174 Ground improvement by deep vibratory methods
4.7.3 Vibro replacement foundation for the
new international airport at Berlin
The new international airport for Berlin is expected to go into operation in
2017. For the construction of infrastructural measures to connect the central terminal building with the highway and with high speed and regional
rail traffic systems, ground improvement by vibro replacement was necessary. Subsoil at the site consists of marl and sand layers of glacial origin
that are relatively soft to a depth of about 8 m below ground surface, where
generally stiff marl follows, characterized by CPT cone resistance values
typically well in excess of 15 MN/m 2 . Design target of the ground improvement scheme was a reduction of the settlements deriving from the upper
soft layers by a factor of 2 for single footings and a deformation modulus
of 21 MN/m 2 below rafts and approach ramps. The necessary spacing of
the vibro stone columns was determined by load tests on groups of four
columns for structures and on single columns adopting the unit cell concept
for area loads.
All single column load tests were performed with a test load of 200 kN/
m 2 on square test footings of 1.25 × 1.25 m and 2 × 2 m, each supported by
a 9 m long vibro stone column of 0.5 or 0.6 m diameter. Figure 4.39 shows
the load test setup together with a CPT diagram which is representative
of the site. Table 4.12 gives details of the load tests which were evaluated by
the method described in Section 4.4.
By adopting the unit cell concept the settlement of a single column determines the equivalent deformation modulus E* by introducing the equivalent column length l* from Figure 4.28 into Equation 4.47. The necessary
grid spacing for the target modulus can then be determined graphically,
here by extrapolation according to Figure 4.40, where a necessary area A n
= 7.4 m 2 for a square grid of 0.6 m diameter stone columns was obtained
for the required design modulus E d = 21 MN/m 2 . The ground improvement
works were performed using conservatively a square grid of 0.6 m diameter
stone columns of 9.0 m length with 2.5 m spacing.
While the necessary area replacement factor for the infinite grid was
just a c = 0.05 to achieve a modulus of 21 MN/m 2 , the evaluation of the
column group tests resulted in a required a c = 0.09 for single footings
to reduce settlements by 50%. Prior to the load tests, large shear box
tests were performed in the laboratory with the stone column material
anticipated for the ground improvement works. It consisted of crushed
granite rock of 10–35 mm grain size. In the laboratory an average friction angle of 57° was determined for the backfill material at a density of
1.9 t/m 3 , safely allowing the use of column friction angles of 45° in the
design calculation.
An area of about 20,000 m 2 was improved by stone columns on this project in 2008 using Keller bottom feed M vibrators. Quality control measures
included standard data acquisition systems to record the usual parameters
4.7.3 Vibro replacement foundation for the
new international airport at Berlin
The new international airport for Berlin is expected to go into operation in
2017. For the construction of infrastructural measures to connect the central terminal building with the highway and with high speed and regional
rail traffic systems, ground improvement by vibro replacement was necessary. Subsoil at the site consists of marl and sand layers of glacial origin
that are relatively soft to a depth of about 8 m below ground surface, where
generally stiff marl follows, characterized by CPT cone resistance values
typically well in excess of 15 MN/m 2 . Design target of the ground improvement scheme was a reduction of the settlements deriving from the upper
soft layers by a factor of 2 for single footings and a deformation modulus
of 21 MN/m 2 below rafts and approach ramps. The necessary spacing of
the vibro stone columns was determined by load tests on groups of four
columns for structures and on single columns adopting the unit cell concept
for area loads.
All single column load tests were performed with a test load of 200 kN/
m 2 on square test footings of 1.25 × 1.25 m and 2 × 2 m, each supported by
a 9 m long vibro stone column of 0.5 or 0.6 m diameter. Figure 4.39 shows
the load test setup together with a CPT diagram which is representative
of the site. Table 4.12 gives details of the load tests which were evaluated by
the method described in Section 4.4.
By adopting the unit cell concept the settlement of a single column determines the equivalent deformation modulus E* by introducing the equivalent column length l* from Figure 4.28 into Equation 4.47. The necessary
grid spacing for the target modulus can then be determined graphically,
here by extrapolation according to Figure 4.40, where a necessary area A n
= 7.4 m 2 for a square grid of 0.6 m diameter stone columns was obtained
for the required design modulus E d = 21 MN/m 2 . The ground improvement
works were performed using conservatively a square grid of 0.6 m diameter
stone columns of 9.0 m length with 2.5 m spacing.
While the necessary area replacement factor for the infinite grid was
just a c = 0.05 to achieve a modulus of 21 MN/m 2 , the evaluation of the
column group tests resulted in a required a c = 0.09 for single footings
to reduce settlements by 50%. Prior to the load tests, large shear box
tests were performed in the laboratory with the stone column material
anticipated for the ground improvement works. It consisted of crushed
granite rock of 10–35 mm grain size. In the laboratory an average friction angle of 57° was determined for the backfill material at a density of
1.9 t/m 3 , safely allowing the use of column friction angles of 45° in the
design calculation.
An area of about 20,000 m 2 was improved by stone columns on this project in 2008 using Keller bottom feed M vibrators. Quality control measures
included standard data acquisition systems to record the usual parameters