Geoscience Reference
In-Depth Information
22.17 Design shaft resistance in rock
•
The table below combines the concepts provided above by the various authors.
•
The formula has to be suitably factored for a mix of conditions, eg low quality
rock with no slurry and grooving of side used.
Table 22.17
Shaft capacity for bored piles in rock (modified from above concepts).
Typical material properties
Construction condition
τ
=
Ultimate side shear resistance (MPa)
Soil, RQD
25%
0.1 (q
u
)
0
.
5
<<
0.2 (q
u
)
0
.
5
Low quality rock
Slurry used, straight,
RQD
25%, clay seams
smooth sides
<
defects
60 mm
Medium quality rock
<
0.45 (q
u
)
0
.
5
RQD
25%-75%
defects 60-200 mm
High quality rock
=
0.70 (q
u
)
0
.
5
Artificially roughened by
RQD
75%
grooving
>
defects
200 mm
>
22.18 Load settlement of piles
•
Some movement is necessary before the full load capacity can be achieved. The
full shaft capacity is usually mobilized at approximately 10mm.
•
Due to the large difference in movement required to mobilise the shaft and base,
some designs use either the shaft capacity or the base capacity but not both.
•
Reese and O'Neil (1989) use the procedure of movement
>
10mm, then the load
is carried entirely by base while displacement
10mm then the load is carried
by shaft. Therefore calculation of the settlement is required to determine the load
bearing element of the pile.
<
•
Often 50% to 90% of the load is required by the shaft capacity.
•
The base resistance should be ignored where boreholes do not extend beyond
below foundation or in limestone areas where solution cavities are possible.
•
Factor of safety to consider the above relative movements.
Table 22.18
Pile displacements.
Load carrying
Displacement required
element
Typical
Material specific eg bored piers in clay/mudstones
Shaft
0.5% to 2% Shaft
1% to 2% of Shaft diameter
diameter 5-10 mm
10 mm maximum for piles with
diameters
>
600 mm
Base
5% to 10% Shaft
10% to 20% of Base diameter
diameter
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