Investigation of The Properties of Unbound Granular Basecourse Aggregate Using Laboratory Tests

Abstract

Permeable paving has long been researched and implemented in various forms around the world. The primary benefit of permeable paving is its Low Impact Design (LID) which mitigates environmental pollution. Permeable paving can also reduce flooding during peak rainfall events by allowing surface water to infiltrate through the pavement. Good pavement strength and permeability are difficult to achieve simultaneously without the use of binders or expansive drainage systems. The study aims to develop an appropriate permeable structure that provides good strength and permeability while utilising natural and economic aggregate resources. Laboratory tests such as the Repeated Load Triaxial (RLT) test and constant head permeability tests were conducted to determine the strength and permeability of the basecourse respectively. Vibratory compaction is primarily used to compact RLT specimens. This research addresses the applicability of current laboratory compaction techniques and outlines the changes that need to be made to minimise the gap between laboratory and field compaction. The target density achieved in laboratory studies, utilising the New Zealand standard test methodology, that is then used to target field compaction levels, is often too high to realistically achieve in the field. It was identified that there is a maximum aggregate Average Least Dimension (ALD) to specimen diameter and specimen height ratios that must be adhered to when using laboratory testing to evaluate, design or construct pavement basecourse layers. In addition, the power input of laboratory vibratory hammers used for compaction must be controlled to minimise aggregate degradation, especially for test moulds that do not satisfy the minimum diameter to ALD ratio. The goal of laboratory compaction should be to accurately simulate field compaction and thus provide realistic targets that can be achieved in the field. If unrealistic field compaction density targets are specified, then over-compaction can occur, causing aggregate breakdown. The research concluded that changes are required to test standards, so that field target densities are more accurately represented in the laboratory. There is currently a significant lack of research into permeable pavements in New Zealand, which has resulted in lower levels of confidence in permeable pavement design. The risks associated with permeable pavements are not understood. Therefore, traditional pavements with poor drainage qualities are used in situations where well-drained layers can potentially be used. Pavements that ii constructed without effective drainage are deteriorating at much faster rates than if they were well drained. Several constant head tests were conducted on the permeable basecourse developed in this research and a traditional high fines basecourse to compare the permeability between the two types of basecourse. The flow type expected through the basecourse needs to be established to identify an appropriate model that can be used to determine the permeability. Certain models are appropriate for laminar flow within porous media while others are used for turbulent flow. The relationship between apparent flow and hydraulic gradient needs to be determined so that linear or non-linear relationships appropriate for predicting future permeability values can be applied. It was established through a literature survey that testing of course aggregates in small triaxial equipment can result in misleading strength-deformation characteristics, although this practice is very common in laboratories in New Zealand. Appropriate test variables such as specimen dimensions, compaction and testing procedures, and saturation techniques were evaluated, and the effects of each variable were tested using the RLT test. A significant database of RLT tests was conducted to determine the effects of these variables as well as the deformation behaviour of the permeable and M/4 basecourse. The NZTA M/4 basecourse specification was used to determine if the two types of basecourse used in this study were appropriate for pavement construction. Currently, there is inconsistency in RLT test results between laboratories in New Zealand. The inconsistency can be related to following outdated compaction procedures and where a target density method is followed, the discrepancies are a result of insufficient maximum aggregate ALD to specimen diameter ratios adopted. This is shown in this research and by several reputable researchers in this field of study. The target density method of compaction does not offer homogeneity within the specimen. Therefore, a standard method of compaction that simulates realistic field densities is suggested. Standard methods of compaction produce variable densities in the smaller 150mm diameter triaxial apparatus, which is one of the reasons why the smaller apparatus is not recommended to be used to test course aggregate gradings. Using the small-scale apparatus to test basecourse material results in significant scatter in results. Reduction of the grading scale is not an appropriate practice to follow. For more representative test results, the material must first be tested in its natural grading, and secondly, the specimen size iii of the material being tested must be appropriate for the largest aggregates present. Scalping of the grading does not provide results that can be transferred back into field situations, which utilise the full grading. The structural response observed during RLT testing proved to be affected significantly by the grading scale, or the maximum particle size of the aggregate used in the specimen. The experimental outcome of this investigation is in general agreement with the results of previous studies. The analyses of the test results confirm the complexity of the nature and the extent of the impact of grading scale reduction on triaxial test results. It is, therefore, recommended that triaxial specimens be made large enough to allow testing of granular materials at their natural gradings.