A support system for actively controlling foundation pit displacement and its design method

Abstract

The present invention relates to a support system for actively controlling displacement of a foundation pit and a design method thereof. The support system includes a retaining structure, an adjustable force-adding device, a concrete transverse support system, and a column system. The force-adding device is arranged transversely, and its two ends are respectively connected to the retaining structure and the concrete transverse support system. The concrete transverse support system is one or a combination of an integral rod structure or a plate structure. The column system vertically supports the concrete transverse support system. The present invention takes into account both the stability of the support system and the active control of displacement of the retaining structure. The concrete support system is not easy to lose stability, thus ensuring the stability of the entire system. When the force-adding device applies axial force, the concrete support system of the integral rod structure or the plate structure generates less adverse secondary internal force, thus providing high safety. It is particularly suitable for foundation pits with complex shapes and large spans, and fully utilizes the advantages of flexible arrangement of concrete supports.

Description

Technical Field

[0001] The invention relates to a support system in a foundation pit, in particular to a support system for actively controlling the displacement of a foundation pit and a design method thereof, belonging to the technical field of foundation pit engineering. Background Art

[0002] Excavation of foundation pits will cause settlement and displacement of surrounding strata, thus affecting the safety of nearby subway tunnels and buildings. The types of existing support systems and the problems they have are as follows:

[0003] 1. Concrete support system. This system has high stability, but no remedial measures can be taken after displacement occurs, so it cannot actively control the displacement of the enclosure structure. In addition, concrete support systems are mostly statically indeterminate structures. Concrete components are subjected to bending, compression, shear, and torsion, and the stress conditions are very complex. If a jack is added to the components inside the system to apply axial force, secondary stress will be generated in the entire system, which can easily lead to different forces and values ​​of concrete components, making it difficult to meet safety requirements.

[0004] 2. Servo steel support system: This system uses a jack to connect a steel support. It is a long strip structure. Although it can control the displacement of the retaining structure, it is not suitable for foundation pits with complex shapes. In addition, in the case of large-scale foundation pit excavation, the span of the steel support is large and it is easy to become unstable. Once unstable, a chain reaction of failure will occur, increasing the risk of the foundation pit.

[0005] 3. The combination system of steel support and concrete support. The common form is the servo-type steel support combined with concrete purlin, but this still cannot solve the shortcomings mentioned in Article 2. Another common form is to replace the internal components in the concrete support with servo-type steel support. However, the concrete system is an over-static structure. When the servo-type steel support applies axial force, secondary stress will be generated in the system, the safety of the system will be weakened, and the risk of foundation pit will be increased.

[0006] In addition, when dealing with large-area foundation pits, existing technologies will also adopt the method of foundation pit sub-pit construction during construction, dividing the large-area foundation pit into multiple small foundation pits for excavation and support. This solution requires the construction of partition walls between multiple sub-pits, which greatly increases the construction period and cost.

[0007] In summary, the existing technology cannot take into account the advantages of active displacement control and system stability, and the construction period is long and the cost is high. Summary of the invention

[0008] The purpose of the present invention is to provide a support system for actively controlling the displacement of a foundation pit and a design method thereof, which has the advantages of actively controlling the displacement of a retaining structure and stabilizing the support system. The displacement of the retaining structure can be controlled by a force-adding device to ensure the stability of the concrete support system; and, in the case of large-area excavation, zoning can be avoided, thus saving construction costs and construction period.

[0009] The present invention adopts the following technical solutions:

[0010] A support system for actively controlling foundation pit displacement, comprising a retaining structure, an adjustable force-adding device, a concrete transverse support system 3, and a column system; the force-adding device is arranged transversely, and its two ends are respectively connected to the retaining structure and the concrete transverse support system 3; the concrete transverse support system 3 is one or a combination of an integral rod structure or a plate structure; the column system vertically supports the concrete transverse support system 3.

[0011] Furthermore, the retaining structure is an underground continuous wall.

[0012] Furthermore, the enclosure structure is a row of piles 17 , and the row of piles 17 is connected to a force-adding device via a surrounding purlin 18 .

[0013] Furthermore, the concrete transverse support system 3 is composed of reinforced concrete, and the column system is one or more of steel lattice columns, steel pipes, concrete, and steel tube concrete.

[0014] Furthermore, the column system is composed of a plurality of columns 4, and the lower ends of the columns 4 are inserted into column piles or soil.

[0015] Furthermore, embedded parts pads 10 are provided at both ends of the force-adding device, and anchoring steel bars 6 extend from the embedded parts pads. The anchoring steel bars 6 are embedded in the interior of the lateral support system 3 to be cast in situ and the enclosure structure.

[0016] Furthermore, one side of the force-adding device is connected to the enclosure structure, connected to the embedded part pad 10 through a sliding hinge support, and connected to the enclosure structure through an anchoring steel bar 6.

[0017] Furthermore, the retaining structure is a row of piles 17 , and a purlin 18 is arranged on one side of the row of piles 17 close to the inside of the foundation pit. The sliding hinge support is connected to the purlin 18 through an embedded part pad 10 and an anchor steel bar 6 .

[0018] Furthermore, the enclosure purlin is a steel enclosure purlin, which is directly welded or bolted to the sliding hinge support, and the sliding hinge support is connected to the force-adding device.

[0019] Furthermore, the sliding hinge support includes a force-adding device pad, a ball or rod 8 and a slide groove 9. The ball or rod 8 is placed between the force-adding device pad and the slide groove 9, and relative sliding can occur between the force-adding device pad and the slide groove 9.

[0020] Furthermore, the ball or rod 8 is made of high-strength material.

[0021] Furthermore, the force-adding device is controlled by an external control system 12 to apply a lateral thrust to the enclosure structure.

[0022] Furthermore, the force-adding device is a jack.

[0023] Furthermore, concrete pier self-locking devices are provided between some or all of the adjacent force-adding devices.

[0024] Furthermore, the concrete pier self-locking device includes a second concrete pier 14, one end of which is connected to the concrete lateral support system 3, and the other end has a wedge block bearing part, on which is provided a wedge block group consisting of a pair of mutually cooperating wedge blocks, and the wedge block group supports the surrounding structure.

[0025] A design method for a support system for actively controlling foundation pit displacement is disclosed. A concrete transverse support system 3 is configured as an integral rod structure or a plate structure. A force-adding device with adjustable thrust is fixedly arranged laterally between a retaining structure and the concrete transverse support system 3. A column system is used to support the concrete transverse support system 3.

[0026] Furthermore, during construction, the following steps are included:

[0027] S1. Locate and construct underground enclosure structure and column system;

[0028] S2, excavate the first layer of soil to the depth of the bottom of the first layer of support, set up the concrete support formwork, place the steel cage, and connect it with the column system; pour concrete to form the first concrete lateral support system 3;

[0029] S3, when the first concrete transverse support system 3 is cured to the design strength, the second layer of soil is excavated to the depth of the bottom of the second support, a concrete support formwork is erected, a steel cage is placed, and connected to the column system; concrete is poured to form the second concrete transverse support system 3;

[0030] In S4, between steps S2 and S3, and at least one of the steps after S3, when the concrete transverse support system 3 is cured to the design strength, a force-adding device is installed, and the two ends of the force-adding device are respectively connected to the concrete transverse support system 3 and the retaining structure; the force-adding device is connected to the external control system 12 through an oil pipe, and during the excavation process, if the retaining structure has a large displacement, the oil pressure is controlled by the control system 12 to apply a lateral thrust to the retaining structure outside the foundation pit, thereby reducing the displacement of the retaining structure and realizing active control of the displacement;

[0031] S5. If it is necessary to excavate the third layer or the soil below the third layer to the depth of the corresponding support bottom, repeat the steps of S3 and S4;

[0032] Furthermore, in step S1, the retaining structure is an underground continuous wall 2, and anchor steel bars 6 and embedded pads 10 are pre-embedded in the steel cage of the underground continuous wall 2, and the pre-embedded positions of the anchor steel bars 6 and embedded pads 10 are consistent with the positions where the force-adding device is installed; in step S4, the non-active head end of the force-adding device is welded and fixed to the embedded pad 10, and connected to the second concrete transverse support system 3 through the anchor steel bars 6; the active head 13 on the other side is welded to the force-adding device pad, and the force-adding device pad is confined in the slide groove 9, and a ball or rod 8 is placed between the force-adding device pad and the slide groove 9, so that the force-adding device pad and the slide groove can slide relative to each other to form a sliding hinge support, and the slide groove 9 is welded to the embedded pad 10, and connected to the underground continuous wall 2 through the anchor steel bars 6.

[0033] Furthermore, in step S3, after pouring concrete, a purlin 18 and a first concrete pier 20 are formed; a grouting hole 19 is provided on one side of the first concrete pier 20 close to the purlin 18; in step S4, when a gap is generated between the purlin 18 and the first concrete pier 20, grouting material is poured through the grouting hole 19 to close the gap.

[0034] Furthermore, the enclosure structure adopts an underground continuous wall. In step S3, after pouring concrete, a first concrete pier 20 is also formed; a grouting hole 19 is set on the side of the first concrete pier 20 close to the underground continuous wall; in step S4, when a gap is generated between the purlin 18 and the first concrete pier 20, grouting material is poured through the grouting hole 19 to close the gap.

[0035] Furthermore, in step S4, a concrete pier self-locking device is provided between some or all of the adjacent force-applying devices, and a lower wedge 15 and an upper wedge 16 are placed on the second concrete pier 14 of the concrete pier self-locking device. The angles of the upper wedge and the lower wedge are smaller than the self-locking angle of the material. When a gap is generated between the underground continuous wall 2 and the lower wedge 15, the upper wedge 16 is struck to close the gap between the underground continuous wall 2 and the lower wedge 15. The beneficial effects of the present invention are:

[0036] 1) Take into account both the stability of the support system and the active control of the displacement of the retaining structure. Use a force-adding device to apply a lateral thrust to the retaining structure outside the foundation pit to reduce the displacement of the retaining structure; the concrete support system is not easy to become unstable, ensuring the stability of the entire system.

[0037] 2) When the force-adding device applies axial force, the concrete support system of the overall bar structure and plate structure generates less adverse secondary internal force and has high safety.

[0038] 3) It is especially suitable for foundation pits with complex shapes, making full use of the advantages of flexible arrangement of concrete supports.

[0039] 4) During the excavation of large-scale foundation pits, there is no need to carry out sub-pit excavation and construction of partition walls, which reduces project costs, material consumption, shortens construction period, and is green and environmentally friendly.

[0040] 5) The sliding hinge support in this system can ensure the stability of the force-adding device in the event of vertical displacement such as column uplift, thereby reducing the risk.

[0041] 6) The ingenious design of the concrete pier self-locking device further ensures the stability of the entire system.

[0042] 7) The concrete pier grouting hole device is cleverly designed, further ensuring the stability of the entire system. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 It is a top view of the support system for actively controlling foundation pit displacement in the first embodiment of the present invention.

[0044] Figure 2 yes Figure 1 Elevation section view of .

[0045] Figure 3 yes Figure 1 Detailed view of the booster device at the middle AA position.

[0046] Figure 4 yes Figure 1 Detailed view of the booster device in the middle BB area.

[0047] Figure 5 It is a top view of the support system for actively controlling foundation pit displacement in the second embodiment.

[0048] Figure 6 yes Figure 5 Elevation section view of .

[0049] Figure 7 yes Figure 5 Detailed drawing of the concrete pier and grouting hole arrangement at the middle CC position.

[0050] The reference numerals in the figure are: 1. jack; 2. underground continuous wall; 3. concrete lateral support system; 4. column; 5. column pile; 6. anchor steel bar; 7. jack pad; 8. rod; 9. slide; 10. embedded part pad; 11. oil pipe; 12. control system; 13. force device joint; 14. second concrete pier; 15. lower wedge; 16. upper wedge; 17. pile row; 18. purlin; 19. grouting hole; 20. first concrete pier. DETAILED DESCRIPTION

[0051] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments:

[0052] Embodiment 1:

[0053] like Figure 1-4 As shown, this embodiment uses an underground continuous wall as a retaining structure, and when implemented specifically, the following steps are included:

[0054] 1) Locate and construct the underground continuous wall 2, column 4 and column pile 5, wherein the anchor steel bar 6 and the embedded part pad 10 are embedded in the steel cage of the underground continuous wall 2, and the embedded position of the anchor steel bar 6 and the embedded part pad 10 is consistent with the position where the jack is installed. Considering the construction error, the embedded part pad 10 can appropriately use a steel plate larger than the design size. The column 4 is inserted into the column pile 5, and the specific insertion depth can be adjusted according to the design requirements and the force.

[0055] 2) Excavate the first layer of soil to the depth of the bottom of the first support layer, set up a concrete support formwork, place a steel cage, and weld it to the column 4; pour concrete to form the first concrete lateral support system 3.

[0056] 3) When the first concrete lateral support system 3 is cured to the design strength, the second layer of soil is excavated to the depth of the bottom of the second support, a concrete support formwork is erected, a steel cage is placed, and connected to the column 4, and anchor steel bars 6 and embedded pads 10 are placed at the corresponding jack positions; concrete is poured to form a second concrete lateral support system 3 and a second concrete pier 14.

[0057] 4) When the second concrete transverse support system 3 and the second concrete pier 14 are cured to the design strength, the jack 1 is installed, and the non-active head end of the jack 1 is welded and fixed on the embedded part pad 10, and connected to the second concrete transverse support system 3 through the anchor steel bar 6; the active head 13 on the other side is welded on the jack pad 7, and the jack pad 7 is limited in the slide 9. A steel ball is placed between the jack pad 7 and the slide 9, so that the jack pad 7 and the slide can slide relative to each other to form a sliding hinge support. The slide 9 is welded on the embedded part pad 10 and connected to the underground continuous wall 2 through the anchor steel bar 6. A lower wedge 15 and an upper wedge 16 are placed on the second concrete pier 14, and the angle between the upper wedge and the lower wedge is smaller than the self-locking angle of the material.

[0058] like Figure 1 As shown, the spacing between jacks can be determined as needed based on design requirements.

[0059] In addition, the movable head end of the jack can be directly connected to the embedded part pad 10, and then connected to the underground continuous wall 2 through the anchor steel bar 6, but this is not a preferred solution. The second concrete pier 14, the lower wedge block 15 and the upper wedge block 16 are preferred solutions, but not essential components.

[0060] 5) The jack 1 is connected to the external control system 12 through an oil pipe. During the excavation process, if the retaining structure produces a large displacement, the oil pressure is controlled by the control system 12 to apply a lateral thrust to the underground continuous wall 2 outside the foundation pit, thereby reducing the displacement of the underground continuous wall 2 and realizing active control of the displacement; when a gap is generated between the underground continuous wall 2 and the lower wedge block 15, the upper wedge block 16 can be struck to close the gap between the underground continuous wall 2 and the lower wedge block 15.

[0061] 6) Excavate the third layer of soil to the depth of the bottom of the third layer of support, and repeat steps 3, 4, and 5.

[0062] 7) Excavate the fourth layer of soil to the bottom of the pit and complete all excavation.

[0063] Embodiment 2:

[0064] like Figure 3 , Figure 5-7 As shown, this embodiment uses pile rows as the enclosure structure, and when implemented, the following steps are included:

[0065] 1) Position and construct the pile row 17, the column 4 and the column pile 5. The column 4 is inserted into the column pile 5, and the specific insertion depth can be adjusted according to the design requirements and the force.

[0066] 2) Excavate the first layer of soil to the depth of the bottom of the first support layer, set up a concrete support formwork, place a steel cage, and weld it to the column 4; pour concrete to form the first concrete lateral support system 3.

[0067] 3) When the first concrete transverse support system 3 is cured to the design strength, the second layer of soil is excavated to the depth of the bottom of the second support, a concrete support formwork is erected, a steel cage is placed, and connected to the column 4, and anchor steel bars 6 and embedded parts pads 10 are placed at the corresponding jack positions; concrete is poured to form the second concrete transverse support system 3, the purlin 18 and the first concrete pier 20; a grouting hole 19 is set on the side of the first concrete pier 20 close to the purlin 18.

[0068] 4) When the second concrete transverse support system 3 and the first concrete pier 20 are cured to the design strength, the jack 1 is installed. The non-active head end of the jack 1 is fixed to the embedded part pad 10 by bolts, and connected to the second concrete transverse support system 3 by the anchor steel bar 6; the active head 13 on the other side is fixed to the jack pad 7 by bolts, and the jack pad 7 is confined in the slide groove 9. Steel balls are placed between the jack pad 7 and the slide groove 9, so that the jack pad 7 and the slide groove can slide relative to each other to form a sliding hinge support. The slide groove 9 is fixed to the embedded part pad 10 by bolts, and connected to the surrounding purlin 18 by the anchor steel bar 6.

[0069] like Figure 5As shown, the spacing between jacks can be determined as needed based on design requirements.

[0070] In addition, the movable head end of the jack can be directly connected to the embedded part pad 10, and then connected to the purlin 18 through the anchor steel bar 6, but this is not a preferred solution. Figure 7 As shown, the first concrete pier 20 and the grouting holes 19 are preferred solutions, but are not essential components.

[0071] 5) The jack 1 is connected to the external control system 12 through the oil pipe. During the excavation process, if the retaining structure has a large displacement, the oil pressure is controlled by the control system 12 to apply a lateral thrust to the purlin 18 out of the foundation pit, reducing the displacement of the pile row 17 and realizing active control of the displacement; when a gap is generated between the purlin 18 and the first concrete pier 20, the grouting material can be poured through the grouting hole 19 to close the gap.

[0072] 6) Excavate the third layer of soil to the bottom of the pit and complete all excavation.

[0073] The above are two embodiments of the present invention. A person skilled in the art may make various changes or improvements on this basis. Without departing from the general concept of the present invention, these changes or improvements should fall within the scope of protection claimed by the present invention.

Claims

1. A support system for actively controlling the displacement of a foundation pit, characterized in that : It includes a protective structure, an adjustable force-adding device, a concrete lateral support system (3), and a column system; The force-adding device is arranged horizontally, and its two ends are respectively connected to the enclosure structure and the concrete horizontal support system (3); The concrete lateral support system (3) is an integral bar structure; The column system vertically supports the concrete lateral support system (3); The concrete lateral support system (3) is an integrated concrete rod structure, wherein a row of rods adjacent to the force-applying device is in the shape of a continuous triangular rod, and the rest are in the shape of an interval rod; Embedded pads (10) are provided at both ends of the force-adding device, anchoring steel bars (6) extend from the embedded pads, and the anchoring steel bars (6) are embedded in the interior of the lateral support system (3) to be cast in situ and the enclosure structure; The force-adding device is connected to one side of the enclosure structure, connected to the embedded component pad (10) via a sliding hinge support, and connected to the enclosure structure via an anchoring steel bar (6); The enclosure structure is a row of piles (17), a purlin (18) is arranged on one side of the row of piles (17) close to the inside of the foundation pit, and the sliding hinge support is connected to the purlin (18) through an embedded pad (10) and an anchor steel bar (6); The sliding hinge support comprises a force-adding device pad, a ball or a rod (8) and a slide groove (9), wherein the ball or the rod (8) is arranged between the force-adding device pad and the slide groove (9), and relative sliding can be generated between the force-adding device pad and the slide groove (9); Concrete pier self-locking devices are provided between some or all of the adjacent force-adding devices; The concrete pier self-locking device comprises a second concrete pier (14), one end of the second concrete pier (14) is connected to the concrete transverse support system (3), and the other end has a wedge bearing portion, and a wedge block group consisting of a pair of mutually matching wedge blocks is provided on the wedge block bearing portion, and the wedge block group supports the enclosure structure.

2. The support system for actively controlling foundation pit displacement according to claim 1, characterized in that :The retaining structure is an underground continuous wall.

3. The support system for actively controlling foundation pit displacement according to claim 1, characterized in that The enclosure structure is a row of piles (17), and the row of piles (17) is connected to a force-adding device via a surrounding purlin (18).

4. The support system for actively controlling foundation pit displacement according to claim 1, characterized in that The concrete lateral support system (3) is composed of reinforced concrete, and the column system is one or more of steel lattice columns, steel pipe columns, concrete columns, and steel pipe concrete columns.

5. The support system for actively controlling foundation pit displacement according to claim 1, characterized in that The column system is composed of a plurality of columns (4), and the lower ends of the columns (4) are inserted into column piles or soil.

6. The support system for actively controlling foundation pit displacement according to claim 3, characterized in that :The purlin is a steel purlin, which is directly welded or bolted to the sliding hinge support, and the sliding hinge support is connected to the force-adding device.

7. The support system for actively controlling foundation pit displacement according to claim 1, characterized in that : The ball or rod (8) is made of high-strength material.

8. The support system for actively controlling foundation pit displacement according to claim 1, characterized in that The force-adding device is controlled by the control system (12) to apply a lateral thrust to the enclosure structure.

9. The support system for actively controlling foundation pit displacement according to any one of claims 1 to 8, characterized in that: The force adding device is a jack.

10. A design method for a support system for actively controlling foundation pit displacement according to any one of claims 1 to 9, characterized in that: The concrete transverse support system (3) is configured as an integral rod structure or a plate structure, a force-adding device with adjustable thrust is fixedly arranged laterally between the enclosure structure and the concrete transverse support system (3), and a column system is used to support the concrete transverse support system (3); The construction process includes the following steps: S1. Locate and construct underground enclosure structure and column system; S2, excavate the first layer of soil to the depth of the bottom of the first support, erect concrete support formwork, place steel cages, and connect them to the column system; pour concrete to form the first concrete lateral support system (3); S3. When the first concrete transverse support system (3) is cured to the design strength, the second layer of soil is excavated to the depth of the bottom of the second support, a concrete support formwork is erected, a steel cage is placed, and connected to the column system; concrete is poured to form the second concrete transverse support system (3); S4, at least one of steps S2 and S3, and after S3, Implement S4a: When the concrete lateral support system (3) is cured to the design strength, a force-adding device is installed, and the two ends of the force-adding device are respectively connected to the concrete lateral support system (3) and the retaining structure; the force-adding device is connected to an external control system (12) through an oil pipe. During the excavation process, if the retaining structure is displaced, the oil pressure is controlled by the control system (12) to apply a lateral thrust to the retaining structure outside the foundation pit, thereby reducing the displacement of the retaining structure and realizing active control of the displacement; S5. If it is necessary to excavate the third layer or the soil below the third layer to the depth of the corresponding support bottom, repeat the steps of S3 and S4a.

11. The design method of the support system for actively controlling foundation pit displacement according to claim 10, characterized in that: In step S1, the enclosure structure is an underground continuous wall (2), anchoring steel bars (6) and embedded pads (10) are pre-embedded in a steel cage of the underground continuous wall (2), and the pre-embedded positions of the anchoring steel bars (6) and embedded pads (10) are consistent with the positions where the force-adding device is installed; In step S4, the non-active head end of the force-adding device is welded and fixed on the embedded part pad (10), and connected to the second concrete transverse support system (3) through the anchor steel bar (6); the active head (13) on the other side is welded to the force-adding device pad, and the force-adding device pad is confined in the slide groove (9). A ball or a rod (8) is placed between the force-adding device pad and the slide groove (9), so that the force-adding device pad and the slide groove can slide relative to each other to form a sliding hinge support. The slide groove (9) is welded to the embedded part pad (10) and connected to the underground continuous wall (2) through the anchor steel bar (6).

12. The design method of the support system for actively controlling foundation pit displacement according to claim 10, characterized in that: In step S3, after pouring concrete, a perimeter purlin (18) and a first concrete pier (20) are formed; a grouting hole (19) is provided on one side of the first concrete pier (20) close to the perimeter purlin (18); In step S4, when a gap is generated between the perimeter purlin (18) and the first concrete pier (20), grouting material is injected through the grouting hole (19) to close the gap.

13. The design method of the support system for actively controlling foundation pit displacement according to claim 10, characterized in that: The enclosure structure adopts an underground continuous wall. In step S3, after pouring concrete, a first concrete pier (20) is formed. A grouting hole (19) is provided on one side of the first concrete pier (20) close to the underground continuous wall. In step S4, when a gap is generated between the perimeter purlin (18) and the first concrete pier (20), grouting material is poured through the grouting hole (19) to close the gap.

14. The design method of the support system for actively controlling foundation pit displacement according to claim 10, characterized in that: In step S4, a concrete pier self-locking device is arranged between some or all of the adjacent force-applying devices, and a lower wedge block (15) and an upper wedge block (16) are placed on the second concrete pier (14) of the concrete pier self-locking device. The angle between the upper wedge block and the lower wedge block is smaller than the self-locking angle of the material. When a gap is generated between the underground continuous wall (2) and the lower wedge block (15), the upper wedge block (16) is struck to close the gap between the underground continuous wall (2) and the lower wedge block (15).

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