GB2169020A

GB2169020A - A method of reducing pressures on tunnelling shields and tunnel linings in weak rock

Info

Publication number: GB2169020A
Application number: GB08531256A
Authority: GB
Inventors: Kjell Holestol; Bjorn Buen; Arild Palmstrom; Anders Borg
Original assignee: PETROMINE AS
Current assignee: PETROMINE AS
Priority date: 1984-12-21
Filing date: 1985-12-19
Publication date: 1986-07-02
Also published as: DE3545084A1; FR2575219A1; GB8531256D0; JPS61179999A

Abstract

A method of reducing the pressures from weak rock on shields 11 used for tunnel driving, as well as reducing pressures on tunnel linings, by filling the void between the shield 11 and the surrounding rock 3 with an expandable, hardenable and deformable material, said void being filled by injection with the material. The deformation and strength properties of the material will allow controlled deformation of the rock towards the shield 11 and the tunnel support 2 such that the applied rock pressures are reduced. The invention reduces the danger of immobilisation of the shield because of being squeezed by the deforming rock and will allow tunnel supports to be designed for substantially reduced rock pressures. <IMAGE>

Description

SPECIFICATION A method of reducing pressures on tunnelling shields ahd tunnel linings in weak rock This invention relates to a method for reducing the risk of tunnel excavation shields becoming immobilised in weak rocks because of deformations of the rock (squeezing or swelling ground), and for reducing the pressure on the tunnel lining at the same time.

Excavation of a tunnel will cause changes in stress and deformations in the rock surrounding the tunnel. The deformations of the rock into the tunnel are usually a few millimetres in hard rock, but in weak rock the deformations can be several tens of centimetres. The deformations are dependent on time and if not treated in the correct manner, the weak rocks can give unacceptably large deformations of apply unacceptably high loads on tunnel supports. Modern tunnelling practice takes this into account in the design of support for tunnels and caverns and the principles on further systematised in the New Austrian Tunnelling Method (NATM).

From geotechnical and rock-mechanical principles it is generally known that loads on tunnel supports reduce if the suported rock mass is allowed to deform such that the shear strength of the rock mass is mobilised.

According to the present invention there is provided a method of reducing pressures on tunnelling excavation shields and linings in weak rock, in which the void formed during excavation between the excavated rock surface and a shield and between the rock surface and the tunnel support or lining is filled with an expandable, and after setting, deformable material which will upon application, cause reduced rock pressures against the shield and against the tunnel support or lining and as a result will reduce the risk of immobilisation of the shield due to excessive friction as well as reduce the strength required of the tunnel support or lining.

The distinctive feature of the invention is a method of driving tunnels in weak rocks where the void between the shield and the rock is filled by injection of specially adapted fluid materials which expand and harden rapidly and which are compressible. The expanded and hardened material will reduce the stresses in the rock closest to the tunnel wall and will transfer to the tunnel supports only this reduced stress. At the same time, the expanded material will contribute to sealing the tunnel against infiltration by water and gas.

The invention will now be described, by way of an example, with reference to the accompanying drawings, in which: Figure 1 is a graphical representation of the rock load transferred to rigid supports installed at different times after excavation; Figure 2 is a graphical representation of the relationship between the reaction load on a tunnel support and time; Figure 3 shows graphically the relationship between the load on a tunnel support and time and expresses criteria for the design of tunnel support; Figure 4 shows schematically a section through a conventional boring head as used with a tunnel boring machine (TBM); Figure 5 shows schematically a section through a tunnel boring machine in operation will attendant injection of material outside the shield in accordance with the present invention; and Figure 6 shows graphically the relationship between the load curve for the rock (from Fig.

1) and the resultant load for in the injected material installed in accordance with this invention with the resulting load curve for the tunnel support.

The invention is based on the interaction between the response of the rock to unloading and the response of the support to loading as shown on the attached Figures. Fig. 1 shows how the pressure from the rock reduces with time to a minimum value, before the rock ruptures with an ensuing increase in pressure. Fig. 2 shows an example that some rock deformation is allowable before the pressure increases rapidly in the tunnel support and how the load in the support builds up to a value Pmax before the support collapses.

Superposition of these curves gives the basis for optimum design of a support placed at the correct time and with sufficient strength, as shown in Fig. 3. The line marked "a" shows that the contact between the expanding rock and the support occurs early and that to prevent collapse the support should be designed against large loads. The line marked "b" shows that the contact is established too late, after disruption of the exposed rock surface.

The line marked "c" is the optimal line and it shows that if the contact between the rock and the support were established at the appropriate time, the lining could be designed for a minimal load or could have a larger safety margin.

In weak rocks, the stand up time (the time between tunnel excavation and collapse of the unsupported rock), can be very short: the deformations occur rapidly and the shield can become immobilised by friction generated by the rock pressure, see Fig. 4.

Referring to Fig. 4, the support of the tunnel behind the shield 1 of the tunnel boring machine is usually made of prefabricated segmens 2, which form a complete circular ring when erected. In current tunnelling practice cement mortar, pea-gravel or a similarly rigid material 4 is placed between the support 2 and the rock 3 to obtain the necessary combined behaviour and an even transfer of load from the rock 3 to the support 2.

A complete lining ring can also be case in situ. These supports are all so stiff that the pressure from the rock applied to the supports can be very high (curve "a" of Fig. 3).

The invention is to utilise a particular type of material placed between the rock and the support. This material, which will be specially adapted to individual tunnelling conditions, will set rapidly, will transmit to the support some of the load applied by the rock and will be deformable, see Fig. 5.

The material 6 will fill the void which occurs between the rock 3 and the shield 11 which protects the excavation machinery. The same material 6 will fill the void behind the shield 11, corresponding to the thickness of the shield outside the lining 2.

The material and material injection system will have the following properties: 1. Deformation properties and strength which can be adapted to various tunnelling conditions and which permit optimal design of the tunnel support by allowing deformation of the material as the rock moves and with a gradually increasing transfer of load to the relatively stiff tunnel support.

2. The injected material will be capable of expanding into and filling the voids around the shield and lining.

3. The injected material will not reduce the advance of the tunnelling shield by causing increased friction. This is achieved by appropriate design of the shield and apportenant machinery and systems.

4. The injected material will be stable and durable and will maintain its strength with age in order to prevent rock deformations from increasing with the corresponding load increases in the tunnel support.

5. The injected material will not be soluble in water upon completion of injection, nor will the material be susceptible to chemical attack such as may be caused by gas or salts present the rock.

The properties of the material to be injected cannot be specified in greater detail than indicated above without prior quantification, description or specification of the rock conditions and properties. Several types of suitable material are extant as for example polyurethane which can be formulated to yield various compressibilities.

The combined load response curve as a function of time for the injected material 6 and tunnel support is shown in Fig. 6. The notation used in Fig. 6 is as follows: Ps =stabilising load: the load required of the injected mass to stabilise the support.

P,pt = optimum rock load on the support.

k,p = ultimate load capacity of the support.

P =maximum rock load.

topt =time at which the rock load is at a minimum, immediately before collapse.

t =time at which injection commences.

t,0,Sj=time at which the support starts to take load.

The following conditions are required for implementation of the invention: -the rock 3 penetrated by the tunnel is under stress and this stress causes the rock to deform into the tunnel void-the tunnel sur face is strengthened against this deformation with a lining 2 erected or cast in a shield 11 or at the trailing edge of a shield 11.

-The material 6 which fills the void outside the shield 11 and tunnel support has proper ties adjusted to the properties of the rock which will be in contact with it. Relevant properties of the material are rates of gain of strength time time, strength properties, elas tic properties, creep properties and permea bility.

-The tunnel excavator (in the case of a tunnel boring machine, the cutter head) 5, the shi eld 11 and lining segments 2 are integrally designed to enable implementation of the above procedure. Inter alia, the diameter of the shield will have to be designed to leave sufficient space between the shield and the rock to give the desired thickness of injected material.

Claims

1. A method of reducing pressures on tunnelling excavation shields and linings in weak rock, in which the void formed during excavation between the excavated rock surface and a shield and between the rock surface and the tunnel support or lining is filled with an expandable, and after setting, deformable material which will upon application, cause reduced rock pressures against the shield and against the tunnel support or lining and as a result will reduce the risk of immobilisation of the shield due to excessive friction as well as reduce the strength required of the tunnel support or lining.

2. A method for tunnel excavation in weak rock substantially as hereinbefore described with reference to the accompanying drawings.