Geology

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Browsing Geology by Author "Ayenew, Tenalem (Prof)"

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    Engineering Geological Study of the Proposed Entoto Tunnel
    (Addis Ababa University, 1999-06) Wolderufael, Girma; Ayenew, Tenalem (Prof)
    For the devolopmant of Addis Ababa c ity's water suppl y, there needs to des ign a project of constructing two dams and a 4.8 km tran smi ss ion tunnel through the Entoto ridge, by the Addis Ababa Water and Sewrage Authority (AA WSA). On the basis of the demand to construct a detail ed engineering geological and geotechni cal investigation for the proposed Entoto tunne l, the study was ca rried out after defining a tunnel corridor of about 10 km2 The study focuses on the engineering geological in vestigation of this tunnel corridor. The study area lies in the Shoa plateau that is formed of Tertiary rocks be longing to the Trap series. It is composed mainly of tuffs and trach ytic lava flows which are called the Entoto si licics of Addis Ababa area. There a re also basaltic lava flows outcropping on the top of the Entoto silicics as patches which area products of Tarmaber basalt. Undiffe rentiated Quaternary sediments which include residual , colluvial and alluvial depos its occur in v31ious parts of the area. Aerial photo interpretation, field traverses, insitu tests of rocks, as wel l as joint analysis from core orientation and surface observation and subsequent laboratory analysis were conducted. A number of exploratory boreholes drilled along the hlllnelline indicated that a significant number of trachyte / tuff interfaces OCCUlT in the defined tunnel zone. From the borehole drilling and the geophys ical investigation data, the area of the tunnel corridor appeared to be highly fau lted and weathered. The position of the water tab le, which is above the tunne l zone, and the permeability of the rock masses were detelll1ined.Engineering geological map at the scale of I: 10,000 was prepared. The rock mass strength was determined us ing Schmidt halllmer and point load tester perf01111ed during fie ld work and laboratory in vesti gati ons. I n additi on to thi s in the fi eld , rock materi al strength was done with field identification by using geological hammer and scratching by pocket knife. The borehole data a long the tunnel line, whi ch is combi ned with surface mapping, confi rmed the general geological assessment and allowed th e choice of construction methods. The borebole data was used as a basis for detailed des ign of the tunnel. Us ing all the avail able information on the rock mass and its hydrogeological characteristics, rock mass class ifi cation schemes are developed. From all the existing methods of rock mass classification, the Rock Mass Rating (RMR) and the Rock Mass Quality Index (Q - system) are selected. Consideling all the rock mass propelties, using the above classifica ti on systems the Illost economic and suitable excavation Illethod would be by drill and blast. The respective supporting method woul d also be by bolts and shotcretes. It is hoped th at this study will provide a sound geologica l bas is durin g the design, and excavation stages of the tunneli ng. However, it should be emphas ized that detail ed geotechni cal investigation on stress conditions of the rock mass and stability analys is, and furth er works at the pOItal areas and fault zones are very im portant.Engin eering geological map at the scale of I: 10,000 was prepared. The rock mass strength was determined us ing Schmidt hammer and point load tester performed during field work and laboratory investigations. In addition to this in the fie ld , rock material strength was done with field identification by using geological hammer and scratching by pocket knife. The borehole data along the tunnel line, which is combined with surface mapping, confirmed the general geological assessment and a llowed the choice of construction methods. The borebole data was used as a bas is for detailed design of the tunnel. Using a ll the available information on the rock mass and its hydrogeological characteristics, rock mass classification schemes are developed. From all the existing methods of rock mass classification, the Rock Mass Rating (RMR) and the Rock Mass Quality Index (Q - system) are selected. Considering a ll the rock mass properties, using the above classification systems the most economic and suitable excavation method would be by dri ll and blast. The respective supporting method would also be by bolts and shotcretes. It is hoped that this study wi ll provide a sound geological basis during the design, and excavation stages of the tunneling. However, it should be emphasized that detailed geotechnical investigation on stress conditions of the rock mass and stab ili ty anal ys is, and further works at the portal areas and fault zones are very i m po Ita n t.
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    Hydrogeological Investigation of the Upper and Middle Borkena River Catchment, Northern Ethiopia, Wollo
    (Addis Ababa University, 2001-06) Sahele, Mesfin; Ayenew, Tenalem (Prof)
    The geology, hydrology, hydrogeology and hydrochemistry of the Borkena catchment which is located on the western edge of the Afar rift was studied based on conventional geologic and hyd rogeological mapping, river di scharge measurements, water sample anal ysis and assessment and interpretation of hydrometeorological data. The main objective of this work is to understand the different lithologic unit and to characterize and group them into hydrostratigraphic unit and map them, to analyze the water balance and to study the water quality. Four hydrostratigraphic units were recognized and mapped. These are: coll uvial and river channel deposits, alluvial deposits, scoraceous and basaltic lava flow and fractured massive basalt, welded tuff and rhyolite. Generally the volcanic rocks lie with in the recharge zone wllile the Quaternary sediments lie in the discharge zone. Accordingly the hydrostratigraphic unit located on the Quaternary sediments are characterized by many springs and hand dug well where as low or rare in the volcanic regions. From storage, utilization and abstraction point of view, the colluvial and river channel deposits are the most promising units than other units as a result most of the springs, hand dug wells and boreholes are located in this hydrostratigraphic units. Generally seven rock units and three set of fau lts are identified and mapped at the scale of I :50,000. Among the rock units, scoraceous and basaltic lava flow and alluvial deposits cover more than 70 percent of the catchment.Usually the exposure of the volcanic unit is limited to the topographically elevated and steep slope regions of the catchment where as the Quaternary sediments out crop with in the floor of the graben and gentle slope areas. The oldest rock unit (Rhyolite) corresponds to the AJaji Formation (28-32Ma) while basalts are correlated to the Tarmaber Formation (26-28Ma). Structurally the N-S trending faults are more common than the E-W ones. The analysis of precipitation shows that the area experience 1028 mm of mean annual rainfall with 84 percents obtained during the rainy seasons. Potential evapotranspiration was calculated by modified Penman method and is 1339 mm per annum while actual evapotranspiration was computed using Thornthwaite and Mather Soil-Water Balance Model, (1955) and is found to be 860 mm per annum. Computation of water balance was done in two steps: one for each subcatchment and the other for the entire catchment. Accordingly the entire catchment receives 50.6 mcm (million cubic meter) of recharge per annum while the Dessie and Kombolcha Subcatchments receive 18 .2 mcm and 32.4 mcm of recharge per annum respectively. Hydrochemical analysis of water samples indicate that the river water is enriched with respect to both cations and anions than the groundwater bodies. The plotting of ionic concentration of water samples was done through Aquachem software. From the plot it is found that the river water is a sodium-calcium bicarbonate type water where as the groundwater (springs, hand dug well and borehole) is calcium-sodium bicarbonate type water.From hydrochemical analysis, measurement of TDS, EC and other variables, the water bodies are characterized by low total dissolved solids (TDS) and low electri cal conduct ivity (EC) values as a result they are classified as fresh water and their utilization for various purposes scarcely bring danger.