain directions.

2. Soft soils, which can increase the peak ground acceleration (see Sec. 4.6.1). Often a sitethat may be susceptible to liquefaction will also contain thick deposits of soft soil. Thelocal soil condition of a thick deposit of soft clay is the most common reason for increas-ing the peak ground acceleration amax.

3. Basin effects, such as the conversion to surface waves and reverberation experienced bysites in an alluvial basin.

5.7 REPORT PREPARATION

The results of the screening investigation and the quantitative evaluation will often need tobe summarized in report form for review by the client and the governing agency. The itemsthat should be included in the report, per the Guidelines for Evaluating and MitigatingSeismic Hazards in California (Division of Mines and Geology 1997), are as follows:

� Description of the proposed project’s location, topographic relief, drainage, geologic andsoil materials, and any proposed grading

� Site plan map of the project showing the locations of all explorations, including test pits,borings, penetration test locations, and soil or rock samples

� Description of the seismic setting, historic seismicity, nearest pertinent strong-motionrecords, and methods used to estimate (or source of) earthquake ground motion parame-ters used in liquefaction and landslide analysis

� A geologic map, at a scale of 1 : 24,000 or larger, that shows bedrock, alluvium, collu-vium, soil material, faults, shears, joint systems, lithologic contacts, seeps or springs, soilor bedrock slumps, and other pertinent geologic and soil features existing on and adja-cent to the project site

� Logs of borings, test pits, or other data obtained during the subsurface exploration� Geologic cross sections depicting the most critical (least stable) slopes, geologic struc-

ture, stratigraphy, and subsurface water conditions, supported by boring and/or trenchlogs at appropriate locations

� Laboratory test results, soil classification, shear strength, and other pertinent geotechni-cal data

� Specific recommendations for mitigation alternatives necessary to reduce known and/oranticipated geologic/seismic hazards to an acceptable level of risk.

Not all the above information in the list may be relevant or required. On the other hand,some investigations may require additional types of data or analyses, which should also be


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included in the report. For example, usually both the on-site and off-site geologic and seis-mic hazards that could affect the site will need to be addressed. An example of a geotech-nical engineering report that includes the results of the screening investigation andquantitative evaluation for seismic hazards is provided in App. D.

5.8 PROBLEMS

The problems have been divided into basic categories as indicated below:

Standard Penetration and Cone Penetration Tests

5.1 A standard penetration test (SPT) was performed on a near-surface deposit ofclean sand where the number of blows to drive the sampler 18 in was 5 for the first 6 in, 8for the second 6 in, and 9 for the third 6 in. Calculate the measured SPT N value (blows perfoot) and indicate the in situ density condition of the sand per Table 5.2. Answer: MeasuredSPT N value � 17, and per Table 5.2, the sand has a medium density.

5.2 A clean sand deposit has a level ground surface, a total unit weight �t above thegroundwater table of 18.9 kN/m3 (120 lb/ft3), and a submerged unit weight �b of 9.84 kN/m3

(62.6 lb/ft3). The groundwater table is located 1.5 m (5 ft) below ground surface. Standardpenetration tests were performed in a 10-cm-diameter (4-in) borehole. At a depth of 3 m (10ft) below ground surface, a standard penetration test was performed using a doughnut ham-mer with a blow count of 3 blows for the first 15 cm (6 in), 4 blows for the second 15 cm (6in), and 5 blows for the third 15 cm (6 in) of diving penetration. Assuming hydrostatic porewater pressures, determine the vertical effective stress (�v0′ ) at a depth of 3 m (10 ft) and thecorrected N value [that is, N60, Eq. (5.1)]. Answers: �v0′ � 43 kPa (910 lb/ft2) and N60 � 5.1.

5.3 Using the data from Prob. 5.2, determine the N value corrected for both field test-ing and overburden pressure, and indicate the in situ condition of the sand per Table 5.3.Answers: (N1)60 � 7.8 and per Table 5.3, the sand has a medium density.

5.4 Use the data from Prob. 5.2 and assume a cone penetration test was performed ata depth of 3 m (10 ft) and the cone resistance qc � 40 kg/cm2 (3900 kPa). Determine theCPT tip resistance corrected for overburden pressure. Answer: qc1 � 59 kg/cm2 (5800 kPa).

Shear Strength Correlations

5.5 Using the data from Prob. 5.2, determine the friction angle � of the sand using Fig.5.12. Answer: � � 30°

5.6 Using the data from Prob. 5.4, determine the friction angle � of the sand using Fig.5.14. Answer: � � 40°.

5.42 CHAPTER FIVE

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