FIELD OF THE INVENTION

The present invention relates to a vibration-based method for densification of an amount of soil, and a system therefor. Unlocking insights from Geo-Data, the present invention further relates to improvements in sustainability and environmental developments: together we create a safe and liveable world.

BACKGROUND ART

A typical soil is about 50% solids (about 45% mineral and 5% organic matter) and 50% voids (or pores). Certain proportions of the soil are occupied by water and gas (fluid). The percentage of water and gas content in soil is considered highly variable. The pore space allows for the infiltration and movement of air and water in soil. Such pore space can be decreased by densification of soil, which is advantageous for gaining storage space and improving the engineering properties of the ground soil.

Methods based on vibration are known for densification of soil. Vibration can be applied to soils externally via dynamic compaction or internally via vibratory probes (vibrocompaction).

Coarse-grained material or sand, is relatively incompressible and although possible, is not amenable to significant densification by simple compression under increased load. These are also categorized as materials with zero or low plasticity. It is normally densified by applying vibration externally or internally via vibratory probes. The void space within the loose granular material in sand is reduced by such densification. The densification of these materials can be performed by exciting the component particles by vibration and overcoming friction at particle contacts, thus permitting local movement and displacement of particles to achieve a denser packing. The natural permeability of coarse-grained sand materials is sufficient for water to displace within the time scale of the applied vibration loading, permitting densification to occur.

In saturated coarse-grained material or sand, which is the typical case, this requires displacement of water or pore fluid that previously occupied the larger void space. Coarse- grained materials, although relatively incompressible, are sufficiently permeable to permit water flow and water pressure dissipation within a comparable time scale to the densification process, thereby permitting local densification of the particle mass by vibration.

Fine-grained clay soils are compressible under load. In such materials with relatively higher plasticity, the density can be increased by applying a constant external loading, most commonly by physical surcharge on the ground surface. The fine-grained clay soil has low permeability, hence the time taken for water to be displaced, to allow consolidation and densification, may be of the order of months or years, defining the period over which the surcharge loading must remain in place. Typically, such fine-grained clay soils are filled with water. The time required for water displacement can be reduced by introducing artificial drains to reduce the distance through the fine-grained soil that water must flow to reach a permeable channel. Boreholes can be installed into the clay in a regular pattern and filled with sand so that water may flow out from the consolidating clay by means of the vertical drains. More recently, prefabricated drains made from synthetic materials have proved faster and simpler to install, typically by direct push into the (loose) soft clay within a mandrel that is subsequently withdrawn to leave the prefabricated drain in place.

To summarize: there are two key factors in densification known from the prior art:

• The approach used to densify the loose particulate material: by vibration for coarsegrained materials (sands), and by increased loading and consolidation in fine-grained plastic materials (clays).

• Permeability: as water must be displaced to permit densification of saturated particulate materials.

SUMMARY OF THE PRESENT INVENTION

Silt is a class of granular material with a particle size between coarser grained sand-size particles, and finer grained clay-size particles. In general, any silty soil includes a portion of either or both sand-size particles and/or clay-size particles. The clay-size particle content is important because if these particles have clay mineralogy, they will cause plasticity in the soil. The magnitude of plasticity depends on the clay mineralogy and the clay content. In some cases, particles in the clay-size range do not have any clay mineralogy, such material may be called “rock flour”. Particles of non-clay mineralogy, even if clay size, do not cause any plasticity. Therefore, plasticity in a silty soil arises from the clay content rather than just the presence of clay-size particles. Silty soils are found widely in nature: large volumes arise from human activity such as mineral processing or maintenance dredging of rivers and harbours. The fine-grain waste products of mineral extraction, called tailings, are typically transported, and deposited by hydraulic means and stored behind retaining structures or dams (tailings storage facilities). Such hydraulically placed mine tailings are generally in a very loose condition. Maintenance dredging of rivers and harbours also generates large volumes of fine-grained material, again often transported, and placed hydraulically for storage behind retention dikes, usually in a loose condition.

The known approaches to densify clay soils (applied surcharge load with artificial drains) or sandy soils (vibration) are generally not effective for silty soils. This is because silt materials have a relatively low compressibility, more similar to sand, and a relatively low permeability, more similar to clay soils. Therefore, external surcharge loading is not effective for densification of loose silty material because it is relatively incompressible (relative to soft clay soil). However, vibration is also not effective to densify silt materials because of the relatively low permeability that impedes drainage and movement of pore fluid (as required to permit particles to achieve a tighter packing).

Permeability as used herein refers to hydraulic conductivity K (m/s), also known as hydraulic permeability. It is a measure of the ease with which a fluid (usually water) can move through the pore space, or fractures network of soils. It depends on the intrinsic permeability k (m ² ) of the material, the degree of saturation, and on the density and viscosity of the fluid.

Attempts have been made to find a method to densify silt efficiently. However, even though there is a high need for such a technology, no successful attempt has been reported. Hence, there exists a high need in art for a method which densifies silt efficiently. The present invention relates to densification of such silt materials.

According to a first aspect of the invention, there is provided a vibration-based method for densification of ground soil. The method preferably comprises the step of a) providing a draining unit, comprising a drain installation, at a depth in the ground soil and providing a vibration unit. In an embodiment, the method further comprises b) determining one or more ground soil locations for applying vibration to the ground soil based on a position of the draining unit. In a preferred embodiment, the method further comprises c) extracting fluid from ground soil using the drain installation while at least during an overlapping period of time applying vibration with the vibration unit to the one or more ground soil locations in order to densify the ground soil.

Advantages of such a method comprise (1) to reduce the total volume of material by densification of the grain packing so as to reduce storage requirements; (2) to create a more stable dense packing of material to prevent possible strength loss or liquefaction phenomena that can result from sudden shearing; and (3) to provide stable and useable land with suitable engineering properties to permit beneficial use of such areas.

Other advantages of the invention will become more apparent in the detailed description of the technical features.

In an embodiment, the determining one or more ground soil locations comprises determining the location of the draining unit and applying the vibration within about 40 meters, preferably within about 30 meters, more preferably within about 20 meters, still more preferably within about 10 meters of the draining unit.

In an embodiment according to the invention, the ground soil has an initial permeability (hydraulic conductivity) K in the range of 10' ⁴ to 10' ⁸ m/s, preferably as measured by the method described in determination of soil permeability coefficient according to ISO 17892- 11 :2019, and wherein the densification of the soil obtains a ground soil having a decreased permeability K’ by at least a factor of 5 relative to the initial permeability K, preferably having a decreased permeability K’ by at least a factor of 10 relative to the initial permeability K, more preferably having a decreased permeability K’ by at least a factor of 50 relative to the initial permeability K.

In the context of the present invention, the ground soil has a permeability (hydraulic conductivity) K in the range 10' ⁴ to 10' ⁸ m/s, preferably measured according to ISO 17892- 11 :2019, Geotechnical investigation and testing - laboratory testing of soil - part 11 : Permeability tests. The ground soil permeability can also be measured by the one of the methods described in Geotechnical Research 7(1): 58-70, or in VARIOUS METHODS OF THE MEASUREMENT OF THE PERMEABILITY COEFFICIENT IN SOILS - POSSIBILITIES AND APPLICATION, electronic journal of polish agricultural universities, 2005, volume 8, issue 2.

In an embodiment, the drain installation comprises at least one drain. In a preferred embodiment, step a) comprises installing the least one drain vertically beneath an upper surface of the ground soil. Preferably, the at least one drain is a vertical drain comprising an elongated body with at least one inlet and at least one outlet for expelling fluid extracted through the drain.

In an embodiment, step a) comprises installing the least one drain horizontally beneath an upper surface of the ground soil. Preferably, the at least one drain is a horizontal drain comprising an elongated body with at least one inlet and at least outlet for expelling fluid flowing into the drain.

In an embodiment, step a) further comprises providing a vacuum pump, the vacuum pump being provided in the draining unit. Preferably, step c) further comprises applying a vacuum to extract fluid from the ground soil.

In an embodiment, the vibration is applied externally on an upper surface of the ground soil. Alternatively, or additionally, the vibration is applied internally from below the upper surface of the ground soil. Preferably, the vibration is applied at the determined one or more soil locations.

In a preferred embodiment, the vibration unit comprises at least one vibratory probe and the vibration is applied by the at least one vibratory probe in the ground soil.

In an embodiment, the step of providing a draining unit a) comprises installing a plurality of vertical drains below an upper surface of the ground soil and wherein the plurality of vertical drains are spaced apart from one another, wherein the spacing between the drains is in a range of 1.5 m to 8 m. Preferably, a first vertical drain is installed substantially parallel to and at a spacing L from a second vertical drain in an X direction parallel to the upper surface of the ground soil, and the first vertical drain is installed substantially parallel to and at a spacing 2L from a third vertical drain in a Y direction parallel to the upper surface of the ground soil, and wherein the X direction is perpendicular to the Y direction.

Preferably, an elongated body of a first horizontal drain may be disposed substantially parallel to an elongated body of a second horizontal drain, at different depths below the upper surface of the ground soil. Alternatively, or additionally, the spacing between the drains is determined based on at least one of permeability of the soil, a drainage speed, and a vacuum pressure applied to the drains.

In a preferred embodiment, the step of providing a draining unit further comprises connecting an outlet of the at least one drain to at least one vacuum pump, configured to extract fluid. In the vibration-based method for densification of ground soil having an upper surface of the ground soil according to the present invention, the soil preferably comprises soil particles and, at least partially, a fluid between them, the particles having an average particle diameter between 0.01 mm and 0.5 mm as measured in all directions in 3D space. The method preferably comprises providing a draining unit comprising a drain installation in the amount of soil below the upper surface of the ground soil, with drainage enhanced by the application of vacuum, determining one or more soil locations for applying vibration to the amount of soil based on a position of the drain installation, and preferably extracting at least part of the fluid from the amount of soil using the drain installation under vacuum while at least during an overlapping time period applying vibration to the one or more soil locations in order to densify the amount of soil. In an embodiment of the invention, the drains are provided at least partially below an upper surface of the ground soil, and not completely below.

According to a second aspect of the invention, there is provided a vibration-based soil densification system for densification of ground soil having an upper surface. In an embodiment, the system comprises at least one draining unit comprising a vacuum pump configured to be installed in the ground soil below the upper surface of the ground soil. It is noted that the vacuum pump may also be installed above the upper surface of the ground soil. In an embodiment, the draining unit and/or the vacuum pump are configured to be installed at least partially below the upper surface of the ground soil.

In an embodiment, the system comprises at least one vibration unit for applying vibration to densify the ground soil. Preferably, the system further comprises a controller configured to control the draining unit with the vacuum pump, and the vibration unit to extract fluid from the ground soil while at least during an overlapping period of time applying a vibration to the ground soil in order to densify the ground soil.

In an embodiment the draining unit comprises a draining installation having at least one drain. Preferably, the controller is further configured to determine whether the drain is arranged to be vertically or horizontally disposed beneath the upper surface of the ground soil, based on a depth ground soil.

In an embodiment, the draining unit comprises at least one vertical drain, the at least one vertical drain comprising an elongated and permeable body and at least one outlet for expelling fluid extracted via the drain. Alternatively, or additionally, the draining unit comprises at least one horizontal drain, the horizontal drain comprising an elongated and permeable body and at least one outlet for expelling fluid extracted via the drain.

In an embodiment, the vibration unit is an external vibration unit configured to apply vibration externally on the upper surface of the ground soil. Alternatively, or additionally, the system comprises an internal vibration unit configured to apply vibration internally from below the upper surface of the ground soil.

In an embodiment, the system further comprises a plurality of drains below the upper surface of the ground soil, and which may be spaced apart from one another, with a spacing between the drains is in a range of 1.5 m to 8 m. Preferably, the plurality of drains comprises at least a first, second and third vertical drains installed vertically below the upper surface of the ground soil, such that the first vertical drain is installed substantially parallel to and at a spacing L from the second vertical drain in an X direction parallel to the soil surface, and the first vertical drain is installed substantially parallel to and at a spacing 2L from the third vertical drain in a Y direction parallel to the upper surface of the ground soil, wherein the X direction is perpendicular to the Y direction.

In an embodiment, the plurality of drains comprises at least a first and a second horizontal drain installed horizontally below the upper surface of the ground soil, and such that an elongated body of a first horizontal drain is disposed substantially parallel to an elongated body of a second horizontal drain, at different depths below the upper surface of the ground soil.

In a preferred embodiment, the controller is further configured to determine a spacing between the drains based on at least one of the permeability of the ground soil, a drainage speed, and a vacuum pressure applied to the drains.

In an embodiment, the at least one draining unit includes at least one vacuum draining unit, preferably comprising a vacuum pump. In a preferred embodiment, the at least one vacuum draining unit comprises: at least one horizontal or vertical drain; at least one vacuum pump. Preferably, the vertical drain has at least one outlet and the at least one outlet is connected to the at least one vacuum pump. In an embodiment, the horizontal drain has at least two outlets which are each connected to one vacuum pump.

Preferably, the vibration-based soil densification system (or apparatus) for densification of ground soil having an upper surface of the ground soil, comprises at least one draining unit comprising at least one drain, such as at least two drains, at least two drains or a plurality of drains. The vibration-based soil densification system may further comprise at least one vacuum pump within the draining unit, in an embodiment of the invention. The draining unit comprising the at least one drain and vacuum pump is configured to be installed in region of soil below the upper surface of the ground soil, or at least partially below the upper surface of the ground soil. The vibration-based soil densification system further comprises at least one vibration unit for applying vibration to densify the amount of soil, and a controller configured to control the draining unit, the applied vacuum and the vibration unit to extract at least part of the fluid from the amount of soil while at least during an overlapping time period to apply vibration to the one or more soil locations in order to densify the amount of soil.

According to a third aspect of the invention, use of a vibration-based soil densification system for densification of ground soil as described herein is disclosed.

BRIEF DESCRIPTION OF DRAWINGS

Figure la shows compressibility of clay soils and how they are densified (by reducing void ratio) through surcharge loading.

Figure lb shows relatively low compressibility, non-plastic granular material such as sand and silt, which has much less reduction in void ratio (densification) due to the same surcharge loading.

Figure 2 shows densification by vibration in coarse-grained materials with no or low plasticity.

Figure 3 illustrates a plurality of vertical drains installed in an amount of soil comprising silt particles, according to an embodiment of the present invention.

Figure 4a illustrates a typical array of vertical drains installed below the upper surface of the ground soil, according to an embodiment of the invention.

Figure 4b shows a schematic of a vibratory probe used for densification of the amount of soil, according to an embodiment of the invention.

Figure 5 is an illustration of a method of installing a vertical drain in the amount of soil, according to an embodiment of the invention. Figure 6 shows an illustration of a plurality of horizontal drains installed in the amount of soil, according to an embodiment of the invention.

Figure 7 shows a top view of vertical drains installed beneath an upper surface of the ground soil, according to an embodiment of the invention.

Figure 8 shows a top view of horizontal drains installed beneath an upper surface of the ground soil, according to an embodiment of the invention.

Figure 9 shows a vibration-based soil densification system, according to an embodiment of the invention.

Figure 10 shows steps of a vibration-based soil densification method, according to an embodiment of the invention.

DETAILED DESCRIPTION

The matters exemplified in this description are provided to assist in a comprehensive understanding of various exemplary embodiments of the present invention disclosed with reference to the accompanying figures. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the exemplary embodiments described herein can be made without departing from the scope of the claimed invention. In particular, combinations of specific features of various aspects and/or embodiments of the invention may be made. An aspect or embodiment of the invention may be further advantageously enhanced by adding a feature that was described in relation to another aspect or embodiment of the invention.

Further, the functionality associated with any particular means may be centralized or distributed, whether locally or remotely. It may be advantageous to set forth that the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation. In addition, reference to an element by the indefinite article "a" or "an" does not exclude the possibility that more than one of the elements is present, unless the context clearly requires that there be one and only one of the elements. The indefinite article "a" or "an" thus usually means "at least one".

Throughout the description, reference is made to terms “sand”, “silt”, and “clay”. Instead of “sand”, the term “coarse-grained” material is also used. Instead of “silt”, the term “fine-grained material is also used. These are used for ease of understanding; however, the skilled person understands these terms in terms of the particle sizes of each of these materials. As mentioned, grain/particle is classified, however not restrictively, as clay if the particle diameter is <0.002 mm, as silt if it is between 0.002 mm and 0.06-0.07 mm, or as sand if it is between 0.06-0.07 mm and 2 mm. The present invention is addressed to silty soils that may also include a possibly significant portion of sand sized, or larger, particles and/or a portion of clay sized particles.

Initial in-situ testing is generally performed to determine the properties and current state of material to be densified. In-situ testing is an efficient means to provide data on the volume of loose material to be densified, the properties and characteristics of the loose material, and of the underlying natural ground (where relevant). For example, cone penetration testing (CPT), a standard geotechnical test, can provide the basic data required. Measurement of pore water pressure (CPTu), including stages with water pressure dissipation tests, provide data on permeability. The measurement of shear and pressure wave velocities using a seismic sensor on the cone (SCPTu) provides additional data on material properties. Recovery of samples for particle size analysis and other index and laboratory testing provides a more complete assessment of the volume, in-situ density and material properties of the finegrained deposit to be densified. All such methods may be applied in the context of the present invention as well.

Figure la shows the compressibility of clay soils, or how the void ratio is reduced by surcharge load, causing densification.

As mentioned, clay soils are relatively impermeable, therefore, the time taken for water/fluid to be displaced in order to allow consolidation and densification, may be of the order months or years. However, clay soils are intrinsically compressible, and therefore the void ratio changes significantly in response to vertical surcharge load. An increase of vertical load can generate considerable settlement and reduction in volume of such soils. When unloaded such soils/materials respond in a relatively stiff manner causing little rebound or volume recovery.

The graph in Figure la defines a void ratio in a certain amount of soil as Y axis, and an effective stress produced in clay soil by loading as the X axis. As the effective stress increases from due to surcharge loading, the void ratio, or the space between particles in the clay soil decreases from a value “o” to a decreased value “al” on a normal consolidation line (NCL). Once unloaded, when the surcharge load is removed, the void ratio increases in response from value “al” to “bl”. However, since the rebound is much less, the surcharge loading was able to densify the soil from an initial void ratio “o” to a final void ratio “bl”. The densified soil may attain a final void ratio, at the in-situ effective stress in the ground, that is close to or below the critical state line (CSL). The value of the required surcharge loading, the induced stress cr^, is determined by the target void ratio, after consolidation, with respect to the CSL. The terms NCL and CSL are well-known to the skilled person and require no further explanation.

Figure lb shows the relatively low compressibility of low-plasticity granular materials such as sand and silt, or how they react less to possible densification by surcharge loading (with only a relatively small change in void ratio caused by surcharge load).

Similar to Figure la, the graph in Figure lb defines the void ratio in a certain amount of low plasticity soil as Y axis, and an effective stress produced by loading as the X axis. Increasing vertical load by surcharge results in little volumetric compression or settlement in such materials. As shown in Figure lb, even though the effective stress increases from ' _Vg to due to surcharge loading, the void ratio, or the space between materials in the soil is barely reduced. The void ratio decreases minimally from a value “o” to a value “a2” on a normal consolidation line (NCL1). The void ratio remains almost the same when the surcharge is removed, and the material in-situ returns to the previous in-situ vertical effective stress. The value “b2” represents the value of void ratio once the load is removed and will generally remain significantly above the critical state line (CSL). The graphs in Figure 1 are a good manner to understand the relative ineffectiveness of densification by surcharge loading to loose granular materials like sand and silt.

Figure 2 shows densification by vibration in coarse-grained granular materials with low or no plasticity. Figure 2, again, shows a curved NCL1 and a curved CSL, like Figures lb. The zig-zag curve 200 indicates a changing value of effective stress in the soil due the applied vibration, due to the generation of excess water pressure causing a reduction in effective stresses, and the simultaneous dissipation of water pressure causing an increase in effective stress and reduction in void ratio. In coarse-grained granular materials drainage and reduction in excess water pressure caused by vibration will occur naturally so that the void ratio can be reduced from the initial condition “o” on the NCL1 line, to a value “c” typically below (denser than) the CSL line. It is widely known that a vibration process as applied to coarse-grained granular material does not result in an acceptable level of densification when it is applied to finegrained soils. It is also known that a densification method as used to densify soils with high plasticity, like clay, in which a vertical load is applied, possible with some extra water drainage by artificial drains, also does not work well when applied to low (or zero) plasticity, fine-grained soils.

The inventors of the present invention have found however that a combination of vibration and water drainage provides a high level of densification in an acceptable timeframe. The explanation is as follows.

Densification of fine-grained granular materials having low plasticity requires repeated shearing to cause local movement between particles that can find a denser packing. This can be achieved by local vibration that provides enough excitation to cause particle movement, overcoming friction at particle contacts. However, densification of saturated material requires expulsion of pore fluid as the void space between particles is reduced. As mentioned previously, in sand this occurs naturally because of the relatively high permeability. To achieve a similar reduction of pore space in silt within the time scale of applied vibration, such fine-grained silty soils require assistance to drain fluid therefrom.

Suitable drainage can be achieved using prefabricated drains installed in the soil. The rate of drainage may be further enhanced by increasing the hydraulic gradient in the soil by means of applying vacuum pressure to the drain. The type of drains to be installed is determined based on the depth of the soil body and/or on the volume of loose silt material to be densified.

The required dimensions, properties and spacing of the drains may be determined from in-situ measured material properties and permeability characteristics of the loose material to be densified, as determined by testing.

Drains can be installed vertically into the soil to be densified, or horizontally below the upper surface of the soil. Vertical drains are suitable for any soil depth and may typically be installed to a depth of 10 m to 25 m below the upper surface of the ground soil. Horizontal drains are more suited to applications where the depth of the loose silty soil to be densified is of the order 10 m. Drains are installed horizontally within the soil at a desired vertical and horizontal spacing from one another, about 1.5 m to 5 m. Prefabricated drains are disposable and are typically left in the soil after densification. Figure 3 illustrates a plurality of vertical drains 301a, 301b,..301d installed in soil containing a significant amount of silt sized particles, according to an embodiment of the present invention. Here, the term “vertical” is used in the sense that the direction of insertion of the drains 301a, 301b,.., 301d into the soil has a vertical component. It does not need to be exactly perpendicular to the upper surface of the soil.

Drains 301a, 301b,..301d are installed vertically downwards from an upper surface of the ground soil. Each vertical drain 301a, 301b... comprises an elongated body 302a, 302b.., into which pore fluid can flow and with a permeable core allowing the pore fluid to travel up to at least one outlet to the drain 303a, 303b. . . The length of the elongated body is shown in Figure 3 by the arrows on the left side of the elongated bodies 302a and 302b. The drains 301a, 301b,. . ., 301d reduce the distance through the fine-grained soil that water/fluid must flow to reach a permeable channel. The fluid enters the drains laterally and is propelled upwards towards the outlet of the drain, from which it is expelled. In a preferred embodiment, the application of vacuum pressure by at least one vacuum pump 304 to the drain greatly enhances the hydraulic gradient and hence the flow of fluid through the drainage system.

The drains may be implemented as cylindrical pipes 301a, 301b,.., 301d, e.g., made of perforated plastic or a geosynthetic liner filled by sand, with a diameter typically between 50 mm and 150 mm. In current practice, artificial drains are used that typically comprise a geosynthetic sleeve with a synthetic core through which water can flow axially in the drain to the outlet. The selected drain dimensions and geotextile material properties depend on the test results on the soil referred to above. The diameters need not be equal along the entire length of the drains 301a, 301b,.., 301d. Consecutive draining actions may be performed with different drain dimensions. For example, it is sometimes efficient to complete densification in two main phases on very loose and weak deposits. The first phase has a focus on shallower material to a depth of several meters and may use lighter probes and lighter equipment. The second main phase targets the deeper soil layers to densify the loose material.

Each outlet 303a, 303b. . . . may further be connected to at least one vacuum pump 304 to enhance the rate of drainage. The use of one or more vacuum pumps 304 with the drains 301a, 301b,.., 301d serves three main purposes, (a) The applied vacuum increases the hydraulic gradient thereby increasing the flow of water to the drains 301a, 301b,.., 301d from the soil being densified, (b) The pore fluid removed from the ground during densification is piped away for controlled disposal in an environmentally satisfactory manner, (c) The effective (inter-particle) stresses in the ground are increased by the applied vacuum pressure which, near the ground surface, greatly enhances surface bearing capacity for site access.

Figure 4a illustrates a typical a 3D view of array of vertical drains 301a, 301b,.., 301d, 301e, 30 If,.., 301h installed below the upper surface of the ground soil, according to an embodiment of the invention. Axes X and Y lie perpendicular to each other, but parallel to the upper surface of the ground soil. Axis Z is perpendicular to both X and Y lies in a vertical/perpendicular direction towards the soil body (the body of ground soil below the upper surface of the ground soil).

Figure 4a shows a series of vertical drains 301a, 301b,.., 301d, 301e, 301f, .., 301h arranged below the upper surface of the ground soil in adjacent substantially parallel rows to one another. For example, vertical drains 301a, 301b, 301c and 301d form a first row of drains. Vertical drains 301e, 30 If, 301g and 301h form a second row of drains. Drains 301e, 30 If, 301g and 301h are disposed at a horizontal offset to drains 301a, 301b, 301c, 301d, respectively. The drains are installed at suitable horizontal offsets which can be provided to a desired spacing between the drains in a certain direction.

In other words, according to the embodiment of Figure 4a (the feature is not limited to this embodiment, but applies to all embodiments of the invention), a first vertical drain 301a is installed substantially parallel to a second vertical drain 301b in the X direction parallel to the upper surface of the ground soil (along the X axis) and a third vertical drain 301e in the Y direction parallel to the upper surface of the ground soil (along the Y axis). As a result of the mentioned offset, in an embodiment, a first vertical drain 301a is disposed at a spacing L from the second vertical drain 301b and at a spacing of approximately 2L from the third vertical drain 30 le.

The spacing between the vertical drains (or horizontal drains, which are described below) is determined based on at least one of permeability of the soil, a drainage speed, and a vacuum pressure applied to the drains.

As shown in Figure 4a, the vertical drain 301a may include an electrically conductive material 410a, disposed fully or partially on its elongated body. The vertical drain 301e may also include an electrically conductive material 410e. There is no limitation to the nature of the electrically conductive material. Voltage differences in the range 25-100 V DC, can be applied to the electrically conductive materials 410a and 410e of adjacent drains. They then act as electrodes (anodes/cathodes), and with a certain electric potential difference between such adjacent drains, these electrodes direct the water molecules flow to the respective electrode (drain). The induced electrical gradient enhances the permeability of the soil materials and the flow of pore fluid through such materials, especially those fine-grained, in a cost-effective manner.

Although the feature relating to application of voltage is shown using the example of a vertical drain in figure 4a, the skilled person understands that it may also be combined with any embodiment relating to a horizontal drain.

As mentioned, with the pore fluid sufficiently drained, application of vibrations offers an effective manner of densification of soil comprising silt. Vibrations can be applied externally from above the upper surface of the ground soil, e.g. via surcharge loading or by impacting the upper surface of the ground soil using e.g. a mechanical load, or a surface vibrator, or internally from below the upper surface of the ground soil, via vibratory probes. Before applying vibrations, one or more soil locations for applying vibration are determined. The soil locations may lie at one or more locations on the upper surface of the ground soil, at the same depth in the soil body or different depths in the soil body. Such determination may be performed by a manual or an apparatus controller based on results of the earlier mentioned tests.

Figure 4b shows a basic schematic of a vibratory probe 404 used for densification of the amount of soil according to an embodiment of the invention.

At least one vibratory probe 404 may be used for the purpose of densification of the soil. Probing locations of the at least one vibratory probe 404 in the amount of soil are determined based on the position of a drain beneath the upper surface of the ground soil.

Each vibratory probe 404 comprises a circular or elliptical body 405 with a curved cross section, as shown in Figure 4b. Other shapes and forms are not excluded for the purpose of this application. The vibration of the probes 404 may be generated internally, within the probe, or applied externally at the top of the probe 404. The vibration may include combinations of torsional, vertical and horizontal components of movement.

At or near an end portion of the vibratory probe 404 is the vibratory part 407 which vibrates as the probe 404 penetrates the upper surface of the ground soil into the soil body. Fluid pressure generated by vibration reduces the vertical effort required for the probe to penetrate the ground or for subsequent extraction. The vibratory probe 404 may penetrate the soil and be drawn upwards. This process may be repeated multiple times for optimal densification. A total of 2 to 5 repetitions of vibration treatment at any location may be used, depending on the target densification to be achieved and to ensure a uniform settlement across the soil site.

For fine grained silty materials, a relatively high frequency of vibration is often effective. The frequency of vibration may be >10 Hz, e.g., range from 10-50 Hz, 50-100 Hz or even up to about 150 Hz.

The vibratory probe 404 may further comprise a plurality of vanes 406a, . . ., 406d, which form plates extending from the body 405 of the probe equipment. Though shown to be positioned at the end portion near the vibratory part 407, vanes 406a, . . . , 406d may be situated at any portion of the body of the probe 404 as long as they can contact an amount of soil during penetration motion of the probe. The vanes maximize a contact area between the probe and the soil. The vanes may be perforated to further enhance contact between the vibrating probe 404 and the material to be densified. Any number of protruding vanes may be used, although two or four would be a typical number.

A plurality of probes 404 may be operated simultaneously at various locations in the amount of soil. The spacing between multiple probes 404 may be determined in proportion to the spacing between the drains and/or their layout. The vibration may be caused by torsional, vertical and horizontal vibratory displacement of the probe, or any combination of these. The depth of the probe can be adjusted as densification proceeds, and cycles of densification with the probe at different soil depths (Z) may be used.

According to an embodiment, in a multiple probe configuration, the phase of vibration between probes may be adjusted locally to displace the soil in a manner that amplifies the excitation and resulting densification of the soil.

The probe 404 may also include a vacuum drainage system to remove excess water from material immediately adjacent to the probe, enhancing the transmission of vibration from the probe 404 to the surrounding loose material. When the probe 404 also permits drainage of fluid from the amount of soil, such drainage flow may be switched off intermittently. This increases fluid pressure adjacent to the probe 404 and reduces the force needed to move the probe 404 downward or upward. Further, the probes may be equipped with waterjets to overcome locally excessive resistance to penetration or to assist extraction in possibly hard soil layers or in well densified material.

According to an embodiment, like the vertical drains 301a, 301b, . . . the probe 404 may also be provided with an electrically conductive material (not shown). With the probe 404 in an inserted position, a voltage may be applied to this electrically conductive material, to create an electrical gradient, e.g., to an adjacent drain. The skilled person understands that voltages may be applied in said manner also in any embodiment relating to a horizontal drain. Such voltages may also be in the range of the range 25-100 V DC, more preferably, 50-100 V DC.

One or more of the drains 301a, 301b, . . . may be made of a geosynthetic material. Prefabricated drains made from synthetic textile materials have proved to be faster and simpler to install, typically by direct push into the amount of soil using a mandrel. Each drain 301a, 301b, . . . may have a cover and a core (not shown). The material specifications of the geotextile cover and core are selected based on a type of soil to be drained (to prevent clogging) and the maximum volume flow of water/fluid through the drain. It may thus depend on soil permeability and an estimated volume of water/fluid to be removed.

Typically, the drains 301a, 301b, ... are flat rolled, with dimensions typically of the order 100-200 mm width and 3 to 5 mm thick, before they are installed in the amount of soil.

Figure 5 is an illustration of a method of installing a vertical drain in the amount of soil, according to an embodiment of the invention.

The prefabricated drain is normally stored in rolled form and the drain 301a is positioned in a metal guide or mandrel 502. The mandrel containing the prefabricated drain is then inserted into the amount of soil to a predetermined depth Z m in the soil body. In other words, vertical drain 301a is pushed vertically downwards into the amount of soil by mandrel 502. Mandrel 502 is subsequently extracted from the amount of soil while leaving in place the installed drain extending to a depth in the soil Z m.

A mandrel 502 is a metal guide that contains the unrolled drain. It can be cylindrical or rectangular, depending on the shape of the rolled drain or other factors. The drain may be held in place at the bottom of the mandrel by attachment to a false end piece, or by being wrapped around a bar across the end of the mandrel. During insertion the mandrel 502 remains attached to the drain 301a. After insertion, the mandrel 502 is withdrawn and naturally leaves the drain 301a in place with the false end piece or bar.

In case of a horizontal drain, the prefabricated drains are installed horizontally below the upper surface of the ground soil.

As described above, a drain may be vertical or horizontal in configuration.

Figure 6 shows an illustration of a plurality of horizontal drains installed in the amount of soil, according to an embodiment of the invention.

Figure 6 shows horizontal drains 601a, 601b and 601c installed beneath the upper surface of the ground soil. Each horizontal drain, 601a, 601b and 601c, comprises an elongated body 602a, 602b and 602c that is permeable and permits the inflow of pore fluid (usually water). In the embodiment shown, drains 601a, 601b and 601c comprise outlet pairs 6031a-6032a, 6031b-6032b and 6031c-6032c, respectively, where pore fluid can be dispersed. Each horizontal drain 601a, 601b, 601c may further have an elongated horizontal portion which at both ends rises to the ground surface where the outlets 6031a and 603 lb are situated. The use of outlets at each end of the horizontal drain results in faster extraction of pore fluid which can flow in the direction toward the closest outlet. Each outlet 6031a, 603 lb and 6031c may be coupled to a vacuum pump 604A, and each outlet 6032a, 6032b and 6032c may be coupled to a vacuum pump 604B.

Although not shown, the skilled person understands that a single outlet and pump configuration is also possible. The technical effects of using vacuum extraction are described above. Moreover, although three horizontal drains 601a, 601b, 601 c are shown, any other number of drains in any desired configuration may be applied, including at different depths inside the soil.

As shown in figure 6, elongated body 602a of first horizontal drain 601a is disposed substantially parallel to elongated body 602b of a second horizontal drain 601b, at different depths (Z) below the upper surface of the ground soil.

As indicated, before applying vibrations, one or more soil locations for applying vibration are determined. These locations may be determined based on the positions of the drains and/or their layout. Figure 7 shows a top view of vertical drains installed beneath an upper surface of the ground soil, according to an embodiment of the invention.

The figure is a top view representation. Axes X and Y lie perpendicular to each other, but parallel to the upper surface of the ground soil. Axis Z is not shown.

Entities 301a, 301b, 301e, 30 If . . . . represent vertical drains installed below the upper surface of the ground soil. The outlets of these drains may be connected to a vacuum pump 304 positioned on the upper surface of the ground soil for fluid extraction as described above.

In case of multiple probe locations, these locations may be chosen to lie around the elongated body (not shown) of a vertical drain e.g., 30 If at a predetermined depth in the soil body. At any certain depth Z m below the soil surface, the probe locations may lie equidistant from a centre point on the median plane of the elongated body. The points may lie on a transverse plane of the elongated body.

Seen from above as in Figure 7, these points may be considered to be arranged in a circular distribution around vertical drain 30 If. In the case of vertical drains disposed at end rows e.g., 301b, the points may be considered to be arranged in a semi-circular or circular distribution around vertical drain 301b.

According to an embodiment, if vertical drains 00, 01, 10 and 11 are positioned in a 2x2 array, with drains 00, 01 in row 0, 10 and 11 in row 1 without a horizontal offset between a drain 00 and 10, then a vibration point may simply lie at the centre point between the drains 00 and 10.

Figure 8 shows a top view of horizontal drains installed beneath an upper surface of the ground soil, according to an embodiment of the invention.

Entities 601a, 60 Id, 60 le, 60 If represent horizontal drains installed below the upper surface of the ground soil. These drains are shown installed parallel to each other along a Y direction parallel to the upper surface of the ground soil. The drains may be installed at the same depth Z m below the upper surface of the ground soil, with their outlet pairs 6031a- 6032a, 6031d-6032d, 6031e-6032e and 6031f-6032f connected to vacuum pumps 604A and 604B. A first plurality of probing locations 802a..802n are disposed between an elongated body 602a of horizontal drain 601a and an elongated body 602d of horizontal drain 60 Id. Similarly, a second plurality of probing locations may be disposed between an elongated body of horizontal drain 60 Id and an elongated body of horizontal drain 60 le, and so on. Figure 9 shows a vibration-based soil densification system 900, according to an embodiment of the invention. For sake of conciseness, individual features described above shall not be further explained.

The system 900 comprises at least one draining unit 901. The at least one draining unit 901 may comprise a fluid extractor 906 (a horizontal or vertical drains as described above), or a combination of the fluid extractor (drain installation) 906 and a vacuum pump 905. The system further comprises at least one vibration unit 902 for applying vibration to densify the amount of soil. As indicated, the vibration unit 902 may be an external vibration unit 902 which applies vibration to the upper surface of the ground soil or an internal vibration unit 902 which applies vibration below the upper surface of the ground soil. A controller 903 controls the draining unit 901 and the vibration unit 902 to extract at least part of the fluid from the amount of soil, often enhanced by the application of vacuum pressure, while at least during an overlapping time period to apply vibration to the one or more soil locations in order to densify the amount of soil. The controller 903 may be further configured to determine whether the fluid extractor should be arranged to be vertically or horizontally disposed beneath the upper surface of the ground soil in dependence on results of test measurements performed on the soil.

The system 900 may further comprise a voltage generator 904 connected to the controller 903, so that the controller 903 controls the voltage generator 904 to provide a voltage to a first electrically conductive material on a first drain and a second electrically conductive material on a second drain and/or a vibratory unit 902.

Figure 10 shows steps of a vibration-based soil densification method, according to an embodiment of the invention. For sake of conciseness, individual features described above shall not be further explained.

Step 1001 comprises providing a drain installation in an amount of soil with finegrained material below the upper surface of the ground soil, as well as vibration unit 902. The drain installation may be installed in at least one of a vertical or horizontal direction. Step 1002 comprises determining one or more ground soil locations for applying vibration to the amount of soil. This may be determined based on the position and/or type of drain installation. Step 1003 includes extracting at least part of fluid from the amount of soil using the drain installation, preferably enhanced by applied vacuum pressure, while at least during an overlapping time period applying vibration to the one or more soil locations in order to densify the amount of soil.

While the invention has been particularly shown and described with reference to certain exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and detail may be made therein without departing from the scope of the present invention as defined by the appended claims and equivalents thereof.