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By Moneth G. Deposa, Researcher
Conclusion
Navotas Mayor Tobias Tiangco definitely agreed
on the principle behind the Camanava flood-control project, but not
its design.
For Tiangco, building a dike and drainage system
is the right approach in preventing floods in low-lying areas like
his town and neighboring Malabon. “Whether they copied the design
correctly in Holland, where it is based, or they designed it
correctly, I cannot comment on that,” he said.
His only concern is that if flaws are proved,
they should be corrected so that construction can begin. He said the
camanava dike that has been discussed is not the entire design of
the flood-control project, but its first phase.
In the first phase of the project, Navotas would
be protected from waters coming from its river because of the
proposed navigation lock on the river’s mouth. But Tiangco fears
that the water from Manila Bay will continue to pour because the
coastal dike needed in this area was not included in the first
phase, where he thinks it should be.
Tiangco feels that the phases of the project
should be completed at once to prevent financial losses.
He recalled his first meeting with Kin, who said
that the coastal dike along Manila Bay was not necessary. The
designers defended their work and persuaded the public works
department of the implementation of the implementation of the
project. The problem is, “Wala raw silang pondo para ituloy
‘yong second phase ng project. Dapat raw maumpisahan muna ang
first phase bago sila makakuha ulit ng pondo for the second phase
[They have no funds to continue the second phase of the project.
They should start the first phase before they can get funds for the
second].”
On Rodolfo’s assertion that the project has
flaws, Tiangco said it is the department engineers who can answer
that, “because they did it based on engineering computations.”
Unlike Tiangco, however, Malabon Mayor Amado
Vicencio agreed on the project “design” from the very start.
That’s why he expressed disappointment at the delay of its
groundbreaking. Vicencio said, “Whatever changes or adjustments
are made on the design should be completed the soonest possible
time.” He is worried that his constituents may have the same
problem.
Asked about the possibility of cancellation
owing to some unexpected modifications, Vicencio said Fernando
assured him that the project would push through sometime this year.
“My only concern is the immediate
implementation of this project because it is the ultimate hope of
all the residents of Malabon,” Vicenco said.
Why postponed?
Owing to reports that the groundbreaking for
this phase was postponed on March 12 because of Rodolfo’s
criticisms, the Department of Public Works and Highways showed a
letter from the Office of the President informing the department of
the sudden change in her schedule.
The designers waited for any formal
communication from President Arroyo regarding the design of the
project after they sent copies of their response to Rodolfo’s
criticisms, but until now the President hasn’t raised any question
or ordered a re-evaluation of the project. The consultants believe
her attitude implies that she is convinced that the design is
appropriate for the existing condition.
Design concept and conditions
In finalizing the concept of hydraulic
structures to be built in the project area, experts said some
countermeasures should be considered in making the detailed design,
especially those of the pumping station, control gate and floodgate,
since it is recognized that land subsidence has taken place and will
continue for sometime in the project area.
On the other hand, inadequate data make it
difficult to make an accurate projection of future land subsistence,
although the past subsidence occurred in the site. As a result,
experts proposed that projection of land subsidence be tentatively
made on the basis of a rather moderate speed (0.65 cm/year x 30
years = 20 cm) and that the initial projected land subsidence of 20
cm would be incorporated into the structural design.
For the river dike, revetment and polder dike,
it is technically possible to raise them in accordance with the
actual subsidence, which will take place in the future, on condition
that close observation should be made. Therefore, future land
subsidence would not be considered critical for the design of such
structures as the dike, revetment and wall.
The soil mechanical investigations along the
proposed polder dike have revealed a thick layer of Holocene
deposits of 5 cm to 31 m, consisting of loose sand and very weak
clay with high water content. In view of this condition, experts
pointed out that bearing layer for heavy structures as pumping
station, flood-control gates and navigation gate should be a
Pleistocene layer so that pile foundations should be adopted.
Holocene deposits should mainly consist of very soft clay so that
construction of a new polder dike shall be designed based on
stability analysis and consolidation settlement. Experts said there
should be no continuous drainage layer between a lower part of the
soft clay and Pleistocene hard clay layers so that it will take a
long time for the soft layer to attain complete consolidation after
the construction of the polder dike. Therefore, it will be necessary
to use some soft layer stabilization methods to improve the ground
or to conduct proper maintenance for securing the crown height of
the polder dike against settlement.
According to the stability and consolidation
analysis of the basic plan for the proposed polder dike, during the
third and tenth year after the embankment is completed, the
settlement will be 20 cm to 40 cm with over 10 m in thickness.
Therefore, the maintenance every five years is considered to keep
the design crown height. This means that the crown of the polder
dike is expected to be lower by 4 cm to 7 cm a year due to the
consolidation of the foundation layer, or around 6 to 7 times the
moderate sinking speed of 0.65 cm a year, which has been the basis
for hydraulic structures with pile foundations in the project as
well as in conformity with Philippine practices. In the project
therefore, an extra embankment of 0.3 cm is considered a
countermeasure for initial settlement, providing for regular
maintenance work according to the actual settlement in the future.
And, to minimize surcharge loads of self-weight and traffic volume,
a simple and relatively minimal cross-section shape is used on
condition that mobile traffic shall not pass regularly.
Land subsidence, ground-water extraction
relationship
Close relation between land subsidence and
ground-water extraction has been observed and technically recognized
in many countries all over the world. And, as a countermeasure
against land subsidence, regulations and guidelines for restricting
the use of ground water have been drawn up and carried out.
Therefore, such an administrative control system should be
considered strictly in the project area so as to reduce the speed of
land subsidence due to ground-water extraction, although the main
cause of land subsidence in the area is expected theoretically due
to the distribution of thick soft clay layers which are more
significant and serious than ground-water extraction.
Experts find it technically difficult to
accurately project future land subsidence, even though the point
land subsidence observation is conducted regularly. If observation
points are far from the project site, it is rather difficult to
analyze such data for the projection of future land subsidence.
Underground water occupies space between the
sediment particles. When water is pumped from an aquifer, it
develops empty spaces between the particles, which then become
tightly packed. Continued pumping of ground water without adequate
replacement increasingly compresses the sediment, making the upper
surface settle or subside.
The land subsidence/settlement process is sped
up when ground water is extracted from the sedimentary deposits
faster than it can be replenished (i.e. ground-water withdrawal
exceeding recharge capacity). On the other hand, compaction of
sediment, which also causes land subsidence, is sped up by the
weight of upper layer deposits.
Proposed polder dike
The present area to be surrounded by the
proposed polder dike, which is 8.5-km long, is mostly used by
fishponds. This is administered as part of a developing area of
Malabon City. The target for future land use is year 2020, projected
throughout the detailed design study, which shows the areas to be
converted from fishponds to residential and industrial after
reclamation. The land use in 2020 is a practical study basis of
technical and social conditions for the project’s viability.
To meet the structural requirement of the polder
dike, which serves as a water barrier, the engineers studied
alternative designs. According to design experts, the existing
design of the polder dike, providing for the regular maintenance and
a construction cost of P18,000 for the dike a meter long (or a total
of P150 million), is substantially more economical than soil
treatment at foundation with a unit cost of P125,000 a meter long
for the structural design with reliable pile foundation,.
Who has the final say?
Carrying out the first phase of the Camanava
flood-control project depends on Secretary Fernando, and many are
asking the effects the project may bring to the people and the
country’s relationship with Japan. People are worried it may
affect the country’s future requests for economic assistance from
the Japanese government. But residents of Camanava are optimistic
that concerned people and agencies in this project could resolve all
issues to finally get it going.
Part 1
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