Engineering Geological Study of the Proposed Entoto Tunnel
No Thumbnail Available
Date
1999-06
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa University
Abstract
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.
Description
Keywords
Engineering Geological