Bagong banta: Pagguho ng lupa sa gitnang bahagi ng Maguila-guila Siltation Dam o 'Subsidence'

Subsidence, sinking of the Earth’s surface in response to geologic or man-induced causes.  Is it the same as sinkholes?

Despite similarities the two are not the same.  In simplest terms, land subsidence is a lowering of the ground while sinkholes involve a collapsing of the land and usually creating a giant hole. You need certain conditions for each to happen.

Land Subsidence:Occurs when more fluid (water or oil and even gas) is taken from an aquifer than is replaced. When fluids that were filling the pores spaces (space between particles) of the aquifer are removed, compaction of the aquifer occurs.  This can become permanent because of the weight of the land, which through intense pressure keeps the aquifer from reflating even when water is pumped back into it.

Sinkholes:According to the United States Geological Survey USGS), sinkholes occur when the rock below the land is limestone, carbonate rock, salt beds or any kind of rock that can be easily dissolved by (usually acidic) groundwater that flows through them.  This dissolving of rock thus creates spaces underground, which can be small or become so large that sinkholes are created because there is no longer enough rock below to hold up the land surface.  Thus a dramatic collapse can occur, which is especially dangerous if a building or person is above it at the time. 

Subsidence sa Maguila-guila Siltation Dam ang panibagong banta ayon sa MGB-MIMAROPA

May hindi inaasahang pangyayari na naganap sa mismong gitna ng Maguila-guila Siltation Dam sa Mogpog. Ang tawag dito ay subsidence o pagguho at paglikha ng butas sa gitna ng dam na imbakan ng mine tailing. 

Napag-alaman na nagsimula ang laki ng butas na 3-metro ang lapad at ito ay sa ilalim nagmula. Sa kasalukuyan ay lumawak na ang nasabing subsidence hanggang sa umabot na ito sa luwang na 60-metro at maihahambing ang hugis sa isang imbudo.

Ang pagluwang at tuluyang paglalim ng gumuhong bahaging ito ng siltation dam ay naganap noong Hunyo 2016, kasunod ng walang humpay na pag-ulan. Nasundan ito ng mabilis na pagdaloy ng tubig hanggang malimas ang tubig mula sa 8-metrong lalim ng butas. Ang mabilis na pagdaloy ay nagtulak naman para mabilis ding makapasok ang mga madaling matibag na mga sedimento papunta sa loob ng drain tunnel na may habang 90-metro.

Tinatayang ang kondisyon na ito ay mas malaking problema ngayon, sa kadahilanang ang pagguho ay patuloy na lumalawak at lumalalim pa at kakikitaan ng maliit na paglawa sa ilalim ng gumuhong bahagi.

Ito ay naging sanhi rin ng pagbabara sa drain tunnel at ng mga karagdagan pang solidong sedimento sa bahagi ng ilog malapit sa labasan ng tubig. Kapag hindi naagapan, lilikha ito ng banta sa mga katabing istruktura at mga lugar, lalo na ang potensyal na pagguho ng apektadong bahagi ng Maguila-guila earth dam. Kasama na rin dito ang mas malawakang pagbaha papuntang dulo ng Mogpog River.

May mga nirekomendang remediation measures ang  MGB Team para mabawasan ang mga usaping magiging sanhi ng pangyayaring ito. Kasama rito ang pagsasagawa ng karampatang engineering works, at ang maaring pagpapatayo ng mga gabion barriers sa ibabang bahagi ng Siltation Dam malapit sa Brgy. Bocboc. Ito ay para makontrol ang pagbuhos ng siltation mula sa pinagmumulan nito sa itaas na bahagi.

Mababasa sa ibaba ang kabuuhan ng report ng Mines and Geosciences Bureau-MIMAROPA.

Subsidence in the Maguila-guila Siltation Dam of Marcopper: a new threat to Mogpog riverine environment according to MGB-MIMAROPA

by MGB-MIMAROPA Region

The Mines and Geosciences Bureau (MGB)-MIMAROPA is now closely monitoring the development of subsidence caused by sediment collapse in the reservoir floor of Maguila-guila Siltation Dam, a zoned earthen dam of Marcopper Mining Corporation (MMC), located in Sta. Cruz, Marinduque.

The subsidence was first observed on February 2016 as a 3–meter diameter crater that appeared in the middle of the sediment storage facility of the dam, which primarily catches unconsolidated sediments (mostly sand and gravel) from mine waste dumps, and other fluvial materials derived from upstream sources. The collapse initiated at depth but then propagated to the surface producing major subsidence features on the overlying alluvial sediments. Based on several on-site assessments, MGB-MIMAROPA monitoring team deduced that under the influence of water (flood water and groundwater), the high porosity of the sediments gave way to seepage or subsurface flow, which consequently led to piping action and concentrated internal erosion. This is followed by gradual collapse due to backward erosion as sediments and floodwaters entered the damaged down-drain and silted drain tunnel of the Maguila-guila Dam. The damaged section of the drain is attributed to continuous corrosive effect of acid drainage on the concrete wall of the drain structure.

Since then, surface and internal erosion together with active subsurface flow have drastically expanded the size of the alluvial collapse and created a 60-meter wide funnel-like depression on the surface of the impoundment area. Increased runoff and seepage led to eventual ponding of the depression and development of deep narrow incised channels as a consequence of repeated surface erosion by active stream flow. Continuous widening and deepening of the collapsed area occurred on June 2016, following incessant rains, which resulted in major subsurface outflow that nearly drain the water from the 8-meter deep depression. This fluvial action is triggered by high pressure water that created large seepage forces toward the damaged drain structure allowing highly erosive sediments to enter the 90-meter long drain tunnel of the dam.

Recently, the condition of subsidence poses more problems as the collapse continues to widen and deepen with minor ponding at the bottom of the depression. Active erosional processes have caused immediate clogging of the drain tunnel and increased sedimentation of the river channel near the drain outlet.  Such events impose serious threats on adjoining structures and areas, most specifically, potential erosion or damage of the disturbed section of the Maguila-guila earth dam, possible breakdown of the drain structures, and brisk siltation and widespread inundation problems at downstream Mogpog River. 

To help alleviate some of these issues, the team recommended several remediation measures such as immediate replacement or repair of the damaged drain structure, implementation of major dredging activities to manage the flow of water along the Maguila-guila River valley, and employment of suitable engineering works to stabilize the collapse. It was also suggested that gabion barriers be constructed at the downstream portion of the Maguila-guila Siltation Dam near Brgy. Bocboc as an additional measure to control the upsurge of siltation from upstream sources.

Gabions are used in many situations including the stabilization of earth movement and erosion, river control, reservoirs, etc.

With the increasing threat of massive siltation and flooding in the riverine environment of Mogpog, MGB-MIMAROPA is constantly advising Marcopper Mining Corporation (MMC) to address the problems regarding subsidence and siltation in the Maguila-guila Siltation Dam. MMC was also instructed to comply with the action plans and recommendations prepared by the monitoring team. On June 2016, a special seminar was conducted by MGB-MIMAROPA Geohazard Team to educate the officials of LGU- Mogpog on the possible adverse impacts of subsidence caused by sediment collapse in the reservoir floor of the siltation dam. Hazard advisories were then re-issued in critical barangays along Mogpog River.