Particle image velocimetry (PIV), or digital image correlation (DIC), is a widely used technique to measure soil displacements and strains in small-scale geotechnical models. Arrays of single-board computers (SBCs) produced by Raspberry Pi, and their associated 8-MP cameras, are being used at the University of Cambridge to capture the images required for DIC analysis. This alternative to more expensive camera setups has numerous advantages. A single expensive and large camera can be replaced-at low cost-by multiple cameras, adding flexibility and affordability to any experimental setup. Traditionally, the alignment of multiple cameras to each other and the referencing to a known coordinate system required painted or machined markers to be located on the observation windows through which the experiments are viewed. This can obstruct localised soil grain displacement measurements in those areas of the model where such markers are placed. To complement the Raspberry Pi camera system, a markerless calibration method was used during image acquisition. This paper outlines the setup of four of these small computers and associated cameras, provides an overview of the use of the markerless referencing system and reviews two different experimental apparatus used to measure soil displacement and strain. When the cost of additional cabling, connectors and mounting hardware is considered for this system, the total cost to implement was approximately $125 USD per camera plus one-time costs of $175 USD for system peripherals, which represents outstanding value and enables practically all geotechnical laboratories to develop similar capabilities.

Flexible pavement structures are widely used in road construction, especially in circumstances where very high traffic volumes are not expected. These structures comprise of multiple layers of granular material, generally having decreasing strength with depth. Failure of these systems is typically observed as rutting on the ground surface, but the failure instigates at depth. In this research, for the first time, observations can be directly made of the progressive failure of these deeper layers under repeated wheel loading due to the combination of a new test apparatus, the Cambridge Accelerated Pavement Tester (APT), and Digital Image Correlation (DIC) technology. The use of a window allowing observation of soil displacements at depth during repeated wheel loading cycles allows the progressive failure to be observed and the changes in soil displacements and strains with different layer thicknesses to be quantified. It was observed that the critical failure mechanisms for thin and thick surficial layers are different, resulting in changes in the rates of surface rutting. Understanding these deformation mechanisms potentially allows savings to be made in road or airfield construction by using correctly-sized structural layers.

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