【Excerpts】Foundation pit monitoring

       Generally speaking, foundation pit monitoring should be carried out when the excavation depth is greater than or equal to 5m or less than 5m but the site geological conditions and surrounding environment are more complex, as well as other foundation pit projects that need monitoring. Specifically speaking, which foundation pit projects must be monitored? What still needs expert demonstration? What does the foundation pit monitor and measure? Who will test it? And so on these problems, engineers should know should know.

01 Which foundation pit projects need to be monitored?

Foundation pit works to be monitored:

（1）Foundation pit design safety level is grade one and grade two foundation pit.

（2）The following foundation pits with excavation depth greater than or equal to 5m:

① soil foundation pit;

② extremely soft rock foundation pit, broken soft rock foundation pit, extremely broken rock foundation pit;

③ upper part of the extremely soft rock, broken soft rock and extremely broken rock mass.

（3）The excavation depth is less than 5m but the site geological conditions and the surrounding environment are more complex.

02 Which foundation pit monitoring still needs expert demonstration?

The following monitoring scheme of foundation pit works shall be specially demonstrated:

（1）foundation pit engineering with complex engineering geological and hydrogeological conditions.

（2）foundation pit engineering adjacent to important buildings, facilities and pipelines.

（3）foundation pit engineering with serious accidents and reorganized implementation.

（4）primary and secondary foundation pit engineering with new technologies, new processes, new materials and new equipment.

（5）Other foundation pit projects that need to be demonstrated.

03 Who implements the foundation pit monitoring?

       Before the construction of the foundation pit project, the builder shall entrust a third party with corresponding qualifications to conduct on-site monitoring of the foundation pit project.

04 When will foundation pit monitoring start and when end?

       The monitoring work of foundation pit engineering should run through the whole process of foundation pit engineering and underground engineering construction. The monitoring work should start before the construction of the foundation pit works until the completion of the underground works. The monitoring of the surrounding environment with special requirements should be continued until the deformation tends to stabilize.

05 How to implement the foundation pit monitoring work?

       The site monitoring of foundation pit engineering should adopt the method of combining instrument monitoring and on-site inspection. During the whole construction period of the foundation pit project, there should be special personnel to inspect it every day. Inspection and inspection should include the main contents: supporting structure, construction conditions, surrounding environment, monitoring facilities and other inspection contents. The method of inspection is mainly visual inspection, can be supplemented by hammer, drill, drill, ruler, magnifying glass and other instruments as well as camera, photography and other equipment.

The monitoring of foundation pit engineering shall comply with the following provisions:

（1）Before the construction of foundation pit engineering, the monitoring scheme of foundation pit engineering shall be prepared.

（2）The monitoring project, monitoring point layout, monitoring method, monitoring frequency and monitoring and early warning should be determined according to the safety level of foundation pit engineering, surrounding environmental conditions, support type and construction site.

（3）At least, the horizontal displacement and settlement of the top of the retaining wall should be monitored, and the deep horizontal displacement, the internal force of the supporting structure, soil pressure and pore water pressure should be monitored according to the technical design conditions of the project.

（4）The monitoring points should be laid along the top of the foundation pit retaining wall, and the surrounding middle and Yang corners should be located.

（5）When the foundation pit monitoring reaches the deformation warning value, or the quicksand, pipe surge, uplift, collapse of the foundation pit, or the foundation pit supporting structure and the surrounding environment appear large deformation, the early warning should be carried out immediately.

（6）Foundation pit precipitation should be monitored for the depth of water level, and recharge capacity and water quality should be monitored for groundwater recharge construction.

（7）The reverse method construction should be monitored in the whole process.

06 How to deal with the problems found in the foundation pit monitoring?

       When one of the following circumstances occurs, the danger alarm must be made immediately, and the parties concerned shall be notified to take emergency measures against the foundation pit supporting structure and the surrounding environmental protection objects.

（1）The displacement value of the foundation pit supporting structure suddenly increases significantly, or the quicksand, pipe surge, uplift or collapse appear in the foundation pit.

（2）The support or anchor rod system of the foundation pit support structure shows signs of excessive deformation, compression bending, fracture, relaxation or pulling out.

（3）The deformation cracks of the structure around the foundation pit appear.

（4）The ground around the foundation pit appears more serious sudden cracks or underground cracks, the ground subsidence.

（5）The deformation of the pipeline surrounding the foundation pit suddenly increases significantly or appears cracks, leaks, etc.

（6）When the frozen soil foundation pit is subjected to freeze-thaw cycle, the temperature of the soil around the foundation pit increases significantly, and obvious freeze-thaw deformation occurs.

（7）Other hazards occur that need to be alerted.

——Source: Architectural Engineering Luban Union

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