Pile Design and Construction Practice, Fifth edition

The pull-out resistance of the mass of rock (as shown in Figure 6.12b) is the final criterion
of the performance of an individual anchor. The actual shape of the mass of rock lifted (if
failure does not occur due to the failure of the steel-to-grout or rock-to-grout bond) depends
on the degree of jointing and fissuring of the rock and the inclination of the bedding planes.
Various forms of failure are sketched in Figure 6.13. A cone with a half angle of 30 gives
a conservative value for the pull-out resistance and represents conditions for a heavily
jointed or shattered rock. Wyllie(4.39)suggests that the base of the cone should be taken at
the mid-point of the bonded length (Figure 6.12b), but this arrangement would not apply for
the case of a compression fitting at the bottom of the anchor. Because shear at the interface
between the surface of the cone and the surrounding rock is neglected, a safety factor of
unity can be taken on the weight of the rock cone, where the rock is bedded horizontally or
at moderate angles from the vertical (Figure 6.13a). Where the bedding planes or other joint
systems are steeply inclined, as shown in Figure 6.13b to d, either an increased factor should
be allowed, or an attempt should be made to calculate the uplift resistance of the rock mass
by rock mechanics’principles. The submerged weight should be taken for rock below
groundwater level or below the sea. The uplift resistance of the cylinder or cone of soil

Piles to resist uplift and lateral loading 321

Table 6.3Examples of bond stress between grout and rock

Type of rock Bond stress between Reference

grout and rock at

failure N/mm^2

Chalk (Grade I) 0.21 Littlejohn(6.5)

Chalk (Grade III) 0.80 Littlejohn(6.5)

Keuper Marl (Zones I and II) 0.17–0.25 Littlejohn(6.5)

Chalk 1.0 Hutchinson(6.6)

Weathered shaley slate 0.27 Unpublished(6.7)

Hard shaley slate 1.0–1.7 Unpublished(6.7)

Billings shale (Ottawa) 3.0 Freeman et al.(6.8)

Sandstone 0.6 Unpublished(6.7)

Initial bonded length

Pull out

Cone of failure

of rock in uplift

position suggested

by Wyllie

30 ° half angle

of cone

Bondedlength (

L)

Sheathed length

L/2

Cone of rock

lifts out

Maximum stress

in anchor

(a) (b)

Figure 6.12Pull-out of cone of rock (a) Fully bonded anchor (b) Upper part sheathed, lower part
bonded.