Analysis of Surface Settlement in Response to Shaft Excavations in Auckland's Expansive Soils

Abstract

Ground settlement can be a problem in soft ground excavations in urban areas, where buildings and infrastructure can be placed at geotechnical risk. Recently in Auckland, there have been a number of deep shaft and directionally-drilled pipeline projects with the potential to induce ground settlement. One of the main sources of settlement is consolidation from the groundwater drawdown during excavation and, in some cases, over the longer-term. Consolidation will cause localised settlement, but when excavating below the water table, a dewatering system is typically implemented, and this may induce surface soil settlement >100 m away from the excavation. As a result, it is essential to make predictions of ground surface settlement around an excavation so that measures can be taken in order to mitigate the effects of surface settlement on residential housing, in particular. However, experience has shown actual groundwater drawdown-induced settlement contrasts significantly with predicted settlement, calculated during the planning phase of infrastructure projects. Hence, the cost and labour involved in settlement modelling and monitoring programs may not be justifiable. This research investigates the apparent disparities between predicted and observed settlement during excavation of shafts at three sites within the Auckland urban region. Modelling of groundwater drawdown-induced settlement was undertaken using Rocscience's Settle3D. This modelling calculates primary consolidation settlement, based on the linear and nonlinear modelling approaches combined with Westergaard's stress distribution method. The aim of this study is to develop a better understanding of the factors that influence the apparently overly-conservative modelled settlement predictions in Auckland. The results of this study show that the soil behaviour response in the Auckland area to groundwater drawdown is too complex to accurately predict using either the linear or non-linear modelling approaches. Both linear and non-linear modelling approaches are highly dependent on, and affected by, the accuracy and representativeness of soil parameter values. At the same time, it is difficult to quantify the accuracy of these estimated soil parameters, due to the highly variable Auckland subsurface geology. The results highlight that factors such as seasonal variations in ground movement, Auckland soil behaviour and adequate ground investigations are also important. Such factors need to be accurately quantified and specified in order to resolve the disparity between calculated and observed settlements. Further recommendations, such as techniques to better estimate soil stiffness, alternative soil models, and use of the Interferometric Synthetic Aperture Radar (InSAR) monitoring system are briefly discussed.