Value point: The foundation pit of this project is super-large and super-deep, with large support volume and earthwork excavation volume. By adopting deep foundation pit concrete internal bracing support and basin-island combined efficient earthwork excavation technology, the purpose of improving the safety of the foundation pit and the efficiency of earthwork excavation and transportation is achieved.

I. Planning Background

A certain project is a general construction contracting project. The foundation pit area is 56,800 square meters, the perimeter of the foundation pit is 1,084 m, and the depth of the foundation pit is 18-24 m. The surrounding environment of the foundation pit is complex, adjacent to subway tunnel sections, subway stations, viaducts, municipal roads, high-rise buildings, rivers, underground pipelines, etc. The deformation control of the foundation pit is strict. The soil layers of the foundation pit contain multiple layers of peaty soil. The earthwork excavation and transportation volume reaches 1.1 million cubic meters. The earthwork excavation and outward transportation will span a rainy period of up to 6 months. The safety of foundation pit construction and the efficiency of earthwork excavation and transportation will have a significant impact on the construction period and economic benefits of the entire project. Therefore, deep foundation pit concrete internal bracing support and basin-island combined efficient earthwork excavation technology are adopted to ensure the safety of the foundation pit while improving the efficiency of earthwork excavation.

II. Planning Approach

In view of the large support volume of the foundation pit, the complex surrounding environment, the strict deformation control of the foundation pit, the large earthwork excavation volume, and the excavation process will experience an extended rainy period, the project research decided to start with the organization of earthwork excavation to optimize the construction plan, solve construction difficulties, and find a construction plan that suits the characteristics of the foundation pit of this project, so as to achieve safe and efficient excavation.

1. Organize project personnel to conduct benchmarking visits and studies on similar super-deep and super-large deep foundation pits that have been constructed in China.

2. The technical department of the project comprehensively sorts out the construction requirements of the foundation pit, including earthwork transportation equipment, earthwork excavation technology, engineering pile construction technology, etc., records in detail the equipment models, key points of foundation pit deformation control, and experience of similar projects, solicits multi-party discussions with technical personnel from other construction units and professional subcontractors, consults designers and foundation pit construction experts, and combines on-site survey results to determine a detailed and feasible construction plan.

3. Commercial personnel conduct detailed calculations on the construction plan, compare labor and machinery usage, and finally select the basin-island combined earthwork excavation construction plan.

4. The project organizes relevant personnel to conduct construction briefings at all levels for this construction plan. The technical department organizes team leaders and workers to study the plan to ensure construction accuracy and avoid cost increases caused by foundation pit deformation, rainy period effects, etc.

III. Technical Points

The basin-island combined excavation plan is adopted, that is, internal bracing is formed by layers, zones, and blocks, and excavation is carried out by layers, zones, and blocks. After each concrete internal bracing is formed, the next layer of earthwork is excavated. The entire foundation pit adopts basin-style excavation. Each layer of excavation involves large-area excavation in the middle of the foundation pit, leaving soil around the foundation pit. After the middle bracing is formed, the soil is excavated block by block, and the bracing is completed. Soil is left inside the ring bracing of the foundation pit as a soil discharge platform for dump trucks to park and for soil loading, forming a typical island-style excavation. The rings are connected to form a soil discharge channel, avoiding the impact of rainy season construction on soil discharge. This not only ensures priority excavation of earthwork within the central bracing range but also provides a working face for concrete internal bracing construction as early as possible.

1. Construction Process Flow

The foundation pit support and excavation construction of this process is divided into central foundation pit bracing and excavation, foundation pit edge bracing and excavation, and central island bracing and excavation.

(1) Leave soil near the foundation pit perimeter - earthwork excavation within the first section of central foundation pit bracing - construction of the first section of internal bracing in the central foundation pit - earthwork excavation within the m-th section of central foundation pit bracing - construction of the m-th section of internal bracing in the central foundation pit.

(2) Formation of bracing in the corresponding area of the central foundation pit - excavation of the first section while leaving soil near the foundation pit edge - construction of the first section of bracing within the excavation range near the foundation pit edge - excavation of the n-th section while leaving soil near the foundation pit edge - construction of the n-th section of bracing within the excavation range near the foundation pit edge.

(3) Formation of bracing in the first area - earthwork excavation of the first section while leaving soil in the island... Formation of bracing in the i-th area - earthwork excavation of the n-th section while leaving soil in the island - curing of concrete bracing to design strength - entering the next layer of earthwork excavation and the next internal bracing construction.

2. Key Construction Operation Points

(1) ** layer central foundation pit earthwork excavation: the entire foundation pit adopts basin-style excavation, earthwork excavation in the middle of the foundation pit, and soil is left around the foundation pit.

(2) Construction of the first internal bracing in the middle of the foundation pit.

(3) Leave soil inside the ring bracing of the foundation pit as a soil discharge platform for dump trucks to park and for soil loading, forming a typical island-style excavation. Connect the rings to form a soil discharge channel to avoid the impact of rainy season construction on soil discharge.

(4) ** layer earthwork near the foundation pit perimeter is excavated block by block in zones.

(5) The first internal bracing near the foundation pit perimeter is completed block by block in zones.

(6) Excavation of the remaining earthwork of the ** layer, excavation of the soil left in the island, and curing of the first internal bracing concrete to prepare for the second layer of earthwork excavation. Connect the passages within the ring as soil discharge and transportation channels.

(7) Start the second layer of earthwork excavation. In the same way, leave soil around the foundation pit and excavate earthwork in the middle of the foundation pit.

(8) Construction of the second internal bracing in the middle of the foundation pit. Enter the next cycle until earthwork excavation is completed and internal bracing construction is completed.