Excavation pits & pile foundations

VBG-Headquarters Hamburg – 12-metre-deep excavation pit
 

For FRANKI, this project involved building the deepest excavation pit in
its history using a ‚Sealed wall with precast concrete elements‘ system.
The statutory accident insurance organisation VBG is planning to con-
struct a new building for its headquarters that has been located in
Hamburg since 1948. Plans involve constructing a five-storey building
with a 15-storey tower in the Barmbeker Drosselstraße for the 550 em-
ployees. On the approx. 5,000 square metres site of the former bus
station it was necessary to first install a watertight excavation pit. The
FRANKI proposal comprising a sealed wall with precast concrete
elements won over the planners.


Sealed wall as inexpensive solution

Additional ground examinations showed that the layer of marl discovered
at a level of around 3 metres was only the tail-end of an initial layer that
could not be used to seal the excavation pit. The sealing layer of marl
was 28 metres deep; however, this problem was overcome by the sea-
ling wall proposed by FRANKI: the diaphragms were then sunk down
into the deeper second layer of marl. The additional costs required for
this procedure were relatively low compared to the considerable costs that would have been required for a deeper bored pile wall or an additional sealed base. On the western edge of the excavation pit FRANKI also produced 1,000 square metres of in-situ concrete diaphragm walls. This extremely low-deformation installation method was necessary due to the close proximity to residential buildings that were susceptible to settling. To keep the base of the excavation pits dry despite the submerging layer of marl and various aquifers, and to ensure it remain secure, it was necessary to install a relief system for the groundwater aquifer in addition to the planned residual dewatering system. This ensured that the excavation pit base did not break.


Twelve-metre-deep excavation pit

Work is well underway: the excavation work is almost complete. The final excavation depth is around 12 metres on the northern side of the pit – with a dual-layer back-anchoring of the precast concrete parts. This makes this excavation pit the deepest of its kind yet using this system. VIT (a member of the FRANKI Group) is currently installing the anchors. In April 2015 FRANKI will complete the excavation pit and hand it over to the piping experts.

Completed work:
6.700 m² sealed wall,
d = 60 cm, L ≤ 36 m
1.000 m² diaphragm wall,
d = 80 cm, L ≤ 36 m
145 precast concrete
elements, L ≤ 14,6 m
193 temporary strand
anchor, Zk ≤ 700 kN,
L ≤ 25,0 m
60.000 m³ excavated
material and professional
disposal



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