To improve the accuracy of shallow seismic shear wave velocity profiling, we propose a minimally invasive hybrid surface-and-borehole method that enhances the detection of higher modes of Rayleigh wave dispersion data. The new method combines techniques from the multichannel analysis of surface waves and multichannel simulation with one receiver (MSOR) methods to record components of Rayleigh wave motion at the surface as well as at shallow depths within the soil mass. The performance of the proposed method is demonstrated through computational and experimental studies. We show that individual modes of Rayleigh waves can exhibit different dominant depths at which their motion is most significant. This is demonstrated through a numerical study of eigenvectors of layered soil profiles via the stiffness matrix method, and confirmed by a finite element simulation of the apparent dispersion trends recorded at shallow depths using MSOR. Upon superimposing dispersion data recorded via the receivers at various depths, the resulting multimode dispersion data is used in a multi-objective inverse analysis, for which the difference between experimental and theoretical dispersive phase-velocity spectra are minimized for multiple modes simultaneously. In the numerical study, we demonstrate that the resulting inverted profiles and theoretical dispersion data have improved accuracy relative to single-mode inversion. Preliminary field tests are performed using the new hybrid method, and the results are shown to support the conclusions of the numerical study and confirm the feasibility of the proposed technique. Although the use of multiple modes in surface wave testing is not new, the proposed hybrid method can provide more accurate and complete multimodal dispersion data than achieved with surface-only Rayleigh wave methods. As a result, errors because of misidentification or partial measurement of higher modes may be minimized, thus reducing statistical uncertainty in the inverted profiles.

Poor subsurface drainage is frequently identified as a factor leading to the accelerated damage of roadway systems. Geocomposite drainage layers offer an alternative to traditional methods but have not been widely evaluated, especially in terms of the impact of changes on both drainage capacity and stiffness. In this study, both paved and unpaved test sections with and without an embedded geocomposite drainage layer were constructed and tested. The geocomposite layers were installed directly beneath the roadway surface layers to help the rapid drainage of any infiltrated water and thus prevent water entering the underlying foundation materials. The laboratory, field, and numerical analysis results showed that the geocomposite layers increased the permeability of roadway systems by two to three orders of magnitude and that it can effectively prevent the surface and foundation materials from becoming saturated during heavy rainfall events. For the stiffness of the sections, the paved sections with and without a geocomposite layer showed that the composite modulus values measured at the surface were more reflective of the foundation layer support conditions beneath the geocomposite layer than the geocomposite layer itself. The unpaved road section with the geocomposite layer yielded lower composite modulus values than the control section but showed overall better road surface conditions after a rain event due to the improved subsurface drainage condition.

A fundamental experimental research programme on the dynamic behaviour of surface foundations on sand in general planar motion and the use of centrifuge modelling in soil–structure interaction studies is presented. Pursued with the dual purpose of extending the conventional formulation of dynamic test programmes as well as generating a physical database with sufficient parametric variations of the key aspects, an extensive experimental study to explore the dynamic soil–structure interaction problem in the small-amplitude regime is described. Through a comparison of the generated data with those from sequential vertical-centric and horizontal load tests, a novel hybrid-mode test concept by way of eccentric excitations is substantiated in terms of its economy and efficiency in capturing the force–response characteristics of the system. Synthesised in the frequency domain for direct qualitative and quantitative insights, multiple forced-response records of the foundation models on sand subjected to random vertical, horizontal and rocking excitations are summarised. As an illustration of the engineering relevance of the experimental database, a critical evaluation of the commonly used homogeneous half-space dynamic foundation solution pertaining to cohesionless soils is also provided.