I. INTRODUCTION
In recent years, it has been becoming
clear that the extensive exploitation of groundwater resource has been bringing
with it an undesired environmental side effect. At many localities in the
world, groundwater pumpage from unconsolidated aquifer-aquitard system has been
accompanied by significant land subsidence (Osaka: maximal land subsidence: 3
m, area of land subsidence: 190 km2, duration: 40 years; Tokyo: maximal
land subsidence: 4 m, area of land subsidence: 190 km2, duration: 50
years; Mexico City: maximal land subsidence: 9 m, area of land subsidence: 130
km2, duration: 32 years; Santaclara (California): maximal land
subsidence: 4 m, area of land subsidence: 650 km2, duration: 50
years; Las Vegas (Nevada): maximal land subsidence: 1 m, area of land
subsidence: 500 km2, duration: 28 years; Water Supply factory (Pháp
Vân, Hà Nội): maximal land subsidence 228.6 mm, from 1988 to 1994.
Pumping is from sand and gravel aquifers and the pumping always is accompanied by vertical leakage from the adjacent aquitards consisting of clay, which is composed of high-compressibility material. As a consequence, the pumping leads to compaction or consolidation. Consolidation is the gradual reduction in volume of a fully saturated soil, often of low permeability, due to drainage (for example: groundwater abstraction) of some of the pore water. The consolidation process continues until the original pore water pressure has completely disappeared, where it is replaced by an equivalent amount of intergranular stress in order to keep the total stress constant. The increase in intergranular stress causes the compaction.
In this paper, calculation of land
subsidence in the study area is presented.
II. CALCULATION OF LAND
SUBSIDENCE AFTER TERZAGHI’S THEORY
The simplest way of description is that
of one-dimensional consolidation. Consolidation settlement is the vertical
displacement of the surface corresponding to the volume change at any stage of
the consolidation process. Consolidation settlement will result, for example,
if a structure is built over a layer of saturated clay, or if the water table
is lowered permanently in a stratum overlaying a clay layer.
The assumptions made in Terzaghi’s theory
[5] of one-dimensional consolidation are:
- The soil is fully saturated;
- The soil particles are incompressible;
- Compression and flow are
one-dimensional (vertical);
- Strains are small;
- The coefficient of permeability and
coefficient of volume compressibility remain constant throughout the process;
- There is an unique relationship,
independent in time, between void ratio and effective stress.
The main limitation of Terzaghi’s theory
arises from the last assumption. Experimental results show that the
relationship between void ratio and effective stress is not independent in
time, i.e., there is hysteresis.
Suppose that the layer is subjected to a
decrease in groundwater head h.
According to the Terzaghi’s theory, the
vertical settlement L
of the clay layer can be calculated by the formula:
(1)
where: c - coefficient of consolidation.
P - change in effective stress in the solid
material (in kPa), but on other hand:
P = *g*h
- density of water (in kg/m3)
g = 10 m / s2 - gravity constant
h
- drawdown (in metre)
L
- compression of the considered layer (in metre)
L - thickness of the layer (in metre)
P - initial effective stress which is
calculated by the following expression: weight of the overlaying formations +
1/2 own weight - water pressure
Drawdown (h) and coefficient of
consolidation (c) are used according to Geological Party 47 [4].
III. PUMPING ACTIVITY
IN THE STUDY AREA
Mớ Đá mineral water source is
situated in Vĩnh Đồng and Hạ Bì communes, in the
northwest of Kim Bôi townlet, stretching along NW-SE direction. Area of the
source is about 9 km2. A net of pumping wells was put into
operation, withdrawing mineral water for treatment, domestic purposes, etc..
Most of the wells are concentrated around the Kim Bôi Rest Home, consisting of:
LK.14, LK.12, LK.13, LK.7, LK.11, LK.8 and LK.5B. Besides the central part,
there are LK.3 and LK.6 in the NW and LK.4 in the north of the Mớ Đá
mineral source. Most of the wells pump mineral groundwater from Cẩm Thuỷ
(P3 ct) water-bearing
formation, at a depth from 140 to 150 m. The water head in these wells rises 10
m above surface. At the outcrops ĐL.133 and ĐL.145, mineral
groundwater of Cò Nòi (T1
cn) Formation was
revealed. According to the Geological Party 47, just flow discharge of the
ĐL.133 (4.86 l/s) can satisfy the total demand of mineral water for local
use and bottling (about 110 m3/day).
Pumping activity of the well LK.7 gives best quality of the water, because there is a filter clay layer above, that can prevents pollution, but after long time of exploitation can causes land subsidence. It is clear that, the land subsidence can occur in Quaternary deposits in a stratigraphical column of the well. Thickness of the Quaternary deposits is 25 m [4], in which thickness of upper part is 7 m, mainly consisting of clay, sandy clay, clayish sand; the lower part is 18 m of thickness, consisting of spilite boulder, pebble, gravel, etc..
IV. CALCULATION OF LAND
SUBSIDENCE AT SOME PUMPING WELLS
1.
Calculation of land subsidence in the well LK.7
Parameters of the wells LK.7, LK.3 and
LK.9 are illustrated below (Fig.1).
As mentioned above, Quaternary deposits of the well LK.7 is 25 m thick and consists of 7 m of clay layer in the upper part and 18 m of spilite boulder, pebble, gravel, etc. in the lower part.
- Land
subsidence of 7 m of clay layer in the upper part:
