Field of the invention

The present invention relates to a test machine for characterizing artificially frozen soil.

Background art

Artificial ground freezing (AGF) is a soil consolidation and waterproofing technique, obtained by decreasing the temperature of the water present in the ground down to a value reasonably lower than water freezing temperature. It is a technique for the temporary support of excavations in difficult conditions, which may be applied to any type of soil. It is mainly used for the consolidation of medium to high permeability soils when the excavation takes place under high hydraulic heads, to increase the resistance and rigidity of the ground and reduce the permeability thereof, limiting potential instability and subsidence at ground level.

Since this technique is a method to support excavations in urban environments, it is vital to correctly predict the ground displacements induced during the activation of the freezing, the maintenance thereof, the excavation and during thawing. In fact, excessive differential yielding during one of the steps mentioned above may damage the existing buildings.

Given the absolute reversibility and the temporary nature thereof, the technique has a minimal environmental impact; in fact, it does not introduce nor it releases substances in the soil and in the groundwater. Despite the growing awareness of the merits thereof, the technique is still considered with a certain skepticism due to the high costs, connected to the consumption of nitrogen during the activation steps, and of electricity during the brine maintenance steps, and due to perceived uncertainties with respect to more conventional methods, for the difficulty of predicting the mechanical behavior of the land during freezing and thawing.

The understanding of the physical phenomena at the base of the freezing and thawing and the assessment of the thermo-mechanical properties which regulate them are in fact key elements for the optimization of the technique and for the greater diffusion thereof. Furthermore, disadvantageously, existing test machines do not allow to freeze the soil sample in the same manner as this occurs on site.

The need is therefore felt to provide an innovative test machine which overcomes the aforesaid drawbacks.

Summary of the invention

It is an object of the present invention to provide a test machine for improving the mechanical characterization tests of frozen soils, allowing freezing to be applied in the same manner as on site.

It is another object of the present invention to provide a test machine which allows, with a correct mechanical characterization of the frozen ground, to optimize artificial freezing interventions on site and to adopt less extensive safety margins with the consequent reduction in production costs.

The present invention achieves the aforesaid objects by providing a test machine for characterizing an artificially frozen soil, comprising

- a triaxial cell defining a longitudinal axis and having a housing to house a soil sample along said longitudinal axis;

- refrigerating means adapted to freeze the soil sample housed in said triaxial cell; in which said refrigerating means comprise at least one freezing member crossing said housing in the entirety thereof along said longitudinal axis so as to freeze the soil sample from the inside outwards in a radial direction.

Advantageously, the test machine of the invention comprises a triaxial cell with deformation, load and temperature control which allows to freeze the soil sample from the inside outwards, as it happens on site during the application of the AGF. For a correct mechanical characterization of soil artificially frozen on site, the test machine of the invention advantageously applies the thermal load in the same direction as the real scale, i.e. in the radial direction, from the inside outwards, of the specimen. This test machine allows to study the advancement of the frozen front in the radial direction, to quantify the amount of water that the frozen front can recall and to measure the resulting radial deformations.

Preferably, the radial deformations of the soil sample are measured with magnetic sensors placed along a radial belt surrounding the soil sample. Preferably, the axial deformations of the soil sample are measured with local transducers mounted substantially at half the height of the sample and with a transducer external to the triaxial cell.

The temperature in the sample is measured at different radial distances from the freezing pipe and at different heights along the soil sample.

The application of the thermal load in the radial direction from the center outwards allows to correctly study all the phenomena typical of freezing. In fact, when the water in soil pores starts to freeze, it increases in volume inducing swelling deformations; furthermore, the interface between liquid water and ice is curved and attracts further water to the frozen front.

Advantageously, a double drainage system for the liquid water, placed at the base and at the head of the triaxial cell, is positioned in an area peripheral with respect to the longitudinal axis of the cell, to allow the flow of water, from and to the frozen front, for as long as possible. The position of the drainage system allows to better study the phenomena of water recall during the freezing (until the frozen front does not reach the drainage) and of water expulsion during the thawing and to always measure the induced deformations with local transducers placed at different heights.

Further features and advantages of the invention will become more apparent in the light of the detailed description of some exemplary, but not exclusive, embodiments.

The dependent claims describe particular embodiments of the invention.

Brief description of the Drawings

In the description of the invention, reference is made to the accompanying drawings, which are provided by way of explanation and not by way of limitation, in which:

Figure 1 diagrammatically shows the test machine of the invention;

Figure 2 diagrammatically shows part of the machine in Figure 1 ;

Figure 3a shows some details of the upper area of the part in Figure 2;

Figure 3b shows some details of the lower area of the part in Figure 2;

Figure 4 shows a diagrammatic top view of a part of the machine in Figure 1 ;

Figure 5 shows a bottom view of the component in Figure 2. Same elements, or elements having equal function, have the same reference numeral.

Description of exemplary embodiments of the invention

An embodiment of the test machine of the invention, shown in the Figures, comprises:

- a triaxial cell 2 defining a longitudinal axis X and having an inner housing to house a soil sample 9 along said longitudinal axis;

- and refrigerating means 3 adapted to freeze the soil sample 9 arranged in the housing.

The housing of the soil sample 9 is defined by the space comprised between the head 6, or upper head, and the base 7, or lower head, of the triaxial cell 2. Head 6 and base 7 preferably have a cylindrical shape.

Preferably, head 6 and base 7 of the triaxial cell 2 are advantageously made of a semi-crystalline thermoplastic material, preferably combining optimum mechanical properties and excellent chemical resistance. The choice of the material to be used was made after a careful assessment of the evolution of the temperature starting from parameters available in the literature for steel and plastic materials. The assessment of the expected mechanical behavior under thermal load was carried out with thermo-mechanical numerical analysis and led to the choice of PEEK, whose thermal characteristics are A=0.27 W/m ^* K, Cs=1 ,100 J/kg ^* K, p=1 ,310 kg/m ³ and can be used up to 260 °C in hot water or steam or in very low temperature conditions, down to - 60 °C.

The choice of PEEK allows the triaxial cell 2, which works under temperature control, to optimally assure two conditions: low thermal conductivity and low coefficient of thermal expansion.

However, other materials alternative to PEEK can be used.

The soil sample 9, preferably cylindrical, is laid on the base 7 and is wrapped into an impermeable membrane or sheath 30, for example in latex, which is externally fastened both to the base 7 and to the head 6 by means of suitable fastening means, for example by means of a plurality of O-rings.

The refrigerating means 3 comprise a refrigerating system 31 , of a known type, and a relative hydraulic circuit of the refrigerating fluid provided with a pump 32. Preferably, said refrigerating means 3 comprise a single refrigerating system 31 , or refrigerated circulator, and the relative hydraulic circuit of the refrigerating fluid. Advantageously said refrigerating means 3 comprise at least one freezing member 4 which crosses, along the longitudinal axis X, the base 7, the housing, and therefore the soil sample 9, and, preferably at least partially, the head 6, so as to freeze the soil sample 9 from the inside outwards in the radial direction.

Therefore, the freezing member 4, also of a longitudinal shape, crosses the housing of the soil sample 9 in the entirety thereof.

In the variant shown in Figures 1 and 2 there is provided a single freezing member 4, preferably in the shape of a freezing pipe, which forms integral part of the aforesaid hydraulic circuit of the refrigerating fluid.

Preferably, the diameter of the freezing pipe is about 8-12 mm, for example 10 mm.

The freezing pipe is preferably made of copper, or of another suitable thermally conductive material, and allows the refrigerating fluid to circulate from the refrigerating system 31 inside the sample 9, from the bottom upwards.

Alternatively to the freezing pipe, a freezing member may be used in the form of a flexible tube adapted to support the deformation of the soil sample during the application of the axial load.

Both the head 6 and the base 7 have, respectively (Figures 3a and 3b):

- a central hole 12, having as axis the axis X itself, longitudinally crossed by the freezing member 4;

- at least two first side holes 13, diametrically opposite, for a water inlet to the housing or for a water outlet from said housing;

- preferably, a plurality of second side holes 14, arranged at different distances from the central hole 12, for the insertion of thermocouples 8.

Central hole 12, first side holes 13 and second side holes 14 are preferably parallel to one another and have longitudinal axes parallel to the longitudinal axis X.

As shown in Figure 2, the first side holes 13 of the head 6 are aligned with corresponding first side holes 13 of the base 7, having the axis in common. Preferably, also the second side holes 14 of the head 6 are aligned with corresponding second side holes 14 of the base 7.

The second side holes 14 are preferably arranged along radial directions and, in any case, in a position comprised between the central hole 12 and the first side holes 13, the latter being arranged along a same cylindrical side surface.

The thermocouples 8 have different lengths and/or are positioned at different distances/heights from the freezing member 4. For example, the thermocouples can be installed in a number of twelve and arranged on three circular crowns at a growing distance from the freezing pipe, four thermocouples on each circular crown.

The freezing pipe is located into the central sealed hole 12, allowing the replacement thereof in case of wear or subsequent modification with different materials and geometric shapes. In particular, the freezing member 4 is inserted and sealed into the central holes 12 of head 6 and base 7 so that the refrigerating fluid only passes inside the freezing member, and not between the surface of the central hole 12 and the external surface of the freezing member.

The test machine of the invention also comprises:

- axial load applying means 1 , adapted to apply an axial load on the soil sample 9 along said longitudinal axis X, and relative axial deformation measuring means; - and radial load applying means, adapted to apply a radial load on the soil sample

9, and relative radial deformation measuring means 5 arranged about said housing, externally to the impermeable membrane 30.

The axial load applying means 1 comprise, for example, a mechanical press inside which the triaxial cell 2 is arranged.

The mechanical press is a press with a controlled speed/force and is provided with a load cell 43 for measuring the axial force. The force control system is integrated into the press itself.

The contrast element, comprising the load cell 43, acts on the head 6 of the triaxial cell 2, preferably by means of a cover 42, preferably in tempered steel and fastened to the upper end of the head 6, which acts as an element for connecting and distributing the load with the hydraulic jack of the press. The head 6, therefore, also acts as a loading piston. The axial deformation measuring means, i.e. measuring the vertical displacements of the sample 9 during the axial load step, comprise at least one displacement transducer 22, for example an LVDT transducer, external to the triaxial cell 2 and fastened on the head 6 or on the cover 42.

Alternatively, the application of the axial load can be provided on the base 7 of the triaxial cell 2, i.e. from the bottom upwards.

With regard, instead, to the application of the radial load, there is provided a volume surrounding the soil sample 9 wrapped into the impermeable membrane 30. This volume is advantageously filled with a pressurized antifreeze liquid 16 to produce a radial compression on the soil sample 9. The impermeable membrane 30 is rather thin, and in itself known, so as to be extremely compressible and not to influence the measurements of radial deformations suffered by the soil sample 9.

The volume surrounding the soil sample 9 is delimited by a container 15 defined by a tube or cell 25, preferably of a cylindrical shape, coaxial and external to the housing of the soil sample 9.

The tube 25 is closed at the top by a first plate 17 having a central hole thereof crossed by the head 6 of the triaxial cell 2, whereas it is closed at the bottom by a second plate 19, or pedestal, which supports and communicates with the base 7 of the triaxial cell 2.

The plates 17 and 19 are preferably, but not necessarily, of a circular shape and made of stainless steel. A plurality of vertical columns 34, for example made of stainless steel, keeps the plates 17, 19 together, parallel to each other, at a peripheral portion thereof.

Optionally the tube or cell 25 is a hollow cylinder, open at the ends, preferably made of highly resistant, transparent acrylic material, for example Perspex® or another suitable acrylic material. The tube 25 is advantageously provided with metal stripes along the external side surface thereof, to apply a maximum confinement pressure of 2400 kPa.

Advantageously, there is provided a sealed tightening system 18 between the first plate 17 and the head 6. Such sealed tightening system 18 consists of a series of components connected to one another to keep the system hydraulically insulated.

In a variant shown in Figure 3a, the sealed tightening system 18 comprises a first tightening member 35, arranged coaxial and adjacent externally to the head 6 in the central hole of the first plate 17.

Such first tightening member 35, which, with an external annular projection thereof, rests against the first plate 17, has at the upper end thereof:

- on the internal side thereof, an annular groove;

- on the external side thereof, a thread.

At least one O-ring is provided between the first tightening member 35 and the first plate 17.

Advantageously, in said annular groove, proximal to the head 6, a Teflon ring 38, adjacent to the head 6, and an O-ring 37, arranged coaxial and external to the substantially semi-rigid Teflon ring, can be housed. Such elements are tightened to the member 35 by means of an external threaded ring 36, which is screwed onto the thread of the external side of the upper end of the first tightening member 35.

The Teflon ring 38, having a low friction coefficient, is used so as to minimize the resistance on the head 6 during the axial compression step.

The O-ring 37, placed between the Teflon ring 38 and the tightening member 35, has the function of compressing the Teflon ring 38 on the side surface of the head 6 so as to guarantee the hydraulic seal.

Teflon ring 38 and O-ring 37 define a special joint with a low friction coefficient, such as to make the system hydraulically insulated, and therefore to ensure that the antifreeze fluid 16 under pressure does not escape into the space between the member 35 and the head 6 of the triaxial cell 2.

The variations in the volume of the soil sample 9 can be assessed by measuring the radial deformations of the sample with the aforesaid radial deformation measuring means 5.

In a variant of the invention, said radial deformation measuring means comprise a radial belt 5 provided with magnetic sensors 44 (Figure 2 and Figure 4). These magnetic sensors 44 allow the measurement of the radial deformations under any temperature condition.

In addition, the local axial deformations of the soil sample 9 are measured with two displacement transducers 45, for example LVDT transducers (Figure 4).

Preferably, the radial belt 5 is positioned at half the height of the soil sample 9, whereas the displacement transducers 45 are arranged, diametrically opposite to each other, at the recesses 46 present on the radial belt 5, therefore arranged around the housing of the soil sample 9.

The second plate 19, or pedestal, is the base element on which the triaxial system body develops. In the peripheral portion thereof, preferably in the external circular crown thereof, the second plate 19 is provided with holes for tightening the screws 33 which anchor the columns 34 (Figure 5).

In the central portion of the second plate 19 there are, instead, provided (Figures 3b and 5):

- a central hole 12', having as axis the axis X itself, crossed by the freezing member 4;

- at least two first side holes 13', diametrically opposite, for a water inlet to the housing or for a water outlet from said housing;

- a plurality of second side holes 14', preferably arranged at different distances from the central hole 12', for the insertion of the thermocouples 8;

- preferably, a plurality of third side holes 20 for the insertion of further equipment. Central hole 12', first side holes 13', second side holes 14' and third side holes 20 are preferably parallel to one another and have longitudinal axes parallel to the longitudinal axis X.

Central hole 12’, first side holes 13’ and second side holes 14’ of the second plate 19 are aligned and communicate with central hole 12, first side holes 13 and second side holes 14 of the base 7, respectively (Figure 3b).

Preferably, in case of application of the axial load from the top downwards, the freezing member 4, longitudinally extending along the axis X, for example in the form of a rigid pipe or of a flexible tube, is inserted and fastened with a first end thereof into the central hole 12' of the second plate 19, completely crosses the central hole 12 of the base 7 and the housing of the soil sample 9 in the entirety thereof, and only partially crosses the central hole 12 of the head 6. The second end of the freezing member 4 is therefore inside the central hole 12 of the head 6. This allows a sliding of the head 6 along the axis X on the freezing member 4 during the application of the axial load.

In case of applying the axial load from the bottom upwards, the freezing member 4 is inserted and fastened with a first end thereof in a central hole of the cover 42, completely crosses the central hole 12 of the head 6 and the housing of the soil sample 9 in the entirety thereof, and only partially crosses the central hole 12 of the base 7. In this case a sliding of the base 7 along the axis X on the freezing member 4 is allowed during the application of the axial load.

In both cases, the length of the freezing member 4 is, for example, less than or equal to about 90%, preferably equal to a value comprised in the range 75-85%, of the sum of the lengths of the central holes 12 of head 6 and base 7 and of the housing of the soil sample 9 along the axis X in the resting position, i.e. not compressed.

Preferably, in one of the third side holes 20, placed between the second holes 13' and the peripheral holes 33, there is provided the passage for the antifreeze liquid 16 under pressure inside the container 15.

The pressure of the antifreeze liquid 16 inside the container 15, or confinement pressure, is regulated by means of a compressed air system, provided with regulators of pressure, for example of the Watson & Smith type, of an air/antifreeze liquid interface 16 and measured with at least one pressure transducer connected to the hydraulic circuit of the antifreeze liquid.

For example, a compressed air system can be used which allows a pressure of up to 0.8 MPa to be achieved and, using a three-multiplier interface, a pressure of up to 2.4 MPa can be achieved.

In the central portion of the second plate 19 there are further provided a plurality of further holes to house tightening screws 40 for tightening the base 7 of the triaxial cell 2.

In the peripheral portion of the second plate 19 there are also provided holes 41 , transverse to the axis X, for the passage of cables and tubes towards the outside of the triaxial cell, for example for the passage of the thermocouples, of the cables of the displacement transducers, of the means for pressurizing the antifreeze liquid, of the water passage duct, etc.

Advantageously, the positioning of the side holes 13, in the head 6 and in the base 7, and of the side holes 13' in the second plate 19 is provided at the periphery of the housing of the soil sample 9, therefore in a peripheral position with respect to the longitudinal axis X. This positioning allows water to flow from and to the sample 9 even during the freezing, which advances from the center of the sample outwards, until the frozen front reaches the holes 13. The frozen front, advancing from the inside outwards, allows the study of a fundamental aspect of the soil under thermal load, i.e. it allows to follow the flow of water towards the frozen front. In particular, it allows to follow the call and expulsion of water for as long as possible, until the approaching of the frozen front. Understanding the frozen front water exchange mechanism is an important aspect for the correct prediction of the effects on the soil of freezing/thawing, such as displacements.

The test machine of the invention is also provided with pressurizing means for pressurizing the water entering the side holes 13', 13 and means for measuring the volume of water entering and exiting the housing, i.e. entering and exiting the soil sample 9.

Such pressurizing means comprise pressure regulators, for example of the Watson & Smith type, and the water pressure is measured with at least one pressure transducer connected to the hydraulic circuit of the water. The variations in the volume of the sample 9 can be assessed by measuring the water entering and exiting the sample through the side holes 13 by means of a volumometer. This measurement is valid for samples saturated with liquid water (temperatures higher than zero) and partially valid during freezing.

For example, the water pressurizing means comprise a compressed air system which allows to exert a pressure of up to a maximum of 0.8 MPa on an air-water interface which also acts as a volumometer. The water side of the interface is connected to the drainage holes 13 in the base 7 and in the head 6.

With the test machine of the present invention it is possible to vary and measure:

- the confinement pressure, for example up to a maximum of 2.4 MPa, and the consequent radial deformations of the soil sample; - the axial load, applied with controlled displacement and/or with controlled force by means of the press, and measured with a load cell with a full scale of about 100 KN, and the consequent axial deformations of the soil sample;

- the pressure and the volume of water crossing the soil sample 9;

- the temperature in the freezing pipe, varied by means of the refrigerating system

31 and measured by means of the thermocouples 8.

With the test machine of the invention, the following compression tests may be carried out at different temperatures:

- isotropic compression tests, increasing the confining tension and the axial tension by the same quantity;

- anisotropic compression tests, with an obliquity n=(axial tension)/(confining tension) other than one, imposing an axial tension different from the confining tension;

- axial compression tests, with displacement control and with different confining tensions.

The temperature application step in the rigid freezing pipe or in the flexible freezing tube is characterized by a transient until the constant temperature is reached inside the soil sample. The measurement of the temperature in the sample at a different distance from the freezing pipe and at a different height allows, from the backward analysis of the data, to assess the thermal conductivity parameters of the soil at different temperatures.

From the isotropic and anisotropic compression steps, the compressibility parameters are obtained at different obliquities as the temperature changes. From the axial compression step, the resistance parameters are obtained as the temperature and the displacement speed change.