Civil Engineering Reference
In-Depth Information
must be exercised as the factor of safety is determined using nominal values
with no factors of safety applied to individual densities, so one should not
be over-optimistic as to the concrete densities that can be achieved. If moni-
toring data is available for salinity, it may be possible to refine values if, for
example, it is known that the watercourse never runs with fully fresh or fully
saline water. Note, however, that the presence of suspended sediments in the
water will increase its density and this must be taken into account also. The
assumptions made by the designer would normally be verified during con-
struction by taking density test cores from the concrete and making as-built
surveys to confirm the geometry of the structure. This information is used to
verify the actual weight of the structure and the displacement of the structure
as construction progresses and hence, the factor of safety actually achieved.
Tunnel approaches
This bouyancy design process also applies to cut-and-cover approach tun-
nels and approach ramp structures, where the ground water level will be
high in relation to the structure and similar upward forces will apply. The
approach structures are rather more straightforward although the various
stages of construction must still be considered. Often, the critical condition
to be considered is when temporary ground water lowering, which may be
required to construct the structure in a dewatered excavation, is turned off
and the ground water level is first allowed to rise around the cut-and-cover
tunnel. The full weight of the tunnel may not apply if the construction is
not fully complete, so the factor of safety against uplift will likely be less.
Where approach structures are designed as gravity structures and use
only their self-weight to resist the uplift, the following factors of safety
against uplift can be used:
• Approach structures temporary condition 1.10
• Approach structures permanent condition 1.15
Factors of safety are slightly higher than for the immersed tunnel as there
may be greater variation in structural thickness (particularly if constructed
using embedded walls, such as diaphragm walls) and the variability of ground
water levels needs to be taken into account. Design codes vary the factors of
safety required and the values may be higher than stated above in some cases.
Again, partial load factors should be equal to 1.0 and no allowance for friction
between the structure and the backfill should be taken for gravity structures.
Additional conditions may be set if there is a high tidal water level variation
or ground water variation at the site, and it is not uncommon to define a set of
criteria for an extreme high water event, with a factor of safety slightly lower
(1.05) for what could be termed a “survivability” event. Similarly, during con-
struction, a lower factor of safety of this order of magnitude can be considered.
