empirical correlations of shear wave velocity (vs) and

Indian Journal of Geo Marine Sciences Vol. 45 (11), November 2016, pp. 1566-1577

Empirical correlations of shear wave velocity (Vs) and standard penetration resistance based on soil type in Babol city

F. Farrokhzad* & A. J. Choobbasti

Department of Civil Engineering, Babol University of Technology, Babol, Mazandaran, Iran

*[Email: [email protected]]

Received 06 August 2014 ; revised 30 September 2014

Shear wave velocity (Vs) plays a fundamental role in soil dynamic problems and seismic analyses to estimate the site effect for earthquake geotechnical microzonation. During an earthquake the ground motion is significantly affected by the soil type, stiffness and geological condition. The shear wave velocity and Shear modulus are the applied variables in nonlinear site response analysis. As a part of microzonation study for the Babol city a total of 35 boreholes have been drilled in 35 km2 of the research area. The depths of these boreholes ranged about 25 to 30 m. SPT blow counts were taken in each 2 m depth. Many geophysical surveys, and generally 35 downhole logging surveys, are carried out in 35 mentioned boreholes for generation and measurement of shear waves in situ. According to the results of various in-situ tests, the variation of shear wave velocity obtained by downhole tests and SPT-N values were studied and some correlations were developed. It can be said that the developed correlations show acceptable prediction performance and can be used for similar geotechnical and geological site conditions.

[Keywords: Shear wave velocity, Standard penetration test, Earthquake geotechnical, In-situ tests]

Introduction Safety against earthquake hazards has two aspects:

firstly safety against seismic forces and secondly the safety of a site itself related with geotechnical phenomena such as amplification, landslideing and liquefaction1. Geotechnical microzonation is defined as the process of subdividing an area into zones with respect to some geotechnical, geological and geophysical characteristics of the sites. The key issue behind a microzonation study is to use the obtained variation of the selected parameters for land use and city planning. The determination of soil characteristics constitutes one of the most important aspects of geotechnical microzonation2. Local site conditions describe the materials that lie directly beneath the site from the surface to bedrock. Many researchers have also shown that a more refined geotechnical classification is warranted based on measurements of shear-wave velocity3. Shear wave velocity is widely used by earthquake and geotechnical engineers to model the seismic behaviour of the sites which is considerable in geotechnical and earthquake microzonation. One of the most common important problems in geotechnical earthquake engineering is the evaluation of ground response. The influence of the dynamic properties of

soil deposits on the nature of ground motion has been recognized for many years. For detailed studies of site effects and ground motion using nonlinear and equivalent linear methods, shear wave velocity and shear modulus have to be obtained. This requires laboratory and in-situ tests. But it is not often economically acceptable to measure the shear wave velocity in all cases. On the other hand, standard penetration test (SPT) is the most common in-situ test which is almost carried out in every geotechinical investigation plan4. Many regression equations of SPT-N versus shear wave velocity are available in literature for different soils by many researchers. But a common feature of these empirical relations is their applicability for a specific region or site condition5. Most researchers today agree that there is no correlation between SPT-N and shear wave velocity which can be applicable for different soil types and all regions. Different site conditions such as soil and sediment erosion, geological situation, soil structure and fabric and … are the main factors of mentioned uncertainty. So it can be said that the best empirical correlations for a specific region should be assessed based on in-situ tests which are done in that region6.

An investigation to systematize empirical equations for the shear wave of soils was made in terms of four

INDIAN J. MAR SCI., VOL. 45, NO. 11, NOVEMBER 2016 1567

characteristic indexes by Yutaka Ohta and Noritoshi Goto7,8. In mentioned research, 300 sets of available data with a description about shear wave velocity, N-value, Depth, Geological age and soil type were used. The general empirical formula corresponding to optimum coefficients is described as Equation 1:

N

H K E K E

k k

log (Vs) - log (Vs) = C (log N - log N) +

C (log H - log H) + E d (k) + F d (k)

(1)

Where Ek and Fk are coefficients which represent

each category in terms of geological age and soil type, and, and are mean average in a logarithmic sense of the whole set of data. Other unknown coefficients can be obtained by referring to the relation of shear wave velocity and N-value of data sets. Finally the relationships were derived by empirical equation consisting of two indexes of [Epoch-Type]; (Equation 2):

(2)

The authors emphasized that all data in mentioned research are from alluvial plains in Japan, so the direct application of the derived empirical equations to other countries may require circumspection.

Shear wave velocity profiles of Erbaa-Turkey were developed to provide data for site response analyses by Muge K. Akin, Steven L. Kramer and Tamer Topal9. The geological units observed in study area consist of alluvial and Pliocene mostly clayey-sandy units. These layers were evaluated on the basis of drilling, in-situ (SPT, SCPTU and SPT based uphole and laboratory testing). Consequently, new empirical relationships between SPT-N and Vs were proposed for different alluvial and Pliocene soils in the study area (Equation 3).

Empirical relationship Soil Type r

0.109 0.426

sv 59.44N Z

All alluvial soils

0.89

0.176 0.481

sv 38.55N Z Alluvial sand 0.94

0.116 0.35

sv 78.1N Z Alluvial clay 0.92

0.101 0.216

sv 121.75N Z

All Pliocene soils

0.94

0.359 0.177

sv 52.04N Z

Pliocene sand

0.98

(3) The authors noted that the above equations are

valid down to 25 m depth for study area. The multichannel analysis of surface wave

(MASW) tests has been carried out at thirty sites in Channai city by Uma Maheswari R, Boominathan A and Dodagoudar G. R for which SPT-N value profiles were available10. Based on the statistical assessments, an empirical correlation between Vs and SPT-N was developed (Equation 4).

Empirical relationship Soil Type r2

0.301

sv 95.64N All soils 0.83

(4)

The shear wave velocity curve obtained from the proposed equation lies between those obtained from (Ohba & Toriumi, 1970) and (Imai & Tonouchi, 1982)11, 12 (Equation 5 and Equation 6).

Empirical

relationship Soil

Type

Ohba and Toriumi (1970)

0.31

sv 84N

All soils

(5)

Imai and Tonouchi

(1982)

0.314

sv 57N

All soils

(6)

In the study of Thaker and Rao13, in order to develop of statistical correlations between shear wave velocity and penetration resistance using MASW technique, 602 data pairs were used. The correlation were developed using a simple regression analysis for the existing database (Equation 7).

Empirical relationship

Soil Type r2

0.42

sv 59.72N All soils 0.77

0.45

sv 51.21N Sandy Soil 0.78

1 clay

1.23 fine sand

1 alluvium 1.318 medium145.1

1.753 diluvium 1.445 coarse sand

1.566 sand and gravel

1.828 gravel

Vs

————— 1- Corresponding author

1568 FARROKHZAD & CHOOBBASTI: EMPIRICAL CORRELATIONS OF SHEAR WAVE VELOCITY

0.42

sv 62.41N Clayey Soil 0.78

(7) The proposed correlations were compared with the

regression equations proposed by various other investigators and found that the developed correlations exhibit good prediction performance.

A SPT based uphole test, which was simple and economical for determining Vs, is proposed by Eun-Seok and Dong-Soo14. A testing procedure and interpretation method for obtaining the shear wave velocity profile were introduced. The data reduction methods introduced in mentioned research involve inconvenient procedure of finding the refracted ray path along using an interactive calculation procedure. Through a numerical study using the finite element, downhole and SASW methods, the procedure of the method was verified.

Some regression equations of SPT-N versus shear wave velocity are listed below (Table 1), in literature, for different soils. The majority of such relations are developed for a specific region and their applicability for other regions is not clear enough6,15-29.

Table 1— Existing correlations between Vs and SPT-N values

Vs (ft/sec) Vs (m/sec) Soil type Kanai (1966)

VS=62 N0.6 - -

Yoshikawa VS=127 (N+1)0.5 to VS=178 (N+3)0.5

- -

Sakai (1968)

VS=49 (N+1)0.5 to VS=110 (N+3)0.5

- -

Shibata (1970)

VS=104 N0.5 - Sand

Hamilton (1970)

- VS= 128 D0.28 All

Ohba & Toriuma (1970)

VS= 280 N0.31 VS = 85 N0.31 Alluvium

Ohta ,et al. (1972)

VS= 286 N0.36 - -

Fujiwara (1972)

- VS = 92.1

N0.337 -

Ohasaki & Iwasaki (1973)

VS= 268 N0.39 VS = 82 N0.39 All

220*

VS= 195 N0.47 VS = 59 N0.47 Cohesionl

ess Imai & Yoshimura (1975)

VS= 302 N0.329 VS= 92.1

N0.329 -

Imai & Fumoto & Yokota (1976)

VS= 295 N0.341 VS= 89.8

N0.341 -

VS= 137 N0.417 - - Hamilton (1976)

VS= 301 D0.28 - All 29

Campbell & Duke (1976)

VS= 319 D0.386 - New

Alluvial Deposits

VS= 491 D0.358 - Old

Alluvial Deposits

Imai (1977) - VS = 91 N0.337 All

943*

- VS = 102

N0.292 Clay 183*

- VS = 80.6

N0.331 Sand 151*

Ohta & Goto (1978)

VS= 280 N0.348 VS = 85.35

N0.348 All 289

VS= 290 N0.340 VS = 88 N0.34 Sand VS= 309 N0.340 VS = 94 N0.34 Gravel

- VS= 67.79

N0.219 D0.230

Well Graded Sand

- VS= 62.14

N0.219 D0.230 Clay

Fumal (1978)

VS= 471 D0.20 - Sand 59*

Campbell et al. (1979)

VS= 170 (D+3.9)0.45

- Soft Soil

VS= 278

(D+2.4)0.413 -

Medium Soil

VS= 519

(D+2.0)0.349 - Hard Soil

JRA (1980) - VS=100 N0.333 Clay - VS=80 N0.333 Sand Seed & Idriss (1981)

VS = 200 N0.5 VS = 61 N0.5 All

Imai & Tonouchi (1982)

VS=318 N0.314 VS = 97 N0.314 All

1654*

Seed & Idriss & Arango (1983)

Sand

Sykora & Stokoe (1983)

VS=350 N0.27 VS = 107 N0.27 Granual

Soil 229*

VS=330 N0.29 VS = 100.5

N0.29 Sand 97*

Barrow & Stokoe (1983)

VS=476+13.9 N - All 33*

VS=475+13.4 N1 - All 33*

Lin ,et al. (1984)

VS=65.58 N0.502 - All 31*

Jinan (1985)

- VS =

90.91(D+0.6206)0.2126

After Holocene

70*


- VS =

30.75(D+0.4160)0.5906

End of Plistocene

28*

Jinan (1987)

- VS =

116.1(N+0.3185)0.202

Holocene 59*

Okamota ,et al. (1989)

- VS = 125 N0.3 Sand

Imai & Yoshimura (1990)

- VS = 76 N0.33 -

Lee (1990) - VS = 57.40

N0.49 Sand 22*

- VS = 57.40

D0.49

- VS = 114.43

N0.31 Clay 44*

- VS = 70.81

D0.37

- VS = 108.64

N0.32 Silt 22*

- VS = 70.52

D0.39

Yokota ,et al. (1991)

- VS = 121 N0.27 -

* Number of samples

Study Area, Instrumentation and Methodology Iran is located in a relatively active seismic zone

and most regions of the country may experience catastrophic and destructive earthquakes in the future, as many parts of the country have been recurrently destroyed by earthquakes during the past decades. Babol is located in the in the north of Iran, between the northern slopes of the Alborz Mountains and southern coast of the Caspian Sea. Babol city is one of the most important cities in the north of Iran. The city is located approximately 20 kilometers south of Caspian Sea on the west bank of BabolRud River and receives abundant annual rainfall (Fig. 1)30,31.

a) Map of Babol city

4.7 Magnitude 5.1 Magnitude 5.0 Magnitude 4.2 Magnitude 4.3 Magnitude 4.9 Magnitude 4.1 Magnitude 5.7 Magnitude 4.7 Magnitude

4 Magnitude b) Recent earthquakes around study area

Fig. 1— Map of study area in north of Iran

Babol-roud

Caspian Sea


The study area is mainly affected by Alborz and khazar faults. The Khazar fault is the boundary between the Caspian plain and Alborz Mountain. Regional geological studies indicate activity of the fault in Cenozoic time, and active propagation of the fault related folding towards the west.

Alborz range is located in northern part of Iran, parallel to the southern margin of Caspian Sea. Alborz is characterized by the dominance of platform-type sediments, including limestone, dolostone, and clastic rocks. Rock units from Precambrian to Quaternary have been identified, with some hiatuses and unconformities in Paleozoic and Mesozoic (Fig. 2)32.

Fig. 2— Khazar and alborz Faults near the Babol city

The investigation phase of any geotechnical study

undertaken for development, construction, or any other engineering works is by far the most important phase. Exploring the geologic environment and mapping surficial conditions, including rock, soil, water, and geologic hazards; preparing subsurface sections; and obtaining samples of the materials for identification, classification, and laboratory testing are the aim of geotechnical investigations33. Figure 3 presents the outcome of the site investigation carried out in Babol based on the results of a desk study on more than 100 borehole data, geology, hydrogeology and the results of a detailed aerial photograph. Upon the importance of the research, the site investigation

program included the exploration of site subsurface conditions at the study area through the drilling of 35 boreholes to a depth of 30 m each and reviewing of 65 boreholes up to a depth of 20 m each below the existing ground level34,35. Two types of samples were collected: Disturbed soil samples, which do not retain the in-situ properties of the soil during the collection process and is used for soil classification and undisturbed soil samples that retain the structural integrity of the in-situ soil and have a high recovery rate within the sampler. It should be noted that several triaxial, consolidation, unconfined compressive Strength and direct shear tests are carried out on the undisturbed soil samples of study area. As it is obvious in Figure 3. The soil profiles contain clay, silt and sand. In addition, the natural moisture contents, atterberg limits (liquid and plastic), specific gravity and absorption tests were performed to evaluate the engineering properties of the soil samples36,37,38,39.

a) Geotechnical map of study area at surface

Study Area


b) Geotechnical map of study area (Depth=10 m)

c) Geotechnical map of study area (Depth=20 m)

d) Geotechnical map of study area (Depth=30 m)

Fig. 3— Surface and subsurface soil layering of Babol city

One of the most commonly used method for shear

wave velocity profiling is the downhole test. This test method is applied for determination of the interval velocities from arrival times and relative arrival times of compression (P) and vertically (SV) and horizontally (SH) polarized shear (S) seismic waves which are generated near surface and travel down to an array of vertically installed seismic sensors. Sequences for downhole testing in this research is as follows:

At first the Boreholes are drilled. In this study, test depth is 30 meters. The SPT test and other geotechnical tests, as mentioned above, are carried out in all boreholes. The SPT test is repeated at every 2 m interval. PVC tube in the same length of borehole is then installed inside the hole as casing for a geophone (Fig. 4-a). Any gap between perimeter hole and outside tube is then filled with granular soil. Then the probe and geophone are installed into the borehole to any depth. Using hand-pump, air is pumped to inflate the membrane of geophone until the membrane stick on the inside perimeter of tube. Triggering of the source is performed by striking the prepared wooden


plank, placed on approximately about 2 meter of slant distance measured from hole. Wooden plank is struck vertically on the middle of top, and horizontally on the both left side and right side (Fig. 4-b). Test is repeated for any intervals as specified, until sufficient data is obtained40.

a) Borehole logging

b) Downhole test

Fig. 4— A sample of geotechnical tests and geophysical surveys in Babol city

Selecting the right equipment for downhole test is a key to efficiently meeting of test objectives and, ultimately, accurately characterizing of reservoir.

There are several acceptable devices that can be used to generate a high-quality P or SV source wave or both and SH source waves. The technical features of downhole test equipment are shown in Table 2 and Figure 5.

Fig. 5— Downhole test equipment

Table 2— Technical features of 16S-U Number of channels 12

With 2nd unit connected in series

Up to 48

Acquisition boards National Instruments® Resolution 24 bit with proprietor algorithm

Sampling time from 125 μs to 2 ms for all 24

canali Record Length from 32 ms to 65536 ms

Filters

Digital filters: in post-acquisition (50-60 Notch,

250LP) antialiasing: active, LPF, 8th Butterworth order;

attenuation -48dB/oct (-160dB/dec); f0=5/8fnyq;

accuracy ±1% cutting freq.

Enhancement (stacking) With/without total/partial

preview Delay 0-8000ms (step of 1ms)

Geophone polarity inversion

Yes


First Picking

marker to find out the position of the video points on the time scale; possibility to save first

arrivals on file for data transfer to PC

Seismic Wave Display wiggle-trace / variable area

Noise-monitor Yes, with real time “cascade”

display

Trace-size Automatic or manual for each

channel Automatic Calibration Yes

Data format SEG-2, PASI Power supply 12V external battery

Development of Empirical Correlations Between Vs and SPT-N

A total of 35 boreholes have been drilled in study area. An intensive geotechnical sampling and geophysical testing program was applied in these boreholes. Previous geotechnical investigations of the study area, which include 65 borings and laboratory test results, were evaluated in this research, too. The depth of boreholes ranged from 20 to 40 m. Downhole logging surveys and SPT tests carried out through new 35 boreholes, were performed at every 1.5 m and 2 m of depth. The location of drilled boreholes over Babol City, a sample of geotechnical test results and shear wave velocity profile obtained by downhole test are shown in Figure 6.

a) Drilled boreholes in study area for this research

b) A sample of borehole log up to depth 15 m

c) A sample of (S wave) data obtained in downhole test

d) A sample of shear wave and primary wave profile

Fig. 6— Results of laboratory tests and geophysical analysis

Review of shear wave velocity correlations clearly


shows that limited study has been carried out using SPT-N60 values. In this study more than 550 data pairs from 32 boreholes were employed in the assessments. It should be noted that the proposed empirical relationships between Vs and SPT-N60 are evaluated to investigate the effect of energy ratio correction factor. In this research, the authors believed that in order to develop an efficient relationship, the SPT-N values should be corrected for energy ratio using the following relationship ((8).

60. . . .

H R S BN N C C C C (8)

Where CH, CR, CS and CB are the correction factors of hammer type, rod length, sampler type and borehole diameter.

The measured data points are shown in Figure 7. The correlation developed between shear wave velocity and corresponding SPT-N60 for all soils, after a regression analysis is represented in (Equation 9).

Fig. 7— All data pairs obtained by in-situ test

Vs = 73.808(N60)

0.498

r2=0.827 (9)

In the next step of this study, the data pairs were

divided in to 2 categories; cohesive soil (clay and plastic silt) and cohesionless (sand and non-plastic silt). Finally; new relationships were proposed between shear wave velocity and SPT-N60 values for 2 mentioned categories. The data pairs and correlation from present study for each category are shown in Figure 8, Equation 10 and Equation 11.

a) Data pairs; Category of Clay & Plastic Silt

b) Data pairs; Category of Sand & non-Plastic Silt Fig. 8— Division of Data pairs based on soil type

Clay & Plastic Silt Vs = 70.424(N60)

0.514 (10) r2=0.87

Sand & non-Plastic Silt Vs = 83.226(N60)

0.457 (11)

r2=0.84 In order to assess the capability and accuracy of

developed correlations, the Vs profiles obtained by downhole method and estimated by above correlation are compared in Fig. 9. It should be added that these data pairs were not used in development of the correlations. It can be said that the proposed correlation can be used for estimation of soil shear wave velocity in study area and sites with similar geotechnical and geological situations.


a) Comparison of Vs

determined from proposed correlations and downhole test

in test borehole (No.1)

b) Comparison of Vs determined from proposed

correlations and downhole test in test borehole (No.2)

Fig. 9— Assessing the accuracy of proposed correlations

The developed relationships for all 3 categories (all

soils, cohesive and cohesionless) are compared with the earlier regression equations proposed by other researchers as shown in Fig. 10. It can be said that different geotechnical and geological situation, accuracy of the obtained data, SPT test procedure and methods of Vs measurements are the causes of variations in developed empirical correlations.

Fig. 10— Comparisons between proposed and previous correlations

Conclusion Seismic measurements of compressional and shear

wave velocities are of particular importance in assessment of the engineering properties of the ground, as they can be used to determine geotechnical properties of soil, evaluation of liquefaction and analysis of local site effects during an earthquake. Good planning for a geotechnical and geophysical site investigation is the key to obtaining sufficient data with acceptable accuracy for seismic microzonation in a timely manner and with minimum cost.

Most of the correlations between shear wave velocity and SPT-N values are results of case studies based on the field tests in a specific area; so the direct application of the derived correlations for another region may require some changes in original equation.

The aim of this paper is to generate new correlations between shear wave velocity and SPT-N60 values by combining the new data obtained by authors and old available data of Babol city. A detailed site characterization of Babol city is carried out by conducting several SPT and downhole tests in an area of 35 km2. Shear wave velocities in 35 boreholes were measured to provide data for evaluation of local site effects and assessment of liquefaction potential as a part of microzonation study.

Regarding to the previous correlations, the authors believed that the quality of data pairs and accuracy of correlations may be improved by applying the energy correction factors to the SPT-N values. The developed correlations are classified into 3 categories according to soil types (all soils, cohesive and cohesionless).

It can be said that the correlation proposed in this study are compatible with the trend of previous correlations. Based on the results of the analyses, the proposed correlations gave the highest r2 for all 3 categories in comparison with previous correlations.

Finally, it should be noted that the proposed equations provide a reliable empirical correlation for estimating Vs from SPT-N60 which can be used in Seismic Microzonation projects, especially in Babol city.

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empirical correlations of shear wave velocity (vs) and

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