Foundation bed provided with immersed tube tunnel inclined self-silt-discharging furrows and design method of foundation bed

Abstract

The invention relates to a foundation bed provided with an immersed tunnel inclined self-silt-discharging furrow, which comprises a foundation trench, a block stone layer and a rubble layer, the rubble layer is provided with a furrow (3) and a ridge platform (4), the furrow (3) comprises two side slopes, the surfaces of the two side slopes are called side slopes (6), and the foundation bed is characterized in that the position, close to the middle and lower than the ridge platform (4), on the furrow (3) is a furrow starting point, and the ridge platform (4) is arranged on the furrow (3). Two furrow end points are arranged at the two ends of the furrow (3) respectively, a middle slope from high to low extends from the furrow starting point to each furrow end point, and a front inclined plane (5) is formed on the surface of each middle slope; the shapes of the side slopes (6) of the two side slope surfaces are determined according to the middle slope; and in the silt squeezing process of the immersed tube, silt returning objects at the bottom of the immersed tube flow out of the bottom of the immersed tube along the two front inclined planes (5). The invention further provides a design method of the foundation bed.

Description

Technical Field

[0001] The present invention relates to the technical field of immersed tube tunnels, in particular to a subgrade provided with an inclined self-draining ditch for an immersed tube tunnel and a design method thereof. Background Art

[0002] Among immersed tube tunnel foundation leveling technologies, the pre-laying method is commonly used in immersed tube tunnel foundation construction because it enhances the bearing capacity of weak foundations, helps ensure foundation stability, and promotes drainage, consolidation, and settlement of the underlying soil layer, thereby reducing foundation deformation and settlement. Since the incorporation of ridges and trenches in the subgrade bed can promote rapid stabilization of the sunken tube segments and enhance the subgrade bed's silt-absorbing capacity, currently operating pre-laying immersed tube tunnels mostly utilize a gravel subgrade with ridges and trenches. Specifically, a gravel layer is laid on the subgrade bed in a pattern of alternating ridges and trenches, perpendicular to the axis of the immersed tube tunnel, and the trenches are aligned in the same horizontal plane. Because the pre-laying method often requires a period of time between subgrade leveling and placement of the immersed tube segments, a certain thickness of sediment accumulates on the subgrade bed, posing risks and challenges during segment placement. The sediment on the subgrade surface has a high moisture content and good fluidity. The traditional base bed ditch structure has the following shortcomings: (1) The ditch opening is not deep enough, and the ditch depth is in the same horizontal plane, which does not allow the silt to drain automatically after reaching a certain thickness; (2) The silt holding capacity is limited, and the accumulation of silt can easily affect the installation accuracy of the immersed tube; (3) The gravel base bed needs to be desilted before the immersed tube is placed, which increases the construction period and cost. Summary of the Invention

[0003] The present invention overcomes the technical problem of traditional immersed tube tunnel subgrade ditches being unable to achieve self-drainage. It provides a subgrade with an inclined, self-draining ditch for immersed tube tunnels and a design method thereof. The subgrade ditch of the present invention comprises a positive slope and two side slopes from the starting point to the end point of the ditch. The positive slope and the side slopes have a certain slope, enabling silt on the slope to flow out of the ditch along the slope under the action of the silt-squeezing load of the immersed tube and the deadweight of the silt. To achieve the above objectives, the present invention adopts the following technical solutions:

[0004] A subgrade with inclined self-draining ditch for immersed tube tunnel comprises a foundation trench, a block stone layer and a crushed stone layer, a ditch 3 and a ridge platform 4 are provided on the crushed stone layer, the ditch 3 comprises two side slopes, the two side slope surfaces are called side slopes 6, the middle position of the ditch 3 and lower than the ridge platform 4 is set as the ditch starting point, the two ends of the ditch 3 are respectively the two ditch end points, a middle slope from high to low extends from the ditch starting point to each of the two ditch end points, and the surface of each middle slope forms a positive slope 5; the shapes of the side slopes 6 of the two side slope surfaces are determined according to the middle slope; during the immersed tube squeezing process, the silt at the bottom of the immersed tube flows out of the bottom of the immersed tube along the two positive slopes 5.

[0005] The present invention further provides a method for designing a subgrade provided with an inclined self-draining ditch for an immersed tube tunnel, comprising the following steps:

[0006] (1) Determine the underwater angle of repose of the gravel;

[0007] (2) Conduct tests on the properties of the silt to determine the buoyant density γ', cohesion c, and internal friction angle of the silt. and external friction angle The value of

[0008] (3) According to the properties of the silt and the mechanical analysis of the slopes of the ridges, the depth h1 at the starting point of the ridge, the depth h2 at the end point of the ridge, the slope θ1 of the positive slope, and the slope θ2 of the side slope are designed;

[0009] (4) The minimum area S of the ridge under the immersed tube is determined based on the bottom load Q during the use of the immersed tube and the maximum allowable stress M obtained from the load test of the gravel used in the gravel base bed and the safety factor a:

[0010]

[0011] Where: L is the length of the immersed tube segment, D is the width of the immersed tube segment;

[0012] (5) Determine the ridge width B using the minimum ridge area S under the submerged tube and the mechanical analysis of the silt extrusion process of the silt on the ridge;

[0013] (6) A crushed stone layer with ridges is laid on the block stone layer, and the depth increases linearly from the starting point of the ridge to the end point of the ridge.

[0014] Furthermore, the method of step (3) is as follows:

[0015] 1) To ensure that the silt on the side slope of the ditch can slide onto the positive slope, the slope of the side slope of the ditch is designed. Let p be the silt-squeezing load at the bottom of the immersed tube when the tube segment is sunk. θ2 should meet the following two conditions:

[0016]

[0017] θ2 is smaller than the underwater angle of repose of the gravel and larger than the external friction angle of the silt.

[0018] 2) To ensure that the silt at the starting point of the ditch can flow down along the positive slope of the ditch, the depth h1 of the starting point of the ditch and the slope θ1 of the positive slope should satisfy:

[0019]

[0020] 3) Calculate the depth h2 at the end of the ditch based on the ditch length d, the slope θ1 of the ditch's positive slope, and the depth h1 at the ditch's starting point:

[0021] h2=h1+d·tanθ1

[0022] 4) The ridge width C is determined by the depth h2 at the end of the ridge and the slope θ2 of the side slope:

[0023]

[0024] Furthermore, the method of step (5) is as follows: calculate the area S1 of a single ditch at the bottom of the immersed tube by multiplying the ditch width C by half of the immersed tube width D:

[0025]

[0026] 2) Assume there are 2n furrows below the immersed tube section, and one platform corresponds to two furrows. Then the number of platforms is n+1. The maximum value of n, n1, is calculated using the following formula:

[0027]

[0028] In the above formula: [*] means rounding down the internal calculation result;

[0029] 3) Limit the width of the ridge platform to prevent the ridge platform from being too wide, which will prevent the silt on the ridge platform from being squeezed into the ditch by the immersed tube silt-squeezing load. The maximum width of the ridge platform B2 is obtained by the following formula:

[0030]

[0031] In the above formula, K is the lateral pressure coefficient, and H is the thickness of the silt on the ridge platform;

[0032] 4) Determine the minimum value n2 of n by the maximum ridge width B2, the formula is:

[0033]

[0034] In the above formula: {*} means rounding up the internal calculation result;

[0035] 5) Determine the value of n based on n1 and n2. The value of n is within the closed interval formed by n1 and n2. Then, calculate the ridge platform width B based on the number of furrows and ridge platforms. The formula is:

[0036]

[0037] Furthermore, in step (4), the safety factor a is between 1.5 and 2.

[0038] Furthermore, the thickness of the gravel layer is not greater than 1.5 m.

[0039] The present invention has the following advantages:

[0040] (1) The silt-absorbing capacity of the base bed ditch is improved by designing it to be high in the middle and low on both sides and increasing the depth of the opening end. At the same time, the ditch has a self-draining function. The silt in the base bed ditch is automatically cleared by utilizing the weight of the silt, the silt-squeezing load of the submerged tube and the slope of the ditch. This can not only ensure the smooth sinking of the submerged tube, but also save the time and cost of desilting the submerged tube base bed.

[0041] (2) The deposited silt falls into the foundation trench along the base bed ridge, which is simple to operate and easy to clean, saving engineering time. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 Schematic cross-sectional view of a subgrade with an inclined self-draining ditch for an immersed tube tunnel according to the present invention

[0043] Figure 2 Schematic diagram of ridge layout plan

[0044] Figure 3 Stress conditions of silt on the slope

[0045] Figure 4 Stress conditions of silt on ridges

[0046] Figure 5 Three views of ridges and ditches

[0047] Figure 6 Schematic diagram of ridges and ditches

[0048] In the figure, 1-sunk tube; 2-base bed; 3-ridge ditch; 4-ridge platform; 5-front slope; 6-side slope DETAILED DESCRIPTION

[0049] The present invention will be described below with reference to the accompanying drawings and embodiments.

[0050] This invention improves the gravel bed ditch structure of an immersed tube tunnel, providing a subgrade with an inclined, self-draining ditch for the immersed tube tunnel. It also provides a design method. By increasing the depth and width of the subgrade ditch opening, this method allows silt deposits of a certain thickness on the subgrade surface to flow out of the ditch, while also increasing the silt holding capacity of the ditch. This is now explained with reference to a specific example.

[0051] like Figure 1 and Figure 2As shown, the base bed of the immersed tube tunnel of the present invention is provided with an inclined self-draining ditch, including a foundation trench, a block stone layer and a crushed stone layer, and a ditch 3 and a ridge platform 4 are provided on the crushed stone layer, the ditch 3 includes two side slopes, and the two side slope surfaces are called side slopes 6, which is characterized in that the middle position of the ditch 3 and lower than the ridge platform 4 is set as the ditch starting point, and the two ends of the ditch 3 are respectively the two ditch end points, and a middle slope from high to low extends from the ditch starting point to the two ditch end points respectively, and the surface of each middle slope forms a positive slope 5; the shapes of the side slopes 6 of the two side slope surfaces are determined according to the middle slope; during the immersed tube squeezing process, the silt at the bottom of the immersed tube flows out of the bottom of the immersed tube along the two positive slopes 5.

[0052] See also Figure 3-6 The design method is as follows:

[0053] (1) Use a riprap boat to drop the boulders into the foundation trench through a grab bucket. The boulders are dumped in layers. The total thickness of the boulders is 2m, and the thickness of each layer is ≤0.5m to avoid uneven density caused by excessive accumulation. After each layer is completed, a hydraulic vibratory hammer is used to vibrate and compact the boulders layer by layer. Use a scraper-type leveler or manual adjustment to ensure that the surface flatness error is controlled within ±30mm.

[0054] (2) Use the natural accumulation method to calculate the underwater angle of repose of the gravel. Slowly and evenly pour the material through a funnel onto a horizontal substrate, forming a naturally accumulated cone. The maximum stable slope of the cone is the underwater angle of repose of the gravel. In this example, the underwater angle of repose of the gravel is 1:1.5.

[0055] (3) Conduct indoor tests on the silt to determine the buoyant density of the silt. The buoyant density of the silt is γ' = 12.6 kN / m 3 , cohesion c=0.3kPa, internal friction angle External friction angle

[0056] The silt-squeezing load of the immersed tube is 5 kPa. To ensure that the silt on the side slope 6 of the ditch 3 can fall smoothly into the ditch 3, the slope θ2 of the side slope 6 of the ditch 3 needs to be designed. The minimum value of θ2 should meet the following requirements:

[0057]

[0058] Since the external friction angle of the silt is small (less than θ2), the formula That is, the gravity of the silt has a promoting effect on the silt on the side slope 6 falling onto the positive slope 5. At the top of the side slope 6, the thickness of the silt is 0. At this time, the silt only slides down under the action of the bottom load of the immersed tube, that is, The minimum value of the slope θ2 of the side slope 6 is 4°. The greater the slope of the side slope 6, the easier it is for the silt to slide off the side slope 6. Therefore, the slope θ2 of the side slope 6 is set to 30°, which is less than the underwater angle of repose of the gravel.

[0059] In the entire ditch 3, the depth and thickness of the silt at the starting point of the ditch 3 are the smallest. In order to ensure that the silt at the starting point of the ditch 3 can flow smoothly downward along the positive slope 5 of the ditch 3, the depth h1 at the starting point of the ditch 3 and the slope θ1 of the positive slope 5 should satisfy:

[0060]

[0061] First, in order to ensure that the main force of silt extrusion is greater than the shear strength of the silt, the slope θ1 of the positive slope 5 should be greater than When the depth at the starting point of ditch 3 is 0.32m and the slope of the positive slope 5 is θ1 = 2.4°, the main force of silt extrusion at the starting point of ditch 3 is greater than the shear strength of the silt, and the silt flows smoothly down the positive slope 5. At the same time, the depth of 0.32m at the starting point of ditch 3 is the maximum thickness h of the silt on the positive slope 5 of ditch 3. f When the thickness of the silt is greater than 0.4 m, the deadweight load γ'h1 of the silt exceeds the silt squeezing load at the bottom of the immersed tube and becomes the main factor in the main force of silt squeezing.

[0062] Then, the depth h2 at the end of ditch 3 is calculated based on the length d of ditch 3 perpendicular to the axis of the immersed tube, the slope θ1 of the positive slope 5 of ditch 3, and the depth h1 at the starting point of ditch 3. The gravel base is 46.5m wide at the bottom, 42m wide at the top, and 1.5m thick. The slope is 1:1.5. The length d of ditch 3 should be a value between half the width of the bottom of the gravel base and half the width of the top of the gravel base. d is taken as 23m. The depth h2 at the end of ditch 3 is:

[0063] h2=h1+d·tanθ1=1.28m

[0064] The width C of the ditch 3 is determined by the depth h2 at the end of the ditch 3 and the slope of the side slope 6. The width of the positive slope 5 gradually decreases from the start point to the end point of the ditch 3. At the end point of the ditch 3, the width of the positive slope 5 becomes 0. At this time, the width of the ditch 3 is the same as the top view width of the side slope 6. The calculation method of the ditch 3 width is:

[0065]

[0066] At the end of ridge 3, the width of the positive slope 5 is 0, and at the starting point of ridge 3, the width of the positive slope 5 is

[0067]

[0068] (4) The minimum area of ​​platform 4 is determined based on the maximum allowable stress value and safety factor obtained from the bottom load during the use of the immersed tube and the load test of the gravel used in the gravel base. In this example, the bottom load during the use of the immersed tube is 63.2kPa, the length of the immersed tube is 180m, the width of the immersed tube is 37.95m, the maximum allowable stress value is 500kPa, and the safety factor is 2. Therefore, the maximum stress borne by platform 4 at the bottom of the immersed tube is 250kPa. The minimum area S of platform 4 at the bottom of the immersed tube is calculated to be 1727m 2 .

[0069] (5) The width of the ridge platform B is determined by using the minimum area S of the ridge platform under the immersed tube and the mechanical analysis of the silt extrusion process on the ridge platform.

[0070] The area of ​​a single ditch at the bottom of the immersed tube, S1, is calculated by multiplying the ditch width C by half the immersed tube width D. The area S1 of a single ditch at the bottom of the immersed tube is 84.06m 2 .

[0071] Assuming that there are 2n furrows 3 under the pipe section (since the furrows 3 are arranged symmetrically along the axis, one ridge platform 4 corresponds to two furrows 3), the number of ridge platforms 4 is n+1. The maximum value of n is obtained by dividing the maximum furrow area at the bottom of the submerged pipe by 2S1 and rounding down. The maximum furrow area at the bottom of the submerged pipe is obtained by subtracting the minimum area of ​​the ridge platform under the submerged pipe, S, from the bottom area of ​​the submerged pipe. The formula is: [*] indicates that the internal calculation result is rounded down, and n1 is 30.

[0072] During the silt-squeezing process of the submerged tube, the silt on the platform 4 will flow to the surrounding area under the action of the submerged tube silt-squeezing load. Compared to the submerged tube boundary, the distance between the ditch 3 and the platform 4 is closer, so the silt will preferentially fall into the ditch 3. In order to ensure that the silt on the platform 4 can be smoothly squeezed into the ditch 3 under the action of the submerged tube silt-squeezing load, it is necessary to limit the width of the ridge platform 4 to prevent the ridge 4 from being too wide and the silt from being squeezed into the ditch 3 by the submerged tube silt-squeezing load. The maximum width of the ridge platform B2 is obtained by the following formula:

[0073]

[0074] Where K is the lateral pressure coefficient of the immersed tube silt-squeezing load, which is taken as 1.0, H is the thickness of the silt on the ridge platform 4, the left side of the formula is the main force for silt squeezing of the immersed tube on the ridge platform 4, and the right side of the formula is the silt squeezing resistance of the silt on the ridge platform 4, as shown in the following example: Figure 4 As shown in Figure 2, the ridge platform reaches its maximum width B2, reaching its limit state, where the silt-squeezing resistance equals the main silt-squeezing force. When the silt thickness on ridge platform 4 is H = 0.2 m, the maximum width B2 of ridge platform 4 is 2.82 m.

[0075] The minimum value n2 of n is determined by the maximum width B2 of the ridge platform, and the formula is: {*} means that the internal calculation result is rounded up, and the minimum value of n n2 is 25.

[0076] By comparing n1 and n2, the value of n is determined. In order to improve the bearing capacity and stability of the base bed, the value of n that makes the ridge platform width larger should be selected. In this embodiment, the value of n is 25. Then the ridge platform width B is calculated by the number of furrows and the number of ridge platforms. The formula is The ridge width B is obtained to be 2.66m.

[0077] A gravel layer with ridges 3 is laid on the block stone layer. The top width of the gravel layer is 42m, and the bottom width is 46.5m. The gravel layer at the bottom of the immersed tube section is provided with 25 symmetrically distributed ridges 3 and 26 ridge platforms 4. The width of the ridge 3 is 4.43m, and the width of the ridge platform 4 is 2.66m. The ridge 3 consists of two side slopes 6 and a positive slope 5. The slope of the side slope 6 is 30°, and the slope of the positive slope 5 is 2.4 degrees. The depth at the starting point of the ridge 3 is 0.32m, and the depth at the end point of the ridge 3 is 1.28m. During the immersed tube squeezing process, the silt on the ridge platform 4 will flow into the ditch 3 under the action of the immersed tube squeezing load. The silt on the side slope 6 in the ditch 3 will then flow to the positive slope 5 under the action of the immersed tube squeezing load and the gravity of the silt. The silt on the positive slope 5 accumulates to the maximum thickness h of the silt. f Then, the water will flow out of the ditch 3 along the positive slope 5, thereby achieving the effect of automatic silt removal by utilizing the ditch 3.

Claims

1. A subgrade with an inclined self-draining ditch for an immersed tube tunnel, comprising a foundation trench, a block stone layer and a crushed stone layer, a ditch (3) and a ridge platform (4) being provided on the crushed stone layer, the ditch (3) comprising two side slopes, the two side slope surfaces being referred to as side slopes (6), characterized in that: The position on the ditch (3) that is close to the middle and lower than the ridge platform (4) is set as the ditch starting point, and the two ends of the ditch (3) are respectively the two ditch end points. A middle slope extending from high to low extends from the ditch starting point to the two ditch end points, and the surface of each middle slope forms a positive slope (5); the shape of the side slopes (6) of the two side slope surfaces is determined according to the middle slope; during the silt squeezing process of the immersed tube, the silt at the bottom of the immersed tube flows out of the bottom of the immersed tube along the two positive slopes (5).

2. The method for designing a subgrade with an inclined self-draining ditch for an immersed tube tunnel according to claim 1, comprising the following steps: (1) Determine the underwater angle of repose of the gravel; (2) Conduct tests on the properties of the silt to determine the buoyant density γ', cohesion c, and internal friction angle of the silt. and external friction angle The value of (3) According to the properties of the silt and the mechanical analysis of the slopes of the ridges, the depth h1 at the starting point of the ridge, the depth h2 at the end point of the ridge, the slope θ1 of the positive slope, and the slope θ2 of the side slope are designed; (4) The minimum area S of the ridge under the immersed tube is determined based on the bottom load Q during the use of the immersed tube and the maximum allowable stress M obtained from the load test of the gravel used in the gravel base bed and the safety factor a: Where: L is the length of the immersed tube segment, D is the width of the immersed tube segment; (5) Determine the ridge width B using the minimum ridge area S under the submerged tube and the mechanical analysis of the silt extrusion process of the silt on the ridge; (6) A crushed stone layer with ridges is laid on the block stone layer, and the depth increases linearly from the starting point of the ridge to the end point of the ridge.

3. The method for designing a base bed according to claim 2, characterized in that: The method of step (3) is as follows: 1) To ensure that the silt on the side slope of the ditch can slide onto the positive slope, the slope of the side slope of the ditch is designed. Let p be the silt-squeezing load at the bottom of the immersed tube when the tube segment is sunk. θ2 should meet the following two conditions: θ2 is smaller than the underwater angle of repose of the gravel and larger than the external friction angle of the silt. 2) To ensure that the silt at the starting point of the ditch can flow down along the positive slope of the ditch, the depth h1 of the starting point of the ditch and the slope θ1 of the positive slope should satisfy: 3) Calculate the depth h2 at the end of the ditch based on the ditch length d, the slope θ1 of the ditch's positive slope, and the depth h1 at the ditch's starting point: h2=h1+d·tanθ1 4) The ridge width C is determined by the depth h2 at the end of the ridge and the slope θ2 of the side slope:

4. The method for designing a base bed according to claim 3, characterized in that: The method of step (5) is as follows: Calculate the area S1 of a single ditch at the bottom of the immersed tube by multiplying the ditch width C by half of the immersed tube width D: 2) Assume there are 2n furrows below the immersed tube section, and one platform corresponds to two furrows. Then the number of platforms is n+1. The maximum value of n, n1, is calculated using the following formula: In the above formula: [*] means rounding down the internal calculation result; 3) Limit the width of the ridge platform to prevent the ridge platform from being too wide, which will prevent the silt on the ridge platform from being squeezed into the ditch by the immersed tube silt-squeezing load. The maximum width of the ridge platform B2 is obtained by the following formula: In the above formula, K is the lateral pressure coefficient, and H is the thickness of the silt on the ridge platform; 4) Determine the minimum value n2 of n by the maximum ridge width B2, the formula is: In the above formula: {*} means rounding up the internal calculation result; 5) Determine the value of n based on n1 and n2. The value of n is within the closed interval formed by n1 and n2. Then, calculate the ridge platform width B based on the number of furrows and ridge platforms. The formula is:

5. The method for designing a base bed according to claim 1, characterized in that: In step (4), the safety factor a is between 1.5 and 2.

6. The method for designing a base bed according to claim 1, characterized in that: The thickness of the gravel layer shall not exceed 1.5m.

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