Md. Zillur Rahman

In this study, 3D terrestrial laser scan data have been used to quantify the surface roughness of rock mass discontinuities, to determine the roughness scale effect and to determine the influence of the range of precision. The Roughness-length method was applied to calculate fractal parameters (fractal dimension and roughness amplitude). The study showed that it is possible to calculate the fractal parameters from laser scan data of discontinuity surfaces. It has been found that the fractal dimension and roughness amplitude are very high for raw laser scan data, due to range measurement error of the used scanner. The discontinuity surface has been reconstructed from the raw point data using a scattered data interpolation technique. The range measurement error can be removed using this data interpolation technique. The reconstructed surface visually resembles the actual discontinuity surface. The fractal parameters of the reconstructed surface have subsequently been calculated. By subtracting the fractal parameters of the raw data from the fractal parameters of the reconstructed surface, the range error of the laser scan data can be estimated. The fractal parameters have also been calculated using different sample sizes. The fractal parameters show an increase with increasing sample size. This means that the roughness complexity of the discontinuity surface is also increasing with increasing sample size. From this observation it can be concluded that the roughness of discontinuity surfaces has a significant scale effect. The maximum resolution of the laser scan data used in this study is 5 mm (1 point per 5 mm). The laser scan data with 5 mm resolution can only acquire roughness information from roughness features larger than 5 mm. Consequently, small-scale roughness (roughness features smaller than 5 mm) can not be obtained from the data. Therefore the resolution of the laser scan data (point density) needs to be increased to quantify discontinuity surface roughness accurately as a smaller scale.

Abstract:

The history of earthquakes in Bangladesh and surrounding areas indicates that many severe earthquakes have occurred in this landmass. The past regional earthquakes caused considerable damage in different parts of the country, including the Dhaka City. The rapid growth of population and urbanization, together with frequent occurrences of earthquakes in the eastern part of Bangladesh has sharpened the need of understanding the affect of earthquake in the city.

A study has been made to infer the pattern of possible ground motion to be occurred during a moderate earthquake in and around Dhaka City. The inferences are made on the basis of tectonic influence, geomorphology and lithological boundary condition of subsurface materials, their generalized geotechnical properties, state of infrastructure design and construction practice in the Dhaka City. The study reveals that the city has been developed on an advantageous geological location consisting of raised Madhupur Clay Formation or older alluvium in respect to the surrounding floodplains of young alluvial deposits. Considering the variations in geomorphology, ground condition, their constituent geological materials and geotechnical properties the Dhaka City and surrounding area is divided into three broad earthquake risk zones. The zones are - Zone 1: The Central high area having Ground Condition Class 1; Zone 2: Complex of high and low areas having Ground Condition Class 2 and Zone 3: Complex of low area having Ground Condition Class 3.


The Central high area forms the axial zone of the terrace and extends northward. The Madhupur Clay Formation is well exposed throughout this zone. This zone is designated as earthquake risk zone 1. The ground condition of this zone is rated as of Class 1 type which is composed of very stiff to hard reddish brown Clay-Silt having good engineering properties of materials and considered to produce less ground motion.

The Complex of high and low areas is considered as earthquake risk zone 2. This zone is formed of small domes or nodes of Madhupur Clay Formation that is either exposed at lower elevation or overlain by young alluvium or fill materials. The ground condition of this zone is rated as of Class 2 type. The elevation of this zone is below the central zone; the materials are moister and have lower shear strength than those of zone 1. 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 during earth-movement. 

The Complex of low area is located in the eastern and western periphery of Dhaka City and considered as earthquake risk zone 3. This zone is formed of very soft clay silt in the east and flood plain silt-sand in the west. The ground condition of this zone is rated as of Class 3 type. The general elevation of this zone is below the adjacent complex of high and low areas; the materials are very soft and susceptible to compression and liquefaction.

Present study on effect on existing infrastructures indicates that there are three types of existing infrastructures in terms of design and construction such as well engineered infrastructures, poorly engineered infrastructures and non-engineered infrastructures. It has been observed that the structures designed by qualified engineers and architects and constructed under close supervision by maintaining proper quality of materials are expected to survive during a moderate earthquake irrespective of earthquake risk zones of Dhaka City.

Cities without the use of underground space are unthinkable in a modern urban environment. Mega-city Dhaka in Bangladesh has yet to step into the underground construction world to take advantage of its underlying unique and firm soil, the stiff-to-hard Madhupur Clay Residuum, which overlies dense-to-very dense sandstone of the Dupi Tila Sandstone Formation. Dhaka is very densely populated and continues to be among the fastest growing city in Asia. Uncontrolled growth including skyward expansion have caused tremendous overcrowding and exasperating traffic congestions. However, use of underground space is still ignored and largely unexplored. The possibility of building an underground rapid transit system will relieve many of the present urban stresses.
The city was developed on an uplifted tectonic block (anticlinorium) of Plio-Pleistocene age, which is isolated from the surrounding floodplains of active Ganges-Brahmaputra delta system. The top 20 ft of ground is composed of stiff-to-hard over-consolidated clayey soil (Layer 1), that overlies thick moderately lithified sand (Layer 2, drilled to 200 ft). A 3D geological model is prepared to illustrate the spatial distribution of these stratigraphic layers. Extensive geophysical and geotechnical exploration data including hydrogeological conditions are compiled to give insights and engineering options for shallow and deep underground construction. The geotechnical and geophysical properties of the geological strata include vertical consistency, density, undrained shear strength, consolidation and settlement characteristics, and shear wave velocity. A plot of geotechnical properties with depth indicates improved ground conditions occur with the increasing depth. The 3D geological model shows that strata are homogenous and uniformly distributed. The increase of density, shear strength and shear wave velocity (>1400 ft/s below 20 ft) with depth is a strong advantage for planning of safe tunnels and underground structures.

Bangladesh is prone to numerous natural disasters, including floods, cyclones, storm surges and earthquakes. In recent years, landslides have become a prominent geological hazard in the southeastern region due to modification of natural slopes, deforestation, urban expansion and prolonged torrential rainfall. During the last 2–3 decades the frequency of devastating landslides has sharply increased, which has resulted in deaths of hundreds of people and significant loss of property. In South-Asia, Bangladesh ranks first in terms of landslide-related deaths. Understanding of landslide processes has become essential for disaster management and sustainable development. Geo-engineering evaluation of landslides in the region has recently been completed for Chittagong, Cox’s Bazar, Rangamati, Bandarban, Teknaf and Moheshkhali Island. The landscapes of these urban clusters are formed by dissected hills and valleys in a tectonically active region. Hills and slopes are underlain by young folded and faulted sedimentary rocks. Folds trend NNW-SSE and rock units are highly fractured, jointed and commonly distorted. Stratigraphic formations most susceptible to mass movements are the Dihing, Dupi Tila, Tipam and Boka Bil Formations. Slope movements in the region are classified as: lateral spreading failures, rotational and translational slides, and planer slides. Man-made slides are classified as rock fall and debris avalanches and are considered to be the most devastating types of mass movement.
For this investigation, rock masses were characterized by following ISRM suggested method and Slope Stability Probability Classification System and divided into eight geotechnical units. The present study reveals that contributing factors to landslides in this region include attitudes of bedrock units, faults and other discontinuities in rock formations, shear strength properties of constituent materials, and unusual hydrological conditions. Increase in the human interference has become a major factor in the slope failures. Landslides in the southeastern Bangladesh can be reduced when proper slope design and management are applied, and a monitoring system for geotechnical control is developed.

Seismic hazard characterization is the foremost module for earthquake risk management in a seismically vulnerable region. The mega city Dhaka in Bangladesh is considered by many researchers as one of the riskiest cities in the world due to many non-engineered construction practices and poorly studied tectonic boundary conditions. The city is built on a Plio-Pleistocene terrace, located within the subsiding Bengal basin. The records of historical earthquakes indicate that three large magnitude earthquakes occurred during the last 150 years within and in close proximity to Bangladesh. Magnitudes of these earthquakes ranged from 6.9 to 8.7 occurring between 1885 and 1918. These events caused moderate damage to buildings and other infrastructures in Bangladesh, but the damage in Dhaka city were negligible. It is believed that the 6.9 magnitude Bengal earthquake occurred at about 50 km from the city, although there are multiple controversies about the location of the epicenter. Many consider that the epicenter of this earthquake was 170 km away from Dhaka city and others inferred the epicenter to be somewhere along Madhupur fault, approximately 50 km away. The 1885 Bengal, 1897 Great Indian, and 1918 Srimangal Earthquakes are considered as the seismic sources for site-specific seismic hazard characterization. The peak ground acceleration (PGA), peak ground velocity (PGV), spectral accelerations (SA) of different periods have been calculated at the ground surface based on recently developed ground motion prediction equations and site amplification factors. The amplification factors are predicted from the average shear wave velocity to a depth of 30 m (Vs 30), which are estimated using various geophysical and geotechnical investigations. The study reveals that the city is built on a very firm ground where seismic risks are manageable provided the engineering structures adhere to the norms of seismic regulations and building codes. 

Citation
Karim, M. F., Rahman, Z. M., Kamal, M., & Siddiqua, S. (2016, 08). Site-specific earthquake hazard characterization for Dhaka City, Bangladesh. Poster Presentation at 2016 SCEC Annual Meeting.