198 Ground improvement by deep vibratory methods
These deformations were incompatible for the envisaged structures, and
it was decided to carry out a vibro concrete column foundation in accordance with existing German regulatory requirements. The vibro concrete
columns were constructed from a stable working platform using the small
diameter Keller N-Alpha vibrator (see Table 3.1 and Figure 5.2b). The minimum column diameter, depending on the vibrator diameter actually used,
was chosen as 40 cm and the diameter of the column toe 80 cm. Based upon
experience from the construction of vibro concrete columns in similar soil
conditions, the safe characteristic vertical load capacity of the columns was
chosen as 710 kN and column spacing was selected accordingly. In column construction, the column toes had to be safeguarded by enhancing the
diameters to at least 80 cm and that all were actually resting on the dense
gravel layer below 8 m depth with envisaged settlements below 1.5 cm.
In total, 103 working vibro concrete columns were necessary for the foundation of the medical center. To verify the selected safe load bearing capacity of the vibro concrete columns, six test columns were installed and load
tested employing the high strain method and analyzed using the CAPWAP
procedure (Rausche et al., 1985). The test columns had a nominal diameter
of 40 cm and lengths between 5.2 and 9.1 m, respectively. Table 5.2 gives the
Table 5.1 Characteristic soil properties
Soil layer and depth
Shear parameters
Stiffness modulus
c u (kN/m 2 )
φ′ (°)
E s (MN/m 2 )
Superficial alluvial layer (0–8 m)
Sandy
—
30
7
Gravelly
—
32.5
15
Silty
30
25
5
Quaternary gravel
—
35
100
Tertiary layers
Very stiff
70
22.5
10
Hard
100
22.5
20
Table 5.2 Load tests on vibro concrete test columns (CAPWAP test results)
Column
no.
Column
Activated
static bearing
capacity (kN)
Skin
friction
(kN)
Point
bearing
(kN)
Settlement
at test blow
(mm)
Diameter
(cm)
Length
(m)
1
40
6.2
1238
424
814
8
2
40
9.0
2204
1104
1100
8
3
40
5.2
834
275
559
14
4
40
6.6
1528
416
1112
8
5
40
8.8
2087
852
1235
8
6
40
9.1
2287
1025
1262
8
These deformations were incompatible for the envisaged structures, and
it was decided to carry out a vibro concrete column foundation in accordance with existing German regulatory requirements. The vibro concrete
columns were constructed from a stable working platform using the small
diameter Keller N-Alpha vibrator (see Table 3.1 and Figure 5.2b). The minimum column diameter, depending on the vibrator diameter actually used,
was chosen as 40 cm and the diameter of the column toe 80 cm. Based upon
experience from the construction of vibro concrete columns in similar soil
conditions, the safe characteristic vertical load capacity of the columns was
chosen as 710 kN and column spacing was selected accordingly. In column construction, the column toes had to be safeguarded by enhancing the
diameters to at least 80 cm and that all were actually resting on the dense
gravel layer below 8 m depth with envisaged settlements below 1.5 cm.
In total, 103 working vibro concrete columns were necessary for the foundation of the medical center. To verify the selected safe load bearing capacity of the vibro concrete columns, six test columns were installed and load
tested employing the high strain method and analyzed using the CAPWAP
procedure (Rausche et al., 1985). The test columns had a nominal diameter
of 40 cm and lengths between 5.2 and 9.1 m, respectively. Table 5.2 gives the
Table 5.1 Characteristic soil properties
Soil layer and depth
Shear parameters
Stiffness modulus
c u (kN/m 2 )
φ′ (°)
E s (MN/m 2 )
Superficial alluvial layer (0–8 m)
Sandy
—
30
7
Gravelly
—
32.5
15
Silty
30
25
5
Quaternary gravel
—
35
100
Tertiary layers
Very stiff
70
22.5
10
Hard
100
22.5
20
Table 5.2 Load tests on vibro concrete test columns (CAPWAP test results)
Column
no.
Column
Activated
static bearing
capacity (kN)
Skin
friction
(kN)
Point
bearing
(kN)
Settlement
at test blow
(mm)
Diameter
(cm)
Length
(m)
1
40
6.2
1238
424
814
8
2
40
9.0
2204
1104
1100
8
3
40
5.2
834
275
559
14
4
40
6.6
1528
416
1112
8
5
40
8.8
2087
852
1235
8
6
40
9.1
2287
1025
1262
8