Aalst is a Flemish town of 83,000 inhabitants situated 25 km northwest of Brussels. The city recently finished the redevelopment of the Hopmark square and surroundings, one of the big projects the municipal council has launched to renovate its historical city center. The project involves the construction of a three level underground car park for 317 cars and the reconstruction of the square with trees and an area for public events with two pavilions housing a bar, a restaurant, a city shop and parking utilities. The construction of the car park, the pavilions and the area for public events was awarded to Interparking NV, who will run the car park in concession for 30 years. Interparking contracted MBG NV for the construction works. The Hopmarkt square is surrounded by buildings without deep foundations. Due to a settlement risk on a subsoil of peat layers and soft loam, the construction work had to be vibration free (< 2 mm/s) and lowering the water table was limited. The lowest water level allowed outside the building pit was -4.5 m below street level (8.5 m TAW). Therefore, the original design included an execution with slurry walls, reaching into the clay layer up to -20.0 m below street level.
Soil & piling contractor Soetaert-
Soiltech proposed a more economical
and faster solution with a better
watertightness: a combination of
Cutter Soil Mix (CSM) and steel sheet
piles. Sheet piles can be lowered in
fresh soil mix walls without generating
any vibrations. After hardening, the soil
mix walls serve as a watertight screen
during excavation (kh
< 10-8 m/s).
The sheet piles act as retaining walls
during and after the construction
phase, as 100% watertight walls after
excavation, and as permanent visible
walls after completion of the car park
transferring vertical loads from the car
park (acting like bearing piles).
Because the calculations of an AZ 26-700
wall indicated a displacement close to
the maximum allowed 60 mm, a mix of
AZ 26-700 and AZ 36-700N profiles
in steel grade S 355 GP was chosen
to be installed in an alternating way.
Measurements after excavation indicated
the displacement of the sheet piles did not
exceed 28 mm.
ArcelorMittal’s in-house design
department carried out a fire resistance
analysis, demonstrating that the sheet pile
sections could resist a 90 minutes ISO fire
caused by burning cars in one level.
The 550 mm thick CSM wall was installed
over the total circumference of the car
park (232 m) up to a depth of 21.0 m
below street level, embedded 1 to 2 m
into the impervious clay layer. The 15 m
long AZ sheet piles were inserted with a
Bauer RG25T rig in the still liquid soil mix
up to -16.5 m below street level.
Once the sheet piles were installed,
excavation started inside the pit down
to level -4.7 m, while the inside of the
building pit was drained. A row of strand
anchors was installed at level -4.3 m. The
anchor blocks were injected and welded
to make them watertight. Next, the
excavation continued to the bottom level
of -10.3 m. The soil mix was removed
on the excavation side and the sheet
piles were cleaned. To achieve a 100%
watertight wall the middle interlocks
were seal-welded at the job-site before
installation over a length of 12.5 m and
Demountable Protection During Assembly
the leading interlocks were seal-welded over 8.8 m after excavation.
To ensure a watertight connection
between the sheet piles and the base slab,
steel plates were welded to the sheet piles
and incorporated in the concrete base slab.
To prevent the floating of the concrete
base slab of the car park, 475 active
micropiles were installed. The piles are
16 m long and can resist 450 kN tension
force each. The base slab was poured in
two phases (40 + 18 cm thickness).
After the construction of the car park
slabs, which serve as permanent struts for
the sheet pile retaining wall, the sheet piles
were sandblasted and coated for aesthetic
The works started in February 2012 and
the car park was completed in July 2013.