Deep excavations: monitoring mechanisms of ground displacement

As construction methods become more advanced and design more refined, better control and smaller magnitudes of ground displacements resulting from deep excavation can be achieved. Mechanisms of ground movements expected from deep excavation and methods for their prediction are briefly reviewed. Such predictions, whether empirical or numerical, are critical when the observational method is implemented during the construction process. Their accuracy can only be verified by correspondingly accurate monitoring of actual vertical and horizontal surface and subsurface ground displacements throughout construction. It is therefore relevant to investigate whether instruments routinely used to determine such displacements are sufficiently accurate. Monitoring techniques involving precise levelling, rod extensometers and torpedo inclinometers were examined in detail over a 3-week investigation period at a greenfield site to quantify the short-term precision and accuracy of the measured displacements obtained from them. Where possible, the influence of external factors (e.g. temperature and weather) on measurements is quantified and recommendations for best practice are made. Providing that the monitoring techniques discussed are performed to a high standard, the data indicate that ground displacements around deep excavations can be monitored to a sufficient accuracy for calibration and validation of numerical analyses and ultimately safe engineering design and construction.