A THESIS SUBMITTED TO THE DEPARTMENT OF CIVIL ENGINEERING, IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN GEOTECHNICAL ENGINEERING.

Abstract

Construction of the various pavement layers is very much dependent on the geotechnical characteristics of the sub grade soils. The aim of this thesis work is to characterize the subgrade soils and stabilize the poor subgrade material of Arba Minch - Wolaita Sodo main road to Zefine town, because this road was constructed without any geotechnical investigation of the subgrade soils, and giving transportation purpose for the community with continuous maintenance on most part of the road; thus, geotechnical characterization and stabilization of problematic subgrade soil is very important. To achieve these objectives, twenty soils sample pits were selected from different representative parts of the existing road at 1km interval. To understand the basic characteristics of the sub grade soil, such as field dry density, natural moisture content, particle size distribution, specific gravity, Atterberg limits (LL and PL), compaction (OMC and MDD), CBR and CBR swelling potential tests were concerned in this thesis. The test results showed that the NMC ranges from 9.29% to 15.86%, the field dry density ranges from 1.67g/cc to 1.85g/cc, percentage finer ranges from 63.36% to 78.31%, LL ranges from 31.53% to 60.27%, PI ranges from 6.49% to 39.02%, GI ranges from 4 to 31, OMC ranges from 14.00 % to 17.74 %, MDD ranges from 1.74 g/cc to 1.98g/cc, CBR ranges from 2.05% to 9.08% and CBR swell ranges from 1.05% to 3.66%. Based on this results the study area was characterized as silt and clay soil, and for the soil classification under A-4, A-5 and A-7-6 according to AASHTO M145 soil classification system. From all soils samples the three test pits samples (from TP1 to TP3) were unsuitable to use as a subgrade material for pavement construction, and the remaining were good bearing capacity of soils for the pavement construction based on ERA (2002) manual specification; thus, for the unsuitable subgrade soils stabilize by mechanical method of stabilization, with different percentages ratio of locally available selected material (10% , 15% and 20%) by weight, were used to get appropriate mixing ratio, finally it was found that 20% mixing of locally available selected material were efficient.