Abstract

While site investigation data is typically focused on the spatial variability for site characterisation, those properties may change with time. Temporal variability needs to be considered in the design and construction of roads and high-level foundations. Seasonal variation occurs in the active zone and long-term changes occur 2 to 5 years after construction using expansive clay materials in adverse climate conditions. The latter may occur below the zone of seasonal variation. A case study for seasonal changes in subgrade strength at an uncovered test site when tested at 5 different times of the year. Measurement accounted for 30% of the variation, while temporal and spatial variation accounted for an additional 30% variation over the 1-year period of testing. An appropriate probability distribution function (PDF) is required for characteristic values. Constructing embankments to the equilibrium moisture content (EMC) is more important than the optimum moisture content (OMC), which is a short-term construction expedient. Time Domain Reflectometry (TDR) probes were used to monitor embankment moisture changes over a 4-year period for a new road construction on expansive clays. This included 1) an existing 30-year-old embankment during the project planning phase, 2) a trial embankment during design, and 3) the new constructed roadway embankments. All demonstrated the importance of understanding the EMC when expansive clays are being used for embankment construction. Results in other climatic environments are compared. The results show subgrade CBR tested at OMC is meaningless for such soils and in those extreme climates.

Abstract

Delineation of areas into a “uniform” harvestable quality material is required for cut and fill earthworks projects. Mixing or inadvertent contamination with onsite high-quality material with adjacent poor quality is unacceptable. Importing material from off-site sources have significant cost associated. Two case studies are presented to highlight the design and contractual interpretation of a “unform” site. The first case study compares when a characteristic design value is used versus the day to day on site requirements to meet these design requirements. The required coefficient of variation (COV) of material parameters is discussed from both a design and construction perspective for section delineation. A major highway road widening had 4 sections as part of the upgrade. The tender documents were based on balanced cut to fill. On site material variability had pockets of good and bad material. Statistical analysis pre and post tender were compared in the contractual dispute which followed. The contractor was obligated to then import material to significant subgrade depths for these sites. To do otherwise would be contra to both the design material requirements specified in the contract documents and the requirements of the Earthworks specifications. The second case study is for a major 13 km rail upgrade to illustrate how the COV can be used in site characterisation and spatial variation at a cutting. The COV values adopted for both design and construction assessment are different as the intent is different. A quality control COV is different for a characteristic design COV.