Geology

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The Geology Geochemistry and Origin of Supplied Mineralization in Katta,Wollega Province
(Addis Ababa University, 1980-07) Mammo, Telahun; Valera, Roberto (Prof.)
The Katta rocks form part of the Birbir group of the upper compl ex s i tuated in the paleo- cal c-alkaline arc that runs northsouth across the country and which compri ses thick sequences of metavolcanic and metasedimen tary r ocks 0 The rocks are intensel y folded and faulted o Their minera l ogy and texture indicate that metamorphism is o f the greenschist facies o Logging of five bore ho l es , their correlation and subs equerr petrographic s tudies on selected samples are conducted and depth variation i n miner a logy i s presented o Geochemical studies on stre am sedime nt s , bed r ock and f l oat samples, soil and core samples , and statistical tr eatment, of the results show that there is lateral and vertical variati ons in the copper and zinc mineralizations . The geochemical s oil maps show that Cu , Zn , Pb , Ni , and Co mineralizations are concentr ated along fo liation and bedding p laneso There is low litho logical c ontro l of mi neralization o Moda l ana l ysis on the core samples s how that main mineralization is a s sociated to carbonate o Ore microscopic studies show that pyrite , chalco pyrite , magnetite , bornite , cubanite) bournonite , cuprite and sphal e r ite are the ore minerals found occuring in bands , as d i ss eminati ons and in veins o Two types of mineralization are recognized syngenetic stratabound and e pigene t ic vein and dissemina t ed types . It was the syn~enetic type that gave rise to epigenetic types as a resul t of later met a morphi(: and supergene remobilization. 'r'he epigenetic type is considered to be the main mineralization in the area o
Evaluation of Cement Raw Materials in Wayou ( New Mugher ) Area
(Addis Ababa University, 1980-07) Lakew, Tesfaye; Asssefa, Getaneh (PhD)
The raw mater i als used in the manu f acture of P ortland cement were s tudied in the Wayu locality (New Mughe r). The study showed th. t there is lime s tone of a good quality in large aMount which justifies exploitation. The c l ay ~ate ri a l studied was fou nd ou t t o be de f icient in silica and neens a si l ica modulus correc t or . Conse quently , a sandstone body which is found b e neath the basalt was investi~ated and the investigation showed that there is enough san stone tha t can be used as a cor r ector . The study of the gyps um deposi t also revealed that sufficient amount of gypsum e x ists . A conclusion was reached to the effect that the installation of a cement p l ant is justifiedo
The influence of Geology , and other Soil Forming Factors in the Inherent Fertility of Soils in the Assela Area, Arssi, Ethiopia
(Addis Ababa University, 1982-02) Abebe, Bekele; Cinque, Ardol (Prof.)
The geology of the Asela area has been studied In de t a ~ l and the fertility status of the soils in relation t o the f actors of soil formation (parent material, climate, top ogr aphy , organisms and time) examined qualitatively and semiquant itatiVielyo The area is associated with Pliocene and younger volcanics (central and fissur al) and is intensively aff e cted by different episodes of faultingo Basal t s, Trachybasalts , trachytes and acidic pyroclastics are the rocks oc curing in the areao The major soils are the dark-dark brown and the reddish brown soils of the plate au and the dark-browndark ye llowish brown soils of the rifto In general, the rif t s oils have higher plant nutrient s t atus than the plateau soil s o Removal of materials is important on the plate au and addition in the rifto Parts of the plateau and mo s t of t he r i f t f l oor are affected by poor drainageo The abundance of surf ace hindrances on the surface of the rift soils might be attr i buted to t he young age of the rocks in the lattero In te r ms of physical limit ations of the land, both t he lowland and t he plateau have their drawbacks o The cumulative influence of the topography and the climate is found to be dominantly r e s p ons ible for the lateral and vertical variations of the inherent fe rtility of the soilso
The Geology and Geochemistry of the Banded Iron Formation in Chago-werekalu, Wellega Province
(Addis Ababa University, 1985-10) Getahun, Abera; Valera, Roberto (Prof.)
The r ocks of the stulied area form nart of the ~ o llepa Birbir proup of the uppe r c08plex and consist of a series of me t ased i me ntary and plutonic proterozoic rocks. The winiera lo~ical assembl age indicate tha t t he metamorphism is of the r r eenshist facies. The variation diagram cons tructed for the va rious plut onic re e ks stronpl y supvest coma pmatic oripin by crysta l f rac ti ona ti on . Major and trace element ana lyses of re ore sentative samples of the Chago iron formation ? t he plutonic ann. YOllnp:er e f f us i ve rocks are presented . The Chago iron f ormation can be c las s i f i ed as nre cambrian banded iron for mation and is in J11any r espects siJrii l ar to the oxide fa cies iron format i on of Lake Superior t ype. The Cha go iron fo r mation consis t s only of Iron oxides and mi nor amoun t of sil i ca with total absence of iron silica t e, s ulfide and carbonate minerals. Trace element pe ochemis try sUPRests that t he iron f orma ti on was chemic ally prec i pitated as ox ida t e s ediments in which the principal i ron mineral, ma gne tite 1 was formed at low te mpera ture in a s ~lallow nea r shore environment .
The Geology and Geochemistry of the Banded Iron Formation in Chago-Werekalu, Wellega Province
(Addis Ababa University, 1985-10) Getahun, Aberra; Valera, R. (Prof.)
The r ocks of the stuJied area form nart of the ~ ol lera Birbir group of the upper complex and cons ist of a s eries of metasedimentary and plutonic proterozoic rocks. The l1'inieralo£"ical assemblage indicate th a t the metamorphism is of the rreenshist facies. The variation diagram constructed for the various plutonic rocks strongly suppest comagmatic origin by crystal f ractionation. Ma jor and trace element analyses of reDresentative samples of the Chago iron for mation, t he plutonic an~ YOllnper effusive rocks are present ed. TIle Chago iron formation can be classified as nrecambrian banded iron formation and is in l1'any r espects similar to the oxide facies iron formation of Lake Superior t ype. The Cha go iron formation consists only of iron oxides and mi nor aIT'ount of si l ica with total absence of iron silicate, sulfide and carbon ate minerals. Trace element peochemistry suppests that the iron f ormation was chemically precipitated as oxidate sediments in wh i ch the principal iron mineral, ma gne tite, was formed at low temperature in a shal low near shore environment .
Shallow Resistivity invist Igaation In the Filwoka Fault, Addis Ababa
(Addis Ababa,University, 1985-12) Selassie, Haile
The effectiveness of the electrical resistivity methods has been studied using various configurations over the Filwoha Fault. It has been opserved that the delineation of the fault was possible under differential weathering. Geoelectric sections across the fault has been prepared on the basis of Schlumberge r sounding results.
The Sedimentology of the Diatomite Bearing Lacustrine Deposits of Adami Tulu Area, Arsi, Ethiopia
(Addis Ababa,University, 1986-06) Amare, Ketema
Sedinents of Late ouaternary Period are exposed in Adami Tulu area. they represent deposition in a fault controlled Basin in the northern part of the main Ethiopian Rift vallay- They form thick successions of coarse and fine grained sedi- ncnts, gravcle,sands, silt and diatomitas awong which grevels and diatopitas are volunetrioally important.
Petrology and Geochemistry of Racrustal and Intrusive Rocks form the Rdopain of the Gore- gambela Geotra Verse
(Addis Ababa,University, 1986-07) Teklay, Mengist
The studied aren, in the Precanbrian Shibld of western Ethiopia , is characterized .by low anpbilite grade metamorphism and by supracrustal rocks of sedimentary and volcanic oringin. the oastern part of the aran is conpased of well- preserved turbidites (wackes and oelitas), volcaniclastic rocks,
Engineering geological study of Omorati Area
(Addis Ababa University, 1990-06) Mehari, Germaye; Olszewska, B. Grabowska (Prof.)
Eng ineering geological study of Omorati area
(Addis Ababa University, 1990-06) Mehari, Germaye; Olszewska, Prof . B. Grabowska
Engineering geological Study of an area amounting to 25km living on the western bakn of Gamo river at winch a town is proposed to be located.
Hydrology and Hydrochemistry of Bishoftu Crater Lakes (Ethiopia) Hydrological, Hydrochemical and oxygen isotope modeling
(Addis Ababa University, 1990-06) Kebede, Seifu; Leng, Melani(PhD)
The Bishoftu Crater Lakes, located on the western shoulder of the main Ethiopian Rift can provide a sedimentary record, which has regis!ered the changes in the balance between precipitation input and evaporative water loss. However, groundwater fluxes may reduce sensitivity of the lakes to a change climate. In order to test the possibility to use the lakes as paleohydrometers, the modem balance between precipitation, evaporation and groundwater flux has been determined. In the Bishoftu area where hardrock form the terrain geochemical and isotope mass balance has been found suitable to be used to constrain the water budget. Although stable isotopes of oxygen and deuterium as aid in quantifying water balance of the lakes have limitations due to the lack of measurement of environmental parameters such as h, E, I1E ; the same approach has been found appropriate for this study. The varying hydrology of the Bishoftu crater lakes has provided a suitable base for determining water budget using stable isotope method without reverting to the measurement of environmental parameters. Accordingly the lakes have been found suitable for quantitative isotope hydrology and quantitative reconstruction of paleoclimate from lake carbonate records. Of the existing five crater lakes, lake Hora, lake Arenguade and lake Kilole are least affected by groundwater outflow while lake Babogaya and lake Bishoftu are more influenced by groundwater outflow. The hydrology of lake Kilole is recently influenced by the diversion of the Mojo river into the lake. Geochemical mass balance results show that lake Arenguade might be affected by geothermal water flux . Of all crater lakes, lake Hora is therefore selected as the most suitable for paleoclimate studies.
Engineering Geological Characteristics of the Clay Soils of Bole Area their Distribution and Practical Importance
(Addis Ababa University, 1990-06) Ayalew, Lulseged; Olszewska, Prof . B. Grabowska
On the basis of the cement to construct new engineering objects.
National Labour Information Network for Ethiopia (NALLNET)
(Addis Ababa University, 1992-06) Alemayehu, Nega; Neelameghan, A. (Prof.)
Information is a vital resource in the economic development process. Information about labour enables us to utilise human resource and planning and implementation of projects. A number of organisation deal with matters directly or indirectly related to labour issues. Information is produced and used in those organisations. Labour information is also needed and used by other organisations and individuals in discharging productively their functions and responsibilities. Hence bringing together these parties in a network-like structure will be greatly help in coordination of efforts in processing data and sharing of information and resources. For the purpose of data and fact collection to study the existing situation and infrastructure facilities in the country, survey by questionnaire and interviews was conducted besides examining appropriate documents. A simple descriptive technique and systems approach are applied for data analysis purpose. The survey results indicate that existing labour information fac ility is inadequate and deficient. The organisat ion of data and information is very poor.~i ~ ~,i !'!'J t:lY1eth~!:" cr~.:!~ isa~~ j ('\ l:; concarnad with labour matters in a netwoLk-: ike structure could help to solve some of the existing problems and provide better services. It will be a single-window shopping for users of information. A plan for National Labour Information Network for Ethiopia (NALINET) is discussed. In order to initiate the networking formation of a Coordinating Body and Steering Committee are very suggested. The Committee will work on a number of technical and organisational issues. In the first phase, the Committee-will be involved in reviewing existing facilities and recommending meas?res for strengthening the infrastructure. as appropriate; and drafting a charter under which all participants of the network will work. The study identifies a number of functions that would be provided by each participant and focal point of the network and other issues that deserve the attention of concerned organisations and recommends measures to be taken.
Mineralogical and Geochemical Studies on the Sakaro Primary Gold Deposit (Sidamo, Southern Ethiopia): Zonation in Ore Bodies And Host Rocks
(Addis Ababa University, 1993-06) Glrma, Mesfin; A., Anatoli (PhD)
The Sakaro deposit is a vein-type fault-hosted gold deposit in the Adola area I southern Ethiopia. It is composed of qua rtz veins up to 9.3 m wide which occupy fault planes aloog foliation and petrographic contacts in precambrian low-grade metamorphic rocks. The veins are gold-rich and sulfidepoor, forming a mineralized zone of 760 m in strike length and up to at least 150 m in vertical extent. Veins Nos . 1 and 2 are the richest shoots with an estimated total ore reserves of 760,800 tons capable of producing 2,551 kg of gold at an average grade of 3.4 gft and 5.7 gft respectively. Geochemical and mineralogical studies of the ores and the bost rocks reveal that the Sakaro deposit might include tungsten other than gold as a potential exploration target. Tungsten mineralization is the earliest to be formed. Tungsten content increases up to > 1% in the ore veins and in the wall rocks (mainly in amphibolites proximal to the veins), forming a distinct zone that assumes W-As -Cu, a geochemical zone or wolframite-scheelite-iron sulfides zone, a mineralogi c al zone. Gold is mostly associated with Ag and Pb, showing asymmetrical dispersion pattern that starts in the hanging wall side of the vein and decays towards the foot wall rocks. Its tight relation with Ag and Pb is expressed by forming a Au-Ag-Pb zone though it i s a l so related at a lesser degree with other elements especially with Cu. Outward from this zone, Ag-PbCu, W- As-Cu, and Fe-Co-Ni-Mo-Cu follow in respective distances from the are veins toward enclosing rocks. Gold distribution in the ore veins is control led by thickness variation of the quartz fillings in both strike and dip directions, poor in swell s and rich in constrictions. In the plane of the ma in vein, five are columns are identified with marked contrast in gold mineralization: (1) obliquely running, EW dipping gold rich zone, (2) north e rly trending, vertically dipping gold rich zone, ( 3 ) intermediate low gold zone, (4) oxidation zone, and (5) surf i cial weathering zone. The first two zones are are s hoots representing the fronts of gold depositing mineralizing fluids . Gold contents follow a systematic pattern o f more or less regular course within and away from these front s which otherwise are perceived as very irregular and erratic distributions . Moreover, statistica l treatment on the meta l content shows that gold distributions in populations separated according to the rock types and environments obey lognormal law , with mean values highest in ore veins, less in amphibolites, and least in mica schists. Studies on the vein development reveal three distinct types of hypogene mineralization from veins margins to the center, of marcasite + chalcopyrite in the peripheral parts, galena + chalcopyrite + marcasite in the following portion, and sphalerite + h igh galena + c halcopyrite in the centra l part. Gold is present in all of the above associations, however, i t is highly enriched in sphalerite-high galena-chalcopyrite zone, followed by hanging wall side marcasite-chalcopyrite zone , and least in foot wall side marcasitecha l copyrite zone. The gold enrichment in spha lerite-high galena-chalcopyrite zone indicates the close association of Au, Ag, Pb , Cu and Zn . The selective enrichment of gold in the hanging wall side (marca s ite-chalcopyrite) zone is most likely due to intense shear remobiliz a tion effects. The ve ins di splay a crude ascending mineral sequence, from depth to the surface, of pyrrhotite, gold + galena + sphalerite + chalcopyrite + iron sulfides (productive zone), and marcasite. A des cending sequence in the zone of weathering shows that Fe-hydroxides a nd Fe-disulfides grade downward where go ld is en ri c hed with secondary minerals of Pb , Cu, Zn. I n the enclosing rocks, wolframite + sc heelite + Fe-sulfides form a zone adjacent to the ore veins a nd marcasite + pyrrhotite + chalcopyrite in di stal areas. These zones are in agreement with geochemical zones W-As-Cu and Fe-Co-Ni-Mo-Cu respectively.Disulfidization process of iron monosulfides increases in the upper levels of the deposit and is more intense a t the vein margin s especially in the hanging wa ll side of the ore veins due to shearing which is also responsible for recrysta ll ization of quartz a t the vein selvages. The vein mineralogy shows as the vein development is the result of multiple episodes of vein opening and filling characterized by wolframitescheelite- quartz, sulfide - gold-quartz and disulfide-carbonate stages. statistical parameters of distribution of the components, ore geochemistry, peculiarities of horizontal (along the strike and across the thickness), vertical and three dimensional geochemical and mineralogical zonation of the mineralized rocks and the general sequence of formation of minerals in ores of the Sakaro deposit considered and proved in the thesis are the features of its genetic model. These features are a lso of practical importance for are blending and further improvements of are dressing tec hnological schemes. They are of value and use in prospecting for similar mineral deposits in the region and their successfu l exp lo ration
Hydrogeology of Jimha Area
(Addis Ababa University, 1994-06) Tadesse, Nata; Billi, P.(Prof.)
The studied area is located 335 Km. southwest of Addis Ababa in the southwestern Ethiopian plateau . Over half of the map-area is underlain by thick basalt flows inter layered with tuffs and volcanoclastic sediments . The Jimma Volcanics, which is mainly composed of massive rhyolite in thick flows alternating wjth trachytes, tuffs, ignimbrites and subordinate basalts, underlain small area in the eastern and northeastern part of the basin. Based on the hydrometeorologic data collected for the last 38 years an attempt has been made to calculate the hydrological parameters l i ke precipitation, evapotranspiration, and runoff. The ann ual mean rainfall in the basin is 1476.89 mm . The values of the annual potential evaportanspirtion and actual evapotranspiration are 837.51 mm and 822.29 mm respectively. The only runoff that leaves the studied area comes through Boye river and is 90 .77 million cubic meter of water. The amount of water which is actually available to recharge the groundwater circulation within the hydrogeological basin is 51.14 million cubic meter . The hydrogeological character of the outcropping lithotypes was discussed with particular reference to infiltration and their water bearing capacity. The hydr ogeology of the area varies in different parts of the basin. There is a great potential of groundwater in the valleys which is partly filled with alluvial sediments while aless significant ameunt exist within the velcanic recks ferming the uplifted berders ef the valleys. Determinatien ef the hydraulic parameters like transmissivity and permeability have been carried eut. Due to. the absence ef ebservatien well nearby a discharging wells it was impessible to. calculate the ceeff icient ef sterage, a significant earameter which measures the velume ef water the aquifer releases er take into. sterage. Fer basaltic aquifers a transmissivity value ef 18.39 m2/day and a permeability value ef 3.31 x 10-1 cm/ sec. have been calculated. A transmissivity value ef 1.89 m2 /day . and a permeability value ef 1.62 x 10-5 cm / sec. have been co.mputed fer beth alluvial and rhyelitic aquifers. Because ef the lack ef bere wells in the seuthwest, seuth, and seutheast lewer mest part ef the basin a thereugh study ef the actual variatien ef greundwater level was net pessible. Fer the ether part ef the basin, apprepriate greundwater level centeur lines was censtructed on the basis of the available static level data. Preper chemical analysis ef beth the greundwaters and surfaces water carried eut in the basin shew lew tetal disselved selids. All of the samples have less than 1000 mg / l disselved selids. The analysed samples also. cempared with the acceptable standard for human, agr icul tural, and industr ial use.
The Sedimentology of the Diatomite Bearing Lacustrine D_posits of Adami Tulu Area
(Addis Ababa University, 1996-06) Amare, Ketema
Hydrological and Hydrogeological Study on the Kulfo River Basin
(Addis Ababa University, 1996-06) Masresha, Paulos; Jeschofnig, Paulos( Prof.)
The study area, the Kulfo River basin, is located 500 km south of Addis Ababa. It is bounded on the basis of the surface divide of the Kulfo River. The area is composed of part of the rift floor and part of the western escarpment. More than 85% of the area is underlain by Oligocene basalt and the lowland areas by allu vium and Quaternary basalt. Using three meteorological stations;-the weighted mean annual rainfall of 1232 mm was computed by using the Thiessen Polygon method. The Thornthwaite method was applied to calculate the potential and actual evapotranspiration of the area, which are 971 mm and 940 mm, respectively. From the above data, the surplus water was obtained , which is 291 mm. The mean annual runoff depth of the Kulfo river is 361 mm which is less than the calculated surplus water. The reason might be that there is an inflow of groundwater from an adjacent basin. Concerning the aquifer characterstics of the different rock units, the alluvium is more productive than the other rock units. One representative well from AWTI was taken to calcualte the hydraulic parameters of this unit. Using the discharge data from Arbaminch Springs, the general hydrological conditions of the springs were computed. This includes the computation of the recession constants and the recession cu rve for the spri ngs. The hydraulic volume which feeds the springs in stant by instant were also computed. The study of the chemistry of the springs was based on data from WSSA's Arbaminch water su pply , project. This study confirms that the quality of the water is suitable for drinking and agricu ltura l purposes according to intern ational standards. However, the presence of E. coli bacteria indicates that the water needs to be chlorinated on a regular basis.
Geology and Geochemistry of the Negash Pluton and their Metallogenic Significance, Central Tigrai
(Addis Ababa University, 1997-01) Asrat, Asfawossen; Peccerillo, Angelo (Prof.)
The Negash area is located in Central Tigrai, Northern Ethiopia. The area consists of metamorphic rocks th at are cut by some granitoid intrusions. The present investigation was directed to one of such intrusions, the Negash granitoid stock. Geological mapping at a scale of I: 50 000, and geochemical studies led to the assessment of the petro logy, geochemistry and tectonic setting of both the intrusion and the country rocks and the economic potential of the area. The meramorphic rocks are mainly low - grade metavolcanic rocks with a wide compositional variation from tholeiitic to calc - alkaline mafic to felsic composition formed in an island arc environment. There are also minor intercalations of metased imentary rocks. Hornblende - hornfels facies rocks are found in proximi ty to the intrusion. The Negash granitoid body is unde fo rmed, discordant, circular stock that consists of diorites, quartz diorites, tonalites, monzonites and granodiorites with minor aplitic, grani tic and pegmatitic dikes. Its western boundary lies along a fault line (the Suluh Fault) that forms the Suluh River valley. The intrusion is dominantly of mafic and intermediate composition rocks, even though the felsic rocks crop out extensively at the top most part of the intrusion. Geochemical and petrological data suggest that the intrusive rocks were diffe rentiated from a mafic magma by fract ional crystallization and assimilation with fractional crystallization (AFC). Hornblende was the main fractionating mineral phase. This process resulted in relatively dry residual magma, as can be evidenced from the scarc ity of pegmatites and related mineralization. Preliminary geochemical survey of the Negash granitoid stock show no significant anomalies except for Zn and Sn. The country rocks show anomalies for several elements including Au, Zn, U, Sb, As and Ba. This indicates that the intrusive magma did not play an important role in generating mineralization in the area but it furnished the heat which triggered fluid circulation in the country rocks. The geochemical anomalies for most of the elements are concentrated along the western boundary of the , intrusion, the Suluh fault line. The fault presumably provided the ease through which the metasomatic fluids enriched the area with metals. Further geological and geochemical investigations should be directed to this zone.
Hydrogeology and Hydrochemistry of Lake Ziway Area and the Surrounding
(Addis Ababa University, 1998-06) Gashaw, Haile; Barbieri, Guilio (Prof.)
The studied area is located in the northerrt part of the central s~cto r of the Main Ethiopian Rift. It is bounded by 7° 35' N & 8 12' N latitudes and 38 25' E & 39°15' E longitudes. It is covered by Pliocene to Recent volcanic rocks and lacustrine sediments. Volcanic rocks are dominantly acid ignimbrites, pumices and lavas with a few basaltic lava flows and strombolian scoriae. The maximum altitude in the area is 4080 meters above sea level at the tip of Chillalo mountain and the lowest point is 1636 meters above sea level at the western shore of Ziway lake. The effective annual depth of precipitation in the area is 936.62 mm, the actual and potential evapotranspirations are 735.87 mm and 893.64 mm respectively in the lowlands and 639.06 mm and 695.83 mm respectively in the highI,Pndp. From the total mean annual volume of water entering thlla~e about 123. 3 x 10 m of water leaves the lake as surface runoff, about 88.4 x 10m of water leaves the lake as groundwater outflow and about 643.91 x 106 m3 of water leaves the lake in the form of evaporation. ~be main aquifers in the_area are coarse_grainedJ acustrine sediments, scoria cones~ _ hyaloclastites and highly weathered and fractu red basalts and ignimbrites. Depth of ground water level ranges from less than two metres up to 130 meters and the general groundwater flow direction is towards the lake from east, west and north of the area and out of the area from the southern part of the lake . The water in the area is mainly of sodium bicarbonate type and a few are of calciumsodium bicarbonate type. The chemical analyses of ground waters of the lowlands show high total dissolved solids and high fluoride concentrations. High fluoride concentrations may result from input from fumarolic activity, water-rock interaction and low Ca concentrations. Generally ground waters from the lowlands have poor quality while surface waters and ground waters in the highlands have good chemical quality for different purposes.
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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