Users’ Manual
EDS-71V/P Single point borehole extensometer
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6
2.1.3
Connecting rod assembly
2.1.3.1 Fibreglass connecting rod assembly
Fibreglass connecting rod assembly (2) comprises of a fibreglass rod of specified length protected inside
an outer continuous nylon tube. The rod has two end connectors. One end connector has a male thread
and fits into anchor (1). Other end connector with a female thread is for connecting to reference button
(15). Displacement of connecting rod end is transferred through a link plate (16) to the displacement sensor
(20) with the help of a sensor-tightening nut (17) and a sensor lock nut (18). Components 15, 16, 17 and
18 are packed inside the reference head assembly in a plastic bag.
The fibreglass assembly is illustrated below in figure 2.3
Anchor end of nylon tube is firmly sealed with anchor to prevent any grout from leaking in. Other end of
nylon tube is firmly sealed in reference head assembly with help of rubber bush (10), washer (11) and
hollow bolt (12) and long hollow bolt
Proper sealing in reference head assembly is especially necessary for upwards to horizontally slanting
holes to prevent any grout leaking into reference head assembly during grouting.
The outer nylon tube allows free movement to fibreglass rod and reference button even after borehole is
grouted.
NOTE:
Fibreglass rods are generally more suitable for vertically upward holes as compared to
downward holes. This is because in vertically upward holes the connecting rod is in tension;
whereas in downward holes it is in compression, resulting in sagging or buckling. Longer the
extensometer, greater may be the sagging or buckling.
Shorter the extensometer, better is accuracy of measurement of relative displacement
between anchors. In deeper boreholes, use of stainless steel connecting rods is
recommended, as they are more rigid.
Design Engineer should carefully consider what type of connecting rod to use taking into
consideration field conditions and accuracy of measurement required. Actual performance of
rods is determined by site conditions and it is recommended to conduct some field tests to
arrive at a proper conclusion.
2.1.3.2 Stainless steel AISI 410 connecting rod assembly
Stainless steel AISI 410 connecting rods of 8 mm diameter are available in standard lengths of 1m, 2 m
and 3 m with a M6 x 12 mm male thread at one end and a M6 x 15 mm female thread at other end. These
are connected together at site for positioning anchor at the correct depth from mouth of borehole. Thread
sealant Loctite 290 or equivalent is used between threads to firmly grip connecting rods to each other.
To give an example, in case depth of a particular anchor from the mouth is 14 m, use four connecting rods
of 3 m length and one of 2 m length. Similarly, in case depth of anchor from the mouth is 25 m, use eight
connecting rods of 3 m length and one connecting rod of 1 m.
Nylon Tube (OD - 10 x 1 thk)
Fibre glass rod assly.
Nylon Tube (OD - 12 x 1 thk)
Figure 2.3 – Fibreglass connecting rod assembly
