On August 16, 2004, a portion of the eastern concrete wall of Santa Barbarra Subdivision II collapsed following heavy rains. Report of the incident came out in newspapers on August 18, (Tempo, p. 2), prompting the DENR-MGB to send a team of geologists on the same day to investigate the incident.

The team found that the incident occurred in a small, isolated portion of the subdivision—within the water tank area located 100 meters away from the houses being built. The wall that collapsed was part of the eastern concrete wall that was temporarily built to protect the water tank area. Directly attached to the wall, were occupied shanties. Another concrete wall on the northern side remained unbreached.

DENR-MGB Director Horacio C. Ramos said the team found no evidence that would point to landslide as the culprit of the concrete wall-collapse, specifically since the unprotected walls of soil did not collapse.

“Natural landslides would normally yield tension cracks, and manifest on vegetation covers atop an indicated landslide area, such as tilted trees and disturbed crops; but apparently there was none of such occurrences,” Ramos said.

“Besides, seeing the portion of the wall with bare soil and uncovered with concrete—which is just immediately adjacent to the concrete wall—still intact, would already give you a hint that the problem is isolated to that concrete wall.”

Alvin Matos, one of the geologists sent to the area, said the eastern concrete wall designed and constructed “sub-vertically at about 80 degrees” without any other reinforcing structures was prone to collapse.

“If the eastern wall was more inclined similar to the northern wall, the water pressure may not have been enough to topple the wall as experienced by the unbreached northern wall of the water tank area,” Matos said.

Explaining how the rains could have triggered the wall-collapse, Matos said: “What most probably occurred was that soil horizon became supersaturated with water due to heavy downpour and then the more [solid] bedrock and retaining wall itself trapped the water pressure which became greater than the confining pressure of the eastern wall pushing it to collapse.”

Aside from Matos, the two other geologists who conducted investigation in the area were: Brian Zoilo A. M. Esber, Senior Science Research Specialist; and Edward B Aguinaldo, Science Research Specialist II. Matos is also a Senior Science Research Specialist.