If high-tensile steel (which has a yield stress of 347 N/mm^2 ) is used
factor of safety at yield (which is satisfactory).The anchor will be installed in a 215 mm diameter drill-hole. Thus for an ultimate bond stress
between grout and strong rock of 0.8 N/mm^2 (Table 6.5), allowable bond stress 0.8/2.5
0.3 MN/m^2.
required bond lengthDrill the hole to say 7 m below the toe of the pile and grout the annulus fully. Then
working stress between steel and grout
(which is satisfactory)The bond length should be increased by approximately L/2 to comply with Figure 6.12b to
give a bond length over the cone of 10 m below the surface of the weathered chalk and space
the piles at 4 m centres. Then in Figure 6.17a, Vc 0.35 103 350 m^3. Since m/L 0.61,
m 10 0.61 6.1 m. In Figure 6.17b, S 4 m, so that S/m 4/7 0.66, and thus
. M 2, N 1, and P 0, and therefore
Rock volume anchored by pair of anchors m^3.
Weight of rock resisting uplift 560 0.59.81 2747 kN.
Factor of safety against uplift 2747/2800 0.98.
This is insufficient for compliance with the EC7 recommendations (see below) but the
frictional resistance on the sloping surfaces of the overlapping cones can be taken into
account. As a rough approximation, assume that the two cones act as a rectangular block
having a volume of 560 m^3 , say 10 8 7 m deep, and take the angle of shearing
resistance of the chalk as 30 ̊ and take Koas 1.5.
Average unit frictional resistance on the vertical surfaces of the block
1.5tan 30 9.810.53.5 14.9 kN/m^2
Frictional resistance to uplift 2 (108) 7 14.9 3755 kN
Total resistance 2747 3755 6502 kN
Factor of safety against uplift 6502/2800 2.3Checking the design of the anchorage for compliance with the EC7 requirements, the
axial uplift load on a single anchor pile is a variable unfavourable action. From Table 4.1
take G 1.5.
Therefore design value of uplift Fdt 1.5 1400 2100 kN.
The value of 71.5 kN as the uplift resistance of the 600 mm pile within the weathered
chalk was based on a number of tests in compression and uplift (ref. 4.43). Take the results
of three tests as applicable to the selected unit resistance of 30 kN/m^2 to give a correlation
factor of 1.2 (Table 4.7).
Design resistance in weak chalk 71.52.5/1.2 148 kN.
The grout to strong chalk ultimate bond stress of 0.8 N/mm^2 (Table 6.3) was based on pull-out
tests, for which the standard practice of cycling the load would have been adopted. Table 6.2
gives an anchorage resistance partial factor of 1.4, therefore for a 10 m bond length:
350[(2 1)(1 0.40)] 560
VmVc 0 VnVc 0.
1329 1000
168.3 7 1000
0.36 N/mm^21320 1000
215 0.3 1000
6.6 m.347
175
2.0
Piles to resist uplift and lateral loading 361