Deep foundation pit supporting system and construction method suitable for different stratigraphic structures

Abstract

A deep foundation pit supporting system suitable for different stratigraphic structures, comprising a sealed bag (1), freezing pipes (2), vacuum pipes (3), a reinforcing cage (4), and sand (5). The sealed bag (1) is disposed in a groove section formed in a foundation around a foundation pit, and the reinforcing cage (4), the sand (5), the multiple freezing pipes (2), and the multiple vacuum pipes (3) are all disposed in the sealed bag (1). Upper ends of the freezing pipes (2) and the vacuum pipes (3) pass through an upper end surface of the sealed bag (1), and are then connected to a cold source and a negative pressure source, respectively. The sealed bag (1), having undergone vacuum evacuation and freezing treatment, together with the reinforcing cage (4), the sand (5), the freezing pipes (2) and the vacuum pipes (3) inside, forms a frozen vacuum reinforced sand wall, and multiple sections of frozen vacuum reinforced sand wall are connected end to end to form a retaining wall distributed around the foundation pit.

Description

A deep foundation pit support system and construction method suitable for different geological structures Technical Field

[0001] This invention belongs to the field of foundation pit support technology in civil engineering, and specifically relates to a deep foundation pit support system and construction method suitable for different geological structures. Background Technology

[0002] Chinese invention patent application No. 201210295813.9 discloses a vacuum-acting gravity foundation pit support system and construction method. This method uses a vacuum-acting gravity pit wall support structure for support, which can achieve a greater excavation depth. However, its prerequisite is that there must be sufficient space outside the excavation range to set up gravity retaining walls and slopes. However, the space in modern cities is small, which greatly limits its application scope.

[0003] Chinese invention patent application No. 200810236142.3 discloses a deep foundation pit excavation method using vacuum curtain water-stopping and atmospheric pressure support. This method uses vacuum curtain water-stopping and atmospheric pressure for foundation pit support, which has advantages such as not requiring internal support structures and dewatering wells inside and outside the foundation pit. However, it has three shortcomings: the method does not set up internal support structures, which is convenient for excavation but cannot be adapted to large excavation depths; the method requires the setting up of side wall solidification walls, which increases the amount of work and is not conducive to reducing costs; the method is essentially a gravity-type pit wall support structure, which requires a large amount of implementation space outside the excavation range.

[0004] Chinese invention patent application No. 117090221A discloses a vacuum support excavation method suitable for deep foundation pits in complex strata near existing buildings. The method includes setting column piles in the soil of the foundation pit wall; setting vertical drainage bodies, vertical reinforcement bodies, external vertical sealed curtains, grouting bags, horizontal drainage bodies and horizontal sealed layers according to the design plan; evacuating the foundation pit by vacuuming with a vacuum pressure source and excavating the foundation pit; laying a slope sealed layer and vacuum nails on the newly excavated foundation pit wall and setting an internal support system in the foundation pit; sealing the slope sealed layer by the vertical sealed curtain at the bottom of the slope. Although this invention organically combines the vacuum reinforcement of the pit wall soil with the internal support system, supporting deep, non-slope, and vertical excavation without traditional reinforced concrete piles or diaphragm walls, it has certain advantages in terms of cost and construction period. However, due to the complex and variable actual geological conditions and the difficulty in achieving strict sealing of the deep underground layers of the pit wall, the vacuum degree in the pit wall soil and the resulting increase in soil strength are not easy to guarantee, thus limiting its application scenarios and practical application effects. Summary of the Invention

[0005] To address the aforementioned problems, the present invention aims to provide a deep foundation pit support system and construction method suitable for different geological structures.

[0006] To achieve the above objectives, the present invention provides a deep foundation pit support system suitable for different geological structures, comprising a sealing bag, freezing pipes, vacuum pipes, reinforcing cages, and sand. The sealing bag is placed within a trench section opened in the foundation surrounding the foundation pit. The reinforcing cages, sand, multiple freezing pipes, and multiple vacuum pipes are all placed inside the sealing bag. The upper ends of the freezing pipes and vacuum pipes penetrate the upper surface of the sealing bag and are respectively connected to a cold source and a negative pressure source. The sealing bag, after being vacuumed and frozen, along with its internal reinforcing cages, sand, freezing pipes, and vacuum pipes, together constitute a section of frozen vacuum reinforced sand wall. Multiple sections of frozen vacuum reinforced sand wall are connected end-to-end to form retaining walls distributed around the foundation pit.

[0007] The sealed bag is made of a biodegradable material with heat-insulating properties.

[0008] The reinforcing cage is made of steel bars, bamboo fibers, or jute fiber materials.

[0009] When the depth of the foundation pit exceeds the support capacity of the cantilever retaining wall, the deep foundation pit support system suitable for different geological structures also includes walers and internal supports installed on the inner side of the retaining wall.

[0010] The multiple freezing tubes and multiple vacuum tubes are spaced apart inside the sealed bag.

[0011] The gaps between adjacent sections of the frozen vacuum reinforced sand wall are filled and connected with mortar.

[0012] The construction method for a deep foundation pit support system suitable for different geological structures provided by this invention includes the following steps performed in sequence:

[0013] 1) The caisson construction method is adopted to sink the connected support formwork section by section into the foundation, so as to realize the segmented trenching in the foundation around the foundation pit. The trenching depth should meet the embedment depth requirements of the foundation pit support design.

[0014] 2) If there is water or mud inside the support formwork of the trench section, use a pump to pump it out, and then put a sealed bag with an open top into the inside of the support formwork. The shape and size of the sealed bag should be adapted to the support formwork.

[0015] 3) Inject clean water into the sealed bag to make the sealed bag fit the support template better. Then, hang the reinforcing cage into the sealed bag, insert multiple vacuum tubes and freezing tubes vertically downwards, and fill it with sand until the top of the accumulated sand reaches the design elevation of the foundation pit support.

[0016] 4) Seal the upper end of the sealed bag and make the upper ends of the vacuum tube and the freezing tube penetrate the top surface of the sealed bag. Then seal the contact parts between the vacuum tube, the freezing tube and the sealed bag. After that, connect the vacuum tube to the negative pressure source and the freezing tube to the cold source. First, start the negative pressure source and evacuate the inside of the sealed bag through the vacuum tube. Then, use the cold source to freeze the pore water in the sand through the freezing tube, thereby making a section of frozen vacuum reinforced sand wall.

[0017] 5) While the support formwork is lifted by a crane at the top of the trench section, mortar is injected into the bottom of the support formwork through the grouting pipe installed on the side of the support formwork and opened at the bottom of the support formwork. The support formwork is lifted out of the trench section by the combined force of the lifting force of the crane and the pushing force of the mortar, thereby achieving the purpose of recycling the support formwork and filling and connecting adjacent frozen vacuum reinforced sand wall sections with mortar.

[0018] 6) Repeat steps 1) to 5) to make the next section of frozen vacuum reinforced sand wall until a retaining wall consisting of multiple sections of frozen vacuum reinforced sand wall is made around the foundation pit.

[0019] 7) Within the excavation area of ​​the foundation pit, excavate the soil in the foundation pit step by step from the ground. When the depth of the foundation pit exceeds the support capacity of the cantilever retaining wall, use walers and internal bracing to support the inner side of the retaining wall. After excavating to the design elevation of the bottom of the pit, construct the underground permanent structure and backfill the trench step by step until the permanent structure emerges from the ground. Recycle the reinforcing cage, vacuum pipe and freezing pipe in the retaining wall for recycling. The foundation pit support project is then completed.

[0020] The deep foundation pit support system and construction method suitable for different geological structures provided by this invention have the following beneficial effects:

[0021] 1. Supports vertical excavation of deep foundation pits;

[0022] 2. Application scenarios are not limited by the soil conditions of the foundation;

[0023] 3. It does not consume expensive building materials such as concrete and steel bars, but only uses a small amount of recyclable steel and biodegradable materials such as bamboo and jute fiber. The cost is significantly lower than that of bored piles or reinforced concrete diaphragm walls with internal bracing. It is also in line with the concept of low-carbon, green and environmentally friendly sustainable development and conforms to the development trend in the world today.

[0024] 4. The construction period is shortened by about half compared to traditional methods;

[0025] 5. Because underwater concrete pouring is not required, the construction quality control is better;

[0026] 6. No special waterproofing is required for the foundation pit sidewalls; it has good self-waterproofing effect. Attached Figure Description

[0027] Figure 1 is an elevation view of the deep foundation pit support system suitable for different geological structures provided by the present invention.

[0028] Figure 2 is a partial planar structural diagram of the deep foundation pit support system suitable for different geological structures provided by the present invention. Detailed Implementation

[0029] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0030] A foundation pit project has dimensions of 35m long × 31m wide × 10m deep.

[0031] The foundation soil layers can be divided into 5 layers, as follows:

[0032] ① Plain fill: yellowish-brown, soft plastic, moist, mainly composed of clay and fine sand; depth 0-1.9m.

[0033] ② Low liquid limit clay: yellowish-brown, soft plastic, with moderate dry strength and toughness, containing a small amount of grayish-green lumps; depth 1.9–5.1 m.

[0034] ③Silty sand: grayish-yellow, dense, saturated, mainly composed of quartz and feldspar, with a small amount of mica; depth 5.1~6.6m.

[0035] ④ Low liquid limit clay: brownish-yellow, stiff plastic, with high dry strength and toughness, locally containing a small amount of fine sand, and a small number of grayish-green clumps can be seen; depth 6.6~9.2m.

[0036] ⑤Silty sand: grayish-yellow, very dense and saturated, mainly composed of quartz and feldspar, with a small amount of mica; depth 9.2~28.3m.

[0037] As shown in Figures 1 and 2, the deep foundation pit support system suitable for different geological structures provided in this embodiment includes a sealing bag 1, a freezing pipe 2, a vacuum pipe 3, a reinforcing cage 4, sand 5, a waler 6, and an internal support 7. The sealing bag 1 is placed in a trench section opened in the foundation around the foundation pit. The reinforcing cage 4, sand 5, multiple freezing pipes 2, and multiple vacuum pipes 3 are all placed inside the sealing bag 1. The upper ends of the freezing pipes 2 and vacuum pipes 3 penetrate through the upper end face of the sealing bag 1 and are connected to a cold source and a negative pressure source, respectively. The sealing bag 1, which has been vacuumed and frozen, and the reinforcing cage 4, sand 5, freezing pipes 2, and vacuum pipes 3 inside it together form a section of frozen vacuum reinforced sand wall. Multiple sections of frozen vacuum reinforced sand wall are connected end to end to form a retaining wall distributed around the foundation pit.

[0038] The construction method for a deep foundation pit support system suitable for different geological structures provided in this embodiment includes the following steps performed in sequence:

[0039] 1) The hydraulic excavation caisson construction method is adopted to sink the connected support formwork sections into the foundation, so as to realize the segmented trenching in the foundation around the foundation pit. The trenching depth should meet the embedment depth requirements of the foundation pit support design. In this embodiment, the trenching depth is 15m, and the three-dimensional dimensions of each support formwork section are 3m long × 2.5m wide × 2m high, with a wall thickness of 0.1m.

[0040] 2) Pump out the mud from the support template in the trench section, and then place the sealed bag 1 with an open top inside the support template. The shape and size of the sealed bag 1 are adapted to the support template. The sealed bag 1 is made of a biodegradable material with heat insulation properties.

[0041] 3) Inject clean water into the sealed bag 1 to make the sealed bag 1 fit the support template better. Then, hoist the reinforcing cage 4 made of φ22@200c / c steel bars into the sealed bag 1, insert two φ50@2000c / c vacuum tubes 3 and six φ40@400c / c freezing tubes 2 vertically downward, and then fill in sand 5 until the top of the accumulated sand 5 reaches the design elevation of the foundation pit support. The reinforcing cage 4 is made of steel bars, bamboo bars or jute fiber materials.

[0042] 4) Seal the upper port of the sealing bag 1, and make the upper ends of the vacuum tube 3 and the freezing tube 2 penetrate the top surface of the sealing bag 1. Then seal the contact parts between the vacuum tube 3, the freezing tube 2 and the sealing bag 1. After that, connect the vacuum tube 3 to the negative pressure source and the freezing tube 2 to the cold source. Turn on the negative pressure source and evacuate the inside of the sealing bag 1 through the vacuum tube 3. When the pore pressure in the sand 5 reaches above -95kPa, use the cold source to make the temperature inside the sealing bag 1 below -15℃ through the freezing tube 2 so that the pore water in the sand 5 freezes, thereby making a section of frozen vacuum reinforced sand wall.

[0043] 5) While the support formwork is lifted by a crane at the top of the trench section, mortar is injected into the bottom of the support formwork through the grouting pipe installed on the side of the support formwork and opened at the bottom of the support formwork. The support formwork is lifted out of the trench section by the combined force of the lifting force of the crane and the pushing force of the mortar, thereby achieving the purpose of recycling the support formwork and filling and connecting adjacent frozen vacuum reinforced sand wall sections with mortar.

[0044] 6) Repeat steps 1) to 5) to make the next section of frozen vacuum reinforced sand wall until a retaining wall consisting of multiple sections of frozen vacuum reinforced sand wall is made around the foundation pit.

[0045] 7) Within the excavation area, the soil in the foundation pit is excavated downwards from the ground level. When the excavation depth reaches 2.5m, the retaining wall at a depth of 2.0m is supported by walers 6 and internal supports 7. After excavating to the design elevation of the pit bottom, the underground permanent structure is constructed and the trench is backfilled. Once the permanent structure emerges from the ground, the reinforcing cage 4, vacuum pipe 3, and freezing pipe 2 inside the retaining wall are recovered for recycling. This completes the foundation pit support project. In this embodiment, the walers 6 are made of double-section 400×400×13×21 H-beams Q235B, and the internal supports 7 are made of Q235B steel pipes with a diameter of 800mm and a wall thickness of 15mm.

[0046] The foundation pit support project, using the system and construction method of this invention, cost 480,000 yuan in materials, 4.96 million yuan in machinery, and 3 million yuan in labor, for a total cost of 8.44 million yuan and a construction period of 5 months. If the traditional support scheme of bored piles with internal concrete bracing and cement-soil mixing piles for water stoppage had been used, the cost would have been 8.1 million yuan in materials, 4.86 million yuan in machinery, and 3.24 million yuan in labor, for a total cost of 16.2 million yuan and a construction period of 10 months. Therefore, the system and construction method of this invention have only half the cost and half the construction period of the traditional method, saving a significant amount of steel and cement, reducing carbon emissions, and resulting in substantial social and economic benefits.

Claims

1. A deep foundation pit support system suitable for different stratum structures, characterized in that: The deep foundation pit support system suitable for different stratum structures comprises a sealing bag (1), frozen pipes (2), vacuum pipes (3), a reinforcement cage (4) and sand (5). The sealing bag (1) is arranged in a groove section formed in the ground around the foundation pit. The reinforcement cage (4), the sand (5), the frozen pipes (2) and the vacuum pipes (3) are arranged in the sealing bag (1). The upper ends of the frozen pipes (2) and the vacuum pipes (3) penetrate the upper end surface of the sealing bag (1) and are connected to a cold source and a negative pressure source respectively. The sealing bag (1) and the reinforcement cage (4), the sand (5), the frozen pipes (2) and the vacuum pipes (3) inside the sealing bag (1) together form a frozen vacuum reinforced sand wall. A plurality of the frozen vacuum reinforced sand walls are connected end to end to form a retaining wall around the foundation pit.

2. The deep foundation pit support system suitable for different stratum structures according to claim 1, characterized in that: The sealing bag (1) is made of a degradable material with heat preservation performance.

3. The deep foundation pit support system suitable for different stratum structures according to claim 1, characterized in that: The reinforcement cage (4) is made of steel bars, bamboo bars or jute fiber material.

4. The deep foundation pit support system suitable for different stratum structures according to claim 1, characterized in that: When the depth of the foundation pit exceeds the support capacity of the cantilever retaining wall, the deep foundation pit support system suitable for different stratum structures further comprises a surrounding purlin (6) and an inner support (7) arranged on the inner side of the retaining wall.

5. The deep foundation pit support system suitable for different stratum structures according to claim 1, characterized in that: The plurality of frozen pipes (2) and the plurality of vacuum pipes (3) are arranged at intervals in the sealing bag (1).

6. The deep foundation pit support system suitable for different stratum structures according to claim 1, characterized in that: The gaps between adjacent frozen vacuum reinforced sand walls are filled and connected by mortar.

7. A method of constructing a deep foundation pit support system suitable for different stratum structures as claimed in any one of claims 1 to 6, characterized in that: The construction method comprises the following steps performed in sequence: 1) A support formwork connected section by section is sunk into the ground by the sinking well construction method to form groove sections in the ground around the foundation pit. The groove depth should meet the embedded depth requirement of the foundation pit support design. 2) If there is water or mud in the groove section, the water or mud is pumped out. Then, a sealing bag (1) with an open upper end is placed inside the support formwork. The shape and size of the sealing bag (1) are adapted to the support formwork. 3) Clean water is poured into the sealing bag (1) to make the sealing bag (1) better adhere to the support formwork. Then, a reinforcement cage (4) is hoisted into the sealing bag (1), and a plurality of vacuum pipes (3) and frozen pipes (2) are vertically inserted into the sealing bag (1). Then, sand (5) is filled into the sealing bag (1) until the upper end of the sand (5) reaches the design elevation of the foundation pit support. 4) The upper end of the sealing bag (1) is sealed, and the upper ends of the vacuum pipes (3) and the frozen pipes (2) penetrate the top surface of the sealing bag (1). Then, the contact parts between the vacuum pipes (3), the frozen pipes (2) and the sealing bag (1) are sealed. Then, the vacuum pipes (3) are connected to a negative pressure source, and the frozen pipes (2) are connected to a cold source. First, the negative pressure source is started to perform vacuumization on the inside of the sealing bag (1) through the vacuum pipes (3). Then, the cold source is used to freeze the pore water in the sand (5) through the frozen pipes (2) to form a frozen vacuum reinforced sand wall. 5) While the support formwork is lifted by a crane, mortar is injected into the bottom of the support formwork through a mortar injection pipe installed on the side of the support formwork and opening at the bottom end of the support formwork. The combined force of the lifting force of the crane and the pushing force of the mortar lifts the support formwork out of the groove section and onto the ground, thereby achieving the purpose of recycling the support formwork and filling and connecting adjacent frozen vacuum reinforced sand wall sections with mortar. 6) repeat steps 1) to 5) to make the next segment of the frozen vacuum sand wall, until the retaining wall is made by connecting the multiple segments of the frozen vacuum sand wall around the foundation pit; 7) excavate the soil in the foundation pit from the ground step by step, when the depth of the foundation pit exceeds the supporting capacity of the cantilever retaining wall, support the inside of the retaining wall with the surrounding purlin (6) and the inner support (7), excavate to the design elevation of the pit bottom, then construct the underground permanent structure and backfill the trench step by step, until the permanent structure comes out of the ground, recycle the reinforcement cage (4), the vacuum pipe (3) and the freezing pipe (2) in the retaining wall, and thus the foundation pit supporting project is completed. 8.

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