Bored Pier Design Australia

[addt]

Hi,

We have been asked to review a structural design for a proposed 2-storey residential build, both brick veener (supported on slab with high level edge beams on bored piers i.e no drop edge beams used) and cladded walls (bearers and joists suppoted bored piers). The building will be located on a fill site with the substrate approx 600mm (silty clay; low to medium plasticity) below finished ground level for the most part. The builder has concerns over the engieers proposed footing design may not have taken into account the heavy floor loading from floor tiles.

A geotechnical engineer has provided the following design parameters
- Class M - Moderately reactive(expected surface movement 23-30mm)
- end bearing pressure of 100kPa if founded in stiff or better clay soils or controlled fill.

Current footing design
Dia 400mm x 750mm deep bored piers @ MAX 2100 c/c (i'm assuming the 750mm depth is to allow for a 150mm embedment into the substrate)

Proposed un-factored loading conditions (loads calculated using material and live loads over an area of 2.1m x 2.1m)
Typical external wall with piers at 2100 c/c
G = 8kN
Q = 8kN
Wup_ult = -6.44kN
Wdn_ult = 4.1 kN

Typical internal wall with piers at 2100 c/c
G = 5kN
Q = 8kN

Isolated 350x350 brick column 3m high supporting deck and roof
G = 26kN
Q = 18kN
Wup_ult = -12kN
Wdn_ult = 8kN
Brick column is be supported on a single bored pier which is tied back into the slab edge beam using a 300mm (W) x 400 (D) strip footing.

Not taking into account the self weight of the bored pier and i assume that wind loads are not to be taken into account, i caclulate the following pier end bearing pressure.
Dia = 0.4m
Area = 0.13m^2
P = 8 + 8 (external wall piers) = 16kN

Pier end bearing pressure = P / A = 16/0.13 = 123 kPa > 100 kPa

If wind load to be taken into account
P = 8 + 8 + (0.7 * 4.1) = 18.9 kN ==> 18.9/.13 = 145 kPa > 100 kPa

Question is, Does it appear that the engineer has under sized the piers or not, if so, how would one go about designing a suitable bored pier foundation system?
Is there an allowance in AS2870 and the NCC that can be used for bored piers without doing the maths on bearing pressure and use tables similar to the strip footing, edge beam tables/figures in both of these documents?
Is it correct to assume that the end bearing pressure supplied is an allowable pressure i.e. unfactored loads to be used?
Do we need to take into account a factory of safety and reduce the 100kPa or has the geotechnical engieer already taken that into account?
Do you also calculate the brickwork, edge beam and slab weight and calculate the pier bearing pressure or does the slab pier combination share the load evening over the ground?
Do we take into account skin friction, if so how do you go about doing that?
Given wind load is short term loading, do you need to include that in the design working load?
Do we need to take into account the 23-30mm settlement, if so, how do you go about doing that?

Thanks in advance

 

[sdz]

Why have you been asked to review the design? What's your experience?

Is that end bearing a design capacity or an allowable bearing pressure? You would need to read the geotech report.

So far it doesn't look good.

 

[addt]

Hi sdz,

I'm a timber design engineer specialising in frame/truss/floor system design. I have not been involved in foundation design for more than 20years, so when it comes to design combinations relating to bearing pressues i get a little lost.

One our builders had a concern over the proposed footing design and requested that we have a look over it and provide comment.

The geotech report does not state design or allowable. Below is the extract from the report.

"Piered Footings
Piered footings are considered as an alternative to deep edge beams or high level footings. It is suggested
that piered footings, founded in stiff or better clay soils or controlled fill could be proportioned on an end
bearing pressure of 100kPa. Where uniformly founded in the underlying bedrock, an end bearing pressure of
600kPa could be adopted.
Where piered footings are utilised, the potential for volume change in the subsurface profile should be taken
into consideration by the designer, along with potential settlement.
All footings should be founded below any topsoil, deleterious soils, uncontrolled fill (if encountered) or soils
affected by tree roots. All footings for the same structure should be founded on strata of similar stiffness and
reactivity to minimise the risk of differential movements.
Inspection of high level or pier footings excavations should be undertaken to confirm the founding conditions
and the base should be cleared of fall-in prior to the formation of the footing."

Thanks again.