Pile Design and Construction Practice, Fifth edition

Thus the load to be carried by anchorage in shale � 1100 – 72 � 1028 kN. This load can be
resisted by an anchor cable made up from seven 15.2 mm 7-wire Dyform strands. The
breaking load of each strand is 300 kN. Therefore
working load � 1028
� 0.49�breaking load of strand
7�300
which is satisfactory. The approximate overall diameter of the 7-strand cable is 45 mm.
Therefore for an allowable bond stress between steel and grout of 1.0 N/mm 2 :
required bond length of cable � 1028�1000
� 7.3 m
��45�1.0�1000
Piles to resist uplift and lateral loading 359
Drill the cable hole to 9 m and provide an unwrapped and cleaned bond length of 7 m with
compression fittings swaged on to the lower end. The cable can be fed down a 150 mm
borehole for which,
1028�1000
working bond stress between rock and grout �
�0.31 N/mm2
��150 �7.0 �1000
which is satisfactory for a strong shale (Table 6.3). The stress is not excessive if the anchors
are stressed to 1.5 times the working load during installation.
From Figure 6.17a, the volume of a rock cone with a 30� half angle lifted by single anchor
cable is 0.35 � 9 3 � 255 m 3 . The submerged weight of the rock cone � 1.3 � 9.81
� 255/1000 � 3.25 MN.
Factor of safety against uplift � 3.25
� 3.1 (which is satisfactory)
1.03
The anchorage of the whole dock floor requires 70 lines of anchors (at right-angles to the
centre-line of the dock) and 20 ranks of anchors ( parallel to the centre-line of the dock) to
form the 3 m square grid. Therefore in Figure 6.17b, N � 70, M � 20, and P � S � 3 m.
From Figure 6.17a, m/L � n/L � 0.57, and therefore, m � n � 0.57 � 9 � 5.1 m. Then
P/n � s/m � 0.59 so that, from Figure 6.17b, �Vn�Vc ��Vm�Vc � 0.45.
Because
(P/n) 2 � (S/m) 2 � 2 � 0.59 2 � 0.7 is less than 4, there is composite overlapping of the rock
cones, and the charts are not valid. The intersecting cones represent a rock volume roughly
estimated to be 69 � 3 � 19 � 3 � 6 � 70794 m 3 .
The sum giving the total force resisting uplift is thus as follows:
210 � 60 � 0.8 � 2.4 � 9.81
Weight of dock floor � � 237.3 MN
1000
210 � 60 � 8.0 � 0.8 � 9.81
Submerged weight of soft clay � � 791.1 MN
1000
70 794 � 1.3 � 9.81
Submerged weight of anchored rock � � 902.8 MN
1000
Total � 1931.2 MN