an engineering geological characterisation of tropical clays - GBV

14
as degree of drainage and wetness. The black cotton soils could easily be distinguished and
separated from intercalated patches of the dark grey mottled clays through the presence of a
characteristic shrubby vegetation which appears as specks on the photos in areas of the former soil
type.
A field investigation plan and/ or map showing sampling sites is presented in a later chapter (Fig.
4.1). A total of 336 survey points and 36 sampling sites were covered along planned field profiles
within black clays in the study area. Geotechnical logging of soil profiles was effected at
representative soil sections obtained by hand-excavation as well as those exposed at cut-sections of
stream courses, roads and quarries. The soil sections facilitated correlation of soil profiles across the
study area. They also enabled determination of soil thickness as well as depth position and nature of
underlying bedrock. A standard description of the nature of underlying bedrock (in accordance with
ISRM, 1978a/ b) is included in a later chapter dealing with the geology of the study area.
Undisturbed U100 and disturbed soil sampling was done in the excavated pits. Besides excavated
and / or exposed soil sections, the nature and lithological variation of soils were also investigated by
employing soil augering techniques. Cone penetration sounding was also used to determine depth
variation of soils across the study area. In situ shear strength characteristics of soils were
investigated by means of vane shear testing.
The red soils were also investigated in this study (but to a lesser extent than the black clays). Their
inclusion in this study was mainly for comparison purposes. A total of 6 excavation, soil sampling
and/ or field investigation sites were covered for these soils.