Conclusions and recommendations

IRIS

The IRIS database was examined to provide an indication of how structural maintenance schemes perform over a five year period following completion. Information collected as part of regular trunk road inspections was analysed to estimate the number, type and timing of defects recorded between January 2015 and December 2019. Cat 1 defects are typically associated with potholes and failed patches. Cat 2 defects are much more varied and include faults such as fretting, cracking, edge deterioration and other miscellaneous features. Category 2 defects are not judged to signify an immediate or imminent hazard and are more associated with wear and tear of the road materials. The findings of the analysis were as follows:

Occurrence of defects on structural maintenance schemes

• The annual frequency of Cat 1 defects being recorded in the first year of construction for schemes constructed between 2015 and 2019 ranged between 5 and 11%.
• The annual frequency of Cat 2 defects being recorded in the first year of construction for schemes constructed between 2015 and 2019 ranged between 5 and 17%.
• From the 40 schemes built in 2015, Cat 1 and Cat 2 defects were recorded within the same scheme extents after five years of service in 30% hand 40% of cases respectively.
• Fretting and cracking, and to a lesser extent edge deterioration, appear to be the dominant defects recorded in the northern parts of Scotland.
• The occurrence of potholes was the main defect recorded in the south of Scotland.

Early life defect sites

Based on a review of IRIS and information supplied by OCs, 19 schemes were identified as having the potential to provide useful information on the cause of early life defects. All the sites were visited and inspected, and the findings were analysed along with design scheme data supplied by Transport Scotland. Based on this analysis, the sites were split into four broad groups, namely: external or isolated defect; inadequate inlay depth; edge deterioration and drainage; and urban/utilities. The schemes that fell into these groups were studied to determine the likely reasons that contributed to early life defects. The main findings were as follows:

• The site visits highlighted that 42% of the schemes where defects had been recorded within the scheme extents contained defects that were external to the original maintenance works or were isolated defects.
• Additional analysis corroborated the above finding by showing that 30% of schemes constructed in 2015 where a Cat 1 defect had also been recorded, required no further maintenance treatment following repair of the CAT 1 defect.
• When the findings from Phase 1 (using IRIS database) are adjusted for external and exceptional defects, the incidence of Cat1 defects coinciding with maintenance schemes falls from 6% to 3% after one year and 30% to 17% after five years.
• The condition of surface courses, particularly TS2010, was regarded to be good.
• The most common early life defects were associated with inadequate inlay depth and this was evidenced by the fact that deeper inlays used on the same scheme showed no defects.
• For schemes treated with shallow inlays the available information suggested that the primary cause of failure was related to poor substrate stability.
• The failure at some schemes was related to inadequate pavement width and drainage.
• Information collected on urban schemes suggested that full reconstruction was required in some instances and that defects were due to weak subbase and foundation conditions.

Recommendations

Section 4 of the report discusses the likely failure mechanisms that caused the early life failures on the schemes visited and recommends potential solutions to reduce the incidence of early life defects. General advice is provided on reducing the risk of early life defects occurring, including optimising the selection of inlay depths.

It is also recommended that the site on the A82, near Aonach Mòr, be monitored to determine the ongoing performance of pavement reinforcement geogrid.