Construction method for local deepening foundation waterproof support in deep foundation pit

Abstract

The invention discloses a local deepening foundation waterproof support construction method in a deep foundation pit. The method comprises the steps that the soil texture condition of a to-be-excavated land is obtained, and the foundation pit is excavated and leveled; after a first concrete cushion layer is poured at the bottom of the foundation pit, a cement mortar bonding layer is laid; installing a single-wall bottomed steel jacket box and then injecting water; concrete is poured on the outer wall of the single-wall steel jacket box with the bottom; a waterproof layer and a waterproof protection layer are arranged after a second concrete cushion layer is poured; binding foundation steel bars; an inner formwork is installed on the foundation steel bars in the single-wall bottomed steel jacket box; and foundation concrete is poured, and construction is completed. The invention relates to a construction method for a local deepening foundation waterproof support in a deep foundation pit, which effectively solves the problems of high difficulty, poor effect, long period and high cost of the traditional local deepening support construction of the deep foundation pit through a core technical scheme of a single-wall steel jacket box with a bottom, micro-expansion concrete filling, refined waterproof joints and synchronous construction.

Description

Technical Field

[0001] This invention belongs to the field of foundation pit construction and support technology, and relates to a construction method for locally deepening the foundation and providing waterproof support in deep foundation pits. Background Technology

[0002] Locally deepened foundation pits within deep foundation pits are often referred to as "pits within pits." These refer to areas created by over-excavation below the overall excavation surface due to needs such as elevator shafts, sump pits, or equipment foundations. This area is the key and challenging part of deep foundation pit engineering. The locally deepened pit is the lowest point of the entire pit and a "collection zone" for groundwater and surface runoff. If dewatering measures are not implemented effectively, water accumulation can easily occur, softening the soil at the bottom and reducing its strength. This can lead to seepage damage such as bottom heave, piping, or quicksand, seriously threatening construction safety. Common dewatering and support methods for locally deepened deep foundation pits include local wellpoint dewatering and Larssen sheet pile support. These methods are characterized by high difficulty, long development cycles, and high costs. When the geological conditions of the locally deepened foundation pit are silty clay or silt mixed with silty clay, the permeability coefficient is approximately 3.9 × 10⁻⁶. -6 cm / s to 2.3×10 -3 The water flow rate is between cm / s, which is considered slightly permeable to moderately permeable. Under these geological conditions, there is generally no obvious open water after the foundation pit is excavated. However, usually after about 4 hours, there will be obvious water accumulation in the foundation pit. The water accumulation mainly comes from the lateral stagnant water runoff between the same layers.

[0003] In summary, existing technologies suffer from the problems of high difficulty and poor effectiveness in the construction of deep foundation pit support for local deepening. Summary of the Invention

[0004] The purpose of this invention is to provide a construction method for locally deepening the foundation water-proof support in deep foundation pits, which solves the problems of high difficulty and poor effect in the construction of locally deepening foundation pit support in existing technologies.

[0005] The technical solution adopted in this invention is a method for constructing a locally deepened foundation waterproofing support in a deep foundation pit, comprising: Step 1: Obtain the soil conditions of the site to be excavated, and proceed with the excavation and leveling of the foundation pit; Step 2: After pouring the No. 1 concrete cushion layer at the bottom of the foundation pit, lay the cement mortar bonding layer. Step 3: Install the single-walled, bottom-mounted steel casing and then fill it with water; Step 4: Pour concrete on the outer wall of the single-walled, bottom-supported steel casing. Step 5: After pouring the No. 2 concrete foundation, install the waterproof layer and waterproof protective layer; Step Six: Binding of foundation reinforcement; Step 7: Install the inner formwork on the foundation reinforcement inside the single-walled, bottom-mounted steel casing; Step 8: Pour the foundation concrete. Construction is complete.

[0006] The invention is further characterized by: The foundation pit is excavated using a slope excavation method, and the slope ratio is determined based on the soil conditions of the site to be excavated. A 200-300mm thick soil layer is reserved during the excavation of the foundation pit, and the bottom is cleaned manually.

[0007] The surface flatness error of the No. 1 concrete foundation layer is within ±5mm.

[0008] After the strength of the No. 1 concrete cushion layer reaches greater than 1.2 MPa, a cement mortar bonding layer is laid on top of the No. 1 concrete cushion layer.

[0009] Step three includes: Using the lifting rings on the single-walled bottomed steel casing, the single-walled bottomed steel casing is lifted to the designated position on the cement mortar bonding layer using hoisting equipment. After adjusting the position to meet the design specifications, water is injected into the single-walled bottomed steel casing.

[0010] Step four includes: Micro-expansion concrete is poured into the gap between the outer wall of the single-walled bottomed steel casing and the soil layer. After the pouring is completed and the set strength is reached, the lifting ring on the single-walled bottomed steel casing is removed.

[0011] Step five includes: Simultaneously pour the concrete cushion layer around the locally deepened part of the deep foundation pit and the No. 2 concrete cushion layer on top of the micro-expansion concrete. After the pouring is completed and reaches the set strength, the external corner of the No. 2 concrete cushion layer at the eaves of the single-walled bottom steel casing is corrected. A waterproof layer is laid on the concrete cushion layer, the No. 2 concrete cushion layer, and the inner eaves of the single-walled bottomed steel casing around the locally deepened foundation pit. After the waterproof layer is fixed to the inner wall of the single-walled bottomed steel casing, a waterproof coating is applied to the inner wall of the single-walled bottomed steel casing, and a waterproof protective layer is constructed on the waterproof layer.

[0012] The external corner of the No. 2 concrete pad at the eaves of the single-walled bottom steel casing is a rounded chamfer with a radius greater than 50mm; The waterproof layer extends at least 250mm below the inner eaves of the single-walled, bottomed steel casing.

[0013] Step six includes: tying the foundation reinforcement bars and the main base slab reinforcement bars simultaneously, according to the specifications and spacing requirements of the design drawings.

[0014] Step eight includes: after simultaneously pouring the foundation concrete for the locally deepened parts and the foundation base slab concrete, the concrete surface is kept warm and moist for curing. The curing time shall not be less than 14 days. Construction is completed after the concrete reaches the set strength.

[0015] The beneficial effects of this invention are as follows: This invention, through its core technical solution of "single-walled bottomed steel casing + micro-expansion concrete filling + refined waterproofing nodes + synchronous construction," effectively solves the problems of high difficulty, poor effect, long cycle, and high cost in traditional deep foundation pit local deepening support construction. Specifically, it uses water pressure to tightly adhere the single-walled bottomed steel casing to the cement mortar bonding layer, blocking the bottom seepage channel by injecting water into it during hoisting and positioning. C20 micro-expansion concrete is poured into the gap of no less than 300mm between the outer wall of the steel casing and the soil layer to form a "steel casing-concrete" composite water barrier. The external corner of the No. 2 concrete pad at the eaves of the steel casing is then modified to a rounded chamfer with a radius greater than 50mm, and the waterproofing layer extends at least 250mm into the inner eaves of the casing. Combined with waterproof coating and a waterproof protective layer, a complete water-proof system is constructed, completely blocking groundwater and lateral perched water recharge, avoiding the risks of pit bottom heave, piping, and other seepage damage. Simultaneously, it adopts slope excavation and only pre-excavation... A 200-300mm thick soil layer is left for manual cleaning of the bottom. Precast steel caissons are quickly installed using lifting rings, and their outer walls serve directly as permanent external formwork, eliminating the need for dismantling and erection. Simultaneously, the surrounding bedding layers of the locally deepened areas, the No. 2 concrete bedding layer on top of the steel caisson, the locally deepened foundation concrete, and the foundation slab concrete are poured, significantly simplifying the construction process and shortening the overall cycle by more than 30% compared to traditional techniques. In addition, by controlling the flatness error of the No. 1 concrete bedding layer to within ±5mm and ensuring a strength of 1.2Mpa before laying a cement mortar bonding layer, the foundation reinforcement of the locally deepened areas and the foundation slab reinforcement are constructed simultaneously. The poured concrete is covered with heat preservation and moisture retention for at least 14 days within 12 hours, ensuring the integrity of the structure and the quality of construction. Furthermore, the use of "steel caisson + concrete filling" to replace Larssen steel sheet piles and "composite waterproof structure" to replace the local wellpoint dewatering system reduces the manual labor for formwork procurement and dismantling, as well as the number of concrete pours, significantly reducing resource input and construction costs. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the structure of the single-walled bottom steel casing in this invention; Figure 2 This is a schematic diagram of the installation of a single-walled, bottom-mounted steel casing in this invention; Figure 3 This is a cross-sectional schematic diagram of the installation of the single-walled bottom steel casing in this invention; Figure 4 This is a schematic cross-sectional view of the outer wall of the single-walled bottomed steel casing after concrete pouring in this invention. Figure 5 This is a partially enlarged schematic diagram of the waterproof node at the eaves of the single-walled bottomed steel casing in this invention; Figure 6 This is a schematic cross-sectional view of the foundation concrete after pouring in this invention.

[0017] In the diagram: 1. Eaves reinforcing rib; 2. Horizontal reinforcing rib; 3. Vertical reinforcing rib; 4. Outer wall of single-walled bottomed steel casing; 5. Casing bottom; 6. Lifting ring; 7. Single-walled bottomed steel casing; 8. Foundation pit slope; 9. Soil layer; 10. Concrete filling area; 11. No. 1 concrete cushion layer; 12. Cement mortar bonding layer; 13. Micro-expansion concrete; 14. Waterproofing node at the eaves of the single-walled bottomed steel casing; 141. Waterproofing protective layer; 142. Waterproofing layer; 143. No. 2 concrete cushion layer; 144. Inner wall of the single-walled bottomed steel casing; 15. Main base slab reinforcement; 16. Foundation reinforcement in the deepened area; 17. Inner formwork; 18. Adjustable support rod for inner formwork; 19. Foundation main base slab concrete; 20. Foundation concrete. Detailed Implementation

[0018] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0019] Construction methods for locally deepening the foundation and providing waterproofing support in deep foundation pits include: Step 1: Obtain the soil conditions of the site to be excavated, and proceed with the excavation and leveling of the foundation pit; The foundation pit is excavated using a slope excavation method, and the slope ratio is determined based on the soil conditions of the site to be excavated. A 200-300mm thick soil layer is reserved during the excavation of the foundation pit, and the bottom is cleaned manually. Step 2: After pouring the No. 1 concrete cushion layer 11 at the bottom of the foundation pit, lay the cement mortar bonding layer 12. The surface flatness error of the No. 1 concrete cushion layer 11 is within ±5mm; After the strength of the No. 1 concrete cushion layer 11 reaches greater than 1.2 MPa, a cement mortar bonding layer 12 is laid on top of the No. 1 concrete cushion layer 11. Step 3: Install the single-walled, bottom-mounted steel casing 7 and then fill it with water; Using the lifting rings 6 on the single-walled bottomed steel casing 7, the single-walled bottomed steel casing 7 is lifted to the designated position on the cement mortar bonding layer 12 using hoisting equipment. After adjusting the position to meet the design specifications, water is injected into the single-walled bottomed steel casing 7. Step 4: Pour concrete on the outer wall 4 of the single-walled bottom steel casing; Micro-expansion concrete 13 is poured into the gap between the outer wall 4 of the single-walled bottom steel casing and the soil layer 9. After the pouring is completed and the set strength is reached, the lifting ring 6 on the single-walled bottom steel casing 7 is cut off. Step 5: After pouring the No. 2 concrete foundation 143, install the waterproof layer 142 and the waterproof protective layer 141; Simultaneously pour the concrete cushion layer around the locally deepened part of the deep foundation pit and the No. 2 concrete cushion layer 143 in the top area of ​​the micro-expansion concrete 13. After the pouring is completed and the set strength is reached, the external corner of the No. 2 concrete cushion layer 143 at the eaves of the single-wall bottom steel sleeve box 7 is corrected. A waterproof layer 142 is laid on the concrete cushion layer, the No. 2 concrete cushion layer 143, and the inner eaves of the single-walled bottomed steel casing 7 around the foundation pit in the locally deepened part. After the waterproof layer 142 is fixed to the inner wall 144 of the single-walled bottomed steel casing, a waterproof coating is applied to the inner wall 144 of the single-walled bottomed steel casing, and a waterproof protective layer 141 is constructed on the waterproof layer 142. The external corner of the No. 2 concrete pad 143 at the eaves of the single-walled bottom steel casing 7 is a rounded chamfer with a radius greater than 50mm; The waterproof layer 142 extends at least 250mm into the bottom of the single-walled steel casing 7 below the inner eaves; Step Six: Binding of foundation reinforcement; According to the specifications and spacing requirements of the design drawings, the foundation steel bars 16 and the base slab steel bars 15 of the locally deepened parts are tied simultaneously. Step 7: Install the inner formwork 17 on the foundation steel reinforcement 16 inside the single-walled bottom steel casing 7; Step 8: Pour the foundation concrete; construction complete. After simultaneously pouring the foundation concrete 20 for the locally deepened section and the foundation base slab concrete 19, the concrete surface is kept warm and moist for curing. The curing time shall not be less than 14 days. Construction is completed after the concrete reaches the set strength.

[0020] After construction, the support structure uses a "single-walled bottomed steel casing 7 + micro-expansion concrete 13" as the core load-bearing and waterproofing frame. The single-walled bottomed steel casing 7 is retained as a permanent outer formwork. Its main body consists of the outer wall 4 and the bottom 5 of the single-walled bottomed steel casing. The prefabricated transverse reinforcing ribs 2, vertical reinforcing ribs 3 and eaves reinforcing ribs 1 on the outer wall 4 still play a role in strengthening the structural rigidity and preventing the steel casing from deforming due to stress or soil pressure during subsequent use. The gap between the outer wall 4 of the single-walled bottomed steel casing and the soil layer 9 has been filled with C20 micro-expansion concrete 13 to form a dense concrete filling part 10. This filling part 10 is closely integrated with the single-walled bottomed steel casing 7 to form a "steel-concrete" composite barrier, which not only bears the lateral soil pressure of the foundation pit but also blocks the lateral seepage path, becoming the core load-bearing and waterproofing carrier of the support structure.

[0021] The bottom of the support structure forms a stable support system based on the No. 1 concrete pad 11 and the cement mortar bonding layer 12. The No. 1 concrete pad 11 is poured on top of the soil layer 9, and its surface flatness error is controlled within ±5mm. Its strength has been stabilized to meet the design requirement of greater than 1.2Mpa, which can provide a uniform and stable support surface for the structure above. The cement mortar bonding layer 12 above the No. 1 concrete pad 11 is tightly bonded to the bottom 5 of the single-walled bottomed steel casing 7. During construction, the water pressure generated by injecting water into the steel casing further enhances the tightness of the bond between the bottom 5 and the cement mortar bonding layer 12. This not only avoids water seepage caused by gaps at the bottom, but also allows the bottom load to be evenly transferred to the No. 1 concrete pad 11 through the cement mortar bonding layer 12, ensuring the stress balance at the bottom of the entire support structure.

[0022] After construction, the support structure forms a multi-layered, synergistic waterproofing system. Besides the core composite barrier of "single-walled bottomed steel casing 7 - micro-expansion concrete 13," it also includes refined waterproofing nodes 14 and related protective layers: the No. 2 concrete pad 143 at the eaves of the single-walled bottomed steel casing 7 has been poured synchronously with the surrounding concrete pad of the locally deepened area, forming a unified whole. Its external corner has been corrected to a rounded chamfer with a radius greater than 50mm, effectively preventing damage to the waterproofing layer 142 due to stress concentration at this location; one end of the waterproofing layer 142 extends into the inner eaves of the single-walled bottomed steel casing 7. The bottom is no less than 250mm, and the other end covers the surface of the No. 2 concrete pad 143 and the surrounding concrete pad. It is also tightly fixed to the inner wall 144 of the single-walled bottomed steel box with a special adhesive. The inner wall 144 of the single-walled bottomed steel box is also coated with waterproof paint to further enhance the seepage prevention effect of the inner wall. The waterproof protective layer 141 constructed on the outside of the waterproof layer 142 forms a physical protection for the waterproof layer 142 to prevent the waterproof layer 142 from being damaged during subsequent use. The multiple waterproof structures jointly block the seepage path of groundwater and lateral stagnant water from the bottom, sides and eaves.

[0023] The support structure and the main foundation form an integrated load-bearing system. The foundation reinforcement 16 and the base slab reinforcement 15 in the locally deepened section are tied synchronously according to the design specifications and spacing, and formed into a unified reinforcement skeleton by welding or mechanical connection to ensure the continuous transfer of force between the two. The foundation concrete 20 in the locally deepened section and the base slab concrete 19 have been poured synchronously. The two types of concrete are seamlessly connected without obvious construction joints, so that the foundation concrete 20 can evenly transfer the load to the base slab concrete 19. The single-walled bottomed steel casing 7 serves as a permanent outer formwork, which is closely integrated with the internal foundation concrete 20. This not only enhances the lateral displacement resistance of the outer side of the foundation, but also allows the lateral earth pressure borne by the steel casing to be transferred to the concrete structure. Finally, an integrated structure of "reinforcement skeleton-concrete-steel casing" is formed to meet the load-bearing and stability requirements of the foundation during long-term use.

[0024] During construction, the present invention adopts a slope excavation method. The excavation slope ratio needs to be determined according to the specifications based on the site soil conditions, and is generally not steeper than 1:1. During the excavation process, a soil layer 9 of 200-300mm thickness needs to be reserved. The subsequent bottom cleaning operation is carried out manually to avoid mechanical over-excavation from disturbing the original soil of the foundation. Finally, a foundation pit slope 8 with a compliant slope, flat base and meeting construction requirements is formed, creating conditions for subsequent subbase construction.

[0025] After the foundation pit passes the acceptance inspection, the pouring of C20 No. 1 concrete cushion layer 11 will be carried out immediately. The thickness of this concrete cushion layer is usually 100mm. After the pouring is completed, aluminum alloy screeds are used to level the surface of the cushion layer, and the flatness error is strictly controlled within ±5mm. At the same time, the laying range of No. 1 concrete cushion layer 11 should be 100mm wider on each side than the outer perimeter of the single-wall bottomed steel casing 7 to ensure that a stable and close-fitting foundation support surface is provided for the subsequent installation of the single-wall bottomed steel casing 7.

[0026] First, wait for the strength of the No. 1 concrete cushion layer 11 to reach a level greater than 1.2 MPa. Then, pump out the water remaining in the foundation pit and evenly lay the cement mortar bonding layer 12 on top of the No. 1 concrete cushion layer 11. Next, use the lifting rings 6 on the single-walled bottomed steel casing 7 to lift the single-walled bottomed steel casing 7 to the designated position and make careful adjustments to ensure that its position meets the design specifications. Before lifting the single-walled bottomed steel casing 7, temporary internal supports need to be pre-installed inside it to enhance structural stability. After it is lifted into place, water is injected into the single-walled bottomed steel casing 7. The water pressure makes the single-walled bottomed steel casing 7 fit tightly against the cement mortar bonding layer 12 and keep it stable to prevent displacement during subsequent construction.

[0027] Before construction, first confirm the width of the gap between the outer wall 4 of the single-walled bottomed steel casing 7 and the soil layer 9, ensuring that the gap is not less than 300mm. Then, pour C20 micro-expansion concrete into the gap. The filling height of the micro-expansion concrete 13 should be flush with the bottom of the large base slab pit to form the concrete filling part 10. After the micro-expansion concrete 13 is poured and reaches a certain strength, use professional tools to cut off the lifting ring 6 on the single-walled bottomed steel casing 7 to complete the fixed connection between the single-walled bottomed steel casing 7 and the surrounding soil, thereby enhancing the stability of the overall support structure.

[0028] To ensure the continuity and stability of the overall structure, the concrete cushion layer around the locally deepened part of the deep foundation pit and the No. 2 concrete cushion layer 143 on top of the micro-expansion concrete 13 are poured simultaneously during construction. During the pouring process, it is necessary to ensure that the concrete is evenly distributed and compacted, so that the two concrete cushion layers form a seamless overall structure, providing a flat and solid working foundation for subsequent waterproofing and main foundation construction.

[0029] First, the external corner of the No. 2 concrete pad 143 at the eaves of the single-walled bottomed steel casing 7 is treated. Using professional tools, it is corrected into a rounded chamfer with a radius greater than 50mm to prevent the waterproof membrane forming the waterproof layer 142 from being damaged due to stress concentration at this point. Next, the waterproof layer 142 is laid, extending at least 250mm below the inner eaves of the single-walled bottomed steel casing 7. The waterproof layer 142 is then tightly bonded to the inner wall 144 of the single-walled bottomed steel casing with a special adhesive. At the same time, a waterproof coating is applied to the inner wall 144 of the single-walled bottomed steel casing. All joints of the waterproof membrane and internal and external corners are reinforced with additional waterproof membrane or waterproof coating. Finally, a waterproof protective layer 141 is constructed on the outside of the waterproof layer 142 to completely construct the waterproof node 14 at the eaves of the single-walled bottomed steel casing, achieving a reliable water-proof effect.

[0030] Construction workers strictly followed the specifications and spacing requirements in the design drawings to tie the foundation reinforcement bars in the locally deepened areas. During the tying process, it was necessary to ensure that the reinforcement bars were accurately positioned and firmly tied. At the same time, the foundation reinforcement bars in the deepened areas were reliably connected to the reinforcement bars of the main base slab by welding or mechanical connection, so that the two formed a unified whole load-bearing system. This ensured that after the subsequent foundation concrete was poured, the structure could bear the load evenly and meet the design bearing capacity requirements.

[0031] During construction, the outer wall 4 of the single-walled bottomed steel casing 7 is used directly as the permanent outer formwork, eliminating the need for subsequent removal. This saves construction steps and enhances the stability of the outer structure of the foundation. Subsequently, the inner formwork is erected inside the single-walled bottomed steel casing 7 according to design requirements. The inner formwork is made of materials that meet strength requirements. During the erection process, the formwork is firmly fixed by components such as supports and tie rods. At the same time, the joints between the formwork are sealed to ensure tightness and prevent grout leakage during subsequent concrete pouring. This ensures that the appearance and dimensions of the foundation after completion meet the design standards.

[0032] To ensure the integrity of the foundation and the base slab structure, the foundation concrete in the locally deepened areas and the base slab concrete are poured simultaneously. During pouring, a layered pouring and layered vibration method is adopted. Special attention is paid to fully vibrating the concrete inside the single-walled bottom steel casing 7 and the connection between the concrete and the base slab to ensure the density of the concrete and avoid quality defects such as honeycomb, pitting, and voids, thus ensuring the strength and durability of the foundation structure.

[0033] After the foundation concrete is poured, it is necessary to cover the concrete surface with geotextile or plastic film within 12 hours to keep it warm and moist. During the curing process, the integrity of the covering and the humidity of the concrete surface should be checked regularly, and water should be added in time to ensure that the concrete is always moist. The curing time shall not be less than 14 days. Through continuous and standardized curing operations, the normal growth of concrete strength will be promoted, and the shrinkage cracks caused by excessive evaporation of water will be prevented, so as to ensure that the foundation construction quality meets the design requirements.

[0034] Example 1 This embodiment proposes a construction method for locally deepening the foundation and providing waterproof support in deep foundation pits, including: Step 1: Obtain the soil conditions of the site to be excavated, and proceed with the excavation and leveling of the foundation pit; The foundation pit excavation adopts the slope excavation method, and the excavation slope ratio is determined according to the soil conditions of the site to be excavated; A 200mm thick soil layer was reserved during the excavation of the foundation pit, and the bottom cleaning was carried out manually.

[0035] Step 2: After pouring the No. 1 concrete cushion layer 11 at the bottom of the foundation pit, lay the cement mortar bonding layer 12. Step 3: Install the single-walled, bottom-mounted steel casing 7 and then fill it with water; Step 4: Pour concrete on the outer wall 4 of the single-walled bottom steel casing; Step 5: After pouring the No. 2 concrete foundation 143, install the waterproof layer 142 and the waterproof protective layer 141; Step Six: Binding of foundation reinforcement; Step 7: Install the inner formwork 17 on the foundation steel reinforcement 16 inside the single-walled bottom steel casing 7; Step 8: Pour the foundation concrete. Construction is complete.

[0036] Example 2 This embodiment proposes a construction method for locally deepening the foundation and providing waterproof support in deep foundation pits, including: Step 1: Obtain the soil conditions of the site to be excavated, and proceed with the excavation and leveling of the foundation pit; The foundation pit is excavated using a slope excavation method, and the slope ratio is determined based on the soil conditions of the site to be excavated. A 300mm thick soil layer is reserved during the excavation of the foundation pit, and the bottom is cleaned manually.

[0037] Step 2: After pouring the No. 1 concrete cushion layer 11 at the bottom of the foundation pit, lay the cement mortar bonding layer 12. The surface flatness error of the No. 1 concrete cushion layer 11 is within ±5mm; after the strength of the No. 1 concrete cushion layer 11 reaches greater than 1.2Mpa, a cement mortar bonding layer 12 is laid on top of the No. 1 concrete cushion layer 11.

[0038] Step 3: Install the single-walled, bottom-mounted steel casing 7 and then fill it with water; Step 4: Pour concrete on the outer wall 4 of the single-walled bottom steel casing; Step 5: After pouring the No. 2 concrete foundation 143, install the waterproof layer 142 and the waterproof protective layer 141; Step Six: Binding of foundation reinforcement; Step 7: Install the inner formwork 17 on the foundation steel reinforcement 16 inside the single-walled bottom steel casing 7; Step 8: Pour the foundation concrete. Construction is complete.

[0039] Example 3 This embodiment proposes a construction method for locally deepening the foundation and providing waterproof support in deep foundation pits, including: Step 1: Obtain the soil conditions of the site to be excavated, and proceed with the excavation and leveling of the foundation pit; Step 2: After pouring the No. 1 concrete cushion layer 11 at the bottom of the foundation pit, lay the cement mortar bonding layer 12. Step 3: Install the single-walled, bottom-mounted steel casing 7 and then fill it with water; Using the lifting rings 6 on the single-walled bottomed steel casing 7, the single-walled bottomed steel casing 7 is lifted to the designated position on the cement mortar bonding layer 12 using hoisting equipment. After adjusting the position to meet the design specifications, water is injected into the single-walled bottomed steel casing 7. Step 4: Pour concrete on the outer wall 4 of the single-walled bottom steel casing; Step 5: After pouring the No. 2 concrete foundation 143, install the waterproof layer 142 and the waterproof protective layer 141; Step Six: Binding of foundation reinforcement; Step 7: Install the inner formwork 17 on the foundation steel reinforcement 16 inside the single-walled bottom steel casing 7; Step 8: Pour the foundation concrete. Construction is complete.

[0040] Example 4 This embodiment proposes a construction method for locally deepening the foundation and providing waterproof support in deep foundation pits, including: Step 1: Obtain the soil conditions of the site to be excavated, and proceed with the excavation and leveling of the foundation pit; Step 2: After pouring the No. 1 concrete cushion layer 11 at the bottom of the foundation pit, lay the cement mortar bonding layer 12. Step 3: Install the single-walled, bottom-mounted steel casing 7 and then fill it with water; Step 4: Pour concrete on the outer wall 4 of the single-walled bottom steel casing; Micro-expansion concrete 13 is poured into the gap between the outer wall 4 of the single-walled bottom steel casing and the soil layer 9. After the pouring is completed and the set strength is reached, the lifting ring 6 on the single-walled bottom steel casing 7 is cut off. Step 5: After pouring the No. 2 concrete foundation 143, install the waterproof layer 142 and the waterproof protective layer 141; Step Six: Binding of foundation reinforcement; Step 7: Install the inner formwork 17 on the foundation steel reinforcement 16 inside the single-walled bottom steel casing 7; Step 8: Pour the foundation concrete. Construction is complete.

[0041] Example 5 This embodiment proposes... Construction methods for locally deepening the foundation and providing waterproofing support in deep foundation pits include: Step 1: Obtain the soil conditions of the site to be excavated, and proceed with the excavation and leveling of the foundation pit; Step 2: After pouring the No. 1 concrete cushion layer 11 at the bottom of the foundation pit, lay the cement mortar bonding layer 12. Step 3: Install the single-walled, bottom-mounted steel casing 7 and then fill it with water; Step 4: Pour concrete on the outer wall 4 of the single-walled bottom steel casing; Step 5: After pouring the No. 2 concrete foundation 143, install the waterproof layer 142 and the waterproof protective layer 141; Simultaneously pour the concrete cushion layer around the locally deepened part of the deep foundation pit and the No. 2 concrete cushion layer 143 in the top area of ​​the micro-expansion concrete 13. After the pouring is completed and the set strength is reached, the external corner of the No. 2 concrete cushion layer 143 at the eaves of the single-wall bottom steel sleeve box 7 is corrected. A waterproof layer 142 is laid on the concrete cushion layer, the No. 2 concrete cushion layer 143, and the inner eaves of the single-walled bottomed steel casing 7 around the foundation pit in the locally deepened part. After the waterproof layer 142 is fixed to the inner wall 144 of the single-walled bottomed steel casing, a waterproof coating is applied to the inner wall 144 of the single-walled bottomed steel casing, and a waterproof protective layer 141 is constructed on the waterproof layer 142. The external corner of the No. 2 concrete pad 143 at the eaves of the single-walled bottom steel casing 7 is a rounded chamfer with a radius greater than 50mm; The waterproof layer 142 extends at least 250mm into the bottom of the single-walled steel casing 7 below the inner eaves; Step Six: Binding of foundation reinforcement; Step 7: Install the inner formwork 17 on the foundation steel reinforcement 16 inside the single-walled bottom steel casing 7; Step 8: Pour the foundation concrete. Construction is complete.

[0042] Example 6 This embodiment proposes a construction method for locally deepening the foundation and providing waterproof support in deep foundation pits, including: Step 1: Obtain the soil conditions of the site to be excavated, and proceed with the excavation and leveling of the foundation pit; Step 2: After pouring the No. 1 concrete cushion layer 11 at the bottom of the foundation pit, lay the cement mortar bonding layer 12. Step 3: Install the single-walled, bottom-mounted steel casing 7 and then fill it with water; Step 4: Pour concrete on the outer wall 4 of the single-walled bottom steel casing; Step 5: After pouring the No. 2 concrete foundation 143, install the waterproof layer 142 and the waterproof protective layer 141; Step Six: Binding of foundation reinforcement; According to the specifications and spacing requirements of the design drawings, the foundation steel bars 16 and the base slab steel bars 15 of the locally deepened parts are tied simultaneously. Step 7: Install the inner formwork 17 on the foundation steel reinforcement 16 inside the single-walled bottom steel casing 7; Step 8: Pour the foundation concrete; construction complete. After simultaneously pouring the foundation concrete 20 for the locally deepened section and the foundation base slab concrete 19, the concrete surface is kept warm and moist for curing. The curing time shall not be less than 14 days. Construction is completed after the concrete reaches the set strength.

[0043] Figure 1 The schematic diagram of the single-walled, bottomed steel casing 7 clearly shows the complete structure of this prefabricated component. Its core components and functions are highly compatible with the prefabrication and installation requirements of the steel casing in the technical solution. As shown in the diagram, the single-walled, bottomed steel casing 7 consists of an outer wall 4 and a bottom 5 forming the main frame. To enhance structural rigidity and adapt to hoisting and subsequent stress requirements, horizontal reinforcing ribs 2 and vertical reinforcing ribs 3 are provided on the outer wall 4. Additionally, an eaves reinforcing rib 1 is added at the top eaves to prevent deformation of the eaves during hoisting and water injection. Lifting rings 6 are welded to both sides of the top of the steel casing 7. These lifting rings 6 are key components in step three of the technical solution, "lifting the steel casing to the designated position on the cement mortar bonding layer 12 using hoisting equipment," ensuring precise positioning of the steel casing. The overall structure is prefabricated, reducing on-site processing steps and ensuring the structural stability of the steel casing, laying the foundation for the subsequent construction of a "steel casing-concrete" composite water barrier.

[0044] Figure 2 The diagram presents the overall scene of the installation stage of the single-walled bottomed steel casing 7, which directly corresponds to the excavation requirements of the foundation pit in step one of the technical solution and the installation process of the steel casing in step three. The foundation pit shown in the diagram adopts the slope excavation method specified in the technical solution to form a foundation pit slope 8 that meets the soil requirements. A 200-300mm thick soil layer 9 is reserved at the bottom of the foundation pit, which will be manually cleaned later to avoid disturbing the original soil of the foundation due to mechanical over-excavation. At this time, the single-walled bottomed steel casing 7 has been hoisted to the designated area in the foundation pit by the lifting ring 6 and is in a state of waiting for adjustment and water injection. The No. 1 concrete cushion layer 11 and cement mortar bonding layer 12 of the lower structure, which are not shown in the diagram, have been constructed according to step two of the technical solution. The flatness error is ±5mm and the strength is >1.2Mpa, providing a stable support surface for the steel casing 7. This schematic diagram intuitively shows the connection between the installation of the steel casing and the results of the foundation pit excavation, and is the preparatory state for subsequent water injection and pouring of outer wall concrete.

[0045] Figure 3The sectional view shows the layered structural relationship during the installation of the single-walled bottom steel casing 7, fully presenting the construction results of steps two and three of the technical solution, with the position and function of each component clearly defined. Looking upwards from the bottom of the cross-section, the lowest layer is the soil layer 9 reserved after the foundation pit excavation; above the soil layer 9 is the No. 1 concrete cushion layer 11, which has been poured and whose surface flatness is strictly controlled within ±5mm and whose strength has reached the requirement of >1.2Mpa; above the No. 1 concrete cushion layer 11, a cement mortar bonding layer 12 is laid, and the bottom 5 of the single-walled bottomed steel casing 7 sits directly on the cement mortar bonding layer 12. By injecting water, the water pressure is used to make the bottom 5 of the casing and the cement mortar bonding layer 12 fit tightly together, blocking the bottom seepage channel; at the same time, the cross-section also clearly shows that a gap of not less than 300mm is reserved between the outer wall 4 of the single-walled bottomed steel casing and the soil layer 9. This gap is the reserved space for "pouring micro-expansion concrete 13" in step four of the technical solution, which will later form the concrete filling part 10. This intuitively reflects the synergistic effect of each structural layer in the installation stage of the steel casing, preparing for the construction of a composite water barrier.

[0046] Figure 4 This is the cross-sectional view of the outer wall of the single-walled, bottom-mounted steel casing after the pouring of micro-expansion concrete. It corresponds to step four of the technical solution, and its core element is the formation process of the "steel casing-concrete" composite water-resistant barrier. Figure 3 Compared to the previous diagram, the biggest change is that the gap between the outer wall 4 of the single-walled bottomed steel casing and the soil layer 9 has been filled with C20 micro-expansion concrete 13, forming a complete concrete filling part 10. The filling height of the micro-expansion concrete 13 is flush with the bottom of the large base slab pit, forming a composite waterproof structure together with the single-walled bottomed steel casing 7, completely blocking the lateral seepage path. At this time, the No. 1 concrete cushion layer 11 and the cement mortar bonding layer 12 still serve as the bottom support structure, ensuring the overall stability of the steel casing 7 and the micro-expansion concrete 13. Although the state of the removal of the lifting ring 6 is not clearly shown in the diagram, according to step four of the technical solution, after the micro-expansion concrete 13 is poured and reaches the set strength, the lifting ring 6 will be removed. This cross-sectional view intuitively proves the completion of the construction of the composite waterproof barrier, laying the foundation for the subsequent waterproof node construction.

[0047] Figure 5This enlarged view of the waterproof node 14 at the eaves of the single-walled bottomed steel casing shows the detailed construction of the "refined waterproof node" in step five of the technical solution, which is a key link in building a complete waterproof system. As shown in the figure, the base layer of the waterproof node is the No. 2 concrete cushion layer 143. This cushion layer has been poured simultaneously with the surrounding concrete cushion layer of the locally deepened part according to the technical plan requirements, and its external corner has been corrected to a rounded chamfer with a radius greater than 50mm to prevent the waterproof layer 142 from being damaged due to stress concentration. The No. 2 concrete cushion layer 143 and the inner wall 144 of the single-wall bottomed steel casing are covered with the waterproof layer 142. The waterproof layer 142 extends at least 250mm below the inner eaves of the single-wall bottomed steel casing 7 and is tightly bonded to the inner wall 144 of the single-wall bottomed steel casing with a special adhesive. At the same time, the inner wall 144 of the single-wall bottomed steel casing has been coated with waterproof paint. A waterproof protective layer 141 is constructed on the outside of the waterproof layer 142 to effectively protect the waterproof layer 142. The layers are tightly connected to form a tight eaves waterproof structure, completely blocking the water seepage path at the eaves, and forming a complete waterproof system in conjunction with the steel casing-concrete composite barrier.

[0048] Figure 6 The diagram presents the final cross-sectional state after the foundation concrete pouring is completed, corresponding to the construction content of steps six to eight in the technical solution, visually demonstrating the structural integrity and construction results. As seen in the diagram, the single-walled, bottom-mounted steel casing 7 is retained as a permanent outer formwork, with the foundation reinforcement binding and concrete pouring already completed inside. The foundation reinforcement 16 and the base slab reinforcement 15 in the locally deepened section are tied synchronously according to the technical solution requirements, forming a unified overall load-bearing system. The inner formwork 17, installed before pouring, is fixed by adjustable support rods 18 to ensure that the foundation concrete 20 meets the design dimensions. At this point, the foundation concrete 20 in the locally deepened section and the base slab concrete 19 have been poured synchronously, with seamless connection between the two types of concrete, ensuring the structural integrity of the foundation and the base slab. Although the curing measures are not shown in the diagram, according to step eight of the technical solution, the concrete was covered with insulation and moisture-retaining curing within 12 hours after pouring, and the curing time is no less than 14 days to promote strength growth and prevent shrinkage cracks. This cross-sectional diagram fully presents the foundation structure after construction, proving that the goals of "synchronous construction" and "ensuring structural integrity" in the technical solution have been achieved.

[0049] This invention, through its core technical solution of "single-walled bottomed steel caisson + layered support + synchronous construction," specifically addresses the problems of "high difficulty, poor effectiveness, and high cost" in existing deep foundation pit local deepening support. Its beneficial effects, combined with specific technical details, can be summarized in the following four points: I. Enhance the effectiveness of waterproofing support to avoid the risk of seepage damage. This invention achieves efficient water isolation through the synergistic effect of multiple water-proof barriers. First, leveraging the structural characteristics of the single-walled, bottom-mounted steel casing 7, water is injected into its interior after hoisting and positioning. The water pressure causes the single-walled, bottom-mounted steel casing 7 to tightly adhere to the underlying cement mortar bonding layer 12, directly blocking the bottom seepage channel. Subsequently, C20 micro-expansion concrete 13 is poured into the gap of no less than 300mm between the outer wall 4 of the single-walled, bottom-mounted steel casing 7 and the soil layer 9, forming a composite water-proof barrier of "single-walled, bottom-mounted steel casing 7 - micro-expansion concrete 13," further blocking lateral seepage. Simultaneously, the No. 2 concrete at the eaves of the single-walled, bottom-mounted steel casing 7... The external corner of the concrete cushion layer 143 is treated by modifying it into a rounded chamfer with a radius greater than 50mm to prevent the waterproof layer 142 from being damaged due to stress concentration at this point. The waterproof layer 142 is also made to penetrate at least 250mm below the inner eaves of the single-walled bottomed steel casing 7. Combined with the waterproof coating applied to the inner wall 144 of the single-walled bottomed steel casing and the waterproof protective layer 141 constructed on the outside of the waterproof layer 142, a complete and tight water-proof system is constructed, which completely cuts off the replenishment path of groundwater and lateral perched water, effectively avoiding the risk of seepage damage such as pit bottom heave and piping, and solving the problem of poor water-proofing effect of traditional support methods.

[0050] II. Simplify the construction process and significantly shorten the construction period. In terms of optimizing the construction process, this invention reduces redundant steps and improves work efficiency through several technical details: the foundation pit excavation adopts a slope excavation method, and only a 200-300mm thick soil layer 9 is reserved for manual cleaning, which not only meets the excavation requirements of different soil types, but also avoids the disturbance of the original soil of the foundation by mechanical over-excavation, simplifying the post-excavation processing procedures; the single-walled bottomed steel casing 7 is a prefabricated component, which can be quickly and accurately positioned by hoisting equipment through the lifting ring 6 on it, without the need for complicated on-site assembly, and the outer wall 4 of the single-walled bottomed steel casing directly serves as a permanent outer formwork, eliminating the cumbersome procedures of traditional formwork erection and dismantling; at the same time, the pouring of the concrete cushion layer around the locally deepened part in the deep foundation pit and the No. 2 concrete cushion layer 143 in the top area of ​​the micro-expansion concrete 13, as well as the pouring of the foundation concrete 20 and the foundation base slab concrete 19 in the locally deepened part are carried out simultaneously, reducing the waiting time for multiple pouring. Through the simplification and synchronization of these procedures, the overall construction cycle is shortened by more than 30% compared with traditional technology.

[0051] III. Ensure structural integrity and improve construction quality To ensure the stability of the foundation structure and the quality of construction, this invention sets strict technical standards for key stages: When pouring the No. 1 concrete cushion layer 11, the surface flatness error is controlled within ±5mm, and the cement mortar bonding layer 12 is laid only after its strength reaches greater than 1.2Mpa. This provides a flat and stable support foundation for the installation of the single-walled bottom steel casing 7, preventing displacement or water-proofing failure of the steel casing due to uneven foundation. During the foundation reinforcement construction stage, the foundation reinforcement 16 of the locally deepened parts is tied to the main base plate reinforcement 15 simultaneously according to design requirements. A unified overall force-bearing system is formed through welding or mechanical connection to ensure that the structure can bear the load evenly after the subsequent concrete pouring. After the concrete is poured, it is promptly covered with geotextile or plastic film for heat preservation and moisture retention within 12 hours, and the curing time is not less than 14 days. This effectively promotes the normal growth of concrete strength, prevents drying shrinkage cracks due to excessive moisture evaporation, ensures the strength and durability of the foundation structure, and improves the overall construction quality.

[0052] IV. Reduce resource input and significantly lower construction costs This invention significantly reduces construction costs through material and process optimization: It replaces traditional Larssen sheet piles with a combination of "single-walled bottomed steel casing 7 + micro-expansion concrete 13," eliminating the costs of purchasing, renting, and removing sheet piles; it replaces local wellpoint dewatering systems with "composite waterproof barrier + refined waterproof nodes," reducing the investment and operating costs of wellpoint equipment; the outer wall 4 of the single-walled bottomed steel casing serves as a permanent formwork, eliminating the need for additional formwork purchases and reducing labor costs associated with formwork removal; and the synchronous pouring process reduces the number of concrete pours, lowering related costs for concrete transportation and pouring. Through these cost control measures, resource input is significantly reduced, resulting in improved economic benefits.

Claims

1. A construction method for locally deepening the foundation and providing waterproof support in a deep foundation pit, characterized in that, include: Step 1: Obtain the soil conditions of the site to be excavated, and proceed with the excavation and leveling of the foundation pit; Step 2: After pouring the No. 1 concrete cushion layer (11) at the bottom of the foundation pit, lay the cement mortar bonding layer (12). Step 3: Install the single-walled bottom steel casing (7) and then fill it with water; Step 4: Pour concrete on the outer wall (4) of the single-walled bottom steel casing; Step 5: After pouring the No. 2 concrete cushion layer (143), install the waterproof layer (142) and the waterproof protective layer (141). Step Six: Binding of foundation reinforcement; Step 7: Install the inner formwork (17) on the foundation steel bars (16) inside the single-walled bottom steel casing (7); Step 8: Pour the foundation concrete. Construction is complete.

2. The construction method for locally deepening the foundation and providing waterproof support in a deep foundation pit according to claim 1, characterized in that, The foundation pit is excavated using a slope excavation method, and the excavation slope ratio is determined based on the soil conditions of the site to be excavated. A 200-300mm thick soil layer (9) is reserved during the excavation of the foundation pit, and the bottom is cleaned manually.

3. The construction method for locally deepening the foundation and providing waterproof support in a deep foundation pit according to claim 1, characterized in that, The surface flatness error of the No. 1 concrete cushion layer (11) is within ±5mm.

4. The construction method for locally deepening the foundation and providing waterproof support in a deep foundation pit according to claim 1, characterized in that, After the strength of the No. 1 concrete cushion layer (11) reaches greater than 1.2 MPa, a cement mortar bonding layer (12) is laid on top of the No. 1 concrete cushion layer (11).

5. The construction method for locally deepening the foundation and providing waterproof support in a deep foundation pit according to claim 1, characterized in that, Step three includes: Using the lifting rings (6) on the single-walled bottomed steel casing (7), the single-walled bottomed steel casing (7) is lifted to the designated position on the cement mortar bonding layer (12) by the lifting equipment and the position is adjusted to meet the design specifications before water is injected into the single-walled bottomed steel casing (7).

6. The construction method for locally deepening the foundation and providing waterproof support in a deep foundation pit according to claim 1, characterized in that, Step four includes: Micro-expansion concrete (13) is poured into the gap between the outer wall (4) of the single-walled bottom steel casing and the soil layer (9). After the pouring is completed and the set strength is reached, the lifting ring (6) on the single-walled bottom steel casing (7) is cut off.

7. The construction method for locally deepening the foundation and providing waterproof support in a deep foundation pit according to claim 1, characterized in that, Step five includes: Simultaneously pour the concrete cushion layer around the locally deepened part of the deep foundation pit and the No. 2 concrete cushion layer (143) at the top area of ​​the micro-expansion concrete (13). After the pouring is completed and the set strength is reached, the external corner of the No. 2 concrete cushion layer (143) at the eaves of the single-wall bottom steel sleeve box (7) is corrected. A waterproof layer (142) is laid on the concrete cushion layer, the No. 2 concrete cushion layer (143), and the inner eaves of the single-walled bottom steel box (7) around the foundation pit of the locally deepened part. After the waterproof layer (142) is fixed to the inner wall (144) of the single-walled bottom steel box, a waterproof coating is applied to the inner wall (144) of the single-walled bottom steel box, and a waterproof protective layer (141) is constructed on the waterproof layer (142).

8. The construction method for locally deepening the foundation and providing waterproof support in a deep foundation pit according to claim 1, characterized in that, The external corner of the No. 2 concrete pad (143) at the eaves of the single-walled bottom steel box (7) is a rounded chamfer with a radius greater than 50mm; The waterproof layer (142) extends at least 250 mm below the inner eaves of the single-walled bottomed steel casing (7).

9. The construction method for locally deepening the foundation and providing waterproof support in a deep foundation pit according to claim 1, characterized in that, Step six includes: according to the specifications and spacing requirements of the design drawings, the foundation steel bars (16) of the locally deepened parts are tied together with the main base plate steel bars (15) simultaneously.

10. The construction method for locally deepening the foundation and providing waterproof support in a deep foundation pit according to claim 1, characterized in that, Step eight includes: after simultaneously pouring the foundation concrete (20) of the locally deepened part and the foundation base slab concrete (19), the concrete surface is kept warm and moist for curing. The curing time shall not be less than 14 days. Construction is completed after the concrete reaches the set strength.

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