THE MIBATAS (MABATAS) TAILINGS
IMPOUNDMENT FACILITY
Sitio
Mibatas, Brgy Upper Ulip, Municipality Of Monkayo, Compostela Valley Province
The
site of the Phase I Mibatas (Mabatas)
Interim Tailings Disposal Facility1 of the Diwalwal Direct State
Development Project (DDSDP) will cover some three (3) hectares
of the Mibatas Creek drainage area within public forest land (NOT A PROTECTED FOREST) in a logged over area
of PICOP Resources, Inc.. An earth dam across the valley in the creek’s upstream
portion is under construction. The
upstream of the dam will be used as an impoundment area for tailings coming
from the processing of the ores from the Diwalwal mines. At a later time, Phase II Mibatas (Mabatas) Final Tailings Disposal Facility will be
constructed and will cover some 35 has. in the
same general area. It shall cover the 3
has. of the interim facility. The
larger dam axis of the final facility will be constructed downstream of the
interim tailings dam axis, about 1 km. from the confluence of the Mibatas Creek
and the Naboc River.
The
8,100 has. of the Diwalwal Mineral Reservation and Environmentally Critical
Area2 was part of the 1.9 million has. Agusan-Davao-Surigao Forest Reserve established in 1931 under Proclamation 369. The portion removed is only 0.42% of the
Forest Reserve's original area. The
Mineral Reservation falls within the expired Timber License Agreement (TLA) No.
43 of PICOP Resources, Inc. which is being applied for conversion into an
Integrated Forest Management Area (IFMA).
Only 0.004% (35 has. of the 8,100 has.) of the mineral reservation will
eventually be covered by tailings.
The
proposed tailings impoundment structure is a zoned embankment with an
impervious clay core and outer shell of random fill (2 lines of defense against
seepages)3,5. It will be
provided with a cut-off trench, filter, blanket drain, toe buttress and spillways.3 All these features and appurtenant structures are measures to
control any contamination/pollution that may come from the dam area.
The
dam will be designed to comply with the National Structural Code of the
Philippines. Proper seismic design
parameter of at least 0.4 will be used in the slope stability analysis of the
proposed tailings dam under seismic loading.
The
Monkayo Municipal Government had considered five (5) sites as zones for mineral
processing plants and tailings facilities.
All are along/close to the national highway within the Monkayo
Municipality. Three of the sites
(Pasian, Rizal and Jagumitan) are northwest of Mibatas while two (Banlag and
Olaycon) are west of Mibatas. On technical
study by the MGB, the sites evaluated were found to be unsuitable. There is lack of appropriate area for a
tailings facility in Pasian, Rizal and Jagumitan and limited storage volume in
Banlag. The integrity of an existing
tailings dam in Olaycon needs to be reviewed3.
In
view of the lack of basic requirements for a good mineral processing zone and
tailings impoundment area, a search for other potential sites was done.
The
MGB made an investigation of the general area of Mibatas about 3.5 kms. west of
the Diwalwal mining area. After a
thorough investigation/analysis by MGB of the general area and three possible
locations of the dam axis, a site was selected. This site is the best/most
environmentally sound choice as it meets most of the criteria set for tailings
dam siting. The site’s near surface is semi impervious as indicated by the
swampy conditions and the occurrence of argillized (clayey) and silicified
zones.3 This means that any
liquid from the tailings will not easily percolate to the ground and affect the
groundwater. Additionally, the site is much closer to the Diwalwal mine
area compared to the approximately 30 kms. distance to the other sites
considered, thereby keeping possible environmental impact within a smaller zone
and reducing hauling costs.
An
area east of the reservoir shall be developed for a government owned and
operated Carbon-in-Pulp/Carbon-in-Leach gold processing plant, and other
mineral processing facilities that will be allowed to be relocated from their
present locations. The centralized MPZ
and tailings impoundment will mitigate siltation/contamination along Naboc
River, tributaries in the mining area and the downstream farmlands. As the Mibatas area drains to the
already-affected Naboc River, it will not affect any other stream and
watershed. For instance, the adjacent
watershed of Ulip River will not be affected since it is not downstream of the
tailings impoundment area.
There
is no active fault in the Mibatas area.
Neither is there an active fault capable of generating a near-field
earthquake. There is no risk of ground
rupture. The foundation material is
bedrock that is neither liquefiable nor prone to differential settlement. There is no geomorpholgical evidence to
suggest the presence of old landslides.4 Since any earth movement associated with the construction of the
tailings dam might alter the present stability, the modification of the
landscape has been considered in the design of the tailings impoundment
facility.
Mibatas
is a plateau-like area3 within the Mineral Reservation. The topography is ideal. This is the site
that has the best storage volume and embankment volume to storage volume ratio3. The terrain and a sufficiently narrow
valley allows for the construction of a dam that can be used to hold back the
expected volume of tailings from the Diwalwal mines. The engineered dam can withold a maximum of about 5.3 million
cubic meters of tailings3 at maximum design height.
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1 The Phase
I Mibatas (Mabatas) Interim Tailings Disposal Facility is covered by ECC
12-03-07-0146-5010 issued on 29 July 2003 by EMB XI RD and Special Land Use
Permit (SLUP) No. RXI-024-2003 issued 29 July 2003 by DENR XI RED
2 EO 297 dated 25 Nov. 2002
3 Initial
Site Selection for a Proposed Mabatas
Tailings Dam, Diwalwal, Monkayo, Compostela Valley Province (Jan. 2003) -
by Ramon D. Quebral
4 Seismic
Hazard Assessment for the Proposed Mabatas Tailings Dam, Diwalwal, Monkayo,
Compostela Valley Province (Unfinished draft, July 1 2003) - by
Ramon D. Quebral
5
Engineering Geological and Geotechnical Engineering Report for the Proposed
Mabatas Tailings Dam, Diwalwal, Monkayo, Compostela Valley Province (Unfinished
draft, July 1 2003) - by Ramon D. Quebral