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GOVERNMENT OF THE PEOPLES REPUBLIC OF BANGLADESH

Ministry of Energy and Mineral Resources

Geological Survey of Bangladesh

May 2005

A NOTE ON SOME GEOLOGICAL ADVANTAGES FOR

CONSTRUCTION OF UNDERGROUND RAILWAY TRANSIT SYSTEM

IN THE CITY OF DHAKA

Mir Fazlul Karim

Director

Geological Survey of Bangladesh,

E-mail: azadkk@gmail.com and mfk@dhaka.agni.com

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A note on some geological advantages for construction of underground railway transit

system in the City of Dhaka

Mir Fazlul Karim, Director, Geological Survey of Bangladesh, Dhaka

Foreword

A request was made to the Director General, Geological Survey of Bangladesh (GSB) by The Louis

Berger Group, INC and Bangladesh Consulting Ltd. to provide a geological evaluation of Dhaka City

for understanding of and planning of underground transportation system under Strategic Transportation

Planning Project of the Government of Bangladesh. This evaluation report is prepared using geological

investigation reports and technical papers available in the GSB. Emphasis is given to provide some

concept on the sub-terrain environment and ground condition of Dhaka city. An interpretation on the

engineering geological advantages and disadvantages for planning, designing and development of

underground transportation system is done.

Summary

The City of Dhaka occupies a unique geological location in terms of the geological set up and ground

condition. Geologically the city landmass is a part of an old terrace that is raised considerably up than

the surrounding floodplains. The general elevation in the axial part of the city reaches up to about 6 to

7m above mean sea level (AMSL) whereas the surrounding floodplains are at about 4m AMSL. The

ground of the terrace is composed of Madhupur Clay (Pleistocene), consisting of very stiff to hard,

reddish brown, highly ferruginous, plastic Clay-Silt with some minor contents of fine quartz-sand and

iron concretions. The shear strength properties are considerably high and the average value of un-

drained shear strength is ≈ 2 kg/cm2. The settlement characteristics indicate that the red clay to be an

over-consolidated stratum. The geological history indicates that due long run erosion about 20m of

materials have been removed from the top of the city ground. This indicates that an amount of ≈ 1

kg/cm2 of vertical stress has removed from the top of the existing ground surface. At present the

average thickness of Madhupur Clay in the main urban area is about 7 meters. The upper clay-silt

stratum overlies a firm sand bed, geologically known as Dupitila Formation (Upper Tertiary). The

geological and geotechnical properties of Dupitila Sand (Upper Dupitila Sandstone) tends to behave as

soft rock. This soft sedimentary rock acts as dense bedrock to hold most of the pile tips of all deep

foundation which is uniformly distributed throughout the axial part of the city.

In terms of hydro-geological environment the Madhupur Clay Formation acts a wonderful cap over the

Dhaka terrain. The top material is low in permeability and no ground water recharge occurs in the city

area at ambient condition. The surface materials and natural topography with unique natural drainage

system the flash-flood water has excellent run-off characteristics. The permanent and confined ground

water level is located below 70m from the surface. The existing geo-hydrological environment provides

better options for underground excavation.

The existence of Madhupur Clay and Dupitila Sand below the urban feet turns the ground condition of

Dhaka city to the best comparing the ground condition of many other cities of the world. Though

Bangladesh is located at edge of most seismically active global zone, in terms of the ground condition

the apparent seismic hazard* is relatively low in Dhaka city than most of other parts of the country

(expected maximum Peak Ground Acceleration is 0.5g). A correlation on Shear Wave Velocity (Vs) of

the geological materials underneath Dhaka city indicates that the value of Vs would be > 350 m/sec.

Considering the geological and geotechnical aspects, the city of Dhaka has advantageous ground

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condition for construction of underground engineering structures including underground rapid

transportation system.

(*Caution: The low seismic hazard does not mean lower seismic risk, as the risk is directly related to

human activity and response to seismic issues.)

Introduction

Dhaka is the capital city of Bangladesh. The history of the city dates back to he 7th

century, due

to various political and social changes the city could not flourish with freedom till the creation

of Bangladesh. After independence the city has been grown physically by at least 5 times in 25

years where the population has increased by at least 6 times. The present population of the city

is nearer to 12 millions, by virtue of that it has become a member of world’s mega-cities

family. The area of the Greater Dhaka City is 1353 sq. km where the main city is about 500 sq.

km. The urban distribution of the city is shown in figure 1.

Figure 1. Urban and industrial distribution of Dhaka City.

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Development of underground railway or rapid urban mass transit systems is one of the main

targets in most urban areas in all modern cities of the world as it helps to avoid the daily traffic

jam. The implementation involves severe financing risks throughout the development,

construction and monitoring and maintenance of system. The major form and the main

elements of the proposed system are geological condition and earthquake or environmental

risk, design, construction methodology and cost estimation.

Construction of underground railway transit for the Dhaka City is a fascinating idea for the

people of the country. As the rapid growth of population, urbanization, communication and

infrastructure development in Bangladesh has sharpened the need of improved fast

transportation system in the cities. It will be a challenging step for the government, engineers

and geologists as underground constructions are always difficult and require clear

understanding of subsurface geological structure and ground conditions.

Building of underground transportation system is essentially a costly and time-consuming

affair. Many factors go into the decision of whether or not to proceed with such a huge

underground activities that involves highest level of risk in the different construction phases.

Like real time observation and monitoring of geological changes.

Question may be raised, why the Dhaka city would invest huge amount of public money and

much time into a rapid transportation system?

The city of Dhaka is considered as one of the densely populated cities of the world and there

are about 80 cities in the world where underground transportation systems are already

developed. Dense traffic jam in the roads of Dhaka city has become a serious everyday-

problem and hampering normal movement of city dwellers and causing acute environmental

pollution. Development of underground transportation system will reduce traffic jam, less

traffic on the streets will conserve energy and reduce import of fuel, fewer vehicles on the road

emit less exhaust and noise pollution and reduce the stress of driving and traveling to the work

place and for a long run it would be cheaper mode of transportation. Government will earn

genuine revenue from the system.

Considering these, present evaluation is done to understand and delineate the geological

advantages for construction of special engineering structures like underground railway or

tunnel railway. This paper delineates and describes the geological conditions for underground

excavation and construction of surface transportation tunnels in Dhaka city.

Geology

The Dhaka City occupies a greater part of the Dhaka-Gazipur Terrace. The terrace is a part of

Madhupur Tract, located in the central part of Bangladesh. The tract is a structural high that

extends from the folded hills in the eastern fringe of Bengal basin. It is exposed as a

monoclonal limb of a large fold and appears as a terrace. Due to higher elevation than the

surrounding alluvial plains the terrace has become a seat of urban and industrial development.

For the continuing expansion and sustainable development, it is necessary to understand the

geomorphology, geotechnical behaviour and tectonic or structural setting of the terrace.

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Present study indicates that the terrace is formed of number of blocks separated by plastic

deformation due to compression forces during last phases of Himalayan upliftment. A series of

dendrite to trellis drainage system has been developed on the terrace following the fractures or

shear zones that has produced a unique geomorphic outlook. Tectonic setting of the region

influences the terrace morphology and erosion. Three tectonic bench levels are identified that

affect variation in state of weathering of Madhupur Clay Residuum including surface run-off

and annual flood level. The terrace is formed of three broad geomorphic units consisting of

raised plain land, moderately dissected or rugged complex of low plains and depressions.

The city is formed in the raised plain land which is composed of Madhupur Clay Formation of

Pleistocene age, its average thickness is about 8m. The clay formation is underlain by thick

dense sand bed of Dupi Tila Sandstone Formation. There are three distinct geomorphologic

units of greater Dhaka city, figure 2.

a) Central high area: This unit is an elongated tableland above flood level that stretches from

south to north. The elevation is 7m above mean sea level (AMSL).

b) Complex of low and high areas: This unit consists of narrow strips of benches or foot

slopes, rounded to elongated saddle of central unit, narrow and shallow erosion gullies and

incised valleys or abandoned channels.

c) Complex of low areas: It consists of flood plains of the rivers Burhiganga, Sitalkhya and

Meghna, which is annually flooded. The unit is flat and the elevation is below 2m AMSL.

Figure 2. Three distinct geomorphologic

units of greater Dhaka city. These

divisions are consistent to seismic zone

of the city

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TABLE 1. Generalized stratigraphic succession in and around Dhaka-Tongi area (After Alam,

1988 and Karim, 1991).

AGE FORMATION LITHOLOGY THICKNESS

in meters

H

olo

cene

All

uviu

m

Lowland:

Riverbed deposit: Grey sand and silty sand,

medium to fine-grained.

0 – 9

Unconfor

mity

Natural Levee and inter-stream deposit: Sandy

silt, silt and loam, grey and friable. 1.21-4.7

Backswamp & depression deposits: Clay and

silty clay, grey, bluish grey to dark grey, 0.61-1.5

High Land:

Silt, sand and clay above the present flood level. 0-3.5

Local

Unconformity

P

leis

toce

ne

Madh

upu

r C

lay

Red clay: Light brown to brick and massive,

oolitic and contains fossil woods, ferruginous

and calcareous nodules and surfacial deposit of

slag.

Mottled clay: Earthy grey with patches of

orange, brown colour, massive and contains

calcareous & ferruginous nodules.

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Unconform

ity

P

lioce

ne

D

upi

T

ila

Sandstone: Yellow to yellow grey, massive,

cross bedded, fine to medium grained mostly and

contains gravels, moderately consolidated. 90+

Depending on geomorphologic situations and consistency of subsurface materials the ground

conditions are rated as hazardous, poor and favourable for underground engineering activity

(Table 2).

Geo-Hydrology

In terms of hydro-geological environment the Madhupur Clay Formation acts a wonderful cap

over the Dhaka terrain. The top material is low in permeability and no ground water recharge

occurs in the city area at ambient condition. The surface materials and natural topography with

unique natural drainage system the flash-flood water has excellent run-off characteristics. The

permanent and confined ground water level is located below 70m from the surface. The

existing geo-hydrological environment provides better options for underground excavation.

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Seismic state and Evaluation of Ground Condition for Underground Excavation and

tunneling

A study has been made to infer the pattern of possible ground motion to be occurred during a

moderate earthquake in Dhaka (Karim and Rahman, 2002). The inferences are made on the

basis of tectonic set-up, structural geology and lithological boundary condition of subsurface

materials, their generalized geotechnical properties, state of infrastructure design and

construction practices in Dhaka City. The study reveals that the city has been developed on an

advantageous geological location consisting of raised older alluvium in respect to the

surrounding floodplains, which is formed of young alluvial deposits.

To determine the state and suitability of underground excavation by considering the geological

ground conditions and geomorphic attitudes, the Dhaka City and surrounding area can be

divided into three broad geological-geotechnical zones (Figure 2) which are Zone 1: The

Central high area, Zone 2: Complex of high and low areas and Zone 3: Complex of low area.

The Central high area forms the axial zone and extends northward up to Gazipur and then

further up. Madhupur Clay Formation is well exposed throughout the zone. This zone is rated

as Class 1 type ground condition, it is composed very stiff to hard reddish brown Clay-Silt

(Madhupur Clay Formation), having better engineering properties of the materials (Table 3)

and considered to produce less ground motion than the other two zones. The Complex of high

and low areas consist of small domes or nodes of Madhupur Clay Formation which is either

exposed at lower elevation or buried under thin cover of young alluvium or fill materials. This

zone is rated as Class 2 type. The elevation of this zone is below the central zone; the materials

are moister and have lower shear strength than the elevated materials. The inter-depressions of

this zone are some times filled up with very soft clay, organic clay or peat deposits. The

materials are often compressible and may suffer strong ground motion to cause severe

destruction. Complex of low area is located in the eastern and western periphery of Dhaka

City. This zone is formed of very soft clay silt in the east and flood plain of silt-sand in the

west. This zone can be rated as Class 3 type. The general elevation of this zone is below the

Complex of high and low areas; the materials are very soft and susceptible to compression and

liquefaction. The generalized soil logs are shown in Figure 4.

The Dhaka City and surrounding area is divided into three broad seismic zones having distinct

variations in ground condition and constituent geological materials. The seismic zones are

consistent to geo-morphological environment of the city ground mass.

The proposed Rapid Transport System is located mostly in the Central High Area and part of

adjacent geological unit and the subsurface has reasonably excellent ground condition.

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Table 2. Rating of ground condition for different geomorphologic units of Greater

Dhaka city.

Rating Factors GLU Areas

a. Liquefaction III Flood plain of river Burhiganga

b. Slope failure II & I1 Along streams and depression edges

1. Hazardous c. Flood III & II Areas below normal flood level

d. Water logging I & II Areas where natural drainage is blocked

e. Fill collapse III Filled up water wells, ponds and depressions

a. Ground

collapse

and subsidence

III Low areas with partially saturated sand/silt

(elevation <2m)

2. Poor b. Swelling clays I Mostly central or high area

c. Soft to firm clay,

organic soils, and

municipal waste

II & III

Eastern part of Greater Dhaka city

3. Favourable Elevated Virgin

land

I & II Central high area and adjacent mix area.

The proposed underground railway is located

with the GLU I and GLU II

GLU 1 LESS GEOLOGICAL PROBLEMS

GLU II HAS MODERATE GEOLOGICAL

PROBLEMS

GLU III HAS MORE GEOLOGICAL PROBLEMS

TONGI

OLD

DHAKA

GULSHAN

Figure 3. The geomorphologic units of greater

Dhaka city is divided in to three units in terms of

ground condition rating for different geotechnical

land-use.

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.

STIFF CLAY

DENSE SAND

SOFT CLAY

STIFF SILT

TYPE A A TYPE A B TYPE A C TYPE B TYPE C

SOFT CLAY

STIFF CLAY

DENSE SAND

SOFT CLAY

DENSE SAND

0 m

10 m

Figure 4. The proposed Rapid Transport System

is aligned in the Central High Area having the

best ground condition.

Generalized geological log at different location

is shown and the vertical ground conditions are

classified as A, B and C. The Type A is further

divided in to AA, AB and AC depending on the

soil type and quality of upper strata. Type AA

Type B

Type B

Type C

Type AB

Type AC

20 m

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Alignment of The proposed Rapid Transport System and existing geological condition

The proposed Rapid Transport System is aligned in the Central High Area having the best

ground condition. The area forms the axial zone and extends northward up to Gazipur and then

further up. The Madhupur Clay Formation is well exposed throughout the zone. The average

elevation is 7 meters above mean sea level. The localities lying in this zone includes the region

from the old city part in the south, Tejgaon, Gulshan, Uttera, Tongi to Gazipur. The average

width is about 2.5 km.

The zone is drained by numerous large and long streams, which were once connected with the

rivers flowing through the outskirts of the city. Most of the long streams are deep enough to cut

through Madhupur Clay Formation. There are number of depressions in this zone, mainly

along the abandoned streams.

Table 3. The generalized engineering properties of Central High Area are as follows:

Formation Average

Thickne

ss,

meter

Materials Average

Moisture

content,

wn %

Dry unit

Weight

d

kN/m3

Average

Undrained

Shear

Strength,

su, kPa

N value

Range

(SPT

count)

State and

suitability of

underground

excavation

and tunneling

Madhupur

Clay

6 Stiff

Clay-SILT

25

16

150

10 - 30

Highly

suitable

Dupi Tila

Sandstone

> 90 Very dense

Sandstone

with

occasional

clay beds

< 20

> 16

> 30o

Often

Refusal

Excellent for

underground

activity

The alignment of the propose underground railway system may be considered to follow under

the existing and abandoned railway lines. For understanding the three dimensional distribution of

geological materials and sub-geologic structure a schematic 3D- Box Diagram of Dhaka City is

prepared. Following the vertical and lateral extent of Madhupur Clay Residuum and Dupitila

Sandstone Formation three possible vertical locations for placement of Underground Railway

System is proposed here.

A. At surface – to be constructed by Cut and Fill Method.

B. Near Surface – to be constructed below the existing roads by Tunnel Excavation, where

maximum depth of the roof of the tunnel would be 8m below the existing road surface (+6m

AMSL) of Kazi Nazrul Islam Avenue and

C. Deep Tunnel – at a depth of at least 15m from the existing surface of Kazi Nazrul Islam

Avenue. The subsurface geology indicates that there is wide range of freedom of choice for

selection of any method for tunneling and removal of waste materials. Application of Tunnel

Boring Method (TBC) would be the most suitable method for construction of underground

transportation system in Dhaka.

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Figure 5: Schematic 3D- Box Diagram of Dhaka City showing the possible vertical locations for

placement of Underground Railway System

Dhan

mondi

Kam

ragir

Char

Isla

mpur

Dem

ra

Ram

na

Bas

hab

o

Mir

pur

Kaf

rul

Bar

idhar

a

Utt

era

Utt

er K

han

6m

4m

2m

0m

- 2m

- 4m

- 6m

- 8m

- 10m

- 12m

- 14m

- 16m

DT - DUPITILA SANDSTONE

MC -

MADHUPUR

CLAY

RESIDUUM

DT

DT

DT

MC

MC

MC

A B

C

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Table 3. A generalized comparison of geological advantages and disadvantages existing other

cities of the world.

City Engineering geology Disadvantages Geological advantages

Kolkata and

Bangkok

Young deltaic

sediments, soft clay,

silt and sand and peat

at near surface

Only choice - cut and fill as no

firm stratum at a considerable

depth. heavy dewatering

Easy for excavation and easy for

removal of waste

Manhattan and

many European

cities Hard rock

Difficult to drill or excavate..

removal difficult Requires less protection during

progression of tunnel

Dhaka

Terrace of

sedimentary rock or

stiff clay and very

dense sand

Will require moderate

protection to retain tunnels

before rimming

Freedom of choice for any

method of tunneling and removal

of waste materials. Application of

Tunnel Boring Method (TBC)

would be the most suitable

method for construction of

underground transportation

system in Dhaka.

Conclusion

The City of Dhaka occupies a unique geological location considering the geological set up and

ground condition of surrounding flood plains. Geologically it is an old terrace that has been raised

considerably about 7m above sea level (AMSL). The surrounding floodplains are at about 4m

AMSL. The ground is composed of Madhupur Clay. The Clay is Over-consolidated. The shear

strength properties are considerably high. The Thickness of Madhupur Clay is about 6 m and it

overlies a firm sand bed, geologically known as Dupitila Formation (Upper Dupitila Sandstone). It

is may be considered as very dense bedrock. The Madhupur Clay and Dupitila Sandstone are very

much suitable for construction of underground structures like underground transportation system

for our growing Mega City.

The permanent and confined ground water table is situated below 70m of depth from the surface

and it will not cause any significant constraint if the tunneling is planned within this depth. The

raised platforms of the stations will help to avoid runoff water.

Though Bangladesh is located at edge of most seismically active global zone, in terms of the

ground condition the apparent seismic hazard is relatively low (<0.5g Peak Ground Acceleration is

expected) for Dhaka city than most of other parts of the country. Considering the geological and

geotechnical aspects, the city of Dhaka has advantageous ground condition for construction of

underground engineering structures including underground rapid transportation system.

The sequence of sedimentary rock or stiff clay and very dense sand beneath the city surface

provides in freedom of choice for selection of any method of tunneling and removal of waste

materials during construction. Application of Tunnel Boring Method (TBC) would be the most

suitable method for construction of underground transportation system in Dhaka.

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References

1. Alam, M. K. Geology of Madhupur Tract and its adjoining areas in Bangladesh. Records of the

Geological Survey of Bangladesh. 5(3): 1-8, Dhaka, 1988.

2. Karim, M.F. and Haider, M.J. Impact of Geo-Engineering Mapping for Waste Management of

Greater Dhaka City. In Proc: First International Congress on environmental Geotechnics,

Alberta, Canada. July, 1994.

3. Amin, M.N., Karim, M.F. and Uz Zaman, M.A. Some experience on Bored Cast-In-Situ

Reinforced Concrete Piles in Bangladesh - A Case Study. In Proc. of International Conference on

Design and Construction of Deep Foundations, US Federation of High Ways Administration ,

December, 1994, Orlando, Florida, USA.

4. Amin, M. N. and Karim, M.F. Model procedure and specification for construction of Bored Cast in

Situ R.C.C. pile foundation in Bangladesh. Paper presented at the 39th Annual Convention, 1995.

Institute of Engineers Bangladesh, Chittagong, Bangladesh.

5. Karim, M. F. State of earthquake research and monitoring in Bangladesh. UNESCO arranged

International Workshop on Seismic Analysis in the South Asia Region Workshop, Kathmandu,

Nepal, 10-13 September 2001.

6. Karim, M. F. Necessity for Development of Micro-Seismic Monitoring System for Dhaka City,

Bangladesh. UNESCO arranged Second International Workshop On Seismic Analysis for the South

Asia Region, 20-23 May, 2002, Kunming, China.

7. Khan, M.R. and Karim. M.F. State of geological aspects in land use planning in Bangladesh: An

overview. Paper prepared for Expert Group Meeting on Geological Aspects of Land Use Planning.

ESCAP, Bangkok, Thailand, 10-13 May 1994.

8. Karim, M. F. Necessity for Development of Micro-Seismic Monitoring System for Dhaka City,

Bangladesh. Second International Workshop On Seismic Analysis for the South Asia Region

Organized by UNESCO , 20-23 May, 2002, Kunming, China.

9. Karim, M.F., Haider, M.J. Chowdhury, M. and Kabir. Engineering Geomorphology for Ground

Improvement in Part of Dhaka City and Tongi Area. . In The Ground Beneath Our Feet: A Factor

In Urban Planning. Atlas of Urban Geology, Volume 14, UN ESCAP New York, 2003, pp 187 –

194.

10. Karim, M.F. Geological aspects in urban planning and infrastructure development: An increasing

awareness in Bangladesh. In The Ground Beneath Our Feet: A Factor In Urban Planning. Atlas of

Urban Geology, Volume 14, UN ESCAP New York, 2003. pp 195 – 207.

11. Karim, M. F. and Rahman M. Z. Possible Effect of Moderate Earthquake on Existing

Infrastructures of Dhaka City: A Geological-Geotechnical Overview. Bangladesh Journal of

Science and Technology, Vol. 4(2), July 2002. pp 193-203.