NSL General HdW.update nov09

NORTH / SOUTH LINE AMSTERDAM Tunnel Engineering Consultants

NORTH / SOUTH LINE AMSTERDAM - a challenge in desig n and construction J.C.W.M. de Wit MSc, Royal Haskoning – Tunnel Engineering Consultants / Adviesbureau Noord/Zuidlijn Amsterdam 1. General and brief description The North/South Metro Line in Amsterdam, containing eight stations and measuring 9-km in length, will connect the northern and southern suburbs with the city centre (figure 1).

Figure 1 Alignment North/South Line The northern part of the line will be mainly on surface level; as the line approaches the IJ river, the line goes underground using traditional cut and cover techniques. An immersed tunnel will be applied for the crossing of the river. The southern part of the line will be just below surface level, also applying the traditional cut and cover techniques. For the metro lines running through the city centre, construction is treated quite differently. For the purpose of protecting the historic city centre and restricting the disruption of city life to a minimum, special construction techniques are required that are not frequently applied in the Netherlands. This includes for example a bored tunnel for the running tunnels, and diaphragm walling and jet grouting for the deep station boxes. A special design approach was adopted in order to meet boundary conditions for this part of the line. 2. Design approach in general The North/South Line contains a number of designs based on advanced and innovative construction techniques that have to be applied within the historic city centre. Since application on this scale has never been applied before, a special design approach has been adopted for this vulnerable part of Amsterdam. This approach, in general, is valid for both the bored tunnel as well as the station boxes. An important starting point in the design process is a sturdy design; this means it does not only depend on advanced calculation models and is not sensitive to small variations in design parameters. Feasilibility and parametric studies, with a focus on environmental impact, were carried out to develop this sturdy design. In addition, all aspects of the design had to be completed and confirmed, which included the following:

- extensive soil investigations including special tests that can accommodate data for the advanced calculation models that have been applied;

- extensive desk studies to investigate building response (brick buildings on wooden pile foundations) to settlement induced deformations due to construction – figure 2.

- investigating the present state of the buildings alongside the alignment followed by a building classification, and in a number of cases suggestions for foundation reinforcement;

- development of FE models considering the construction process for predictions and back analyses;

- a monitoring program to measure deformations of the buildings, subsoil and North/South Line structures (e.g. building pit walls);

- development of back up measurements to implement in the design if anticipation on monitoring data is required.


Figure 2 : Typical historic Amsterdam building There was a lack of knowledge when applying the innovative design aspects; in these cases international recognised experts or institutes were consulted and the designs were reviewed. Furthermore, full scale tests were carried out to investigate the impact of construction techniques as a means of verifying and validating the calculation models that had been developed. The next set of tests that have been carried out are as follows:

- the test pile project to investigate the impact of tunnel boring activities to pile foundations; - the environmental impact of diaphragm wall installation; - the effect of grouting techniques (jetgrouting, permeation grouting, compensation grouting); - freezing test; - test to investigate lining design; - tail injection test for the bored tunnel with a focus on limiting the environmental impact

This approach, based upon the observational method, was applied for several designs within this project, and fine tuned to a tailor made concept to meet the specific boundary conditions for each location.


3. Bored Tunnel (Kaalberg et al)

For the running tunnels, a twin bored tunnel will be applied that follows the street pattern as closely as possible and is lowered to a great depth to minimise the construction effects. The total length of the bored tunnel part is approx. 3.2 km; the outer diameter is approx. 7 m. Since the streets are very narrow the tunnels will be bored with only 0,5 x diameter clearance. In one case the tunnels even have to be bored above each other because positioning the tunnels next to each other is not possible (figure 3a). Figure 3a : Alignment bored tunnel

It is obvious that the assessment of possible construction risks in this environment is of high importance. Therefore a combination of the principles of 3D value engineering and design by testing is adopted. 3D value engineering is used to optimise the design by means of parametric studies thereby applying advanced 4D FE models to predict the impact due to tunnelling and for which the advanced tunnel boring machine (TBM) is considered (figure 3b).

Figure 3b : 3D model of Amsterdam building block and advancing TBM Since the interaction between the ground and structure plays an important role, the FE models are not always considered to be reliable up to the level that is required. In these cases, full scale tests are carried out for verification reasons meaning that the design is confirmed by way of testing.


Furthermore, advanced TBM techniques are developed to limit damage to historic buildings. It includes: - a compact shield system (Vario) that can be adjusted during boring depending on the varying ground

conditions; - a TBM process control system that will be supported by means of the GIS Settlement Risk

Management tool (paragraph 6) - an innovative concept of the tail of the TBM with an injection principle that was tested on a scale of

1:5. The tests proved that assessment of the settlements improved significantly. This information was provided to the contractor during the tender phase. The selected contractor decided to implement all techniques in the TBM that is going to be used. On a number of locations, along the metro line, there will be support to several buildings by means of the compensational grouting technique, while the TBM is passing. The lining design consists of 0.35 m thick segments and includes a special joint design that is accommodating an optimised structural interaction between the ring sections (figure 3c).

Figure 3c : Concept for segment joint


4. Deep stations in the city centre Since the bored tunnel is installed at a deep level, the underground stations are at a great depth as well. The stations, 15-25 m wide and 200-260 m long, will be constructed at very busy locations and close to historical buildings. To keep construction disturbance to a minimum, a cover will be put over the building pit. Traffic flows can then be maintained by implementing the construction of diaphragm walls and the building pit cover in several phases (figure 4a).

Figure 4a : Construction of diaphragm walls east phase Rokin (photo G. Dubbelman) For the construction of the underground station, a building pit will be constructed having 1.2 -1.5 m-thick braced diaphragm walls. These walls reach a depth of 40 m below ground level. Since the diaphragm walls continue down to a sealing clay layer, the building pit can be excavated in the dry. From surface level, a deep jet grout slab is installed under the final excavation level. The excavations of over 30 m will be carried out in soft soil conditions with high ground water levels. After the excavation is completed the concrete structure can then be built. The reinforced concrete diaphragm wall which is originally the retaining wall, will then be integrated into the permanent body. One of the most important aspects of designing underground stations is the influence of the construction on the pre-existing surroundings. This is of high importance because there are historical buildings close by and along the building pit. Considering the resistance of these buildings is often limited, the next aspects of the building process needs to be examined: 1. installation of diaphragm walls; 2. installation of building pit cover of deck; 3. excavation of building pit in combination with placing struts; 4. construction of permanent body in combination with removing struts. To get a proper picture of deformations, it is necessary to use a program based on the Finite Element Method (FEM), in which the full building sequence with a time schedule is processed. The installation of the deck, the excavation of the building pit and the construction of the permanent body can be analysed with a 2D FE model as a combination of staged construction and water flow calculation.(figure 4b) .


Figure 4b Installation of the deck

Figure 4c: Excavation of the building pit underneath the deck


The excavation of the diaphragm wall trench is modelled with a 3D FE model that is validated as a part of a full scale diaphragm wall test that was carried out. The design philosophy of the deep station boxes is also based upon the observational method which resulted in the development of a settlement risk management tool. Figure 4d : FE modelling deep station box

5. Station Island Station Island is an artificial island (land reclamation) that was created at the end of the nineteenth century in order to build the railway station for Dutch Rail including the squares and the access roads (figure 5a); it will now be crossed by the North South Line. The metro station “Central Station” is under construction here as well as the tunnel part between this station and the bored tunnel including the TBM launching shaft in which several special and innovative construction techniques are applied. Figure 5a : Station Island / commencing construction North/South Line (photo Your Captain luchtfotografie)


5.1 Central Station The North/South Line crosses underneath the central station of the Dutch Railways. The Central metro sta tion is planned at this exact location (figure 5b). Th e platforms of the underground metro station are situated directly underneath the railway station and concourses and entrances are being constructed on both sides.

Figure 5b : Metro station “Central Station” On the south side, a spacious concourse as well as entrances will be constructed underneath the Station Square, one of the busiest squares in Amsterdam. The building pit of this part of the station consists of 1.2 m-thick braced diaphragm walls with lengths varying form 30 (building pit wall) to 60-m (building pit wall and foundation element). To limit the disturbance to city life a building pit cover will be installed in different stages. This deck will be founded on bored piles and diaphragm wall panels with a length of 60-m. A jet grout prop is installed underneath the final excavation level. When the building pit has been excavated in the dry to the required depth of 22-m, the concrete structure can be built and the diaphragm walls will be integrated into the permanent body. On the north side, a concourse as well as metro entrances will be built. The construction of this part is combined with the construction of a 2x2 lane traffic tunnel (passing over the metro) and a bus station platform at level +1. The building pit consists of braced combi pile walls (diameter 1.6 m) and an underwater concrete slab (excavation will be carried out in the wet). After the excavation is completed the concrete structure, which is founded on bored piles with a length of 60-m, can be built. In the construction phase the main building pit will also serve as a lock pit (in connection to the IJ river) for the floating transportation of a tunnel element (platform part) that is immersed underneath the railway station. The platforms of the underground station are located underneath the railway station. The design can be briefly described as a tunnel immersed in a building pit (like a canal) underneath the railway station (figure 5c). First of all, a building pit must be constructed underneath the railway station. A thorough investigation must be taken of the existing, relatively vulnerable, pile foundations under the railway station buildings and traffic flows within the railway station (passengers and trains).


Figure 5c : Construction underneath railway station (illustration E. Verdult) Disturbance is only permissible to a limited degree, and compensatory measures will be taken where necessary. Special tailor made types of building pit walls are applied: - Underneath the central building a composite building pit wall, the so-called “sandwich wall” will be

constructed, that has to provide strength and water and ground tightness. This wall consists of two rows of steel tubex piles of φ457/16, with the space between the pile rows being grouted, thus creating a wall with a thickness of about 2.5-m, suitable for absorbing both horizontal and vertical loads (figure 5c-f). One steel tubex pile is assembled with lengths of about 2 to 5 m (total length 30 to 60 m).

Figure 5d Installation tubex pile Figure 5e. Jetgrouting


The requirements that have to be met are high, but the layout of the wall is quite delicate and vulnerable to construction deviations. Therefore, at the construction of the wall, an intensified version of the observational method, including supervision, is used. The construction of all the different components, such as steel piles and jet grout columns, are carefully monitored and subsequently included in an as built design and construction plan (figure 5f). Both the design as well as the construction parameters (e.g. for jetgrouting) can be adjusted based on the latest as built information. On a predefined moment (25%, 50%, 100% completion) the wall is tested on watertightness; in doing so it detects possible leakages that can be treated in the next stage. In this way, a ground and watertight wall can be produced. The first part of the wall is produced using this approach and appeared to be effective.

Figure 5f : Plan view sandwich wall - The building pit wall underneath the platforms consists of tubular piles (diameter 1.82 m) connected

together with watertight interlocks. The piles with lengths of 30 to 60 m are installed with a limited head room (2.80-m) using a vertical micro tunneling method. This method is proposed by the contractor and worked out in an alliance with the contractor and the clients’ advisor. Special equipment (TBM, press frame, service crane) is developed.

Figure 5g : Micro tunnelling for building pit wall underneath railway station (illustration CMM/IHC)


Sandwichwall installed and partly excavated

Microtunneling wall


When the building pit walls have been put in place, the support structures will be made; these will be laid upon the building pit walls in order to support the station components above the building pit. The building pit wall will be braced at the top by the support structures and stabilised at the bottom by a jet grouting strut underneath the excavation level. Subsequently, the building pit will be excavated wet. Once the building pit is completed, a sort of canal-like structure is created underneath the railway station in which a tunnel element (lxbxh = 130x21x8 m) will be immersed, containing the platform part of the metro station. This canal is connected to the IJ river by means of the building pit structures for the northern part of the metro station. The tunnel is constructed in a casting basin opposite the river and will be floated up once it is completed and the casting basin is inundated. From the north side it will be transported afloat underneath the railway station and immersed in two stages against the southern part of the metro station (figure 5h); afterwards the tunnel will be back filled. On the north side, the construction of the concrete structure of the station box can then begin. The joints between the immersed tunnel and the in-situ cut and cover parts are constructed using the traditional immersion joint layout at the south side and freezing techniques at the north side.

Figure 5h. Immersion of tunnel element for platform underneath railway station


Tunnel element under construction in casting basin Tunnel element floated out of construction basin


5.2 Tunnel section between metro station “Central S tation” and bored tunnel

Areal view with location of pneumatic caissons The tunnel section between metro station “Central Station” and the bored tunnel is incorporated into an Amsterdam canal and crosses two bridges which are essential to the city centres’ road system. For the construction of this section the pneumatic caisson method is applied (can be considered as a special form of trenchless tunnelling); therefore a temporary land fill within the canal is installed (figure 5i). On this land fill, three concrete tunnel boxes are constructed (figure 5j).

Figure 5i: Temporary landfill for construction of southernmost caisson


Figure 5j : Pneumatic caisson layout The southern most caisson (60 x 25 x 20 m) will also serve as TBM launching shaft (figure 5k). Underneath each tunnel section a working chamber is provided; from this chamber, the ground underneath the tunnel can be excavated, sinking the tunnel into the ground as a result. When the working chamber is under groundwater level, the excavation (hydraulic) is carried out under compressed air conditions. The joint between the tunnel sections will be constructed using freezing techniques. The pneumatic caisson method was selected because of the typical ground conditions and possible obstacles in the ground.

Figure 5k : Caisson (TBM launching shaft) under construction


Excavation underneath caisson 6. Settlement risk management To anticipate ground movement, possible damage to buildings during construction, and to enable timely and appropriate actions, an effective monitoring concept has been designed. Buildings will be monitored using a fully automated system of total stations and sensors. This on-line system will provide direct data for interpretation in the design office. The monitored information is analysed and processed in such a way that during the construction period changes to the design/process can be made. Settlement risk management is an important tool in controlling the effects of the construction of bored tunnel and station boxes. Within the design phase, GIS is used to combine settlement predictions and building classifications along the station in order to confirm or adjust the design or to implement mitigating measures. It includes a number of design modules: - the building response that is investigated by means of desk studies in which the typical Amsterdam

brick houses are considered in FE models and in which the impact due to construction activities are implemented as settlement induced deformations.

- Various functions that have been programmed to determine damage parameters (settlements, differential settlements, angular distorsion) for each building.

- Settlement predictions in all stages of construction; these predictions will be constantly improved by means of back analyses of preceding stages during construction.

During construction the GIS allows for rapid interpretation of the monitoring data by using the information from the design modules. If necessary, back analyses can be carried out and be implemented in the GIS, in this way, an early decision can be made about active measures.

References Kaalberg F.J., Bosch J.W., 3D Value Engineering & Design by testing for bored tunnel design, Cement 2004 nr.2

NSL General HdW.update nov09

Documents

Transcript of NSL General HdW.update nov09