Benefits of a granular construction interface in highway construction over pyritic subgrades for mitigating against deleterious ground effects

摘要

Construction and improvement of the arterial highway infrastructure in the UK may often encounter areas of pyritic ground conditions. Exposure during construction provides conditions suitable for the oxidation of pyrite, which may become destructive to highway pavements and associated infrastructure if not appropriately mitigated for. Deleterious reaction will occur when the more reactive microscopic form of pyrite known as framboidal pyrite is found. This is present in ancient marine mudrock deposits found near many of the built up areas of the UK. Protection to road pavements may be best achieved by the inclusion of a granular construction interface separating the road from underlying deleterious pyritic subgrade. The granular interface serves as a flexible buffer against differential movement and as a capillary break preventing upward migration of sulfate bearing water. Road construction will have traditionally avoided such consequences by the inclusion of a granular capping layer, as typically required in previous Highways Agency standards to mitigate for lower subgrade strengths. Revision to the guidance in the last decade allows the decision for inclusion (or not) of a granular capping by the designer. Therefore, if pyritic subgrade is present the absence of a granular separator layer may expose the road pavement to the deleterious consequences of ground movements and aggressive ground conditions. The paper presents and discusses three case studies. Two examples demonstrate the benefits of inert granular interfaces. The third outlines an example where a granular interface was not included leading to destructive consequences to a road pavement.