Evaluating the Geo-Mechanical Properties of the Jimma Volcanic and Makkonnen Basaltic Rock Formations: A Specific Guideline for use in the Construction industry (A Case study on Wabe Hydropower Project, Ethiopia)
No Thumbnail Available
Date
2020-06
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa University
Abstract
In the present study, the engineering properties of the Jimma volcanic and Makkonnen basaltic
rock formations located in the south-western part of Ethiopia in Wolkite Town are assessed. It is
aimed to propose a guideline useful in the early stages of site investigation. A systematic
methodology is followed which includes review of secondary reports on investigations and design
and; collection of primary data from subsurface and surface outcrops of the geological formation.
The physical and mechanical properties of rock materials such as water absorption, specific
gravity, porosity, flakiness index (FI) and elongation index (EI), Uniaxial Compressive Strength
(UCS), Aggregate Crushing Value (ACV), Aggregate Impact Value (AIV) and Los Angeles
Abrasion (LAAV) have been determined with reference to the various specifications and standards
(AASHTO, ASTM and BS) in terms of suitability in the construction industry. WA value ranges
from 0.2-5.52%, porosity values range from 1.17-13%, SG values range from 2.02-2.92%, FI and
EI values range from 5.26-32.24% and 14.65-22.57% respectively; UCS value ranges from 1.37237.29MPa,
ACV value ranges from 16.23-20.33%; AIV values range from 10.02-12% and
LAAV values range from 24.32-29%.
The data analysis from the geological logs and the seismic refraction survey suggests that the
bedrock is located at a deeper depth represented by fresh, massive and slightly fractured basalt of
high quality (RMR>65) represented by high P-wave velocity (Vp>5000m/s). Some very soft tuff
rocks and heavily jointed media have Vp<1500m/s and RMR<35. The majority of the rock mass
can be classified as an excellent foundation material.
The drilled boreholes are also checked for in-situ rock mass permeability conditions using packer
test. According to the test results the test sections have a very low to medium permeability Lugeon
value ranging from 0.03-22.8. The relatively high lugeon values are obtained at BH-07 (21 and
22.8) which is located at the right abutment of the dam site
The rock mass along the tunnel section is divided into a number of geological structural units in
order to determine the RMR and support requirement. Accordingly, GC1(geotechnical class one)
has an RMR of 85 and categorized under a very good rock mass class; GC2 (geotechnical class
two) has an RMR of 71 and categorized under a good rock mass class; GC3 has an RMR of 81 and categorized under a very good rock mass class; GC4 has an RMR of 66 and categorized under
a good rock mass class and GC5 has an RMR of 13 and categorized under a very poor rock mass
class.
Finally, on the basis of the findings of the present research, a specific guideline is proposed based
on rock index property tests and geotechnical parameters. Initially, the rock mass is divided into
five rock grades (A, B, C, D and E) using weathering (alteration), observed number of joint, joint
spacing together with RQD as a judgment criterion. A rock mass classification guideline is then
proposed based on the combination of the geotechnical parameters that is based on the overall rock
mass quality and the intensity of P-wave velocity for the mapped geological units.
Description
Keywords
Basaltic Rock Formations, Jimma Volcanic, Volcanic and Makkonnen Basaltic Rock, Geo-mechanical properties