Engineering geological studies for the preparation of maps and plans have been carried out since the early years of the 20th century, mainly in Europe. The purpose of these maps was to provide information about characteristics of the natural environment for planning land use and engineering structures of all kinds.
Rapid urbanization due to population growth in developing countries requires infrastructure and housing development over large areas. Problems resulting from unfavorable engineering geological conditions have confirmed that engineering geological mapping is a fundamental prerequisite for planning. Various authors have developed concepts about engineering geological maps or mapping. Accordingly, an engineering geological map is the best way to depict the natural environment for engineering purposes. The purpose of engineering geological maps is to show the distribution of specific geological phenomena and characteristics of rocks and soils affecting engineering use of different terrains.
In Ethiopia, the preparation of engineering geological maps is not well known; perhaps the country’s demand and requirement of the map for the development of engineering structures and planning purpose in the near future became critical. From the report of even the Geological mapping of the country is not fully addressed. Consequently, in many towns of Ethiopia, very little is known about the soil and rock conditions or engineering geology of the towns, researchers in the area claim.
As population number grows, the demand of constructing buildings for residential, commercial, industrial, and other purposes also increases. Mekelle town is presently rapidly and geometrically expanding towards all directions, and several civil engineering structures such as single to multi-storey buildings, roads, bridges, etc. are under construction. Located about some 783km north of Addis Ababa, geographically the town is positioned at 390 28′ East longitude and 130 32′ North latitudes, situated in the extension of the central highlands of Ethiopia. Mekelle’s approximate geographic location is 39o 28′ Eastern Longitude and 13o 32′ Northern latitude. It has an average altitude of 2000 meters above mean sea level.
The engineering geological and geotechnical properties of soils and rocks, and develop detailed multi-purpose engineering geological map at a detail scale. Engineering geological and geotechnical characterization of soils and rocks are based on their index and engineering properties, and their classification is according to the standard proposed by Unified Soil Classification System (USCS) and International association of Engineering Geologists (IAEG (1981).
According to USCS; soils are classified into: fat clay type; inorganic silt, elastic silt and (silty of clayey fine sand with slight plasticity) of silt soil types; lean sandy clay, inorganic silts, elastic sandy silt of sandy clay/ silt type; and clayey/ silty sand soil types. According to IAEG (1981); soils are classified into: clay of intermediate and high plasticity; silt of intermediate, high and very high plasticity; SAND clayey of intermediate plasticity, SAND silt of high plasticity; and CLAYEY sand of intermediate, high plasticity and SILT sand of intermediate plasticity). Classification of rock masses is based on their strength, degree of weathering and joint characteristics; classified into three major engineering geological subunits: rocks with low mass strength, rocks with medium mass strength and rocks with high mass strength.
The city is growing into different directions and has to adjust its planning strategies to accomplish the demands of an abruptly growing population. The spatial effects of the surrounding mountain ridges and enclosing landforms have special importance for reservation of specific areas and facilitate development of the city. At present, significant concentration of engineering structures mainly multi-story buildings, roads, bridges are under construction, especially they are concentrated in the north, northwest, and west directions of the city. There is a need for systematic work of detailed engineering geological and geotechnical mapping for Mekelle town and surrounding area.
Engineering geological mapping, plays a vital tool in most current civil engineering and mining works, has not been given the attention it deserves in most countries lying within the tropics. According to the reasons for this might be partially found in the unavailability of personnel and facilities in these countries and lack of awareness of decision makers and planners. A major reason, however, is the absence of methods specifically designed to cope with the special problems of the tropical regions.
Densely populated urban centers are often founded on steep slopes, or on poor soil conditions, and even in some cases with high seismicity areas. Knowledge of the geological and geotechnical conditions of an urban area is necessary to provide basic information of the ground condition to the local authorities, engineers and contractors. This necessary information also constitutes the basic concept for the evaluation of both the geological hazards, which are encountered in the area in question, and the dynamic response of the construction in the case of seismic activity. According to the current research work which was conducted by considering the present expansion of the town towards the northern and north western parts, the results of the research provides detailed engineering geological information that help for planning, design and maintenance of engineering structures on these areas.
In order to provide easy, adequate and well organized geological, geotechnical and engineering geological information of a particular site (for planners, engineers, designers and others that require it), it is recommended to prepare a database for the engineering geological map of the Mekelle town, where the database should demonstrate the attribute and spatial values of lithological, structural, geomorphological condition, geodynamic processes, hydrogeological conditions, geotechnical and engineering geological characteristics and classification of the soils and rocks of the Mekelle area.
If there are quantitative geotechnical and engineering geological property descriptive records, there is the possibility to prepare the 3-Dimentional engineering geological map of the Mekelle town. It is recommended to undertake further research in order to prepare the 3-Dimensional engineering geological map of the town. This could further benefit subsurface investigations for engineering structures built in the Mekelle town. It can provide data on the lateral and vertical lithological, geotechnical and engineering geological information which could be used for the design and construction of large-scale engineering works.
BY HAFTU GEBREZGABIHER
The Ethiopian Herald May 22/2021
