Table 5.

Observations, conclusions and lessons learnt from the four case studies

Case studyObservations, conclusions and lessons learnt
1, Km 282(1) Metre-by-metre geotechnical investigation and detailing of design was required to yield as robust as possible a construction in such a geomorphologically aggressive environment
(2) Minor realignment into the hillside at critical locations provided space for below-road retaining walls and a more stable subgrade for the reinstated road
(3) Drainage control and scour protection proved of paramount importance
2, Km 314(1) Without drilling investigations, and particularly subsurface movement and groundwater monitoring, the ground model was arguably incomplete
(2) Attempts at low-cost reinstatement, based on earthworks and bio-engineering alone, although appropriate for ‘normal’ operating conditions, were unable to accommodate the extreme rainfall of Typhoon Pepeng. It is debatable, however, whether any level of investment would have survived such conditions, underlining the need to strike a suitable balance between cost and serviceability
3, Km 267(1) Drilling investigations and inclinometer readings proved the only way to fully appreciate the depth and nature of ground movements
(2) Again, drainage management and erosion control proved of paramount importance in tipping the balance from instability towards stability
4, Km 264(1) Although the potential for slope erosion and instability to affect the road was recognized and (to an extent) allowed for in the design, disturbance to road drainage systems up-gradient, combined with severe typhoon rainfall, caused significant damage at this location
(2) As with case studies 1 and 3, attention to detail regarding drainage management and erosion control, and the founding levels of retaining walls and scour protection works, has been proven critical in allowing the balance to be tipped in favour of improving stability
(3) Even minor shortcomings in the road drainage system can quickly lead to a worsening situation and eventual failure of the road itself if not remedied immediately