By: Edgar Manaay
THE Jalaur River Multi Purpose Dam (JRMP II) was born in the 1960s after the Fourth Congress passed Republic Act 2651 authorizing the construction of the Jalaur River Dam in Iloilo. No wonder why our beloved Ilonggo Senator Frank Drilon jubilantly declared that; “it’s finally happening.” The good senator was referring to the signing of the contract between the National Irrigation Administration (NIA) and Korean Contractor Daewoo Engineering and Construction Company for the Dam construction last week.
It took nearly half a century to finally realize that the dam construction would soon commence and perhaps setting a world record as the slowest mega project a government has undertaken. The NIA in Western Visayas was already bragging that it will need 17,000 workers for the construction phase. If indeed this is true, then it will be a bonanza for many Ilongos who are unemployed, even for just two to four years only. But even before the euphoria dies down, let us be sober and be reminded of the lessons learned from the failure of the St. Francis Dam in Los Angeles California.
In 1928, the St. Francis Dam near Los Angeles, California broke only a year after it was completed. The concrete dam was about 60 meters (200 ft.) high, and the wall of water that roared down the valley killed about 400 people in two Counties. The failure of this ill-conceived dam destroyed the reputation of its chief designer, William Mulholland. Earlier in the 1900s, he had built an unsurpassed but controversial system of aqueducts to bring (some say, steal) water from Eastern California and Colorado River to irrigate an arid Southern California. This led to the booming growth of Los Angeles and San Diego.
The investigation following the disaster pointed to a number of possible causes. SOME WERE DESIGN AND CONSTRUCTION BLUNDERS. Three geologically related problems, each of which would have caused the dam to break up, were IGNORED by the builders. These were:
- The northwestern abutment (basement) of the dam was built on conglomerate (loose stones and pebble deposit). Investigators were astonished to discover that the samples of that conglomerate would disintegrate when placed in water!
- The dam was on a geologic fault separating sedimentary and metamorphic (the transition stage when sedimentary rock is transformed to volcanic rocks due to intense depth pressure and temperature) rock. Water, under high pressure (due to height or depth of water in the reservoir) could have blasted out ground-up rock within the fault. As experienced by the writer working in underground mines, rocks or soil strata along fault lines are loose due to the constant collision between two crustal plates.
- The southeastern abutment had been built on laminated mica schist (a metamorphic rock). The laminations are foliation (parallel alignment of textural and structural feature of a rock) planes. These planes of weakness are parallel to this side of the valley where the reservoir is located.
Landslide scars in the valley should have been ample warning to the builders that the metamorphic rock moved even under only the force of gravity. A competent engineer/geologist worries as much about the stability of the rock against which a dam is built as about the strength of the dam itself. Water pressure at the base of the dam itself exerted a force of 5.7 tons per square foot against the dam.
With such pressure, the dam and part of the bordering foliated rock could easily slide. Movement would be parallel to the weak foliation planes, JUST AS IF THE DAM HAVE BEEN ANCHORED AGAINST A GIANT DECK OF CARDS! In the 1980’s, Professor David Rogers of the University of Missouri-Rolla reinvestigated the disaster in detail from a geologic perspective. He presented compelling evidence that a rockslide took place in the schist (a metamorphic rock characterized by coarse-grained minerals oriented approximately parallel) extending from above the dam to below the abutment. Movement took place along the plane of weakness in the schist, just like how a kinder child slides down a concrete slide at school.
The St. Francis Dam disaster is just one of the many instances in which ignorance of geology cost lives or fortune. Had competent professional geologic consultation been obtained, it is UNLIKELY THAT THE BUILDING OF THE DAM WOULD HAVE PROCEEDED. In the case of JRMP II, contractor Daewoo Engineering hired Dasan Consultant also from Korea (where else since the funding is from a Korean bank!) to perform geotechnical investigations in order to assure the Ilongos that there will be NO DAM FAILURE just like what happened to the St. Francis Dam.
Dasan Consultant SHOULD HAVE conducted a series of core drilling up to maybe 2,000 to 3,000 ft., read and perform lab tests on the cores retrieved, then subject the holes to various geo-physical surveys such as electrical resistivity, electromagnetic and gravity methods. The purpose of all of these is to determine WHAT IS THE CONDITION UNDERGROUND where the dam and reservoir is located. It will identify minor and major faults, fabric and type of the soil strata to assure that there is none of the condition that will cause failure as in the St. Francis Dam.
But the problem here is that the Provincial Government have NOT provided a third party preferably an Ilongo as a geo-tech auditor who could have looked “over the shoulder” of the Consultant. Kay pareho lang sila Koreano sang contractor, basi ginkalokalo lang nila ang geo-tech investigation. Let us just hope and pray that this has not happened so that people living downstream of the dam can sleep soundly at night.
Note: The author is the Hydrogeology Consultant of the Municipality of Pavia, Iloilo and a Professional Member of the Geological Society of America (GSA).