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4) The landslide in the pit leads to the instability of the support in the foundation pit.
After the excavation of deep foundation pit, the soil was damaged by overall sliding instability along the arc sliding surface or weak interlayer formed under the retaining wall.
Before excavation, a certain amount of materials and equipment shall be prepared, including 2-3 sets of sand bags, steel pipes, anchor rod hole forming machines and grouting machines.
Emergency measures for foundation pit.
During the slope excavation in the long strip deep foundation pit such as railway station and subway station, the landslide caused by steep slope, rainfall or other reasons may wash away the support and column constructed in advance in the foundation pit, resulting in the damage of the foundation pit.
The two pictures below show the failure of the support system caused by the soil landslide in the pit of Fengqi Road Station of Hangzhou Metro Line 1 in 2009.
The picture shows the breaking and failure of diaphragm wall during the construction of deep foundation pit of Hangzhou Metro in 2008.
The following figure shows a deep foundation pit accident in Suzhou in 2008.
If there is water leakage between supporting piles, if the leakage point is small, the leakage can be blocked with early strength cement or steel mesh shotcrete after conduit drainage; When the water inflow is large, in addition to the above methods, combined with local compaction grouting outside the pit and other measures.
Roughly divided, the accident forms of deep foundation pit engineering can be divided into surrounding environment damage, support system damage and soil seepage damage.
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Surveying and setting out of enclosure works, focusing on controlling the spacing between enclosure structure and basement exterior wall; Performance and safe operation of foundation pit support construction equipment and machines; The raw materials of enclosure works are qualified; Carry out construction control according to the enclosure design requirements to ensure that all control indicators meet the design requirements; At the same time, the construction emergency plan and rescue measures shall be prepared.
The notice on safety management measures for dangerous divisional and subdivisional works issued by the Ministry of housing and urban rural development stipulates that deep foundation pit engineering refers to the excavation depth of more than 5m (including 5m) or more than 3 floors in the basement (including 3 floors), or although the depth does not exceed 5m, However, the foundation pit earthwork excavation, support and dewatering works with particularly complex geological conditions, surrounding environment and underground pipelines.
Therefore, how to predict and reduce the ground settlement caused by construction has become a difficult problem to be solved in the field of deep foundation pit engineering.
An important impact is that it will cause uneven subsidence of the surrounding surface, affect the normal use of surrounding buildings, structures and underground pipelines, and seriously cause engineering accidents.
In 2011, the retaining pile of a deep foundation pit in Hangzhou was broken.
There are many types of safety and quality problems in deep foundation pit engineering, and the causes are also complex.
The following figure shows the “skirting” failure of a deep foundation pit.
2、 Key points of foundation pit construction process control Firstly, in combination with the national or local control standards and specifications related to the foundation pit retaining engineering involved in the design scheme and construction scheme, implement various construction measures, including retaining pile wall, water stop curtain, support (concrete or steel support), excavation process, layout and depth of dewatering well, etc.
Under the action of water and soil pressure, the supporting structure may be damaged.
Generally speaking, a foundation pit with an excavation depth of more than 5m can be called a deep foundation pit.
3、 Precautions during foundation pit construction 1.
2) the overall instability of foundation pit enclosure.
Fourth, foundation pit dewatering control.
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Groundwater will affect the strength of soil layer.
In the process of excavation, if the local displacement of the support structure is found to be large, which has exceeded the allowable range, the excavation shall be suspended temporarily, the steel pipe or steel pipe cable shall be used for diagonal bracing in the vertical plane, and the unloading shall be carried out outside the support structure to reduce the active earth pressure, and the anchor rod can also be set for reinforcement.
The following figure shows the overall instability failure accident of a deep foundation pit enclosure.
The groundwater level in soft soil areas is often high, which will involve different hydrological conditions such as phreatic water, micro confined water and confined water.
The failure forms are also different with different types of supporting structure.
The number, depth and hole diameter of well points for dewatering construction shall be reasonably set; When diving dewatering is involved, dewatering shall be carried out first and then excavation, and the water level shall be controlled to be more than 0.5m below the excavation surface; When the dewatering of confined water layer is involved, depressurization and pumping shall be started after excavation to a certain depth according to the design checking calculation, and the excavation can be continued only after the water pressure drops to the safe water level; The water level change shall be monitored through the water level observation well.
I don’t know what a big project it is! 2.
Third, earth excavation shall be carried out when the strength of enclosure structure and water stop curtain meets the design requirements and the conditions for earth excavation are met; Strictly control the excavation depth, retaining slope ratio, construction sequence, location and route of excavation and transportation port, engineering pile and horizontal support, and the deformation of enclosure system to ensure the reasonable progress of earthwork excavation; Over excavation is strictly prohibited; 20cm original soil can be reserved at the pit bottom and excavated manually to reduce disturbance; In case of abnormal conditions, such as water and sand gushing, obstacles, etc., the owner and participating units shall be notified immediately, and the excavation shall be suspended or the predetermined emergency plan shall be adopted for rescue; For the support removal of foundation pit with multiple supports, it is necessary to consider setting temporary support replacement first, and then removing the existing support after it reaches the strength; The support shall be removed by manual or blasting according to the requirements of the surrounding environment and the protection of Engineering finished products, so as to ensure safety.
Real estate engineering concerns What are the control points and precautions for the construction of deep foundation pit in real estate engineering circle? How to strengthen process monitoring? 1、 Failure forms and accident cases of deep foundation pit support 1.
When the water level changes suddenly, the causes shall be analyzed, and the precipitation measures shall be adjusted in time according to the surrounding environmental monitoring; When the dewatering of confined water outside the pit is involved, measures for reinjection outside the pit shall be provided; Ensure that the dewatering measures continue until the demolition time allowed by the design or until the basement is above the ground; During construction excavation, avoid the collision of excavation machinery with dewatering wells, resulting in the failure of dewatering wells; Pumping power supply and pumping equipment shall be provided for construction.
Concept of deep foundation pit.
When cracks appear in the supporting structure, steel pipe or steel pipe cable can be used to reinforce the support between the supporting structure and the supporting pile.
After it is stable, anchor bolts can be driven or grouting reinforcement can be carried out at the pit bottom..
Damage to the support system 1) the breaking accident of the foundation pit support system is mainly due to the rush of construction progress, excessive excavation, and the support erection can not keep up, which is due to the lack of a large number of necessary support in the design of the support system, or the construction unit does not construct according to the drawing, takes chances and adds less support, resulting in the breaking of the support system due to excessive stress or the damage or large deformation due to excessive support axial force.
Seepage may cause flowing soil, quicksand and sudden surge, resulting in damage.
The damage of surrounding buildings and underground pipelines caused by excessive deformation of retaining structure and groundwater loss is also a foundation pit engineering accident.
If there is groundwater seepage, it will increase the risk of foundation pit; When the confined water layer is involved, it may lead to sudden inrush of foundation pit and even instability and failure of foundation pit; Therefore, effective control of foundation pit dewatering is very important.
3) Foundation pit retaining skirting failure due to the small depth of the retaining wall of the deep foundation pit inserted into the bottom of the foundation pit and the low strength of the soil at the bottom, there is a large “skirting” deformation from the bottom of the retaining wall to the foundation pit, resulting in the uplift of the soil in the pit.
The environmental damage around the foundation pit will disturb the surrounding soil in varying degrees during the construction of deep foundation pit.
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According to the permeability characteristics of the stratum where the foundation pit is located, attention should be paid to the cooperation of dewatering and excavation procedures.
In order to prevent sudden accidents and ensure the smooth construction of underground structures of the project, the following measures will be taken to ensure the safety of the whole foundation pit and surrounding environment.
Visually, the foundation pit must be at least tens of meters.
The factors causing the surrounding surface settlement generally include: the displacement of foundation pit wall; Rebound and uplift of foundation pit; Formation consolidation caused by well point precipitation; Sand loss, piping and sand flow caused by pumping.
In this case, if the lower part of the support pile is displaced in the pit, soil or riprap shall be piled on the inner side of the pit immediately, and unloaded on the outer side of the support structure.
After the foundation pit is excavated to the design depth, the block stone cushion shall be made immediately for rescue, and the block stone cushion shall be paved close to the support pile.
Secondly, pay attention to the construction control of enclosure structure.
