Pile Design and Construction Practice, Fifth edition

Top of rock
30˚ 30˚
Figure 6.15 Failure condition at group of anchors in rock.
Piles to resist uplift and lateral loading 323
Anchors
Rockhead Pile
Figure 6.16 Splaying anchors in group to increase uplift resistance.
Shaded area
shows block of
rock resisting
uplift
Drilled
anchor
The calculation of the volume of rock V c in a single cone with a half-angle of 30� at various
angles of inclination � to a horizontal rock surface can be performed with the aid of the curve
for V c/L 3 in Figure 6.17a. The effect of overlapping cones of rock in groups of vertical or
raking anchors can be calculated by reference to Figures 6.17a and b. These charts enable the
overlapping volumes �V m and �V n to be calculated for a group of anchors arranged on a
rectangular grid. They are not applicable to a diagonal (i.e. ‘staggered’) pattern. All the
anchors in the group are assumed to be arranged at the same angle of inclination to the horizontal
and the charts are based on a cone with a 30� half-angle. The charts are not valid if
the sum of (P/n) 2 and (S/m) 2 is less than 4 when composite overlapping occurs. In such a case
the total volume acting against uplift should be estimated from the geometry of the system.
Because of the various uncertainties in the design of rock anchors as described above, it
is evident that it is desirable to adopt stressed anchors with every anchor individually
stressed and hence checked for pull-out resistance at a proof load of 1.5 times the working
load. However, it should be noted that the technique of stressing anchors by jacking against
the reaction provided by the pile does not check the pull-out resistance of the cone of rock:
this is clear from Figure 6.10. The resistance offered by the mass of rock can be tested only
by providing a reaction beam with bearers sited beyond the influence of the conjectural rock
cone. Tests of this description are very expensive to perform and it is usual to avoid them by
adopting conservative assumptions for the dimensions of the cone, and applying a safety
factor to the calculated weight if required.