Since the 1990s, the Doorstopper test has been a widely-used technique for measuring in situ stress at shallow depths in structures such as tunnel linings, retaining walls and rock walls. This is thanks to its small, non-invasive size and economy of execution.
When the measurement involves linear, elastic, isotropic materials, this test effectively determines the normal plane stress state of the the borehole, because the stress component parallel to the borehole (σz) is negligible. Each test provides an independent measurement of horizontal stresses (σx and σy), making this a valuable tool for evaluating stress fields in situ, where interpretation of stress tests is simple and reliable.
The Doorstopper strain gauge cell consists of 4 electrical resistance strain gauges, superimposed at 45° to each other, which are secured to the bottom of a borehole made in rock or concrete with a diameter of 76 mm, previously levelled and clean, using a specific two-component glue. The cell is also equipped with a temperature sensor that allows the thermal compensation of deformations. When the portion of material is overloaded, it causes voltage discharges that are measured by variations in the electrical resistance of each branch of the glued cell.
Through these variations in electrical resistance, it is possible to trace the stress to which the monitored material has been subjected. The cell is supplied on a special plastic structure, with a military-style connector for the resistive measurement of the various branches of the strain gauge.
Final considerations on the Doorstopper method
The Doorstopper method is a valuable tool for measuring shallow in situ stress, offering simplicity and reliability for plane stress analysis. However, its limitations in terms of measuring longitudinal stress and depth make it less suitable for deep or triaxial stress analysis. When used in the right context, it provides critical data for rock mechanics and geotechnical engineering projects. For more comprehensive stress measurement, it can be combined with other methods to achieve a full 3D stress tensor. Earth System is at your disposal for information and advice on the instrumentation needed for measurements using the Doorstopper method.
