FR2627202A1 - METHOD AND DEVICE FOR CONSOLIDATION OF LANDS - 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.

Description

The invention relates to the. consolidation of

  soft soils with low permeability, impregnated with water.

  These types of soil are generally unfit for construction, unless they are made on piles that have been beaten or sunk to a hard underlying rock or

have been consolidated.

  The technique of beaten piles quickly becomes prohibitively expensive if the thickness of the soft ground layer is important. In this case, a consolidation-land technique is used, which essentially involves the removal of a large quantity of water contained in these layers. Since some layers, such as clay layers, have very low permeability, the removal of water is sometimes very

  difficult, and in practice very long.

  Among the various consolidation techniques

  known such lands, include the following.

1. The embankment 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. jacente, a layer 3 of embankment, for example pebbles or earth, thus loading the site to build. The embankment brings on the ground a pressure stress in general greater than that of the projected construction. The presence of the Pemblai causes a settling of the layer 1, forcing the water contained in this layer to escape according to a general path indicated

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by Arrows 4.

  At the end of a usually very long time, generally exceeding several years, one obtains a certain consolidation of the ground. The embankment can then be removed and the land built. Such a technique is expensive because of the loading and unloading operations of the ground and very long to implement, commonly requiring a

  ten years to obtain satisfactory results.

  2. The technique of embankment and drainage.

  It is possible to improve somewhat the backfill loading technique, as described above, by installing in the low permeability layer 1, a network of drains 5 shown vertical in Figure 2. These drains can be constituted for example by drilling small diameters filled with sand, according to

  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 go up to the surface through the whole thickness of the layer 1

  permeable, will "flow" to the nearest drain 5.

  For a tight mesh, the path of the water, as indicated by the arrows 6, will be relatively short. As a result, the speed of consolidation of the ground will be very

significantly increased.

  In practice, it will be possible in particular to use drains formed of plastic tubes, porous of a

  inner diameter of the order of 50 mm.

  3. The technique of drainage and loading by

  application of a partial air vacuum.

  In practice, it appeared that the use of embankment loading techniques posed a number of problems, and in particular - problems of stability of the embankment, - recovery of backfill materials, often expensive, sometimes difficult to obtain, and

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  - placement and disposal of embankments, given the relatively large volumes of materials

usually required.

  Also, some have proposed, as illustrated in Figure 3, another technique of coming to establish a partial air gap, for example by means of a vacuum pump 7, in a layer 8 of permeable material surmounted by an airtight membrane 9 which is impervious to air and covers in

  surface area of land 1 to consolidate.

  As a permeable material, sand is usually provided that is deposited on the ground in a layer of about 20 to 60 cm thick and in this sand mass, a vacuum of about 60 is established. at 80 kPa. As understood, the load applied to the

  terrain depends on the surface thus covered.

  Generally, to ensure the tightness of the membrane 9, there is used a channel 11 which is hollowed at the periphery of the area to be treated and filled with a fluid material, such as a bentonite sludge, in which one comes embed the outer edges 9a of the membrane. Regarding the drains, which have been identified again 5, they will be of the same type as those previously described. They always open in part

  greater than the level of the layer 8 of permeable material.

  Nevertheless, in this case, if the layer 1 to be consolidated is located above a porous layer 10 with good permeability, that is to say "drainable", care will be taken, as shown in FIG. that the drains 5

  do not reach this draining layer.

  It will be noted that with this technique, the water located in the layer 1 near the surface will, because of the depression created in the layer 8, tend to rise

towards it by capillarity.

  In spite of all the major problem which remains posed is that of the evacuation of the water which accumulates in the drains and no interesting solution has been up to now proposed allowing to push back on the surface this water

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  without impairing the efficiency of the vacuum holding system of the permeable layer into which

the drains.

  The subject of the invention is precisely a new method for consolidating a 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. impervious waterproofing and covering the area of land to be consolidated on the surface, a mesh network of hollow drains with permeable walls is made in the ground and stopping before entering a permeable sub-layer of ground and these drains are led out at the level of the layer

  permeable material in question.

  The method of the invention is characterized in that each drain is equipped with a hollow tube connected to an external means for discharging water, such as a pump, said tubes descending into the drains over the greater part the height of the latter with which they communicate in the lower part, and said external evacuation means is controlled, so as to evacuate at the surface, out of

  drains, the water that will have accumulated there.

  In order to avoid any risk of leakage in the sealing membrane, the suction tubes will pass under this membrane or will pass through this membrane in a sealed manner. In fact, the external evacuation means will preferably be ordered intermittently when water has been able to accumulate in the drains. And in the case where a suction pump, of the "vacuum pump" type, is used as a means for discharging the water, the holding of the permeable material under partial air vacuum will be interrupted during the time interval during which we will order

said suction pump.

  It is then expected that in practice the time during which the holding under partial vacuum of the layer of permeable material and the suction pump will be stopped will be about half an hour, with a periodicity of one to one hour. twice by

twenty-four hours.

  In this way, we will be able to significantly improve the efficiency of the last mentioned technique by increasing the efficiency of the drains, while preserving practically in their entirety the advantages ensured by the vacuum maintenance of the surface layer which covers the field area. to be treated, the time necessary for

consolidation will be reduced.

  So far, to the plaintiff's knowledge, backfill 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 land. As is understood, it would then be practical that the loading and unloading of fill materials could be carried out at a cost

reasonable.

  It is in this spirit of combining the mentioned techniques that a characteristic of the invention provides that in addition to the pressure stress exerted on the area of land to be consolidated by the partial evacuation 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

embankment materials.

  In addition to various process features, the invention also provides a device for implementing this method, characterized in that it comprises, to equip each drainun hollow tube which opens at its lower end on an entrance cage d water open on the drain and in which is held prisoner a float equipped with a valve that can close off the lower end of said tube when the water contained in the drain reaches a level below that of the float, all tubes being connected to the same suction pump for

  the evacuation of water, arranged on the surface.

  In this way, once the drained drains, it will avoid aspiration by the tubes the air, or more precisely the rest of air, contained in the drains. In practice, sensors of all known types will be further disposed towards the bottom of at least some of the drains, at the lower end of the tubes, and connected to the control means of the pump, in order to stop it. once

drains emptied of their water.

  The invention and its implementation will appear more

  clearly from the following description made in

  reference to the accompanying drawings, in which, in addition to FIGS. 1, 2 and 3 previously described, relating to the prior art: FIG. 4 shows schematically the process of the invention; FIG. 5 shows in an enlarged view the detail identified; in FIG. 4, and FIGS. 6, 7 and 8 illustrate successively and schematically an overall view of

  three embodiments of the invention.

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

draining layer.

  In this field, a mesh network of drains 13 consisting of, for example, porous plastic sheaths has been drilled and put into place; stranded tubes, that is perforated, or others. These drains 13 form wells having a diameter of the order of 50 mm and stop at a distance above the draining layer 12 they must not reach. The mesh of the network can respect a distance between drains for example from 4 to 6 meters. In practice, the more dense the mesh, the faster consolidation can be. On the surface, the drains 13 open into a superficial layer 14 of relatively thin permeable material, such as a sandstone approximately 30 to 50 cm thick, which has been extended substantially over the entire surface of the land to consolidate. The layer 14 is covered with a tarpaulin or membrane 15 made of a synthetic material that is substantially airtight, at least in air, whose peripheral outer edges 15a are immersed in a channel 16 filled with a material that is sufficiently viscous to hold the edges in position.

  of the membrane and ensure its tightness.

  In the layer 14, further opens a conduit 17 connected to a water discharge means such as a suction pump "vacuum" 18 of the liquid ring type,

  way that we can establish under the membrane, in the -

  layer 14, a partial air vacuum of the order of 60 to 80 kPa.

  Of course, the conduit 17 will leave the membrane without

affect 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 polyamide type approximately 4 to 8 mm in diameter. In the lower part, these tubes are

  in communication with the drains and, in practice,

  will descend into the latter up to about 1 to 2 meters from the bottom, so as to end at a level higher than the deposition layer 20 of various materials which forms in

general at the bottom of the 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 the layer 14 of permeable material.

  In this case at the inlet of the pump 18 are provided switching means, schematically 28, controlled by a control unit, marked 38, allowing the pump

  aspirate through either conduit 17 or conduit 21.

  Referring to Figure 5, a particularly adapted construction of the extreme part will be described.

lower tubes 19.

  This lower part is curved upward hook-shaped and ends with a cage 22 open on the drain, in which cage the tube opens through a hole 23. Inside the cage is held prisoner a float 24 mounted mobile in the cage and equipped with a valve or a ball 25 may come close

the orifice 23.

  Operation and control of the installation

  that we have just described are the following.

  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 prevails in this layer and in the non

submerged drains 13.

  The conduit 21 is substantially at atmospheric pressure.

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

to the conduit 21.

  The partial vacuum hold of the layer 14 is therefore temporarily interrupted. A pressure lower than that prevailing in the layer 14 and at the top of the drains 13 is then established in the tubes 19 to stabilize in

  around 60 to 80 kPa.

  The water contained in the drains is then sucked through the openings of the cage 22, passes into the orifice 23 and back into the tubes to be evacuated at the surface, to

the outside of the membrane 15.

  As soon as the water contained in the drains falls below the float 22 whose density is lower than that of the water, the valve 25 comes under the effect of the

  depression prevailing in the tubes, close the orifice 23.

  Thus, it is avoided that a depression of the drains and the layer 14 "compaction" is obtained by the implementation of

depression of the tubes 19.

  At float 22, at least some drains are preferably provided with another level sensor 31, such as an electrical sensor connected to its supply means (not shown) and to the unit 38. 18, so that as soon as this sensor 31 is no longer immersed in water, the pump "tilts" again conduit 21 to the conduit 17 and thus restores the

partial vacuum in the layer 14.

  The aspiration of the water by the tubes 19

  then breaks off and the drains fill up again.

  It should be noted that such a cyclically controlled installation makes it possible to obtain a good drying efficiency, as well as a limited energy consumption compatible with the

  requirements of field interventions.

  In practice, the tests conducted so far have shown that control cycles in suction of the water by the tubes 19 and interruption of the maintenance in depression of the layer 14, lasting about half an hour , with a periodicity of one to two times per twenty-four hours, ensured a very good efficiency of the drains, without affecting the superficial loading of the ground provided by

the setting in depression of surface.

  Referring now to FIGS. 6 to 8, an alternative embodiment of the invention will be described in which, in addition to the pressure stress exerted on the zone 1 of land to be consolidated by the partial evacuation of the layer 14. we will come to exert on the zone in question an additional pressure, by placing on it, always on the surface, a distributed load such as a load of water or embankment materials. This will increase

  all the faster the ground compaction.

  Figure 6, first of all it is expected to deposit directly on the layer 1 of land, a backfill layer 32 which was covered with the layer 14 of permeable material surmounted by its membrane 15 whose outer edges 15a dive into the peripheral trench 16 sealing. In this case, the drains 13 pass through the layer 1 to be treated, but also the layer 32 of backfill to open under the layer 14 in which, as

  previously, partial vacuum has been established.

  Figure 7, somehow provided the solution "inverse". The backfill layer 32 has here been extended over the membrane 15 which still covers the layer 14

  in which the drains open.

  Finally, FIG. 8 illustrates a third possibility of complementary loading. In this case, it is provided on the surface peripherally border zone 1 of land to be treated by a merlon or mound 33 embankment, so as to constitute, above the zone 1, a bowl 34. The bottom of the bowl and Merlon 33 were covered with the layer 14 of permeable material which was itself covered with its sealing membrane. The bowl 34 thus formed was filled with a liquid 35, such as water, so as to ensure additional loading of the ground, in the manner of the embankment of Figures 6 and 7. In order to avoid any problem, we will choose at least in this case

waterproof membrane.

  Although this has not been specified, it is clear that the installations of Figures 6 to 8 are also

  provided with means for evacuation of the water from the drains.

  Moreover, even if in the foregoing it has precisely described the case where the depression of the layer 14 of permeable material and the tubes 19 was provided by the same pump 18, one could quite

  consider using a pump for each function.

  Finally, a means for discharging the water contained in the drains, other than a vacuum pump could be used. For example, it could be equipped with a drain, towards its base, a discharge pump that can be controlled continuously or intermittently from the surface, cyclically, via a compressed air duct connected to a compressor to evacuate on the surface, by an auxiliary pipe of discharge, the water collected at the bottom of

drains (not shown).

  Such a device is well known, in particular patent FR-A-2 564 500 to which reference may be made, and in which is described a pump comprising essentially 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 water under pressure in the body, a flexible conduit for supplying compressed air connected to said body and a compressor station on the ground, and a chamber of accumulation of water formed in said body, which chamber is connected through a non-return valve to another flexible discharge pipe discharging upstream water, through the drain, to

the surface.

  Of course in this case, the water would always be evacuated out of the membrane and the layer of permeable material placed underneath would still be maintained under

partial vacuum by a vacuum pump.

Claims (12)

  A method of consolidating a low permeability, water-impregnated, soft ter-rain zone of the type in which a partial air vacuum is established in a layer (14) of permeable material surmounted by a membrane (15). ) impervious waterproofing covering the area (1) of land to be consolidated on the surface, a mesh network of permeable-walled hollow drains (13) sinking into the ground and stopping before entering an area is made; sub-layer (12) of permeable ground and said drains (13) are opened at said layer (14) of permeable material, characterized in that each drain (13) is fitted with a tube (19). ) hollow connected to an external means of water discharge (18), such as a pump, said tubes (19) down into the drains over most of the height of the latter with which they communicate in the lower part , and said external evacuation means (18) is controlled so as to evacuate er in   surface, out of these drains, the water that has accumulated there.   2. Method according to claim 1, characterized in that a suction pump is used as external means (18) for discharging the water, which is controlled so as to establish a pressure in the tubes.   less than that prevailing in the drains.   3. Method according to claim 1 or claim 2 characterized in that the control means (18) external evacuation intermittently, when de   water could accumulate in the drains (13).   4. Method according to claim 3 characterized in that during the period of time during which said suction pump is controlled, it interrupts the holding under vacuum   partial air of the layer (14) of permeable material.   5. Method according to claim 4, characterized in that the period during which the partial vacuum hold of the layer (14) of permeable material is interrupted and the vacuum pump (18) is controlled, is about half an hour, with a periodicity of   order one to two times in twenty-four hours.   6. Process according to any one of   Claims 1 to 5, characterized in that   circular drains (13) having a diameter of the order of 50 mm and using tubes (19) of plastic material of about 4 to 8 mm in diameter.   7. Process according to any one of   Claims 1 to 6 characterized in that in addition to   the pressure constraint exerted on the area (1) of land to be consolidated by partial evacuation of the layer (14) of permeable material, it is exerted on the area in question an additional pressure by placing on it, on the surface , a load (32, 35) distributed, such as a load of water or fill material.   8. A method according to claim 7 characterized in that deposited directly on the area (1) of land to be treated a load (32) embankment material on which is deposited the layer (14) of permeable material that   it is covered with its membrane (15) sealing.   9. A method according to claim 7 characterized in that places the layer (14) of permeable material which is covered with its membrane (15) sealing directly on the surface of the area (1) of land to consolidate and is deposited on said membrane a load (32) of fill material.   10. A method according to claim 7 characterized in that on the surface, above the area (1) of land to be consolidated, there is created a bowl (34) peripherally lined with a mound (33) embankment material, it covers The bowl of the layer (14) of permeable material surmounted by its membrane (15) sealing and is filled   the bowl (34) of 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 hollow tube (19) which opens at its lower end into a cage (22) of water inlet open on the drain and in which is held prisoner a float (24) equipped with a valve (25) that can close off the lower end (23) of said tube when the water contained in the drain reaches a level lower than that of the float (24), the set of tubes (19) being connected to a same suction pump (18),   for the evacuation of water disposed on the surface.   12. Device according to claim 11 characterized in that said suction pump (18) is also connected to the layer (14) of permeable material so as to provide partial vacuum, said pump (18) comprising means (28). ) connected to a control unit (38) adapted to sequentially control either the partial air evacuation of the layer (14) of material   permeable, ie the depression of the tubes (19).