July/August 2024 - 116

Case Study II - Sydney
This case is related to the construction of
the Third Parallel Runway, Kingsford
Smith Airport, Sydney. Construction
utilized sand dredged from Botany Bay.
This sand is uniform, fine-medium with
D =300m, 1% to 3% passing #200, and
50
uniformity coefficient Cu=2. Construction
consisted of two distinctive stages.
First, an embankment was formed in
Botany Bay using loose dredged
material. Once the dike was formed,
wellpoint pumps were installed along
the perimeter to keep the phreatic
surface within the loose embankment.
This sand was then excavated enabling
the construction of an MSE wall from
the bottom up in a dry environment. The
same dredged sand was then placed in
layers, allowing installation of facing
and reinforcement. It was compacted to
specified density. Once the wall
construction was completed, the
wellpoint pumps were removed and the
excess dredged sand on the seaside was
hydraulically pumped away.
Metallic reinforced walls were
selected for this marine environment.
Two types of walls were constructed.
The first was for the Millstream
(drainage) canal using standard
cruciform 5 by 5 ft (1.5 by 1.5 m)
concrete facing panels. The reinforcement
was 0.4 in (10 mm) thick black
steel strips (not galvanized). Each side
of the canal was about 0.63 mi (1 km)
long. The second wall, the seawall,
utilized special double-T precast
concrete panels, about 10 ft (3 m) long
and about 6.6 ft (2 m) or 3.3 ft (1 m) tall.
Black steel strips, 0.6 in (15 mm) thick,
having length equal to wall height,
were used. Wall height was either
about 16.4 ft (5 m) or about 13.1 ft (4 m)
utilizing the needed combinations of
stacked double-T prefabricated concrete
panels. The double-T had an
aesthetically pleasing frontal surface;
the T shape was precast by forming two
buttresses, extending from the back of
each panel about 2 ft (0.6 m) long. These
116 * DEEP FOUNDATIONS * JULY/AUG 2024
Airport runway, constructed walls and Botany Bay
buttresses enabled quick connection of
reinforcement strips as well as adding
stability to the stacked facing during
construction, before backfill and
reinforcement were placed. This
seawall was about 4.3 mi (7 km) long.
Minimum relative density for compaction
of the sand was specified as
Dr≥80%. A 10 T roller was used at a
distance greater than 3 ft (0.9 m) from
the back of the panels. Within 3 ft
(0.9 m) from the panels, either a 1 T
handheld compactor or jumping jack
was used to compact the sand next to
the facing. To prevent loss of sand
through joints between the panels, first,
no-fines blocks were stacked against
the joints and then nonwoven geotextile
with apparent opening size of
AOS<80 µm was glued at spots to the
panel on either side. The no-fines
blocks were presumably placed to
break water energy under wave action.
Considering the dogleg geometry of the
joints and the probable height of waves
in Botany Bay, the need for these blocks,
added after the project was approved, is
ques t i onabl e , espec i a l ly when
considering the added complexity of
compaction around such blocks.
Construction started in 1992 and
completed in 1994, nine months ahead
of schedule.
The contractor has guaranteed
maintenance-free performance of 100
years for the seawall and 50 years for the
Millstream wall. Two years after
construction, sinkholes appeared next
to some joints. Around 2002, 30% of the
joints had significant sinkholes. Infilling
these holes with sand could not resolve
the fundamental problem of sand
leakage. It also accelerates corrosion.
Repair was done between August 2005
and September 2006. Two parallel rows
of jet grouting piles were used from the
back of the face to the rear of the reinforcement.
In essence, a gravity wall,
about 5.7 mi (9 km) long and about 15 ft
(4.5 m) deep, was formed thus rendering
the joints and reinforcement unimportant.
This rectification replaced an
impressive MSE wall with an oldfashion,
expensive gravity wall.
July/August 2024

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