Subsidence Phenomena - Environmental Geomechanics - page 115

evident. Depending on the nature of the exploited layers, intensive pumping can
cause a decrease in thickness, which usually results in surface lowering and
occasionally leads to spectacular consequences for public property and
infrastructures.
Several cases of subsidence caused by water extraction have been recorded and
well documented by field data: in this chapter we refer to the most spectacular ones.
Inside the brackets the measured subsidence, depth of predominantly exploited
layers, surface effected by lowering and time span during which the effects have
been observed are indicated, respectively. When the last year is followed by a +, the
phenomenon was still occurring at the time.
We begin with the county of Cheshire in England (15 m, 100-300 m, 1,500 km 2 ,
1533-1977), then Haranomachi City in Japan (2 m, 100-200 m, 25 km 2 , 1965-
1978+), Tokyo (4.59 m, 0-400 m and 800-2,000 m, 3,240 km 2 , 1918-1975+),
Wairakei, New Zealand (6-7 m, 250-800 m, 30 km 2 , 1952-1975+), Santa Clara
Valley, California (4.1 m, 50-330 m, 650 km 2 , 1918-1970), San Joaquin Valley,
California (9 m, 60-900 m, 6,200 km 2 , 1930-1975), Tulare-Wasco, USA (4.3 m,
60-700 m, 3,680 km 2 , 1930-1970), Houston-Galveston, USA (2.75 m, 60-900 m,
12,000 km 2 , 1943-1973+), Mexico City (9 m, 0-50 m, about 225 km 2 , 1891-1978),
Po River Delta, Italy (3.2 m, 100-600 m, 2,600 km 2 , 1951-1966), Ravenna, Italy
(1.2 m, 80-500 m, about 600 km 2 , 1955-1977+) and Venice, Italy (0.15 m, 70-350
m, 400 km 2 , 1952-1970). Wairakei is a geothermal field while the subsidence in
Ravenna is caused by the combined effect of water withdrawal from subsurface
layers up to a depth of 600 m and gas extraction from deep reservoirs (about
1,800 m), see Chapter 7. In contrast, in the case of Po River Delta, water containing
dissolved gas was pumped out and, once the gas was recovered on the surface by
separation due to the decrease in pressure, the water was discarded in the sea.
For detailed information on these and similar cases, the interested reader is
referred to the UNESCO Guidebook to Studies of Land Subsidence due to
Groundwater Withdrawal [POL 84].
5.1.2. The mathematical model
From the mechanical point of view, a decrease of interstitial pressure caused by
water extraction involves an increase in the effective stress while the total stress
induced by the overburden remains constant. This increase causes a deformation
(compaction) of the strata that continues until the system reaches a new hydraulic
equilibrium. Hence, this is a hydro-mechanical problem for a medium composed of
a deformable solid skeleton and one or several fluids saturating the interstitial voids.