Rock foundations - Geotechnical Investigation and Design Tables - page 292

22.19 Pile refusal
•

Piles are often driven to refusal in rock

•

The structural capacity of the pile then governs.

•

There is often uncertainty on the pile founding level.

•

The table can be used as guide, where all the criteria are satisfied, and suitably
factored when not all of the factors are satisfied.

Table 22.19 Estimate of driven pile refusal in rock.

Rock property

Likely pile penetration
into rock (m)

SPT value, N* Is (50)MPa RQD (%) Defect spacing (mm)

1.0 75% B

400 600

B

0. 3-1.0 50-75%

B 3B

200-400 200-600

2B 4B

0.1-0.3 25-50%

3B 5B

100-200 60-200

5B 7 B

0.1 25%

100 60 5B

•

As the structural capacity and driving energy determines the pile refusal levels,
the table should be factored downwards for timber piles and upwards for steel
piles. For example a 450mm prestressed concrete pile is expected to have arrived
at refusal (set) within 3m of an N ∼ 100 material, but an H pile requires N > 200

to achieve that set.

22.20 Limiting penetration rates
•

The pile refusal during construction may be judged by the penetration rates.

•

This varies according to the pile type.

Table 22.20 Penetration rate to assess pile refusal.

Pile type Maximum blow count (mm/blow)

Concrete 2-3 mm

Timber 6-8 mm

Steel - H 1-2 mm

Steel - Pipe 1-2 mm

Sheet Piles 2-3 mm