Earthquake induced liquefaction phenomena have been recorded and developed in many parts of the world [3, 14, 15]. The methods of liquefaction susceptibility analysis and mapping have further been modified, improved, calibrated and validated by many researchers [16, 17, 18]. Dhaka City is located close to seismically active zone. The eastern, western, southeastern parts of the city are covered by the Holocene sand, silty sand, silty clay, sandy- and clayey-silt up to more than 20 m depth from the ground surface. Even though past studies have been carried out for development of liquefaction potential maps of Dhaka City by different researchers [11, 13]. However, the studies lacked the use of high-quality data and did not reflect on the methodology adopted for development of the maps. In addition, for assessing Liquefaction susceptibility the previous researchers used NCEER workshop [3] and Youd’s [15] guidelines which is currently outdated as the latest databases used by Boulanger [20, 21] and Cetin [6, 19] included more case histories. Therefore, in this study, an attempt was taken to prepare a seismically induced liquefaction hazard map as well as seismic site classification map of Dhaka City Corporation area based on the most updated SPT based method liquefaction susceptibility assessment method developed by Cetin [6] integrating it with provisions of BNBC 2020 [7] for evaluating liquefaction potential and site classification using shear wave velocity data.

## 5.1 Seismic Site Characterization as per provisions of BNBC 2020

BNBC 2020 [7] provides guidelines for estimation of seismic site class according to shear wave velocity of upper 30m of the site profile. For our evaluation we used Wair’s [22] SPT based co-relationship for estimation of shear wave velocity for specific soil layers. The correlation suggested by Wair et. al [22] is as follows.

$$\text{V}\text{s}=30 {{\text{N}}_{60} }^{0.215}{{{\sigma }}_{\text{v}} }^{0.275}$$

1

The age scaling factor for Pleistocene and Holocene deposition suggested for the equation is 1.13 and 0.87 respectively.

In addition to SPT based evaluation of shear wave velocity additional shear wave data from Seismic Cone Penetration Test conducted for Dhaka Sub-way project was also included for estimation of Vs30. Average soil property has been determined by using the following Eq. 2

$$\stackrel{-}{\text{V}\text{s}}=\frac{{\sum }_{\text{i}=1}^{\text{n}}\text{d}\text{i}}{{\sum }_{\text{i}=1}^{\text{n}}\text{d}\text{i}/\text{V}\text{s}\text{i}}$$

2

Where, di = soil layer thickness of layer i, n = number of soil layer in upper 30m and Vsi = Shear wave velocity of i layer

The seismic site class property as per provision of BNBC 2020 using shear wave velocity data of upper 30m soil layer is as follows: SA for shear wave velocity > 800 m/s, SB for shear wave velocity within the range of 360 ˗ 800 m/s, SC for shear wave velocity within the range of 180–360 m/s and SD for shear wave velocity < 180 m/s

## 5.2 Evaluation of Liquefaction Susceptibility

The original simplified procedure for predicting liquefaction resistance of soils [14] was developed by using the Standard Penetration Test (SPT) blow counts correlated with a parameter representing the seismic loading on the soil, called cyclic stress ratio (CSR). The load induced by earthquake (CSR) and resistance against liquefaction (CRR) are variables which compared while evaluating factor of safety against liquefaction. A magnitude scaling factor is multiplied for earthquakes having magnitude of more or, less than 7.5.

$${\text{F}}_{\text{L}}=\frac{{\text{C}\text{R}\text{R}}_{7.5}}{\text{C}\text{S}\text{R}}\bullet \text{M}\text{S}\text{F}$$

3

For calculation of cyclic stress ratio, the following Eq. 4 is used.

$$\text{C}\text{S}\text{R}=\left(\frac{{{\tau }}_{\text{a}\text{v}}}{{{\sigma }}_{{\text{v}}_{0}}^{{\prime }}}\right)=0\cdot 65\left(\frac{{\text{a}}_{\text{m}\text{a}\text{x}}}{\text{g}}\right)\left(\frac{{{\sigma }}_{{\text{v}}_{0}}}{{{\sigma }}_{{\text{v}}_{0}}^{{\prime }}}\right)\cdot {\text{r}}_{\text{d}}$$

4

Since, the inception stage the stress reduction factor, rd used for evaluating cyclic stress ratio went through lot of upgradations as different researchers used updated database from time to time and introduced new equations. Stress reduction factor is a site-specific parameter which depends on different factors such as depth, dynamic characteristics of soil as well as ground motion characteristics [23]. For our study, we used BNBC 2020 [7] recommended stress reduction factor considering local perspectives which is shown in Eq. 5

$${\text{r}}_{\text{d}}=1-0.015 \text{z}$$

5

Where, z is the depth of soil column in meters.

Peak ground acceleration value, PGA for respective boreholes have been calculated according to the provisions of BNBC 2020. BNBC 2020 characterizes seismic site class based on estimation of average shear wave velocity, Vs30 for top 30m of the soil layer. If Vs30 is less than 180m/s then the site is being classified as SC and if the value is greater than 180m/s then it is classified as SD. The seismic zone co-efficient (Z) for Dhaka city according to BNBC 2020 is 0.2 which is the value for maximum credible earthquake having a return period of 2475 and 2% probability of exceedance in 50 years. The surface PGA value considering site amplification scenarios and design basis earthquake considerations have been calculated using the following Eq. 5

$$\text{P}\text{G}\text{A}=\frac{2}{3} . \text{S}.\text{Z}.\frac{\text{I}}{\text{R}}$$

6

where \(\frac{\text{I}}{\text{R}}=1\) has been considered for free field liquefaction assessment. S has been considered 1.15 for seismic site class SC and 1.35 for seismic site class SD according to BNBC 2020. Hence, PGA value used in liquefaction evaluation is 0.15 for locations falling under site class SC and 0.18 for locations falling under site class SD.

Even though past studies considered a moment magnitude of 7 for evaluation of liquefaction susceptibility however, recent probabilistic seismic hazard assessment carried out suggested that a magnitude of 8.02 [24] may be generated by the Dauki fault, on the other faults near the Chittagong Tripura fold belt may generate an earthquake of 8.5[25]. However, for our assessment we used a magnitude of 7.5 for estimation of cyclic resistance ratio which is also a function of clean sand equivalent (N160) as well as fines content (FC). The following Eq. 6 developed by Cetin [6] has been adopted for CRR evaluation.

$$\text{C}\text{R}\text{R}\left({\text{N}}_{\text{1,60}},{\text{M}}_{\text{W}},{{\sigma }}_{\text{v}}^{{\prime }},\text{F}\text{C},{\text{P}}_{\text{L}}\right)=\text{e}\text{x}\text{p}\left[\frac{\left(\begin{array}{c}{\text{N}}_{\text{1,60}}\bullet \left(1+0.00167\bullet \text{F}\text{C}\right)-27.352\bullet ln\left({\text{M}}_{\text{W}}\right)\\ -3.968\bullet ln\left(\frac{{{\sigma }}_{\text{v}}^{{\prime }}}{{\text{P}}_{\text{a}}}\right)+0.089\bullet FC\\ +16.084+2.96. \bullet {{\Phi }}^{-1}\left({\text{P}}_{\text{L}}\right)\end{array}\right)}{11.771}\right]$$

7

where PL = 50% has been considered for deterministic evaluation.

The factor of safety (FL) is not a sufficient parameter for evaluation of liquefaction and its damage potential at any site. However, the thickness and depth of the liquefiable layer and the factor of safety are very important inputs for damage potential based on liquefaction. Accordingly, we used liquefaction potential index (LPI) based approach to evaluate the intensity of damage for each borehole location as it includes the thickness and depth of the liquefiable layer and factor of safety as inputs. The LPI was originally proposed by Iwasaki [26, 27] to evaluate the potential for liquefaction to cause foundation damage. The LPI assumes that the severity of liquefaction is proportional to the thickness of the liquefied layer; proximity of the liquefied layer from the ground surface; and amount by which the factor of safety (FL) is less than 1.

$$\text{L}\text{P}\text{I}={\int }_{0}^{\text{z}}\text{F}\left(\text{z}\right). \text{W}\left(\text{z}\right).\text{d}\text{z}$$

8

Where, F (z) = 1 – FS, for FS < 1.0

F (z) = 0, for FS ≥ 1.0

W (z) = 10–0.5z, for z < 20 m

W (z) = 0, for z > 20