Method and geotechnical structure for protecting an embankment from erosion resulting in the formation of a sand boil

Abstract

A method for protecting a embankment from erosion resulting from the formation of a sand boil comprises a first step, in which there is excavated a trench which interrupts the supply channel (C) of the venting shaft (P) of the sand boil. There is then positioned in the trench a barrier structure (14) which has a filtering capacity and which is arranged to allow the water running in the channel (C) to pass and at the same time to retain the particulate which is dragged by the water upstream of the barrier structure (14). The barrier structure (14) extends over a predetermined length in the direction of the embankment (A) and comprises at least one filtering sheet (26) which confers the filtering capacity on the barrier structure (14). The filtering sheet is arranged at least partially transversely with respect to the channel (C) which supplies the sand boil. The barrier structure (14) with the filtering sheet (26) is part of a geotechnical structure for protecting an embankment from such an erosion.

Description

Field of the invention

[0001] The present invention relates to civil engineering works, in particular to a method and to a geotechnical structure for mitigating the negative effects of sand boils.Technological background

[0002] The term "sand boil" is a term which is used in hydrology to indicate a source which is formed by an infiltration of water at the exterior of a embankment while a river is in flood, particularly in the case in which the river bed is in a raised position with respect to the surrounding alluvial plain. The sand boil is generally caused by the fact that the water, as a result of the increased pressure, generates a route under the embankment, for example, through spaces which are excavated by moles or residues of decomposing roots. The sand boils put at risk the stability of the embankments along the water courses because they promote rapid erosion of the constituent soil and under the embankment itself.

[0003] It is known to combat sand boils by positioning bags of sand around the hole so as to form a ring, or surrounding reinforcement, in order to generate a counter-pressure which causes the speed of the water to be reduced, reducing the erosion of the conduit being formed and the transport of soil or sand. If it is possible to reduce the water gradient and to slow down the flow of water until eliminating the turbulence, the slower, non-turbulent flow will no longer be capable of transporting particles of soil.

[0004] The conventional solution for stopping the erosion of the soil as a result of sand boils is not satisfactory, unless for small or extremely small sand boils. As soon as the dimensions of the sand boil increase, for example, forming a crater measuring a few metres in diameter, the arrangement of bags of sand around the hole requires a substantial workforce. Furthermore, if the arrangement of the sand bags is not done in a workmanlike manner, there is the risk of the surrounding reinforcement failing, compromising the efficacy of the protection work from erosion.

[0005] Another well-known solution is described in Förster et al.

[0006] Vertically inserted geotextile used for strengthening levees against internal erosion, Geotechnical Engineering for Infrastructure and Development, January 2015, pp. 1995-2000. A geotextile shield is inserted vertically into the upper part of the aquifer as a preventive measure against the phenomenon of backward erosion, with the aim of blocking the transport of sand grains by the underground water flow. In this document, doubts are raised about the long-term effectiveness and durability of the proposed solution, especially following consecutive flood events. The solution is also complex to implement in real situations outside of a scientific experiment, as the installation of the geotextile in the described manner is complex and may not be accurate, compromising the effectiveness of the protection structure from erosion.Statement of invention

[0007] An object of the present invention is to solve the problems of the prior art by providing technical tools which can be used to combat sand boils and the erosion of the soil, in particular of embankments, which cause them.

[0008] Another object of the invention is to provide an intervention methodology which can be readily and successfully applied to combat sand boils with some prospect of long-term protection.

[0009] Another object of the invention is to provide a geotechnical structure for protecting an embankment from erosion resulting from the formation of a sand boil, which is simple to implement and which is durable in time, even in case of following consecutive flood events.

[0010] According to a first aspect of the invention, there is described a method for protecting an embankment from erosion resulting from the formation of a sand boil, comprising the steps of: excavating a trench which interrupts a supply channel of a sand boil, positioning in the trench a barrier structure which has a filtering capacity and which is arranged to allow the water running in the channel to pass and at the same time to retain the particulate which is dragged by the water upstream of the barrier structure.

[0011] Therefore, there is described a way to implement a geotechnical structure for protecting an embankment which comprises the barrier structure positioned in the trench in order to interrupt the supply channel of the sand boil.

[0012] The barrier structure can extend over a predetermined length in the direction of the embankment and comprises at least one filtering sheet which confers the above-mentioned filtering capacity on the barrier structure. The filtering sheet is arranged at least partially transversely with respect to the channel which supplies the sand boil. In this manner, the water which supplies the sand boil can continue to flow, and it is slowed down by the filtering effect of the barrier structure. Furthermore, which is much more important, the filtration effect of the filtering sheet prevents the particulate dragged by the water from migrating into the channel which supplies the sand boil, thereby interrupting the phenomena of erosion of the embankment.

[0013] According to a particular aspect, the positioning of the barrier structure in the trench is carried out in such a manner that an upper portion of the barrier structure is positioned in the region of a layer of soil which is substantially impermeable or in any case has low permeability, for example, the clay layer which lies above the underlying aquifer layer, in which there is formed the channel supplying the sand boil and where the lower portion of the barrier structure is instead positioned when it is installed in the excavated trench for this purpose. The upper portion of the barrier structure is thereby "capped" by the layer of argillaceous soil in such a manner that the water which supplies the sand boil cannot avoid the barrier structure at the upper side.

[0014] According to a particular aspect, the above-mentioned upper portion of the barrier structure which is positioned in the layer which has low permeability extends less than the lower portion which is positioned in the underlying aquifer layer. This is intended to keep the barrier structure "immersed" in the underlying aquifer layer to the greatest possible extent, maximizing the filtering efficacy thereof. According to a particular aspect, the trench is then closed again by covering the barrier structure at least partially with low-permeability soil, for example, the same soil as the argillaceous layer previously excavated in order to form the trench itself.

[0015] According to a particular aspect, the barrier structure may comprise a support structure which is elongate in a main length direction which is normally aligned with the embankment approximately in a parallel manner. The barrier structure may comprise a metal framework which defines a containment volume, which can be filled with a permeable filler material. The metal framework may be configured so as to support the above-mentioned at least one filtering sheet. Such a barrier structure is easy to install inside the trench. Furthermore, the filtering sheet is carefully positioned to interrupt the underground water flow. This also has the advantage of preventing deterioration or damage to the filtering capacity of the barrier structure, without the risk of tearing the filtering sheet during thew installation thereof. The barrier structure is also more stable over time, even following any landslides in the surrounding area.

[0016] These barrier structures can be constructed using structures which are known for other applications in the field of geotechnical works, for example, efficiently using gabions which are covered with geotextile or multicellular structures, such as the structures commercially designated FlexMac ®< from the same Applicant, or functionally similar structures.

[0017] Preferably, but in a non-limiting manner, the filler material is material obtained from the excavation of the trench, such as sand, which avoids the costs and expenses of transport from a different extraction site. Furthermore, by using the same material from the trench excavated on site as filler material, the barrier structure is well integrated into the surrounding environment as the material is essentially homogeneous both inside and outside the barrier. This helps to naturally ensure the filtering function of the protective barrier over time.

[0018] According to a particular aspect, the metal framework of the barrier structure may comprise an electro-welded metal net or a metal net of the type with rhomboid or hexagonal mesh. The metal framework can be supported by rods or brackets which contribute to the stiffening thereof.

[0019] According to another particular aspect, the at least one filtering sheet of the barrier structure can be a sheet of nonwoven fabric which covers, internally or externally, the containment volume of the barrier structure at least at one bottom thereof and at one of the sides of the barrier structure, in particular at least at the side that extends over the direction of the embankment, thus being elongated in the aforementioned main length direction, and which is arranged at least partly transversely with respect to the channel that supply the sand boil.Brief description of the Figures

[0020] Additional features and advantages will be appreciated from the following detailed description of a preferred embodiment of the invention with reference to the appended Figures which are given merely by way of non-limiting example and in which: Figure 1 is a schematic illustration of a section of soil in the region of a embankment where a sand boil is formed; Figure 2 is a schematic illustration, similar to Figure 1, of the same section of soil which includes civil engineering work incorporating aspects of the present invention for protecting the embankment from erosion as a result of the sand boil; and Figure 3 is a non-limiting example of a structure for protection to be used for carrying out the civil engineering work which is schematically shown in Figure 2. Detailed description

[0021] Now with reference to Figure 1, there is illustrated a section of soil in the region of the embankment A of a river F, the level L of which exceeds the level of the soil T adjacent to the embankment A. The typical conditions in which the formation of a sand boil can occur involve a situation in which there is present under the embankment A a layer B of soil with low permeability, for example, argillaceous soil, under which there is present a permeable layer, for example, a sandy layer, of the aquifer Q which extends under the bed of the river F and which extends out of the embankment A.

[0022] The head of water resulting from the level L of the river F brings about an increase in the hydrostatic pressure on the aquifer Q. Therefore, the water can find an escape route C under the embankment A through, for example, spaces which are excavated by moles or residues of decomposing roots. There is thereby formed a water route C in the aquifer Q which can pass through the low-permeability layer B and flow out into a venting shaft P, thereby forming the sand boil.

[0023] As illustrated in Figure 2, in order to prevent or in any case to slow down the water flow along the channel C and the dragging of sand or soil from the layer of permeable soil Q to the venting shaft P, there is provided a geotechnical structure comprising a barrier 14 which is formed by a semipermeable or draining structure which allows the water to pass, slowing it down, but retaining the granular material (sand, soil) which would otherwise be dragged towards the venting shaft P, bringing about eroding phenomena under the embankment. The barrier 14 therefore allows a release of the water pressure, allowing the water to flow but stopping the transport of material.

[0024] The barrier 14 is preferably constructed with a drainage structure, which is buried in the aquifer Q so as to interrupt the channel C. The barrier 14 extends in a vertical manner so as to involve over a greater part the aquifer Q but also to remain partially covered by the low-permeability layer B in an upper zone 16. The barrier extends in terms of length, skirting around the embankment A over a distance sufficient to prevent the water which supplies the venting shaft P of the sand boil from avoiding the barrier 14, giving rise to other sand boils further upstream or downstream.

[0025] Figure 3 illustrates a non-limiting example of a barrier 14 which provides for a support structure 18 which is elongate in a main length direction L, comprising a framework 20, for example, made of metal net. The metal net can be an electro-welded net or a metal net of the type with rhomboid or hexagonal mesh which is supported by rods or brackets 22 which contribute to its stiffening. The framework 20 defines a containment volume 24 which is filled with permeable filler material, for example, the same sand or soil which is formed by the aquifer layer Q. The framework 20 further supports one or more filtering sheets 26, for example, sheets of nonwoven fabric, which cover, internally or externally, the barrier 14 at least at the bottom and at least one of the sides thereof, more advantageously at both sides.

[0026] In order to protect the embankment A from erosion resulting from the sand boil, there is carried out the identification of the channel C which supplies the venting shaft P. The method then continues with excavation of a trench, for example with a so called restricted section excavation, namely an excavation with a width equal to or less than the height, which intersects with the channel C, cutting it. The length of the trench will be defined on the basis of geological and engineering assessments in order to ensure interception of the flow of water and to prevent subsequent sand boils from being formed upstream or downstream of the venting shaft P to be combatted.

[0027] There is positioned in the trench which is excavated in this manner the barrier structure 14 which can be, for example, of the type illustrated in Figure 3 or a similarly functional type. For example, rapid-response multicellular structures can be advantageously used, such as, for example, the structure commercially designated FlexMac ®< from the same Applicant. The containment volume 24 of the barrier 14 is filled with permeable material, for example, the same material excavated to form the trench. The excavation depth of the trench and the positioning of the barrier 14 are configured and carried out so that, preferably, an upper portion of the barrier 14 affects the low-permeability layer B which thereby performs the function of an upper "cap" of the barrier 14, preventing the water from avoiding it by passing above it in order to continue to supply the sand boil 12. The barrier 14 is then buried so that the water which flows through the channel C can pass through it but without dragging the particulate which, in the absence of any barrier 14, would be dragged towards the venting shaft P. The erosion of the embankment A is thereby interrupted, at the same time affording the possibility for the water pressure to be vented into the channel C with a flow which is slowed down and without transporting any material.

[0028] Naturally, the principle of the invention remaining the same, the forms of embodiment and details of construction may be varied widely without thereby departing from the scope of the invention.

Examples

Embodiment Construction

[0021]Now with reference to Figure 1, there is illustrated a section of soil in the region of the embankment A of a river F, the level L of which exceeds the level of the soil T adjacent to the embankment A. The typical conditions in which the formation of a sand boil can occur involve a situation in which there is present under the embankment A a layer B of soil with low permeability, for example, argillaceous soil, under which there is present a permeable layer, for example, a sandy layer, of the aquifer Q which extends under the bed of the river F and which extends out of the embankment A.

[0022]The head of water resulting from the level L of the river F brings about an increase in the hydrostatic pressure on the aquifer Q. Therefore, the water can find an escape route C under the embankment A through, for example, spaces which are excavated by moles or residues of decomposing roots. There is thereby formed a water route C in the aquifer Q which can pass through the low-permeabili...

Field of the invention

[0001] The present invention relates to civil engineering works, in particular to a method and to a geotechnical structure for mitigating the negative effects of sand boils.Technological background

[0002] The term "sand boil" is a term which is used in hydrology to indicate a source which is formed by an infiltration of water at the exterior of a embankment while a river is in flood, particularly in the case in which the river bed is in a raised position with respect to the surrounding alluvial plain. The sand boil is generally caused by the fact that the water, as a result of the increased pressure, generates a route under the embankment, for example, through spaces which are excavated by moles or residues of decomposing roots. The sand boils put at risk the stability of the embankments along the water courses because they promote rapid erosion of the constituent soil and under the embankment itself.

[0003] It is known to combat sand boils by positioning bags of sand around the hole so as to form a ring, or surrounding reinforcement, in order to generate a counter-pressure which causes the speed of the water to be reduced, reducing the erosion of the conduit being formed and the transport of soil or sand. If it is possible to reduce the water gradient and to slow down the flow of water until eliminating the turbulence, the slower, non-turbulent flow will no longer be capable of transporting particles of soil.

[0004] The conventional solution for stopping the erosion of the soil as a result of sand boils is not satisfactory, unless for small or extremely small sand boils. As soon as the dimensions of the sand boil increase, for example, forming a crater measuring a few metres in diameter, the arrangement of bags of sand around the hole requires a substantial workforce. Furthermore, if the arrangement of the sand bags is not done in a workmanlike manner, there is the risk of the surrounding reinforcement failing, compromising the efficacy of the protection work from erosion.

[0005] Another well-known solution is described in Förster et al.

[0006] Vertically inserted geotextile used for strengthening levees against internal erosion, Geotechnical Engineering for Infrastructure and Development, January 2015, pp. 1995-2000. A geotextile shield is inserted vertically into the upper part of the aquifer as a preventive measure against the phenomenon of backward erosion, with the aim of blocking the transport of sand grains by the underground water flow. In this document, doubts are raised about the long-term effectiveness and durability of the proposed solution, especially following consecutive flood events. The solution is also complex to implement in real situations outside of a scientific experiment, as the installation of the geotextile in the described manner is complex and may not be accurate, compromising the effectiveness of the protection structure from erosion.Statement of invention

[0007] An object of the present invention is to solve the problems of the prior art by providing technical tools which can be used to combat sand boils and the erosion of the soil, in particular of embankments, which cause them.

[0008] Another object of the invention is to provide an intervention methodology which can be readily and successfully applied to combat sand boils with some prospect of long-term protection.

[0009] Another object of the invention is to provide a geotechnical structure for protecting an embankment from erosion resulting from the formation of a sand boil, which is simple to implement and which is durable in time, even in case of following consecutive flood events.

[0010] According to a first aspect of the invention, there is described a method for protecting an embankment from erosion resulting from the formation of a sand boil, comprising the steps of: excavating a trench which interrupts a supply channel of a sand boil, positioning in the trench a barrier structure which has a filtering capacity and which is arranged to allow the water running in the channel to pass and at the same time to retain the particulate which is dragged by the water upstream of the barrier structure.

[0011] Therefore, there is described a way to implement a geotechnical structure for protecting an embankment which comprises the barrier structure positioned in the trench in order to interrupt the supply channel of the sand boil.

[0012] The barrier structure can extend over a predetermined length in the direction of the embankment and comprises at least one filtering sheet which confers the above-mentioned filtering capacity on the barrier structure. The filtering sheet is arranged at least partially transversely with respect to the channel which supplies the sand boil. In this manner, the water which supplies the sand boil can continue to flow, and it is slowed down by the filtering effect of the barrier structure. Furthermore, which is much more important, the filtration effect of the filtering sheet prevents the particulate dragged by the water from migrating into the channel which supplies the sand boil, thereby interrupting the phenomena of erosion of the embankment.

[0013] According to a particular aspect, the positioning of the barrier structure in the trench is carried out in such a manner that an upper portion of the barrier structure is positioned in the region of a layer of soil which is substantially impermeable or in any case has low permeability, for example, the clay layer which lies above the underlying aquifer layer, in which there is formed the channel supplying the sand boil and where the lower portion of the barrier structure is instead positioned when it is installed in the excavated trench for this purpose. The upper portion of the barrier structure is thereby "capped" by the layer of argillaceous soil in such a manner that the water which supplies the sand boil cannot avoid the barrier structure at the upper side.

[0014] According to a particular aspect, the above-mentioned upper portion of the barrier structure which is positioned in the layer which has low permeability extends less than the lower portion which is positioned in the underlying aquifer layer. This is intended to keep the barrier structure "immersed" in the underlying aquifer layer to the greatest possible extent, maximizing the filtering efficacy thereof. According to a particular aspect, the trench is then closed again by covering the barrier structure at least partially with low-permeability soil, for example, the same soil as the argillaceous layer previously excavated in order to form the trench itself.

[0015] According to a particular aspect, the barrier structure may comprise a support structure which is elongate in a main length direction which is normally aligned with the embankment approximately in a parallel manner. The barrier structure may comprise a metal framework which defines a containment volume, which can be filled with a permeable filler material. The metal framework may be configured so as to support the above-mentioned at least one filtering sheet. Such a barrier structure is easy to install inside the trench. Furthermore, the filtering sheet is carefully positioned to interrupt the underground water flow. This also has the advantage of preventing deterioration or damage to the filtering capacity of the barrier structure, without the risk of tearing the filtering sheet during thew installation thereof. The barrier structure is also more stable over time, even following any landslides in the surrounding area.

[0016] These barrier structures can be constructed using structures which are known for other applications in the field of geotechnical works, for example, efficiently using gabions which are covered with geotextile or multicellular structures, such as the structures commercially designated FlexMac ®< from the same Applicant, or functionally similar structures.

[0017] Preferably, but in a non-limiting manner, the filler material is material obtained from the excavation of the trench, such as sand, which avoids the costs and expenses of transport from a different extraction site. Furthermore, by using the same material from the trench excavated on site as filler material, the barrier structure is well integrated into the surrounding environment as the material is essentially homogeneous both inside and outside the barrier. This helps to naturally ensure the filtering function of the protective barrier over time.

[0018] According to a particular aspect, the metal framework of the barrier structure may comprise an electro-welded metal net or a metal net of the type with rhomboid or hexagonal mesh. The metal framework can be supported by rods or brackets which contribute to the stiffening thereof.

[0019] According to another particular aspect, the at least one filtering sheet of the barrier structure can be a sheet of nonwoven fabric which covers, internally or externally, the containment volume of the barrier structure at least at one bottom thereof and at one of the sides of the barrier structure, in particular at least at the side that extends over the direction of the embankment, thus being elongated in the aforementioned main length direction, and which is arranged at least partly transversely with respect to the channel that supply the sand boil.Brief description of the Figures

[0020] Additional features and advantages will be appreciated from the following detailed description of a preferred embodiment of the invention with reference to the appended Figures which are given merely by way of non-limiting example and in which: Figure 1 is a schematic illustration of a section of soil in the region of a embankment where a sand boil is formed; Figure 2 is a schematic illustration, similar to Figure 1, of the same section of soil which includes civil engineering work incorporating aspects of the present invention for protecting the embankment from erosion as a result of the sand boil; and Figure 3 is a non-limiting example of a structure for protection to be used for carrying out the civil engineering work which is schematically shown in Figure 2. Detailed description

[0021] Now with reference to Figure 1, there is illustrated a section of soil in the region of the embankment A of a river F, the level L of which exceeds the level of the soil T adjacent to the embankment A. The typical conditions in which the formation of a sand boil can occur involve a situation in which there is present under the embankment A a layer B of soil with low permeability, for example, argillaceous soil, under which there is present a permeable layer, for example, a sandy layer, of the aquifer Q which extends under the bed of the river F and which extends out of the embankment A.

[0022] The head of water resulting from the level L of the river F brings about an increase in the hydrostatic pressure on the aquifer Q. Therefore, the water can find an escape route C under the embankment A through, for example, spaces which are excavated by moles or residues of decomposing roots. There is thereby formed a water route C in the aquifer Q which can pass through the low-permeability layer B and flow out into a venting shaft P, thereby forming the sand boil.

[0023] As illustrated in Figure 2, in order to prevent or in any case to slow down the water flow along the channel C and the dragging of sand or soil from the layer of permeable soil Q to the venting shaft P, there is provided a geotechnical structure comprising a barrier 14 which is formed by a semipermeable or draining structure which allows the water to pass, slowing it down, but retaining the granular material (sand, soil) which would otherwise be dragged towards the venting shaft P, bringing about eroding phenomena under the embankment. The barrier 14 therefore allows a release of the water pressure, allowing the water to flow but stopping the transport of material.

[0024] The barrier 14 is preferably constructed with a drainage structure, which is buried in the aquifer Q so as to interrupt the channel C. The barrier 14 extends in a vertical manner so as to involve over a greater part the aquifer Q but also to remain partially covered by the low-permeability layer B in an upper zone 16. The barrier extends in terms of length, skirting around the embankment A over a distance sufficient to prevent the water which supplies the venting shaft P of the sand boil from avoiding the barrier 14, giving rise to other sand boils further upstream or downstream.

[0025] Figure 3 illustrates a non-limiting example of a barrier 14 which provides for a support structure 18 which is elongate in a main length direction L, comprising a framework 20, for example, made of metal net. The metal net can be an electro-welded net or a metal net of the type with rhomboid or hexagonal mesh which is supported by rods or brackets 22 which contribute to its stiffening. The framework 20 defines a containment volume 24 which is filled with permeable filler material, for example, the same sand or soil which is formed by the aquifer layer Q. The framework 20 further supports one or more filtering sheets 26, for example, sheets of nonwoven fabric, which cover, internally or externally, the barrier 14 at least at the bottom and at least one of the sides thereof, more advantageously at both sides.

[0026] In order to protect the embankment A from erosion resulting from the sand boil, there is carried out the identification of the channel C which supplies the venting shaft P. The method then continues with excavation of a trench, for example with a so called restricted section excavation, namely an excavation with a width equal to or less than the height, which intersects with the channel C, cutting it. The length of the trench will be defined on the basis of geological and engineering assessments in order to ensure interception of the flow of water and to prevent subsequent sand boils from being formed upstream or downstream of the venting shaft P to be combatted.

[0027] There is positioned in the trench which is excavated in this manner the barrier structure 14 which can be, for example, of the type illustrated in Figure 3 or a similarly functional type. For example, rapid-response multicellular structures can be advantageously used, such as, for example, the structure commercially designated FlexMac ®< from the same Applicant. The containment volume 24 of the barrier 14 is filled with permeable material, for example, the same material excavated to form the trench. The excavation depth of the trench and the positioning of the barrier 14 are configured and carried out so that, preferably, an upper portion of the barrier 14 affects the low-permeability layer B which thereby performs the function of an upper "cap" of the barrier 14, preventing the water from avoiding it by passing above it in order to continue to supply the sand boil 12. The barrier 14 is then buried so that the water which flows through the channel C can pass through it but without dragging the particulate which, in the absence of any barrier 14, would be dragged towards the venting shaft P. The erosion of the embankment A is thereby interrupted, at the same time affording the possibility for the water pressure to be vented into the channel C with a flow which is slowed down and without transporting any material.

[0028] Naturally, the principle of the invention remaining the same, the forms of embodiment and details of construction may be varied widely without thereby departing from the scope of the invention.

Examples

Embodiment Construction

[0021]Now with reference to Figure 1, there is illustrated a section of soil in the region of the embankment A of a river F, the level L of which exceeds the level of the soil T adjacent to the embankment A. The typical conditions in which the formation of a sand boil can occur involve a situation in which there is present under the embankment A a layer B of soil with low permeability, for example, argillaceous soil, under which there is present a permeable layer, for example, a sandy layer, of the aquifer Q which extends under the bed of the river F and which extends out of the embankment A.

[0022]The head of water resulting from the level L of the river F brings about an increase in the hydrostatic pressure on the aquifer Q. Therefore, the water can find an escape route C under the embankment A through, for example, spaces which are excavated by moles or residues of decomposing roots. There is thereby formed a water route C in the aquifer Q which can pass through the low-permeabili...

Claims

1. A method for protecting an embankment from erosion resulting from the formation of a sand boil, comprising the steps of: - excavating a trench which interrupts a supply channel of a sand boil, - positioning in the trench a barrier structure (14) which has a filtering capacity and which is arranged to allow the water running in the channel to pass and at the same time to retain the particulate which is dragged by the water upstream of the barrier structure (14), wherein the barrier structure (14) extends over a predetermined length in the direction of the embankment (A) and comprises at least one filtering sheet (26) which confers the filtering capacity on the barrier structure (14), the filtering sheet being arranged at least partially transversely with respect to the channel (C) which supplies the sand boil.

2. A method according to claim 1, wherein the positioning of the barrier structure (14) in the trench is carried out in such a manner that an upper portion (16) of the barrier structure (14) is positioned in the region of a layer of soil (B) which has low permeability, a lower portion of the barrier structure (14) being positioned in the region of an underlying aquifer layer(Q), in which there is formed the channel (C) of the sand boil.

3. A method according to claim 2, wherein the upper portion (16) of the barrier structure (14) which is positioned in the layer of soil (B) which has low permeability extends less than the lower portion which is positioned in the aquifer layer (Q).

4. A method according to any one of the preceding claims, comprising the additional step of covering the barrier structure (14) with low-permeability soil (B).

5. A method according to any one of the preceding claims, wherein the barrier structure (14) comprises a support structure (18) which is elongate in a main length direction (L), comprising a metal framework (20) which defines a containment volume (24) which is filled with a permeable filler material (Q), the metal framework (20) supporting the at least one filtering sheet (26), wherein preferably the permeable filler material (Q) is material obtained from the excavation of the trench.

6. A method according to claim 5, wherein the metal framework (20) comprises an electro-welded metal net or a metal net of the type with rhomboid or hexagonal mesh.

7. A method according to claim 6, wherein the metal framework (20) is supported by rods or brackets (22) which contribute to the stiffening thereof.

8. A method according to any one of the claims 5 to 7, wherein the at least one filtering sheet is a sheet of nonwoven fabric which covers, internally or externally, the containment volume (24) of the barrier structure (14) at least at one bottom thereof and at one of the sides of the barrier structure (14) elongated in a main length direction (L) and positioned at least partially transversely with respect to the channel (C) which supplies the sand boil.

9. Geotechnical structure for protecting an embankment from erosion resulting from the formation of a sand boil, comprising: - a barrier structure (14) positioned in a trench which interrupts a supply channel of a sand boil, the barrier structure (14) extending over a predetermined length in the direction of an embankment (A) and comprising at least one filtering sheet (26) which confers a filtering capacity on the barrier structure (14), the filtering sheet (26) being arranged at least partially transversely with respect to a channel (C) which supplies the sand boil, the barrier structure (14) interrupting the channel (C) so as to allow the water running in the channel (C) to pass and at the same time to retain the particulate which is dragged by the water running in the channel (C) upstream of the barrier structure (14).

10. Geotechnical structure according to claim 9, wherein an upper portion (16) of the barrier structure (14) is positioned in the region of a layer of soil which has low permeability and a lower portion of the barrier structure (14) is positioned in the region of an underlying aquifer layer (Q) in which there is formed the channel (C) of the sand boil.

11. Geotechnical structure according to claim 10, wherein the upper portion (16) of the barrier structure (14) which is positioned in the layer of soil which has low permeability extends less than the lower portion which is positioned in the aquifer layer (Q).

12. Geotechnical structure according to any claims 9 to 11, wherein the barrier structure (14) is covered with low-permeability soil (B).

13. Geotechnical structure according to any claims 9 to 12, wherein the barrier structure (14) comprises a support structure (18) which is elongate in a main length direction (L), comprising a metal framework (20) which defines a containment volume (24) which is filled with a permeable filler material (Q), the metal framework (20) supporting the at least one filtering sheet (26), wherein preferably the metal framework (20) comprises an electro-welded metal net or a metal net of the type with rhomboid or hexagonal mesh.

14. Geotechnical structure according to claim 13, wherein the metal framework (20) is supported by rods or brackets (22) which contribute to the stiffening thereof.

15. Geotechnical structure according to claim 13 or claim 14, wherein the at least one filtering sheet is a sheet of nonwoven fabric which covers, internally or externally, the containment volume (24) of the barrier structure (14) at least at one bottom thereof and at one of the sides thereof elongated in the main length direction (L) and arranged at least partially transversely with respect to the channel (C) which supplies the sand boil.

Images