Design methods for low volume roads

This thesis is concerned with producing a simple method to design low volume roads (LVR) by means of a rationale which accounts for permanent deformation development in granular layers. Rutting is regarded as the main distress mode in unsealed and thinly sealed pavements. Hence, it is desirable that it be analytically approached rather than empirically, as in most design methods. The overall aim of this PhD thesis was to look into the behaviour of in-service roads and from a newly developed process, to advance, in a systematic manner, the elements required to produce a simple mechanistic design procedure. The study took as its basis an assessment of the proximity of the stress distribution in the pavement to the material’s failure envelope. After a literature review on unbound granular materials mechanical behaviour and on low volume roads pavement design methods, Chapters 4 and 5 discuss full scale trials carried out in Scotland on typical forest roads. The overall goal of the trials carried out within the Roads Under Timber Transport project was to establish the effect of weather and seasonal effects on the rutting of forest roads and to improve their performance while enabling the roads to be economically constructed and maintained. It appears that most of the rutting occurring in the sites surveyed came shortly after their construction/resurfacing, leading to the assumption that workmanship may be a highly important variable. Lack of compaction of the layer could be one of the likely reasons for the high initial rutting rates. Establishing the effect of weather on rutting further to the existing knowledge was, however, difficult to achieve; this was mainly due to the difficulties faced in monitoring traffic conditions. A newly developed method was needed to quantify permanent deformation development due to wandering traffic on a non-level pavement; this was achieved by the use of wheel path areas, and seemed to be a way forward in the analysis of rutting in unsealed roads. Accelerated pavement trials are reported that aimed to evaluate the performance of aggregate under soaked conditions and the relative pavement deformation caused by different timber haulage vehicles. A road segment simulating a standard forest road section was constructed in a purpose-built facility located at the Ringour Quarry facility. Ten different trials were carried out combining three different aggregate materials and five types of vehicles. Tyre fitment, axle configuration and tyre pressure were assessed and demonstrated to play an important role on the study of rutting development. Conclusions drawn from the results suggest that management of the tyre inflation pressure and axle overload may be one of the most economic means of managing pavement deterioration in the forest road network. A mechanistic analysis of a variety of unsealed pavements was carried out in Chapter 6; and the newly proposed methodology is described in Chapter 7. With changing loading conditions – e.g. as a consequence of the introduction of Tyre Pressure Control Systems and super single tyres – more detailed analyses are required, so that their effect can be analytically assessed. Then an analytical method is introduced for evaluating the stress-strain condition in thinly surfaced or unsurfaced pavements as typically used in LVR structures. It aims to improve the understanding of the effect of tyre pressure and contact area in regard to permanent deformation. To achieve this, several scenarios were modelled using Kenlayer software varying aggregate material, thickness, stiffness, tyre pressure & arrangement. The results usually show a fairly well defined locus of maximum stresses. By comparing this stress envelope with failure envelope, conclusions could be established about the more damaging effect of super singles over twin tyres and, likewise, the greater damage inflicted by high tyre pressures compared to that incurred by lower tyre pressures. Finally, the framework of the proposed method contributes to LVR pavement design procedures mainly due to its simplicity. It still treats the pavement analytically, permitting a more fundamental description of the behaviour of granular layers than in simple linear elastic analysis, but by simplifying the elasto-plastic analysis for routine use it thereby reduces demands of material characterization and computational skills, thus increasing its utility in practical application.