Sustainability Issue #3 June 2010

The way groundwater chemistry is affected by underground installations is a relatively undeveloped area of knowledge. Underground construction in rock generally causes the turnover times of groundwater to be reduced by a forced and mainly downwards groundwater flow, and this, in turn, creates a goundwater chemistry envionment that may be different from that under undisturbed conditions.

Our research shows that groundwater chemistry conditions are made more dynamic by an underground construction than under undisturbed conditions. Asessment of the nature of groundwater chemistry through sampling before the construction phase of an installation often provides an unreliable description of the groundwater chemistry in the immediate vicinity of the tunnel compared with undisturbed conditions. The guidelines applied by Swedish authorities to assess the properties of groundwater in conjunction with underground construction are therefore not entirely reliable.

In Scandinavia, the most  recent glaciation and sea water changes are important for quaternary geological development and for groundwater chemistry. Hydrologically, the landscape is divided into inflow and outflow regions for groundwater. Through leakage of water into an underground installation the hydrological conditions may be changed so that natural outflow regions will instead be inflow regions. In the landscape, outflow regions are represented by wetlands, watercourses, lakes and seas. Sedimentary areas and wetlands can act as organically binding storage places for several chemical compounds, and these may be liberated as a result of the hydrological conditions caused by underground construction.

Within the framework of our research we have noted how the chemical composition of the groundwater can affect degradation of the loadbearing system in underground installations. Important structural components are rock bolts, sprayed concrete, grouting and drainage systems for infiltrating water.

Research results show that the following water chemistry changes may be important for assessing the impact on steel and cement based materials:

A more oxidising environment is created through the inflow of further groundwater into the rock.

An increse in sulphate concentration through oxidation of available sulphur in rock and soil, which may, in turn, result in a lowering of pH (see Fig. 1). This may give rise to a corrosive environment for steel and to sulphate attack on cement based materials.

Fig. 1. Changes in groundwater chemistry in a borehole drilled in rock (left y axis) and in a wetland situated above (right y axis) which were affected by extraction from the borehole over the period December 2000 – April 2005. Extraction gave rise to oxidation processes which caused sulphate to go into solution and sulphuric acid to form. In turn, the formaton of acid caused a lowering of pH. After groundwater extraction had ceased, there was a gradual recovery in both sulphate concentration and pH. The diagram also shows an unaffected observation borehole in rock (full line) in a reference area.

Increased chloride concentrations due to the flow of deep lying relict water towards the underground installation or to chloride containing water of marine origin flowing towards the installation. High chloride concentrations may cause degradation of mainly steel materials. See Fig. 2.

Fig. 2. Example of concrete foundation in part of a tunnel with brackish groundwater. The concrete has been leached by water and exposed to rust bursting. Photographer: Jonas Lersten.

Through inflow of surface water the concentration of organic carbon in rock groundwater may increase. During degradation organic carbon uses up oxygen (if available) or reduces sulphate to hydrogen sulphide. If calcium carbonate is available, this may go into solution. The inflow of organic carbon thus counteracts some of the above processes.

When groundwater contains bicarbonate, calcite may form. This may cause deterioration of bearing strength since the bond between cement and adjacent materials such as rock or bolts decreases. Calcite formation is probably a common process in the vicinity of an underground installation, since the solubility of calcium carbonate in water decreases when carbon dioxide is emitted to air in the installation. The system for calcium carbonate is closely associated with alkalinity. See Fig. 3.

Fig. 3. Alkalinity of rock groundwater in an observation borehole (HGF31) that was affected by groundwater extraction, compared with an unaffected reference. Over the period December 2000 – February 2004 HGF31 was used as reference when a nearby borehole was pumped. From April 2004 to April 2005, HGF 31 was also used for groundwater extraction. When the adjacent borehole was pumped, there was a gradual decrease in alkalinity over time. After pumping had ceased in the spring of 2004 the general level of alkalinity rose but had a declining trend, which is due to changed influence regions. The diagram also shows the criterion of Rail Administration/ Road Administration for aggressive water.

Another common problem that is related to water chemistry conditions is the blockage of drainage systems for water infiltrating into the tunnel. The problem due to blockage of drains mainly occurs when iron and manganese dissolved in the groundwater are oxidised through microbial activity and precipitate. This occurs in the case of iron mainly when water has pH between 5.5 and 7.0, and in the case of manganese with pH between 7 and 8. Redox state and pH are often affected by underground construction since grouting with cement creates an alkaline environment  near an installation, and infiltration into a tunnel may give rise to increased oxygenation. See Fig. 4.

Fig. 4. A typical tunnel section with ample occurrence of iron hydroxide and calcite deposits on the left tunnel wall. Photographer: Christian Butron.

Research results show that the groundwater chemistry encountered during the construction and operation phases of an underground installationare highly dependent on geological and geohydrological conditions and the construction technique selected. It may be stated that good grouting of a tunnel also minimises changes in groundwater chemistry. In the endeavour to choose construction materials that optimise the service life of an underground installation and minimise costs, predictions of groundwater chemistry are of vital importance. In future infrastructure projects it is of the utmost importance to pay greater attention to the water chemistry of the rock.

Research at Chalmers has been funded by Rock Mechanics Research Foundation Befo, Building Industry Development Fund (SBUF), Rail Administartion, Halland Project, Environmnental Protection Agency, Äspö Environment Foundation, the firm Swedish  Nuclear Fuel Disposal AB,  Swedish Environmental Research Foundation (SIVL),  NCC and Vectura Consulting.

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