Detrimental side effects of tunnel construction like damage to buildings and services above can be avoided if the initial state of stress of the surrounding ground is maintained during excavation. The technique of mechanical tunnelling, particularly in loose ground, is now so refined that this ideal can virtually be achieved: it is now possible to support the face continuously with a fluid under pressure so that the primary stress state at the face is maintained.
The steering gap of about 25mm around the shield, which is required to allow it to be steered and is caused by a degree of overbreak at the cutterhead, is partially filled by the support fluid from the face (Fig 1). If the pressure in this steering gap is maintained at the same pressure as that of the face by additional grout lines through the shield skin, the primary stress state around the shield can also be maintained.
Problems with grouting the shield tail gap
The area to the rear of the TBM where the segments are erected continues to present a problem because the ring gap changes constantly as the shield advances. Tunnels driven in loose ground are lined with segments erected within the protection of the shield tail. A gap of 80-200mm, the shield tail gap, remains between the back of the segment and the surrounding ground. If this ground is not stable, the gap continuously formed by the shield advance must be filled at once. Otherwise, the unstable ground fills the gap and relaxes and loosens, resulting in settlement at the surface and a deterioration in the tunnel bed.
Surface structures can be damaged and the tunnel lining exposed to much higher stresses, leading possibly to higher maintenance costs. It is therefore necessary to fill the shield tail gap immediately and maintain a particular constant pressure by grouting with a support medium to balance the surrounding ground and groundwater to maintain the primary stress state (Fig 2).
The essential problem of achieving this is associated with the void to be grouted. This void is caused by the shield’s advance. It is not formed at a steady rate but, even so, must be filled with fluid at a defined constant pressure. If the volume of the pressurised support fluid does not exactly match the volume of the shield tail gap, the result is an immediate change in the pressure of the support fluid, whose purpose is to support the surrounding ground and groundwater. A drop in pressure would allow ground and water to collapse into the shield tail gap, causing settlement at the surface. If the pressure of the fill material rises above the predetermined value, the result is surface heave.
Maintaining the fill material at a constant pressure over the entire shield tail gap is difficult. The pressure in the fill material is measured in the grout pipe just before the outlet and is used to control the double piston pumps which supply the fill material. The pressure at the outlet is considerably reduced by friction in the grout line. With a pump pressure of 8 bar, the pressure at the outlet can be reduced to about 3 bar, depending on the diameter and length of the grout line and the shear strength of the fill material.
In addition, the requirement to maintain a constant pressure is further complicated by the fact that the fill material can only be placed via a limited series of lines around the perimeter of the shield. The pressure in the fill material falls with increasing distance from the injection point and is reduced by the shear strength of the fill material. The composition of this material and the curing process of the hydraulic bonding agents determine the shear strength. Transfer of filter water to the surrounding soil and friction between the soil and the fill material reduce the flow capacity of the fill material. All this means that the shield tail gap cannot be evenly filled at all predetermined pressures.
Controlling the volume of the fill material input is also a problem. The shield tail gap is grouted with double piston pumps. Under ideal conditions, to maintain a constant pressure in the material supplied, both pistons supply a single feed line. But, because they have varying fill rates, control systems based on the number of cycles cannot ensure an absolutely constant feed rate. So, controlling the number of pump strokes leads to inadequate control of the grouted volume. The shield tail gap must therefore be grouted by means of systems based primarily on pressure control. Control of the volume grouted is essentially only of interest for quality control.
The shield tail gap is generally closed in the direction of the shield by a rigid seal arrangement. The gap between the back of the segments and the inner surface of the shield tail is sealed by either a rubber seal attached to the shield tail skin or by a row of wire brushes. The brushes have the advantage of sealing a gap on the longitudinal joints even if the individual segments are offset. With a stiff rubber seal, a residual gap remains which allows the pressurised grout to escape.
With steel brush seals, the spaces between the rows of steel brushes are filled with grease to prevent the grouting material penetrating the brushes and reducing their elasticity. The grease is therefore maintained at a certain pressure. It is generally assumed that one row of brushes can seal a maximum of 1.5 bar pressure drop without excessive grease consumption. This implies that, for a pressure drop of 3 bar, three rows of steel brushes are required. Grease consumption is still considerable. On the French side of the Channel Tunnel, the consumption was about 25kg/m of lining.
As demonstrated above, the grouting material applied under pressure is influenced by many factors that prevent the shield tail gap from being completely grouted at a constant specified pressure, the pre-requisite for maintaining the primary stress state in the ground. Control of the grouting pressure and volume using current systems does not produce convincing results. All these imperfections can be avoided if the seal of the shield tail gap is designed to be moveable and elastically supported. It should have a range of movement of about 400mm along the axis of the tunnel and should be elastically supported by hydraulic jacks mounted on the shield tail.
The support jacks are interconnected and linked to a gas reservoir whose pressure can be varied. The moveable seal assembly is then thrust forward by the grout material. Only when the pressure in the grouting material reaches the pre-set pressure in the support jack gas reservoir can the seal assembly move forward. If the pressure in the grouting material falls below this value, the seal assembly remains stationary, independently of any shield advance. In this way, complete grouting of the shield tail gap is achieved at the predetermined pressure (Fig 3).
The grouting material must have excellent flow characteristics during the grouting process. After erection of about five rings, which can occur after six to eight hours at high advance rates, the grout must develop sufficient strength to prevent displacement of individual segments when they are loaded by the back-up gantries. Finally, a strength should be reached which corresponds with the surrounding ground in order to transfer loads to the segments acting together. It is therefore not necessary to develop shear strengths above that of the surrounding ground. Only small quantities of hydraulic bonding agents such as cement should be added to the grout to prevent the grouting lines clogging in the event of delays.
Grouting material criteria
The essential criteria for the grouting material is the shear strength after six to eight hours. It has to be high enough to prevent the segmental ring floating up in the ring gap and deformation of the ring by point loading from the back-up gantries displacing the fill material. The required shear strength is no higher than that of a sand gravel mix with a maximum grain size of 4-6mm. The internal friction of the sand gravel mix considerably reduces the flow capacity of the fill during grouting and thus endangers the complete grouting of the shield tail gap.
To avoid this detrimental effect on the flow capacity during grouting, a polymer foam should be added to the sand-gravel mixture to fluidise the sand and gravel by almost eliminating the internal friction for about four hours. This is an ideal filling material, meeting all the requirements for complete shield tail gap grouting at a predetermined pressure.
Related Files
Geometry of Shield Tail Gap
Grouting Diagram
Pressure Distribution Diagram
