A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
comes to a stage when the inertial forces of inside soils, together with the
internal frictional forces exceeds the bearing capacity of soils at pile toes.
Consequently, the soil plug formed is brought down by the piles.
It is practically possible to excavate all soils inside steel tubular piles and
replace them completely by reinforced concrete. However, as engineers
strive to produce economical design the extra cost associated with
excavation of soil plug and filling of concrete could be saved in case the
soil plug remains in position. Moreover, from the technical point of view it is
considered unnecessary to remove the soil plugs because it serves to
provide a platform for the placing of on-top infill concrete on one hand and
to fill the void space below the infill concrete on the other hand. In addition,
the soil plug is considered to be sufficiently compacted by pile driving
action and is deemed to be stable during the design life of the piling
system.
10. Why “inadequate pile founding level” commonly occurs in piles of
piers? (MP1)
The most severe load on piers generally is the horizontal load due to
berthing and mooring of large vessels. The design of piers is taken as an
example to illustrate the importance of adequate pile founding level. Since
the widths of open berth piers are relatively small so that they provide a
short lever arm to counteract the moment induced by berthing loads.
Moreover, the dead load of open berth piers are normally quite light and
therefore the resisting moment provided by the dead load of pier structures
may not be sufficient to counteract the moment generated by berthing
loads. To aid in providing adequate resistance to the overturning moment
by the berthing load, the soil resistance above bedrock contributes to the
stabilizing moment. For commonly adopted marine piling type, i.e. driven
steel tubular piles with reinforced concrete infill, driven piles can at most be
founded on top of rockhead surface. In case the rockhead level is shallow
(e.g. near shoreline), the little soil cover may result in inadequate lateral
resistance to the berthing load.
11. For typical pile bents in marine piers, how is vertical loads related
to horizontal capacity of the pile bents? (MP1)
Let’s consider a pile bent with a top slab supported by two ranking piles,
each inclining at an equal angle to the pier slab. In designing such a
system, truss action is normally adopted to analyze the pile bent. When the
reaction forces of these piles, horizontal forces (e.g. due to berthing and
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