6 | Section 7.2 Ground Subsidence

DRRR 11 Q2

Ground subsidence

• _______________ is the relative lowering of the earth’s surface usually with respect to the mean sea level.

1. dissolution and collapse of limestone

2. excessive groundwater withdrawal

3. mining

4. oil and gas extraction

5. earthquakes

6. change of season

• Ground subsidence can be a result of any of the following:

1. Carbonate dissolution and collapse

2. Excessive groundwater withdrawal

3. Extraction of oil and natural gas

4. Earthquakes

5. Isostatic rebound

6. Change in season

• List the causes of ground subsidence

Carbonate dissolution and collapse

• This is a common cause of subsidence especially in the Philippines which has an abundance of limestone terrains due to its geographic setting and extensive coastlines.

mineral calcite (CaCO₃)

• Limestone which is composed of the ____________ and possibly other carbonates and fossils, is easily dissolved by even slightly acidic water.

• carbon dioxide

• water vapor

• Acidic rainwater in nature is formed by the mixing of ___________ and __________ in the atmosphere.

voids

• When acid rainwater seeps into the ground, limestone is dissolved little by little and creates ____.

Acidic fluids

• ___________ could also be introduced by leaking sewage systems or water supply and distribution networks.

CaCO₃ + H₂CO₃ = Ca⁺² + 2HCO₃^-2

• The following simplified equation below shows how limestone is dissolved by carbonic acid (CaCO3) from the atmosphere:

sinkhole

• The subsurface usually becomes extremely weakened due to the creation of voids and reaches a point that it is no longer able to support the volume of rock on top of it.

• As a result, the ground surface either lowers slowly or collapses rapidly and swallows cars, houses, and other property.

• The depression formed is called a _______.

Excessive groundwater withdrawal

• ____________________ is also a main cause of subsidence in the Philippines, particularly in the CAMANAVA area of Metro Manila and in a lot of other highly populated urban centers in developing countries worldwide.

• Caloocan

• Malabon

• Navotas

• Valenzuela

• Enumerate CAMANAVA:

perennial flooding

• Subsidence in CAMANAVA is manifested by _______________.

Extraction of oil and natural gas

• The withdrawal of these fluids is mainly anthropogenic but can also be caused by natural processes such as by outflow through springs.

Earthquakes

• Aside from the relative subsidence measured on the hanging wall of a normal fault or the footwall of a thrust or reverse fault, another situation in which ground subsidence may occur is during liquefaction, which takes place in areas with fine-grained sediments or areas that were reclaimed.

• appearance

• amount

• areal dimensions

• Underground mining methods sometimes intentionally cause the collapse of the surface in which the _________, _______, and __________can be predicted.

Isostatic rebound

• The Earth’s crust which is floating on the asthenosphere, reacts to the addition or removal of load on its surface.

uplift

• If a large mass is removed, through weathering and erosion or melting of large ice sheet glaciers, ____ takes place.

isostatic subsidence

• On the other hand, areas which receive large deposits of sediments like valleys and bodies of water tend to become even lower in elevation because of ____________.

Change in season

• ___________ has an effect on clays which have are types of mineral that have absorptive property.

• They can absorb water and expand by allowing water to occupy (and leave) the space within their sheet structures.

desiccants

• It is for this reason that clays are used as ingredients in the production of ________ for storage of many products in which maintenance of a certain level of moisture is important.

1. expand

2. shrink

1. During wet season, clays in the soil tend to ______.

2. But during the dry season these _____ again.

cracking and foundation failure

• Buildings and other property resting on this type of ground may suffer ________________ if these are not designed to with such situations.

effect

• Due to its slow rate, subsidence may not have the terrifying effects of other natural disasters like volcanic eruptions and earthquakes but its _____ may last for a long period of time.

catastrophic

• Sudden ground collapse events can be _________ but are usually isolated and very limited in expanse.

Slow subsidence

• ___________ can proceed almost unnoticeably but may still cause serious, widespread damage to property (Figure 7.2-5).

real-time subsidence monitoring systems

• It is hard to determine the exact time and location of a subsidence-related disaster unless ______________________ can be deployed in areas which are subsidence-prone.

• map out the areas which are subsidence-prone

• avoid development in the area

• adapt by modifying or strengthening structures

• A more pragmatic approach is to ___________________ and to either ________________________ or _________________.

elevated

• Areas which experience subsidence are prone to severe, extensive, and prolonged flooding so structures and roads, for instance, could be ________.

• Dikes and pumping

• drainage systems

• _______________ or _____________ can also be constructed.