For the devolopmant of Addis Ababa c ity's water suppl y, there needs torndes ign a project of constructing two dams and a 4.8 km tran smi ss ion tunnelrnthrough the Entoto ridge, by the Addis Ababa Water and Sewrage Authorityrn(AA WSA). On the basis of the demand to construct a detail ed engineeringrngeological and geotechni cal investigation for the proposed Entoto tunne l, the studyrnwas ca rried out after defining a tunnel corridor of about 10 km2 The study focusesrnon the engineering geological in vestigation of this tunnel corridor.rnThe study area lies in the Shoa plateau that is formed of Tertiary rocksrnbe longing to the Trap series. It is composed mainly of tuffs and trach ytic lavarnflows which are called the Entoto si licics of Addis Ababa area. There a re alsornbasaltic lava flows outcropping on the top of the Entoto silicics as patches whichrnarea products of Tarmaber basalt. Undiffe rentiated Quaternary sediments whichrninclude residual , colluvial and alluvial depos its occur in v31ious parts of the area.rnAerial photo interpretation, field traverses, insitu tests of rocks, as wel l asrnjoint analysis from core orientation and surface observation and subsequentrnlaboratory analysis were conducted. A number of exploratory boreholes drilledrnalong the hlllnelline indicated that a significant number of trachyte / tuff interfacesrnOCCUlT in the defined tunnel zone. From the borehole drilling and the geophys icalrninvestigation data, the area of the tunnel corridor appeared to be highly fau lted andrnweathered. The position of the water tab le, which is above the tunne l zone, and thernpermeability of the rock masses were detelll1ined.Engineering geological map at the scale of I: 10,000 was prepared. The rockrnmass strength was determined us ing Schmidt halllmer and point load testerrnperf01111ed during fie ld work and laboratory in vesti gati ons. I n additi on to thi s inrnthe fi eld , rock materi al strength was done with field identification by usingrngeological hammer and scratching by pocket knife.rnThe borehole data a long the tunnel line, whi ch is combi ned with surfacernmapping, confi rmed the general geological assessment and allowed th e choice ofrnconstruction methods. The borebole data was used as a basis for detailed des ign ofrnthe tunnel. Us ing all the avail able information on the rock mass and itsrnhydrogeological characteristics, rock mass class ifi cation schemes are developed.rnFrom all the existing methods of rock mass classification, the Rock Mass Ratingrn(RMR) and the Rock Mass Quality Index (Q - system) are selected. Considelingrnall the rock mass propelties, using the above classifica ti on systems the Illostrneconomic and suitable excavation Illethod would be by drill and blast. Thernrespective supporting method woul d also be by bolts and shotcretes.rnIt is hoped th at this study will provide a sound geologica l bas is durin g therndesign, and excavation stages of the tunneli ng. However, it should be emphas izedrnthat detail ed geotechni cal investigation on stress conditions of the rock mass andrnstability analys is, and furth er works at the pOItal areas and fault zones are veryrnim portant.Engin eering geological map at the scale of I: 10,000 was prepared. The rockrnmass strength was determined us ing Schmidt hammer and point load testerrnperformed during field work and laboratory investigations. In addition to this inrnthe fie ld , rock material strength was done with field identification by usingrngeological hammer and scratching by pocket knife.rnThe borehole data along the tunnel line, which is combined with surfacernmapping, confirmed the general geological assessment and a llowed the choice ofrnconstruction methods. The borebole data was used as a bas is for detailed design ofrnthe tunnel. Using a ll the available information on the rock mass and itsrnhydrogeological characteristics, rock mass classification schemes are developed.rnFrom all the existing methods of rock mass classification, the Rock Mass Ratingrn(RMR) and the Rock Mass Quality Index (Q - system) are selected. Consideringrna ll the rock mass properties, using the above classification systems the mostrneconomic and suitable excavation method would be by dri ll and blast. Thernrespective supporting method would also be by bolts and shotcretes.rnIt is hoped that this study wi ll provide a sound geological basis during therndesign, and excavation stages of the tunneling. However, it should be emphasizedrnthat detailed geotechnical investigation on stress conditions of the rock mass andrnstab ili ty anal ys is, and further works at the portal areas and fault zones are veryrni m po Ita n t.