Engineering on the precipice: the performance of mountain road rehabilitation in the Central Cordillera of the Philippines

Parts of the Central Cordillera of Luzon are among the most challenging locations on Earth to build and maintain transport infrastructure, a situation perhaps most epitomized by the Halsema Highway. Since its original construction in the 1920s, the highway has undergone phased improvement and has become a socio-economic lifeline to the rural communities it serves. Unfortunately, in 1990, ongoing road improvement works had to be abandoned due to the outcome of a 7.8 Ms earthquake and the effects of subsequent typhoon damage. Earthquake reinstatement works were designed in 1998 and constructed between 2001 and 2006. The engineering geological challenge this posed was compounded by the effects of severe typhoon rains during the reconstruction period, requiring continual assessment of changing slope and drainage conditions. Since construction, the road has been impacted by several new typhoons including, most-notably, Typhoons Pepeng and Ompong in 2009 and 2018 respectively. Field inspections in 2010 and 2018, combined with the interpretation of satellite imagery available in Google Earth, have allowed the performance of the works implemented between 2001 and 2006 to be assessed and the outcome has been largely favourable. Very little of the recorded damage has occurred in the locations of earthquake reinstatement. Instead, several new areas of slope failure and subsequent blockage and damage to the road have developed. Many of these areas can be explained with respect to their underlying engineering geology and geomorphology. However, there are just as many that owe their origin to the pattern of rainfall and runoff arising during the passage of individual and successive typhoons, modified significantly by drainage management practices in the road corridor, where engineering serviceability and farming practices sometimes have conflicting objectives.