METHOD FOR CONSTRUCTING AN UNDERGROUND TUNNEL OR HOLE TO CREATE AN IMPERVIOUS PLUG FOR THE STORAGE OF HAZARDOUS, PARTICULARLY RADIOACTIVE, WASTE

Abstract

A method for the construction of a gallery or a shaft leading to an underground facility intended for the medium- or long-term storage of hazardous wastes, including an excavation of at least one determined length of the gallery carried out by using a method of prior preparation by protection or reinforcement of the materials prior to the excavation, in a part of the gallery intended to subsequently receive a plug that is sealed to liquids or gasses.

Description

The present invention relates to a method for constructing an underground gallery, or a shaft, for a deep facility in a geological medium intended to receive hazardous and in particular radioactive wastes. This method enables a sealing plug to be constructed during closure of the storage.

The fate of hazardous wastes is a problem which has existed for a long time, in particular for the radioactive wastes originating from nuclear power stations or spent fuel reprocessing plants. In the case of nuclear wastes, in particular the high-level and long-lived wastes, long-term storage in “geological” strata, i.e. in deep strata of a material chosen for its very high mechanical and chemical stability is envisaged. Choice of such strata can vary according to the country and according to the sites, for example at a depth of the order of 300 to 500 m and in materials such as argillites, schists or granite.

These wastes present problems due to their danger to living organisms, combined with a very long lifetime, half-lifes from a few hundred years to a few hundred thousand years. Moreover, some can give rise to additional constraints, for example by producing heat during the early part of their storage, for a few hundred years.

Such storage installations have been envisaged for a long time, for example since 1978 for the Swedish projects, but have not always led to satisfactory technological solutions. This type of installation generally comprises a number of shafts and galleries leading to vertical or horizontal chambers. These chambers are to be backfilled with wastes in containers, surrounded by buffer materials such as bentonite.

Document U.S. Pat. No. 4,725,164 of Swedish origin describes thus an excavation method for constructing an underground installation of the “WP-cave” type in an extremely hard formation such as rock. This structure comprises a main central space of large dimensions receiving the wastes, surrounded by annular tunnels (16) backfilled with bentonite and a series of shafts forming a “hydraulic cage” around the main cavity in order to protect the latter from water seepages.

The methods envisaged for implementing sealing are based on backfilling the access tunnels with bentonite, and making the tunnels watertight by injecting a suspension of bentonite into the walls so that they resist hydraulic pressure in the case of flowing water conduits.

Other projects, in particular currently in France, envisage a distributed structure comprising multiple chambers excavated in extremely stable formations, the access tunnels of which are then closed in a sealed manner by localized plugs.

As shown in FIG. 1, when a chamber 101 is backfilled with wastes it is sealed by a plug 102, for example made of cast concrete in its access gallery. Similarly for the service galleries 103: when the task of backfilling several chambers 101 in a sector is complete, the corresponding service gallery can be sealed by a plug 104 according to the requirements, if any, of reversibility of the storage. The access gallery 105 to the service galleries 103 is plugged in a similar manner 104 as well as the last shaft 107 by a plug 106. All this can extend over many years; the last shaft may only be closed one hundred years after the initial excavation.

The performances with regard to stability and sealing for this type of storage are an important point, in order to avoid water seepages penetrating into the storage chambers, with the risk of causing deterioration of the containers, and flowing out again with the risk of contaminating the surrounding subsoil and the water tables in their underground course.

At a great depth, a fissured area called an “Excavation Damage Zone” (EDZ) or area damaged by the excavation forms all around an underground cavity. These fissured areas do not always occur immediately, but develop over a period of time, according to the nature of the medium and the depth of the storage. Audible cracking may indicate that they are extending. By way of example, with the Mont-Blanc Tunnel, in granite and under heavy overlying strata, this period continued for more than a week.

This damaged area is much more permeable that the surrounding massif and the fissures are practically impossible to close when their faces have slipped in relation to one another. Thus water flows can occur around the different storage works, and possibly carry radioactive products outside the storage. It is therefore necessary to prevent these fissures occurring.

It has been proposed to seal the fissures that may be present in the rock by injecting slurry of cement or sodium silicate. It may be of concern that not all the fissures will be sufficiently treated in order to obtain sealing that is satisfactory or close to the characteristics of the material of origin and/or that the lifetime of the slurry will not be sufficient.

Another solution proposed by document EP 1 760 256 is to use an automated machine equipped with a tool-holder arm in order to re-cut the walls over a certain thickness, in order to remove the damaged portion and thus produce a specific profile before a concrete plug is poured. It is however possible that this new excavation itself causes a damaged area due to the tools used, or even simply producing a fresh relief of the internal stresses which always exist behind newly uncovered surfaces.

A purpose of the invention is to allow effective sealing against gasses or liquids under good conditions of reliability and stability over time, around the galleries or the shaft once the plugs are sealed.

As shown in FIG. 2, during the construction of a tunnel 200, the position of its surface 201 is the result of a movement, from its initial position 202 in the massif, towards the void (in broken lines in FIG. 2)

In order to limit this displacement in a tunnel having a circular cross section, supports 210 are placed therein, reducing the stresses in the massif itself.

If the stresses in the massif are equal to the weight of earth γ·z, the vertical tangential stress on the wall in the massif, according to the elasticity theory, is σ_z =2·γ·z, but the support bears a large part thereof. The main vertical and horizontal tangential stresses at the apex of the face 209 (shown diagrammatically by a half-circle) are:

σ_Z=σ_H=1.5·γ·z

but as there is no support at the face the material is more stressed there than along the tunnel: the displacements and fissuring can be much greater there than in the linear part of the tunnel

Large displacements result locally in large deformations; beyond a certain level of deformations, breaks, fissures or sliding surfaces appear, resulting in voids that are often impossible to close, even under high pressures. Observations conducted in the ANDRA underground laboratory in the Meuse-Haute Marne region of France have shown that these deformations are sufficiently significant to cause fissures and sliding surfaces to appear close to the face, and that corresponding lines of sliding are found along the rectilinear portion 208 of a service gallery ^{[ref. 1]}: the main part of the EDZ is created around the face.

Similar observations have been made during the construction of the Belgian underground laboratory at Mol in clays which were sufficiently stable to enable the walls to be supported simply by sliding arches. However, disturbances at the face were very significant when it was not protected by a shell of sprayed concrete ^{[ref. 2]}

After construction according to the prior art, whether by explosives or by excavator, such a gallery 200 is thus located in a medium which has previously been severely disturbed by excavation of the working face 209, as well as by the relief of the internal stresses due to the pressure experienced by the rock massif.

The invention proposes a method for constructing a gallery or shaft, in particular according to the claims hereinafter.

As the positions of the plugs were chosen before the start of construction, they should be protected and provided as soon as they have been constructed with supporting works and linings installed during excavation or immediately afterwards. In this way leaving the chosen areas uncovered for many years is avoided, during which time these areas would risk degradation due to long-term creep, ageing and deterioration of the materials (in particular for shafts with thermal or hydrometric variations, or at water table levels, etc.).

In order to obtain sealing around the future plug, the invention proposes to provide this plug in a gallery which has not been disturbed by a working face. To this end, however contradictory it may seem, it is proposed to construct a portion of the gallery (or of the shaft) in an area distant from the working face, i.e. ahead of the working face.

A principle of this method is to distance the working face, which is the main source of damage and fissuration of the receiving massif from the area where a plug must be provided, or at least to “construct” the gallery, and possibly its support, even before the gallery is excavated.

More particularly, the invention proposes a method for constructing a gallery leading to an underground installation intended for the medium- or long-term storage of hazardous wastes. According to the invention, this method comprises an excavation of at least one determined length of this gallery by using a method of preparation prior to the excavation. Before excavation, a protection for the massif is constructed that will pass through the gallery or a reinforcement of the massif in the part intended to subsequently receive a liquid- or gas-tight plug.

Usually, any prior preparation method is considered unnecessary in very stable materials such as those excavated during the boring of such storage installations. Moreover, no reinforcement can be obtained with regard to this type of materials, or only to a very limited extent. Furthermore, the stable and often resistant nature of such materials, for example granite or the argillite, means that any additional processing of the material before excavation would be difficult and costly, both in time and in labour and equipment.

In the state of the art, such a preparation prior to excavation is only used in unstable formations in order to guard against the risks of collapse of the working face and of the area not yet supported.

In the materials involved here, such a prior preparation method would be unnecessary if the construction of only a single gallery was involved.

Thus, the invention proposes to use such a method of preparation (protection and/or reinforcement), not to facilitate the construction of the gallery but in order to obtain an improved subsequent sealing of this gallery, in spite of the additional effort that this may represent and despite the assumption that it is unnecessary.

In particular, the invention proposes to minimize as far as possible the dimensions of the holes or bores used in order to carry out this prior preparation, making it possible to minimize the risks of damage caused by these prior bores, or even to avoid them completely. Thus, several small-dimension bores may be used rather than a single bore of larger dimensions, for example in order to excavate the volume necessary to a prior protection ahead of the working face or to allow ties to be installed. For these reasons among others, the protective structures installed prior to the main excavation will preferably have compact dimensions. Preferably, they are solid and the subsequent creation of voids or compressible spaces within them will be avoided, in order to limit the risks of damage that could be created by future collapses. It should be noted that this choice tends to run contrary to the methods often used in order to obtain a greater mechanical strength by limiting the addition of material, in which it is usual to use or produce hollow structures having large external dimensions.

It should be noted that all or part of these prior preparation methods are known in the field of the excavation of mines or tunnelling. These known methods can be used in order to implement the method of the invention, as they are or modified or combined together in different ways.

The following description gives in greater detail the construction methods with prior preparation, by prior protection or by prior reinforcement, that can be used within the scope of the invention. Some of the characteristics of the prior preparation methods described here can be common to the methods used in civil or mining engineering for passing through unstable areas. However, these similarities do not mean that the methods described here are systematically the same as those used in unstable terrains, nor that all the characteristics of the known methods are necessarily applied by the invention. The characteristics described here for the invention can differ therefrom. The invention may also use only a part of these known methods, and/or possibly combine them.

Other advantages and characteristics of the invention will become apparent on examination of the detailed description of an embodiment which is in no way limitative, and the attached diagrams, in which:

FIG. 1 is a diagrammatic perspective view of a storage level having several chambers within an underground facility for the storage of radioactive wastes;

FIG. 2 is a diagrammatic view of a horizontal gallery in the process of excavation according to the prior art and of the surrounding damaged area, respectively in transverse section, horizontal section, and in longitudinal vertical section in the region of the working face;

FIG. 3 is a diagrammatic view in vertical half-section of a portion of gallery constructed according to the first embodiment with prior installation of support elements;

FIG. 4 is a diagrammatic view of a tapered shell 324 constructed around a part of the gallery of FIG. 3;

FIG. 5 is a diagrammatic view in horizontal section of the gallery of FIG. 3;

FIG. 6 is a diagrammatic view in horizontal section of a gallery in the process of excavation according to the second embodiment of the invention using expendable rock bolts installed alternately in three successive groups;

FIG. 7 is a diagrammatic view in horizontal section of a gallery in the process of excavation according to an embodiment combining the prior installation of support elements with a prior reinforcing of the working face by means of expendable rock bolts;

FIG. 8 is a diagrammatic view in longitudinal section of a gallery comprising a sealing plug during seal testing.

FIRST EMBODIMENT

The term gallery is used here to indicate indistinctly a horizontal gallery, or also a vertical shaft, or a ramp, or also tunnels according to all intermediate inclines.

FIG. 3 shows a first embodiment of the invention using an installation of support elements 321 to 324 before excavation of the gallery and as it progresses 219 (cf. FIG. 2).

Three different parts 301, 302 and 303 are represented here, on the path of a gallery 300 in the process of excavation. The part 301 has already been excavated in standard fashion. The intermediate part 302 is in the process of excavation and is intended to subsequently receive a sealing plug. The part 303 is not yet excavated, and this will be done in standard fashion after completion of the intermediate part 302.

The part 302 is constructed under the protection of the tapered shells 321 to 324 that have a horizontal axis and are nested together. The shells 321 to 323 that are already constructed, as well as the shell 324 in the process of construction, are made by means of a series of bores 32410 drilled in the periphery of the working face 309. These bores are contiguous and inclined in order to distance them from the axis of the gallery to be excavated. They are for example excavated using an end-working chain mechanism comparable to a chainsaw. These bores 32410 are then backfilled 32419 with concrete in order to constitute a segment 3241 forming part of a frustum of a cone 324 (shown separately in FIG. 4) surrounding an area 3240 of the future gallery. A start is made by excavating then backfilling the bores of odd rows (example: 32410) in order to produce the corresponding segments (example: 3241), then those of even rows (example: 3242).

In constructing the following shell nested in the previous one, here the shell 324 nested in the shell 323, before having completely excavated the content 3230 of this previous shell 323, it is understood that the material 32409 outside the gallery to be excavated is protected without it being able to deform and thus deteriorate due to the excavation.

FIG. 5 thus shows the result of the operation in longitudinal section: the excavated parts 301 and 303 normally each exhibit a damaged area 391 and 393, unlike the part 302 intended to receive the plug.

The excavation of the recesses 3241 that must receive the concrete for constituting the shell portion 324 only creates negligible damage, perhaps one hundred times smaller that of the zones 391 and 393 caused by the standard excavation of the gallery.

In fact, each of the bores 32410 made has a very small dimension which therefore only results in a very small damaged area, or none at all. These bores for example have a maximum diameter of 10 cm to 15 cm, or even 15 cm to 20 cm in the case of chain boring machines.

The tapered shells 322, 323 and 324 have thus been entirely produced in areas that are not disturbed by a working face, and do not have the drawbacks of the damaged zone (EDZ) known in the current state of the art.

SECOND EMBODIMENT

FIG. 6 shows a second embodiment of the invention using expendable rock bolts installed in the material in advance of the working face and/or on its periphery, and as the work progresses.

Two different parts 601 and 602 are represented here on the path of a gallery 600 in the process of excavation. The first part 601 has already been excavated in standard fashion. The second part 602 is in the process of excavation and is intended to subsequently receive a sealing plug.

The method of preparation by prior reinforcement used here comprises the installation of expendable ties 621 to 623 sealed over their whole length in order to reinforce the working face 609. These ties are embedded in narrow bores drilled in the working face 609 and/or its periphery. These ties are for example made of glass fibres having a length of the order of three times the diameter of the gallery, for example comprised between 2.5 and 4 times this diameter. They are sealed in the material of the working face; they are distributed in a substantially uniform fashion over the surface of the working face and/or its periphery. They are installed preferably parallel to the axis of progression 619 of the gallery, or diverging slightly around this axis, for example so as to occupy in a substantially uniform fashion the volume of material situated behind the working face and which will subsequently be excavated. This reinforced volume can correspond to the substantially cylindrical volume of the future tunnel, but can also be provided in order to occupy a slightly conical volume becoming larger in the direction of progression 609, according to an angle comprised for example between 0° and 10°.

These ties reinforce the area of the working face. They have different lengths 621, 622 and 623 and are installed by successive thirds. They are destroyed during the advance of the excavation over the whole length of the area where it is sought to avoid deformation of the surrounding material and therefore its fissuration. New long ties are installed by one third of the total number of ties each time the excavation advances by on third of the length of these ties.

The working face 609 is therefore retained by anchorages sealed in the materials of an area situated a very long distance in front of it in the massif. This reinforcement makes it possible to avoid or limit the damage in the area 602 provided for the plug.

THIRD EMBODIMENT

A third embodiment, which does not require particular representation here, comprises the use of a method of freezing the massif beyond the working face which makes it possible to increase the strength of the medium when the interstitial water is transformed into ice over approximately ten metres in length. The material is then excavated in its frozen state, which prevents the formation of a damaged area around the excavated gallery.

This freezing can be carried out with the means and according to the methods used in some extreme cases for a tunnel to pass through an unstable area. It can be carried out on all or part of the volume of the tunnel in the location of the future plug. In the event of freezing being used, the method according to the invention preferably comprises a prior thermal, hydraulic and mechanical study of the material to be excavated.

Combining Several Embodiments

Several embodiments of the invention, and in particular those described here, can be combined without exceeding the scope of the invention. For example, FIG. 7 shows an embodiment combining prior installations of support elements and means of anchoring the working face, for example as described above.

As shown in this embodiment, the anchoring means 621 to 623 can be arranged in order to project outside the gallery 600, to a depth EA, for example to further reduce the damage caused by the bores producing the tapered shells 321 to 323.

Complementary Parameters

The modelling envisaged in relation to FIG. 2 assumes that the stresses in a massif are equal to the weight of land γ·z, according to the Heim hypothesis. The thickness of the damaged zone (EDZ) is then constant all around a gallery that has a circular cross section.

This hypothesis is not confirmed in all terrains. For example in Canada, in the granite region of the “Canadian Shield” the horizontal stress close to the surface of the granite, and even at depth, is greater than the weight of land γ·z.

Sometimes the main horizontal stresses are equal to half the weight of land, but most often they are both different. In such cases, the disturbances around a gallery can be localized, for example on the walls, such as in the Mont-Blanc Tunnel, or elsewhere close to bend radii that are too sharp, such as in the corners of the drainage channels of the road surface of a tunnel.

In the argillite of the Callovo-Oxfordien of the Meuse-Haute Marne site, the main stresses are close to the weight of the land, but one of the main horizontal stresses is slightly greater than the other. Such cases of anisotropy of the stresses can justify a specific study in order to assess and locate the extent of the damaged area caused around a gallery by an excavation according to the state of the art, and/or to assess the effect on the performance indicators of the method according to the invention.

Similarly, it can be useful also to take account of the anisotropy of the mechanical properties (resistance and elasticity) of the excavated massifs, in directions depending on the stratification.

Studies can be carried out in advance, by in situ experiments or by calculation. Based on such studies, it is possible to determine more precisely the validity of the use of the method according to the invention in specific cases. Such studies can also allow corrections or adjustments to be provided in the areas to be treated with respect to the basic principles described here. Thus, the invention can then comprise the specific use of a prior reinforcement method in certain areas of the gallery or the working face when there is a higher risk of the creation of an EDZ.

Verification of Sealing

The longitudinal sealing of the walls of a gallery, and in particular the result and the performance of the excavation methods described here are difficult to verify reliably other that by testing, preferably full-size.

The invention also proposes a method for verifying the sealing of a plug, for example installed in a gallery excavated according to the invention.

FIG. 8 thus shows how to test a gallery constructed according to the invention on a part 802 and comprising a plug 805 and the device for testing the seal around the gallery.

The tested gallery comprises the treated part 802, which was constructed according to the invention. It is situated between a part 803 called upstream on one side, for example on the storage side, and a part 801 called downstream on the other side. A central plug 805 is constructed inside the treated part 802, according to known methods.

In the treated part 802, on the upstream side of the central plug 805, an annular chamber 831 called an upstream chamber is created by installing a plug 832 called an upstream plug. In this same treated part 802, on the downstream side of the central plug 805, an annular chamber 811 called a downstream chamber is created by installing a plug 812 called a downstream plug. The upstream 832 and downstream 812 plugs can also form an integral part of the central plug 805, which then has a shape leaving free an annular space forming the upstream 831 and downstream 811 chambers.

A pressure p1 is applied in the upstream chamber 831 and the development of the pressure p2 prevailing in the downstream chamber 811 in order to assess the leakage flow rate and the tightness of the gallery around the central plug 805.

Of course, the invention is not limited to the examples which have just been described and numerous adjustments can be made to these examples without exceeding the scope of the invention.

REFERENCES

• ^[1] ANDRA (2005)—Dossier 2005. Référentiel du site de Meuse/Haute-Marne. Tome 2. Ch. 32. pp. 359 and 364.
• ^[2] Wileveau Y., Bernier F.—Similarities in the hydro-mechanical response of Callovo-Oxfordian clay and Boom clay during gallery excavation. Clays in Natural & Engineered Barriers for Radioactive Waste Confinement. 3^rd International Meeting. Lille, Sep. 17 to 20, 2007. page 149.

Claims

1. A method for the construction of a gallery or a shaft leading to an underground facility intended for the medium- or long-term storage of hazardous wastes, comprising: an excavation of at least one determined length of said gallery carried out by using a method of prior preparation by protection or reinforcement of the materials prior to the excavation, in a part of said gallery intended to subsequently receive a plug that is sealed to liquids or gasses.

2. The method of claim 1, wherein the prior preparation method comprises a prior protection by installing in the material to be excavated, in front of the working face of the gallery, elements positioned so as to support said gallery during the subsequent progression of said working face.

3. The method of claim 2, wherein the support elements comprise elements in the form of a frustum of a cone, positioned so as to form a shell completely or partially surrounding the periphery of the gallery in the process of excavation.

4. The method of claim 1, wherein the preparation method comprises a prior reinforcement by installation in the material to be excavated, in front of the working face of the gallery in the process of excavation, expendable means of anchoring said working face over a determined depth, beyond the surface of said working face.

5. The method of claim 4, wherein the anchoring means comprise rock bolts which are expendable during the excavation, positioned in a manner substantially perpendicular to the working face or diverging slightly in the direction of the excavation.

6. The method of claim 1, wherein that the prior preparation method comprises a prior reinforcement by freezing the materials located in front of the working face of progression of the gallery in the process of excavation.

7. A method for the construction of a gallery leading to an underground facility intended for the medium- or long-term storage of hazardous wastes, comprising: a combination of a prior protection by installing in the material to be excavated, in front of the working face of the gallery, elements positioned so as to support said gallery during the subsequent progression of said working face and a prior reinforcement according to claim 4 for the excavation of a single part of said gallery intended to subsequently receive a plug that is sealed to liquids or gasses.

8. The method of claim 7 wherein the prior preparation method comprises an installation in the material to be excavated, in front of the working face of progression of the gallery in the process of excavation:

on the one hand, of elements positioned so as to support said gallery during the subsequent progression of said working face, and

on the other hand, of means of anchoring said material over a determined depth beyond the surface of said working face.

9. An underground facility intended for the medium- or long-term storage of hazardous wastes, comprising: at least one part of the gallery receiving, or intended to receive, a sealing plug and constructed by a method according to claim 1.

10. The method for testing the sealing performance indicators of at least one part of a gallery, called a treated gallery, obtained by an excavation method according to claim 1, said testing method comprising the following steps:

installing in said treated part at least one sealing plug surrounded on each of its two sides by at least one upstream and downstream test chambers delimited by at least one upstream sealing plug and a downstream sealing plug;

creation of a pressure difference between said test chambers; and

measuring the pressure difference or the change in pressure difference between said test chambers.