EP3445915A1

EP3445915A1 - Method and kit for mitigating the risk of liquefaction of ground to be consolidated

Info

Publication number: EP3445915A1
Application number: EP17727696.1A
Authority: EP
Inventors: Marco Occhi; Andrea Occhi; Daniele Gualerzi
Original assignee: GEOSEC Srl
Current assignee: GEOSEC Srl
Priority date: 2016-04-18
Filing date: 2017-04-18
Publication date: 2019-02-27
Anticipated expiration: 2037-04-18
Also published as: ITUA20162691A1; PT3445915T; EP3445915B1; WO2017182939A1

Abstract

A method and associated injection kit for mitigating the risk of liquefaction of ground T to be consolidated having an upper interface surface S, which provides for the production of at least one hole (2) in the ground T to be reached or towards volumes of the ground T, injecting an expanding resin (3) into the at least one hole (2) by means of the injection device (3), activating the expanding resin (3), conglomerating the ground T by means of the expanding resin (3), connecting portions of the ground T to each other, the expanding (resin 3) having an expansion relationship R between a final free volume Vfin and an initial free volume Vin less than (5).

Description

Method and kit for mitigating the risk of liquefaction of ground to be consolidated

Technical field

The present invention relates to a method for mitigating the risk of liquefaction of ground which includes the production of at least one hole in the ground and the injection of an expanding resin into the hole and an associated kit which operates according to that method.

Technological background

There is known in groundworks engineering the phenomenon of liquefaction of saturated granular materials, and in the strictest sense sands and sandy round, which during the application of dynamic loads and cycles under non-drained conditions exhibit great variations in the tensorial behaviour thereof.

As a result of a liquefaction process, a piece of sandy non-draining ground can be subjected to a progressive reduction of the shearing resistance as far as values close to zero, under which condition the ground tends to behave in the manner of a viscous fluid and no longer like a piece of granular ground. More specifically, studies and knowledge acquired in recent years have shown that the occurrence of phenomena of liquefaction is connected with the combination of "predisposing" factors (that is to say, connected with the non- draining nature of the ground being analysed) and "trigger" factors (that is to say, an earthquake acting on the above-mentioned ground) : in the absence of one of these two factors, the phenomenon of liquefaction is not produced.

Unfortunately, those conditions necessary for the production of the phenomenon of liquefaction of ground have been recorded and assessed often in the last millennium (1117-2015) in many parts of the world (see Paolo Galli, "New empirical relationships between magnitude and distance for liquefaction", in Tectonophysics, vol . 324, 2000).

It is known that liquefaction occurs most often in sandy deposits of the type known as "sands - clean sand to silty sand" and/or "sand mixtures - silty sand to sandy silt", that is to say, deposits which are characterised by granulometry which is tendentially homogeneous and stratified and the thickness of which may also be in the order of metres, under conditions of ground which is normally consolidated and saturated, at times with coverings which are above ground which is not, however, liquefiable. These deposits are readily arranged in the vicinity of fluvial deposits, beaches and other areas which are also flat and have accumulations of sand in the presence of very superficial phreatic layers. The phenomenon under discussion occurs when pieces of ground such as the ones described above are subjected to a seismic event which causes the pressures of the interstitial water contained between the granules of the ground to be increased as far as equalising the tensions above so as to cancel out the shearing resistance levels of the ground (Terzaghi, K., "Theoretical Soil Mechanics", John Wiley and Sons, New York, 1943) triggering a true and correct phenomenon of transition from the solid granular state to the liquid state.

The more recent models evaluated to describe the system of the sandy ground which may tend towards liquefaction provide for the granular systems which are saturated with interstitial water and in which the resistance to applied loads is given by interactions of the "friction" type between the granules (that is to say, friction resistance between the various granules dependent only on the mean contact pressure) and where applicable by "adhesive" type interactions (not dependent on the mean pressure acting and in relation to the types and quantities of bonds present inside the material). Inside this specific type of materials, when an earthquake occurs there are produced distortions of the spatial arrangements of the granules, producing an increase of the interstitial pressure of the liquid between the voids and a consequent loss of contact between the grains with a reduction of the interaction of the friction type and the shearing resistance of the system .

In this step, during the application of the earthquake, a portion of the excess of the interstitial pressures is dissipated by means of fractures of the ground with resultant jets of water and superficial sand which are often clearly visible and detectable. Furthermore, after the earthquake has stopped, it is possible for additional phenomena to develop involving the dissipation of the excess interstitial pressure producing a recovery of the occurrences of contact between granules with a reciprocal re-densification of the granular particles which bring about topographical macroscopic variations such as, for example, oscillations, occurrences of sinking, overturning and/or lateral movement (lateral spreading) of the ground, up to the collapse of the structure itself. If there is a building constructed on those pieces of ground, naturally the building will also be subjected to the same destructive processes undergone by the ground below with results which may in some cases go so far as to be catastrophic.

The equilibrium between friction force and interstitial pressure can change differently over time both as a result of natural actions (for example, variations of the conditions of a layer present in the ground of interest, mechanical actions of radical plant equipment, climate variations, etc.) and as a result of anthropic actions (for example, carrying out excavations in pieces of ground adjacent to the constructions, vibrations, loss of fluid in the ground, etc.). All these actions can increase or give rise to, for example, occurrences of subsidence and/or of subsequent structural collapse of the constructions erected above the ground mentioned, produce physical depressions in the ground and the structures in contact therewith, such as, for example, vertical structures (walls) or horizontal structures (pavements) and which can also occur in ground with momentary and/or periodic good mechanical properties. These phenomena of instability are technically defined as differential settlements and occur partially and locally in pieces of ground positioned below a foundation of a building leading the foundation to subsequently collapse and fail with subsequent lowering.

A method for reducing the liquefaction of pieces of foundation ground is described in WO2004/044335.

In the context of the same sector, there are further known techniques for increasing the load-bearing capacity of the ground on which building structures are erected. These techniques include ones which are based on the concept of pursuing only an improvement of the load-bearing capacity of the foundation ground by means of injections of cement-based products or chemical formulations, including expanding ones, such as, for example, injections of cement at high pressure (jet grouting) or injections of polyurethane foams and the like.

Among the documentation in this technological sector, the document EP0851064 relates to injections of chemical products which expand rapidly, such as polyurethane foams which react by substantially increasing the initial volume thereof and which harden in the ground until hardened columns of ground mixed with the resin are produced, at injection locations which are pre- established and arranged in accordance with a grid-like distribution which is predefined and three-dimensionally regular.

That technique has some disadvantages which are correlated with the fact of using resins which expand up to a final volume which is also equal to twenty times the initial volume thereof. Consistently with the teachings of this document, in fact, there is encountered the risk of excessively expanding the structure of the ground, fracturing it internally and producing a weak conglomeration of the granular components.

Subsequently, as proof of this, this document makes provision for the injected resin to be so expanding as to involve the raising of the ground itself and the building structures or the bonding structures above.

Fundamentally, it may be noted that in the above-mentioned methodology for consolidation, the lifting of the ground being processed and consequently of the building structures connected thereto is a necessary and essential prerequisite in order to evaluate the occurrence of consolidation of the ground (typically this above-ground evaluation of a geometric variation is carried out by means of laser levels and lifting indicators which are positioned so as to be fixed to the walls or to the pavements above the ground which is subjected to the injection operation).

Therefore, the above-mentioned document establishes only in an indirect manner the achievement of the presumed consolidation by means of a criterion which correlates the occurrence of lifting of the structures erected above (similarly to how it is also carried out in the patents EP0941388 and EP1314824) with the increase of the load-bearing capacity of the ground.

Another consolidation method is described in DE3332256, in which there is provision for destruction of solidified ground by means of injection of resin. In this case, therefore, the resin also produces a high-pressure effect on the ground.

The Applicant has established in practice that, by modifying the hydraulic conditions of the pieces of ground, as a result of a high level of mechanical compression which is really obtained with the great expansion of resin which is injected with free diffusion in the ground, it is possible to obtain new concentrations and different distributions of water in the same pieces of ground processed, even after a few kilograms of expanding products have been injected. Consequently, by applying the known teachings, it is possible to obtain, even rapidly, a false increase of load-bearing capacity of the ground, which may be encountered outside the ground with the misleading lifting, sometimes instantaneous lifting (precisely as in the case of sands) of the structures erected thereabove. In fact, it is known that an expanding resin, even more so if with a rapid and voluminous expansion, even for a few kilograms which are injected in the ground, will be by its nature extremely invasive (precisely because it is configured to obtain lifting actions) including up to a production of a temporary shock effect and so as to trigger a misleading idea of lifting often only for the duration of the expansion time. The effect of this is that, once the chemical reaction is complete, it will constrain the grains of sand (if not sufficiently aggregated and consolidated) to carry out a new, inevitable movement (this time, conversely, a falling action instead), referred to as a re-settlement movement in search of a new state of equilibrium .

Another document relating to injections of admixtures based on cement is the Japanese patent application JPH0754357. The technique described therein allows a reduction of the void ratio of granular ground by means of injections of cement-based admixtures which cause the surrounding ground which is compacted to move out of position with a resultant increase in the density, the resistance to liquefaction, the rigidity and reduction of the permeability. Typically, there is provision for a mortar grout to be pumped in conjunction with damping elements, including in an upward direction, by means of steel tubes which are driven or drilled into the ground in accordance with a grid with dimensions from 1.5 to 3.0 m. Given that the volume of mortar grout introduced may vary by from 3 to 20% of the volume of ground processed, this process is very invasive and sometimes destructive as a result of great occurrences of lifting of the pieces of ground, including in the order of several tens of centimetres, which reveal resultant limitations of applicability especially in ground which is already built on.

Furthermore, another disadvantage in relation to the present method is that the pressures necessary for producing an injection of these materials in the ground are very high (up to 3.5 MPa), requiring suitable, dimensioned and expensive pieces of equipment in addition to substantial risks of damage to the structures erected above and the technological installations being used. Not least, the cement grouts unfortunately require many days to be able to mature and therefore solidify, which process leads to a substantial increase in the weight of the final volume of the ground being processed. Finally, as known, injections of cement cannot be monitored in a reliable manner by geological control systems during the work itself as a result of the composition type thereof which is very similar to the ground being processed, and therefore can be distinguished only with difficulty.

Statement of invention

The technical problem addressed by the present invention is to provide a method and a kit for mitigating the risk of liquefaction of ground which are structurally and functionally configured to overcome, at least partially, one or more of the limitations set out above with reference to the prior art cited.

In the context of this problem, a main object of the invention is to provide a method and a kit for mitigating the risk of liquefaction of ground by producing a consolidation of the ground itself by means of expanding resin.

This problem is solved and this object is achieved by the present invention by means of a method and a kit produced according to the appended claims.

The method and the kit according to the present invention advantageously allow the production of a mitigation of the risk of the phenomena of liquefaction in varied types of ground, both built-up and free from building structures.

Another advantage is that the method according to the present invention can be readily and effectively monitored.

Brief description of the drawings

The features and advantages of the invention will be better appreciated from the detailed description of a preferred embodiment thereof, which is illustrated in an exemplary and non-limiting manner with reference to the appended drawings, in which :

• Figure 1 is a schematic illustration of a section of a piece of ground to be consolidated and an injection kit which can be associated therewith, • Figure 2 is a schematic illustration of the flow chart of the claimed method.

Detailed description of a preferred embodiment of the invention

With reference to Figure 1, the method for mitigating the risk of liquefaction of ground according to the present invention can be carried out by using an injection kit which is schematically illustrated therein.

Preferably, the injection kit comprises an injection device 1 including supply tubes 4.

In a preferred embodiment, the supply tubes 4 can be inserted in at least one hole 2 which is formed in the ground T to be consolidated.

Furthermore, the injection device 1 is capable of injecting an expanding resin 3 into the at least one hole 2 produced in the ground T.

A person skilled in the art will be capable of identifying, modifying and adapting various types of tubes which are known in the same sector of the technique in order to optimize the connection to the injection device 1, the supply of the expanding resin 3 and reaching the portions of ground T involved in the process for mitigating the risk of liquefaction.

In general, the injection device 1 comprises at least one tank 5, for containing the components of the resin to be injected, a pumping device 7 and at least one pipe 8 which can be inserted inside the hole 2 for reaching the zone involved in the injection.

In a preferred embodiment, the expanding resin 3 has an expansion relationship R between a final volume Vfin and an initial volume Vin less than 5. Even more preferably, the expanding resin 3 has an expansion relationship R between a final free volume Vfin and an initial free volume Vin less than 5. In an embodiment, the final free volume Vfin and the initial free volume Vin relative to the expansion of the expanding resin 3 are calculated from final and initial density values (for the same mass) measured by means of ISO 845 (defined in regulations as "apparent density"). The above-mentioned values of the final density and initial density and the relevant final free volume value Vfin and initial free volume value Vin obtained are calculated under conditions for which each sample is produced in a form which can be readily dimensioned by means of cutting, the cutting does not involve deformations of the internal cellular structure of the material being examined, the dimension of the sample is as broad as possible and with a total volume of the original sample of at least 100 cm³, the reaction is carried out at a pressure of 101325 Pa (760 mmHg) and an atmosphere having 23±2°C, 50± 10% relative humidity in accordance with ISO 291.

Preferably, the calculation in relation to the final free volume Vfin and initial free volume Vin is carried out by means of ISO 845 or similar standards of a national or international type which can readily be identified by the person skilled in the art. It will be commonplace for the person skilled in the art to calculate, from those values obtained in accordance with regulations, the value of the relationship R between the final free volume Vfin and the initial free volume Vin of the expanding resin 3. It is important to note that these measures are carried out under conditions of free expansion of the expanding resin 3 and in a manner substantially without any application of additional pressures or pressures different from the one provided for under standard conditions. This type of expanding resin 3 has inside the ground T to be consolidated characteristics of permeation which are advantageously slow and with an initial triggering time of the chemical reaction which is at least greater than 15 seconds, technically defined as "pot life" and which can be measured in accordance with the criteria of the pr EN 14319-1 : 2007 Annex D "Determination of the reaction profile and free rise density". It is advantageous to consider that the process of liquefaction occurs mainly in systems of pieces of granular ground which are saturated with interstitial water in which the resistance to loads applied is given by interactions of the "friction" type between the granules (which is equivalent to the friction resistance between the various granules and is dependent only on the mean contact pressure) and optionally by interactions of the "adhesive" type (not dependent on the acting mean pressure in relation to the types and quantities of bonds present inside the material) : in this case, using resins with expansion limited to a factor of five of the final free volume with respect to the initial free volume involves the use of resins which increase the conglomeration between the granules not only by means of the increase of interactions of the friction type between the expanding granules but also by improving the heterogeneous interactions of the adhesive type between granules and expanding resin forming a macro-system which is solidly interconnected and capable of better counteracting the deformations induced in the system itself by the increase of the interstitial pressure during the occurrence of seismic phenomena.

By way of non-limiting example, types of expanding resins 3, such as polyurethane resins, are capable of being used in an injection device 1 according to the claims of the present invention.

In another preferred embodiment, the expanding polyurethane resins used are of the closed-cell type. Preferably, the polyurethane resin has a content of closed cells greater than 50% of the total volume.

In another preferred embodiment, the closed-cell polyurethane resins have an expansion relationship R between 1 and 4.8.

This combination of intrinsic characteristics allows the resin to be optimized for the application by means of the injection device 1 in the ground T to be consolidated and affords the advantages of excellent mechanical characteristics of resistance to traction and compression (in some cases, also greater than 3 MPa) and characteristics of shear resistance from 1.5 MPa and a shear modulus from 15 MPa once completely reacted.

The expanding resins 3 which can be used in the injection device 1 preferably have a free density between approximately 230 and 600 kg/m³.

Furthermore, the expanding resins 3 which can be used under conditions involving high external loads which are or can be applied in a point-like manner have a free density between approximately 255 and 600 kg/m³, benefitting from mechanical characteristics of rigidity and breaking load resistance greater than those of expanding resins having free densities less than 200 kg/m³.

According to a preferred embodiment, the above-mentioned injection kit further comprises at least one investigation means 6 which can be associated with the ground T for monitoring at least one predetermined investigation parameter I.

In a preferred embodiment, the monitoring is brought about by means of electrical resistivity tomography of the ground being processed combined with penetrometer tests.

Preferably, in this combined use of the above-mentioned analysis techniques, the tomography is used to detect the presence, the concentration and the distribution in a spatial environment of water in the ground T and the penetrometer tests serve to verify the reduction of the risk of liquefaction of the above-mentioned ground T.

In a preferred embodiment, the penetrometer tests which have an intrinsic point-like nature are correlated with the measurements of tomography in order to obtain provision of behaviour of the ground T in zones adjacent to the ones measured by means of penetrometer techniques.

Advantageously, this monitoring is carried out before, during and after the step of injecting the expanding resin 3 in the ground T. In any case, it will be appreciated that the monitoring may alternatively be carried out during a single one of those steps.

The operating methods of the injection kit which define the method for mitigating the risk of liquefaction of a piece of ground T of the present invention comprise the steps described below with reference to the flow chart of Figure 2.

In a preferred embodiment of the method 400, a user identifies the ground T to be consolidated having an upper surface S and provides for an injection device 1, which can be operationally associated with the ground T (step 402). Subsequently, the user produces at least one hole 2 in the ground T to be reached or towards volumes of the ground T to be processed (step 404).

The at least one hole 2 in the ground T is produced by means of the known techniques which are most advantageous for a person skilled in the art and may advantageously be produced in a direction perpendicular or transverse with respect to the upper surface S of the ground T.

Preferably, different types of holes (for example, vertical, inclined, superimposed or multiple holes) can be produced in order to achieve in an effective manner the reaching of the desired volumes of the ground T to be processed in order to inject the expanding resin 3.

Subsequently, the method provides for injection of an expanding resin 3, by means of the injection device 1, in the at least one hole 2 (step 406). As indicated above, the resin used in the method according to the present invention is capable of reacting chemically, producing an expansion reaction changing from an initial free volume Vin to a final free volume Vfin at the end of the expansion reaction.

After this injection step, the method provides for activation of the expanding resin 3 (step 408).

Optionally, this activation reaction can be produced spontaneously by the components of the resin itself, by the humidity in the air contained in the tubes and in the ground T or by other factors which are known in this technical field.

In a preferred embodiment of the above-mentioned method, the expansion reaction is activated during or after the injection step of the expanding resin 3 in the at least one hole 2 of the ground T.

In a preferred embodiment, the expanding resin 3 is a multi-component polymer material .

Preferably, the activation step of the above-mentioned multi-component polymer material allows time-based differentiation of a polymerization process of the expanding resin 3 and a cross-linking process of the resin itself by producing, for example, an improvement of the cross-linking step in a cross- linking time Tr which is substantially greater than a polymerization time Tp. That condition involves the additional advantage of being able to permeate the ground T in a more extensive manner over time and more efficiently before the definitive cross-linking occurs, which involves the end of the permeation and the achievement of the final mechanical and physical characteristics of the expanding resin 3.

For a person skilled in the art, it will be commonplace to identify, modify and adapt to specific requirements the cross-linking time Tr and the polymerization time Tp in accordance with the characteristics of the ground T while remaining within the scope of the present invention.

In a following step, the method provides for the conglomeration of the ground T by means of the expanding resin 3, binding to each other portions of the ground T (step 410).

Preferably, the action of binding together portions of the ground is carried out by increasing the heterogeneous interactions of the adhesive type between granules and the expanding resin 3 by forming a macro-system which is fixedly conglomerated and capable of better counteracting the deformations induced in the system (for example, deformations induced by the increase of the interstitial pressure during the occurrence of seismic phenomena).

In the method according to the present invention, the expanding resin 3 advantageously has an expansion relationship R between a final free volume Vfin and an initial free volume Vin less than five. In a preferred embodiment, the above-mentioned method comprises the monitoring, by means of at least one investigation means 6 which can be associated with the ground T, of at least one predetermined investigation parameter I.

In this step, the user activates the at least one investigation means 6 (step 403) and activates the acquisition of the values of the at least one predetermined investigation parameter I (step 414).

Advantageously, the activation step of the investigation means 6 can be carried out at any time between the step of providing an injection device 1 which can be associated with the ground T (step 402) and the step of ending the injection of the expanding resin 3 (step 412).

In another preferred embodiment, the monitoring of the at least one predetermined investigation parameter I is carried out during the step of injecting the expanding resin 3 so as to verify that the final safety factor Fs_fin is complied with.

Preferably, the above-mentioned method comprises the step of interrupting the step of injecting the expanding resin 3 in the ground T when the final safety factor Fs_fin is complied with.

Furthermore, in another preferred embodiment, the spatial formation of the upper surface S of the ground T remains unchanged and continuous even following the expansion of the expanding resin 3 except for minor geometric temporary fluctuations which are then recovered by the ground (for example, fluctuations of the upper surface S of a few millimetres for times in the order of minutes or hours).

This technical solution can advantageously be brought about during the process of mitigating the risk of liquefaction in conjunction with the use of the expanding resin 3 which exhibits a chemical expansion reaction as described above, that is to say, having a free expansion relationship R less than five. In fact, given that the injection, activation and conglomeration of the ground T are produced by means of a slow permeation which is finalized for the synergistic benefit produced by the friction increase of the granules of the ground T at the same time as the increase of the heterogeneous adhesive interaction between the granules and the expanding resin 3, this process of conglomeration and consolidation does not necessarily involve the geometric lifting of portions of the ground T with respect to the upper pre-existing lithological horizon and therefore the spatial formation of the upper surface S of the ground T is kept unchanged and continuous.

The use of the at least one investigation means 6 is therefore advantageous in order to evaluate the state and/or the variations in terms of composition and behaviour of the ground T in a mechanical and/or tribological sense.

Preferably, the at least one investigation means 6 can be used during each step which is provided for by the present method. In fact, the above- mentioned at least one investigation means 6 is an analysis technology which is capable of detecting significant variations of parameters in order to monitor the development over time of the mitigation of the risk of liquefaction of the ground T.

Furthermore, in another preferred embodiment of the above-mentioned method, the at least one investigation means 6 can be activated before, during or after any step which is correlated with the step of injecting the expanding resin 3 in the ground T. Advantageously, the activation of the at least one investigation means 6 allows adequate periodic verification of the final safety factor Fs_fin.

Preferably, the above-mentioned method comprises the step of interrupting the injection of the expanding resin 3 in the ground T once the conditions required in relation to the final safety factor Fs_fin are complied with from the at least one predetermined investigation parameter I.

In this case, the user after activating the at least one investigation device 6 at the desired time (step 403, 405, 407, 409 or 411) carries out the acquisition and updating, according to known and preferred methodologies, of the at least one predetermined investigation parameter I (step 414).

Subsequently, the at least one predetermined investigation parameter I is preferably used to calculate a current safety factor Fs_corr (step 415) which is subsequently intended to be compared with the final safety factor Fs_fin (step 416).

Advantageously, this calculation and/or comparison operation may be carried out by the user himself, by a processing unit or by similar solutions which can readily be identified by the person skilled in the art.

In a preferred embodiment, if the at least one predetermined investigation parameter I is such as to generate a current safety factor Fs_corr which is strictly less than the final safety factor Fs_fin, then the method continues with the step following the last one carried out: this means, for example, that if the last action carried out before the activation of the investigation means 6 and the result Fs_corr < Fs_fin was the production of the at least one hole 2 in the ground T (step 404), then the method continues with the injection of the resin in the at least one hole 2 (step 406). Conversely, that is to say, if the current safety factor Fs_corr is greater than or equal to the final safety factor Fs_fin, then the method continues with the injection step of the expanding resin 3 in the at least one hole 2 of the ground T (step 412) being stopped.

Preferably, there is defined an iterative cycle CI which comprises the steps from 404 to 417 (excluding the steps 412 and 413) in which the at least one predetermined investigation parameter I is acquired and updated (according to step 414) a number n of times (with n being equal to any whole number). This iterative cycle CI allows a calculation of n values of the current safety factor Fs_corr to be compared, in accordance with preferred methods, with the final safety factor Fs_fin so as to be able to monitor the development of the safety condition of the ground T as a function of time or predetermined time intervals.

In a preferred embodiment, the user will acquire the at least one predetermined investigation parameter I in accordance with a given frequency of sampling, or spot sampling or random sampling or in accordance with another advantageous identified sampling methodology.

Advantageously, it is possible for the user also to selectively define as a function of which and how many values of the at least one predetermined investigation parameter I the corresponding current safety factor Fs_corr is calculated (for example, via a mean, via a weighted mean, etc.).

Naturally, a person skilled in the art will know how to adapt and modify the comparison operations between the current safety factor Fs_corr and the final safety factor Fs_fin readily in order to be able to infer in a manner as clear as possible the possible variations of the at least one predetermined investigation parameter I during the activation of the above-mentioned method.

By way of non-limiting example, there are described as investigation techniques, which can be used in order to carry out the method in relation to the present invention, investigation methodologies such as penetrometer tests SPT, CPT, CPTU, SCPTU or DPM and geophysical investigations.

In accordance with the selection of the analysis technique, it will be evident to a person skilled in the art to identify the at least one preferred predetermined investigation parameter I and the corresponding current safety factor(s) Fs_corr which is/are capable of being compared with an adequate final safety factor Fs_fin so as to evaluate the occurrence of the mitigation of the risk of liquefaction and therefore whether or not to terminate the execution of the above-mentioned method.

Exemplary and non-limiting cases of predetermined investigation parameters I and the corresponding current safety factor Fs_corr can be the ones indicated by the conventional calculation methodologies of the potential liquefaction, such as: Robertson and Wride, Andrus and Stokoe, Eurocode 8 (ENV 1998-5) while the final safety factor Fs_fin can advantageously be established with the verification of the safety factor which is technically expressed by the following equation [1]

Fs_fin = CRR / CSR [1]

where CRR indicates the resistance of the ground to the cyclical shearing forces (Cyclic Resistance Ratio) while CSR indicates the maximum shearing stress induced by the system (Cyclic Stress Ratio).

In a preferred embodiment, the final safety factor Fs_fin is less than or equal to 1.25 and is established by means of Eurocode 8. Preferably, the at least one investigation means 6 is electrical resistivity tomography (capable of producing geoelectrical monitoring of the ground T) or a penetrometer test, or both . In the case of tomography, the at least one predetermined investigation parameter I is the resistivity measured in the ground T during the geoelectrical monitoring step of the ground T. In another preferred embodiment, the at least one investigation means 6 is electrical resistivity tomography of the three-dimensional type (3D). In fact, by means of that technique, it is possible to produce an effective monitoring of the reduction of the risk of liquefaction, by carrying out injections which expand in a targeted manner as a result of the results which are progressively obtained by the tomography of the 3D electrical resistivity, where during the operation, by keeping the geoelectrical monitoring always operational, it is necessary to modify as required the parameters for injection and/or spatial distribution inside the ground T of the zones to be conglomerated as a function of the effects sequentially encountered in the ground and so as to make uniform the chemical/physical characteristics of the volume subject to known standards in the relevant national or international standard system .

It is known in fact that the measurement of the electrical resistivity (here used as a predetermined investigation parameter I) as a first approximation is directly proportional to the specific parameters of the ground.

Advantageously, the geoelectrical monitoring step is carried out by acquiring data by means of transmitting/receiving electrodes at the surface and/or at depth in the ground, which are connected to a georesistivity meter. Preferably, the monitoring of the at least one predetermined investigation parameter I can be carried out before, during or after the injection of the expanding resin 3 in the ground T.

In a preferred embodiment, the expanding resin 3 includes closed-cell expanded polyurethane.

According to another preferred embodiment, the above-mentioned method comprises a step of carrying out at least one penetrometer test for defining the local correlation of the site and the characteristics in terms of composition or physical characteristics thereof with the tomography of electrical resistivity. Advantageously, the injections are carried out both in surface layers and in deep-lying layers of the ground T even in the absence of a building structure erected above, for examining or counteracting the expansion of the expanding resin 3. This operating condition is evidently possible only as a result of the use of resins with limited expansion which are capable of not continuing permanent and significant geometric stresses of the upper surface S (except for minor temporary fluctuations), maintaining the spatial formation thereof unchanged and constant, and consequently not requiring the application of counter-weights or opposing structures which are capable of limiting an excessive vertical movement and/or structural fracturing thereof.

Preferably, the ground T described in all the steps of the present method is a sandy ground. In fact, it must be observed that in this case the characteristics of the resin are particularly advantageous because the resin is prevented from producing an excessive pressure on the ground.

Naturally, a person skilled in the art may apply additional modifications and variants to the above-described invention in order to comply with specific and contingent application requirements, which variants and modifications are in any case within the scope of protection as defined by the appended claims.

Claims

1. A method for mitigating the risk of liquefaction of ground (T) to be consolidated having an upper interface surface (S), including :

- producing at least one hole (2) in the ground (T) to be reached or towards volumes of that ground (T) to be processed,

- injecting an expanding resin (3) into the at least one hole (2) by means of the injection device (1),

- activating the expanding resin (3),

- conglomerating the ground (T) by means of the expanding resin (3), connecting portions of the ground (T) to each other,

characterized in that the expanding resin (3) has an expansion relationship (R) between a final free volume (Vfin) and an initial free volume (Vin) less than 5.

2. A method according to claim 1, comprising the monitoring of at least one predetermined investigation parameter (I) by means of at least one investigation means (6) which can be associated with the ground (T).

3. A method according to claim 2, wherein the monitoring of the at least one predetermined investigation parameter (I) is carried out during the step of injecting the expanding resin (3) so as to establish that a final safety factor (Fs_fin) is complied with.

4. A method according to any one of claims 1 to 3, comprising the step of interrupting the step of injecting the expanding resin (3) in the ground (T) when the final safety factor (Fs_fin) is complied with.

5. A method according to any one of claims 2 to 4, wherein the at least one investigation means (6) is electrical resistivity tomography or a penetrometer test, or both.

6. A method according to one or more of claims 2 to 5, wherein the at least one investigation means (6) can be activated before, during or after any step correlated with the step of injecting the expanding resin (3) in the ground (T).

7. A method according to one or more of the preceding claims, wherein the expanding resin (3) includes closed-cell expanded polyurethane.

8. A method according to one or more of the preceding claims, wherein the expanding resin (3) has an initial time for triggering the chemical reaction greater than 15 seconds.

9. A method according to one or more of the preceding claims, wherein the ground (T) is a sandy ground.

10. A method according to one or more of the preceding claims, wherein the expanding resin (3) has a free density between approximately 230 and 600 kg/m³.

11. An injection kit which is provided to carry out the operations provided for by a method according to any one of claims 1 to 10, comprising :

• an injection device (1) including supply tubes (4) for injecting an expanding resin (3) in at least one hole (2) which is formed in a piece of ground (T),

• the expanding resin (3) having an expansion relationship (R) between a final free volume (Vfin) and an initial free volume (Vin) less than 5,

• an investigation means (6) which can be associated with the ground (T) for monitoring at least one predetermined investigation parameter (I) necessary for calculating the safety factor (Fs).