Method variations and related processes 197
For verification of the load bearing capacity of single columns or column
groups, standard static load tests are necessary. Dynamic load tests or high
strain tests may only be carried out on vibro concrete columns whose column geometry has already been established and when sufficiently calibrated
against static load tests. It needs to be ensured that the columns can bear
the tensile stresses developing during the impact of the dynamic load test. For
this reason, column heads of test piles can be safeguarded by a special steel
reinforcement. Alternatively, the dynamic impact of the high strain test can
artificially be elongated by packages of large steel springs ensuring that no
detrimental tensile stresses can develop which otherwise result during stress
wave propagation.
5.2.5 Suitable soils and method limitations
We have already seen that the fresh concrete of the column shaft requires
lateral support from the surrounding soil during the curing process. This
is particularly important in soils with thixotropic characteristics which
temporarily lose their strength during installation as a result of the vibrator
motion. In addition, influences from the construction of adjacent columns
acting on each other during installation cannot easily be excluded.
Consequently, a minimum column distance and a threshold for the minimum
soil strength need to be defined and observed for the safe construction of
these columns. In soils which are prone to creep which may result in negative
skin friction and additional vertical forces on the vibro concrete columns,
special care is necessary when using this foundation method.
When extensive heave develops during vibro concrete column construction, which cannot be controlled by changing the column pattern and which
may have a negative impact on the column integrity; partial or complete
pre-drilling at the column location can often provide remedy to the situation
at a relatively low cost.
5.2.6 Case history: Foundation on vibro
concrete columns in soft alluvial soils
In the vicinity of an existing hospital in southern Germany, a new radiotherapy center was to be built in early 2015 consisting of three additional
one-storey structures measuring 19 × 18 m. One of the buildings encompassed the radiation treatment room, a heavy concrete structure, 6 m high,
and with wall and roof thicknesses of 1.75 and 3.00 m, respectively.
Site investigations revealed the presence of a soft alluvial layer to about 8 m
depth consisting primarily of sandy silt overlaying dense quaternary sandy
gravel to about 12 m depth, where tertiary very stiff to hard silts and clays were
found. Table 5.1 provides average soil characteristics established in site and
laboratory tests. These show that the superficial soft layer is unsuitable to carry
foundation loads without excessive settlements, which had been estimated to
4 cm for the light radiation buildings and to 9 cm for the heavy radiation room.
For verification of the load bearing capacity of single columns or column
groups, standard static load tests are necessary. Dynamic load tests or high
strain tests may only be carried out on vibro concrete columns whose column geometry has already been established and when sufficiently calibrated
against static load tests. It needs to be ensured that the columns can bear
the tensile stresses developing during the impact of the dynamic load test. For
this reason, column heads of test piles can be safeguarded by a special steel
reinforcement. Alternatively, the dynamic impact of the high strain test can
artificially be elongated by packages of large steel springs ensuring that no
detrimental tensile stresses can develop which otherwise result during stress
wave propagation.
5.2.5 Suitable soils and method limitations
We have already seen that the fresh concrete of the column shaft requires
lateral support from the surrounding soil during the curing process. This
is particularly important in soils with thixotropic characteristics which
temporarily lose their strength during installation as a result of the vibrator
motion. In addition, influences from the construction of adjacent columns
acting on each other during installation cannot easily be excluded.
Consequently, a minimum column distance and a threshold for the minimum
soil strength need to be defined and observed for the safe construction of
these columns. In soils which are prone to creep which may result in negative
skin friction and additional vertical forces on the vibro concrete columns,
special care is necessary when using this foundation method.
When extensive heave develops during vibro concrete column construction, which cannot be controlled by changing the column pattern and which
may have a negative impact on the column integrity; partial or complete
pre-drilling at the column location can often provide remedy to the situation
at a relatively low cost.
5.2.6 Case history: Foundation on vibro
concrete columns in soft alluvial soils
In the vicinity of an existing hospital in southern Germany, a new radiotherapy center was to be built in early 2015 consisting of three additional
one-storey structures measuring 19 × 18 m. One of the buildings encompassed the radiation treatment room, a heavy concrete structure, 6 m high,
and with wall and roof thicknesses of 1.75 and 3.00 m, respectively.
Site investigations revealed the presence of a soft alluvial layer to about 8 m
depth consisting primarily of sandy silt overlaying dense quaternary sandy
gravel to about 12 m depth, where tertiary very stiff to hard silts and clays were
found. Table 5.1 provides average soil characteristics established in site and
laboratory tests. These show that the superficial soft layer is unsuitable to carry
foundation loads without excessive settlements, which had been estimated to
4 cm for the light radiation buildings and to 9 cm for the heavy radiation room.