e-mosty December 2018 e-mosty December 2018 | Page 57

Figure 6: Mast base following second stage concrete pour
and dead-man and 18m long underneath the mast.
Steel H piles were designed as “maintenance-free”
with sacrificial thickness of steel (assumed to corrode
during the life of the structure).
The reinforced concrete mast abutment is cast above
ground and is 6.25m long x 1.2m deep x 2m wide. The
reinforced concrete dead-man is cast 2.5m below
ground and is 7.3m long x 1.5m wide x 1.5m deep.
The western abutment beam is 3.5m long x 1m deep x
0.8m wide.
Transverse seismic forces are transferred by the plan
cross bracing and taken at both abutments
transferred by the hold down bolts. In the longitudinal
direction the west abutment is free to move because
of hold down bolts in slotted holes.
The top 5m of the site has the potential to liquefy in a
large seismic event. Since the cable stay structure
relies on a passive dead-man anchorage which is
embedded in liquefiable layers there is a potential risk
during a liquefaction event that the passive dead-man
can pull through the liquefied soil layers leading to
large displacements and significant loss of cable
stress.
To accommodate the above scenario, the design
includes 3 raked piles in the dead-man. By raking each
pile with a 1H:6V incline, the design is able to mobilize
more compressive forces when anchoring the dead-
man into the non-liquefiable layers lower down.
CONSTRUCTION
The mast base (eastern abutment beam) was
constructed in stages to reduce the influence of
vertical demands on the existing Leith Wall.
The first stage was a 300mm concrete pour. The
300mm of concrete has rebar passing through the
piles so that the subsequent 1100mm of wet
abutment concrete can be assumed taken by the
300mm concrete raft and transferred directly to the
piles (i.e. will not load the wall).
On this basis the eastern abutment beam is treated as
a composite beam with staged construction
accounted for in the accumulation of SLS cracking
stresses and ULS reinforcing demands.
During construction the interface between the two
pours was scabbled and prepared as a “construction
joint”.
4/2018