Frequentely asked questions concerning sewer tunnels

Are the incoming and outgoing sewer tunnels near each other excavated at the same time? 

Yes, because the work stages can be appropriately scheduled and the equipment is effectively used. At the end of every tunnel some stage of the work is being carried out.  

If damage occurs, will HSY or the contractor compensate for the damage? What if there is any damage to the road? 

The party causing the damage is responsible for repairing it. The excavation contractor has so-called strict liability, i.e. the contractor must demonstrate that no damage has been caused by the excavation.  It is advisable to contact the contractor immediately if something occurs. The contractor will investigate the situation. Ultimately, HSY is liable for damages caused to third parties for work contracted by HSY. 

How is dust on the nearby streets prevented?  

The streets are washed and wetted more often than usual. Dust prevention is the contractor's responsibility. 

Is the increase in traffic taken into account?  

Increased traffic is most noticeable at the point where the blasted rock is brought to the surface. After that, the vehicles will merge into other traffic.

What kind of work is done during the night? 

At night, only work is done that does not exceed the noise limits in bedrooms established by the environmental authority.  If noise causes excessive disturbance, noise measurements are carried out, and the results of which, if necessary, will limit the work. 

Is there compensation for damages even if the vibration values are not exceeded? 

When the measured vibration remains below the pre-set threshold values, no structural damage should arise and, as a rule, no compensation is paid. The safety coefficient between the pre-set threshold value and the damage limit value is significant, and virtually excludes the possibility of damage caused by excavation. In cases where the damage could not be caused by something other than the excavation according to the vibration expert, compensation is paid even if the vibration values are below the set limit.

Is it possible to insure my house for a month, that is, during the time of the blasting? 

You can ask the insurance companies. HSY does not know that such insurance would have been taken. 

What happens to the groundwater? 

The pressure level in the sewer tunnels is lower than the pressure level of the surrounding groundwater. Therefore, groundwater seeps into the tunnel, and the water flowing in the tunnels does not leak into the bedrock.

Can you receive compensation if you lose the opportunity  to build a geothermal heat well? 

Based on current information, a place can be found on each property where a geothermal heat well can be installed after the excavation. Together we can look for a possible location for the heat well.  

What if the geothermal heat well causes some damage to the tunnel after the tunnels have been built? Who is responsible?  

The tunnels and their reinforcements are so strong that such a situation is highly unlikely. The locations of the geothermal heat wells are carefully considered after the construction of the tunnel to prevent such a situation from occurring.   

What is the impact of tunnel excavation on the building's foundation? 

Preliminary environmental assessment prior to excavation and a more detailed review at the beginning of the work are important in order to identify sensitive structures. If the structure is found to be sensitive, the vibration limits are set accordingly. The vibration is monitored with continuous measurements throughout the excavation. Not even the foundation should be damaged.

The tunnel runs underneath my whole property. What happens if the easement that HSY obtains is annulled?  

The easement is permanent and it will not be annulled. When constructing the tunnel, the geology of the rock is mapped and the rock is strengthened by injecting a sand-water-cement mixture into the rock by spraying and bolting. In many cases, it can be agreed with HSY to focus construction activity on the property up to 5 meters from the tunnel. 

What kinds of risks are associated with vibration caused by excavation, for example in terms of housing structures? 

Excavations have been carried out in urban areas much closer to the ground surface and buildings than the sewer excavations that are now being carried out. Experts will conduct a review according to the standards. Vibration values on the structures and their monitoring are objective means of tracking vibration effects. Construction experts have defined threshold values for different types of structures.

What kinds of disturbances does the blasting cause to residents? 

The sounds of blasting are the loudest in tunnel construction. The sound of drilling can be heard when the excavation area is nearby. However, it is very case-specific how well the sound of blasting and drilling can be heard. If the building is constructed directly on rock, the sound is carried further. Noise can be heard less in a building built on thick soil layers. According to the 'noise permit' issued by the Espoo Environment Center, blasting can be carried out on weekdays from 7am to 10pm and on Saturdays from 9am to 6pm. At 7am to 6pm no limits have been set for any other noise levels. If work is done at other times, there is a dB limit in living quarters of 30 ... 40, depending on the time.

How do tunnels affect construction on the properties?

The tunnel alignment is surrounded by the easement area which is defined as 10 meters from the tunnel surfaces. The protection zone is marked on the maps provided to residents. Normally, most of the activities can be done within the easement area, but for certain activities, the building control department must request a statement from HSY. Measures that do not require a statement from HSY on the site are, among others, ground bearing new building and land drainage in the area shown on the map, as well as a well drilled into the rock outside the easement area when the tunnel is built. The building control department should ask for a statement from HSY if the case in-volves:   

  • Excavation and excavation vibrations: As a general rule, HSY states that an action can be carried out taking into account the tunnel.      
  • Significant load (that is, a load that is much larger than a normal house, for example a multi-storey apartment building). As a rule, there is no problem with apartment buildings either, but certain things have to be taken into account in the execution.  
  • The geothermal heat well within the easement area when the tunnels are complet-ed when the location of the pump must be carefully planned.    
  • Neither the rock on top of the tunnel nor the tunnel reinforcement zone must ever be drilled.   

How are buildings in subsidence risk areas taken into account in the excavation of the tunnels?

Before and during tunnel work, subsidence monitoring is also performed. The height and location of structures are monitored in areas where there is a risk of subsidence. Work on rock terrain or non-cohesion soil (sand, moraine) do not pose a risk of subsidence.  Subsidence can occur on clay areas if the surface of the groundwater falls.  Then the clay soil can dry which can cause subsidence. Injection blocks significant leakage into the tunnel and the possible decrease in groundwater level is minimised. In critical locations, the height of the buildings is monitored. Groundwater monitoring is also carried out in the area, meaning groundwater levels are measured from observation pipes at regular intervals.