08/0169 - Response to Information Request [As Published] · 132008.169

All piers must be designed for lateral stability and their depth will be governed by their ability to achieve
the loads nominated on the associated structural plans.

7.1.1 Deep Foundations :
For cast in-situ piers founded into the NATURAL MEDIUM DENSE SAND, assuming a safety factor of

2.5 the following bearing capacities can be used as a design guide: The footing masses have not been
considered.

 

 

 

 

 

450mm Diameter Piers 600mm Diameter Piers
Depth * (m) End Side friction Total safe End Side Total safe
bearing (kPa) capacity of bearing friction capacity of
(kPa) pier (KN) (kPa) (kPa) pier (KN)
3.0 474 8.3 98 497 8.3 172
40 608 10.3 141 631 10.3 236

 

 

 

 

 

 

 

 

* Piers excavated in the sandy soils will collapse if the walls are not supported. Grout replacement
techniques must be used to achieve the capacities indicated above.

Steel screw piers are to be designed by a specialist contractor.

7.2 Earth pressures.

Lateral Earth pressures can be estimated from the equation: P=0.5y HK

Where
ty x 16 KN/m3
H = height m
K selected from the table below.
Earth pressure coefficients for vertical wall designs and friction angle of 32° degrees are as follows;

 

 

 

 

 

 

Angie of backfill (degrees) Ka (active) Ko (at rest) Kp (passive)
0 0.294 0.482 3.25
5 0.310 0.508
10 0.328 0.538

 

 

 

 

Active earth pressures Ka are generated where the wall allows some movement away from the backfill
and the soil gains internal strength in this process. At rest earth pressures Ko are produced where walls
allow very little movement (such as well braced walls not capable of allowing movement). Kp is a
passive earth pressure coefficient.

7.3 Construction:
7.3.1 Excavations

All excavations both temporary and permanent must be adequately retained to prevent the occurrence
of land slip. Where a retaining wail is required, an exposed cut batter of 1 to 2 (vertical to horizontal)
taken from the base of any foundation can be excavated. These earthworks must incorporate shallow
surface spoon drains at the base and along the high side of the cut. Drains must have an efficient
outflow down-slope and clear of the construction area and should utilise absorption as a means of
disposal to minimise the risk of surface scour. Any exposed face should also be protected against the
potential for stormwater erosion.

All earthworks must ensure the stability of neighbouring structures is maintained through not
undermining foundations that support them. This is achieved through the restriction of any earthworks
to above a critical depth fine that radiates downwards at an angle of 1 in 2 (vertical to horizontal) from
the base of the foundation/structure to be protected.

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SCG Consulting Engineers 3 SCG Ref: 19064(S}