J001783-001-R-Rev1

Ashley and Caroline Hermes

Where high-level footings are not viable, bored piles may be adopted.
Table 4 shows representative values of ultimate (unfactored) geotechnical parameters for pile design. A
sensitivity analysis should be undertaken to assess effects for the range of parameters. Settlement of the
piled footings is expected to be in the order of 1% of the pile width/diameter. Detailed design should be
carried out by the piling contractor.
Ultimate geotechnical capacities (Rdug) determined from the parameters provided must be multiplied by a
geotechnical strength reduction factor (øg) in accordance with Section 4 of AS 2159-2009 Piling – Design
and installation to determine the design geotechnical strength (Rdg). The geotechnical strength reduction
factor should be determined in accordance with AS 2159-2009 and is expected to be in the range of 0.45 to
0.55. An upper bound value may be adopted if supervision and certification of pile construction is carried out
by a geotechnical engineer.
The soils to a depth 1.5 times the pile diameter below the excavated surface should be ignored in the
assessment of lateral capacity. The lateral capacity of piles may be determined using various methods, and
which generally require an estimation of the Young’s modulus (Es) of the founding stratum.
Table 4: Pile design values.
                               Unit weight        Elastic       Ultimate        Ultimate
                                Ƴ (kN/m3)        Modulus          skin            base
         Material type
                                                  (MPa)         friction       resistance
                                                                 (kPa)            (kPa)

 Stiff clay                         18             8 - 20          20              NR
 Very stiff clay                    19            15 - 40          40              NR
 Hard clay                          19            40 - 70          80              NR
 Very low strength
                                    20            50 - 250         100            3000
 mudstone
Notes:
    1)     NR – Not recommended
    2)     Skin friction should be discounted for sections of pile shafts where liners are in place.
    3)     Indicative values taken from published paper, Site Characterisation, Terence Wiesner, Australian
           Geomechanics – Dec 1999 and ‘Foundation Analysis & Design (Bowles, 4th Edition)


To achieve the recommended design values, founding materials must be dry and cleaned of any loose,
disturbed or saturated materials prior to pouring concrete. Footing excavations should be inspected by an
geotechnical RPEQ to confirm the foundation material.
If any soil conditions encountered during the footing excavations are found to differ from those noted in the
geotechnical investigation, an inspection of the footing excavations should be carried out by a geotechnical
RPEQ to assess if changes to footing design are required.
All proposed footings should found such that they are not adversely affected by any adjacent excavations,
batter slopes, trenches, or retaining walls that are not designed to support building or fill loads.
To minimise the potential for any adverse interaction effects, footings should found at least below a plane
extending 1 m horizontally from the base of trenches/batter slopes/excavations/ retaining walls, then rising
up at 1V:1H. This requirement is illustrated in the following diagram.




J001783-001-R-Rev1