Probable Liquefaction Map for Purbachal New Town, Dhaka, Bangladesh

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International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 6, Issue 3, March 2016)

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Probable Liquefaction Map for Purbachal New Town, Dhaka,

Bangladesh

Mohammad Atikur Rahman

1

, A S M Woobaidullah

2

, Chowdhury Quamruzzaman

3

, Md. Mahfujur Rahman

4

,

Asad Uz Zaman Khan

5

, Fansab Mustahid

6

1,4,5_{M.S. Department of Geology, University of Dhaka, Dhaka-1000, Bangladesh} 2,3

Professor, Department of Geology, University of Dhaka, Dhaka-1000, Bangladesh

6_{M.Phil Student, Department of Disaster Science and Management, University of Dhaka, Dhaka-1000, Bangladesh} Abstract— Preparation of liquefaction potential map of

prone area has high importance for decision makers or city planners to reduce loss of lives and resource. The research gives a primary approach to grab attention on liquefaction hazard in Purbachal New Town. In this study, liquefaction hazard map of Purbachal New Town is prepared using geomorphic data, PGA (peak ground acceleration) at ground surface, Mw (moment magnitude) of a scenario earthquake and groundwater depth is used. For area-wise evaluating liquefaction susceptibility are suitable in this study. There are 2 steps for the liquefaction analysis in accordance with HAZUS. At first, liquefaction susceptibility is evaluated by geologic / geomorphic data and information of geological age. Secondary, liquefaction probability is estimated by inputting PGA, Mw and groundwater level into the above evaluated liquefaction susceptibility map. In this thesis Geomorphic unit map edited by GSB (2008) is used to have geomorphological data and “Edushake” software is used to calculate PGA (peak ground acceleration, at surface). To have share wave velocity which is used in PGA calculation by “Edushake” ohta-goto (1978) equation is used. Earthquake moment magnitude 7.5 (Mw) and .15 g peak ground acceleration of bed rock (as study area fall into seismic zone 2, Figure ۶) has taken in this study. After data processing and analysis a liquefaction probability map is produced which shows that the Purbachal New Town under low level of liquefaction hazard.

Keywords— Liquefaction, Peak Ground Acceleration (PGA), Plasticity Index, Share Wave Velocity, Standard Penetration Test (SPT), Unit Weight.

I. INTRODUCTION

Earthquake risk is a public safety issue that requires appropriate risk management measures and means to protect citizens. Earthquake induced liquefaction problem became important when it started to affect human and social activities by disturbing the function of facilities and also after rapid urbanization by expanding the cities in reclaimed areas.

Ground failures generated by liquefaction had been a major cause of damage during past earthquakes e.g., 1964 Niigata, 1971 San Fernando, and 1989 Loma Prieta earthquakes. The loss of soil stiffness and strength in loose- to medium-dense, saturated sandy soils due to liquefaction has been the leading cause of damage to bridge foundations during earthquakes. Soil liquefaction can result in a variety of failure modes that compromise (a) loss of foundation stability due to reduced bearing capacity, (b) deep-seated instability and damage to deep foundations, (c) increased lateral earth pressures on earth retention structures, (c) loss of passive soil resistance against walls, anchors, and laterally loaded piles, (d) reduction of axial capacity of piles, and (e) post-liquefaction settlement of soils. Liquefaction generated ground failure can affects citizen in various way such as damaging building, bridge, buried pipeline, train station, lifeline facilities etc. Historical earthquake data and recent seismic activity of Bangladesh and adjoining area indicate that Bangladesh is at strong seismic risk. There are few active fault identified in and around Bangladesh, one of which is ―Modhupur Fault‖ close to purbachal area.

Earthquakes history of this subcontinent indicate that destructive earthquake regularly occure around Bangladesh (Bilham and England, 2001; Ambraseys and Bilham, 2003; Bilham and Wallace, 2005). The occurred historical earthquakes in and around Bangladesh are listed in Table I. Some of these earthquakes such as the 1885 Bengal Earthquake (Middlemiss, 1885), 1897 Great Indian

Earthquake (Oldham, 1899) and 1918 Srimangal

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TABLE I

HISTORICAL EARTHQUAKES IN AND AROUND BANGLADESH (Ms and MMI are after Sabri, 2001; MODIFIED FROM MORINO,2009)

Year Ms Depth (km)

Source Area MMI

Dhaka Chittagong Sylhet Bangladesh

1548? ? ? Sylhet

1664? ? ? Shillong Plateau?

1762 ? ? Chittagong-Arakan 3? 8? 2? 8?

1 1858 6.5 ? Sandway, Myanmar - 5? - 6

2 1869 7.5 48 Cachar, India 5 4 8 8

3 1885 7.0 72 Sirajganj, Bangladesh 7 3 4 8

4 1897 8.1 60 Assam, India 8 6 8 9

5 1906 5.5 ? Calcutta, India 3 _ _ 5

6 1912 7.9 25 Mandalya, Myanmar ? 2 ? ?

7 1912 7.6 14 Srimangal, Bangladesh 5 5 7 8

8 1930 7.1 60 Dhubri, India 5 4 5 8

9 1934 8.3 33 Bihar, India-Nepal ? ? ? ?

10 1938 7.2 60 Mawlaik, Myanmar _ 5 _ 5

11 1950 8.6 25 Assam, Himalya 7 3 7 8

12 1954 7.4 180 Manipur, India 5 4 6 6

13 1975 6.7 112 Assam, India 4 3 5 6

14 1984 5.7 4 Cachar, India _ _ 3? 3

16 1997 5.6 35 Sylhet, Bangladesh 5 3 6 7

17 1997 5.3 56 Bangladesh-Myanmar 4 6 3 7

Therefore, moderate to high earthquakes magnitudes may occur in this region due to continuing tectonic deformation along the plate boundaries and active faults (CDMP, 2009). Purbachal New Town is a residential city which is under development by RAJUK and it is situated close to the seismically active zone. Study area is an alluvial plain consisting of fine sand and silt deposits with shallow ground water table in most places. Although the older alluvium is less susceptible to liquefaction, the deposits along the river flood plains may liquefy during a severe earthquake. Human-made soil deposits also deserve attention.

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According to the legal framework of many countries, regular building is restricted in areas adjacent to active faults.

[image:3.612.51.567.200.535.2]

Similar restrictions apply to soils susceptible to liquefaction due to strong earthquakes, to areas with slope stability problems, to unconsolidated embankments, etc. (e.g. Greek Earthquake Resistant Design Code, 2000).

Figure ۱. Map showing location of the study area.

Therefore, building should be away from liquefaction susceptible area. However exceptions are also possible, in special cases, when special research and damage scenario analysis are conducted and advanced engineering technology are available. In the case of Purbachal area of Dhaka, we mainly focused on the investigation of liquefaction hazard. Information derived from Standard Penetration Test was incorporated into the evaluation of safety factor of the liquefaction hazard at specific sites, since evidence was found for susceptibility to soil liquefaction.

II. LANDFORMS

The study area is divided into two major physiographic or landform units (FAO/UNDP, 1998) 1) High land, 2) Low land, 3) Medium land.

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Unlike the adjoining floodplains, the surface features of Madhupur tract are not determined by the original sedimentary patterns, but rather by the depth of weathering and subsequent erosion and probably also the interaction of the latter two processes with movement on the many faults that bound or bisect the tract on a regional scale.

Madhupur Tract has been divided into three sections by Alam (Alam M.K, 1988).

[image:4.612.47.562.215.579.2]

The Madhupur Garh has higher elevation and consists of elongated hillocks. The Bangsi, Turag, Banar and Khiro rivers dissect it. The topography of the Bhawal Garh is less pronounced and broad flat terrace areas are characteristics.

Figure ۲. SPT borehole location map of study area.

Tributaries of the Turag and Sitalakhya drain the area. The Dhaka terrace is the lowest part of the Madhupur Tract. It is on south of the Tongi Khal and slopes to the south and southeast. Tributaries of the Turag, Buriganga and Balu Rivers have dissected the Dhaka terrace. The study area falls within the Dhaka Terrace. The sites for conduction of the SPT tests were distributed throughout the Purbachal New Town. 19 boreholes were drilled for SPT under required observation and all of them were available for this study.

III. TECTONIC SET-UP

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These form two boundaries where plates converge– the India-Eurasia plate boundary to the north forming the Himalaya Arc and the India-Burma plate boundary to the east forming the Burma Arc. The moving rate of Indian plate is ~6 cm/yr in a northeast direction and subducting under the Eurasian (~ 45 mm/yr) and the Burmese (~ 35

mm/yr) plates in the north and east, respectively (Sella et

al., 2002; Bilham, 2004). This continuous motion of plates

[image:5.612.321.567.121.316.2] [image:5.612.49.292.288.463.2]

is taken up by active fault which is responsible for earthquakes. There are few regional Active faults are present in and around Bangladesh which can generate moderate to great earthquakes.

Figure ۳. Regional tectonic setup of Bangladesh with respect to plate Configuration and Fault.

IV. DRAINAGE OF THE STUDY AREA

Drainage system of study area is dominated by one river of national importance- the Sitalakhya. The Turag river borders western boundary of the area. The Sitalakhya river flows from the north and joins with Meghna river further south of the study area. The Balu river also flows from the north and joins with Sitalakhya river near Demra. The area is traversed few small khals and beels. Most of the Khals are seasonal and feed by the rainwater.

Figure ۴. Drainage pattern of the study area.

V. AQUIFER SYSTEM OF THE STUDY AREA

In the study area the Mio-Pleistocene sediments of the Dupi Tila Formation, which form the aquifer systems, lie beneath the Pleistocene Madhupur Clay Formation, (Table II) Master Plane Organization (MPO) in 1986, have divided the Mio-Pleistocene and Holocene aquifers of Bangladesh into upper and lower aquifer sequences on the basis of differing hydro-geological characteristics. The upper aquifer sequence is a heterogeneous assemblage of sands, silts and clays all essentially in hydraulic continuity. This upper aquifer has three subdivisions and the lower aquifer sequence may be subdivided into five aquifers separated by impervious clay layers, shown in (Table II) below.

The upper aquifer sequence is annually refilled by recharge from rainfall, floods and rivers. The lower aquifer sequence is recharged from outside Bangladesh plains on its eastern unconfined outcrop in the Tripura and Sylhet hills. In the study area the upper aquifer sequence as groundwater storage reservoir has three sub-units:

- An Upper silty clay layer;

- A middle composite aquifer of fine to very fine

sands;

- At bottom, the main aquifer of medium, medium

to coarse sand with layers of clay and silt. The main and composite aquifer divisions are connected

together hydraulically and lie under the upper silty clay cover.

Banglades

h

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TABLE II

AQUIFER SYSTEM OF BANGLADESH (MPO,1986)

Aquifer

Sequence Sub-units Thickness (m)

Upper Aquifer Sequence

Silty Clay Composite Aquifer Main Aquifer

0 to +120 3 to 60 30 to +60

Lower Aquifer Sequence

Clay Aquitard Aquifer No. 2 Clay Aquitard Aquifer No. 3 Clay Aquitard Aquifer No. 4 Clay Aquitard Aquifer No. 5 Clay Aquitard Aquifer No. 6

20 to 80 60 to 120 0 to 170 140 to 180 110 to 140 100 to 170 100 to 160 80 to 150 30 to 50 110 to 190

VI. GEOMORPHIC MAP RIVISION

[image:6.612.318.571.172.680.2]

Geomorphic unit map edited by GSB (2008) is used to evaluate geomorphological data of study area. Moreover, liquefaction hazard depend on rock density, because these areas are high risk for the liquefaction hazard due to distribution of loose sand caused by uncompacted work and high groundwater level. Purbachal New Town fall into upper modhupur terrace and show very low level of liquefaction susceptibility (Table III).

Figure ۵. Surface geomorphological map of Purbachal New Town (modified from GSB, 2008).

TABLE III

Susceptibility for Geomorphic Unit (CDMP, 2009)

Geomorphic Unit Type of Deposit Geological Age Susceptibilit y Meander Channel River

channel Modern Very High

Back Swamp Flood plain Holocene Moderate

Swamp /

Depression Flood plain Holocene Moderate

Flood Plain Flood plain Holocene Moderate

Shallow Alluvial

Gully Colluvium Holocene Moderate

Deep Alluvial

Gully Colluvium Holocene Moderate

Gully Head Talus Holocene Low

Valley Fill Colluvium Holocene Moderate

Channel Bar

Dunes / Delta and

fan-delta

Modern High

Point Bar

Dunes / Delta and

fan-delta

Modern High

Natural Levee

Dunes / Delta and

fan-delta

Modern High

Lateral Bar

Dunes / Delta and

fan-delta

Modern High

Lower Modhupur

Terrace

Residual

soils Pleistocene Very Low

Upper Modhupur Terrace

Residual

soils Pleistocene Very Low

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VII. SEISMISITY

Bangladesh has only few of seismic networks such as network of DUEO, BUET etc., but their history is short. Some Bangladeshi researchers have summarized seismicity data using the data of India and world observatory.The seismicity classified in depth and historical earthquakes by Bilham (2004) shows that seismicity in Bangladesh is high along the plate boundary between the Indian and Eurasian Plate and the Dauki Fault.

In 1979 Geological Survey of Bangladesh (GSB) through an inter-ministerial national committee prepared a seismic zoning map of Bangladesh and outline of a code for earthquake resistant design of structures based on historical earthquakes. Latter on a revised seismic zoning

map (Figure ۶) is papered in 1993. This map is included in

[image:7.612.81.535.257.704.2]

Bangladesh National Building Code (BNBC, 1993). In BNBC map Bangladesh is divided into three zones based on maximum ground acceleration. The zones are Zone 1,

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Zone 2 and Zone 3 where the ground accelerations are 0.075g, 0.15g and 0.25g respectively.

The Zone 1 which is the seismically least active zone includes Rajshahi, Pabna, Koshtia, Faridpur, Jossore, Khulna, Barisal, Noakhali and Patuakhali. Dinajpur, Pnchagarh, Thakurgaon, Nilphamari, Bogra, Tangail, Dhaka, Munshigonj, Comilla, Rangamati, Chittagong and Cox’s Bazar are included in Zone 2. Sylhet, Mymensingh,

Jamalpur, Netrokona, Kishoreganj, Kurigram and

Lalmonirhat are in included in Zone 3 which is the most seismically active zone.

Purbachal New Town fall into Zone 2, thus bed rock acceleration 0.15g is used to calculate peak ground acceleration by ―Edushake‖ software.

VIII. CALCULATION OF PGA

To calculate peak ground acceleration of surface rock layer ―Edushake‖ software is used. To have PGA one has to input layer number, lithology, ground water table, thickness, unit weight, share wave velocity, plasticity index and damping ration 5% for all rock layer. In ―Modulus Reduction Curve‖ option ―Vucetic- Dobry‖ is used for clay and silt; ―Seed and Idriss 1970‖ is used for sand and soil

and ―Linear‖ for bed rock. ―TREAS.EQ‖ for ―File Name‖

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option was taken as study area fall in ZONE-2 in the Seismic Zoning Map of Bangladesh. Then have averaged all peak ground acceleration value to have a single value for a borehole.

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IX. EVALUATION OF LIQUEFACTION PROBABILITY AND

RESULT DISCUSSION

Evaluation of liquefaction potential requires two sets of parameters: parameters for the seismic loading and parameters to represent the characteristics of soil deposit. Each parameter influences the evaluation of liquefaction potential to a different degree, and there is considerable uncertainty associated with each of them. Youd and Perkins (1978) have estimated the liquefaction susceptibility of different types of soil deposits by assigning a qualitative susceptibility rating based on general deposit environment and geologic age of the deposit. The Historical liquefaction at a specific location is strongly influenced by the susceptibility of the soil, the amplitude and duration of ground shaking and the depth of ground water. Thus, the probability of liquefaction for a given susceptibility category can be determined by the following relationships [HAZUS97].

[ ] [ | ] Where,

P[Liquefactionsc|PGA=a]: Conditional liquefaction

probability for a given susceptibility category at a specified level of PGA (Table IV). Purbachal New Town fall in upper modhupur terrace (Geomorphic map edited by GSB, 2008). So ―very low‖ susceptibility category is used in this research.

TABLE IV

CONDITIONAL PROBABILITY RELATIONSHIP FOR LIQUEFACTION

SUSCEPTIBILITY CATEGORIES

Susceptibility Category P[Liquefaction|PGA=a]

Very High 0 ≤ 9.09a – 0.82 ≤ 1.0

High 0 ≤ 7.67a – 0.92 ≤ 1.0

Moderate 0 ≤ 6.67a – 1.00 ≤ 1.0

Low 0 ≤ 5.57a – 1.18 ≤ 1.0

Very Low 0 ≤ 4.16a – 1.08 ≤ 1.0

None 0.0

Chart ۱. Conditional Liquefaction Probability Relationships for Liquefaction Susceptibility Categories (after Liao, et. al., 1988).

KM: MW correction factor, calculated by the following

equation

KM= 0.0027Mw

3

– 0.0267MW

2

– 0.2055MW + 2.9188

Where,

MW = Moment magnitude of the seismic event

Chart ۲. Moment Magnitude (M) Correction Factor for Liquefaction Probability Relationships (after Seed and Idriss, 1982).

KW: Groundwater depth correction factor, calculated by

the following equation

KW = 0.022dW + 0.93

Where,

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Chart ۳. Ground Water Depth Correction Factor for Liquefaction Probability Relationships.

Pml: Proportion of map unit susceptible to liquefaction (Table V)

Study area fall into Upper Modhupur Terrace (Table III) which shows very low liquefaction susceptibility. So ―very low‖ susceptibility in taken to have proportion of map unit (Pml) from table V.

TABLEV

PROPORTION OF MAP UNIT SUSCEPTIBLE TO LIQUEFACTION (AFTER

POWER, ET. AL.,1982)

Mapped Relative

Susceptibility Proportion of Map Unit

Very High 0.25

High 0.20

Moderate 0.10

Low 0.05

Very Low 0.02

None 0.00

With all these processed data, peak ground acceleration ―a‖ has been generated by ―Edushake‖ software in purpose of liquefaction hazard analysis. Then, ―a‖ of 0 to 15 meter of soil profile is used to have conditional liquefaction probability for a given susceptibility category at a specified

level of PGA. Corresponding KM (MW correction factor)

and KW (Groundwater depth correction factor) is also been

[image:10.612.47.294.123.281.2]

calculated for 15 meter depth. Afterwards, probabilities of liquefaction have been evaluated for every borehole with this depth range.

TABLE VI

CALCULATED LIQUEFACTION PROBABILITY

Bore Hole No.

a (avg)

(g)

P[Liquefactionsc|PGA=a] MW KM

dw

(ft)

KW Pml P[Liquefactionxsc]

PBH-01 0.3 0.1680

7.5 1.015

19.03 1.35

.02

0. 002455177

PBH-02 0.38 0.5008 22.31 1.42 0. 006947064

PBH-03 0.26 0.0016 23.29 1.44 0. 000021863

PBH-04 0.39 0.5424 24.61 1.47 0. 007265393

PBH-05 0.42 0.6672 21.33 1.39 0. 009397962

PBH-06 0.28 0.0848 19.23 1.35 0. 001235250

PBH-07 0.37 0.4592 15.75 1.27 0. 007090178

PBH-08 0.34 0.3344 20.67 1.38 0. 004759640

PBH-09 0.34 0.3344 18.70 1.34 0. 004913422

PBH-10 0.42 0.6672 18.37 1.33 0. 009856681

PBH-11 0.36 0.4176 17.71 1.32 0. 006237171

PBH-12 0.42 0.6672 17.06 1.31 0. 010074306

PBH-13 0.32 0.2512 11.81 1.18 0. 004161160

PBH-14 0.31 0.2096 13.78 1.23 0. 003350024

PBH-15 0.42 0.6672 17.72 1.32 0. 009963475

PBH-16 0.48 0.9168 18.70 1.34 0. 013470769

PBH-17 0.39 0.5424 22.31 1.42 0. 007524137

PBH-18 0.45 0.7920 24.93 1.48 0. 010558244

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Figure ۸. Liquefaction probability map of Purbachal New Town.

The whole analysis procedure has been done with MS Excel. The calculated value demarcate according to rank, from 1 (very low) to 5 (very high). Threshold of each rank from 1 to 5 are set as less than 0.05, 0.10, 0.15, 0.20 and equal to / more than 0.20, respectively. Hence all calculated value laid bellow 0.05 (Table VI) show very low liquefaction susceptibility. For convenience of the result discussion, the liquefaction probability is presented visually by probability map by using the ArcGIS-10 software.

X. CONCLUSIONS

The liquefaction Probability map of Purbachal New Town offers a quantitative approach for mapping liquefaction Probability. The liquefaction potentiality of a specific location has been predicted by probable

liquefactionsusceptibility values at each borehole location

and map has been drawn by interpolation method ArcGIS-10 software.

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REFERENCES

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[2] Ambraseys, N., Bilham, R., 2003. MSK isoseismal intensities evaluated for the 1897 Great Assam earthquake. Bull. Seismol. Soc. Am. 93 (2), 655–673.

[3] Bilham, R., England, P., 2001. Plateau pop-up during the great 1897 Assam earthquake. Nature 410, 806–809.

[4] Bilham, R., Wallace, K., 2005. Future Mw N 8 earthquakes in the Himalaya: implications from the 26 Dec 2004 Mw = 9.0 earthquake on India's eastern plate margin. Geol. Surv. India Spec. Publ. 85, 1– 14.

[5] BNBC (1993), ―Bangladesh National Building Code.‖ House and Building Research Institute, Bangladesh Standards and Testing Institutions.

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Models for Evaluationg Liquefaction Probability, Journal of Geotechnical Engineering, vol. 114, No. 4, April.

[9] Middlemiss, C.S., 1885. Report on the Bengal earthquake of July 14, 1885. Rec. Geol. Surv. India 18 (4), 200–221.

[10] Morino, M. 2009. Active Fault Mapping and Modeling in Bangladesh.

[11] Oldham, R.D. 1899. Report of the Great Indian Earthquake of 12th June 1897. Mem. Geol. Surv. India 29.

[12] Power, M. S., A. W. Dawson, D. W. Streiff, R. G. Perman, and S. C. Haley, 1982. Evaluation of Liquefactin Susceptibility in the San Diego, Clfornia Urban Area. Proceedings 3rd_{International} Conference on Microzonation, II, pp. 957-968.

[13] Seed, H. B., and Idriss, I. M. 1982. Ground Motions and Soil Liquefaction During Earthquakes, Earthquake Engineering Research Institute, Oakland, Calfornia, Monograph Series, P. 13.

[14] Sella, G. F., T. H. Dixon and A. Mao, 2002. REVEL: A model for recent plate velocities from space geodesy. Journal Geophysical. Research, Vol. 107, B4, 1029/2000JB000033.

[15] Stuart, M. 1920. The Srimangal earthquake of 8th July 1918. Mem. Geol. Surv. India 46 (1).

[16] Youd, T.L. and Perkins, D. M., 1978. Mapping liquefaction-induced ground failure potential, Porc.ASCE Civil Eng., v.104, no.GT4, p.433-446.