889
Technical Committee 104 /
Comité technique 104
Tests were performed under saturated conditions with 0.15
m of water on top of the sand bed. The anchor was pulled
through the sand bed until it was on the top of the berm. During
the test the displacement of the plunger and the force of the
plunger were measured continuously. After the test the water
table was lowered to create some capillary forces to keep the
anchor in position during spinning down. Back at 1-g the
position of the anchor was carefully measured, see Figure 7.
Figure 7. Carefully measuring the position of the anchor after a test.
3.2 1-g tests
The set-up for the 1-g tests was exactly the same as for the
centrifuge test. The same soil preparation technique, container,
plunger and pulley system were used only now the tests were
run outside the centrifuge at normal 1 g conditions under
atmospheric pressure. Measurements performed during the tests
and after the tests were the same as in the centrifuge. Three tests
were performed.
4 TEST RESULTS
4.1 Corrections on measurement data
The parameters of importance are the penetration and the
displacement of the anchor and the pulling force on the
anchor.The penetration was measured after the test. The other
parameters were determined during the test from the
displacement of the plunger and the force that was measured on
the plunger. The cable used in the pulley system was
3 mm
dyneema cable with a maximum pulling strength of 5 kN. In
order to limit elongation during the test, the cable was pre-
stressed with a force of 2.8 – 3.0 kN. However, there still was
some elongation of the cable. Furthermore, there will be friction
in the pulley system. A dummy test was performed to correct
for the friction both at 1-g and 80-g. In this test the Dyneema
cable was connected with a spring connected in the centrifuge
and an extra force transducer was located between the spring
and the cable. Such a transducer could not be placed between
the anchor and the cable during the real tests because the
dimensions of the transducer and the necessary electrical cables
would influence the test results. In the tests, the force on the
cable at the spring and the force on the plunger were measured.
The results of the measurements are presented in Figure 8. Due
to friction in the system, the results differ depending on the
direction of movement. The movement from left to right in the
plot is the movement during anchor pulling. It appears that,
apart from very small puling forces at plunger displacements
around -120 mm, during pulling the pulling force as measured
in the cable with the force transducer near the spring is always
about 0.75 times the force measured with the force transducer at
the plunger (and divided by 5 to correct for the pully system).
This is only possible when the friction in the system increases
linearly with the pulling force. This correction was applied in
Figure 9.
-0.30
-0.25
-0.20
-0.15
-0.10
-0.05
0.00
0.05
0.10
force in cable and friction (kN)
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
Force on plunger, blue line (kN)
-130 -120 -110 -100
-90
-80
-70
-60
displacement plunger (mm)
Force measured on cable
Force on plunger (right Y- axis)
friction
Figure 8: Comparison forces measured in a dummy test on the cable and
on the plunger at 80g.
The correction for the elasticity of the cable was only
performed for the 80-g tests. Due to the much smaller forces
this was not necessary for the 1-g tests. The elasticity of the
cable can be seen at the end of a test. When the anchor is pulled
to its final position (on top of the berm) the pulling force is
decreased retracting the plunger, while the anchor remains at the
same position (controlled by the cameras). This allowed for
higher pulling forces to measure is the elastic deformation of the
cable. For low pulling forces there is an additional mechanism,
the cables sag due to gravity. The last mechanism is only of
importance for low pulling forces. Only the elastic relaxation is
of importance during anchor pulling. Figure 9 shows the
movement of the plunger as a function of pulling force during
relaxation as measured in a test.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
pulling force on plunger (kN)
135
140
145
150
155
160
displacement (mm)
Figure 9. Relaxation of cable and sagging at the end of a 80 g test. The
slope of the steep vertical part of the measured plunger force is
determined by the elastic strain. The flatter part at low pulling force is
caused by sagging of the cables.
0.00
0.05
0.10
0.15
0.20
0.25
0.30
pulling force on anchor (kN)
-600 -400 -200
0
200
400
600
800
displacement (mm)
force corr.
force not corr.
Figure 10. Pulling force and displacement with and without corrections
on both force and displacement for a 80 g test.
Young’s modulus is about 4 kN/m, measured at the plunger,
thus Young’s modulus of the cable is 4/25=0.16 kN/m.
The influence of the corrections for both the displacement
and the force on the results are shown in Figure 10.
It is clear that the correction for the displacement hardly
influences the results even at 80 g, but that the influence of the
correction for the friction force is considerable.
