Page 181 - Numerical Analysis and Modelling in Geomechanics

162 C.L.RAMSHAW AND A.R.SELBY
with the beams but not rotation. This model could not compute geometric
attenuation of the ground waves, so the strategy used was to input reasonable
impact or vibratory excitation around a pile space, and to compute pipe strains
when the ppv’s at the ground surface directly above the pipe were 10 mm/s. This
relates to Transco’s field practice. Figure 5.25 shows an FE mesh for a 10 m
deep pipe and an impact hammer, and an FE/IE mesh for a 2 m deep pipe with
vibrodriver.
A number of cases were analysed, and some peak bending stresses induced in
the pipe cross-section are summarised in Table 5.3.
The stresses induced into the pipe by bending deformation of the cross-section
are very small, and are negligible in comparison with the yield stress of the pipe
steel of 413 MPa or 552 MPa respectively. There is strong evidence that the
stresses induced by deformation of the cross-section are negligible. However,
other forms of distress to pipelines may be caused by wave-generated
mechanisms, such as joint pull-out or pipe-branch joint damage.
The deformed shapes of the pipe cross-section in response to P, S and R-
waves were particularly interesting and not entirely expected. The passage of a P-
wave was fairly predictable in causing ovalling of the cross-section with the
major axis normal to the direction of travel of the wave front, Figure 5.26. This
deformation caused the lowest stress during passage of ground waves.
The response of the cross-section of the pipe as an S-wave passed through
showed a maximum deformation with a major axis at 45° to the direction of the
wave travel. This can be verified by consideration of the effect of an S-wave
upon a square element, as shown in Figure 5.27.
Finally, deformation of the pipe section near to ground surface caused by a
Rayleigh wave is even more complex, as can be anticipated, because of the
combination of vertical and horizontal movements of a soil particle in a
retrograde elliptic path, see Figure 5.28.

Conclusions

Impact driving of a pre-formed pile causes significant transient ground
vibrations, which attenuate non-uniformly with distance from the pile. Vibration,
expressed as ppv’s, is a function of hammer energy, hammer-pilehead impact,
and, to a lesser extent, of soil type.
A three-stage computational model has been shown to be capable of good
quality back-analysis of the impact and of the definition of outgoing ground
vibrations. The method is less reliable in pre-estimating vibrations because of the
variability of driving parameters, of soil properties and of site construction
features. The selection of an appropriate dynamic soil modulus has been
discussed.
Pile installation and extraction by vibrodriving is highly effective in saturated
fine sands and silty soils. A two-stage computational procedure has been
developed which shows close agreement with site records of ground surface