Unsupported support structure and method for deep foundation pits in saturated soft clay areas

Abstract

This invention discloses a support structure and method for deep foundation pits in saturated soft clay areas without internal support. The support structure includes SWM piles, continuously arranged along the edge of the foundation pit. The steel sections inserted into the SWM piles are recyclable steel sections, and the tops of the steel sections extend beyond the tops of the SWM piles. A reinforced concrete cap beam is horizontally enclosed on the SWM piles and fixedly connected to the extension ends of each steel section to connect all the steel sections into a whole. Multiple prestressed jet grouting anchors are set along the edge of the foundation pit. Each prestressed jet grouting anchor has an interconnected free section and an anchoring section. The free section is used for fixed connection with the reinforced concrete cap beam, and the anchoring section extends downward in an inclined direction and is inserted into the soil outside the foundation pit. This invention overcomes the drawbacks of traditional foundation pit support technology by combining SWM piles, prestressed jet grouting anchors, and reinforced concrete cap beams.

Description

Technical Field

[0001] This invention relates to the field of foundation pit support technology in building engineering, and in particular to a deep foundation pit support structure and method without internal support in saturated soft clay areas. Background Technology

[0002] For deep foundation pit projects in coastal areas with complex geological conditions such as water-rich soft soil strata, silt, and weak soil layers, the following core problems will arise if traditional foundation pit support technology is used:

[0003] 1) Serious waste of resources: The cast-in-place pile system consumes a large amount of concrete (the material cost of a single pile is ≥ 1200 yuan / linear meter), and the steel is not recyclable.

[0004] 2) Insufficient deformation control: Under complex geological conditions such as soft soil and sand layers, the horizontal displacement of the pile top of the single SMW method pile + cap beam system in soft soil areas cannot meet the requirement of displacement ≤40mm in GB50497-2019.

[0005] 3) Environmental drawbacks: Traditional anchor bolt construction generates 2 m³ / -3 m³ / bolt of mud discharge, and concrete waste is difficult to dispose of. Summary of the Invention

[0006] This invention provides a support structure and method for deep foundation pits in saturated soft clay areas without internal support. It overcomes the drawbacks of traditional foundation pit support technology by combining SWM piles, prestressed jet grouting anchors, and reinforced concrete cap beams.

[0007] To achieve the above objectives, the present invention provides a deep foundation pit support structure without internal bracing in saturated soft clay areas, comprising:

[0008] SWM method piles are continuously arranged along the edge of the foundation pit. The steel sections inserted into the SWM method piles are recyclable steel sections, and the top of the steel sections extends beyond the top of the SWM method piles.

[0009] A reinforced concrete cap beam is horizontally enclosed on the SWM method pile and fixedly connected to the extension end of each of the steel sections to connect all the steel sections into a whole.

[0010] Multiple prestressed jet grouting anchors are installed along the edge of the foundation pit. Each prestressed jet grouting anchor has an interconnected free section and an anchoring section. The free section is used to fix the anchor to the reinforced concrete cap beam, and the anchoring section extends downward in an inclined direction and is inserted into the soil outside the foundation pit.

[0011] Preferably, the angle between the extension direction of the anchoring section and the horizontal direction is in the range of 25°-35°.

[0012] Preferably, the free segment and the anchoring segment are detachably connected to enable the free segment to be recyclable.

[0013] Preferably, a reinforcing retaining wall is provided in the area of ​​the foundation pit near the sump and / or elevator shaft, and at least one side of the reinforcing retaining wall is attached to the SWM method pile.

[0014] Preferably, the reinforced retaining wall is formed by at least one set of biaxial cement-soil mixing piles, and each set of biaxial cement-soil mixing piles is driven into the soil within the foundation pit.

[0015] A method for unsupported deep foundation pits in saturated soft clay areas, used for constructing unsupported deep foundation pit structures in such areas, includes the following construction steps:

[0016] Conduct pile driving tests to ensure that all construction parameters during the pile driving process meet the design requirements and the geological conditions of the saturated soft clay area;

[0017] According to the construction parameters of the pile formation test, SWM method piles are continuously arranged at the edge of the foundation pit, and the steel section inserted in the SWM method pile is recyclable steel section, and the top of the steel section extends out of the top of the SWM method pile.

[0018] The construction of the reinforced concrete capping beam ensures that the extended ends of each steel section are fixedly connected to the capping beam, thereby connecting all the steel sections into a whole. During the construction of the reinforced concrete capping beam, multiple prestressed jet grouting anchors are simultaneously installed along the edge of the foundation pit, so that the free section of each prestressed jet grouting anchor is fixedly connected to the reinforced concrete capping beam, and the anchoring section extends downward along the inclined direction and is inserted into the soil outside the foundation pit.

[0019] Preferably, when conducting pile driving tests, the construction parameters should include at least the following: cement grout water-cement ratio, mixing speed, timing of steel section insertion, grouting pressure, rotation speed, pipe lifting speed, cement content, rotation speed, lifting speed, and number of mixing cycles.

[0020] Preferably, after the reinforced concrete cap beam and multiple prestressed jet grouting anchor cables are constructed, at least one set of biaxial cement-soil mixing piles are driven into the soil near the sump / or elevator shaft area in the foundation pit to form a reinforced retaining wall with at least one side conforming to the SWM method piles.

[0021] Preferably, after the reinforcement of the retaining wall is completed, the steel section is pulled out of the interior of the SWM method pile and the free section of the prestressed jet grouting anchor cable is removed, so as to realize the recycling of the steel section and the free section of the prestressed jet grouting anchor cable.

[0022] By adopting the above technical solution, the present invention has the following beneficial effects:

[0023] 1) Vertical retaining stiffness is provided by SMW method piles, the reinforced concrete cap beam constrains the pile top uniquely, and the prestressed jet grouting anchor cable balances the lateral earth pressure, thus achieving complementary stiffness. The anchor cable drilling and SMW method pile construction are carried out simultaneously, and the biaxial mixing pile retaining platform is independently piled to reinforce the local deep pit and avoid construction interference.

[0024] 2) The use of recyclable steel and anchor cable strands improves the steel recycling rate and saves construction costs.

[0025] 3) Through pile formation tests, various construction parameters in the pile formation process, such as grouting pressure, lifting speed, and sinking speed, were optimized to ensure the pile formation effect and reduce the amount of mud discharge. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a plan view of the unsupported structure for deep foundation pits in saturated soft clay areas according to an embodiment of the present invention.

[0028] Figure 2 This is a cross-sectional view of the unsupported deep foundation pit structure in a saturated soft clay area according to an embodiment of the present invention.

[0029] The correspondence between the numbers in the attached diagram is as follows:

[0030] 1- SMW method pile; 2- Reinforced concrete cap beam; 3- Reinforced retaining wall; 4- Prestressed jet grouting anchor cable; 5- Steel section; 6- Edge line of sump pit to be excavated and / or elevator shaft. Detailed Implementation

[0031] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings. It should be noted that these descriptions are for the purpose of aiding understanding the present invention, but do not constitute a limitation thereof. Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0032] Please see Figure 1 and Figure 2As shown, this embodiment of the invention provides a deep foundation pit support structure without internal support in saturated soft clay areas, including SWM method piles 1, reinforced concrete capping beams 2, and multiple prestressed jet grouting anchors 4; wherein, the SWM method piles 1 are continuously arranged along the edge of the foundation pit, and the steel sections 5 inserted into the SWM method piles 1 are recyclable steel sections, and the top of the steel sections 5 extends beyond the top of the SWM method piles 1; the reinforced concrete capping beam 2 is horizontally enclosed on the SWM method piles 1 and is fixedly connected to the extended end of each steel section 5 to connect all the steel sections 5 into a whole; multiple prestressed jet grouting anchors 4 are evenly arranged along the edge of the foundation pit, each prestressed jet grouting anchor 4 has an interconnected free section and an anchoring section, and the free section is used to be fixedly connected to the reinforced concrete capping beam 2, and the anchoring section extends downward in an inclined direction and is inserted into the soil outside the foundation pit. It should be noted that in this embodiment, the design parameters for SMW method pile 1 are Φ850 type triaxial mixing piles, constructed using the single-hole method with interlocking; the cement used is P.0 42.5 grade ordinary Portland cement, with a cement admixture of 20% and a water-cement ratio of 1.5~2.0; the cement admixture is increased to 25% in the backfill of the underground ditch; a two-spraying and two-mixing process is adopted for construction; the sinking speed of the triaxial mixing pile should be controlled within the range of 0.5~1.0 m / min, and the lifting speed should be controlled within the range of 1.0 m / min~2.0 m / min, maintaining a uniform sinking or lifting speed. For areas with pile foundations without surrounding basements, a mixer head with a small soil displacement should be selected, the mixing sinking speed should be controlled within the range of 0.5 m / min~0.8 m / min, the lifting speed should be less than 1.0 m / min, and the spraying pressure should not exceed 0.8 MPa. The triaxial mixing pile is required to have an unconfined compressive strength ≥0.8 MPa after 28 days. The steel section 5 is made of H-beams or I-beams (H500×300×11×18 or H700×300×13×24), with a steel strength grade of Q235B. The steel section 5 is inserted into the SMW method pile 1 to form a retaining and water-stopping structure. The length of the top of the steel section 5 extending beyond the pile top is 300 mm - 500 mm. The extended end is connected to the reinforced concrete cap beam 2 by welding, mechanical connection or other connection methods to form a rigid node. The steel section 5 is pulled out by a vibratory pile extractor when it is recovered, with a recovery rate of ≥95%. The cross-sectional dimensions of the reinforced concrete cap beam 2 are 1200 mm × 800 mm. The SMW method pile 1 provides vertical retaining rigidity. The reinforced concrete cap beam 2 can constrain the displacement of the pile top of the SMW method pile 1. The prestressed jet grouting anchor cable 4 provides horizontal resistance to balance the lateral earth pressure.

[0033] Furthermore, in this embodiment, the angle between the extension direction of the anchorage section and the horizontal direction is 25°-35°, and the prestressed jet grouting anchor cable 4 is a recyclable anchor cable, that is, the free section and the anchorage section are detachably connected to achieve the recyclability of the free section. The pile body is a high-pressure jet grouting pile formed by high-pressure jet grouting; 42.5 grade ordinary Portland cement is used, with a cement admixture ratio of not less than 30%, and the water-cement ratio of the cement grout is 1.0; the anchor cable uses high-strength low-relaxation steel strand with a standard strength value of 1860MPa; after the jet grouting anchor cable body strength reaches 2MPa, it can be tensioned and locked, and locked to the reinforced concrete cap beam 2 with jacks and anchorages. The jetting pressure for hole enlargement should not be less than 20MPa, the nozzle drilling or lifting speed can be 10 cm / min ~ 25 cm / min, and the nozzle rotation speed can be 5 r / min ~ 15 r / min.

[0034] Furthermore, in this embodiment, a reinforced retaining platform 3 is provided in the area of ​​the foundation pit near the sump and / or elevator shaft (where 6 in the figure represents the edge of the sump and / or elevator shaft to be excavated). At least one side of the reinforced retaining platform 3 is attached to the SWM method pile 1. The reinforced retaining platform 3 is formed by at least one set of biaxial cement-soil mixing piles, and each set of biaxial cement-soil mixing piles is driven into the soil in the foundation pit. The biaxial cement-soil mixing piles are 2Φ700 type with an overlap length of 200mm. The cement is P.0 42.5 grade ordinary Portland cement with an admixture of 15%. The two-spray four-mixing process is adopted. The drilling bit spraying and mixing lifting speed should not exceed 0.5m / min, the drilling bit mixing sinking speed should not exceed 1.0m / min, and the lifting or sinking amount per revolution of the drilling bit should be 10mm~15mm. The reinforced retaining platform 3 can independently reinforce local deep pits.

[0035] This invention also discloses a method for unsupported deep foundation pits in saturated soft clay areas, used for constructing the aforementioned support structure. The support method includes the following construction steps:

[0036] Step 1: Conduct pile driving tests to ensure that all construction parameters during the pile driving process meet the design requirements and the geological conditions of the saturated soft clay area.

[0037] Step 2: Construct continuously arranged SWM method piles at the edge of the foundation pit according to the construction parameters of the pile test, and ensure that the steel section inserted into the SWM method pile is recyclable steel section, and that the top of the steel section extends beyond the top of the SWM method pile.

[0038] Step 3: Construct the reinforced concrete capping beam, ensuring that the capping beam is fixedly connected to the extension ends of each steel section to form a unified structure. During the construction of the reinforced concrete capping beam, multiple prestressed jet grouting anchors are simultaneously installed along the edge of the foundation pit. The free section of each prestressed jet grouting anchor is fixedly connected to the reinforced concrete capping beam, and the anchoring section extends downwards along the inclined direction and is inserted into the soil outside the foundation pit. During construction, the capping beam trench is excavated and the anchors are installed: anchor drilling is performed simultaneously (with a 10° angle reduction at intersections), and high-pressure jet grouting is used to form an enlarged head after the steel strands are inserted. Then, the capping beam is poured: the capping beam reinforcement is tied and welded to the steel section, C30 concrete is poured, and it is cured to the design strength. Tensioning is performed after the high-pressure jet grouting anchors and concrete capping beam reach their strength.

[0039] Furthermore, during pile formation tests, the construction parameters must include at least the following: cement slurry water-cement ratio, mixing speed, timing of steel insertion, grouting pressure, rotation speed, pipe lifting speed, cement content, rotation speed, lifting speed, and number of mixing cycles. Specifically, the SMW method pile test area tests the cement slurry water-cement ratio, mixing speed, and timing of steel insertion; the prestressed jet grouting anchor cable test area verifies the grouting pressure, rotation speed, and pipe lifting speed; and the biaxial mixing pile test area optimizes the cement content, rotation speed, lifting speed, and number of mixing cycles.

[0040] Furthermore, after the construction of the reinforced concrete cap beam and multiple prestressed jet grouting anchor cables is completed, at least one set of biaxial cement-soil mixing piles are driven into the soil near the sump pit / or elevator shaft area in the foundation pit to form a reinforced retaining wall with at least one side conforming to the SWM method piles.

[0041] It should be noted that after the reinforcement of the retaining wall is completed, the steel section will be pulled out of the interior of the SWM method pile and the free section of the prestressed jet grouting anchor cable will be removed in order to realize the recycling of the steel section and the free section of the prestressed jet grouting anchor cable. It should be noted that the high-pressure jet grouting pile anchor cable and the concrete cap beam will be tensioned after they reach the required strength. The steel strand can be removed after the foundation pit is backfilled. The steel strand recycling rate after removal can be ≥90%.

[0042] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and these variations still fall within the protection scope of the present invention.

Claims

1. A deep foundation pit support structure without internal bracing in saturated soft clay areas, characterized in that, include: SWM method piles are continuously arranged along the edge of the foundation pit. The steel sections inserted into the SWM method piles are recyclable steel sections, and the top of the steel sections extends beyond the top of the SWM method piles. A reinforced concrete cap beam is horizontally enclosed on the SWM method pile and fixedly connected to the extension end of each of the steel sections to connect all the steel sections into a whole. Multiple prestressed jet grouting anchors are installed along the edge of the foundation pit. Each prestressed jet grouting anchor has an interconnected free section and an anchoring section. The free section is used to fix the anchor to the reinforced concrete cap beam, and the anchoring section extends downward in an inclined direction and is inserted into the soil outside the foundation pit.

2. The deep foundation pit support structure without internal support in saturated soft clay areas as described in claim 1, characterized in that, The angle between the extension direction of the anchoring section and the horizontal direction is in the range of 25°-35°.

3. The deep foundation pit support structure without internal support in saturated soft clay areas as described in claim 1, characterized in that, The free section and the anchoring section are detachably connected to enable the free section to be recycled.

4. The deep foundation pit support structure without internal support in saturated soft clay areas as described in claim 1, characterized in that, The foundation pit is equipped with a reinforced retaining platform in the area near the sump and / or elevator shaft, and at least one side of the reinforced retaining platform is attached to the SWM method pile.

5. The deep foundation pit support structure without internal support in saturated soft clay areas as described in claim 1, characterized in that, The reinforced retaining wall is formed by at least one set of biaxial cement-soil mixing piles, and each set of biaxial cement-soil mixing piles is driven into the soil inside the foundation pit.

6. A method for deep foundation pit support without internal bracing in saturated soft clay areas, characterized in that, For constructing the unsupported deep foundation pit support structure in saturated soft clay areas as described in claim 1, the support method includes the following construction steps: Conduct pile driving tests to ensure that all construction parameters during the pile driving process meet the design requirements and the geological conditions of the saturated soft clay area; According to the construction parameters of the pile formation test, SWM method piles are continuously arranged at the edge of the foundation pit, and the steel section inserted in the SWM method pile is recyclable steel section, and the top of the steel section extends out of the top of the SWM method pile. The construction of the reinforced concrete capping beam ensures that the extended ends of each steel section are fixedly connected to the capping beam, thereby connecting all the steel sections into a whole. During the construction of the reinforced concrete capping beam, multiple prestressed jet grouting anchors are simultaneously installed along the edge of the foundation pit, so that the free section of each prestressed jet grouting anchor is fixedly connected to the reinforced concrete capping beam, and the anchoring section extends downward along the inclined direction and is inserted into the soil outside the foundation pit.

7. The method for deep foundation pit support without internal bracing in saturated soft clay areas as described in claim 6, characterized in that, When conducting pile driving tests, the construction parameters should include at least the following: cement grout water-cement ratio, mixing speed, timing of steel section insertion, grouting pressure, rotation speed, pipe lifting speed, cement content, rotation speed, lifting speed, and number of mixing cycles.

8. The method for deep foundation pit support without internal bracing in saturated soft clay areas as described in claim 6, characterized in that, After the reinforced concrete cap beam and multiple prestressed jet grouting anchor cables are constructed, at least one set of biaxial cement-soil mixing piles are driven into the soil near the sump / or elevator shaft area in the foundation pit to form a reinforced retaining wall with at least one side conforming to the SWM method piles.

9. The method for deep foundation pit support without internal bracing in saturated soft clay areas as described in claim 8, characterized in that, After the reinforcement of the retaining wall is completed, the steel section is pulled out of the interior of the SWM method pile and the free section of the prestressed jet grouting anchor cable is removed in order to realize the recycling of the steel section and the free section of the prestressed jet grouting anchor cable.

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