Centrifugal test device for simulating interaction between tunnel and embedded strip-shaped foundation structure

Abstract

The invention relates to the field of geotechnical engineering physical simulation, in particular to a centrifugal test device for simulating interaction of a tunnel and an embedded strip-shaped foundation structure. Comprising a centrifugal model container composed of a centrifugal model box and a transparent front side plate, a sandy soil sample is filled in the foundation, and a model tunnel and an embedded strip-shaped foundation structure are embedded in the sandy soil sample; in the first state, external baffles are arranged between the foundation stand columns and the side plates on the left side and the right side, and internal baffles are arranged between the adjacent foundation stand columns. In the second state, an upper supporting block is arranged at the upper part of the upper-layer horizontal laminate; and the internal baffle is suspended between the lower-layer horizontal laminate and the sandy soil sample. According to the centrifugal test device and the preparation method disclosed by the invention, the properties of a soil body around the embedded strip-shaped foundation structure can be accurately represented, and the disturbance of a traditional insertion mode to a soil layer is avoided, so that the deformation response of the embedded strip-shaped foundation structure caused by tunnel construction is accurately simulated.

Description

Technical Field

[0001] The invention relates to the field of geotechnical engineering physical simulation, and in particular to a centrifugal test device and a test preparation method for simulating the interaction between a tunnel and an embedded strip foundation structure. Background Art

[0002] With the rapid advancement of urbanization, the available space on the urban ground is becoming increasingly tight, and underground space represented by tunnels has been developed and utilized on a large scale. When constructing tunnels in urban areas with dense ground buildings and underground structures and infrastructure, tunnel excavation technology represented by shield machines has been widely adopted. Although shield tunnel construction technology has made great progress, when encountering special geological conditions or due to operational errors, large-scale ground subsidence or even collapse disasters often occur, which has a great negative impact on public safety and social stability. Therefore, studying the mechanism of action of tunnel construction on adjacent buildings and structures and predicting and evaluating building deformation have important engineering value and social development significance.

[0003] The evaluation of the deformation effect of tunnel construction on adjacent structures is carried out throughout the design and construction stages of tunnel projects. In the design stage, evaluating the impact of tunnel construction on surrounding structures can help optimize the tunnel route, thereby avoiding adverse environmental impacts during the tunnel construction stage. During the shield tunnel construction stage, when encountering special geological conditions or due to operational errors, it is easy to cause deformation and damage to surrounding structures. How to evaluate the deformation degree of buildings in this situation is crucial for subsequent maintenance and reinforcement. However, accurately predicting the impact of tunnel construction on surrounding buildings still faces many difficulties, such as complex soil characteristics, highly nonlinear soil-structure interaction mechanisms, and various types of building foundation types and building materials.

[0004] At present, there are three main methods for studying the impact of tunnel construction on surrounding buildings. The first is numerical simulation or simplified theoretical calculation method. Although this method has low research cost, the reliability of the research results is relatively low. The second is field measurement or full-scale test. This method can consider complex factors and obtain accurate results. However, the related research cost is high and it is impossible to conduct large-scale prediction and evaluation. Physical model test provides a more reliable research method, which can more accurately study the influence of tunnel construction on surrounding buildings. In order to further improve the accuracy of model test, centrifugal model test using hypergravity environment to simulate real soil stress has been widely recognized. Since the gravity acceleration of the entire model box needs to be increased to dozens or even hundreds of times the conventional earth gravity acceleration, the centrifugal model test box is generally small, and the sizes of various models inside the model box are also small. This brings great challenges to the production of the model and the preparation of the test.

[0005] The research on the impact of tunnel construction on surrounding buildings using centrifuge model tests generally considers a two-dimensional plane strain model, and then uses particle image velocimetry to collect test data through a transparent window on one side. In order to simulate the uniform soil properties, the researchers used the sand spreading method to prepare the sand sample, that is, the sand was dropped into the model box at a constant speed from a certain height. Therefore, in the two-dimensional plane strain model, the model box needs to be placed upside down to ensure that the plane strain tunnel model or building model is parallel to the direction of sand falling (i.e. vertically downward). At the same time, a partition is needed on one side of the top of the model box for temporary support, which is the top position of the sand sample when the model box is placed vertically. For the raft foundation building model located on the surface, the building model can be placed directly on the top of the soil layer after the sand sample is prepared and the model box is placed vertically. However, for buried structures such as buried strip foundation buildings (buried strip foundation structures), there is currently no effective centrifuge model test device and preparation method. Existing studies generally place the buried foundation in a simplified manner on the surface, or press the structure into a specified depth above the prepared sand sample. Both methods are unable to simulate the uniform soil properties around the buried foundation, thus affecting the accuracy of the model test results. Summary of the invention

[0006] In view of the above technical problems, the present invention proposes a centrifugal test device and a test preparation method for simulating the interaction between a tunnel and an embedded strip foundation structure. The test device includes a centrifugal model box and a transparent side panel. The centrifugal model box is a rectangular box with four faces, including a bottom plate and a left side plate, a rear side plate and a right side plate connected to the bottom plate. The transparent side panel is detachably connected to the front opening of the centrifugal model box as a front side panel.

[0007] It also includes a sand sample, which is filled in a centrifugal model container assembled from a centrifugal model box and a transparent side panel, and a model tunnel arranged in the front-to-back direction is buried in the sand sample; a plurality of strip foundations are buried at intervals along the left-right direction at a certain depth on the surface of the sand sample, and a foundation column is fixedly arranged on each strip foundation, and the bottom of the foundation column is buried in the sand sample, and a horizontal layer plate is fixedly arranged on the top of the foundation column, which is the lower horizontal layer plate, and an interlayer column is fixedly arranged on the upper part of the lower horizontal layer plate corresponding to each foundation column directly above, and another horizontal layer plate is fixedly arranged on the top of the interlayer column, which is the upper horizontal layer plate;

[0008] In the first state, the second state is a state in the test device preparation process, an external baffle is set between the leftmost foundation column and the left side plate, and an external baffle is set between the rightmost foundation column and the right side plate; an internal baffle is set between adjacent foundation columns;

[0009] In the second state, which is the state when the test device is fully prepared, it also includes an upper support block and a rubber block. A rubber block is arranged on the upper part of the upper horizontal layer plate corresponding to each interlayer column, and the left and right widths of the rubber blocks are smaller than the width of the interlayer columns; an upper support block is arranged between adjacent rubber blocks and is tightly attached to the rubber blocks; and the internal baffle is suspended between the lower horizontal layer plate and the surface layer of the sand sample.

[0010] Preferably, the front end of the model tunnel abuts against the transparent side panel, and the rear end abuts against the rear side panel; the front ends of the strip foundation, foundation columns, horizontal layer panels, and interlayer columns leave a small gap with the transparent side panel, and the rear ends leave a small gap with the rear side panel.

[0011] Preferably, a circular protrusion whose outer diameter is consistent with the inner diameter of the model tunnel is provided on the rear side plate, or a circular groove whose inner diameter is consistent with the outer diameter of the model tunnel is provided on the rear side plate.

[0012] Preferably, in the first state, the outer baffle, the inner baffle and the structures on the left and right sides thereof are sealed and fixed by foam tape.

[0013] Preferably, in the first state, it also includes a top baffle, which is detachably fixedly connected to the inner sides of the left and right panels respectively; the outer baffle is fixed to the top baffle by a threaded screw, and the inner baffle is fixed to the top baffle by a threaded screw, and the threaded screw connected to the internal baffle passes through the horizontal layer plate.

[0014] Preferably, in the first state, the front ends of the outer baffle plate and the inner baffle plate abut against the transparent side plate, and the rear ends abut against the rear side plate.

[0015] Preferably, in the second state, a small gap is left between the front end of the upper support block and the rubber block and the transparent side plate, and a small gap is left between the rear end and the rear side plate.

[0016] Preferably, in the second state, a threaded screw is fixedly connected to the top of the internal baffle, and the upper part of the threaded screw passes through the horizontal layer plate and the upper support block and is positioned by a nut.

[0017] Based on the centrifugal test device for simulating the interaction between a tunnel and an embedded strip-shaped foundation structure, the present invention further proposes a centrifugal test preparation method for simulating the interaction between a tunnel and an embedded strip-shaped foundation structure, comprising the following steps:

[0018] Step 1: First, push the centrifugal model box backwards so that the rear side plate is flat on the ground, place the model tunnel at the set position, and mark the position where the sand sample needs to reach; place the embedded strip foundation structure including horizontal layer plates, interlayer columns, foundation columns and strip foundations at the designated position; fix the inner baffle and the outer baffle at the position where the sand sample needs to reach, and the outer baffle and the inner baffle play the role of top and side plates;

[0019] Step 2: Use the sand spreading method to drop dry sand from a certain height at a uniform speed, with the falling direction parallel to the axial direction of the tunnel model. After the sand sample is filled, fix the transparent side plate;

[0020] Step 3: Place the centrifugal test device upright, that is, place the bottom plate flat on the ground; remove the external baffle and its fixing device, and suspend the internal baffle between the lower horizontal layer and the surface of the sand sample.

[0021] Preferably, in the third step, the upper support block is placed on the upper horizontal layer plate, and the nut is screwed into the upper part of the upper support block from above the threaded screw, and the rotation is continued to gradually lift the inner baffle.

[0022] The present invention has the following innovative features:

[0023] 1. The present invention breaks through the technical bottleneck that the existing centrifuge model test technology cannot achieve uniform soil layer layout of the buried strip foundation structure model, can accurately reproduce the soil properties around the buried strip foundation structure, avoid the disturbance of the soil layer by the traditional insertion method, and thus achieve accurate simulation of the deformation response of the buried strip foundation structure caused by tunnel construction.

[0024] 2. The centrifugal test device of the present invention not only realizes uniform soil layer arrangement, but also ensures that the overall design is reasonable, simple and easy to operate. Moreover, the slender threaded screw that penetrates through the device has small size and light weight, and will not affect the stability and strength of the embedded strip foundation structure.

[0025] 3. The present invention lifts the internal baffle plate by means of a slender screw rod that penetrates through the upper support block, and the entire weight is supported by the upper support block, thereby achieving the purpose of not changing the rigidity of the embedded strip foundation structure itself, and both ends of the upper support block are placed on the horizontal layer plate just above the interlayer columns, and adjacent support blocks are flexibly connected by rubber blocks with good compressibility, ensuring that the upper support block can dynamically adjust its orientation as the embedded strip foundation structure deforms, ensuring that the upper support block constantly and stably suspends the internal baffle plate.

[0026] 4. The internal baffle designed in the present invention is provided with sufficient gap between the foundation column and the internal baffle in the hoisted state, which can ensure that the horizontal displacement of the foundation column during the centrifugal test is not affected by the internal baffle, and can more accurately achieve the test purpose of simulating the interaction between the tunnel and the buried strip foundation structure.

[0027] 5. The upper support block designed in the present invention can be used as a platform for adjusting the self-weight load of the embedded strip foundation structure. The loading of the embedded strip foundation structure can be achieved by placing additional fixed weights or other heavy objects on the upper support block without changing the stiffness of the embedded strip foundation structure itself or affecting the stress state of its horizontal layer plate. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 1 is a schematic diagram of the overall structure of the centrifugal test device of the present invention in the first state (front view);

[0029] Figure 2 1 is a schematic diagram of the overall structure of the centrifugal test device of the present invention in the first state (front view);

[0030] Figure 3 Schematic diagram of the operation process of the centrifugal test preparation method of the present invention (side view);

[0031] In the figure: 1. interlayer column; 2. transparent side panel; 3. centrifugal model box; 4. model tunnel; 5. sand sample; 6. horizontal layer plate; 7. foundation column; 8. strip foundation; 9. inner baffle; 10. outer baffle; 11. foam tape; 12. top baffle; 13. threaded screw; 14. L-shaped angle aluminum; 15. upper support block; 16. nut; 17. rubber block; 18. screw; 19. bolt. DETAILED DESCRIPTION

[0032] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific working methods.

[0033] like Figure 1-3 As shown, the present invention proposes a centrifugal test device for simulating the interaction between a tunnel and an embedded strip foundation structure, wherein the embedded strip foundation structure is an embedded strip foundation building, and the centrifugal test device comprises a centrifugal model box 3 and a transparent side panel 2, wherein the centrifugal model box 3 is a rectangular box with four faces, comprising a bottom plate and a left side panel, a rear side panel and a right side panel connected to the bottom plate, wherein the left side panel, the rear side panel and the right side panel are connected in sequence, and the transparent side panel 2 is detachably connected to the front opening of the centrifugal model box 3 as a front side panel;

[0034] It also includes a sand sample 5, which is filled in a centrifugal model container assembled from a centrifugal model box 3 and a transparent side panel 2, and is filled to a certain height but does not reach the top surface; a model tunnel 4 arranged in the front-to-back direction is buried in the middle or lower middle part of the sand sample 5, and the sand sample 5 is arranged inside the model tunnel 4; a plurality of strip foundations 8 are buried at intervals along the left-right direction at a certain depth on the surface of the sand sample 5, and the spacing between adjacent strip foundations 8 can be the same, and a foundation column 7 is fixedly arranged on each strip foundation 8, the bottom of the foundation column 7 is buried in the sand sample 5, and the upper part leaks out of the sand sample 5, and a horizontal layer plate 6 is fixedly arranged on the top of the foundation column 7, which is the lower horizontal layer plate, and the lower layer An interlayer column 1 is fixedly arranged on the upper part of the horizontal layer plate 6 corresponding to each foundation column 7, and another horizontal layer plate 6 is fixedly arranged on the top of the interlayer column 1, which is the upper horizontal layer plate 6, and the upper horizontal layer plate 6 is parallel to the lower horizontal layer plate 6; wherein, the front end of the model tunnel 4 abuts against the transparent side plate 2, and the rear end abuts against the rear side plate, and the model tunnel 4 is preferably plugged into the rear side plate, such as setting a circular protrusion with an outer diameter consistent with the inner diameter of the model tunnel 4 on the rear side plate, or setting a circular groove with an inner diameter consistent with the outer diameter of the model tunnel 4 on the rear side plate; the front ends of the strip foundation 8, the foundation column 7, the horizontal layer plate 6, and the interlayer column 1 are slightly spaced from the transparent side plate 2, and the rear ends are slightly spaced from the rear side plate;

[0035] like Figure 1 As shown, in the first state, which is the state before the top baffle 12 is removed, an external baffle 10 is connected between the leftmost foundation column 7 and the left side plate by a foam tape 11, and an external baffle 10 is connected between the rightmost foundation column 7 and the right side plate by a foam tape 11, and one side of the foam tape 11 is adhered to the side of the external baffle 10; an internal baffle 9 is connected between adjacent foundation columns 7 by a foam tape 11, and the foam tape 11 is adhered to the side of the foundation column 7; and a top baffle 12 is also included, and the left and right ends of the top baffle 12 are respectively fixedly connected by L-shaped angle aluminum 14 On the inner side of the left plate and the inner side of the right plate, each L-shaped angle aluminum 14 is connected to the top baffle 12 and the side plate of the centrifugal model box respectively by screws 18; the lower end of the threaded screw 13 (cylindrical rod, the same below) is fixedly connected to the top of the external baffle 10, and the other end is fixedly connected to the top baffle 12; the lower end of the threaded screw 13 is fixedly connected to the top of the internal baffle 9, and then passes through the horizontal layer plate 6 and the top is pressed against the bottom surface of the top baffle 12; at least two threaded screws 13 are arranged at intervals along the front and rear directions; wherein, the front ends of the external baffle 10 and the internal baffle 9 are in contact with the transparent side plate 2, and the rear ends are in contact with the rear side plate;

[0036] like Figure 2As shown, in the second state, which is the state when the test device is ready, the top baffle 12, L-shaped angle aluminum 14, and screws 18 are removed; the outer baffle 10 between the leftmost foundation column 7 and the left side plate and the threaded screw 13 and foam tape 11 on the outer baffle are removed, and the outer baffle 10 between the rightmost foundation column 7 and the right side plate and the threaded screw 13 and foam tape 11 on the outer baffle are removed; in the second state, it also includes upper support blocks 15 and rubber blocks 17, one upper support block 15 is arranged on the upper part of the upper horizontal layer plate 6 corresponding to each internal baffle plate 9, and the upper part of the upper horizontal layer plate 6 corresponds to the upper part of the interlayer column 1 and A rubber block 17 is respectively arranged between adjacent upper support blocks 15, and the left and right widths of the rubber block 17 are smaller than the width of the interlayer column 1; the upper support block 15 is tightly attached to the rubber block 17, and the rubber block 17 is placed between adjacent upper support blocks 15 to flexibly fill the gap between adjacent upper support blocks 15; the upper part of the threaded screw 13 fixedly connected to the top of the internal baffle plate 9 passes through the horizontal layer plate 6 and the upper support block 15 and is positioned by the nut 16, at this time, the internal baffle plate 9 is suspended between the lower horizontal layer plate 6 and the surface layer of the sand sample 5; the front end of the upper support block 15 and the rubber block 17 is slightly spaced from the transparent side panel 2, and the rear end is slightly spaced from the rear side panel.

[0037] Based on the centrifugal test device for simulating the interaction between a tunnel and an embedded strip-shaped foundation structure, the present invention also proposes a centrifugal test preparation method for simulating the interaction between a tunnel and an embedded strip-shaped foundation structure, such as Figure 3 As shown, the following steps are included:

[0038] Step 1: If Figure 3 As shown in the side view (a), first, push the centrifugal model box 3 backwards so that the rear side plate is flat on the ground, place the model tunnel 4 at the set position, and mark the position that the sand sample 5 needs to reach; place the embedded strip foundation structure including the horizontal layer plate 6, the interlayer column 1, the foundation column 7 and the strip foundation 8, and the internal baffle plate 9 connected with the threaded screw 13 at the specified position, and the threaded screw 13 passes through the horizontal layer plate 6; fix the external baffle plate 10 and the top baffle plate 12 together through the threaded screw 13, and then The top baffle 12 is fixed to the side plate of the centrifugal model box 3 by L-shaped angle aluminum 14 and screws 18; the positions of the inner baffle 9 and the outer baffle 10 are the positions that the sand sample 5 needs to reach; the outer baffle 10 and the inner baffle 9 are connected by the foam tape 11; at this time, the threaded screw 13 connected to the inner baffle 9 abuts against the top baffle 12; at this time, the foam tape 11, the outer baffle 10 and the inner baffle 9 together constitute a temporary lateral support for the sand sample 5, which is equivalent to the top side plate;

[0039] Step 2: If Figure 3(b) As shown in the side view, dry sand is dropped from a certain height at a uniform speed by the sand spreading method, and the falling direction is parallel to the axial direction of the tunnel model to avoid interference when the sand falls; after the sand sample 5 is filled, the surface of the sand sample 5 is scraped and leveled with a tool, and then the transparent side plate 2 is installed and fixed, and the transparent side plate 2 is fixed to the model box 3 by bolts 19;

[0040] Step 3: If Figure 3 (c) As shown in the side view, the centrifuge test device is placed upright, that is, the bottom plate is placed flat on the ground; Figure 3 (d) As shown in the side view, the top baffle 12, L-shaped angle aluminum 14, and screws 18 are removed; the external baffle 10 and the threaded screw 13 and foam tape 11 on the external baffle are removed and taken out; the upper support block 15 is placed on the upper horizontal layer 6, and the nut 16 is screwed from the threaded screw 13 to the top of the upper support block 15, and the rotation is continued to gradually lift the internal baffle 9 to a distance of at least 2 cm from the upper surface of the sand sample 5. After the internal baffle 9 is lifted, it is separated from the foam tape 11 pasted on the side of the foundation column 7, thereby forming a certain gap with the foundation column 7, so that the internal baffle 9 has no contact with the embedded strip foundation structure and the sand sample 5.

[0041] Subsequently, the centrifuge test device was placed on the centrifuge and centrifuge model tests were carried out to study the impact of tunnel construction on the buried strip foundation structure.

[0042] Although the present invention has been described above with preferred embodiments, it is not intended to limit the present invention. A person skilled in the art of the present invention may make various modifications and improvements without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be determined by the definition of the claims.

Claims

1. A centrifugal test device for simulating the interaction between a tunnel and an embedded strip foundation structure, characterized in that: The test device comprises a centrifugal model box and a transparent side panel, wherein the centrifugal model box is a rectangular box with four faces, comprising a bottom plate and a left side plate, a rear side plate and a right side plate connected to the bottom plate, and the transparent side panel is detachably connected to the front opening of the centrifugal model box as a front side panel; It also includes a sand sample, which is filled in a centrifugal model container assembled from a centrifugal model box and a transparent side panel, and a model tunnel arranged in the front-to-back direction is buried in the sand sample; a plurality of strip foundations are buried at intervals along the left-right direction at a certain depth on the surface of the sand sample, and a foundation column is fixedly arranged on each strip foundation, and the bottom of the foundation column is buried in the sand sample, and a horizontal layer plate is fixedly arranged on the top of the foundation column, which is the lower horizontal layer plate, and an interlayer column is fixedly arranged on the upper part of the lower horizontal layer plate corresponding to each foundation column directly above, and another horizontal layer plate is fixedly arranged on the top of the interlayer column, which is the upper horizontal layer plate; In the first state, an external baffle is arranged between the leftmost foundation column and the left side plate, and an external baffle is arranged between the rightmost foundation column and the right side plate; and an internal baffle is arranged between adjacent foundation columns; In the second state, which is the state when the test device is fully prepared, it also includes an upper support block and a rubber block. A rubber block is arranged on the upper part of the upper horizontal layer plate corresponding to each interlayer column, and the left and right widths of the rubber blocks are smaller than the width of the interlayer columns; an upper support block is arranged between adjacent rubber blocks and is tightly attached to the rubber blocks; and the internal baffle is suspended between the lower horizontal layer plate and the surface layer of the sand sample.

2. The centrifugal test device according to claim 1, characterized in that: The front end of the model tunnel is in contact with the transparent side panel, and the rear end is in contact with the rear side panel; the front ends of the strip foundation, foundation columns, horizontal layer panels, and interlayer columns are slightly spaced from the transparent side panel, and the rear ends are slightly spaced from the rear side panel; the front ends of the upper support blocks and rubber blocks are slightly spaced from the transparent side panel, and the rear ends are slightly spaced from the rear side panel.

3. The centrifugal test device according to claim 1, characterized in that: A circular protrusion with an outer diameter consistent with the inner diameter of the model tunnel is arranged on the rear side plate, or a circular groove with an inner diameter consistent with the outer diameter of the model tunnel is arranged on the rear side plate.

4. The centrifugal test device according to claim 1, characterized in that: In the first state, the outer baffle, the inner baffle and the structures on the left and right sides thereof are fixed by means of foam tape.

5. The centrifugal test device according to claim 1, characterized in that: In the first state, it also includes a top baffle, which is detachably fixed to the inner side of the left plate and the inner side of the right plate respectively; the outer baffle is fixed to the top baffle by a threaded screw, and the inner baffle is fixed to the top baffle by a threaded screw, and the threaded screw connected to the inner baffle passes through the horizontal layer plate.

6. The centrifugal test device according to claim 1, characterized in that: In the first state, the front ends of the outer baffle plate and the inner baffle plate abut against the transparent side plate, and the rear ends abut against the rear side plate.

7. The centrifugal test device according to claim 1, characterized in that: In the second state, a threaded screw is fixedly connected to the top of the internal baffle, and the upper part of the threaded screw passes through the horizontal layer plate and the upper support block and is positioned by a nut.

8. A method for preparing a centrifuge test for simulating the interaction between a tunnel and an embedded strip foundation structure, based on the centrifuge test device according to any one of claims 1 to 7, comprising the following steps: Step 1: First, push the centrifugal model box backwards so that the rear side plate is flat on the ground, place the model tunnel at the set position, and mark the position where the sand sample needs to reach; place the embedded strip foundation structure including horizontal layer plates, interlayer columns, foundation columns and strip foundations at the designated position; fix the inner baffle and the outer baffle at the position where the sand sample needs to reach, and the outer baffle and the inner baffle play the role of top and side plates; Step 2: Use the sand spreading method to drop dry sand from a certain height at a uniform speed, with the falling direction parallel to the axial direction of the tunnel model. After the sand sample is filled, fix the transparent side plate; Step 3: Place the centrifugal test device upright, that is, place the bottom plate flat on the ground; remove the external baffle and its fixing device, and suspend the internal baffle between the lower horizontal layer and the surface of the sand sample.

9. According to the centrifugal test preparation method of claim 8, in the third step, the upper support block is placed on the horizontal layer plate, and the nut is screwed from the top of the threaded screw into the top of the upper support block, and the rotation is continued to gradually lift the internal baffle.

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