IntroductionJust in front of the Central Station of Amsterdam,a large excavation was needed, to connect theexisting East-metro connection with the newNorth-South connection. This building pit,roughly 3000 m2 has been build with diaphragmwalls of 30 to 60 m length and has been excavatedin the dry. In the early stages, 5 layers of concretestruts were planned to prevent the wall from tomuch deformation. During the contracting phase,these have been replaced by two concrete strutsand a jet grout strut, just below the excavation(see figure 2).

Design of the jet grout strutThe design and tender documents did not prescri-be a specific diameter. The layout of the groutstrut should be designed with 5% opening to pre-vent water pressure building up. The minimumthickness of the grout strut was 1,0 m in themiddle, 2,0 m near the diaphragm wall and 3,0 maround some of the steel piles (see figure 3). The layout of the individual jet grout columns wasdesigned by the subcontractor and afterwardsapproved by the client. In principle a triangulargrid was adapted which allowed an average

inclination of 0,5 degrees and a minimum overlapof 10 cm. Due to several causes the demand wasnot met and a smaller grid was required.Furthermore, old sheet piles, old structures, thenew piles and a former trial pit had to be takeninto account (see figure 4).

The design value of the compressive strength was4,5 N/mm2 and was negotiated to an averagevalue of 10 N/mm2. The desired E-modulus was2500 MPa and both values to be reached after28 days.

Jetgrouting with the DS-system.Jetgrouting is a cement soil stabilisation andapplicable in all kinds of soils. With the aid of highpressure cutting jets, the borehole is eroded withthe cement suspension. The erosion capability canbe enlarged by shrouding the jet with air. In thiscase Smet-Keller used the DS-system which usesthe energy more efficient and allows bigger dia-meter. This enabled it to use a diameter of 3,2 mwhich limited the amount of columns drastically.The columns were made in two stages, first cutting the diameter (and measuring it) and afterwards increasing the cement content.

Before starting the execution, three trial columnswere executed. After fine tuning of the parameters,the third column was perfect.

Column productionBecause all traffic at street level should proceedwithout any blockages, the whole building pit wasdivided in three parts. After the works of one partwas finished (diaphragm wall, piles, grout strutund concrete slab) all traffic was diverted and thewhole operation started again. The actual columnproduction started in May 2007 where the lastcolumn was finished in October 2008. In total521 columns were produced.

The grouting was performed with two tall rigswhich could drill the maximum depth of 26 m inone time (see figure 1). Because of the number of return spoil, two largespoil silos were installed, enabling regular concre-te trucks to transport the spoil to an especiallyprepared site. The silos served as temporary sto-rage facility and limited the interference of trafficproblems.The sequence of the columns was determined bytwo requirements: in a virgin area, at least two

Jet grout strut prevents deformationdiaphragm-wall

Summary

In order to limit the amount of steel strutsin the excavation, a jet grout strut wasdesigned and executed. Just beneath theexcavation level a 1 to 3 m thick groutlayer resists the horizontal forces to limitdeformation of the diaphragm wall. Excessive diameter measurements as wellas compressive strength tests have beenperformed to assure that the design requi-rements have been met.

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Rob M. van der Ploeg ProjectbureauNoord/Zuidlijn

Patrick van HoltenCombinatie StruktonBetonbouw / Van Oord

Henk DekkerSmet-KellerFunderingstechniekenVOF

Figure 1 Groutingrigs of Smet-Keller in action. Photo Smet-Keller.

columns should be left before the next columncould be grouted on the same day. After a columnwas over 24 hours old, columns on the oppositeside could be grouted on the same day.

Diameter measurementsDuring the execution of the so called Sandwichwall (another part of the same project) beneaththe Central Station, extensive experience wasbuilt up with measuring diameter at large depth(see [1]). For this project, a new challenge wasfound in the diameter of 3,2 m. The device is screwed on the drilling rig and lowered to the desired depth. The first step ishydraulically opening both arms, like an umbrella.After the full opening of the arms, which wasafterwards verified by checking a damaged plasticbolt, both arms where enlarged, like a telescope(see figure 6). The values of the measurementswhere drawn in a diagram and compared with thecalibration of the device, before and after themeasurement. In total 38 measurements wheresuccessfully executed.

Compressive strengthOne of the main issues during execution was the

Figure 3 Cross-section over the jet grout strut.

Figure 4 Top view of as-built (partly).

Figure 5 Overview of the building site. Photo Projectbureau Noord/Zuidlijn.

GEOtechniek – Special 11th DFI/EFFC – London – United Kingdom 11

Figure 2 Plaxis cross-section over the diaphragm wall.

quality of the grout itself. This was done by performing triple core drillings, which enablesgetting perfect samples for testing and gives aperfect view through the column itself. The drillings were performed both in the hart of the column and at 1/3 of the radius.

Representatives of the clients would choose thedesired samples on which unconfined compressivestrength tests were performed. In total 30 coredrillings and over 200 tests were performed.The average value of all tests landed just overthe desired value.

Further quality controlFurther quality control consisted of the measure-ment of the inclination of the drilling with a mobileinclinometer (218 measurements), the monitoring(100 %) + sampling (~1 %) of the fresh grout and

the monitoring (100 %) + sampling (~5 %) of the return spoil.Because not every column could be measured orcore drilled to prove the diameter or strength, itwas inevitable to show the client all productiondetails. In this way birth certificate of a measuredcolumn nearby could be compared with the actualcolumn. In case of an anomaly or, for example, a large inclination, appropriate measures weretaken.

Behaviour during excavation During the excavation excessive deformationmeasurements have been executed. The measured deformations are less than calcula-ted because the soil reacted stiffer, creep effectsare incorporated in the calculation but not yetmeasured and three dimensional effects were not taken into account in the calculation.

Conclusions Despite the large depths, a jet grout strut wassuccessfully executed and measurements haveshown that the design requirements have beenmet. Both the column diameters of 3,2 m and thestrength reached the desired values. Deformationmeasurements showed that the calculated valueswere on the conservative side. The application of a grout strut proved to be a valuable additionto this building pit.

Reference[1] Langhorst O.S., Schat B.J., Wit J.C.W.M. de,Bogaards P.J., Essler R.D., Maertens J., ObladenB.K.J., Bosma C.F., Sleuwaegen Y.J. and DekkerH., Design and validation of jetgrouting for theAmsterdam Central Station, Geotechniek ECSMG in Madrid 2007.

Figure 6 Calibration ofthe diametermeasure-ment device. Photo Smet-Keller.

Figure 8 Deformationsduring excavation.

Figure 7 Cores madeby triple core drilling.

12 GEOtechniek – Special 11th DFI/EFFC – London – United Kingdom