IMPROVING THE ENGINEERING CHARACTERISTICS OF THE SUBGRADE SOIL USING LIME AND SAWDUST ASH: A CASE STUDY ON BODITI-BEDESSA ROAD PROJECT

Abstract

Subgrade soils is the top surface of the roadbed that has been compacted to resist the loads upon it. These soils are variable due to different environmental factors such as moisture, temperature, and formation of the soil itself also. Based on a field survey, a road built along Boditi-Bedessa road project shows significant problems such as rutting, potholes, wash boarding, water crossing, swell-shrink nature, and roughness of the road. Hence, scientific investigations and understanding for better futuristic design as well as construction are quite important for subgrade materials. It is located in SNNPRS in Wolayita Zone. This study aims to improve the subgrade soil with lime (2,4, 6, 8) %, sawdust ash (5, 10, 15, 20) %, and their combination until the achievement of design standards. To meet these study objectives, eighteen (18) disturbed, and undisturbed test samples were extracted from each kilometer with a depth of (1-1.5) m to conduct Laboratory tests. As per ASTM all indexes and some engineering properties were carried out to characterize the subgrade soil. It was observed from the results that the Field dry density, Natural moisture content, Specific gravity, and free swell ranges from 1.17-1.37 g/cm3, 23-32%, 2.46-2.76, and 20-70% respectively. For grain size analysis, gravel, sand, silty and clay ranges 0.1-1.4%, 2.8-6.15%, 33.73-54.11%, 38.70-59.11% respectively. For Atterberg limits, liquid limit, plastic limit, plasticity index, liquidity index and consistency index ranges from 30-53%, 18-38%, 5-26%, -1.71-0.25, 0.69-2.71 respectively. And also, for modified compaction (MDD & OMC), CBR and CBR Swell ranges from (1.44-1.65 g/cm3 , 21-27%), 1.29-14.95% and 2.60-15.75% respectively. According to the AASHTO system, the soil has fallen under A-4, A-5, A-6, A-7-6 groups whereas as per USCS, the soil is the most likely to be silt (ML) and the more likely to be clay (CH & CL). And also, S1- S5 for ERA strength class. Based on plasticity, Soils under CH and CL groups are generally unsuitable for pavement purposes, therefore it is necessary to improve soils by using industrial and agricultural wastes like lime and SDA. As seen from the result that the use of lime and sawdust ash (SDA) reduced PI, Gs, CBR swelling, and MDD. While increased in OMC and CBR values. Further, the soil shifted from A-7-6 to A-2-4, and Fat clay (CH) to Silty sand (SM) after the improvement of lime and SDA respectively for uncured or cured periods. Therefore, it can be concluded that lime and SDA can improve the engineering properties of subgrade soil. Finally, the results indicate that the combined effects of 6% lime and 15% SDA were optimum contents for unsuitable subgrade material because it satisfies AASHTO (1998) & ERA (2002) standard. And also, SDA can be considered a cheap and acceptable stabilizing agent in road construction for improving the geotechnical properties of soft clayey.