Soil stress measurement by load cell probes as influenced by probe design, probe position, and soil mechanical behaviour

Knowledge of soil stress induced by vehicle traffic is a premise for assessment of soil compaction risks, but obtaining absolute values of soil stress is not trivial because stress transducer probe readings are influenced by probe characteristics and soil properties. However, little research has quantified the magnitude of the deviations between measured and true stress (defined as soil stress in the absence of a probe). The objectives of this study were to use a finite element model to quantify how stress readings by cylindrical probes are influenced by probe design (material properties, dimensions), probe spacing in multi-probe set-ups, probe installation depth, and soil mechanical behaviour. We show that soil stress is typically overestimated by transducer probes. Overestimation decreased with probe diameter to height ratio, and increased with the ratio of probe to soil elastic modulus. Overestimation was smaller but dependent on soil strength under plastic soil conditions and higher but independent of soil strength under elastic soil behaviour. Probes interfere with each other when the vertical gap between adjacent probes is closer than about three times probe diameter. Moreover, we found that measured stress is influenced by soil depth and size of the loaded area. Some of the factors influencing stress readings are easily controlled (e.g. probe material properties, dimensions and spacing). However, other sources of deviations between measured and true soil stress are more difficult to account for. Our findings show that the ratio of measured to true soil stress is dependent on soil deformation behaviour as well as on relationships between probe depth, radius of loaded area and ratio of soil stress to applied surface stress implies that the ratio of measured to true stress is not constant when measuring soil stress at various depths in situ under field conditions. We therefore suggest that future studies should especially address how stress readings are affected by soil mechanical behaviour, soil depth, the size of the loaded area, and interactions among these factors.

Automated summary

The study found that soil stress is typically overestimated by transducer probes. The overestimation decreases with probe diameter to height ratio, and increases with the ratio of probe to soil elastic modulus. Probes interfere with each other when the vertical gap between adjacent probes is closer than about three times probe diameter. Measured stress is influenced by soil depth and size of the loaded area.