New method to reduce risk of damage to buildings during tunnel constructions

23.01.2018 / Jonas Sundell and Ezra Haaf

Groundwater leakage into subsurface constructions, such as tunnels and shafts, can cause a drop of pore pressure in clay resulting in land subsidence with the potential to seriously damage buildings. New research gives better tools for mitigating the risks.

Even at large distances from the construction, groundwater leakage into tunnels can cause a drop in pore pressure, subsidence and damage to buildings.

The costs can be huge and with the increasing need for tunnels to reduce traffic congestion in the world’s growing cities, these potential costs are on the rise. 

Recently, we finalised a research study resulting in a new, improved mapping method to analyse the risks. In brief, we investigated areas where there is a risk of subsidence if you lower the groundwater level. Even today, the issue of subsidence is addressed in a rather fragmented way; the geo-technicians take samples and make assumptions at individual locations, whereas the hydrogeologists investigate and make assumptions for a larger scale, but they could never tie the information together effectively.

With the new method the gap can be bridged, because it describes where in an area the risks may be found. The model combines a probabilistic soil stratification model with statistical analysis of compression parameters for simulation of subsidence on a large area with a simple nonlinear 1D compression model.

The result of this simulation is used for creating risk maps where areas with significant risk for subsidence are distinguished from low-risk areas.

The suggested method is useful in cases where the following criteria are fulfilled:

  • Soils demonstrate significant heterogeneous properties
  • Data on soil stratification are abundant, but data on soil properties are sparse
  • Dependencies between soil properties have been taken into account
  • A relationship between soil depth and soil properties exist
  • The compressible layer is normal or slightly over-consolidated
  • Soil layers can be assumed to be continuous throughout the study site

20,000 boreholes investigated

A case study of the City Link high voltage utility tunnel in Stockholm, Sweden, demonstrates the efficiency and usefulness of this new modelling approach as a tool for communication to stakeholders, decision support for prioritisation of risk-reducing measures and identification of the need for further investigations and monitoring.

Specifically, we looked at 20,000 boreholes in central Stockholm. Some were for the utility tunnel covered by the case study, and some for other projects that we found in the city archive or acquired from authorities.

Out of the 20,000 holes, 14,300 were selected for modelling and the rest used as a validation data set. From the modelling data set, different data sources were combined: 6,500 boreholes contained information on bedrock levels, 7,800 did not reach the bedrock, and 4,000 contain information on soil stratification.

Definition of risk areas 

From the obtained calculation results, a risk map is produced, distinguishing areas with significant risk from areas with low risk of subsidence. A risk area is created for each of the three uniform groundwater drawdown scenarios: 0.5 m, 1 m and 2 m.

To exemplify how a risk area can be defined, calculation points where the 95th percentile of the simulations has a land subsidence exceeding 2 cm, are selected.

Two-centimetre subsidence has previously been set as an upper limit for acceptable damages in other studies. Of course, the probability that the subsidence levels will exceed the 95th percentile value at all locations is much lower.

When interpreting the different risk areas, it is important to remember that the subsidence magnitude can vary at different locations within the same area. Using the 95th percentile is therefore a conservative estimate.

We looked at 20,000 boreholes in central Stockholm. Some were for the utility tunnel covered by the case study, and some for other projects that we found in the city archive or acquired from authorities.
Jonas Sundell Engineer, COWI

Next step – evaluation of economic consequences

The mapped risk areas and the result of the sensitivity analysis can be used, together with information on the vulnerability of surrounding constructions, for supporting decision makers regarding prioritisation of further investigations, risk-reducing measures and monitoring.

In the case of the planned City Link tunnel in Stockholm, the maps have been successfully used for risk communication in legal courts when applying for a permit to modify groundwater conditions.

For a better understanding of the cause and effect chain from drawdown to subsidence, future research on connecting subsidence models with groundwater models and economic valuation of consequences is recommended.

With an improved understanding, the risk for making misguided decisions on risk-reducing measures, monitoring and further investigations can be reduced.

We have expanded our research further and are currently working on an improved subsidence model that accounts for time dependencies in order to extend the soil stratification and subsidence model with a 3D groundwater model and to improve the decision-making by means of economic valuation of consequences.

The study ‘Risk mapping of groundwater-drawdown-induced land subsidence in heterogeneous soils on large areas’ was recently published in the journal Risk Analysis.

The full article can be downloaded at: http://onlinelibrary.wiley.com/doi/10.1111/risa.12890/full

Get in contact

JOSU

Jonas Sundell
Engineer
Geo Engineering, Sweden

Tel: +46 706548188

Get in contact

Ezra Haaf

Ezra Haaf
Engineer
Geo Engineering, Sweden

Tel: +46 708239815

MEET JONAS SUNDELL

Industrial PhD student at Chalmers University of Technology in Sweden.

My field of expertise is groundwater-related issues in infrastructure projects.

The scale of hydrogeological problems implies large heterogeneities and uncertainties with poorly known materials formed and affected by complex geological and anthropogenic processes.

To address these issues, you need a broad range of skills from both natural science and engineering. I find that particularly interesting and challenging.

MEET EZRA HAAF

PhD student in Hydrogeology at University of Gothenburg, Sweden.

My expertise lies in statistical analysis and management of spatio-temporal datasets – large and small. My special focus is modelling and understanding subsurface processes in infrastructure and water resources management.

I enjoy the detective work needed to piece together historic data with new field measurements and understand the process to arrive at a solid conceptualisation of your study site.

What I particularly find engaging is the development of technical solutions that lead to improving services and reducing risks for society.