Pile Design and Construction Practice, Fifth edition

The calculation of the volume of rock Vcin 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 Vc/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 Vmand Vnto 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 hor-
izontal 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.

Piles to resist uplift and lateral loading 323

Anchors

Shaded area

shows block of

rock resisting

uplift

Top of rock

30 ̊ 30 ̊

Figure 6.15Failure condition at group of anchors in rock.

Rockhead Pile

Drilled

anchor

Figure 6.16Splaying anchors in group to increase uplift resistance.