SEISMIC GROUND HAZARDS
FIGURE 84
FIGURE 84 Use of seismic cone peneUse of seismic cone penetrometetrometer for evaluatir for evaluating site-speng site-specific soilcific soil
liquefaction concerns.
DETERMINE LEVEL OF GROUND SHAKING DETERMINE LEVEL OF GROUND SHAKING
In liquefaction analyses, the level of ground shaking from In liquefaction analyses, the level of ground shaking from seismic loading is expressed in terms of the CSR. The CSR seismic loading is expressed in terms of the CSR. The CSR can be estimated using seismic ground hazard maps pub- can be estimated using seismic ground hazard maps pub- lished by the U.S. Geological Survey, state geological agen- lished by the U.S. Geological Survey, state geological agen- cies, the IBC, or the National Earthquake Hazard Research cies, the IBC, or the National Earthquake Hazard Research Program, or alternatively evaluated more properly using site- Program, or alternatively evaluated more properly using site- specific
specificGGmaxmaxdata within commercial codes (e.g., RASCALS,data within commercial codes (e.g., RASCALS,
SHAKE, EduSHAKE, SHAKE2000, or DEEPSOIL). Using SHAKE, EduSHAKE, SHAKE2000, or DEEPSOIL). Using the conventional simplified procedures, the CSR is expressed the conventional simplified procedures, the CSR is expressed as (Seed and Idriss 1971):
as (Seed and Idriss 1971):
(79) (79) where
whereaveaveis the average equivalent uniform shear stress gen-is the average equivalent uniform shear stress gen-
erated by the earthquake (assumed to be 65% of the maxi- erated by the earthquake (assumed to be 65% of the maxi- mum induced stress),
mum induced stress), aamaxmax is the peak ground acceleration,is the peak ground acceleration,
g
gthe gravitational acceleration constant (the gravitational acceleration constant (gg9.8 m/s9.8 m/s223232
ft/s ft/s22),),
vo
voandandvovoare the total and are the total and effective vertical stresses,effective vertical stresses,
respectively, and
respectively, and r r d d is a stress reduction coefficient thatis a stress reduction coefficient that
accounts for the flexibility of the model soil column (0.5 accounts for the flexibility of the model soil column (0.5
r
r d d 1.0). By using the recommendations of the National1.0). By using the recommendations of the National
Center for Earthquake Engineering workshop on soil lique- Center for Earthquake Engineering workshop on soil lique- faction (Youd et al. 2001),
faction (Youd et al. 2001), r r d d can be obtained with depthcan be obtained with depth z z
(meters) as follows: (meters) as follows: For depth For depth z z9.15 m:9.15 m: r r d d (131(131 z z))//113311 ((8800aa)) For 9.15 m For 9.15 m z z23 m:23 m: r r d d (44(44 z z))//3377 ((8800bb)) For 23 m For 23 m z z30 m:30 m: r r d d (93(93 z z))//112255 ((8800cc)) For For z z30 m:30 m: r r d d 0.50 0.50 (80d)(80d)
The value of
The value of aamaxmaxis taken from the is taken from the appropriate design eventsappropriate design events
for a given project (i.e., the 2%, 5%, or 10% probability for a given project (i.e., the 2%, 5%, or 10% probability earthquake for a certain period of
earthquake for a certain period of time, the maximum credi-time, the maximum credi- ble event for a known fault located a certain distance from ble event for a known fault located a certain distance from the site, or a
the site, or a code-based response spectrum).code-based response spectrum).
The CRR is the threshold for liquefaction and used to The CRR is the threshold for liquefaction and used to compare the available soil resistance with level of ground compare the available soil resistance with level of ground shaking represented by the CSR. Therefore, if the CSR shaking represented by the CSR. Therefore, if the CSR value is higher than the CRR, the soil has a high likelihood value is higher than the CRR, the soil has a high likelihood of liquefaction. It the CSR falls beneath the CRR, the like- of liquefaction. It the CSR falls beneath the CRR, the like- lihood of liquefaction is small. The CRR can be expressed lihood of liquefaction is small. The CRR can be expressed using conventional deterministic approaches that give a using conventional deterministic approaches that give a binary decision (liquefaction or no liquefaction), or alterna- binary decision (liquefaction or no liquefaction), or alterna- tively, in terms of probabilistic curves of increasing risks of tively, in terms of probabilistic curves of increasing risks of liquefaction.
liquefaction.
Deterministic approaches include procedures based on Deterministic approaches include procedures based on stress-normalize
stress-normalized tip resistance (e.g., Stark and Olson d tip resistance (e.g., Stark and Olson 1995;1995; Robertson and Wride 1998; Youd et al.
Robertson and Wride 1998; Youd et al. 2001) and/or stress-2001) and/or stress- normalized shear wave velocity (e.g., Andrus and Stokoe normalized shear wave velocity (e.g., Andrus and Stokoe 2000; Youd et al. 2001). For the CPT-based method shown 2000; Youd et al. 2001). For the CPT-based method shown in Fi
in Figurgure 85e 85((upper upper ), the cone tip resistance is normalized as), the cone tip resistance is normalized as CSR CSR
= =
aveave==
⎛ ⎛
⎝
⎝ ⎜⎜
⎞
⎞
⎠
⎠ ⎟
⎟ ⎛ ⎛ ⎝ ⎝ ⎜⎜
⎞ ⎞ ⎠ ⎠ ⎟ ⎟
vo vo vo vo vo vo d d a a g g r r 0 0 65..65 maxmax 68 68 a function of thea function of the effective stress (actual normalization crite-effective stress (actual normalization crite- ria depends on the CPT
ria depends on the CPT soil classification) and is designatedsoil classification) and is designated q
qcc11 N N . For clean quartz sands:. For clean quartz sands:
(81) (81)
where atmospheric pressure is used to make
where atmospheric pressure is used to make the form dimen-the form dimen- sionless (note: 1 atm
sionless (note: 1 atm1 bar1 bar100100kPa). For sikPa). For silty sands, lty sands, thethe
stress-normalized cone tip resistance is modified to the stress-normalized cone tip resistance is modified to the adjusted tip resistance, designated (
adjusted tip resistance, designated (qqcc11 N N ))cscs, which is its equiv-, which is its equiv-
alent clean sand value, by
alent clean sand value, by the relationship:the relationship: ( (qqcc11 N N ))cscsk k ccqqcc11 N N (82)(82) q q qq q q q q c c N N t t vo vo t t vo vo 1 1
= =
00 55==
(( // )) (( // )) .. atm atm atmatm atmatm
FIGURE
FIGURE 8585 DetermiDeterministic anistic approapproaches foches forr
liquefaction analysis of clean sand based
liquefaction analysis of clean sand based
on
on((upper upper ) normalized cone tip resistance) normalized cone tip resistance
(after Robertson and Wride 1998) and
(after Robertson and Wride 1998) and
(
(lower lower ) normalized shear wave velocity) normalized shear wave velocity
(after Andrus and Stokoe 2000).
where
whereK K ccis the correction factor for the apparent fines con-is the correction factor for the apparent fines con-
tent and is empirically calculated from a modified CPT soil tent and is empirically calculated from a modified CPT soil classification index,
classification index, I I cc. Here, the index is redefined by. Here, the index is redefined by
Robertson and Wride (1998) using only CPT
Robertson and Wride (1998) using only CPT QQ andandF F datadata because porewater pressures are often near hydrostatic for because porewater pressures are often near hydrostatic for loose to firm clean sands (thus both
loose to firm clean sands (thus both uu andand B Bqq 0) (see0) (see
Table 11). The modified CPT soil type index is: Table 11). The modified CPT soil type index is:
(83) (83) Specifically,
Specifically,K K ccis evaluated from:is evaluated from:
For
For I I cc1.64:1.64: K K cc11..00 ((8844aa))
For
For I I cc1.64:1.64: (84b)(84b)
This requires iteration as the value of
This requires iteration as the value of QQis adjusted tois adjusted toqqcc11 N N forfor
stress normalization if
stress normalization if I I cc2.6 (see Robertson and Wride2.6 (see Robertson and Wride
1998). The level of ground motion (CSR) and the adjusted 1998). The level of ground motion (CSR) and the adjusted tip resistance (
tip resistance (qqcc11 N N ))cscsare compared with the CRR to deter-are compared with the CRR to deter-
mine whether liquefaction will or will not occur. For clean mine whether liquefaction will or will not occur. For clean sand, the CRR is
sand, the CRR is calculated by the following equation for ancalculated by the following equation for an earthquake moment magnitude of 7.5 (Youd et al. 2001; earthquake moment magnitude of 7.5 (Youd et al. 2001; Robertson and Wride 1998):
Robertson and Wride 1998): If 50
If 50((qqcc11 N N ))cscs116600 ((8855aa))
If (
If (qqcc11 N N ))cscs5500 ((8855bb))
For liquefaction evaluation based on shear
For liquefaction evaluation based on shear wave velocity,wave velocity, a deterministic
a deterministic chart procedure is shown in Fichart procedure is shown in Figure 85gure 85((lower lower )) using the stress-normalized shear wave velocity that is des- using the stress-normalized shear wave velocity that is des- ignated
ignatedV V ss11and determined as:and determined as:
(86) (86) where
where V V ssis in meters/second. The CRR for an earthquakeis in meters/second. The CRR for an earthquake
moment magnitude of 7.5 is found in Andrus and Stokoe moment magnitude of 7.5 is found in Andrus and Stokoe (2000) and Youd et al. (2001):
(2000) and Youd et al. (2001):
(87) (87) CRR CRR7 57..5
== ( (
aa VVss11 110000)) +
22+
bb⎡⎡⎣⎣
11 V( (
Vss11cc V−−
V ss11)) −−
11 VVss11cc⎤⎤⎦⎦
V V ss V V ss vo vo 1 1==
00 2525 (( // )) .. atm atm CRR CRR77 55 00 883333 11 1000 1000 00 0505 .. .. (( )) ..==
⎡⎡
⎣⎣⎢⎢
⎤⎤
⎦⎦⎥⎥++
q qc N c N ccss CRR CRR77 55 11 3 3 93 93 1000 1000 00 0808 .. (( )) ..==
⎡⎡
⎣⎣⎢⎢
⎤⎤
⎦⎦⎥⎥
++
q qc N c N ccss K Kcc==−−
0 40..40033IIcc44++
5 55..58811IIcc33−−
2211 6..633IIcc22++
3333 7..755IIcc−−
1717 8 .. 88888 I I QQ FF cc==
[[ ..33 4477−−
lloogg ]]++
[[ ..11 222++
2 lloogg ]] 2 2 22 V V ss11cc220 m/s for220 m/s forFC FC 5%;5%; VV ss11cc210 m/s for210 m/s forFC FC 20%; and20%; and
V
V ss11cc200 m/s for200 m/s forFC FC 35%.35%.
A calculated factor of safety (
A calculated factor of safety (F F ss) can be defined as) can be defined as F F ss
CRR/CSR for a particular earthquake magnitude and set of CRR/CSR for a particular earthquake magnitude and set of data. In more recent evaluations, CRR curves of different data. In more recent evaluations, CRR curves of different probabilities of occurrence have been developed
probabilities of occurrence have been developed from map-from map- ping functions (Chen and Juang 2000; Juang and Jiang 2000) ping functions (Chen and Juang 2000; Juang and Jiang 2000) to relate the safety factor
to relate the safety factorF F ssto the to the liquefaction probabilityliquefaction probabilityPP L L..
Based on a database of 225 CPT-based cases reported by Based on a database of 225 CPT-based cases reported by Juang and Jiang (2000) for
Juang and Jiang (2000) for qqcc11 N N probability curves:probability curves:
(88) (88) P P L L
= =
1 11⎣⎣⎡⎡
1++ ( (
F F ss 11 00))
⎤⎤⎦⎦
3 3 3434 .. .. SoilSoil Classification Classification Zone Zone No.* No.* RanRangge of Ce of CPPT IndexT Index I I ccValValuueses
Or
Orgganic anic Clay Clay Soils Soils 22 I I cc> 3.60> 3.60
Clays
Clays 3 3 2.95 2.95 << I I cc< 3.60< 3.60
Silt Mixt
Silt Mixtuures res 4 4 2.60 2.60 << I I cc< 2.95< 2.95
Sand Mixt
Sand Mixtuures res 5 5 2.05 2.05 << I I cc< 2.60< 2.60
Sands
Sands 6 6 1.31 1.31 << I I cc< 2.05< 2.05
Gravelly
Gravelly Sands Sands 77 I I cc< 1.31< 1.31
After Robertson and Wride (1998).
After Robertson and Wride (1998).
*Note: Zone n
*Note: Zone nuumber per Robertson SBT (1990).mber per Robertson SBT (1990).
FIGURE 8
FIGURE 866 ProbabProbabilistic cilistic cyclic resyclic resistance istance ratiosratios
(CRRs) for cle
(CRRs) for clean sands baan sands based onsed on((upper upper ))
normali
normalized cone tip resistazed cone tip resistance andnce and((lower lower ))
normali
normalized shear wave velocityzed shear wave velocity(after Juang(after Juang
and
70 70
For the normalized shear wave velocity (
For the normalized shear wave velocity (V V ss11), there is a), there is a
similar mapping function (Juang et al. 2001): similar mapping function (Juang et al. 2001):
(89) (89) Separate CRR
Separate CRR curves corresponding to curves corresponding to different probabilitdifferent probabilitiesies of liquefaction ranging from 10% to 90%
of liquefaction ranging from 10% to 90% usingusingqqcc11 N N andandV V ss11areare
presented in Figures 86 (
presented in Figures 86 (upper upper ) and 86 () and 86 (lower lower ), respectively.), respectively. P
P L L
= =
1 11⎡⎡⎣⎣
1++ ( (
F F ss 00 7..722))
3 13..1⎤⎤⎦⎦
Alternate methods of post-processing CPT data to obtain Alternate methods of post-processing CPT data to obtain probabilistic assessments of soil liquefaction potential have probabilistic assessments of soil liquefaction potential have been recently proposed by Moss et al. (2003, 2006). These been recently proposed by Moss et al. (2003, 2006). These include special stress-normalizati
include special stress-normalization procedures for on procedures for the CPTthe CPT resistances and have been specifically developed to better resistances and have been specifically developed to better address the reliability of seismic ground hazards in sands address the reliability of seismic ground hazards in sands having various percentage fines contents.
This section discusses a variety of other applications for This section discusses a variety of other applications for CPT, including slope stability investigations and landslide CPT, including slope stability investigations and landslide forensics, pavement investigations, sinkholes, and environ- forensics, pavement investigations, sinkholes, and environ- mental investigations.
mental investigations.
In certain circumstances, cone penetrometer technology In certain circumstances, cone penetrometer technology has been employed to assist in delineating and detecting has been employed to assist in delineating and detecting anomalous conditions or unusual features in the ground. anomalous conditions or unusual features in the ground. Because traditional drilling and sampling is intermittent at Because traditional drilling and sampling is intermittent at say 5-ft-depth increments (1.5 m), the continuous nature of say 5-ft-depth increments (1.5 m), the continuous nature of CPT helps provide detailed logging with three or more chan- CPT helps provide detailed logging with three or more chan- nels. Although downhole probes (e.g., geophysical tools or nels. Although downhole probes (e.g., geophysical tools or video cameras) can be lowered down a predrilled borehole, video cameras) can be lowered down a predrilled borehole, the process often requires casing of the hole and is much the process often requires casing of the hole and is much more destructive than CPT invasion (i.e., an augured 8-in. more destructive than CPT invasion (i.e., an augured 8-in. oror 200-mm-diameter hole versus a 1.4-in. or 36-mm pushed- 200-mm-diameter hole versus a 1.4-in. or 36-mm pushed- place hole). A listing of select special applications by CPT is place hole). A listing of select special applications by CPT is presented in Table 12, with a cited reference source given presented in Table 12, with a cited reference source given should additional details be desired.
should additional details be desired.
CPT has enjoyed particular use on geo-environmental site CPT has enjoyed particular use on geo-environmental site investigations because the test produces no samples, no cut- investigations because the test produces no samples, no cut- tings, and no spoil, thereby minimizing the generation of tings, and no spoil, thereby minimizing the generation of above-ground cleanup in sensitive areas and contaminated above-ground cleanup in sensitive areas and contaminated ground. Of well-known acclaim, the conductivity cone is ground. Of well-known acclaim, the conductivity cone is commercially available from manufacturers and CPT service commercially available from manufacturers and CPT service firms as an expedient means to map contaminant plumes and firms as an expedient means to map contaminant plumes and detect the presence of underground pollutants (Campanella detect the presence of underground pollutants (Campanella and Weemees 1990). Resistivity (ohm-meter) is the recipro- and Weemees 1990). Resistivity (ohm-meter) is the recipro- cal of electrical conductivity; therefore, the device is also cal of electrical conductivity; therefore, the device is also referred to as the resistivity cone (see Figure 87). The elec- referred to as the resistivity cone (see Figure 87). The elec- trodes are provided as an array of either four axial rings at set trodes are provided as an array of either four axial rings at set vertical spacings or with a button array (positioned diametri- vertical spacings or with a button array (positioned diametri- cally). The special membrane interface probe offers a single cally). The special membrane interface probe offers a single button electrode for an index determination of in situ resistiv- button electrode for an index determination of in situ resistiv- ity penetration and gas sampling. An example resistivity ity penetration and gas sampling. An example resistivity piezocone sounding (RCPTu
piezocone sounding (RCPTu11) from downtown Memphis,) from downtown Memphis,
Tennessee, is presented in Figure 88. Electrical conductivity Tennessee, is presented in Figure 88. Electrical conductivity can be used to identify soil types. It is also employed in can be used to identify soil types. It is also employed in coastal areas to distinguish the upper freshwater table from coastal areas to distinguish the upper freshwater table from the lower salt water regime.
the lower salt water regime.
Whereas resistivity induces a direct current electrical cur- Whereas resistivity induces a direct current electrical cur- rent into the ground, a similar approach can be provided rent into the ground, a similar approach can be provided using alternating current and thus established to obtain using alternating current and thus established to obtain
dielectric measurements (permittivity). These dielectric dielectric measurements (permittivity). These dielectric readings can be interpreted to provide direct real-time readings can be interpreted to provide direct real-time profiles of volumetric water content. For portions of the profiles of volumetric water content. For portions of the sounding that extend below the groundwater table, the sounding that extend below the groundwater table, the gravim
gravimetric water coetric water content can be mappedntent can be mapped. Figure 87. Figure 87 ((cen-cen- ter
ter ) shows a special dielectric CPT penetrometer developed) shows a special dielectric CPT penetrometer developed for this purpose (Shinn et al. 1998).
for this purpose (Shinn et al. 1998).
New developments in sensors and testing procedures for New developments in sensors and testing procedures for CPT have been introduced to enhance the capabilities of CPT have been introduced to enhance the capabilities of direct-push technologies. Selected instruments and innova- direct-push technologies. Selected instruments and innova- tions are listed in Table 13. Illustrative examples of these tions are listed in Table 13. Illustrative examples of these CPT technologies include the use of cableless systems to CPT technologies include the use of cableless systems to transmit or store data, as shown in Figure 89, including: ( transmit or store data, as shown in Figure 89, including: (left left )) memocone and (
memocone and (right right ) audio-signal cone. Another cableless) audio-signal cone. Another cableless system utilizes special glass-lined rods to
system utilizes special glass-lined rods to allow transmissionallow transmission by infrared signals. These systems are advantageous in the by infrared signals. These systems are advantageous in the following situations: (1) when conducting CPTu with drill following situations: (1) when conducting CPTu with drill rigs where the crew is not sensitive to working with elec- rigs where the crew is not sensitive to working with elec- tronic cables, (2) offshore deployment, and (3) wireline sys- tronic cables, (2) offshore deployment, and (3) wireline sys- tems and deep soundings. In the case of the memocone, the tems and deep soundings. In the case of the memocone, the data are stored downhole until the penetrometer is retrieved data are stored downhole until the penetrometer is retrieved back at the ground surface and the readings of time
back at the ground surface and the readings of time t t ,,qqt t ,, f f ss,,
and
anduu22are matched with the depth logger readings of timeare matched with the depth logger readings of time t t
and depth
and depth z z. In the audio-signal cone, the data are transmit-. In the audio-signal cone, the data are transmit- ted by sound waves up
ted by sound waves up through the center of the rods in realthrough the center of the rods in real time and a special microphone used to capture the sounds time and a special microphone used to capture the sounds that are digitally decoded for the data logger. A similar that are digitally decoded for the data logger. A similar approach is used for infrared signals.
approach is used for infrared signals.
With standard analog systems, the basic logging was With standard analog systems, the basic logging was restricted to depth (
restricted to depth ( z z), cone tip stress (), cone tip stress (qqt t ), sleeve friction), sleeve friction
(
( f f ss), porewater pressures (), porewater pressures (uu), and inclination (), and inclination (ii), often), often
because the electronic cable was of the 10-pin type (10 because the electronic cable was of the 10-pin type (10 wires). Although 12-, 16-, 24-, and even 32-pin wires have wires). Although 12-, 16-, 24-, and even 32-pin wires have been available, they are fragile with short lives because been available, they are fragile with short lives because of of the restrictive inner diameter of the cone rods that the cable the restrictive inner diameter of the cone rods that the cable must be threaded through. A few analog systems could must be threaded through. A few analog systems could circumvent the 10-wire limitations by either forgoing circumvent the 10-wire limitations by either forgoing the inclinometer or friction readings. The multi-piezo- the inclinometer or friction readings. The multi-piezo-