ENGINEERING GEOLOGICAL INVESTIGATION OF SOILS AND ROCKS: A CASE STUDY OF BUILDING CRACKS AND ROAD FAILURES IN ARBA MINCH TOWN, ETHIOPIA

Abstract

The study area is located in the southern Ethiopia, Gamo Zone which is 465 km far from Addis Ababa. The main objective of the research was to study the causes and mechanisms of crack development on buildings & other infrastructures from geological & engineering geological perspectives & to produce engineering geological map at a scale of 1:25,000. To achieve these objectives, field observation & description of soils and rocks were done based on ASTM & ISRM guidelines. Fifteen disturbed and undisturbed soil samples were collected from test pits & trenches based on geology & existence of frequent problems. Laboratory works were conducted based on ASTM laboratory standard method. Schmidt hammer test & Scanline methods were used to determine UCS & RQD of rocks. Rock mass description & classification were done based on Bieniawski classification system. The percent of gravel, sand, silt & clay ranges from 0 to 5.61, 1.71 to 58.31, 16.94 to 69.07 & 19.14 to 69.48 % respectively. The LL & PL values ranges from 28.45 to 88.19 & 17.43 to 45.94 respectively with PI value of 9.22 to 42.25 %. The FS value ranges from 2 to 140 %. The internal friction angle, ɸ & cohesion, C of direct shear test ranges from 3.051 to 12.134о & 15.273 to 30.913 kPa. The qu value ranges from 64.48 to 109.97 kPa. The modified compaction MDD & OMC ranges from 1.626 to 1.672 g/cm3 & 22.3 to 25.7 %. CBR & its swell value ranges from 0.66 to 2.97 % & 0.48 to 3.87 % respectively. The consolidation test result of Cc, Cs, Pc & O.C.R. ranges from 0.166 to 0.266, 0.049 to 0.052, 98 to 150 kPa & 1.96 to 3 respectively. Based on USCS & AASHTO, the soil is classified as CH, CL, SC-SM and ML & the soil falls under A-4, A-6, A-7-5 and A-7-6. Based on UCS, intact rocks show low to high rock mass strength. The RQD and RMR value ranges from 12.05 to 96.83 % & 42 to 73 % of very poor to excellent with III & II classes of fair to good quality respectively. The main geotechnical problems that causes infrastructure failures in most parts of the town are found to be existence of potentially expansive soils. Finally, engineering geological map was produced. Based on the above results, this research recommended that excavating & backfilling with selected geologic materials & preloading is essential to increase the strength of underlying soils & reduce infrastructure failures caused by potentially expansive soils.