170 Ground improvement by deep vibratory methods
4.7 CASE HISTORIES
4.7.1 Wet vibro replacement stone columns
for a thermal power plant
For a new 1500 MW coal-fired thermal power plant in southern India, all
ancillary structures such as cooling towers, service buildings, and special storage tanks were founded on improved ground, because the original soil had
insufficient characteristics to carry even moderate loads. It consists of soft
clays to a depth of 7–10 m followed by layers of silty sands and silts whose
density increases with depth, and that are resting on dense sand at a depth of
about 20 m below grade. The water table is generally close to ground surface.
Standard penetration test (SPT) N-values are between 0 and 2 in the upper
7 m rising to N = 5 at about 10 m depth and then gradually increasing to
well over N = 30 at a depth of 20 m with values in excess of N = 50 below.
The vibro replacement scheme that was proposed for structures such as
tanks, cooling towers, and service buildings consisted of stone columns of
lengths between 7 and 10 m, which were arranged in triangular grids of 2
and 2.5 m equal spacing. Table 4.10 provides details of the structures, their
dimensions and loads, and of the ground improvement scheme adopted.
In 2008, the construction of the stone column foundation was finished, amounting to 285,000 lin.m for the whole project. The vibro stone
columns were executed with a diameter of 0.9 m by the wet vibro replacement method employing the Keller M vibrator and using 12/75 mm crushed
stone aggregate as backfill material. Its gradation was characterized by
D 60 /D 10 value of 4, with less than 5% of the material passing the 12 mm
sieve. Process water was channeled into silt ponds before it was released
into a nearby creek. Quality control was performed with a standard data
0.5
8
1.0
0.0
10
6
4
2
0
4
3
2
1
0
t/l
n = σ
c /σ
s
n
c = σ
c /σ
λ = 1
λ = 0.56
λ = 0.22
λ = 1
λ = 0.56
λ = 0.22
(a)
0.5
1.0
0.0
t/l
(b)
Figure 4.36 Stress concentration factors (a) n and (b) n c for a rigid square foundation
(B = 8 m) on 25 stone columns for σ c = 0.32 and φ c = 35°.
4.7 CASE HISTORIES
4.7.1 Wet vibro replacement stone columns
for a thermal power plant
For a new 1500 MW coal-fired thermal power plant in southern India, all
ancillary structures such as cooling towers, service buildings, and special storage tanks were founded on improved ground, because the original soil had
insufficient characteristics to carry even moderate loads. It consists of soft
clays to a depth of 7–10 m followed by layers of silty sands and silts whose
density increases with depth, and that are resting on dense sand at a depth of
about 20 m below grade. The water table is generally close to ground surface.
Standard penetration test (SPT) N-values are between 0 and 2 in the upper
7 m rising to N = 5 at about 10 m depth and then gradually increasing to
well over N = 30 at a depth of 20 m with values in excess of N = 50 below.
The vibro replacement scheme that was proposed for structures such as
tanks, cooling towers, and service buildings consisted of stone columns of
lengths between 7 and 10 m, which were arranged in triangular grids of 2
and 2.5 m equal spacing. Table 4.10 provides details of the structures, their
dimensions and loads, and of the ground improvement scheme adopted.
In 2008, the construction of the stone column foundation was finished, amounting to 285,000 lin.m for the whole project. The vibro stone
columns were executed with a diameter of 0.9 m by the wet vibro replacement method employing the Keller M vibrator and using 12/75 mm crushed
stone aggregate as backfill material. Its gradation was characterized by
D 60 /D 10 value of 4, with less than 5% of the material passing the 12 mm
sieve. Process water was channeled into silt ponds before it was released
into a nearby creek. Quality control was performed with a standard data
0.5
8
1.0
0.0
10
6
4
2
0
4
3
2
1
0
t/l
n = σ
c /σ
s
n
c = σ
c /σ
λ = 1
λ = 0.56
λ = 0.22
λ = 1
λ = 0.56
λ = 0.22
(a)
0.5
1.0
0.0
t/l
(b)
Figure 4.36 Stress concentration factors (a) n and (b) n c for a rigid square foundation
(B = 8 m) on 25 stone columns for σ c = 0.32 and φ c = 35°.