Soil foundations - Geotechnical Investigation and Design Tables - page 277

Table 21.19 Influence of driven piles (after Broms, 1996).

Location Influence zone at which density increases Typical pile spacing

Along shaft 4-6 pile diameters 3B for frictional piles with lengths 10 m =

5B for frictional piles with lengths

=

25 m

At base of pile 3-5 pile diameters below pile 2B for end bearing piles

•

The above should be considered when driving piles in groups or adjacent to
existing piles.

•

Pile groups in a granular soil should be driven from the centre outwards to allow
for this densification effect.

•

Bored Piles have 2B or 750mm minimum spacing, while driven piles are 2.5B
spacing in sands.

•

Screw piles would be nominally less than for end bearing piles, approximately
1.5B.

•

10 pile diameters is the distance often conservatively used to avoid the effects of
pile installation on adjacent services and buildings.

21.20 Point of fixity
•

The point of fixity needs to be calculated to ensure suitable embedment when
lateral loads apply. For reinforced concrete piles this point is required to determine
the extent of additional reinforcement at the top of the pile.

•

The point of fixity is based on the load, pile type, size, and soil condition. The
table below is therefore a first approximation only.

Table 21.20 Typical depth to the point of fixity for pile width (B).

Soil condition Strength Depth to point of fixity

Sands Very loose 11B

Loose 9B

Medium dense 7B

Dense 5B

Very dense 3B

Clay Soft 9B

Firm 7B

Stiff 6B

Very stiff 5B

Hard 4B

21.21 Uplift on piles
•

The uplift capacity is taken as 75% of the shaft resistance due to cyclic softening.

•

Piles on expansive clay sites experience uplift. The outer sleeve (permanent casing)
may be used to resist uplift in the active zone.