Oborn, L.E. 1978 Tongonan geothermal development : Engineering geology of powerhouse sites. Preliminary report of visit 15-25 July 1978. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 314 17 p.
Abstract: The Tongonan Geothermal Field id one of the most tectonically active parts of the Philippines. Although no major earthquake has been recorded in Leyte in the past eighty or so years, there is abundant topographic evidence to suggest that damaging earthquakes have accompanied active faulting in the recent geological past. Site B has low to moderate relief, and is about 70 metres above the Mahiao and Samboloran rivers. Steep slopes on its northern and western flanks descend into valleys incised by these rivers. The basement rock is andesite, which in places is weathered or hydrothermally altered. Overlying the rocks in many places are alluvium and colluvium of various kinds. Neither the subsurface configuration of the hard andesite, nor the thicknesses or physical properties of the overlying materials, are known with certainty. The site is bounded by the Mahiao Fault, and lies on the projection of the Central Philippine and Sambaloran faults. Whether the Central Philippine and Sambaloran faults do extend along their projection, has not been resolved. Regional geological mapping in the past few years has produced no unequivocal version of faulting near the site, mainly because the faulting pattern is complex, and basic field data are sparse and in many places ambiguous. One of the immediate objects of the proposed engineering geological investigations is to determine whether these, or any other faults, cut the site, and are likely to produce surface displacement in the life-time of the project. Both Kingston Reynolds Thom and Allardice (KRTA) and New Zealand Geological Survey (NZGS) have prepared interpretive geological fault maps which show the Central Philippine Fault to cross the site in a position which would be hard to avoid in siting. Surface displacements of up to 7 metres, which must be considered possible, would be difficult if not impossible to design against. Both organisations have also produced maps which indicate that the fault does not cross the site. Until it can be demonstrated that active faults do not extend through the site, or that the fault zones are sufficiently narrow to enable safe setback distances to be achieved, the site cannot be recommended. The paucity of basic data also limits the value of estimates of slope stability. Investigations are needed that will give information on the kinds of rock and soil underlying the surface, their physical properties, and their thicknesses. Until this information is obtained and analysed, estimates of slope stability, especially during earthquakes, must remain subjective. The chances of achieving acceptably low slope angles are selected, and advanced engineering design and construction methods are employed. The seismic record is too short, and the geological record is as yet too uncertain, to enable a reliable quantitative assessment to be made of the severity of attack of earthquake induced ground motions. It is significant, however, that Leyte has been assigned to the most seism ically active region in the Philippines, and that there is, adjacent to the site, topographic expression of active faulting in the geologically recent past. Although the risk of damaging ground motions occurring during the life of a structure cannot be quantitatively assessed, it must be assumed to be moderate to high. Site C was visited only briefly, and the opinions formed are largely intuitive. The previous ranking of this site was necessarily a coarse one, based on the existence of faults postulated mainly from air photograph interpretation. A finer ranking to choose between Sites B and C is not possible without subsurface investigations. It would first be necessary to determine, as a minimum, the existence, location, attitude, width and nature of the faults near Site C. Sites E1 and F must be ranked lower than sites B and C. Site E1, should be eliminated from further consideration for the reason that it is on the Central Philippine Fault, if for no other reason. Site F is founded on alluvium, and is adjacent to both the Mahiao Fault, and moderately steep slopes. From an engineering geological, and engineering points of view it compares unfavourably with Sites B and C.
