The Romeriksporten railway tunnel — Drainage effects on peatlands in the lake Northern Puttjern area

doi:10.1016/j.enggeo.2008.04.002

Engineering Geology

Volume 101, Issues 3–4, 17 October 2008, Pages 75-88

https://doi.org/10.1016/j.enggeo.2008.04.002 Get rights and content

Abstract

This study presents the results of an investigation of drainage effects of the Romeriksporten railway tunnel on peatlands in the Lake Northern Puttjern area. Drainage impacts on peatlands occurred in marked depressions reflecting faults or weakness zones in bedrocks. The impacts were most evident near the tunnel trace, but dry peatlands and streams were observed as far as 600 m from the tunnel trace. Several types of drainage effects were observed on peatland surfaces after tunnelling; surface subsidence and formation of local depressions, peat slides, peat cracking, dry peat holes, and death of vegetation. The effects depended clearly on peatland properties. The greatest and most damaging effects were associated with thick peat layers around open water surface. Here lowering of the water table caused peat slides. Such areas will generally be extremely vulnerable to lowering of the water level. The magnitude of subsidence of peatlands depends on peat thickness, peat decomposition and groundwater lowering. Even after seven years with artificial infiltration in bedrock wells in dry periods to counteract water table lowering, peat subsidence and peat slides were evident, demonstrating that temporary draw down of water can result in irreversible impacts on peatlands. To better be able to avoid damaging effects of tunnel drainage on environment, investigations of properties of peatlands above possible tunnel traces should be integrated in the planning procedure for tunnel constructions. In mires above such traces, the occurrence of floating peat mats, degree of peat decomposition, peat thickness, deliveries of water from surrounding catchments in dry periods and possible pathways of leakage through subsoils and bedrocks should be examined as basis for assessment of vulnerability of peatlands to tunnel drainage and selection of tunnel traces that evade the most vulnerable peatlands. When such areas can not be avoided, requirements should be set to the tightening of the tunnel to prevent damaging water draw downs in dry periods. Observations of drainage effects on peatland surfaces can complement hydrological information from measurements of tunnel leakages and groundwater levels and thus contribute to provide a broader picture of hydrological impacts and bedrock hydrogeology.

Introduction

Peatlands cover extensive areas in northern Europe, America and Asia. These areas are particularly important for landscape and biological diversity in the boreal forests (Korpela, 1998). As the peatlands are physically and ecologically adapted to stable water tables fluctuating near the surface, these areas are expected to be particularly sensitive to tunnel leakage and ground water draw downs. During the last decades mire drainage for forestry and agriculture purposes has reduced the areas of peatlands, and the increasing awareness of the environmental importance of wetlands has brought peatlands and the potential effects of bedrock tunnelling on such areas into focus in Norway.

Groundwater decrease due to leakage from bedrocks is known from several tunnels (Olofsson, 1993, Cesano et al., 2000, Mabee et al., 2002, Kim and Lee, 2003). Drainage of surface water and flow of groundwater from soils to rocks is also described from several tunnels (Ishizaki, 1979 cited in Olofsson, 1993, Skjeseth, 1982, Cherkauera and Carlson, 1997). In contrast to the extensive literature dealing with engineering aspects of underground tunnelling, little attention has been focused on environmental effects on the surface. Although drainage of peatland water storage to tunnels has been observed (Skjeseth, 1982), the character and extent of effects of tunnelling on peatland surfaces are seldom described. Improved knowledge about the effects of tunnel drainage on peatlands and how such effects are influenced by peatland properties are desired to be able to evaluate potential impacts of future tunnel projects.

During the construction of the railway tunnel Romeriksporten considerable leakage occurred. A drop down of the water table in the Lake Northern Puttjern was discovered in 1997, demonstrating leakage of water from the surface to the tunnel. It was therefore decided to examine the effects of tunnel leakage on peatland surfaces in this area. This study presents the observed drainage effects on peatland surface in the areas around the Lake Northern Puttjern. The objective of this study is to improve the knowledge of how peatlands can be affected by groundwater draw downs and tunnel leakage, and thus improve the basis for future planning of projects, both regarding delimitation of areas particularly vulnerable to tunnel leakage, selection of tunnel traces and the need for tightening or mitigation efforts. This will be accomplished by: (1) describing the different kind of drainage effects on peatland areas; (2) describing the spatial distribution of different drainage effects; (3) describing the mechanisms causing different kinds of effects; (4) relating the different kinds of drainage effects to peatland properties; (5) discussing the possible use of monitoring of peatland surfaces.

Section snippets

Study cite

The study has been conducted in the Lake Northern Puttjern area in Østmarka, above The Romeriksporten railway tunnel, northeast of Oslo in Southern Norway. The construction of the 13.7 km long tunnel between Oslo and Lillestrøm was commenced in 1994. Tunnelling under the Lake Northern Puttjern area started in autumn 1996 and excavation was finished 4th September 1997. The railway tunnel was officially opened 21st August 1999.

In the Lake Northern Puttjern a decline of the water level was

Overview of the influence of tunnel drainage on peatlands between Lake Lutvann and Tørrgranåsen

Drainage effects on peatland morphology (surface subsidence, peat cracks, peat slides) were observed in the Lake North Puttjern valley and in the mire Kjerringmyr (Fig. 2). Neither such effects nor dry peatlands or streams were found on the hills east of the Lake Northern Puttjern, and opposed to the streams in the Lake Northern Puttjern valley and the outlet stream from the mire Kjerringmyr, the streams from the peatlands on these hills did not dry out during summer 1997.

In the Lake Northern

The mechanisms causing different types of effects on peat surface

As saturated peat contains 80–97 volume % water and peat structure is supported by water, rapid peat surface subsidence after drainage is mostly caused by the physical collapse of the peat matrix when the water is removed (Paavilainen and Päivänen, 1995). The peat surface subsidence during 1997 in the Lake Puttjern valley and at the mire Kjerringmyr thus revealed physical collapse of thick peat layers, and demonstrate deep drainage of peatsoils. As the peat surface level along the southern and

Conclusions

Peatland morphology and ecosystems are adapted to stable high water levels and thus particularly sensitive to drainage and changes in water balance. Drainage first affects such systems by changing surface morphology and peat structure. Over time also peat substrate and vegetation composition will be susceptible to changes.

Deep drainage caused by tunnelling can lead to other and more severe effects on peatland surface than surface ditches. Several types of effects can occur when tunnelling

Acknowledgements

This work has been financially supported by NSB Gardermobanen AS and Bioforsk – Norwegian Institute for Agricultural and Environmental Research, Soil and Environment Division.

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¹: Present position Asplan Viak AS, Raveien 2, N-1430 Ås, Norway.

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