The shaft length is taken as the overall depth of the pile, from which the depth of the
enlargement and any allowance made for the shrinkage of the soil away from the pile at the
ground surface are deducted. Where piles in clay have to carry long-term sustained uplift
loading, and the ratio of the depth of these piles to the width of the enlarged base is less
than 5, the uplift resistance, as calculated by equation 6.5 or the methods in Chapter 4,
should be reduced by one-half.
Where piles with base enlargements are installed in groups the uplift resistance of the
group can be calculated as described in Section 5.2.1.
6.2.4 Anchoring piles to rock
Rock anchors are provided for tension piles when the depth of soil overburden is insufficient
to develop the required uplift resistance on the pile in shaft friction. In weak rocks such as
chalk or marl it is possible to drive piles into the rock, or to drill holes for bored piles so that
the frictional resistance can be obtained on the pile shaft at its contact surface with the rock.
However, driving piles into a strong rock achieves only a small penetration and so shatters
the rock that no worthwhile resistance can be obtained. The cost of drilling into a strong rock
to form a bored pile is not usually economical compared with that of drilling smaller and
deeper holes for anchors as described below, although drilling-in large-diameter piles to
carry ship berthing forces in marine structures is sometimes practised (see Section 8.2).
Anchorages in rock are formed after driving an open-ended tubular pile to seat the toe of
the pile into the rock surface. The pile must not be driven too hard at this stage as otherwise
the toe will buckle, thus preventing the entry of the cleaning-out tools and the anchor drilling
assembly. The soil plug within the pile is removed by baling, washing or ‘airlifting’. If a
bored pile is to be anchored, the borehole casing is drilled below rock level to seal off the
overburden. All the soil within the piling tube is cleaned out, and drilling pipes with
centralizers are lowered down to the rock level. The anchor hole is then drilled to the required
depth. The cuttings washed out of the hole are removed by reverse circulation up the drilling
pipe or through a conductor tube up to the surface. The anchor, which can consist of a high-
tensile steel bar or a stranded cable, is fed down the hole. A small-bore nylon tube is taped to
the anchor and used to inject the grout at the bottom of the drilling hole (Figure 6.10). At this
level the bar or cable is provided with a compression fitting to ensure that the full bonded
length of the anchor acts in resisting uplift. Stranded cables are parted (after removing the
sheath) and the strands are degreased over the lower part which is bonded to the grout.
Grout is injected through the nylon tube to fill the annulus completely, and it is also
allowed to fill the piling tube to the required level. Where the anchors are stressed, the bar
or cable is carried up to the top of the pile or pile cap to which the stress from the anchor
is transferred by a stressing head and jack. The anchor is greased and sheathed in a plastic
tube supplemented by wrapping with waterproof tape to protect the unbonded length from
corrosion. The space surrounding the sheathed length can be filled with grout or concrete,
or left as a void. The latter is usually required in the case of piles in marine structures to
allow them to flex under lateral loading.
Unstressed or ‘dead’anchors can consist of steel tubes installed by drilling them down
into rock. On reaching the required depth, grout is pumped down the drilling pipe where it
emerges at the drilling bit and fills the annulus between the anchor tube and the rock. A seal-
ing plate prevents the grout from entering the space between the anchor tube and the drilling
pipe, as shown in Figure 6.11. The grout is allowed to fill the pile to the height necessary to
cover the top of the anchor tube, so as to protect it from corrosion and to serve as the
Piles to resist uplift and lateral loading 315