Design of large diameter microtunnelling shafts in soft ground Daniel, C Parsons Brinckerhoff Australia Abstract This paper examines parametric studies in the geometrical dimensioning, configuration and spatial efficiencies of large diameter shafts. Structural design aspects, construction solutions for excavation stability and parametric studies on ground improvement for deep shafts are also discussed. 1. Introduction This paper considers microtunnelling for tunnels of internal diameters (IDs) from 1000 to 3000 mm. Depths assumed range up to 35 m, and the entire proposed tunnel length can range from 100 m to several kilometres. This entire length may comprise a single drive, or several, possibly between permanent reinforced concrete (RC) access/maintenance points like manholes, valve chambers or monitoring stations. Figure 1 is a schematic example of a multi-drive layout plan. Launching shafts are denoted by L, and arrows indicate direction of launch - others are receiving shafts. A single project could have as many as 30 shafts.

Proposed microtunnelled sewer lineExisting

Sewer Line

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Proposed Manhole 1 L

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Figure 1: Generic microtunnelling works layout plan for a sewerage project (Daniel, 2006) Figure 2 shows a typical launch shaft. Prefabricated tunnel/pipe segments are lowered from ground level and positioned on the thrust stand in front of the thrust ring (left of jack). The jacks push the tunnel segment forward, following the microtunnelling machine's advance to the left. The thrust ring and jack retract, a new segment is lowered, and the process repeated. Tunnel spoil may be mucked out via the slurry discharge pump and pipe work, to slurry treatment systems at ground level. The treated slurry (minus the spoil) is recycled back into the tunnelling process, assisting face stabilisation and lubrication processes. Figure 2: Typical microtunnelling launching shaft set-up (Rasa, 2000)

2. Assumptions affecting shaft design in soft ground Soft ground often coincides with high groundwater tables - shafts must act as cofferdams facilitating dry conditions inside, and limiting adjacent ground/structure movement outside. Shaft walls require openings for launching and receiving the microtunnelling machine, and must resist high thrust forces. Buried services and structures are also often in the vicinity. Thus, the usage of external shaft anchorages like ground anchors or tie-rods may be risky, uneconomical, or both. Shafts often serve only temporary functions whether permanent access/maintenance points are required or not. Clients and authorities often require the eventual removal of shafts and associated works to maximise future land potential. Any permanent works are generally much smaller than the shafts required for microtunnelling. Top-down construction of permanent works doubling as shafts first, is unusual in soft ground (unless ground conditions are particularly poor) due to the higher construction costs and the large clear internal working space required during microtunnelling. Thus, shafts are often internally braced excavations using either structural steelwork or steel caissons/liner plates, and ground improvement where necessary. 3. Shaft geometrical dimensioning and configuration design 3.1 Launching shafts The minimum working area required during microtunnelling is governed by the longer dimension in the longitudinal shaft direction (direction of microtunnelling). Figure 3 shows indicative element footprints whose dimensions and relative positions require incorporation within the launching shaft's geometry and sizing plan. Microtunnel advance is from right to left.

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(a) Equipment footprints on plan (b) Indicative equipment footprint dimensions (m) Figure 3: Schematic launching shaft equipment footprint plan and indicative dimensions Although the author has used dimensions generally pertaining to 2400 mm ID tunnels, Figure 3 is conceptually indicative of all large diameter shafts. Furthermore, longitudinal shaft dimensions shown above are generally not linearly proportional to microtunnel diameters because many of the footprint dimensions could apply to shafts for a range of tunnel IDs (Akkerman, 2006). For example, the thrust jack, precast tunnel segment, thrust block, and working clearance longitudinal dimensions shown could broadly apply to shafts between 1650 and 2600 mm ID (Rasa, 1996/2000). However, increments in microtunnelling machine longitudinal dimensions do often broadly equal increments in tunnel ID (Rasa, 2000; Herrenknecht, 2003; AGD Iseki, 2005). Not all launching shafts need to accommodate microtunnelling machines (Figure 4). This type of ‘man/excavator-shield’ microtunnelling is used for short drives in soft ground, or where microtunnelling machines are not extricable at the drive’s end due to the presence of in-service structures (for example, before proposed manhole 1 and after proposed manhole 8 in Figure 1). It is also used

where product pipes cannot be jacked. This arrangement removes 1 m from the shaft’s long dimension in Figure 3, due to the machine’s absence. Based on the author's experience and others (Herrenknecht, 2003; Wirth, 2005; ASTT, 2003) concerning typical pipe lengths, Figure 3 indicates that trying to shorten shafts by reducing precast tunnel segment or jacked pipe lengths is ineffective as the machine length generally governs. However, Figure 4's arrangement would benefit from shorter pipe/tunnel segments particularly since these drives are typically short, reducing the negative programme impact of the shorter segments. Transverse dimension reduction is dependent on the slurry discharge pump, ventilation duct and access footprint dimensions. For safety reasons, deep shafts will require crane-operated man-cages or mechanised lifts, increasing space requirements. Figure 5 considers circular shaft arrangements with and without machines. As shown, circular shaft sizes are governed by the longitudinal length (direction of tunnelling), thrust block and entrance ring dimension requirements. Ancillary equipment sizes could increase greatly, without effect. Shaft diameter reductions of up to about 900 mm could be possible depending on thrust block thickness requirements. Figure 4: Shaft without microtunnelling machine (Kubota 1998)

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(a) Circular shaft with machine (b) Circular shaft without machine Figure 5: Circular shafts enclosing required equipment footprints on plan (m)

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(a) Rectangular shaft with machine (b) Rectangular shaft without machine Figure 6: Rectangular shafts enclosing required equipment footprints on plan (m) Figure 7 shows biaxially-symmetrical shaft configurations optimised in terms of excavation area and temporary works (proportional to perimeter) - using sheet piles, or prefabricated liner plates . Table 1 indicates that circular machine shafts would likely require more temporary works and significantly greater excavation volumes than rectangular ones because the length requirements significantly exceed width ones. In non-machine shafts, the disparity is less because the governing length dimension reduces without corresponding width reduction - as a rectangle approaches square

dimensions, its area approaches those of inscribed or circumscribed circles, thereby increasing the excavated space efficiency of circular shafts. This holds true for non-machine shafts because the governing dimension is the tunnel/pipe segment length. Thrust jack lengths do not decrease in tandem with ID reductions – hence, circular shafts would only increase their efficiency if precast tunnel/pipe segments were dramatically shortened. This is not generally done, for programming, precasting and commercial reasons.

(a) 'Optimised' shaft uniaxially symmetrical (b) 'Optimised' shaft biaxially symmetrical Figure 7: Regular optimised shaft areas enclosing footprints on plan (with machine) Table 1: Geometry summary table for shaft shapes when tunnelling near-linearly

Case Excavation Volume/m³/m depth

Perimeter/m Reference Figure

Circular shaft with machine 60.8 - 73.9 27.6 - 30.5 5(a) Circular shaft without machine 47.8 - 59.4 24.5 - 27.3 5(b) Rectangular shaft with machine 45.0 28.0 6(a) Rectangular shaft without machine

40.0 26.0 6(b)

'Optimised' shaft uniaxially-symmetrical

41.5 25.1 7(a)

'Optimised' shaft biaxially-symmetrical

43.3 26.0 7(b)

Therefore, it may be concluded that the circle is the least space-efficient launching shaft geometry for a linear or near-linear series of microtunnelling drives. These drives occur when off-takes, junctions or feeds to/from existing networks require that the entire route be straight, or when manholes are required for access/maintenance at fixed intervals along a straight drive. A linear series of drives also occurs within an intended straight route when shafts are required at locations dictated by maximum machine/pipe single drive capacities, and intermediate jacking shafts are not used. Curved drive microtunnelling is possible, but presents its own difficulties (Daniel, 2006). If the route is like Figure 1's, shafts will be required to launch tunnels bi-directionally. The governing criteria may then include avoiding cutting shaft corners for tunnel entrance ring installation, to avoid possible additional internal bracing, and losses of stability (Lee et al, 1998) and watertightness within the shaft. Watertightness is critical in soft ground and for slurry pipe-jacking, and is enabled by the tunnel entrance ring and seal. Cutting through corners complicates the usage of these items. Figure 8 (a) represents modelling output considering equipment footprint requirements varying according to different tunnel IDs. Circular shafts become advantageous if the inter-drive angle is less than about 155º – when excavation volumes of rectangular and circular shafts (Figure 5 (a)) equalise. Non-machine shafts were not considered because of the low likelihood of such bi-directional tunnelling without machines. 3.2 Receiving shafts Receiving shafts contain only three equipment footprints – working space (including cut pipe/ tunnel projections into the shaft for junction construction), microtunnelling machine and tunnel exit ring. The principal footprint concerning size and variability is the machine's. Figure 8 (b) shows that rectangular receiving shafts require both less excavation volumes and temporary works than circular shafts. However, the ratio of rectangular shaft areas to circular ones is

higher than in launching shafts depending on thrust block requirements. For example, Table 1 has the low-end ratio at 45/73.9 = 0.61. Figure 8 (b)'s corresponding ratios are (5.4x3.9)/(πx6.1²/4) = 0.72 and 0.70 – circular receiving shafts can be more space-efficient than circular launching shafts for linear routes if thrust block requirements are rigorous. Comparing Figure 8 (b) and Table 1, it should be noted that receiving shafts are always smaller than launching ones, often by a factor of 2 or more. Furthermore, as shown earlier, circular shapes are preferred for bi-directional receiving shafts as well.

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(a) Rectangular shaft for non-linear routes (b) Circular versus rectangular receiving shafts for linear routes Figure 8: Bi-directional rectangular launching shafts and linear route receiving shafts (m) 4. Structural steelwork shaft design in soft ground As per Section 2, structural steelwork is often a temporary shaft solution in soft ground. Figure 9 provides a plan and elevation of a generic rectangular shaft. Figure 10 shows a circular shaft with concrete walers resisting earth pressures via hoop stress. The sheet pile walls are vibrated, impact-driven or jacked in to the required toe penetration below formation level. Composite corner sheet pile units are fabricated by cutting and welding sheet piles to suit site conditions (Figure 9 (a)). Sheet piles may project above ground level to act as safety railing. Walls are internally braced with structural steel elements acting both as beams and columns. Stiffener plates ensure proper axial load transmission. For construction ease, knee braces 'increase' waler capacities by reducing effective lengths, thereby enabling element uniformity at all levels. Though heavy, microtunnelling machines are generally lowered fully assembled due to shaft space constraints – note how knee brace positioning is governed by the envelope (Figure 9 (a)). The top bracing element should be located typically within the top 2 m due to the likely tension crack zone in clays and to limit the sheet pile cantilever length, reducing ground movement. Subsequent bracing spacings generally decrease with depth due to greater earth pressures, and the need to limit adjacent ground movement by controlling sheet pile deflection between bracings. The lowest bracing level (GL – 11.5) will be the highest-loaded (Figure 9 (b)) because of the vertical clearance required for the microtunnelling machine, the tunnel 'eye' in the wall, and the thrust block. This lowest bracing and the base slab will together resist the earth pressures in this zone – note the temporary strut installed within the 4.2 m height prior to base slab casting, to enable final excavation. Manual earth pressure calculations and estimates of wall and ground movements can be performed with reference to Peck (1969), British Steel Piling Handbook (1997) and Mana et al (1981). From the resulting pressure diagrams, the pressure distributions applicable to each bracing level can be determined, considering each bracing’s vertical tributary load height. Uniformly distributed load (UDL) values are derived, specific to each bracing’s level. These UDLs are used to calculate both moments and axial compressions in the bracing elements. Similarly, seepage may be estimated reasonably accurately from manual flow nets. Vertical tunnel wall opening (Figure 9 (b)) stability was investigated by Broms et al (1967), with the conclusion that failure is likely when the total overburden pressure exceeds about 6 to 8 times the undrained shear strength of the soil. This would suggest that tunnel eye openings are thus only reasonably feasible to about 9 m and 15 m (depths to tunnel axes) in soft and firm clays respectively. Beyond these depths, additional measures would be required to enhance eye stability.

Jet grout piles (JGPs) or an additional sheet pile row are two such successful measures in the author's experience. The JGP solution (Figures 11 (a)/12 (a)) involves jet-grouting two rows of interlocking piles in front of the tunnel eye. The eye may then be cut in the wall without danger of soil collapse into the shaft prior to machine launch. Two rows ensure adequate pile interlocking sealing off of earth and groundwater. The additional short sheet pile row solution (Figures 11 (b)/12 (b)) has the row pulled up by a vibrohammer after machine launch through the shaft wall. This enables its passage without having to

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(a) Launching shaft plan (b) Shaft elevation (m) Figure 9: Schematics of rectangular shaft in plan and elevation

Figure 10: Rectangular (left) and circular (right) shafts bore through the steel (very damaging to most roller bits and cutter discs). While cheaper than JGPs, it does not prevent soil and groundwater ingress into the shaft – it only confines it to the soil/water enclosed between the additional sheet pile row and the shaft wall. The aim is to limit adjacent ground movement due to the tunnel eye opening.

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(a) JGP solution (b) Additional sheet pile row solution Figure 11: Solutions for enhancing wall vertical opening stability (plan view)

Machine bores through JGPs after sheet pile cutting

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(a) JGP solution (b) Additional sheet pile row solution Figure 12: Solutions for enhancing wall vertical opening stability (elevation)

Parametric comparison of Models 1-3

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Figure 13: Results of Plaxis modelling of Figure 9 (b) shaft with/without ground improvement

Finite element parametric studies were performed in Plaxis, modelling Figure 9 (b) with results shown in Figure 13. Model 1 was of the 16.2 m deep shaft in firm clay with groundwater at 3 m depth. Models 2 and 3 were the same but with ground improvement - a 3 m and 3.8 m thick JGP layer starting from GL-17 and GL-16.2 respectively. The JGP layer clearly has great benefits for ground settlement, base heave, slab axial load and wall bending moment. These plane strain model wall results are also applicable to circular shaft walls. Other modelling (not shown) suggests that these effects improve with thickness increases, but more so with proximity to formation level. The JGP layer also redistributed soil pressures between the third to fifth layer struts and base slab - the slab axial load dropped sharply, and the fourth and particularly fifth strut layer (GL-11.5) loads rose similarly. Figure 15 shows the JGP layer bending, significantly lowering sheet pile toe movement and base heave (Figure 14). The models suggest ground improvement may be critical to deep shafts in soft soils with high water tables.

Figure 14 : Wall toe movement and base heave failure mode development in 16.2 m deep shaft

Figure 15 : Beneficial effects of JGP layer below base slab of 16.2 m deep shaft 5. Conclusions Launching shaft sizes for machine and non-machine microtunnelling are governed by machine and tunnel segment lengths respectively. The most and least space-efficient launching shaft shapes for linear microtunnelling are Figure 7 (b) and circular shapes respectively. For bi-directional launching shafts, circular shapes are preferred if the inter-drive angle is less than about 155°. The most efficient shapes for receiving shafts are rectangular and circular for linear and bi-directional microtunnelling respectively. Key rectangular shaft structural details are shown in Figure 9. Jet grout piles (JGPs) and additional sheet piling can enhance shaft vertical wall opening stability. JGP layers can benefit deep shafts by reducing settlement, heave and wall bending moments. Their benefits increase with proximity to formation level and thickness increases. 6. References AGD Equipment Ltd 2005, Iseki Microtunnelling. Retrieved: August 24, 2006, from http://www.agd-equipment.com/html/microtunnelling.htm Akkerman Inc 2006, Products. Retrieved: August 29, 2006 from http://www.akkerman.com/index.html Australasian Society for Trenchless Technology 2003, Pipejacking and Microtunnelling. Retrieved: August 24, 2006, from http://www.astt.com.au/Pipejacking.pdf British Steel General Steels 1997, Piling Handbook Seventh Edition pp. 7/1-7/36.

Broms, B.B and Bennemark, H. 1967, 'Stability of clay at vertical openings', Journal of Soil Mechanics & Foundation Engineering, ASCE, SM1, pp 71-94. Daniel, C 2006, 'Construction Planning of Microtunnelling Works in Urban Areas', Australian Geomechanics, vol. 41, no. 1, March 2006 pp. 63-72. Herrenknecht AG 2003, Tunnelling Systems. Retrieved: August 24, 2006, from http://www.herrenknecht.de/en/dyn_frameset.php3 Kubota 1998, Manual of Kubota Ductile Iron Pipe for Jacking Method, Kubota Corporation, Osaka. Lee, F.H.,Yong, K.Y., Quan, K.C.N. and Chee, K.T. 1998, 'Effect of Corners in Strutted Excavations: Field Monitoring and Case Histories', Journal of Geotechnical and Geoenvironmental Engineering, Vol. 124, No. 4, April 1998, pp. 339-349. Mana, A.I. and Clough, G.W. 1981, 'Predictions of movements for braced cuts in clay', Journal of Geotechnical Engineering, American Society of Civil Engineering, Vol 107, No GT6 pp 759-777. Peck, R.B. 1969, 'Deep excavations and tunnelling in soft ground', State of the art report. Proc. of VIIth International Conference on Soil Mechanics and Foundation Engineering, Mexico, 1969 Rasa Industries, Ltd 2000, technical data booklet, Rasa Industries, Ltd, Tokyo. Rasa Industries, Ltd 1996, Unicorn Slurry Pipe Jacking Method, Rasa Industries, Ltd, Tokyo. Wirth Microtunnelling 2005, Machines - for any type of soil. Retrieved: August 24, 2006, from http://www.microtunneling.com/soltau/Slurry3000.htm