Marco teórico

For the purpose of using them in design, the results of dynamic load tests on piles have to be transformed into an equi val ent static load–settlement curve up to failure. This is par ticu larly cum ber some and un cer tain for the dynamic load test proper, while for the rapid or Statnamic test significant sim pli fica tions are provided by the longer duration (lower frequency) of the load application.

As illus trated in Figure 7.21, the range of nat ural frequencies of a typical pile–soil system is inter medi ate between the loading frequency of high strain dynamic tests and that of Statnamic tests. For this reason the Statnamic test, as the traditional static load test, does not gen erally need to be in ter preted through the applica tion of the stress wave theory to the pile while this is mandatory for high energy dynamic testing.

Due to the variety of soil con ditions, geometry of piles and Statnamic pro ced ures, this statement can better be speci fied using some quantitative para meter. To this aim Middendorp and Bielefeld (1995) defined a wave number, Nw = cT / L, where T is the

load duration, c the wave velocity in the pile and L its length. There is a broad the­ or etical and experimental evid ence that stress wave phenomena in a pile body can be neg lected for Nw > 12.

The wave velocity c in a concrete pile is around 4000 m/s and in a steel pile 5200 m/s. For a Statnamic load test typical value of T is not less than 0.15. If a pile is 10 m long the wave number Nw ranges between 40 and 52 depending on the pile

mater ial: the stress wave phenomena down the pile body can be thus neg lected. In the case of a very long pile, for instance 50 m or longer, the wave number Nw ranges

between 8 and 10.4 and the in ter pretation of the Statnamic test should include the stress wave analysis.

The in ter pretation of the test without an ana lysis of the stress wave propagation in the pile body is often associated to a pile modelled as rigid body. This can be mis­ leading, since significant strains de velop in the pile during a Statnamic load test. In fact, for values of Nw > 12, the signals along the pile shaft are substantially all in

phase, as it occurs in the extreme case of the static load tests. If on the contrary Nw < 12, the above statement does not hold true; this occurs for very long piles

during rapid load tests, and for dynamic high energy tests whatever the pile.

To support these statements, Figure 7.22 reports the results of a Statnamic and a dynamic load test on a bored cast in situ pile with a dia meter d = 1.07 m and a length L = 27.3 m (Pando et al. 2000). The displacement at the top and bottom of the pile,

Figure 7.21 Typical frequency of load testing methods compared to the natural frequency of pile–soil system (after Bermingham and Janes 1989).

Static Range of natural frequency of piles Dynamic Statnamic Energy 0.00001 1 10 Frequency in Hertz 100 1000

170 Present practice: vertical loads

obtained by double integration of the meas ured acceleration, is reported; it can be clearly seen that in the Statnamic test (Figure 7.22.a) the displacement at the top and bottom of the pile are substantially in phase, while the same does not occur during the dynamic load test (Figure 7.22.b). The displacement at the top of the pile obtained by direct laser meas ure ment in the Statnamic test are also reported; there is a satisfactory agreement between the direct meas ure ment and the double integration of the acceleration.

Figure 7.22 Typical displacement signal at the top and at the bottom of a pile during (a) a Statnamic load test and (b) a dynamic load test.

For large values of Nw the time needed for the com pres sion wave to travel down­

ward and back is much smaller than the duration of loading; during the Statnamic tests, hence, the pile usually does not ex peri ence significant tensile stresses. On the contrary during a dynamic load test, significant tensile stresses are de veloped along the pile shaft and structural damages are pos sible. This is one of the reasons why prefabricated high quality reinforced concrete piles or steel piles are more often sub­ jected to dynamic testing than ordinary cast in situ bored piles; in fact dynamic tensile forces which de velop during driving opera tions are always accounted for at the design stage of this kind of pile. Furthermore, dynamic tests are usually ex ecuted for such piles for the availability of the current driving equipment. The exact know­ ledge of the cross section of prefabricated piles makes easier the in ter pretation of dynamic test results while, for bored cast in situ piles, structural defects could cause misin ter pretation of the test. The structural defects and the ordinary load trans fer along the pile shaft are in fact prac tically undistinguishable causes of changes in pile impedance along its axis.