Pile Design and Construction Practice, Fifth edition

Table 8.3 Minimum safety factors for various loading conditions
Piling for marine structures 419
Loading condition Minimum safety factor
1. Design environmental conditions with appropriate drilling loads 1.5
2. Operating environmental conditions during drilling operations 2.0
3. Design environmental conditions with appropriate producing loads 1.5
4. Operating environmental conditions during producing operations 2.0
5. Design environmental conditions with minimum loads 1.5
equipment mounted on girders cantilevering from the completed pile bents, or as an operation
from a floating or jack-up barge. In tidal waters there is usually sufficient water depth
to float a barge with a draft of 1 to to a location close inshore. However, this can be
inconvenient where tidal flats or saltings cover a long depth of the approach or where it is
unsafe to ground the barge on the sea bed at low water.
Where the ‘end-on’ method is used the spacing between pile bents is limited by the ability
of the girders to cantilever when carrying the weight of the piling frame, hammer, and
suspended pile. Loading can be minimized by utilizing the buoyancy of tubular piles with
permanently or temporarily closed ends, or by using trestle guides of the types shown in
Figures 3.6 and 3.8 in conjunction with a pile-mounted hammer and a crane barge for lifting
and pitching the piles.
Piling barges for deep-water locations range in length from about 60 to 120 m with a width
of one-third to one-half of the length and an overall depth of to of the length. Adequate
depth is necessary to provide sufficient strength for towing the barge to the site from a distant
location, and to give sufficient freeboard for safe operation when moored at the work site.
These barges are normally self-contained with accommodation for the barge and rig crew.
Jack-up barges operate most efficiently when provided with mechanically adjustable pile
guides installed either by cantilevering from the side of the barge or spanning a ‘moon-pool’
inset in the barge hull.
If possible, piles should be driven to their full design penetration without the need to
weld-on additional pile lengths, to drive insert piles, or to clean out the soil plug or drill
below the initial refusal level of an open-ended tubular pile. Gerwick (8.21) gave an example
of times required for welding add-on lengths of 1.37 m OD tubular piles; they varied from
hours for 25 mm wall thickness to 10 hours for 64 mm thickness. Such delays cause
increased driving resistance due to ‘take-up’ (i.e. the increase of shaft friction). However,
there are many situations where piles cannot be driven to their full penetration without the
need for lengthening or for ‘drilling-and-driving’ techniques.
Cleaning out the soil plug is an effective way of reducing the driving resistance, thus
obtaining deep penetration, because of the elimination of base resistance. It is particularly
advantageous for obtaining deep penetration into coarse soils, say to develop uplift resistance,
to avoid excessive settlement due to vibration effects, or to reach rockhead. This is
because the base resistance in a coarse soil represents the major proportion of the total
resistance to the driving of the pile. Removal of the soil plug is not particularly effective for
piles penetrating deeply into clays where the base resistance is only a very small proportion
of the total resistance. Drilling out the soil within the pile does not reduce the external shaft
friction of the surrounding clay.
1
3 2
1
1
1 1
12 15
4
1
2 m