aluminium-cast joint surface prints. The
tested cable was installed across two
concrete blocks with the desired shear
surface roughness produced as off
prints of aluminium joint surface
moulds. The study reported that a cable
bolt placed across a joint more than
doubled shear resistance of shear blocks
with both smooth and rough joints.
Testing of the cable bolt in shear
using the double embedment assembly,
as recommended by the British
Standard (BS 7861-2:2009),
5
is an
unrealistic approach. Guillotining of the
cable tendon, leading to true shearing
of the metal elements is not what occurs
when a cable bolt is sheared across a
rock joint. In reality, past laboratory
experiences have shown that the failure
of the cable bolt in rock or composite
material is a combination of both tensile
and shear failure, manifested with
crushing of the rock or concrete
surrounding the zone of the sheared
plains.
6,7
These findings were also
demonstrated by numerical simulations
in both rock bolts.
8
Accordingly, the
application of the double shear system
for testing cable and rock bolts in
concrete moulds is the subject of this
paper.
Cable bolt description
The tested cable bolts were Hilti 19 wire
HTT-UXG plain strand and Hilti 19
wire HTT-IXG spirally indented strand,
which only has indentation on the
surface of the outer strands. Both cable
bolts were of sealed wire construction
type, consisting of an outer 5.5 mm dia.
strand layer, overlying the middle
3 mm dia. wire strands. Both layers are
wrapped around a single sold
7 mm dia. strand wire core, generally
known as king wire of cable strand
construction. Both Hilti spiral (Figure
1a) and plain (Figure 1b) cable-bolts
consisted of 19 strands (9 x 9 x 1) of seal
construction wire rope (Figure 1c). The
cables were both 22 mm dia. with the
rope thread profiles being “Left Hand
Lang’s Lay” construction type. The lay
length of the strands in both cables was
300 mm. Details of both cables
specifications are shown in Table 1.
Concrete samples
preparation and cable bolt
installation
Concrete block casting
Figure 2 shows a general layout of an
assembled double shear apparatus with
installed cable bolt. Each double shear
testing process required three
cement/mortar concrete blocks with
two outer 300 mm side cubes and a
central rectangular block 450 mm long.
The casting of the concrete blocks for
the test was carried out in the steel
frame of the double shear apparatus.
Once mixed, the cement mortar was
poured into each section of the 20 mm
thick steel frame. A plastic conduit with
a 20 mm dia. was set through the centre
of the mould, lengthways, to create a
hole for cable installation in the
concrete blocks. Once the concrete was
set, the plastic conduit was removed
from the centre of the blocks. The centre
hole was subsequently reamed to a
27/28 mm dia. rifle-shaped hole, using
twin wing drill bits. The UCS value of
the concrete used in this study was
40 MPa, determined from testing the
representative 100 mm dia. cylindrical
concrete samples. Lately, the holes are
cast rifled instead of being drilled for
rifling.
Cable bolt installation in
concrete blocks
The installation of the cable bolt in a
three-piece concrete mould was carried
out using Orica (formerly Minova)
FB400 cable bolt grout. The cable bolt
was inserted into the central hole of the
assembled concrete blocks. A 60 t load
cell was then mounted on one side of
the protruding cable bolt using a barrel
and wedge assembly. This was
followed by the addition of the grout
Table 1. Specification of Hilti cable bolts
Performance data
Hilti HTT-UXG (plain strand) Hilti HTT-IXG (indented strand)
Ultimate yield load
495 kN (50 t)
425 kN (43 t)
Ultimate failure load
573 kN (58t)
510 kN (52 t)
Bolt diameter
21.8 mm
21.8 mm
Cross-sectional area
312.9 mm
2
277 mm
2
Mass
2.482 kg/m
2.2 kg/m
Outer strand diameter
5.5 mm
5.5 mm
Inner lay strand diameter
3.0 mm
3.0 mm
Core strand diameter
7.0 mm
7.0 mm
Hole dia./collar reaming size
28 mm/55 mm
28 mm/55 mm
Figure 1a. Hilti 19 wire HTT-UXG
indented strand cable bolt.
Figure 1b. Hilti 19 wire HTT-IXG plain
strand cable bolt.
Figure 1c. 19-strand seal construction
wire rope.
26
|
World Coal
|
January 2016
