For design environmental conditions with dead loads and maximum or minimum live loads
appropriate for combining with extreme conditions:
(6.9)
where fbais the allowableaxial load transfer strength (steel to grout bond strength) and other
terms are as previously defined.
It is evident from equations 6.8 and 6.9 that the first term represents the bond strength
value for the plain pipe. The equations are again subject to limitations as follows:
Grout compression strength to be greater than 17.25 N/mm^2 but less than 110 N/mm^2
Ratio Dp/tpnot more than 40
Ratio Ds/tsnot more than 80
Ratio Dg/tggreater than 7, not more than 45
Ratio Dp/sgreater than 2.5, not more than 8
Ratio h/snot greater than 0.10
Ratio w/hgreater than 1.5, not less than 3
Product fcuh/sequal to or not less than 5.5 MN/m^2The allowable bond stress between grout and rock depends on the compressive strength
of the intact rock, the size and spacing of joints and fissures in the rock, the keying of the
rock effected by the drilling bit, and the cleanliness of the rock surface obtained by the
flushing water. The size of the drill hole and the size of the annular space between the anchor
and the wall of the hole are also important. Usually the diameter of the drill hole is taken as
1.7 to 2.5 times the diameter of the anchor. The lower end of the range is used in a strong
massive rock and the higher end in a weak fractured rock. A large-diameter hole or a thick
annulus gives rise to problems due to the shrinkage of the grout and the consequent
weakening of the grout-to-rock bond. These difficulties can be overcome to some extent by
using a special bonding grout as described above. Also, by using a compression fitting at the
bottom of the anchor, part of the grout column is put in compression. The smaller the annu-
lus and the shorter the bonded length, the higher is the compressive stress on the grout and
hence its ability to lock into the surrounding rock. The value of the bond between grout and
rock will be small if the rock softens to a slurry under the action of drilling and flushing.
This occurs with chalk, weathered marl, and weathered clayey shales. Some observed values
of bond stress at failure for drill holes of up to 75 mm in diameter are given in Table 6.3.
If the bond stress between the grout and the rock is a critical factor in designing the
anchors, the allowable value should be obtained by increasing the length of the anchor rather
than by increasing the diameter of the drill hole, for the reasons already stated. However, in
certain conditions it is possible that the bond stress will not be reduced in direct proportion
to the increase in bond length. This is because of the possibility of progressive failure in a
hard rock. The maximum stretch in the anchor occurs at the top of the bonded length, and
this may cause local bond failure with the rock or the pulling out of a small cone of rock
(Figure 6.12a). Progressive failure then extends down to the bottom of the anchor. By
limiting the bond length and sheathing the upper part of the anchor, referred to in EC7
(Section 8) as the tendon free length, within the rock (Figure 6.12b), the pulling-out of a
cone of rock is prevented and the whole column of grout is compressed and acts in bond
resistance with the rock.
fba 0.1840.67fcu hs (MN/m^2 )320 Piles to resist uplift and lateral loading