CHAPTER 7 SITE CLASSIFICATION -...
Transcript of CHAPTER 7 SITE CLASSIFICATION -...
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CHAPTER 7
SITE CLASSIFICATION
7.1 GENERAL
Site classification has been carried out through experimental data
the geophysical method of electrical resistivity test data and based on the geology
data. Spectral acceleration at ground level was evaluated using correlation
approach. It has been experimented out at 173 locations and respective velocity
profiles are obtained.
The average shear wave velocity for 30 m depth (Vs30) has been
calculated and is used for the site classification of the study area as per
NEHRP (National Earthquake Hazards Reduction Program). Based on the
Vs30 values major part of the study area is classified as “site class D”.
Further, seismic hazard analysis has been done to map the seismic hazard in
terms spectral acceleration (Sa) at rock and the ground level considering the
site classes and seismogenic sources. The quantified hazard values in terms of
spectral acceleration for short period and long period are mapped for rock
level. These spectral acceleration and uniform hazard spectrums are used to
assess the design force for important structures and also to develop the design
spectrum.
In seismic hazard analysis, the shear wave velocity at a site of
interest is important because it gives an indication of the expected shaking in
response to an earthquake rupture. For instance, at a bed rock site (high shear
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wave velocity) there will be little amplification of seismic waves, where as in
a sedimentary basin (low shear wave velocity) one might expect intense
amplification.
7.2 EFFECTS OF SHEAR WAVES IN BUILDINGS
The first waves to arrive at any place after earthquakes are
P-Waves. These are followed by S-Waves. Body waves are high frequency
waves like all other high frequency waves, their amplitude attenuates very
fastest as distance increases. Any structure in the epicentral region, which has
a natural frequency of vibration in the same range is liable to be set in to
vibration, sometimes in near resonance mode. If the structure cannot
withstand these vibrations, it may deform, damage, or even collapse. Since
small height structures are short period of structures, they fall in this category.
In epicentral region, body waves inflict maximum damage to small
height structures. Damage to such low height structures decreases as
epicentral distance increases. Seismic performance of houses made of random
rubble stone masonry is more dismal than that of brick masonry. In Mandvi, at an
epicentral distance of 100 km, gable walls were damaged in several stone masonry
houses.
7.3 SHEAR WAVE VELOCITY
Shear wave velocity (Vs) is one of the most important input
parameters to represent the stiffness of the soil layers. It is preferable to
measure Vs by insitu wave propagation tests, however it is often not
economically feasible to perform the tests at all locations. The propagation of
seismic waves near the surface is strongly influenced by the presence of
unconsolidated loose sediments overlying the bedrock resulting in
modifications of the ground motion characteristics at the surface.
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The ground motion parameters at the surface are generally obtained
by conducting one dimensional ground response analysis considering only the
upward propagating shear waves. In these analyses, the shear wave velocity
(Vs) is one of the most important input parameter to represent the stiffness of
the soil layers. Hence, it is important to determine the shear wave velocity for
the estimation of ground motion parameters at the surface.
7.4 CALCULATION OF SHEAR WAVE VELOCITY
Table 7.1 shows a few of the relations used to determine the shear
wave velocity for the Indian soil conditions. Uma Maheshwari et al (2010)
proposed relationship for shear wave velocity calculated for all soil categories
and it is the most recent developed relation for all categories of soils in our
peninsular India. This relationship with their correlation coefficients is
wished-for between VS (m/s) and SPT-N values for all soil categories.
Table 7.1 Relationship for the shear wave velocity calculation
Soil Type VS (m/sec) Author Region
Clay VS = 89.31 N0.358 Uma maheshwari et al. (2010) Chennai, India
Sand VS = 100.53 N0.265 Uma maheshwari et al. (2010) Chennai, India
All soils VS = 95.64 N0.301 Uma maheshwari et al. (2010) Chennai, India
All soils VS = 82.6N0.43 Hanumantha rao and Ramana (2008) Delhi, India
Sand VS = 57((N1)60CS)0.44 Anbazhagan and Sitharam (2010) Bangalore, India
Clay VS = 80N0.33 Anbazhagan and Sitharam (2010) Bangalore, India
For all Soils: VS = 95.64 N0.301 (7.1)
Where,
Vs = shear wave velocity
N = SPT N value
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The correlations between shear wave velocity and SPT test
blow counts with and without energy corrections were
developed for three categories of soil, all soils sand and clay
These empirical equations can also be used for all soil
categories where a similar ground conditions exist. They
checked against measured Vs values
The developed correlation for different type of soils such as
all types of soils, sand and clay, can be effectively utilized for
the seismic hazard analysis studies for the study area.
7.4.1 Calculation of Shear Wave Velocity Using N - Value
The following relationships with their correlation coefficients are
proposed between Vs (m/s) and SPT-N values for all soil categoriesgiven in
equation 7.2. Figure 7.1 shows the shear wave velocity distribution for the
study area.
For all types of soils: Vs = 95.64N301 (7.2)
Figure 7.1 Shear wave velocity distributions for study area
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7.4.2 Identification of Type of Soil Using Resistivity Range of Soil
As the conductivity in the soil or rock depends upon the
characteristics of the pore water (electrolyte) within it, the measured
resistivity can be related to the type of electrolyte within it. There exists a
relation between salinity of pore water and measured resistivity (Guyod
1964). Table 7.2 provides the typical values of electrical resistivity values for
different soil and rock types.
Table 7.2 Values of resistivity for different types of soils and rocks
MaterialResistivity
( ohm m)
Clay@ 3-30
Saturated organic clay or silt$ 5-20
Sandy clay@ 5-40
Saturated inorganic clay or silt$ 10-50
Clayey sand@ 30-100
Hard, partially saturated clays and silts, saturated sands and
gravels$ 50-150
Shales, dry clays, silts$ 100-500
Sand, gravel@ 100-4000
Sandstone@ 100-8000
Sandstones, dry sands and gravels$ 200-1000
Crystalline rocks@ 200-10000
Sound crystalline rocks$ 1000-10000
Rocksalt, anhydrite@ >1100$Values from Sowers and Sowers (1970)
@ Values from Dohr (1975)
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7.4.3 Site Class Definitions
Classification of sites based on the average shear wave velocity of
the top 30 meters of the subsoil is popular among engineers as a quick way of
understanding how ground motion during an earthquake differs on rock sites
and soil sites. Standard documents such as IBC- 2009 can be referred for
classifying sites based on borehole data or velocity profiling. The standard
site classification definitions are shown in Table 7.3.
Table 7.3 Site class definitions (International Building Code IBC-2009)
Site class
Average shear
wave velocity
(vs1)
Average standard
Penetration resistance
(N1 or Nch
1)
Average undrained
shear strength in the
case of cohesive soils
(su1)
A : Hard Rock >1500 m/s Not applicable Not applicable
B: Rock 760 to 1500 m/s Not applicable Not applicable
C:Very dense
soil or soft rock
370 to 760 m/s >50 >100kPa
D: Stiff soil 180 to 370 m/s 15 to 50 50 to 100 kPa
E: Soft soil <180 m/s <15 <50 kPa
Any profile with more than 3 m of soil having Plasticity Index
PI>20, Moisture content 40%
Average undrained shear strength su < 24 kPa
F:Soils requiring
site-specific
evaluation
Soils vulnerable to potential failure or collapse (liquefiable, quick-
or highly sensitive clays, collapsible weakly cemented soils)
More than 3 m of peat and/or highly organic clays
More than 7.5m of very high plasticity clays (PI>75)
More than 37m of soft to medium clays
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7.5 CORRECTIONS APPLIED FOR N VALUES
For engineering use of site response study and liquefaction
analysis, the SPT N values need to be corrected with various
corrections and seismic bore log has to be obtained
The N values are measured in the field using Standard
penetration test procedure need to be corrected for various
corrections such as;
Over burden pressure (CN)
Hammer energy (CE)
Borehole diameter (CB)
Presence or absence of liner (CS)
Rod length (CR)
The above corrections are listed in Tables 7.4, 7.5, 7.6, 7.7 and 7.8.
The Corrected ‘N’ value (N1)60 is obtained using the following
equation;
(N1)60=N x [(CN) x (CE) x (CB) x (CS) x (CR)] (7.3)
Where,
N = SPT N value
CN= Over burden pressure
CE = Hammer energy
CB = Borehole diameter
CS = Presence or absence of liner
CR = Rod length
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7.5.1 Correction for Overburden Pressure (CN)
Kayen et al (1992) suggested the following equations given
the correction for overburden pressure
CN = (2.2/ (1.2+ ( vo’/Pa))) (7.4)
Where,
CN=overburden correction factor, it should not exceed a value of 1.7
This empirical overburden pressure correction factor is also
recommended by Youd et al (2001)
vo= effective overburden pressure
If SPT N value recorded in the field increase with increases,
effective overburden stress comes up
If the depth of bore hole increase with increasing the effective
overburden stress
d> vo’>N
Effective stress vo=1.7d
Where,
Pa=> Atmospheric pressure in Kpa
d => depth in “m”
The overburden pressure correction for different depths are given in Table 7.4
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Table 7.4 Correction for overburden pressure in different depths
Sl. No. DEPTH (m) CN value
1 1.5 1.517
2 1.6 1.499
3 1.7 1.482
4 1.8 1.460
5 1.9 1.450
6 2.0 1.434
7 2.1 1.419
8 2.2 1.404
9 2.3 1.384
10 2.4 1.374
11 2.5 1.360
12 2.6 1.346
13 2.7 1.333
14 2.8 1.319
15 2.9 1.306
16 3.0 1.293
17 3.1 1.281
18 3.2 1.269
19 3.55 1.227
20 3.8 1.571
21 1.2 1.474
22 1.75 1.534
23 1.4 1.534
24 4 1.178
25 6 1.127
26 7.5 0.999
27 9 0.897
28 10 0.814
29 10.5 0.767
30 12 1.227
31 13.5 0.745
32 9.34 0.687
33 15 0.594
34 16.5 0.556
35 18 0.523
36 19.5 0.494
37 21 0.467
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7.5.2 Correction for Hammer Energy Ratio (CE)
Split-spoon samples generally are taken at intervals of about
1.5m
They are commonly dropped by a rope with two wraps around
a pulley
The field donut type of hammers used in the SPT test
The various correction factors are given in Table 7.5 to Table 7.8.
Table 7.5 Hammer correction factor (Seed et al 1985, Skempton 1986)
Country Hammer Type Hammer Release CE
JapanDonut Free fall 78
donut Rope and pulley 67
United States safety Rope and pulley 60
donut Rope and pulley 45
Argentina donut Rope and pulley 45
Chinadonut Free fall 60
donut Rope and pulley 50
Table 7.6 Correction for borehole diameter (CB)
Diameter(mm) CB
60-120 1
150 1.05
200 1.15
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Table 7.7 Correction factor for sampling method (CS)
Variables CS
Standard sampler 1.0
With liner for dense sand and clay 0.8
With liner for loose sand 0.9
Table 7.8 Correction factor for rod length (CR)
Rod length(m) CR
>10 1.0
6-10 0.95
4-6 0.85
0-4 0.75
The various correction factors of Over burden pressure (CN),
Hammer energy (CE), Borehole diameter (CB ), Presence or absence of liner
(CS ) and Rod length (CR ) are obtained from Tables 7.4, 7.5, 7.6, 7.7 and 7.8
respectively. The Corrected ‘N’ value (N1)60 from equation 7.3 is used in
equation 7.6 and shear wave velocity is calculated for each location.
7.5.3 Calculation of Shear Waves Velocity
For all soils, the liners are used in the SPT test and given in the
following equation;
Vs=78 [(N1) 60cs] 0.40 (Anbazhagan and Sitharam 2010) (7.5)
But, mostly crystalline rock is available in the study area. So, liners
are not used in the SPT test. So the equation 7.5 is formed as following,
Vs=78 [(N1) 60]0.40 (7.6)
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where,
Vs => shear wave velocity (m/s)
(N1) 60cs => SPT corrected N value using the liners
(N1) 60 => SPT corrected N value using the without liners
7.5.4 Correction Applied By Using the Code Book IS 2131-1981
Standard Penetration Test
The following are available through standard penetration test
The N value which is the number of blow required to achieve
300mm penetration of the soil, indicates the relative density of
sand /gravel, the consistency of other soil such as sites (or)
clays and the strength of weak rocks
The test is described in IS 2131-1981
The split spoon sampler is attached to stiff drill rod lowered to
the bottom of the borehole
A standard blow consists of dropping a mass of 65kg free fall
through 760mm on to an anvil at the top of the rods and
ensuing that this amount of dynamic energy is transformed to
the sampler as much as possible
The number of blows required to achieve each 150mm
penetration of recorded for a fall penetration of 450mm
The initial 150mm penetration is referred to as a seating drive
and the blows required for this penetration are not considered
at this zone in disturbed soil
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The next 300mm of penetration is referred to as the test drive
and the number of blows required to achieve this fully is
termed the penetration resistance / N-value
7.5.5 Correction
Due to Overburden Correction
The N-value for cohesion less soil shall be corrected for
overburden. Correction should be applied for only cohesion
less soils, it should not be applied for cohesive soils
Calculating the effective vertical overburden pressure, the
following regression equation is given,
Effective stress p = 1.7d
where,
d = depth in ‘m’
To obtain the corrections of N-value in cohesion less soil for
overburden. Its corresponding to the effective vertical
overburden pressure in Kg/cm2
This factor is taken from the IS 2131-1981(by using graph)
Overburden correction = N values * correction of N values in
cohesion less soil for overburden
Due to Dilatancy
The obtained value shall be corrected for dilatancy if the stratum
consists of fine sand and silt below water table for values of N’ greater than
15, as under N’’
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N” = 15+1/2(N’-15) (7.7)
where,
N’=> overburden correction
N’’=> dilatancy correction
The calculation of liquefaction potential is to determine if the soil
has a potential to liquefy during the earthquake. shear wave velocity
calculated from the N value is used To determine the average shear wave
velocity.
7.6 AVERAGE SHEAR WAVE VELOCITY
The average shear wave rate intended for the depth of “d” of soil is
referred the same as VH. The average shear wave velocity to a depth of H is
computed as follows:
VH= di/ (di/vi) (7.8)
Where H = d = cumulative depth in m
For 30m average depth, shear wave velocity is obtained from equation 7.9.
Vs30 = 30/ Ni=1(d i/Vs) (7.9)
Where di and vi stand for the thickness (in meters) plus shear-wave
velocity (at a shear pull stage of 10 5 otherwise less, m/s) of the ith structure
or else layer correspondingly, in a full of N layers, obtainable in the top 30 m.
Vs30 is established for site categorization as for each NEHRP classifications
and Uniform Building Code (Uniform Building Code 1997, Dobry et al 1995,
Kanli et al 2006). The average velocity has been calculated using the equation
7.9 for each borelog locality.
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A spread sheet has been created for the calculation, as shown in
Table 7.9. The Vs average have been calculated for each 5m depth to a depth
of 30m and also average Vs for the soil overburden has been calculated based
on the borelog information. More often for amplification and site response
study, the 30m average Vs is well and good.
However, if the rock is contained by depths of about 30m, near
surface shear wave velocity of soil have to be selected. If not, Vs30 obtain will
be superior due to the velocity of the rock accumulation. Site characterization use
the SPT data and soil overburden thickness in Coimbatore vary from 1m to
about 15m. Hence, for overburden soil alone, Vs is calculated based on the
soil thickness, which is shown in Table 7.9. The velocity profile with respect to
the depth of soil layer has been shown in the Figure 7.2.
Table 7.9 Typical average shear wave velocity calculation
Soil
depth
in m
Shear
wave
velocity
Vs
Soil
thickness
Average
Vs soil at
5m depth
Average
Vs soil at
7.5m
depth
Average
Vs soil at
10m depth
Average
Vs soil at
13.5m
depth
Average
Vs soil at
30m depth
1.5 171.796 1.5
204.189 220.936 226.706 249.111 254.145
3 216.094 1.5
4.5 235.64 1.5
6 248.933 1.5
7.5 255.004 1.5
9 260.756 1.5
10.5 310.477 1.5
12 310.477 1.5
13.5 310.477 1.5
15 310.477 1.5
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Figure 7.2 Velocity profile with depth
7.7 SHEARWAVE VELOCITY DISTRIBUTION IN STUDY AREA
The rock depth/soil overburden thickness distribution has been
studied, which shows that the north western part has lesser overburden
thickness. However, eastern part and other areas have the overburden
thickness of 0.3m to 1m. The calculated average shear wave velocities are
grouped according to the NEHRP site classes as shown in Table 7.10 and map
has been generated as shown in Figure 7.3. The average shear wave velocity
calculated for 5m, 7.5m, 10m, 13.5m, and 30m depth are mapped.
Table 7.10 Site classes for average shear wave velocity
Site class Range of average shear
wave velocity (m/s)
A – hard rock 1500 < VS30
B – rock 760 < VS30 1500
C – very dense soil and soft rock 360 < VS30 760
D – stiff soil SPT-N <50 180 < VS30 360
E – soft soil SPT-N <15 VS30 < 180
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The average velocity up to a depth of 5m covers most of the study
area having velocity range of 180m/s to 360m/s. Few locations in south
western part and a smaller portion of Northeastern part of Coimbatore have
the velocity less than 250m/s indicating soft soil. The depth also may extend
beyond 5m, matching with the rock level map as shown in Figure 7.3.
The average shear wave velocity for 10m depth varies from 180m/s
to 360m/s in the 10m average map. In this location, the rock depth is found
within 10m show that the area covered has a very dense soil/soft rock.
In this map, northwestern part having the average velocity is more
than 360m/s, matching with the rock depth. When depth of average velocity
increases, the rock velocity influences the average velocity values. The area
covered by the velocity of 360m/s gets reduced with increasing depth. Similar
increased area of higher velocity is found in average depth of 5m to 15m
shear wave velocity profiles. Figure 7.3 shows the map of average shear wave
velocity for a depth of 30m. Even though the average shear wave velocity is
calculated for every 5m depth intervals and up to a maximum depth of 30m,
these maps does not show the average shear wave velocity of soil because of
the wide variation in the soil overburden/ rock level. Hence, the average shear
wave velocity of soil has been calculated based on the overburden thickness
above obtained from bore holes close
to the testing locations. Study region has medium to solid soil with a
velocity varying from 180m/s to 360m/s declining into site class D as per
Table 7.10.
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Figure 7.3 Average shear wave velocity for 30m depth
7.8 CONCLUSION
Shear wave velocity calculated from the N value is used to
determine the average shear wave velocity.
The shear wave velocity profiles (versus depth) and ground layer
anomalies have been presented. The average shear wave velocity has been
estimated for 5m, 7.5m, 10m, 13.5m and 30m depth in the study area. Based
on the overburden thickness value the average shear wave velocity for the soil
depth is estimated.
Site soil classification has been carried out by considering the
NEHRP and IBC classification. NEHRP classification based on the 148
geotechnical data and IBC classification based on the geophysical
investigations conducted at 25 locations. A map has been created for the
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average shear wave velocity of 30m depth using GIS. The estimated Vs30 for
the study area soil can be classified as Site Class D as per NEHRP and IBC
classification.
Further, seismic hazard analysis has been made to map the seismic
hazard in terms spectral acceleration (Sa) at rock and the ground level
considering the site classes and eight seismogenic sources identified.