GEOTECHNICAL INVESTIGATION OF SUBGRADE SOIL AND STRUCTURAL DESIGN FOR RECONSTRUCTION OF ELIAS EGBOKA-NWAKPADOLU ROADS

Abstract

The need to design a pavement structure that will facilitate easy movement of goods and services and also minimize the difficulties faced by both private and commercial car owners and motorcyclist plying Elias Egboka-Nwakapadolu roads particularly during the rainy season formed the basis for the study. The study was undertaken to design Elias Egboka- Nwakapadolu roads using AASHTO method and detail using ArchiCad. Three different samples of soils collected within the study area designated as A, B and C was subjected to geotechnical testing. The tests conducted on the samples were sieve analysis test, specific gravity test, atterberg limit (liquid and plastic limit) test, compaction and CBR test. Result obtained from sieve analysis test classified the samples as A-2-6, A-2-6 and A-2-4 according to AASHTO Soil Classification System, according to Unified Soil Classification System; the samples were classified as SC (sand mixed with clay). The specific gravity of the samples ranged from 2.6-2.66, the liquid, plastic and plasticity index of the samples ranged from 37.8-42.4%, 17.4% - 24.2% and 16.5% - 24.2% respectively, plasticity rating of the three samples revealed that the samples were medium – highly plastic. The maximum dry unit weight and optimum moisture content of the samples ranged from 18.53kN/m3 to 20.23kN/m3 and 9.85% - 11.54% respectively, the soaked and un-soaked CBR of the samples ranged from 11% - 15% and 25% - 31% respectively. The soaked CBR of 11% was used to support a total equivalent single axle load (ESAL) of 25.8 x 106 for a design period of 20 years. Results obtained from the design of Elias Egboka-Nwakpadolu roads using AASHTO method revealed that the thickness of the sub-base, base course and surface course were 150mm, 200mm and 160mm respectively with a total thickness of 510mm superimposed on the sub-grade layer. Detailing of the designed pavement layers was done using architectural software commonly referred to as Archicad.