Using hydrogeological numerical modelling to predict drainage to excavations – findings from four Norwegian case studies

Urban ground work such as deep excavations performed in soft clay may cause damage to neighbouring buildings and structures as a consequence of settlements partly caused by drainage. The costs related to settlement damage can be substantial and there is a considerable potential for reducing these costs. To assess the risk of drainage caused by a particular set of construction methods, as well as the effect of various mitigation measures, it is necessary to quantify the expected groundwater drawdown in the zone of influence surrounding the groundwork site. This may be done with hydrogeological modelling. As part of the work towards general recommendations regarding the use of hydrogeological numerical modelling to predict the pore pressure reduction caused by drainage to excavations, four modelling cases are presented and summarised. The case studies vary from being purely research-based with the purpose of assessing a model's ability to predict or replicate pore pressure reductions, to being part of consulting work with the purpose of assisting in the decision-making regarding the use of mitigation measures

Time series modelling: applications for groundwater control in urban tunnelling

Water ingress to tunnels may result in pore pressure drawdown and consolidation settlements in areas above tunnels founded on soft soil deposits, potentially causing damage to buildings and infrastructure. To limit pore pressure drawdown, requirements are set on water ingress to bedrock tunnels. To meet these requirements, pre-excavation grouting is often performed to reduce the hydraulic conductivity of the rock mass surrounding the tunnel. Real-time pore pressure monitoring may be used to document pore pressure drawdown during construction. However, the effect of tunnel water ingress can be difficult to distinguish from natural pore pressure fluctuations. This paper presents a tunnel case in Oslo, Norway, where time series modelling was applied to local pore pressure data using the transfer function model framework. The input to the models was daily meteorological data considering precipitation and evapotranspiration, and the output was simulated pore pressure levels with impulse response functions. The models were optimised with data from before tunnel excavation, and simulations were run during the tunnel excavation period. Simulated pore pressure levels were compared with observed pore pressure levels to assess tunnelling-induced drawdown. Model uncertainty ranges were used to produce upper, lower, and best estimates of the drawdown. The findings show that time series modelling with transfer function models may be used in tunnel projects to continuously assess the impact on the local groundwater environment, for better evaluation of the pre-grouting performance, and for quantifying both the temporary and long-term drawdown with increased accuracy.Time series modelling: applications for groundwater control in urban tunnellingpublishedVersio

Numerical hydrogeological modelling of drainage to an excavation

The report regards the conceptual and numerical modelling of drainage scenarios to a planned excavation in a deep clay deposit in Oslo, Norway. The modelling was performed in SEEP/W 2D. Ground investigations indicate that the clay lies directly on top of the bedrock, without a layer of moraine between them. The upper metres of the bedrock is assumed to be weathered and serve as a draining layer. The drainage of groundwater to the excavation along steel core piles, and the mitigating effect of cut-off walls of various lengths, were modelled. There were made several versions of the numerical model, with a variation of conductivity and upstream boundary conditions. The simulation results were compared with empirical pore pressure data presented by the R&D project BegrensSkade I (2012-2015). The simulation results deviated notably from the empirical data, leading to the conclusion that the model itself is unreliable. Although there were found weaknesses with the model that could have been improved and resulted in better compliance, some major uncertainties are considered to remain and to affect the results to a significant degree. The uncertainties are in particular related to the modelling of the (weathered) bedrock conductivity, i.e. how the groundwater flows horizontally, vertically and across large distances within the bedrock joints. The discussion of these uncertainties is only introductory, and there is first of all a need to study the literature in greater detail to find out whether these issues are as important as argued here, and whether researchers already have studied them sufficiently. If the literature study comes to the conclusion that more research is needed, two modelling tasks are proposed with the aim to get a better understanding of how the groundwater flow could have been modelled more realistically in a continuum two-dimensional model.Norges Forskningsråd Research Council of Norwa

Work Package 3 – Hydrogeological methods, drainage and grouting

Ground works such as deep excavations and foundation works performed in soft clay can cause damage to neighbouring buildings and structures. Drainage causes pore pressure lowering, followed by consolidation settlements. The costs related to settlement damage can be substantial and there is a considerable potential for reducing these costs. The risk of settlement damage caused by drainage and pore pressure reduction can be reduced during the early design phase of a project by undertaking the correct type of investigations and understanding the hydrogeological conditions. Furthermore, one may select construction methods, which reduce risk of drainage. In case the selected construction solution yields an unacceptable risk for settlement damage to surrounding buildings and infrastructure, remedial measures may be designed to mitigate the effects, followed by implementation and monitoring during the construction phase. This report provides State-of-the-Art related to hydrogeological investigation methods, hydrogeological modelling and numerous measures to mitigate the effects of drainage to excavations. In addition, governing Norwegian rules and regulations are discussed, as well as the causes of drainage to excavations in Norwegian ground conditions.Norges forskningsråd / The Research Council of Norwa

Plaxis 2D Comparison of Mohr-Coulomb and Soft Soil Material Models

Soft soils such as clays, clayey silts and peats demonstrate a high degree of compressibility compared to other soils. In oedometer testing, normally consolidated clays behave up to ten times softer than normally consolidated sands. The task of this master’s thesis has been to investigate the primary consolidation behaviour of soft clay by comparing the results obtained from finite element analysis computations in Plaxis 2D with analytical calculations and survey measurements. Two different material models were utilized during the finite element computations, comparing the performance of the more advanced constitutive Soft Soil material model against the Mohr-Coulomb material model commonly used today. Two case studies provided by Subsea 7, a global contractor for the offshore oil and gas industry, have been investigated with regards to primary consolidation settlements. The case studies comprise rock berms installed on soft clay in the North Sea. A common feature for both case studies were that after installation of the rock berm, parts of the subsoil experienced virgin compression. Survey measurements were obtained for 240 days of consolidation for case study 1, and 31 days of consolidation for case study 2. Results indicate that performing design with the Mohr-Coulomb material model is inadequate when trying to predict the primary consolidation behaviour of soft clay. The most accurate results, when compared to actual survey measurements, were achieved when computing with the Soft Soil material model in Plaxis 2D. For case study 2, the computational results obtained with the Soft Soil model correlated accurately with actual survey measurements of 5 cm, in contrary to the Mohr-Coulomb model, which only predicted a 1 cm consolidation settlement for the same time period. Accurate results were not obtained for case study 1, most likely due to poor quality of the soil parameter data. However, the Soft Soil model still performed better when compared with the Mohr-Coulomb model and analytical calculations. The result from the Soft Soil computation indicated 6 cm of settlement whereas the survey measurement suggested a settlement in between 8 to 12 cm. The study performed in this master’s thesis conclude that projects including soft clay subjected to virgin compression should benefit from the use of the Soft Soil model during Plaxis consolidation computations rather than with the Mohr-Coulomb model.Validerat; 20130930 (global_studentproject_submitter

Predicted and observed settlements of a subsea rock installation : comparison between field measurements and FE simulations of soft North Sea clay

In this paper, a study is presented on the possibility to numerically simulate observed settlements of an offshore subsea rock foundation installed on normally consolidated clay. Settlements were monitored at 31 days and 200 days after installation and compared against predictions computed by means of finite element analysis. The results highlight that computations with a material model incorporating a logarithmic stress-strain relationship can accurately predict the consolidation settlements of normally consolidated clay. For long-term settlements, a material model that includes a creep factor is likely to increase the prediction accuracy of the total settlements. This study was performed in order to improve the prediction of soft clay settlements, as this phenomenon may have a large impact on design for sensitive subsea infrastructure installed on the seabedGodkänd; 2015; 20150605 (hmat