Pathology of a research error: Coefficient of earth pressure at-rest for cohesionless soils

ABSTRACT: The magnitude of lateral earth pressures is required input to geotechnical design and analysis in a wide realm of issues relevant to soil-structure interaction. The determination of lateral earth pressure and its dependence on the over-consolidation ratio (OCR) is often based upon the coefficient of earth pressure at-rest, (Ko), for which no rational method of determination is available. This paper addresses three issues: i) What is the quality of the data which was the basis for the formulations often noted in the literature, taught in our classrooms and referenced in national standards and guidelines? ii) What is the outcome when rigorous testing, accounting for issues which have usually negatively affected the determination of Ko, is carried out? These issues include: Pressure measurement technique, Boundary conditions and Stress history. iii) To what other areas/scenarios have these erroneous measurements and resulting faulty concepts of Ko been propagated, and how does this affect engineering and research decision making? The testing procedures implemented to produce the data used to develop commonly applied relationships for Ko do not meet the basic requirements of at-rest conditions. The small deflections required to produce output signals corrupt the actual measurement. Boundary conditions of the test set-up must be considered when determining Ko. The boundary conditions affect the vertical soil pressure, and as a result the development of lateral soil pressure. Formulations often used in estimation of Ko in cases of unloading do not properly emulate the response of granular soils. The concept that the OCR has overwhelming influence on the development of horizontal soil pressure has been used in the past to develop models which attempt to determine/estimate the development of horizontal soil pressure due to compaction and may, consequently, result in significant over-estimation of residual pressure due to soil compaction activities. Is this approach a correct one ?