an engineering geological characterisation of tropical clays - GBV

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Chapter 4
Field methods
4.1 Introduction
The principal objectives of the field study were to:
(i)
(ii)
(iii)
(iv)
Provide information on the nature and distribution of black cotton soils (expansive
and reactive soils) and red soils of the study area.
Acquire site specific information on environmental characteristics which impact
on in situ soil behaviour (groundwater conditions, climate, vegetation).
Investigate and model depth variation of soils across the study area.
Compare in situ soil properties, behaviour and derived parameters with those
predicted and/ or derived from laboratory studies.
The cone penetration sounding, soil augering, in-situ description of soils and underlying rock,
tests for possible carbonate content, as well as undisturbed and disturbed soil sampling were
carried out, among others. Documentation of results was done according to known standards
(BGR, 1996; DIN 4021/ 4096, 1980; IAEG, 1981; ISRM, 1978; NLfB & BGR, 1991).
The sampling technique was selected and planned in such a way as to facilitate collection and
provision of high quality and detailed data for validating models and assessing variation of
engineering soil properties across the project area.
4.2 Investigation and sampling scheme
A number of site investigation and sampling schemes have been devised by practical
statisticians, and are available for use by geologists (Koch and Link, 1970; Krumbein and
Graybill, 1965; Meyer, 1975; Wetherill, 1966; ). According to Cheeney (1983), the principal
motivation for the diversity of available procedures is the need to achieve maximum
efficiency and precision within constraints of cost, time, effort, available facilities, manpower
skills and specimen accessibility. However, the application of these schemes may be
hampered and/ or limited by the fact that, geological materials and observations are frequently
hard won under difficult circumstances.
A site investigation/ sampling plan is a fundamental part of the study of soils, especially as
may regard the assessment, analysis and evaluation of the spatial distribution of important
engineering geological characteristics and/ or parameters. The use of an unsuitable and
inadequate procedure could lead to bias in the observation and selection of specimens, with a
consequent distortion of the outcome of important soil studies and evaluations such as
hypothesis testing and statistical tests. It is therefore essential that the field investigation/
sampling method adopted is such as to allow for the acquisition of necessary information and
soil samples which are free as much as possible from bias. This is because a biassed set of
observations and/ or sample would only serve to give a biassed impression of the parent
population from which it was drawn (Cochran, 1977).
In the present study, field investigations and sampling were carried out with the aim of
achieving both descriptive and analytical objectives of soils. Among the analytical objectives
is hypothesis testing, the methods of which are based on samples. Attempts were therefore
made to avoid choosing and applying defective site investigation and sampling practices that