Manila and Pasig City are under the threat of soil liquefaction, but Philippine Institute of Volcanology and Seismology (Phivolcs) director Renato Solidum assures that buildings that are well designed and properly constructed are safe from soil liquefaction in the event of a strong earthquake.
Liquefaction is the process by which wet sediment starts to behave like liquid. It occurs because of the increased pore pressure and reduced effective stress between solid particles generated by the presence of liquid. It is often caused by severe shaking, especially that associated with earthquakes.
Solidum said coastal areas in the cities of Marikina, Pasig, Taguig, Caloocan, Malabon, Navotas, Marikina, Muntinlupa, Pasay, Las Piñas and the municipality of Pateros are prone to liquefaction.
In an interview, Solidum said the threatened communities include tens of thousands of people living in communities along Manila Bay such as the Baseco compound in Tondo. He said the areas prone to liquefaction also include reclaimed lands such as portions of Roxas Boulevard.
These findings, he said, were part of the 2004 study conducted by Phivolcs along with the Metro Manila Development Authority (MMDA).
Soil liquefaction mitigation techniques
A structure that possesses ductility, has the ability to accommodate large deformations, adjustable supports for correction of differential settlements, and having foundation design that can span soft spots can decrease the the amount of damage a structure may suffer in case of liquefaction (Committee on Earthquake Engineering, NRC, 1985). To achieve these features in a building there are various aspects to consider.
Shallow foundation Aspects
well constructed foundation It is important that all foundation elements in a shallow foundation is tied together to make the foundation move or settle uniformly, thus decreasing the amount of shear forces induced in the structural elements resting upon the foundation. The photo to the right shows a house wall under construction in Kobe, Japan. The well-reinforced perimeter and interior wall footings (KG) are tied together to enable them to bridge over areas of local settlement and provide better resistance against soil movements. A stiff foundation mat (below) is a good type of shallow foundation, which can transfer loads from locally liquefied zones to adjacent stronger ground.
Strong foundation mat
broken pipes Buried utilities, such as sewage and water pipes, should have ductile connections to the structure to accommodate the large movements and settlements that can occur due to liquefaction. The pipes in the photo connected the two buildings in a straight line before the earthquake (KG).
Deep foundation Aspects
Liquefaction can cause large lateral loads on pile foundations. Piles driven through a weak, potentially liquefiable, soil layer to a stronger layer not only have to carry vertical loads from the superstructure, but must also be able to resist horizontal loads and bending moments induced deep foundation by lateral movements if the weak layer liquefies. Sufficient resistance can be achieved by piles of larger dimensions and/or more reinforcement. It is important that the piles are connected to the cap in a ductile manner that allows some rotation to occur without a failure of the connection. If the pile connections fail, the cap cannot resist overturning moments from the superstructure by developing vertical loads in the piles (see figure below).
Pile cap failure