Metro Cityringen Copenhagen: Innovative Groundwater Management

Copenhagen is increasing the size of its existing Metro system by two-thirds through the con-struction of the new “Cityringen” Metro ring in order to improve the infrastructure of the Danish capital. The following report deals with the project in general and concentrates especially on the innovative groundwater management system.

Project Presentation

The Cityringen comprises a 15.5 km long twin-bore tunnel system as well as 17 Underground stations and a surface maintenance depot. As from 2018, the driverless Metro trains will transport up to 240,000 passengers 24/7. The total project budget for the client, Metroselskabet I/S amounts to 2.87 billion euros.

Hölscher Wasserbau GmbH was commissioned to execute the groundwater management by the general contractor CMT I/S.

The tunnel consists of 2 bores each 5.78 m in diameter and is being produced by 4 EPB TBMs driving simultaneously. The bores run through Copenhagen Limestone down to a depth of 25 m below the surface. In addition to the TBM drives, a total of 21 shafts averaging 65 x 20 m with depths of up to 30 m are to be created. These are required for the 17 stations as well as TBM starting shafts and other special structures (Fig. 1).


Geology

Copenhagen Limestone forms the predominant ground layer through which tunnels are to be driven. This layer is to be found at a depth of approx. 10 to 15 m beneath the surface. It is divided into a 2 to 5 m thick layer of highly weathered Upper Copenhagen Limestone with Middle Copenhagen Limestone – affected by tunnelling – underneath followed by Lower Copenhagen Limestone. This limestone formation forms a groundwater horizon in conjunction with roughly 2 m thick superordinated quaternary gravel sands of glacial origin. This aquifer is critical to the water control operations at the construction shafts for the Metro stations and the starting shafts for the TBMs and the special structures. The hydraulic capability of the limestone greatly depends on the separation plane and fissure formation as well as its orientation. The average horizontal permeability is 3 to 10 times greater than the vertical one. The lower aquifer is separated by a quaternary overconsolidated marl layer from the aquifer close to the surface, which comprises younger quaternary thin gravel sands, marl layers and fills.

The System

The tunnel shafts are designed as diaphragm walls with overcut drilled piles down to a depth of 46 m providing substantial penetration into the Copenhagen Limestone. Internal groundwater lowering is needed to allow shaft sinking and particularly to relieve the shaft floor. As there is no impermeable layer this would of course mean that the groundwater level would also drop outside the shaft. However it is not permitted to lower the normal groundwater level in the centre of Copenhagen as it is set up on old wooden piles, which must on no account be allowed to dry out (Fig. 2).

It is intended to reinfiltrate treated groundwater into the aquifer to ensure that the groundwater level does not change outside the shafts. The existing groundwater is observed at a number of defined measuring points and the required amount of water infiltrated by means of infiltration wells. For this purpose the Hölscher Wasserbau GmbH developed an innovative concept, which possesses both technical and economic advantages. The system contains water treatment plants, which enable the groundwater to be reinfiltrated once treated so that expensive drinking water is saved. In addition, the system can control the groundwater level in the city by fully automatic, computer-regulated means thus enhancing safety.

Altogether the groundwater management system for the City Ring consists of approx. 600 extraction and reinfiltration together with as around 600 groundwater measuring sensors with GPRS data loggers, 21 water treatment plants, approx. 25 km of pipeline and a fully automatic SCADA system.

Set-up of the Groundwater Management System

Wells and Well Drilling

Drilling wells capable of functioning represents the basic essential for successfully installing the system. Activities involve creating extraction, reinfiltration and monitoring wells. Separate well galleries are drilled from the various types of well mentioned for the individual stations and the further shaft structures. The well depths vary from 15 to 45 m depending on requirements.

The DTH (down-the-hole) method is applied throughout. The drill string and casing are sunk together in a single operation. The drilled material is removed by means of compressed air via the annular gap between the string and the casing. In this way an economic drilling process can be pursued in this technically difficult geology which includes many natural obstacles in the quaternary and the limestone formation.

Enhanced demands are placed on the drilling process in Copenhagen. Special attention must be paid to safeguarding traffic as well as noise and fume emission in order to ensure that the quality of life in the downtown area remains unaffected. All drilling appliances were equipped with soot particulate filters to reduce the impact of fumes.

The pump wells are in each case located within the shaft structures in order to lower the groundwater level there from the aquifer, which is freely accessible underneath. Extraction wells are filtered along the entire well length so that water is collected from both aquifers during all construction phases in order to ensure that lowering is carried out continuously. PVC filters with a diameter of 165 mm are used. The filter slot widths and the filter grain fractions are adapted to the given geological situation.

After the extracted water is collected and treated, it is returned to the ground via the reinfiltration wells, which are located outside the construction pit at distances of up to 500 m. Reinfiltration takes place via wells, whose reinfiltration sections are to be found in the fissure water aquifer. In this way, the groundwater horizon, from which the water is primarily extracted, is to be returned to its natural level. As a result, a permanent, time-delayed influence on the upper aquifer via the marl layer, which would exert a suboptimal effect on the surrounding structural foundations, is prevented. Supporting is undertaken by means of 165 mm diameter full and filter pipes.

Monitoring wells with piezometers are installed in the vicinity of both aquifers to secure and control the lowering and reinfiltration targets. For this purpose, 63 mm diameter HDPE pipes with a 2 m filter section are installed. Fig. 3 displays an excerpt from the documented compilation of the drilling operations including the layer structure and the lining for a monitoring well.

Three-step tests are carried out on the pump and reinfiltration wells and capacity tests on the monitoring wells to check the efficacy of the wells, which greatly depend on the global and local fissure formation and orientation of the limestone. The three-step tests comprise 3 directly integrated pump stages involving an increasing pump rate each lasting 1 hour followed by a 3-hour long rerise observation phase. Capacity tests for the monitoring wells are executed with an hourly single-stage pump rate and observation of rerising over 1 hour. Water levels during pumping and rerising are compiled electronically via loggers and referred to by manual measurements (Fig. 4).

SCADA Control System

The groundwater management system is controlled and monitored centrally by means of a supervisory control and data acquisition system (SCADA). For this purpose, each of the 21 local sites is controlled by a Siemens SPS. The frequency-controlled well pumps, the water treatment and reinfiltration processes are controlled and monitored via SPS.

The 21 SPS stations are connected to a main server via UMTS. Continuous data transmission is maintained between the stations and the main server, by means of which the stations are controlled, with data being transmitted also for documentation purposes. For example, desired groundwater lowering targets can be introduced via the main server, which are then transmitted via SPS to the frequency-controlled well pumps.

The 21 stations are visualised, as a result of which the most essential operating modes for the system are identifiable. The SCADA system generates variably reproducible reports of operational data and ensures that corresponding messages are provided via an online system or SMS in the event of malfunctions or if limit values are not observed. Current operating parameters can be called up by service staff on iPads. The overall system can also be controlled with iPads. In this way it is possible for the service staff to control the system permanently 24/7 from any location. Furthermore maintenance procedures planned on a regular basis such as e.g. back-flushing the water treatment plants can be undertaken with a simple “click” (Fig. 5).

Monitoring Wells

Roughly 600 groundwater monitoring wells distributed around the individual shaft structures have to be installed. These 8” diameter wells take the form of a double or triple piezometer. The loggers in the monitoring wells continuously display the groundwater level and transmit the data to the SCADA system for documentation as well as for controlling the groundwater management system. The wells located close to the shaft are connected with the appropriate SPS via a data line laid in the ground.

The wells, which are located further away from the control facilities, are equipped with battery-operated GPRS data loggers. These GPRS data loggers transmit the recorded data via the mobile communications network directly to the SCADA system of the groundwater management facility.

Monitoring

The SCADA system automatically facilitates monitoring of the operating parameters. Predetermined alarm signals are directly displayed on the system’s user interface and transmitted to as many mobile phones as desired in SMS form. In order to ensure that the transmitted data corresponds with the effective groundwater levels, these are checked against manual measurements carried out at regular intervals. The data thus obtained can be entered into the system and displayed as a diagram together with the data automatically transmitted by the sensors thus enabling a suitable comparison. The amount of water extracted by the pump wells is compared with regular manually read ratings.

Turbidity sensors indicate the amount of suspension carried by the extracted water. Should limits be exceeded, a signal is automatically initiated, which controls the valve for the reinfiltration line or the drainage system to open or close them. The turbidity sensor is calibrated when the system is commissioned. Water samples are taken at regular intervals and checked to establish their chemical components to ensure the water quality within the system and prevent any premature aging  through deposits forming.

Water Treatment Plant

The pumped groundwater from the shafts is treated to provide a quality better than drinking water prior to being reinfiltrated. Towards this end, the authorities set the highest standards to make absolutely sure that the communal wells that provide drinking water, which are located in the Metro catchment area, remain unaffected by the reinfiltration process. As a result, in some cases even flocculants, which are permitted for drinking water, are not allowed to be used.  Only special compositions developed jointly with the authorities are permitted for use.

The groundwater is pumped to the water treatment plant from the extraction wells in the interior of the shafts. In addition pump sumps to remove the prevailing groundwater are installed in individual shafts, in which special structures have been created by the cavern construction method. This water is first subjected to a pre-sedimentation phase on account of the relatively high sediment content. The water is cleaned to reach a sediment content of < 0.02 mg/l via plate clarifiers and gravel filters. This treated water is then combined with groundwater pumped from extraction wells and passes through to the water treatment plant.

In addition to a chemical treatment step, the treatment plant mainly consists of an oxidation plant, in which bivalent iron is oxidised to form the trivalent variety, and subsequently removed from the groundwater by means of gravel filters. The treated water is then pumped to the reinfiltration plant.

The ground conditions at the “Sonder Boulevard” tunnel shaft present a special challenge. The groundwater there is polluted with benzene and cyanides. In this case the standard treatment plant is extended to include activated carbon filers, strippers, ion exchangers as well as centrifugal technique. The water from the groundwater management system passes through this plant along with the process water from the installation at the diaphragm walls as well as the process water, which ensures during well drilling. In addition to the existing complexity of the treatment plant as such, logistical challenges for managing this process water have to be tackled (Figs. 6 + 7).

Infiltration Plant

The necessary water pipelines and control cables are laid underground, mainly in roads, between the infiltration wells and the control/treatment facilities. The prevailing geology indicates relatively different capacities for the individual infiltration wells. In this connection it should be mentioned that the horizontal permeability of Copenhagen Limestone is roughly 3 to 10 times greater than the vertical permeability.

The capacity of each infiltration well was determined by a test run to establish the optimal operating conditions. The test results were taken as the basis for programming the groundwater management system. The water treatment plant and the infiltration plant in conjunction with the control system form a complex, interactive system. The infiltration plant constitutes the core of the system. It consists of valves and automatic flaps, which are controlled via the SPS. The automatic flaps are initiated in keeping with the current groundwater conditions recorded by the piezometers thus regulating the rate of inflow to the infiltration well. The maximum rate of inflow to a well is established in advance in keeping with the determined capacity. The resultant tolerances for the current groundwater level amount to a few centimetres. Such results are practically impossible to attain with conventional, manually operated infiltration wells.

A total of around 20 million m³ of groundwater is being prepared via the water treatment plants and reinfiltrated into the Copenhagen subsoil.

Reporting System

The SCADA system enables the recorded data to be displayed in lucid fashion in the form of individually presented reports. In this connection, the selection of parameters can be summarised as hourly or daily averages and exported as a daily, weekly or monthly report. Additional data in diagram form can be displayed alongside water balances and further tabular listings.

Summary and Outlook

The fully automatically controlled groundwater management system, as is applied for building the Metro Cityringen project, affords the ultimate degree of safety in conjunction with an inner-urban tunnelling scheme. Furthermore the authorities and client’s representatives can place great faith in the measure implemented as they are able at all times to obtain an insight into the groundwater situation thanks to direct online access to the groundwater management system.

Apart from the safety aspect, this groundwater management system affords sustainable advantages regarding economy and environmental compatibility. Alone owing to the fact that through the treatment and reutilisation of groundwater, high drinking water and disposal charges can be avoided and the savings can be directly assessed monetarily. Furthermore, it is possible to constantly regulate the facility in a flexible manner in accordance with the current capacity of the infiltration wells.

In conclusion the ever increasing significance of environmental compatibility tests at international level should be mentioned, whose general conditions require innovative solutions to protect groundwater resources in conjunction with construction projects.

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