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.</p

The application of seismic surface wave testing on stiff low-height embankments for the construction of high-speed railway’s earthworks: a case study in the UK

The speed of seismic surface waves generated by the passage of high-speed trains is an important consideration in the design of railway earthworks. To ensure track stability and good earthwork performance, it should significantly exceed the train speed. Traditionally this requirement has been satisfied by specifying a minimum stiffness of earthwork, empirically shown to give acceptable performance. With train speeds increasing, it has been preferable to predict and then check (during construction) that minimum specified Rayleigh and shear wave velocities are achieved. This requires suitable geophysical tests and an understanding of their reproducibility and repeatability in defining wave velocities for compliance assessment. This paper presents the results of comparative tests to evaluate differences in estimated shear wave velocities, using Multichannel Analysis of Surface Waves (MASW) and Continuous Surface Waves (CSW) on a trial railway embankment. The results show that both methods estimated shear wave velocities to similar depths, but CSW produced more consistent shear wave velocity profiles when a stiff embankment overlies natural ground. The variation observed in the MASW testing was attributed to the additional complexity resulting from this unusual stiffness profile. This needs to be considered when specifying appropriate tests for shear wave compliance in earthwork design. </p

Seismic surface waves and borehole methods to determine shear wave velocity: A review of measurement practice by contractors in the UK

In high-speed railways if the soil surface wave velocity approaches the train speed this can cause dynamic issues with the track. For the new high-speed railway (HS2) in the UK, Rayleigh and shear wave velocities (Vs) are determined as part of construction compliance testing using seismic surface waves methods (typically Multichannel Analysis of Surface Waves (MASW) and Continuous Surface Waves (CSW)). However, as these methods are not standardised, there is the potential for differences in how contractors design, collect and process test data which could lead to differences in assessment for any given site. As part of a wider project investigating such tests, a qualitative interview study was undertaken of contractors testing HS2 earthworks to understand how they design and undertake such investigations. The interviews focus on data capture, data processing and interpretation of seismic surface waves methods, but are also compared to the methods used of measuring Vs in boreholes. This show there is variation in testing and analysis protocols for similar sites and experience is important in undertaking such work. Contractors use experience from other projects and review their processes on site to check the data is suitable. They use both commercial and inhouse software for data analysis and the final velocity profiles produced strongly depend on the assumptions made around soil conditions. While there are many similarities in approach used, the differences lead to variability in results. The paper concludes by proposing elements of the testing that could (with further work) be included in best practice guidance around data collection and processing.</p

Blind field tests to determine Rayleigh wave velocity on a high-speed railway environment: The reliability of seismic surface waves methods

To ensure track stability and good earthwork performance for high-speed railways the surface wave velocity in the earthworks should exceed the train’s speed. Specifications for high-speed rail are now stating minimum Rayleigh wave (VR) velocities to be checked during construction. This requires suitable reliable geophysical tests, however there are no defined standards or data collection and processing protocols. Additionally, the analysis of these data traditionally relies on the practice and experience of those undertaking the work which can introduce variability in results. This paper presents the results of a blind comparison trial investigating Multichannel Analysis of Surface Waves test (MASW) for use in high-speed rail earthworks compliance evaluation, concentrating on assessment of Rayleigh wave velocity (VR). Testing was undertaken by four companies, at the same site on natural ground and a stabilised trial embankment. Defined tests protocols and a test of their own design against a specification to assess VR were used. The anonymised VR results show reasonable agreement in the dispersive character of the soil if higher modes are carefully considered when picking a dispersion curve. The VR results were then investigated against depth using a rule of thumb. Such an approach avoids the inversion processing step (to get more traditional Vs against depth) which potentially introduces variability. This suggests that direct Rayleigh wave data could be used by earthworks engineers to give routine compliance assessment and then if required further investigation undertaken in areas of compliance concern within the overall earthwork.</p

Seismic surface waves methods for high-speed rail earthworks compliance: a review of measurement practice

In high-speed railways if the soil surface wave velocity approaches the train speed this can cause dynamic issues with the track. For the new high-speed railway (HS2) in the UK, Rayleigh and shear wave velocities are determined as part of construction compliance testing using seismic surface waves methods (typically Multichannel Analysis of Surface Waves (MASW) and Continuous Surface Waves (CSW)). However, as these methods are not standardised, there is the potential for differences in how contractors design, collect and process test data which could lead to differences in assessment for any given site. As part of a wider project investigating such tests, a qualitative interview study was undertaken of contractors testing HS2 earthworks to understand how they design and undertake such investigations. The interviews focus on data capture, data processing and interpretation. The interviews show there is variation in testing and analysis protocols for similar sites and that experience is important in undertaking such work. Contractors use experience from other projects and review their processes on site to check the data is suitable. They use both commercial and in-house software for data analysis and the final velocity profiles produced strongly depend on the assumptions made around soil conditions. While there are many similarities in approach used, the differences lead to variability in results. The paper concludes by proposing elements of the surveys that could (with further work) be included in best practice guidance around data collection and processing