Monitoring of soil stiffness during ground improvement using seismic surface waves

The continuous surface wave (CSW) technique relies on the propagation properties of vertically polarized seismic surface waves, or Rayleigh waves. Surface waves have depths of penetration related empirically to their wavelength and hence frequency. Surface wave velocity is determined over a range of frequencies and used to calculate the variation in soil stiffness with depth. The nature of the propagation of seismic waves dictates that the stiffness calculated is the maximum value occurring at very small strain (<0.001%). At such strain levels most soils are believed to behave elastically, as a result of which stiffness is independent of strain. The introduction of ground improvement techniques such as vibro and dynamic compaction has led to different methods of monitoring and quality control being used with varying degrees of success. Despite the wide range of testing available, none is ideally suited to the huge variation in grain size found in the semi-cohesive superficial deposits that predominate as overburden throughout the UK. The coarse nature of such material often militates against invasive testing methods such as continuous cone penetration or pressuremeter testing. Surface wave testing, being non-intrusive, does not suffer the same disadvantages. Following extensive trials carried out on numerous sites, clear evidence of stiffness improvement after treatment is beginning to emerge as illustrated in this paper by case studies.