Norra Länken (the northern link) NL51
Project and objectives
The northern link is a major road network project in Stockholm which aims to reduce congestion in the inner city and improve access to the districts of Värtahamnen and Frihamnen. In total, the northern link will be around 5 km in length. The part which is now under construction will be around 4 km long and will run from Norrtull to Värtan. The majority of the new road will run in underground tunnels and a total of around 11 km of road tunnels will be built. As there is little or no rock coverage, for example in the areas of the slip roads leading to and from the northern link, concrete cut-and-cover tunnels will be built.
It will be necessary to lower the groundwater table in order to build these tunnels, as a large part of the work will be below the existing groundwater level. In the case of deep excavation work, the walls of the excavations must be supported by some kind of structure which is watertight and stable and keeps the environmental impact and possible movement to a minimum.
In order to be able to monitor the environmental activities, the Swedish Transport Administration has produced a monitoring programme together with the cities of Stockholm and Solna. Environmental requirements for the implementation of the contract have been drawn up for every subcontract which was negotiated and have been included in the contracts signed with subcontractors.
NL 51 is one of the largest subcontracts on the northern link and will connect Norrtull with Värtahamnen and Lidingö. It consists of a cut-and-cover tunnel around 370 m in length. In order to be able to build the tunnel, a sheet pile wall has been constructed in the loose clay. On behalf of the joint venture H.O.T. (Hochtief and Oden), Geosigma has taken responsibility for the monitoring programme, for groundwater, vertical deformations (inclinometers) and tie-back forces. This means identifying leaks and following up movements around the excavation, plus taking the necessary measures.
Background
The content of the local monitoring programmes is determined by the relevant contract and the monitoring measures required are carried out in some of the contracts by both the client and the subcontractor. On behalf of the joint venture H.O.T. (Hochtief and Oden), Geosigma has taken responsibility for the monitoring programme for NL51 with regard to groundwater, inclinometers (vertical earth movements/deformations) and tie-back forces. This involves procuring, designing and implementing the ongoing work and analysing the data etc.
Groundwater
Before the excavations below the groundwater table begin, it must be demonstrated that the excavation cell is sufficiently watertight, in other words, that the injection and sheeting work has had the desired effect.
Geosigma carried out pumping tests in a number of wells in each excavation cell. The groundwater level was measured in a number of groundwater pipes both inside and outside the excavation. The cells were assumed to be watertight when the groundwater level outside the excavation did not fall below the lowest permitted level, while the groundwater level inside the cell could be reduced to the necessary foundation level and there was no risk of bottom heave.
Groundwater problems resulting from construction in soil and rock have become increasingly prevalent over recent decades, as larger and larger excavation and tunnel works have taken place in built-up areas. The groundwater can be found in the fractures in the rock and the pores of the soil and these often interact in complex ways. Groundwater is extracted from rock primarily via rock tunnels which often results in the drainage of the soil layer above. A reduction in the groundwater level can cause subsidence, in particular in clay soils. As the clay gradually dries out, houses and other buildings with foundations on the clay can begin to sink. By preventing leakage and allowing the infiltration of surface water, via wells in the soil or directly in the rock, it is possible to counteract the falling groundwater level outside the excavation and raise the level of the water.
Inclinometers and tie-back force sensors
When the excavation cell is watertight, the excavation can continue in stages down to the floor of the tunnel. During the excavation, wale beams are installed at each level which are anchored with tie-backs. In order to determine the load on the tie-backs, tie-back force sensors are fitted in a number of selected sections. Inclinometers are also used in the same sections a few metres behind the sheet pile wall to monitor possible horizontal earth movements, which could result in the wall moving in towards the excavation and causing a collapse.
The excavation cells are therefore being monitored using three independent systems. The monitoring process continues until the concrete work is finished and the excavation is refilled, which allows measures to be taken immediately to achieve the desired effect. The result is that problems can be prevented as early as possible, in order to minimise the risk of delays and other negative consequences.
Because Geosigma was responsible for every stage, from the procurement through to the monitoring and analysis, it was able to link together the geotechnical and hydrogeological information, which allowed for broader-based evaluations of the process in the context of both disciplines.
The increased cost of these preparatory measures can be balanced out against the improvement in efficiency, because action can be taken immediately if level changes, movements or increases in load are identified. This allows for countermeasures to be put in place quickly which will reduce the extent and the consequences of damage to the soil, buildings and water and sewage facilities over larger areas.