EP0329500A1 - Process and apparatus for consolidating the terrain - Google Patents

Abstract

The invention relates to the consolidation of terrain, on the surface of which a partial air vacuum is established in a layer (14) of permeable material surmounted by a sealing membrane (15), and in which a network of drains (13) is installed. According to the invention, each drain is equipped with a hollow pipe (19) connected to an external water-discharge means (18), such as a suction pump, which is controlled so as to discharge onto the surface from the drains the water which has accumulated there. The invention is used particularly for the consolidation of loose soils of low permeability saturated with water. <IMAGE>

Description

The invention relates to the consolidation of soft soils with low permeability, impregnated with water.

These types of soil are generally, in the state, unfit to receive constructions, unless these are made on piles beaten or driven to a hard underlying rock or that the grounds have been consolidated.

The technique of driven piles quickly becomes prohibitively expensive if the thickness of the layer of soft ground is important. In this case, we rather use a land consolidation technique, which technique essentially involves the elimination of a large amount of water contained in these layers. Since certain layers, such as clay layers, are very little permeable, the elimination of water is sometimes very difficult, and in practice very long.

Among the various known consolidation techniques for such terrains, the following will be mentioned in particular.

1. The backfill technique .

According to this technique, and as illustrated in Figure 1 attached, is deposited on the layer of soil to be treated, for example a layer 1 of soft clay about twenty meters thick resting on a layer 2 of subsoil overlying, a layer 3 of embankment, for example pebbles or earth, thus loading the site to be built.

The backfill brings to the ground a pressure constraint generally higher than that of the planned construction. The presence of the embankment causes a compaction of layer 1, forcing the water contained in this layer to escape according to a general path indicated by arrows 4.

At the end of a time usually very long, generally exceeding several years, one obtains a certain consolidation of the ground. The backfill can then be removed and the land built.

Such a technique is expensive due to the operations of loading and unloading the land and very long to implement, commonly requiring ten years to obtain satisfactory results.

2. The backfill and drainage technique .

It is possible to improve somewhat the technique of loading by an embankment, as described above, by installing in the weakly permeable layer 1, a network of drains 5 shown vertical in FIG. 2. These drains can be constituted for example by drilling small diameters filled with sand, in a fairly tight mesh, for example from 2 to 8 meters.

After which, the layer 1 is loaded by the embankment 3, as before.

The water to be expelled, instead of having to rise to the surface by crossing the entire thickness of the layer 1 which is not very permeable, will "flow" to the nearest drain 5. For a tight mesh, the water path, as indicated by the arrows 6, will therefore be relatively short. Consequently, the speed of consolidation of the ground will be very markedly increased.

In practice, it will in particular be possible to use drains formed from porous plastic tubes with an internal diameter of the order of 50 mm.

3. The drainage and loading technique by applying a partial vacuum.

In practice, it appeared that the use of backfill loading techniques posed a certain number of problems, and in particular
- backfill stability problems,
- recovery of backfill materials, often expensive, sometimes difficult to obtain, and
- placement and removal of embankments, taking into account the relatively large volumes of materials generally required.

Also, some have proposed, as illustrated in Figure 3, another technique consisting in establishing a partial air vacuum, for example by means of a vacuum pump 7, in a layer 8 of permeable material surmounted by 'An airtight waterproofing membrane 9 covering the surface of the ground area 1 to be consolidated.

As a permeable material, sand is usually provided, which is deposited on the ground in a layer of about 20 to 60 cm thick, and in this mass of sand, a vacuum of around 60 is established. at 80 kPa. As can be understood, the load applied on the ground depends on the surface thus covered.

Generally, to ensure the sealing of the membrane 9, recourse is had to a channel 11 which is hollowed out at the periphery of the zone to be treated and then filled with a fluid material, such as a bentonite mud, in which one comes to drown the outer edges 9a of the membrane.

As regards the drains, which have been again identified 5, these will be of the same type as those previously described. They always open at the top at the level of layer 8 of permeable material.

However, in this case, if the layer 1 to be consolidated is located over a porous layer 10, with good permeability, that is to say "drainable", care will be taken, as illustrated in FIG. 3, to avoid that the drains 5 do not reach this draining layer 10.

It will be noted that with this technique, the water located in layer 1 near the surface, will, due to the depression created in layer 8, tend to rise towards it by capillarity.

Despite everything, the major problem which remains posed is that of the evacuation of the water which accumulates in the drains and no interesting solution has so far been proposed making it possible to pump this water to the surface. without compromising the efficiency of the vacuum maintenance system of the permeable layer into which the drains open.

The subject of the invention is precisely a new method of consolidating soft, low-permeability soil impregnated with water, of the type in which a partial air vacuum is established in a layer of permeable material surmounted by a membrane. airtight waterproofing and covering the area of land to be consolidated on the surface, a mesh network of hollow drains with permeable walls sinking into the ground is created in the ground and stopping before entering a sub- layer of permeable soil and said drains are unblocked at the level of the layer of permeable material in question.

The method of the invention is characterized in that each drain is fitted with a hollow tube connected to an external means for discharging water, such as a pump, said tubes descending into the drains for the most part. the height of the latter with which they communicate in the lower part, and said external evacuation means are controlled, so as to evacuate on the surface, outside the drains, the water which will have accumulated there.

In order to avoid any risk of leakage at the waterproofing membrane, the suction tubes will pass under this membrane or pass through it sealingly.

In fact, the external means of evacuation will preferably be controlled intermittently, when water may have accumulated in the drains. And in the case where a suction pump, of the "vacuum pump" type, is used as the means for discharging the water, the maintenance of the permeable material under partial air will be interrupted during the time interval during which said suction pump will be controlled.

It is then expected that in practice the period during which the vacuum maintenance will be interrupted partial of the layer of permeable material and where the suction pump will be controlled will be about half an hour, with a periodicity of once or twice every twenty-four hours.

In this way, we will be able to significantly improve the efficiency of the last mentioned technique by increasing the yield of the drains, while retaining almost in their entirety the advantages ensured by maintaining the surface layer which covers the area under vacuum. to be treated, the time required for consolidation will be reduced.

To date, to the knowledge of the applicant, the backfilling and "partial vacuum" techniques with drainage have never been combined. However, in the context of the invention, the combination of these two techniques could further increase the speed of consolidation of the land. As will be understood, it would then be necessary in practice for the operations of loading and unloading of fill materials to be able to be carried out at reasonable costs.

It is in this spirit of combining the techniques mentioned, that a characteristic of the invention provides that in addition to the pressure constraint exerted on the area of land to be consolidated by the partial vacuum of the layer of permeable material. , one comes to exert on the zone in question an additional pressure by placing on it, on the surface, a distributed load, such as a load of water or fill materials.

In addition to various process characteristics, the invention also relates to a device for implementing this process, characterized in that it comprises, to equip each drain, a hollow tube which opens at its lower end onto a cage of water inlet open on the drain and in which a float is held captive, a valve capable of closing the lower end of said tube when the water contained in the drain reaches a level lower than that of the float, all the tubes being connected to the same suction pump arranged on the surface for the evacuation of water.

In this way, once the drains have been emptied, the air, or more precisely the rest of the air, contained in the drains will be avoided. In practice, sensors of all known types will also be arranged towards the bottom of at least some of the drains, at the lower end of the tubes, and connected to the pump control means, in order to stop the latter. once the drains are emptied of their water.

• - la figure 4 montre schématiquement le procédé de l'invention,
• - la figure 5 montre en vue agrandie le détail repéré 5 à la figure 4,
• - et les figures 6, 7 et 8 illustrent successivement et schématiquement une vue d'ensemble de trois variantes de réalisation de l'invention.

The invention and its implementation will appear more clearly from the description which follows, made with reference to the appended drawings in which, in addition to FIGS. 1, 2 and 3 previously described, relating to the prior art:

• FIG. 4 schematically shows the method of the invention,
• FIG. 5 shows in enlarged view the detail marked 5 in FIG. 4,
• - And Figures 6, 7 and 8 successively and schematically illustrate an overview of three alternative embodiments of the invention.

If we refer first to Figure 4, we see in 1 the layer or area of land to consolidate, for example a thick layer of twenty meters of soft clay. In the example illustrated, this layer rests on a sublayer 12 of any type, for example a draining layer.

In this field, we went ahead and set up a mesh network of drains 13 constituted for example by porous plastic sheaths, strainer tubes, that is to say perforated, or others. These drains 13 form wells having a diameter of the order of 50 mm and stop at a certain distance above the draining layer 12 which they must not reach. The network mesh can respect a distance between drains for example from 4 to 6 meters. In practical, the denser the mesh, the faster the consolidation can be. On the surface, the drains 13 open into a surface layer 14 of relatively thin permeable material, such as a mass of sand about 30 to 50 cm thick, which has been extended substantially over the entire surface of the zone of land to consolidate.

The layer 14 is covered with a tarpaulin or membrane 15 of synthetic material which is substantially airtight, at least in air, the outer peripheral edges 15a of which are immersed in a channel 16 filled with a material sufficiently viscous to hold the edges in position of the membrane and ensure its tightness.

In layer 14, moreover, there is a conduit 17 connected to a means of evacuating water such as a "vacuum" suction pump 18 of the liquid ring type, so that it is possible to establish under the membrane. , in layer 14, a partial air vacuum of the order of 60 to 80 kPa. Of course, the conduit 17 will come out of the membrane without adversely affecting the seal.

According to the invention, inside each drain 13, a tube 19 of smaller diameter will be lowered, such as a plastic tube of the polyamide type of approximately 4 to 8 mm in diameter. In the lower part, these tubes are in communication with the drains and, in practice, will descend in the latter to approximately 1 to 2 meters from the bottom, so as to terminate at a level higher than the deposit layer 20 of various materials. which generally forms at the bottom of drains.

Each tube 19 is essentially connected to the surface to a common flexible conduit 21, itself connected to an external means for discharging water, which, in the example illustrated, is the vacuum pump 18 which serves to maintain the partial vacuum in layer 14 of permeable material.

In this case, at the inlet of the pump 18, switching means, shown diagrammatically 28, are provided, controlled by a control unit, marked 38, allowing the pump to suck through either the conduit 17 or the conduit 21.

Referring to FIG. 5, a particularly suitable construction of the lower end part of the tubes 19 will be described.

This lower part is bent upwards in the shape of a hook and ends in a cage 22 open on the drain, in which cage the tube opens out through an orifice 23. Inside the cage is held captive a float 24, mounted movable in the cage and equipped with a valve or a ball 25 capable of closing the orifice 23.

The operation and control of the installation just described are as follows.

Suppose that the switching means 28 is controlled so that the pump 18 ensures the depression of the layer 14. A pressure of about 60 to 80 kPa therefore prevails in this layer and in the non-submerged part of the drains 13.

The conduit 21 is substantially at atmospheric pressure.

The water coming from the ground 1 and which has traveled essentially along the arrows 26, has reached the nearest drains, in which it has accumulated until reaching level 27, level where a level sensor 30 is placed, such as a contactor connected to an electrical supply means (not shown) and which may have been fixed to the tube 19. This sensor 30 which is also connected (not shown) to the pump control unit 38 triggers the switching of this pump, the connection of which "switches" from line 17 to line 21.

Maintaining the partial vacuum of layer 14 is therefore temporarily interrupted. A pressure lower than that which prevails in the layer 14 and at the top of the drains 13 is then established in the tubes 19 to stabilize in general around 60 to 80 kPa.

The water contained in the drains is then sucked through the openings of the cage 22, passes through the orifice 23 and rises in the tubes to be evacuated on the surface, outside the membrane 15.

As soon as the water contained in the drains descends below the float 22, the density of which is lower than that of the water, the valve 25 comes, under the effect of the vacuum which prevails in the tubes, to close the orifice 23 Thus, it is avoided that a depression of the drains and of the layer 14 "of compaction" is not obtained by the depression of the tubes 19.

At the level of the float 22, there is preferably provided, on at least a few drains, another level sensor 31, such as an electrical sensor connected to its supply means (not shown) as well as to the unit 38 pump 18, so that as soon as this sensor 31 is no longer immersed in water, the pump again "switches" from line 21 to line 17 and thus restores the partial vacuum in the layer 14.

The suction of the water through the tubes 19 is then interrupted and the drains are filled again.

It will be noted that such a cyclically controlled installation makes it possible to obtain good drying efficiency, as well as a limited energy consumption compatible with the requirements of interventions in the field.

In practice, the tests carried out so far have shown that cycles of control in suction of water by the tubes 19 and of interruption of the maintenance in depression of the layer 14, lasting approximately half a hour, with a frequency of once or twice every twenty-four hours, ensured very good efficiency of the drains without harming the surface loading of the ground provided by the depressurization of the surface.

Referring now to Figures 6 to 8, we will describe an alternative embodiment of the invention in which, in addition to the pressure stress exerted on the area 1 of land to be consolidated by the partial vacuum of the layer 14 , we will come to exert on the area in question an additional pressure, by placing on it, still on the surface, a distributed load such as a load of water or fill material. We will thus increase the speed of compaction of the ground.

FIG. 6, first of all, provision has been made to deposit directly on the layer 1 of ground, a layer of backfill 32 which has been covered with the layer 14 of permeable material surmounted by its membrane 15 whose outer edges 15a plunge into the peripheral sealing channel 16. In this case, the drains 13 pass through the layer 1 to be treated, but also through the backfill layer 32 to open out under the layer 14 in which, as before, partial vacuum has been established.

Figure 7, we have somehow provided the "reverse" solution. The backfill layer 32 has here been extended over the membrane 15 which always covers the layer 14 into which the drains open.

Finally, FIG. 8 illustrates a third possibility of additional loading. In this case, provision has been made on the surface for peripherally bordering zone 1 of land to be treated with a fill or mound 33 in backfill, so as to constitute, above zone 1, a bowl 34. The bottom of the bowl and the merlon 33 were covered with the layer 14 of permeable material which was itself covered with its sealing membrane 15. The bowl 34 thus formed was filled with a liquid 35, such as water, so as to ensure an additional loading of the ground, in the manner of the embankment of FIGS. 6 and 7. In order to avoid any problem, at least in this case, a waterproof membrane will be chosen.

Although this has not been specified, it is clear that the installations of FIGS. 6 to 8 are also provided with means for discharging water from the drains.

Furthermore, even if in the foregoing we have only described precisely the case where the depressurization of the layer 14 of permeable material and of the tubes 19 was ensured by the same pump 18, one could completely envisage use a pump for each function.

Finally, a means of evacuating the water contained in drains, other than a vacuum pump could be used. For example, each drain could be equipped, towards its base, with a discharge pump which can be controlled from the surface continuously or intermittently, cyclically, by means of a compressed air duct connected to a compressor for evacuating on the surface, by an annex discharge pipe, the water collected at the bottom of the drains (not shown).

Such a device is well known, in particular from patent FR-A-2 564 500 to which reference may be made, and in which is described a pump essentially comprising a cylindrical body of diameter generally equivalent to that of the drains, an inlet valve. of water mounted on the body in question and allowing the entry of overpressure water into the body, a flexible compressed air supply duct connected to said body and to a ground compression station, and a chamber accumulation of water formed in said body, which chamber is connected through a non-return valve to another flexible conduit for discharging the water back up, through the drain, to the surface.

Of course in this case, the water would always be removed from the membrane and the layer of permeable material placed below it would still be maintained under partial vacuum by a vacuum pump.

Claims (11)

1. A method of consolidating an area of soft ground with low permeability, impregnated with water of the type in which a partial air vacuum is established in a layer (14) of permeable material surmounted by a membrane (15) d waterproof seal covering the area (1) of land to be consolidated on the surface, a mesh network of hollow drains (13) with permeable walls sinking into the ground and stopping before entering a subsoil is created in said ground layer (12) of permeable ground and said drains (13) are unblocked at the level of said layer (14) of permeable material, characterized in that each drain (13) is fitted with a connected tube (19) to an external means for discharging water (18), such as a pump, said tubes (19) descending into the drains over the greater part of the height of the latter with which they communicate in the lower part, and controls said external means (18) of evacuation so as to evacuate on the surface, out of these drains, the water which therein is accumulated. 2. Method according to claim 1 characterized in that an external pump (18) for evacuating the water uses a suction pump which is controlled so as to establish in the tubes a pressure lower than that reigning in the drains. 3. Method according to claim 1 or claim 2 characterized in that one controls the means (18) external discharge intermittently, when water may have accumulated in the drains (13). 4. Method according to claim 3 characterized in that during the time interval during which said suction pump is controlled, the maintenance of partial air vacuum of the layer (14) of permeable material is interrupted. 5. Method according to claim 4 characterized in that the period during which the maintenance of the partial vacuum maintenance of the layer (14) of permeable material is interrupted and where the suction pump (18) is controlled, is d '' about half an hour, with a frequency of once or twice a day. 6. Method according to any one of claims 1 to 5 characterized in that one carries out circular drains (13) with a diameter of the order of 50 mm and plastic tubes (19) are used. about 4 to 8 mm in diameter. 7. Method according to any one of claims 1 to 6 characterized in that in addition to the pressure stress exerted on the area (1) of land to be consolidated by the partial vacuum of the layer (14) of material permeable, the pressure in question is exerted on the zone in question by placing on it, on the surface, a distributed load (32, 35), such as a load of water or of fill material. 8. Method according to claim 7 characterized in that directly deposited on the area (1) of land to be treated a load (32) of backfill material on which we just deposit the layer (14) of permeable material that the its sealing membrane (15) is covered. 9. Method according to claim 7 characterized in that the layer (14) of permeable material which is covered with its sealing membrane (15) is placed directly on the surface of the area (1) from ground to consolidate and a load (32) of backfill material is deposited on said membrane. 10. Method according to claim 7 characterized in that on the surface, above the area (1) of land to be consolidated, a bowl (34) is created, peripherally bordered by a mound (33) of backfill material, the bowl is covered with the layer (14) of permeable material surmounted by its sealing membrane (15) and the bowl (34) is filled with a liquid, such as water. 11. Device for implementing the method according to any one of claims 1 to 10 characterized in that it comprises, to equip each drain (13), a tube (19) which opens towards its lower end in a cage (22) water inlet open on the drain and in which a float (24) fitted with a valve (25) capable of closing the lower end (23) of said tube when the water contained in the drain reaches a level lower than that of the float (24), characterized in that all of the tubes (19) are connected for the evacuation of water to the same suction pump (18) disposed on the surface, said pump (18) being further connected to the layer (14) of permeable material, so as to ensure partial vacuum therein, and comprising a switching means (28) connected to a control unit (38) capable of sequentially controlling either the partial vacuum of the layer (14) of permeable material, or the depression of the tubes (19) .

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