How to do well the safety management of deep foundation pit construction

1. Definition of deep foundation pit engineering

According to the relevant provisions of the Measures for the Safety Management of Dangerous Division Projects (J.Z. No.200987), the definition of deep foundation pit is as follows:

(1) Earthwork excavation, support and dewatering of foundation pit (trench) with excavation depth exceeding 5m (including 5m). (2) Although the excavation depth is less than 5m, the geological conditions, surrounding environment and underground pipelines are complicated, or the earthwork excavation, support and dewatering project of foundation pit (trough) that affects the safety of adjacent buildings (structures).

Deep foundation pit engineering is a sub-project with risks greater than a certain scale. Before engineering investigation, the construction unit shall investigate the present situation of adjacent facilities and provide survey data (including the foundation of surrounding buildings, structural forms, underground pipeline distribution map, etc.). ) to the survey and design unit. The investigation scope extends outward from the upper edge of foundation pit and slope, which is equivalent to twice the excavation depth or height of foundation pit and slope. After the construction and supervision units enter the site, they should be familiar with the design documents, determine whether the project belongs to the category of deep foundation pit according to the definition of deep foundation pit, and do a good job in deep foundation pit construction.

2. Safety management of deep foundation pit construction

2. 1 natural slope

Natural slope placement is suitable for deep foundation pit engineering with empty surroundings and no important buildings around. It generally occurs in the suburbs, and the safety risk is relatively small. Because of its large area and large amount of backfill, it is rarely used, so it will not be discussed here for the time being.

2.2 Retaining structure support

The concrete forms of retaining structures are pulling anchor retaining structure, supporting retaining structure and cantilever retaining structure. The retaining structure is generally composed of one or more of row piles, underground continuous walls, anchor rods (cables) and support rods.

2.2. 1 safety management of row piles and underground continuous wall construction

The row piles of enclosure structure include concrete cast-in-place piles, steel piles, steel pipe piles, steel sheet piles and special-shaped concrete mixing piles. When using manual digging pile, the following items should be paid attention to:

(1) Manual digging piles need to prepare a special scheme, and those exceeding 16m need expert argumentation. (2) Hole wall support. For the first time, the retaining wall should be 30cm higher than the natural ground; When excavating non-rock stratum, formwork concrete shall be poured for wall protection every drilling about 1m; When there is water seepage and water gushing in the soil layer, concrete wall protection shall be carried out every 50cm; If there are thin layers of quicksand and muddy soil, reinforced concrete wall protection should be adopted at intervals of 50 microns or less; Under unfavorable geological conditions, pier support adopts steel lining or precast concrete lining. (3) Gas supply in the cave to prevent poisoning. For example, in the construction site of a pharmaceutical factory in Chuxiong Economic Development Zone, Yunnan Province, when the reinforcing cage was put into the pile, due to the lack of ventilation in advance, the carbon dioxide content in the hole exceeded the standard by 70 times, resulting in 4 deaths and 3 injuries in the underground. Therefore, it is stipulated in the Technical Specification for Building Pile Foundation that the hole must be tested before lowering the hole, and artificial gas supply must be used when the well depth exceeds lOm. (4) Install the protection plate in the hole. In order to prevent people in the well from being injured by objects, a protective plate with a hole section of 1/3 should be set at a position about 2m above the working layer, and it will gradually move down with the deepening of the working depth. (5) Install splash and leakage protection device. For water-rich soil layers, it is necessary to set up diving.

When the pump drains water, a splash-proof leakage protector should be installed, and the leakage current should not be greater than 15mA. In principle, drainage and construction are not allowed to prevent electric shock.

Due to mechanical hole formation, major accidents such as mechanical injury may occur during the construction of underground continuous wall. In this regard, mechanical construction should pay attention to the following items:

(1) Construction machinery shall have certificate or annual inspection report, entry acceptance procedures and installation acceptance. Ensure that the safety insurance and limit devices are complete and effective. (2) The mechanical operation area shall be leveled and tamped to ensure that the construction machinery is placed stably and will not be tilted or even overturned due to construction vibration. (3) When the existing buildings, underground pipelines and underground structures near the pile row are sensitive to ground motion, protective measures should be taken to control the foundation deformation. Including interval piling, setting isolation and sound insulation grooves, using low vibration and low noise construction equipment and other construction sequences. (4) Before construction, the operators should carry out safety education and safety technical disclosure, and conduct pile testing.

2.2.2 Safety management of anchoring construction

In the process of anchor construction, major safety accidents such as foundation pit collapse and operator falling from a height may occur. Due to over-excavation, the strength of anchor consolidated body did not reach 15MP, the tensile locking design strength did not reach more than 75%, the uplift bearing capacity of anchor met the design and specification requirements, and the working platform was unstable. In this regard, matters needing attention in anchor construction:

(1) Construction shall be carried out in strict accordance with the requirements of design documents and standards, and overbreak is strictly prohibited. In general, the depth of earthwork excavation should be controlled below the anchor to be constructed about 1m, and a suitable working surface should be reserved for anchor construction. (2) After the strength of the anchor consolidated body reaches 15MP and the design strength reaches more than 75%, and the pull-out force of the anchor is detected, the tension locking can be carried out. Only when the detection value of anchor pullout force meets the design and standard requirements can the earthwork excavation of F layer be carried out. (3) Set a safe and stable anchoring construction platform. The bottom of the platform is flat and compacted, and supports can be set around it according to the situation, and protective railings are set around the platform.

2.2.3 Safety management of internal support pole construction

The inner support bar includes steel support, concrete support and combined support of steel and concrete support. Internal bracing varies according to the shape and size of foundation pit, including water semi-bracing, diagonal bracing, angle bracing and ring bracing. Reasonable internal support mode is the key to ensure the stability of foundation pit retaining structure. In the process of installation (or pouring) and demolition, major safety accidents such as collapse and falling from a height may occur. Example: In August 20001year, during the foundation pit construction of a subway test project in Shanghai, local earthwork collapsed, resulting in four deaths. The accident investigation found that the excavation scope of the foundation pit of the project was basically muddy soil, but the construction unit did not adopt continuous vertical support or steel frame support according to the specification requirements, and the collapse accident occurred due to unreasonable support methods. (Literature, Warning Records of Major Safety Accidents in Construction Engineering, p7, Sichuan Publishing Group, Sichuan Science and Technology Publishing House) In this regard, attention should be paid to F during the construction of internal support members:) ① The construction of internal support structure should be carried out symmetrically to keep the stress balance of the members. (2) For steel supports, when the construction in summer produces great temperature stress, measures should be taken to cool the supports in time; When the shrinkage caused by cooling in winter construction causes cracks in the broken end of the support, iron wedges or other reliable connection measures should be used in time. (3) The construction and demolition sequence of the internal supporting structure should meet the design conditions, and the principle of supporting before digging must be followed. (4) Earthwork excavation shall be carried out evenly by layers. In the process of excavation, a large height difference can not be formed in the foundation pit, which leads to uneven stress distribution in the retaining structure and support bars and stress concentration. At the same time, in the process of earthwork excavation and transportation, it is necessary to prevent the earth-moving machinery from colliding with the inner support rod. ⑤ Build a safe and stable anchorage construction platform. The bottom of the platform is flat and compacted, and supports can be set around it according to the situation, and protective railings are set around the platform.

2.3 soil nailing wall supporting soil nailing wall is generally composed of steel bar or steel pipe soil nail, steel mesh and sprayed concrete surface. Under normal circumstances, when the stable soil is deformed, the lateral pressure generated by the deformation is transferred to the deep soil through sprayed concrete steel mesh and soil nails to ensure the stability of the slope. Major safety accidents such as slope collapse, falling from a height and electric shock may occur during construction. Therefore, soil nailing wall should pay attention to the following items:

(1) The construction unit shall set deformation observation points near the slope to observe the slope deformation, arrange full-time security guards, set up warning lines, and formulate emergency rescue measures to ensure the safety of construction and pedestrians. ② Build a safe and stable soil nailing construction platform. The bottom of the platform is flat and compacted, and supports can be set around it according to the situation, and protective railings are set around the platform. (3) In the welding process of steel mesh, because the slope is long and inclined, and the site is wet, it is not convenient to place welding machines, switch boxes and other equipment, and it is easy to generate electricity hidden dangers. Therefore, the field welding machine should be placed on a stable, dry and insulated platform, and the equipment switch box should meet the requirements of "one machine, one brake, one leakage and one box". The rated leakage action current of the leakage protector is not more than 15mA, and the action time is less than 0.1s.

2.4 Gravity cement soil wall

Gravity cement-soil wall generally adopts the structure form that cement-soil mixing piles overlap each other to form a grid or three-dimensional, which is generally large in volume and mass, and relies on the weight of cement-soil itself to resist the deformation of slope. Generally, mechanical construction is adopted. In the process of construction, equipment with complete equipment safety, limit and safety devices shall be selected, and the procedures of equipment entry, installation and acceptance shall be performed, and the safety operation procedures shall be strictly implemented to ensure the construction safety.

3. Follow-up safety management of deep foundation pit engineering

During the foundation construction, the follow-up safety management of foundation pit engineering should be strengthened: ① The construction unit should entrust a qualified unit to strengthen the deformation observation of foundation pit slope, adjacent buildings and facilities according to the overall requirements of the design documents for deep foundation pit construction. (2) Project construction, supervision and construction units should carry out daily inspection of foundation pit safety and conduct drills of emergency rescue plan for foundation pit collapse. (3) If the deep foundation pit project cannot be completed on time, and the exposure time exceeds the service life specified in the supporting design, the construction unit shall entrust the design unit for review and take corresponding measures. Due to the shutdown of the project, the service life of the deep foundation pit exceeds 1 year specified in the supporting design, so the construction unit should take backfill measures. The construction unit that needs to excavate the deep foundation pit again shall reorganize the design and construction.

In a word, deep foundation pit engineering involves the safety and quality of engineering construction, the safety of adjacent buildings, the operation safety of other municipal underground pipe networks and other facilities, and is closely related to the lives of ordinary people. Therefore, the safety of deep foundation pit is the "cornerstone" of the proposed building (structure), and it must be carried out in strict accordance with the requirements of national laws, regulations, norms, standards and design documents.