Pile Design and Construction Practice, Fifth edition

318 Piles to resist uplift and lateral loading
diameter of the anchor and the influence of the bottom compression fitting in short anchors.
The anchor diameter is of significance since with large-diameter high-capacity anchors there
is an appreciable diminution of diameter caused by the inward radial strain that occurs under
the tensile load. This creates a tendency to weaken the bond between the steel and the grout.
Specifications for anchorage materials and grouting cements and recommendations
for the bond strength at the grout to tendon interface are given in British Standard 8081,
Code of Practice for Ground Anchors. This code provides a wealth of useful and practical
information on the design, installation, and testing of anchors in soils and rock. The code
recommendations for ultimate bond strength are
Plain bar not greater than 1 N/mm 2
Clean strand or deformed bar not greater than 2 N/mm 2
Locally noded strand not greater than 3 N/mm 2
Mixes of pumpable normal Portland cement grout have compressive strengths in the
range of 14 to 21 N/mm 2 at 3 days. Special grouts are formulated for injection into the
annulus between an anchor and a tubular pile or between a pile and a surrounding sleeve.
The grouts incorporate plasticizers, expanding agents and fibrous bonding materials. By
adopting a water/cement ratio of about 0.5 compressive strengths of the order of 24 N/mm 2
are attainable at 3 days. Alternatively for marine work a mix consisting of 100 parts of API
Oilwell B cement to 34 parts of seawater will develop a characteristic cube strength of about
22 N/mm 2 at 3 days. However, when such special grouts are used to transfer the load
between large-diameter piles and a surrounding sleeve, correspondingly high bond stresses
cannot be achieved. This is because of the shrinkage of the grout in the relatively wide annulus
and the diminution in the diameter of the inner member due to the inward radial strain
when under tensile load.
The transfer of load from a pile to the sleeve can be effected wholly through shear keys
formed on the inner surface of the sleeve, and outer surface of the pile, and these should be
in the form of beads of weld metal or welded-on steel strips.
The ultimate grout to steel bond strength on the surface of tubular piles on pile sleeves
either with or without mechanical shear connectors can be calculated by an equation
recommended by the UK Department of Energy (6.3) as follows:
f buc � K·C L(9C s � 1100h/s) � ( f cu) 0.5
where f buc � characteristic bond strength in N/mm 2
f cu � characteristic grout compressive strength in N/mm 2
K � stiffness factor (see below)
C L � coefficient for grout length to pile diameter ratio
C s � surface condition factor
h � minimum shear connector outstand in mm
s � nominal shear connector spacing in mm
The stiffness factor is given by
K � 1 m� D t � �1
g
��� D t � � � p
D t � s� �1
(6.6)
(6.7)