(1) Before foundation pit excavation, the supporting scheme shall be determined according to the soil conditions, the depth of foundation pit and the surrounding environment, and its contents shall include: requirements for slope setting, design of supporting structure, selection of machinery, excavation time, excavation sequence, excavation depth in layers, ramp position, vehicles entering and leaving roads, dewatering measures and monitoring requirements.
(2) The formulation of construction scheme must aim at the combination of construction technology and working conditions, the possible collapse factors in the construction process, the safety of working conditions and the prevention of uneven settlement of surrounding buildings and roads, design and formulate concrete and feasible measures, and put them into practice in construction.
(3) The box foundation of high-rise building actually forms the basement of the building. With the increase of superstructure load, it is often required to set three or four basements below the ground, so the depth of foundation pit is often more than 5 ~ 6m, and the area is large, which brings great difficulties and dangers to foundation engineering construction. Safety measures must be carefully formulated to prevent accidents.
1) The construction site is narrow and there are many adjacent buildings. Large-scale foundation pit excavation often causes cracks or uneven settlement of these old buildings;
2) The depth of foundation pit is different, the main building is deep and the podium is shallow, so the construction procedure needs to be carefully arranged. Sometimes, some shallow pits are dug first, and then cantilever sheet piles are used for support.
3) Take reasonable dewatering measures to reduce the earth pressure on sheet piles;
4) When using steel sheet piles, reasonably solve the displacement and bending;
5) In addition to lowering the groundwater level, open ditches and collecting wells should be set up in the foundation pit to eliminate open water in sudden rainstorm;
6) Large area foundation pit should be equipped with two power supplies. When one power supply fails, the other power supply can be used in time to prevent accidents caused by stopping precipitation.
In a word, due to the deepening of foundation pit, the emergence of soil lateral pressure and groundwater, special support design must be carried out to ensure construction safety.
(4) Whether the supporting design scheme is reasonable not only directly affects the construction period and cost, but also directly relates to the safety in the construction process, so it must be approved by the superior.
2. Border protection
(1) When the construction depth of the foundation pit reaches 2m, it will cause danger to the pit-side operation. According to the provisions of aerial work and border operation, border protection facilities should be set up.
(2) The material selection, erection method and firmness of tower protective railings around the foundation pit shall comply with the Technical Specification for Safety of Working at High Places in Building Construction.
3. Foundation pit support
The function of foundation pit support mainly includes the following aspects: protecting the safety of adjacent existing buildings and underground facilities; When the supporting structure is used to control groundwater, the precipitation caused by grouting may lead to excessive settlement of adjacent buildings and affect their normal use function. At this time, local recharge technology should be adopted. Save the construction space, when the construction site is not allowed to have a slope, the supporting structure can be used to limit the excavation space to a small range outside the foundation surface of the main structure; Reduce the uplift at the bottom of the foundation. Due to excavation unloading, the foundation pit and surrounding soil will rebound and uplift, which will lead to the failure of foundation pit uplift in serious cases. Reasonable design and construction of supporting structure can greatly reduce this deformation. Use permanent structure as a part of supporting structure, such as basement exterior wall as the main structure.
The safety grade and important coefficient of the side wall of foundation pit supporting structure can be divided into:
(1) security level 1. The damage result is the damage of supporting structure, and the instability or excessive deformation of soil has a serious impact on the surrounding environment of foundation pit and the construction of underground structure. At this time, the importance coefficient r0 is 1. 1.
(2) The safety level is Grade II. The damage result is the damage of supporting structure, and the instability or excessive deformation of soil has a general impact on the surrounding environment of foundation pit and the construction of underground structure. At this time, the importance coefficient r0 is 1.0.
(3) The safety level is level 3. The damage result is the damage of supporting structure, and the instability or excessive deformation of soil has no serious influence on the surrounding environment of foundation pit and underground structure construction, and the importance coefficient r0 is 0.9.
Different supporting methods are adopted for foundation pits with different depths and working conditions.
(1) undisturbed soil slope. Generally, when the depth of foundation pit is less than 3m, a slope can be put at one time. When the depth reaches 4-5m, graded sloping can also be used. Open cut slope must ensure the stability of the slope, and the safety factor is determined by the stability calculation of shallow foundation pit. Undisturbed soil slope is suitable for shallow foundation pit. For deep foundation pit, piling, soil nailing wall or underground continuous wall can be used to ensure the stability of slope.