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The Land subsidence problem in the coastal area in Taiwan
had raised the attention of land owners,government officials,
engineers,and reseachers. Sinceearly 1980's on, the fish farm
owners in the coastal area began pumping tremen-dous amount of
ground water. This particular practice lead to the disastrous
result of land subsidence. According to statistics, nearly 2/3
of the total coastal areas are affected. Although ground water
pumping in these areas has reduced due to the recession in the
fish farming business, the subsidence did not stop as a
consequence. A lot of reseachers used traditional elastic theory
or consolidation theory to predict the magnitude and the rate of
subsidence,butin most cases they under predict both.This reseach
focus on the physiochemicalcharacteristics of soil, i.e., the
effect of the change in pore water chemistryon the soil
properties and the consolidation behavior, which is one of the
area that all of the previous reseaches failed to include. The
concentrationof salts in pore water undergo an increase as sea
water intrudes into thecoastal area due to the lowering of fresh
ground water table as a result ofover pumping. The research
looks into both the magnitude and the rates ofosmotic
consolidation of fine grained soils due to the increase of
salinityof pore water. Through the understanding of the
physiochemical mechanism of consolidation, the endeavor to find
a solution to land subsidence problem ismore likely to succeed.
After performing fundamental physical characteristicstests,
consolidation tests, and permeability tests with three fine
grained soilsamples, we found that when the pore water changed
into sea water or brine water, the values of liquid limit and
the plastic limit of all soils decreases. Osmotic consolidation
took place during the process in which the pore waterchanged
into sea water or brine water in the consolidation tests.The
volumetricstrain of osmotic consolidation related to the
thickness of soil samples isabout 0.5%-1.5%. Therefore if there
is a thick clay layer that of highly plastic minerals, the
settlement that caused by osmotic consolidation should be taken
into account.Except osmotic consolidation, the compressibility
of soil increases with the change of pore water properties
caused by intrusion of sea water. On the other hand, recharging
fresh water into the soil layer that intruded by seawater would
not hardly change the compressibility of the soil. In addition,
the amount of rebound of the soil layer is minimal.
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