Compensation Grouting
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Transcript of Compensation Grouting
Reprint of:Response of buildings to excavation-inducedground movements, Imperial College,London, 17th–18th July 2001
Technical paper 61-56 E
Central StationAntwerp
Compensationgrouting under highloaded foundations
Dr.-Ing. Gerhard Chambosse,Dipl.-Ing. Reiner Otterbein
Keller Grundbau GmbH, Bochum
Presented by
Keller Grundbau GmbHKaiserleistr. 44
D-63067 Offenbach
Tel. 069 / 80 51- 0Fax 069 / 80 51 -244E-mail [email protected]
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1 Introduction
By hydraulic fracturing, the soil is artificially fractured and the fissures (Soil-Fracs) are filled withgrout under pressure. In this way an improvement of the soil and/or a controlled heave is obtained.During tunnelling or excavation works this method is often used to compensate for ground loss.In the case which is the subject of this paper, the method is called compensation grouting.Experience has been made with almost any type of soil such as: clay, silt, sand, gravel. Goodresults were achieved in normally consolidated as well as in over consolidated soils. The methodis not economically viable in saturated very soft clays, organic soils, as well as very loose fills. Themethod was introduced more than fifty years ago and in the beginning it was applied tocompensate settlements of buildings. BERNATZIK (1951) was the first to report on this. Withthe development of urban tunnelling the method was optimised and combined with real-time
Abstract
With the redevelopment of the railway line Brussels – Antwerp - Amsterdam - to a high-
speed line it was decided to tunnel under the historic Central Station of Antwerp. The main
piers of the buildings are founded at shallow depth on large footings (12 x 27 m) with pressures
up to 800 kN/m². The tunnel underneath is constructed in a traditional way beneath a
protective pipe umbrella roof which is some 6 m below the foundations. To avoid undue
settlements Compensation Grouting was carried out in between the pipe umbrella and the
underside of the foundations. It was the first time that this method had been applied in Belgium.
The grouting works were controlled by a newly developed and well tested water level system
on the basis of pressure monitoring rather than direct level measurements. This paper reports
on the different grouting phases and the observed efficiency of the system.
Keywords: Compensation grouting, water level system, preconditioning, efficiency
Central Station Antwerp
Compensation grouting under high loaded foundations
Dr.-Ing. Gerhard Chambosse · Dipl.-Ing. Reiner OtterbeinKeller Grundbau GmbH, Bochum
G. Chambosse · R. Otterbein
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City Country Year Soil Project Tunnel face
Essen D 1986 Silt AEG Factory Building / Subway Tunnel 110 m²
Bielefeld D 1989 Clay 6 story Buildings / Road Tunnel 220 m²
Dortmund D 1993 Silt Housing Block / Subway Tunnel 37 m²
Düsseldorf D 1993 Gravel/Sand Bridge Abutments / Main Sewer Tunnel 20 m²
Table 1: Selection of compensation grouting projects for tunnelling works in Germany
monitoring systems. The first application of the combined method in Germany was for a tunnelin the city of Essen (Germany). GABENER, RAABE and WILMS (1989) have reported the projectin detail. Here, it was the first time that a water level system was used in order to monitor andcontrol the settlements. Due to the good results the technique was extended to larger tunnelprojects (Table1).
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Central Station Antwerp · Compensation grouting under high loaded foundations
A B G HC D E F
23
44 5 6 7 11
50m
0m
–23m
5 Excavation6 Tunnel ceiling
7 Tunnel slab
4 Diaphragm wall
2 Grouting area
1 Dewatering
3 Pipe jacking
Existingtunnel
New tunnel
Foundation
Sand (Antwerpiaan)
Boomse clayFigure 1: Cross section and
scope of work
The Central Station of Antwerp with its main building reaching a height of nearly 70 m, was builtfrom 1899 –1905 as a dead-end station. As the Belgium railway NMBS is extending the high-speed line from Brussels via Antwerp to Amsterdam, a tunnel under the Central Station Antwerpwas planned to enable the through-passing of the trains. The required tunnel is constructedaccording to the well-proven Belgium Method. This meant, that after constructing a protectivepipe umbrella roof, diaphragm walls with a depth of 15 m were excavated manually from out ofthe outer pipes. This is followed by further excavation works and the construction of the finalroof and floor of the tunnel. The space between the pipe umbrella roof and the foundationsamounts to 5–6 m. The main foundations of the station buildings, measuring 12 x 27 m, exertpressures of approximately 800 kN/m² on the ground whereas the soil below the entrance hallis hardly loaded. Settlement predictions for all construction phases gave values of 60 to 120 mm.To avoid damage, the Soilfrac –System was applied whereby after the pre-stabilization andprestressing of the soil, a compensatory heave can be achieved while building the tunnel. Themaximum allowed settlement was limited to 5 mm for critical construction phases and maximuminclination of 1:2000. The subsoil in the area of the station consists of a tertiary slightlyoverconsolidated fine sand with 10% silt. Cone Penetration Tests showed a point resistance of18–24 MN/m². At a depth of 20 m so-called Boomse-Clay is underlying. During the constructionworks the groundwater was lowered by approximately 15 m. At the depth of the TAMs (tube àmanchette) a concentration of strong shell was encountered. Figure I shows the individualconstruction phases.
2 Central Station Antwerp
G. Chambosse · R. Otterbein
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Point 468
Point 231
Shaft Shaf
t
Figure 2: Phase I areas with low grout pressure
2.1 Real-time Monitoring
To control the settlement performance 8 different measuring systems were installed and had tobe surveyed. In total 480 measuring points exist.
Changes in the height were measured by an automatic water level system (System GeTec). Thissystem uses pressure sensors instead of measuring level changes. With a precision of 0.02 mmof the sensors the guaranteed accuracy of the system is 0.3 mm. The measuring scope coversthree levels. A special software program monitors the measuring results every 30 seconds, thedata from which may be graphically displayed. The system has worked reliably and without failuresince its installation 22 months ago. It is maintained by a remote control system and any excessin settlement limits is transmitted automatically by SMS-message via mobile telephone service.
2.2 Installation of TAMs
Some 3.500 m steel valve tubes of 50 mm diameter and valve spacings of 0.5–1m were installed.The maximum boring length was 45 m. The position of 35% of the borings was measured byinclinometers. The deviations in homogenic sand were in the order of 1% only. Within the shelllayer deviations of up to 2% were measured. The array of TAMs is about 3.50 m below thefoundations. The maximum settlement during the boring works was less than 1mm.Therefore,injections at this stage were not required.
2.3 Grouting
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Central Station Antwerp · Compensation grouting under high loaded foundations
Project Soil-type Grout Range Grout Average Foundation Load( l/m² ) ( l/m² ) ( kN/m² )
Antwerp 1 Sand 42–46 45 60Antwerp 2 Sand 87–115 100 800
Essen, AEG Silt 35–55 45 80
Bielefeld Clay 56–64 60 280
Dortmund 1 Silt 65–75 70 200Dortmund 2 Silt 84–96 90 200
Düsseldorf Gravel/Sand 42–52 50 500
Hamburg [4] Sand/Silt 31–70 45 50
Table 2: Amount of injected Grout in Phase I
The Grouting process principally consists of two phases:a) Phase 1 Preconditioningb) Phase II Heave or compensation of settlements
At Phase I the soil is prestressed and compacted and thereby, in normal consolidated soils, thehorizontal stress is increased. Any loose zones are detected by observing the grouting pressureand are stabilized accordingly. Figure 2 shows 15 zones within the treated area with less than50% of the pressure of 20–25 bar. This value was estimated according to overburden pressureand line loss. In total 15% of the valves were affected.The combination of steel TAMs, injected material and stress increase raises the stiffness of thesoil considerably. The improvement depends on the soil conditions. RAABE and ESTERS (1993)report an improvement of silty soil by a factor of 2–3.Accordingly, Phase I provides substantial reduction in settlements. MARTAK and LIEBSCH (1993)mention a reduction of up to 50% of predicted values. At the construction of a tunnel (NATM)in the city of Bielefeld (Germany) a reduction of 35% was measured. BOECK and SCHELLER(2000) report a measured reduction of 30%. By reducing the settlements any differential settlementwill also be reduced. In some cases, Phase I in connection with a pre-heave will enable tunneldriving without problems. In general, Phase I will be terminated if a pre-determined pressureand a heave of 1–2 mm is achieved. Injections in Phase I depend on the type of soil, the state ofstresses, foundation pressures, relic foundations, loose zones, etc.Since all these influences are not exactly calculable, the requirements for Phase I are hard toestimate. Table 2 shows projects with different quantities of injected grout between 42 and 115l/m² in order to obtain a contact heave. The values indicated refer to stiff to medium dense soil.The type of soil appears to be less important in comparison to other boundaries. As for theCentral Station Antwerp there is quite a difference between low and high loaded foundations.
The required contact heave of 2 mm at the high loaded foundations resulted in heaves up to5 mm in adjacent non-loaded areas. At two places in the dirt floor of the basement heaves of upto 150 mm were registered. Although the works were carried out with the utmost care, thisphenomenon could not be avoided. KUDELLA and GUDEHUS (1992) have reported similar resultsin trials.
G. Chambosse · R. Otterbein
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Figure 3: Profile of injected quantity of grout material for Phase I + II
600
400
200
Shaft
Shaft
Grouting volume
Foundation
Shaft
Shaft
New Tunnel
Lite
r/m
²
average heave [mm]
Volume (Grout) [l/m²]EC [%] = 100 · (1)
2.4 Efficiency of Compensation Grouting
The success of the measures is governed by the quantity of injected grout. As shown in chapter2.3 at Phase I (Preconditioning) the consumption of grout depends mainly on boundaries whichare difficult to assess. As for the efficiency, it is advisable to consider Phase II only.The efficiency EC of compensation Grouting is expressed by the ratio between heave and volumeof grout.
According to formula1 an average efficiency of 11% was determined for the Antwerp CentralStation from Phase II. Figure 3 shows the distribution of the registered quantities of grout.
According to the varying consumptions in the different zones, the efficiency varies considerably.Whereas for the highly loaded foundations a value of EC = 5% was computed, less loaded areasgave values of as much as 15–20%Figure 4 shows the course of settlement and stages of compensation grouting at measuring pointsN° 468 (Location see Figure 2). On the whole, settlements of approximately 23.5 mm werecompensated up until February 2001. Point N° 231, outside of treated area shows the influenceof dewatering.
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Central Station Antwerp · Compensation grouting under high loaded foundations
Figure 4: Settlement and heave at selected measuring points
1999 2000 200112.02. 29.07.05.07.11.06.18.05.24.04.31.03.07.03.19.01.26.12.02.12.08.11.15.10.21.09.28.08. 22.08. 15.09. 09.10. 02.11. 26.11. 20.12. 13.01. 06.02.
0.0
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-1.0
1.0
2.0
3.0
4.0
5.0
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Dewatering Dewatering (vacuum)
Pip
e B
Pip
e D
Pip
e E
Pip
e C
Pip
e H
Pip
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Tunnel excavation
Pipe jacking andcompensation grouting
Tunnel excavation andcompensation grouting
DrillingTAM's
Preconditioning
Point 231
Point 468
It was proved that at the highly loaded foundations considerably higher settlements had to becompensated than at the less loaded floor of the hall. However, the Soilfrac®-technique is suitablefor both low as well as highly loaded foundations. There are merely differences in the degree ofefficiency and, therefore, in the required quantity of grout to compensate settlements.
G. Chambosse · R. Otterbein
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3 References
[1] W. BERNATZIK Anheben des Kraftwerkes Hessigheim am Neckarmit Hilfe von Zementunterpressungen,Der Bauingenieur, Heft 4, 1951
[2] E.W. RAABE and K. ESTERS Soilfracturing techniques for terminating settlementsand restoring levels of building and structures,In: Ground Improvement, M.P. Moseley,Hayward Baker Inc./ USA,Chapman & Hall, Glasgow, 1993
[3] L. MARTAK and H. LIEBSCH Soil-fracturing in silt and clay –New applications in the Vienna Underground,Internationale Konferenz betreffend Injektionenin Fels und Beton, Salzburg, 1993,Österreichische Gesellschaft für Geomechanik
[4] TH. BOECK and P. SCHELLER 4. Röhre Elbtunnel – Sicherung der Bebauung amNordhang der Elbe,Baugrundtagung in Hannover, 2000,Deutsche Gesellschaft für Geotechnik e.V.
[5] GABENER, RAABE and WILMS Einsatz von Soilfracturing zur Setzungsminderungbeim Tunnelvortrieb,Taschenbuch für den Tunnelbau, 1989,Deutsche Gesellschaft für Erd- und Grundbau,Verlag Glückauf GmbH, Essen
[6] KUDELLA and GUDEHUS Bodenverdrängung durch Einpressen von Fluiden,Vorträge der Baugrundtagung in Dresden, 1992,Deutsche Gesellschaft für Erd- und Grundbau