The reliability of seismic surface waves methods for material characterisation and earthworks construction for high-speed railways

The speed of seismic surface waves generated by the passage of high-speed trains is an important consideration in the design of railway earthworks, since if the soil surface wave velocity approaches the train speed this can cause dynamic issues with the track. To ensure track stability and a good earthwork performance, surface wave’s velocity should significantly exceed the train speed. Traditionally this requirement has been satisfied by specifying a minimum stiffness of the earthwork, empirically shown to give acceptable performance.

With train speeds increasing, for the new high-speed railway (HS2) in the UK, it has been preferable to predict and then ensure that minimum specified Rayleigh and shear wave velocities (Vs) are achieved. This can be achieved through suitable geophysical tests and an understanding of their reproducibility and repeatability in defining wave velocities for compliance assessment. The methods mostly used are typically Multichannel Analysis of Surface Waves (MASW) and Continuous Surface Waves (CSW)). However, as these are not standardised, there is the potential for differences in how contractors design, collect and process data which could lead to differences in assessment for any given site.

Based on the above, and since Seismic Surface Waves methods used to estimate Vs through inversion of Rayleigh wave velocity present some known limitations (i.e., the non-uniqueness of the inverse problem solution and the way data acquisition and preliminary processing is done by one geophysicist to another), this project investigates and evaluates the method’s reliability and establishes specific guidelines based on empirical tools, for quicker soil characterisation from Rayleigh wave velocity by engineers and non-experts in geophysics.

The research undertaken herein included a series of qualitative interviews with experienced geophysicists from companies who apply these methods (as well as borehole seismic methods) in the UK and have tested HS2 earthworks. It aimed to understand how they design and undertake such investigations, focusing on data acquisition, processing and interpretation. This study showed there is variation in testing and analysis protocols for similar sites and that experience is important in undertaking such work.

Interview study informed the design of a blind comparative field survey, including a series of seismic line protocols to be tested on a trial embankment, made of both stabilised and natural ground material. The results showed that in treated soil CSW produced more consistent Vs-depth profiles and MASW results presented high variability, attributed to the additional complexity resulting from this unusual stiffness profile, which needs to be considered when specifying appropriate tests for Vs compliance in earthwork design. Due to this variability, further analysis was done, focusing on the dispersion of Rayleigh waves instead, attempting to overpass the mathematically ill-posed inverse problem for Vs. From the result’s analysis it was found that dispersion data showed a good agreement and lead to a proposed tool to translate frequency data into approximate depths. Furthermore, raw data re-analysis showed that changing the dispersion picking approach and the a-priori model parameters mostly affected Vs in natural ground rather than in stabilised soil, proving that processing in treated materials must be done with caution. Finally, validation of the proposed guidelines was done in two other HS2 sites where MASW was also compared to Cross-hole test, indicating higher Vs difference in untreated soil, attributed to the complexity of applying MASW in non-normally dispersive soils where Vs is not augmenting with depth due to stiffer layers on top.