The effect of sample preparation and testing procedure on the geotechnical properties of tropically weathered residual laterite soils of Ethiopia

Abstract

Identification and classification of the various grades of tropically weathered residual soils for engineering purposes is a problem that geotechnical engineers repeatedly face in Ethiopia, as well as in other tropical countries. This paper investigates the applicability of conventional laboratory testing, classification and evaluation systems, which were principally developed for temperate climate sedimentary soils, to the tropically weathered residual laterite soils of Ethiopia. The construction of a new highway project from Bako to Nejo in western Ethiopia, with extensive laterite soils, forms a study site that has allowed detailed inspection and sampling of soils for this research project. Testing of these laterite deposits has revealed that they are sensitive to handling and disturbance. Revisions to sample preparation and testing procedures are considered necessary and are proposed in utilizing these soils for civil engineering purposes.

It is concluded that the plasticity chart should be regarded as a guidance tool only for evaluating the likely properties of laterite soils. It should not be used to classify them based on conventional methods without due deference to their genesis. Unlike soils for which the Casagrande Plastcity Chart, as adopted in the United States Classification System (USCS), was developed, laterite soils that plot well below the A-line generally have good engineering properties. A combination of classification based on structure, mineralogical composition, and geo-morphological impact on soil formation together with conventional classification systems is suggested and can result in a good indication of the engineering properties of laterites.

Keywords: laterite, air-dried, as-received, oven-dried, sesquioxide, mineralogy