This section presents the setup of the forced vertical and horizontal dynamic testing and free vibration testing of helical and driven piles.
5.2.1 Test Piles
The tested helical piles included eight closed-ended, single and double-helix piles with outer steel pipe shaft diameter of 0.324 m, and helix diameter of 0.61 m. Four piles were 6.0 m long and four were 9.0 m long. The two driven piles were closed-ended, of similar geometrical and material configurations as those adopted for the helical piles’ shaft. Table 3.1 provides the geometrical properties of the test piles, while Table 5.1 gives their material properties. The test piles protruded 0.6 m above the ground surface (i.e. unsupported length), with a test body mass attached at their top.
5.2.2 Test Body Mass
To ensure that the resonant frequencies are well defined and fall within the frequency range of the exciting machine, a steel test body was added to the cap of each test pile. This also allowed simulating the effects of a superstructure on the response of the pile- soil system. Upon completing the pile installation, the pile head was machined providing a clean and levelled edge to facilitate placing the cap perpendicular to the pile centreline. The pile cap was a rectangular machined steel plate, 700 × 460 mm and 38.0 mm thick, and weighed approximately 88.0 kg. A circle equal to the diameter of the pile was drawn at the centre of the pile cap plate. The plate was then slipped over the pile head and its position adjusted until the drawn circle coincided exactly with the pile circumference in order to assure that both the plate and pile shared the same vertical axis. The plate was levelled in both directions and fillet welded to the test pile head. The schematic diagram of the experimental setup of piles is illustrated in Fig. 5.1. The setup for vertical and horizontal vibration tests on piles is shown in Fig. 5.2.
After the cap plate was securely affixed to the pile head, the steel test body plates were stacked on its top. The test body comprised of 29, 49 or 59 machined circular steel plates each with diameter of 724.0 mm, thickness of 25.4 mm, and mass of 79.0 kg. The steel plates had machined contact surfaces to prevent slippage. The lower and upper test body plates were centered and welded to the pile cap plate and the Lazan oscillator base plate, respectively. The entire stack of the test body plates was rigidly fastened together with four threaded steel rods so that the whole setup acts as a rigid body. The properties of the test bodies, oscillator, and pile cap are given in Table 5.2.
Table 5.1. Material properties of the test piles
Property Value
Moment of inertia 1.164×10-4 (m4) Young’s modulus 210 (GPa) Poisson’s ratio 0.3
Damping ratio 0.01
Unit weight 78.46 (kN/m3) Coefficient of rigidity 1.11
Table 5.2. Properties of test body, oscillator, and pile cap
Vibration direction
Properties Vertical Horizontal Vertical Horizontal Horizontal
No. of plates 49 49 59 59 29
Mass of cap-test body-
oscillator (kg) 4059.5 4062.5 4849.5 4852.5 2482.5
Height of centre of gravity (CG) , Zc, (m)
a 0.6644 0.6667 0.791 0.793 0.415
Height of excitation above centre of gravity (CG), Ze,
(m)
0.733 0.811 0.861 0.938 0.554
Mass moment of inertia about
y (kg.m2) b 710.7 710.7 1152 1152 238
a
Height of CG, Zc, is measured from the bottom surface of the pile cap plate, which is located at 0.6m
above ground surface.
b
y-axis lay horizontally through the CG and is parallel to the flexible shaft direction.
5.2.3 Excitation Mechanism
The vertical and horizontal excitation forces were produced by means of a Lazan mechanical oscillator, Model MO2460, mounted over the test body mass. The excitation force was harmonic, characterized as a quadratic function proportional to the square of driving frequency. The oscillator is comprised of two counteracting shafts each carried a set of eccentric masses to generate the harmonic excitation. The magnitude of the excitation force can be varied by altering the degree of eccentricity of the rotating unbalanced masses via an external knob at the end of one shaft assembly. The oscillator had a mass of 51.5 kg and was driven by a 7.5 HP 220 V three phase motor capable of generating sinusoidal force of 23.5 kN peak-to-peak. The speed of the motor was controlled by a variable frequency AC speed drive, yielding stable operating speeds
between 3 and 60 Hz. A well-balanced, flexible drive shaft was utilized to connect the oscillator to the motor through its end couplings, see Fig. 5.2.
The oscillator was placed on the top of its base plate, which was welded to the upper steel plate of the test body mass. Four holding rods were used to connect the holding channel frame to the oscillator base plate in order to keep the oscillator stable under vibration. The excitation was applied in the vertical and horizontal directions. For horizontal vibration, the oscillator was placed vertically on its base plate and in reverse for vertical vibration. Table 5.2 gives the properties of the test body, oscillator, and pile cap.
5.2.4 Instrumentation
To evaluate the dynamic performance of the pile, it was necessary to determine the displacement amplitude of the test body mass at each loading frequency accurately. The vibration measuring equipment consisted of two uniaxial piezoelectric accelerometers Model 333B50, one triaxial accelerometer Model 356B18, frequency measurement unit (tachometer), Model KM2235B, to monitor excitation frequency, and National Instruments data acquisition system Model SCXI-1520. The accelerometers were located on the test body in such way that two uniaxial accelerometers were mounted at equidistant positions from the foundation centre on the axis of symmetry. The triaxial accelerometer was mounted on one side of the test body, at the height of the centre of gravity in order to measure horizontal vibration with no effect from rocking mode of vibration on measurements. For vertical vibration, the displacement responses derived from the three accelerometers were averaged to eliminate the rocking mode component. For horizontal vibration, the displacement was derived from acceleration measurements of the triaxial accelerometer, while the other two accelerometers provided measurements for the rocking component (Figs. 5.1 and 5.3). To monitor the strain and force distribution along the helical piles, half-bridge strain gauge circuits were affixed on the pile shaft at specified locations as was discussed in Chapter 3.
1 2 3 Data acquisition system 4 5 6 7 8 0.60m 1.45m 1.55m 1.45m 1.50m 1.50m 0.30m 0.30m 0.60m 9.0m Inter-helix spacing 0.9m Helix diameter 0.61m 0.324m G.S. 1. Oscillator 2. Flexible shaft 3. Motor
4. Speed control unit 5. Test body mass 6. Pile cap 7. Two uniaxial
accelerometers (one on each side) 8. Triaxial accelerometer
(at C.G. of the static mass) Half-bridge strain gauges (4 half-bridges at each level)
Figure 5.1. Schematic diagram of dynamic test setup
Figure 5.2. View for test setup: a) vertical vibration; and b) horizontal vibration
Figure 5.3. Location of accelerometers