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GROUTING OF THE ANNULAR GAP IN SHIELD TUNNELLING – AN IMPORTANT FACTOR FOR MINIMISATION OF SETTLEMENTS AND PRODUCTION PERFORMANCE

Markus Thewes, Christoph Budach

Institute for Tunnelling, Pipeline Technology and Construction Management,

Ruhr-University Bochum, Bochum, Universitätsstr. 150, 44780, Germany Keywords: Grout, annular gap, shield tunnelling INTRODUCTION When tunnelling with shield machines, prefabricated segments are commonly installed for a tunnel lining. A circular annular gap remains behind the shield tail, which is limited on the inside by the lining segments and on the outside by the surrounding ground. The width of the annular gap is caused by overcut, conicity of the shield skin and design of the seal; its width ranges between 13 and 18 cm. Figure 1 shows the factors influencing the width of the annular gap.

segmental lining

overcut

cutting wheel

wire brush

ca. 1

3-18

cm

tailskin

width of sealwidth of tailskin

width of conicity

width of overcut

Figure 1 - Factors of influence on the width of the annular gap In order to minimise settlements at the ground surface and to ensure good embedment of the segmental lining, the annular gap has to be filled with grout continuously during tunnelling. During tunnel advance the grouting of the annular gap and the embedment of the segmental lining are necessary to transmit the forces from the tunnel into the surrounding ground. The grouting material should at least have the same properties (or better) as the surrounding ground in the final state.

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GROUTING MATERIALS Different types of grouting materials have been used in order to ensure good embedment and to minimise settlements. Hydraulically setting mortar and two-component grout are typically used as grouting materials. In the case of shielded hard rock tunnel boring machines the annular gap is backfilled with pea gravel instead of grout. These materials are described in the following. Hydraulically setting mortar Since the beginning of shield tunnelling mortar is the most common grouting material in Europe. There are high demands on the mortar concerning

- embedment of the segment lining - minimisation of settlements - sealing against ground water and leakage water - good plasticity to gain ideal workability and pumpability - high stability - erosion stability - temporary behaviour with respect to compression and shear strength

The mortar should in the first place have an excellent pumpability in order to minimise plugging problems in the grout system. However, it should also have good properties of stiffness at the beginning for filling the annular gap to ensure good embedment. The demand for stiffness at the beginning and the demand on good plastic deformation partly contrast each other. Mortar with good features of embedment sometimes have bad pumping properties and vice versa. The stiffening behaviour of the mortar must be regulated in such a way that it is possible to start tunnelling after an interruption. The demands on the hydraulically setting mortar are listed in the following table.

Table 1 - Typical demands on hydraulically setting mortar to fill the annular gap

Workability

Spread diameter (t = 0 h): 15 cm ± 5 cm Flow diameter (t = 0 h): 20 cm ± 5 cm Flow diameter (t = 8 h): 15 cm ± 5 cm (Flow table test: DIN 18555, T2, 3.2.1.1 and 3.2.1.2; ASTM C124)

Compression strength Compression strength (24 h): small, but measurable sometimes: 0,5 N/mm²

Stiffness modulus Similar to a soil that is suitable for tunnelling with segment linings (e.g. 5 – 10 MN/m²)

The properties of mortar are governed by its cement constituents. It is possible to divide mortar into active, reduced active and inert systems. In inert systems there is no cement, while reduced active systems have a fraction of cement usually varying between 50 kg/m³ and approximately 200 kg/m³. Only in active systems the binder component develops full hydration. There is an active system by using cement with over approximately 200 kg/m³. In table 2 examples for grout mixtures with an active system, reduced active systems and an inert system are shown.

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Table 2 - Grout mixtures with an active system, reduced active systems and an inert system for a major traffic tunnel

Cement Sand 0 - 1 [mm]

Sand 0 - 2 [mm]

Gravel 2 – 8 [mm]

Bentonite-slurry (concentration 6 %)

Fly ash Water

[kg] [kg] [kg] [kg] [kg] [kg] [kg] Active system 194 169 674 454 153 194 207 Reduced active

system 120 169 674 454 183 268 177

Reduced active system 60 169 674 454 166 328 164

Inert system 0 169 674 454 183 420 135 Two-component grout Grout with two components was developed to achieve good pumpabilty / workability and a quick setting. Both components have slurry consistency in order to pump them close to the annular gap where they are mixed. One component is a cement bentonite slurry. The other component is a hardener or activator. After a short reaction time a gel is generated. The reaction time of the grout can be influenced by controlling the volume flow of those both components. The stiffening behaviour of the two-component grout, applied to the second tube of the Dutch Botlekspoortunnel, was similar to the stiffening behaviour of hydraulic setting mortar (Jonker, Maidl 2001).

Table 3 - Grout mixture of two-component mortar Component A Component B

Cement Bentonite Stabilisator Water Activator Water

[kg] [kg] [kg] [kg] [kg] [kg] Two-component grout

(Bäppler, 2008) 482 46 4 742 89 7

Pea gravel When using shield machines in hard rock without pressure at the working face combined with a mortar injection to fill the annular gap, the penetration of mortar into the excavation chamber might occur and cause damage. Also the crown area of the annular gap is difficult to fill, so that only partial embedment of the rings are achieved. Therefore, in hard rock the annular gap usually is filled with pea gravel as bedding for segmental lining. Washed gravel with a diameter between 8 and 12 mm (broken or rounded) is required as grouting material. The gravel should have no fines in order to minimise clogging. A combination of mortar and gravel is also used commonly as grouting material. Firstly, mortar is placed in the annular gap at the bottom to ensure good embedment of the segmental lining. After that gravel can be inserted through the segmental lining prevent settlements. Finally the crown of the tunnel is grouted because the gravel will not achieve a full round embedment. It is important to note that in jointed or fractured hard rock with a high water inflow the annular gap could serve as a sort of drainage layer, where water is collected and flows forward to the excavation chamber of the shield machine. In these cases from time to time a full round injection i.e. with waterstopping PUR resin has to be done from inside the segment rings.

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GROUTING METHODS Two different methods to fill the annular gap with grouting material have been established. On the one hand it is possible to transport the grouting material through segmental lining into the annular gap, on the other hand grout supply lines can be used to fill the annular ring. The latter method is also called grouting through the tailskin. Grouting through grout holes in the lining segments The lining segments have to be equipped with holes to fill the annular gap with grouting material. The grout holes should have a mechanism to retain the grouting material in the annular gap like non-return valves or plugs. The number of grouting holes depends on the plastic deformation of the grouting material. There is usually one grouting hole per lining segment. After the segments are fitted tubes are attached to the grouting holes in order to fill the annular gap. The grouting process should start as soon as possible for minimising settlements. A seal at the end of the tailskin prevents the penetration of the grouting material into the shield machine. By controlling the grouting pressure and the pressure in the excavation chamber, the contact between the mortar and the tunnel boring machine is prevented. Spring steel sheets prevents the penetration of grouting material into the steering gap and into the excavation chamber. Cavities in the tunnel crown in the former annular gap with the high of 1 % of the shield diameter can be the result of slump of the primary grout. Secondary grouting is usually necessary and is carried out 40 to 100 m behind the shield at the end of the back-up system (Maidl et. al, 1996). As mentioned before secondary grouting is also required when using gravel as grouting material.

spring steel sheetgrout hole

wire brush

segmental lining

graveltailskin

Figure 2 - Grouting through grout holes in the lining segments (e.g. hard rock)

Grouting through the tailskin In soft ground it is necessary to fill the annular gap continuously with mortar. Therefore grouting technology through the tailskin was developed. Using this method, the mortar is pumped through a grout supply line into the gap. Generally, the width of the grout supply line changes from a diameter of 65 mm to an oval cross-section which has with the same cross-sectional area. Figure 3 shows the principle of grouting through the tailskin.

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grout supply line suspension wire brush mortarsegmentallining

tailskin

Figure 3 - Grouting through the tailskin (e.g. soil)

Two-component mortar was already grouted into the annular gap through the tailskin and through the grout holes of the lining segments (Bäppler, 2008). The mortar is kept from setting within the grout lines by rinsing the lines regularly. EQUIPMENT FOR THE GROUTING OF THE ANNULAR GAP To fill the annular gap with the above described materials, different methods are applied to pump the material. Piston pump Piston pumps are the most common pumps to grout the annular gap with hydraulic settled mortar. Piston pumps push the material through a supply line and convey the mortar. The volume of delivered mortar is regulated by the pace of the piston. Piston pumps exist as single or double piston pumps. Double piston pumps are usually installed in tunnel boring machines due to their compact design. Each piston fills one grout supply line with mortar.

Figure 4 - Double piston pump with installed grout supply line

Peristaltic pump The main component of a peristaltic pump is a flexible tube fitted inside a circular pump casing. The pump draws the material inside the tube by causing a vacuum inside the tube under simultaneous pushing of the material forward by the rotation of rollers on the flexible tube. Due to

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this process the pump is called peristaltic pump. Normally, the grout supply is at the bottom of the pump and the outlet of the material on the top. Peristaltic pumps are normally used to transport fluids, because a fluid medium is not stressed very much. The pumps are also used to pump ready-mixed concrete to the installation location. Peristaltic pumps were used at Botlekspoortunnel in the Netherlands to transport one component of the two-component mortar. This component was a fluid and without any aggregates (Jonker, 2001). With mortar these pumps are regarded to be neither economically nor practical due to the higher wear and the higher amount of time of maintenance compared with a piston pump.

Figure 5 - Peristaltic pump with supply line at the bottom and extraction of the material at the top

Progressive cavity pump Progressive cavity pumps contain a horizontal spiral within a tube to transport the material due to the friction at the spiral blade. Between the inside of the tube and the outside of the spiral blade there is little space. By using a progressive cavity pump a continuous flow of material is achieved. Figure 6 shows a progressive cavity pump with funnel that can be used to fill in the grouting material.

Figure 6 - Progressive cavity pump

Progessive cavity pump were used for tunnelling projects only to transport two-component mortar. Just as for peristaltic pumps, the transport of mortar is not economical and not practical due to high wear and the extensive time needed for maintance.

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Pressurized air to backfill gravel The backfill of the annular gap through holes in the lining segments using pea gravel is normally done using compressed air. Powerful compressors are needed to guarantee the spreading of the material and to backfill the gap. DEVELOPMENTS CONCERNING THE GROUTING OF THE ANNULAR GAP High demands on projected tunnels leads to new concepts in order to grout the annular gap. Newly developed grouting materials are presented in the following. Mortar without cement Hydraulically setting mortar can involve primary interruptions in case of above-average usage and no supplies on mortar timely. Mortar with active systems can cause secondary interruption by hydrating within the grout supply lines in case of stoppages. Setting of the grout leads to clogging within the lines and can cause interruption of the tunnelling advance. Arrangements to rinse the grout supply lines are required to guarantee the next operation of grouting. To reduce the risk of interruption two-component grout and mortar without cement were developed. Mortar without cement uses quartz flour, limestone flour or fly ash instead of hydraulic setting cement to reduce the danger of hydrating and clogging. Mortars without cement components are also referred to as inert systems. A grout mixture with an inert system is shown in table 2. The systems have got high amount of time of workability. In soft ground inert systems can drain within the annular gap under pressure. The embedment of the segmental lining depends only on shear strength. Fresh produced and placed active systems have no compression strength in the first ten hours, so that the embedment also depends on the properties of shear strength of the mortar. Inert systems were used building the Dutch Leidingentunnel and the Westerscheldetunnel and for mortar in front of a weekend to guarantee the tunnelling advance after the weekend without interruptions. Deforming mortar Building tunnels with segmental lining in high-pressure rock deformations can damage the lining. In order to reduce the pressure on the lining special deforming mortars were developed. The mortars allow a deformation of the rock without a fundamental increase in the pressures on the segmental lining. The mortars are able to compress by up to 50 per cent (Schneider et al., 2005; Billig et. al, 2008). By using these mortars the forces on the linings are lower compared to the forces when using conventional mortar. Currently the material is still under development. OVERVIEW OF APPLICATION RANGE AND BACKFILLING SYSTEM FOR CERTAIN GROUTS In the previous paragraphs different types of grout were analysed, their application range was defined and possibilities to fill the annular gap were shown. As a decision guidance for planners and construction managers, the results have been summarized in a table 4.

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Table 4 - Overview of application range and backfilling system for certain grouts

Application range Backfilling system Required Equipment

Material

Hard

rock

Soil

Gro

utin

g th

roug

h gr

out h

oles

in

the

linin

g se

gmen

ts

Gro

utin

g th

roug

h th

e ta

ilski

n

Pist

on p

ump

Peris

taltic

pum

p

Prog

ress

ive c

avity

pum

p

Pres

suris

ed a

ir

Specifics / remarks

Mortar – active system x x x x x Conventional mortar, stiffness behaviour depends on using of additives

Mortar – reduced active system x x x x Stiffness behaviour depends on using of additives

Mortar – inert system x x x Stiffness behaviour depends on using of additives

Two-component grout x x x (x) x Stiffness behaviour just after mixing

Deforming mortar x x x x Only usable in hard rock (material under development)

Pea Gravel x x x

Often used in hard rock, increasing of bedding by using mortar at the bottom, normally lower modulus of deformation

and lower properties of embedment than for an active mortar

x = applicable (x) = limited applicability

Settlements Settlements at the ground surface can occur while using tunnel boring machines. An analysis on suitable grouting pressures has to be done in order to minimise or prevent settlements at the surface. The forces acting on the tunnel crown have impact on the grouting pressure. The reduction of settlements or setting up of lifting can be the result of a good performance of the grouting process. A qualitative description of settlements and lifting under the influence of grouting the annular gap are shown.

Figure 7 - Schematic development of settlements and percentage of total settlements

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By controlling the pressure of the grout material the pressure in the grout supply lines are commonly higher than in the gap. Further information on measured pressures in the annular gap are given in (Babendererede et. al, 2002). CONCLUSIONS The grouting of the annular gap is necessary in shield tunnelling in order to minimise settlements at the surface and to establish a bedding of the segmental lining. The described types of grout satisfy these requirements under different boundary conditions. Therefore, different backfilling systems and pumps can be used. New developments inspire to enlarge the application range of shield tunnelling and to optimise the procedure to grout the annular gap. REFERENCES Babendererde, S.; Babendererde, L.; Holzhäuser, J., (2002), „Verpressen der Schildschwanzfuge hinter einer Tunnelvortriebsmaschine mit Tübbingausbau“, Taschenbuch für den Tunnelbau 2002, Verlag Glückauf GmbH, Essen, pp. 228 – 254

Bäppler, K., (2008), „Entwicklung eines Zweikomponenten-Verpresssystems für Ringspaltverpressung beim Schildvortrieb“, Taschenbuch für den Tunnelbau 2008, Verlag Glückauf GmbH, Essen, pp. 263 – 304

Billig, B.; Gipperich, C.; Wulff, M.; Schaab, A., (2008), „Ausbausysteme für den maschinellen Tunnelbau in druckhaftem Gebirge“, Taschenbuch für den Tunnelbau 2008, Verlag Glückauf GmbH, Essen, pp. 223 – 262

Jonker, J.; Maidl, U., (2001) „Betuweroute: Erfahrungen mit dem Einsatz innovativer Schildvortriebstechnik“, Forschung + Praxis, Vol. 39, Bauverlag, pp. 52 – 57

Jonker, J., (2001) „Einsatzverfahren bei Schildvortrieben an der Betuwelinie in den Niederlanden“, Tunnelbau, Berichte des 6. Internationalen Tunnelbau-Symposiums München, Verlag Glückauf, Essen, pp. 109 – 120

Maidl, B.; Herrenknecht, M.; Anheuser, L., (1996) „Mechanised Shield Tunnelling“, Berlin, Ernst & Sohn Verlag

Schneider, E.; Rotter, K.; Saxer, A.; Röck, R., (2005) “Compex Support System”, Felsbau, Nr. 5, pp. 95 – 101