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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.