An innovitave approach for jet grouting under difficult circumstances is creating a sandwich wall under Amsterdam Central Station
The new Noord/Zuidlijn underground will link Amsterdam Noord (Amsterdam North) and Amsterdam Zuid/WTC (Amsterdam South/WTC) to the city centre, passing under the IJ waterway. To further connect up the Netherlands capital, a new underground station will be built beneath Amsterdam Central Station.
The overland station, a national monument, was built around 1880 on an island dredged from the adjacent waterway and is supported on 9000 wooden piles. It has undergone about 180mm subsidence over 100 years.
Although tunnelling has yet to start on the metro line, groundworks for the underground station are well under way. Fundamental to the project is keeping trains running and passenger inconvenience to a minimum during construction beneath the station.
The current work is being carried out over a relatively small footprint within the largest interchange station in the Netherlands, making the project highly complex.
The construction process will fi rst involve removal of timber piles within the footprint of the sandwich wall using a specially developed technique. The pile void is being fi lled at the same time as the pile is extracted, minimising the impact on the historic station. Steel Tubex piles are then screwed to depth in segments of 2m to 5m, due to limited headroom, and the zone around them jet grouted to create the sandwich wall - upon which the building foundation loads will be transferred.
Site workers will then lower groundwater by about 5m for an initial excavation phase to allow jet grouting of a base strut at about 20m and the construction of a steel top strut. Next, the groundwater is raised back to normal levels and the area between the sandwich walls excavated to around 20m and fi nally the new tunnel will be floated in to be immersed into place and the ground backfilled.
Ground conditions are complex and range from soft clays to very dense sands.
The design of the sandwich wall is based on a robust, stiff wall, built from smaller components. As a result, the equipment required is relatively light and can be used within the existing station without excessive modifi tion to the building structure.
The sandwich wall consists of two rows of 457mm diameter steel Tubex piles with wall thicknesses of 16mm to 25mm and lengths up to 60m. Rig crews will put them in at 1m centres with the pile rows spaced apart at about 2.5m centres. The Tubex piles are provided with welded rings at 500mm depth intervals to ensure full mobilisation of shear strength within the grout body.
Jet grout columns will fill space between the piles, varying in diameter from 800mm to 1200mm and a 28.5m length. Ground workers will install further jet grout columns in the space between the pile rows with diameters from 1400mm to 2200mm and lengths from 26m to 28.5m. The retained height of the wall is about 18m.
Structural interaction between the steel Tubex piles and the grout body, supplemented with temporary struts at three levels, will provide horizontal stability. This will give an extremely stiff wall, with predicted deformations of 20mm to 30mm.
The sandwich wall also transfers the vertical building foundation loads at the location of the construction pit, through the combination of the grout base of the sandwich wall and a number of Tubex piles within it that are extended to 60m.
Although the system is extremely stiff and is based on the assumed interaction between the Tubex piles and the jet grout, it has a relatively brittle failure behaviour.
To cater for this, safety factors have been increased considerably and resistance to failure is guaranteed by the strength of the two rows of Tubex piles.
Because of the sensitivity of the location, a jet grout trial was carried out in early 2004 to confirm that diameters, deviations and strengths could be achieved. The results were disappointing with variability in diameter and strength measurements casting doubt over the design. So the client decided to set up a steering group from the summer of 2004 until May 2005, which met twice a week to evaluate and discuss the numerous technical issues concerning the jet grouting solution.
The solutions that evolved mean that when work started on the south side of the wall in May 2005, technical control of the jet grouting was probably to the highest standard seen on a site to date. Work on this side has now completed with work on the north side yet to begin.
For each column to be built, details of all the different jet grout parameters in the fi ve or six strata present were set out and the corresponding depths defined.
After each column was drilled, but before jet grouting started, engineers surveyed the hole with an inclinometer lowered down the centre of the drill rods and the as built position determined.
This was immediately compared with the design location using a CAD system and the final approval to jet the column given. For a number of columns where the deviation was not acceptable, either parameters were adjusted to give larger columns or only the upper part of the column was jetted with a new design produced for the lower part to be carried out at a later date with an additional column. Each jet grout rig was fi tted with a recording system that kept a complete record of all jet grouting operations.
In addition, both jet grout column callipers (a Keller system) and hydrophones (provided by Gtec) measured insitu diameter with both systems being extensively developed during the works. The callipers were lowered to measurement depth on rods and the arms extended until the resistance from the edge of the column was felt.
The hydrophones consisted of a number of units suspended on cables either within the Tubex piles or within specially drilled small diameter boreholes. When the grout jet impacts on the hydrophone hole, it is detected, confi ming that the column diameter has reached the offset between the column centre and the hydrophone.
For some of the columns, jet grouting parameters were altered in real time based on hydrophone feedback. This system offers a promising method for determining the correct parameters to create specifi c diameters. In later stages, a direct interface between the Gtec system and the rig instrumentation was made to allow complete synchronisation of depth.
Following completion of the southern phase in late 2005, engineers used a Texplor system to get insitu measurements of wall continuity. Texplor is an innovative method of leak detection based on measurement of electrochemical potential.
The north section of the sandwich wall will restart in February 2007 and the project is due to complete in 2010.
J de Wit, J Bogaards: Adviesbureau Noord/Zuidlijn Amsterdam, Royal Haskoning, Rotterdam;
R D Essler, J Maertens: advisers; O Langhorst: VOF Stationseiland Amsterdam, Holland Railconsult, Utrecht; B Obladen, C Bosma: Main contractor CSO, Combinatie Strukton Betonbouw van Oord ACZ;
Y Sleuwaegen, H Dekker: Jet grouting subcontractor Smet Keller.