Site investigation - Geotechnical Investigation and Design Tables - page 7

Table 1.8 Guideline to extent of investigation.

Development Test spacing Approximate depth of investigation

Building 20 m to 50 m 2B-4B for shallow footings

(Pads and Strip, respectively)

•

3 m or 3 pile diameters
below the expected founding
level for piles. If rock
intersected ensure -
N ∗ >

•

100 and RQD

>

25%

1.5B (building width) for rafts or closely
spaced shallow footings

•

1.5B below 2/3D (pile depth) for pile rafts •

Bridges At each pier location 4B-5B for shallow footings

•

10 pile diameters in competent
strata, or

•

Consideration of the following if
bedrock intersected
- 3 m minimum rock coring
- 3 Pile diameters below target
founding level based on
N ∗ >

•

150

RQD

50%
Moderately weathered or better
Medium strength or better

>

Embankments 25 m to 50 m (critical areas) Beyond base of compressible

100 m to 500 m as in roads alluvium at critical loaded/suspect

areas, otherwise as in roads.

Cut Slopes 25 m to 50 m for H 5 m 5 m below toe of slope or

>

50 m to 100 m for H 5 m 3 m into bedrock below toe

whichever is shallower.

<

Landslip 3 BHs or test pits Below slide zone. As a guide (as the

minimum along critical slide zone may not be known) use 2

×

section height of slope or width of zone

of movement. 5 m below toe of slope
or 3 m into bedrock below toe
whichever is shallower.

Pavements/roads 250 m to 500 m 2 m below formation level.

Local roads 150 m 2 to 3 locations <

Local roads 150 m 50 m to 100 m

(3 minimum)

>

Runways 250 m to 500 m 3 m below formation level.

Pipelines 250 m to 500 m 1 m below invert level.

Tunnels 25 m to 50 m 3 m below invert level or 1 tunnel

diameter, whichever is deeper: greater
depths where contiguous piles for retentions.

Deep tunnels need Target 0.5-1.5 linear m drilling per route

special consideration metre of alignment.

Lower figure over water or difficult
to access urban areas.

( Continued )