Table 2.

Summary of alignment geology, geomorphology, geohazards and construction

KmGeologyGeomorphologyInstability1998 road conditionMain elements of construction
257–263Sedimentary rocks (including limestone and metasediment), deeply weathered and faultedNear ridge top, and gently sloping sidelong groundNot significant(1) Existing concrete pavement badly cracked(1) Repair pavement
(2) Inadequate drainage(2) Improve road drainage and construct new outfall protection works
263–277Metavolcanic rocks, intruded by diorite, deeply weathered, faulted and fracturedNear ridge top and across saddlesMajor zone of landslides (debris avalanches) and severe erosion(1) Road bench collapse in places(1) Reconstruct road bench with large retaining walls
(2) Existing concrete pavement cracked(2) Repair pavement
(3) Inadequate drainage(3) Improve road drainage and construct major new outfall protection
277–290Metavolcanic rocks, deeply weathered in places, extensively faulted and fracturedRoad traverses moderately steep sidelong groundMajor zone of landslides (debris and rock avalanches) and severe erosion(1) Mostly gravel road, short sections of concrete pavement(1) Construct new concrete pavement
(2) Major road cuts(2) Reconstruct road bench with large retaining walls or by realignment
(3) Major erosion and landslides affect road(3) Construct new road drainage and major new outfall protection
290–297Volcaniclastic rocks, horizontally bedded, massive, blocky conglomerates and agglomeratesRoad cut into steep rock slopes near knife edge ridgeLocalized rock falls(1) Narrow (<6 m wide) road bench formed in cut, no pavement(1) Widen road bench to 7 m in steep cut and provide edge beam on outer edge
(2) Existing gravel road in extremely poor condition(2) Concrete pavement
297–305Metavolcanic rocks, intruded by diorite, deeply weathered, faulted and fracturedNear ridge top, ground below over-steepened owing to downcuttingRoad traverses major landslides (deep-seated and channelized debris flows)(1) Mostly gravel road, short sections of concrete pavement(1) Construct new concrete pavement
(2) Some narrow sections of gravel road(2) Reconstruct road bench with large retaining walls or by minor cut
305–325Metavolcanic rocks, intruded by diorite, deeply weathered in places, extensively faulted and fractured (volcaniclastic rocks Km 322–323)Road traverses moderately steep sidelong groundMajor zone of landslides (debris avalanches and deep-seated slides) and severe erosion(1) Fully paved(1) Reconstruct road bench with large retaining walls or by realignment
(2) Major road cuts(2) Construct new road drainage and major new outfall protection
(3) Major erosion and large landslides affect road
325–337Metasedimentary rocks and volcaniclastic rocks, deeply weathered (weathered to residual soil from Km 328 to 334)Along flat ridge in main agricultural areaNot significant(1) Concrete pavement badly cracked(1) Repair pavement
(2) Inadequate drainage(2) Improve road drainage and construct new outfall protection works
337–341Volcaniclastic rocks, deeply weathered in placesNear ridge top, and gently sloping sidelong groundNot significant(1) Gravel road bench(1) Construct new concrete pavement
(2) Inadequate road drainage(2) Construct new road drainage and major new outfall protection

Km references relate to road distances from central Manila. Km locations are given in Figure 1.