Some problems in deep foundation pit monitoring

Talking about some problems existing in deep foundation pit monitoring

first, the significance of deep foundation pit monitoring

with the development of urban construction, the excavation depth of foundation pit construction is getting deeper and deeper, from the initial 5 ~ 7m to the current maximum depth of more than 20m. Due to the complexity of underground soil properties, load conditions and construction environment, the monitoring of soil properties, environment, adjacent buildings and underground facilities caused by construction has become an essential part of engineering construction

for complex large and medium-sized projects or projects with strict environmental requirements, it is often difficult to learn from previous experience, and it is difficult to find quantitative analysis and prediction methods in theory, which must rely on on on-site monitoring during construction. First of all, the design strength of the foundation pit is understood by the on-site monitoring data, so as to provide the design basis for reducing the project cost index in the future. Second, we can timely understand the impact and degree of the construction environment - underground soil layers, underground pipelines, underground facilities, and ground buildings in the construction process. Third, the occurrence and development degree of dangerous situations can be found and predicted in time, so as to act as an ear and eye for taking safety remedial measures in time

II. Contents and methods of deep foundation pit monitoring

deep foundation pit construction must have a certain retaining structure to retain soil and water. Enclosure facilities must be safe and effective. The retaining structure of shallow foundation pit used to be steel sheet pile or concrete sheet pile; The deep foundation pit mostly adopts the underground continuous wall structure or row pile cast-in-place pile structure poured on site, and is equipped with concrete mixing piles or tree root piles to stop water. During excavation, underground water must be pumped out of the pit. For the foundation pit with a depth of 7~15m, two to three horizontal supports must be provided in the middle, and the horizontal support adopts steel tube structure or reinforced concrete structure. The enclosure structure must be safe and reliable, and can ensure the stability of the construction environment. From an economic point of view, a good enclosure design should take the safety index near the critical point, and then adjust the construction scheme by relying on the dynamic information feedback provided by on-site monitoring

1. The following contents are the items that can and should be achieved by foundation pit monitoring at present:

(1) settlement and displacement of underground pipelines, underground facilities, ground roads and buildings

(2) lateral displacement of the underground pile body of the retaining pile (pile inclination measurement), settlement and horizontal displacement of the top of the retaining pile

(3) stress changes of retaining piles and horizontal supports

(4) lateral displacement of soil outside the foundation pit (soil measurement accelerates product upgrading)

for the hammer handle with handle sleeve

(5) layered settlement of the underground soil layer outside the pit

(6) underground water level monitoring inside and outside the foundation pit

(7) earth pressure and pore water pressure in underground soil mass

(8) bottom rebound monitoring in foundation pit

2. Layout of observation points

reasonable layout of observation points can be economical and effective. The selection of monitoring items must be based on the needs of the project and the actual situation of the base. Before determining that the survey can take advantage of the opportunity to obtain the layout of the development points, we must know the geological conditions of the base and the retaining design scheme of the foundation pit, and then consider the layout range and density of the survey points according to the previous experience and theoretical prediction

in principle, the measuring points that can be buried should be buried before the commencement of the project, and a certain stability period should be guaranteed. Before the formal commencement of the project, all static initial values should be measured. The measuring points of settlement and displacement should be directly installed on the monitored object. 1. Functional characteristics of the drop hammer impact tester: on the body, only the underground pipeline of the road. If the sample hole is unconditionally excavated and set points, the cement pile can be buried on the sidewalk as the simulation monitoring point. At this time, the depth of the simulation pile should be slightly greater than the depth of the pipeline, and the surface should be protected by a well cover, which will not only affect the safety of pedestrians; If there are pipeline equipment (such as pipeline wells, valves, etc.) on the road, you can directly set points on the equipment for observation

installation of inclinometers (measuring the lateral displacement of underground soil and retaining piles): inclinometers should be buried in those parts that are prone to collapse according to geological conditions, and generally arranged at a distance of 20~30m parallel to the retaining structure of the foundation pit; The inclinometer pipe of the enclosure pile shall be placed when the enclosure pile is concreted; The buried soil inclinometer pipe must be drilled with a drill, and then the hole wall is filled with yellow sand after the pipe is placed. The surface pipe orifice is sealed with concrete, and the pipe orifice is capped or protected by a well frame. When burying the inclinometer pipe, pay attention to that the cross groove must be perpendicular to the edge of the foundation pit

the groundwater level must be reduced before the excavation of the foundation pit, but after the groundwater level is reduced, the groundwater level outside the pit may leak into the pit. The flow of groundwater is the main factor causing the collapse, so the monitoring of groundwater level is an important content to ensure the safety of the foundation pit; According to the underground hydrological data, the water level monitoring pipe should be buried in the place with large water content and strong water permeability, close to the outside of the foundation pit, with a spacing of 20~30m parallel to the edge of the foundation pit. The embedding method is the same as that of the underground soil inclinometer pipe

the embedding method of layered settlement pipe is also the same as that of inclinometer pipe. When embedding, pay attention to the copper ring outside the bellows not to be damaged; Generally, it is appropriate to place one copper ring every 1m. Layered settlement pipes can also be used to monitor the rebound at the bottom of the foundation pit. Of course, the rebound of the foundation pit can also be solved by precise leveling

earth pressure gauge and pore water pressure gauge are the means to monitor the changes of underground soil stress and water pressure. Projects with high environmental requirements must be installed. The installation of pore water pressure gauge also requires drilling with a drill. Multiple pressure gauges can be put into the hole according to different depths as required, and then filled with dry clay balls. After the clay balls absorb enough water, the holes are blocked. The earth pressure gauge shall be installed together with the foundation pit retaining structure during construction, and its pressure surface shall be outward; According to the principle of mechanics, the pressure gauge should be installed at the lateral stress point of the retaining pile at the hidden danger of the foundation pit. When installing these two kinds of pressure gauges, pay attention to the number and protection of the outgoing line

stress meter is an instrument used to monitor the stress changes of foundation pit retaining piles and horizontal supports. Its installation also requires the cooperation of the construction unit during the construction of the enclosure structure. Generally, select convenient parts, select several sections, and install two pressure gauges on each section to take the average value; The stress gauge must be led out by cable and numbered

3. Data observation

according to experience, the impact range of foundation pit construction on the environment is 3~4 times of the pit depth. Therefore, the back point of view selected for settlement observation should be outside the impact range of construction; The backsight point should not be less than two points. The instrument for settlement observation shall be a precision level, and the second-order precision level observation method shall be used to measure the second-order return, and the calibration error of the return shall be less than ± 1mm The initial values of underground pipelines, underground facilities and ground buildings shall be measured before the foundation pit starts. During the construction period, data should be continuously measured as needed, from once a few days to several times a day; The comparison between each observation and the initial value is the cumulative measurement, and the comparison with the previous observation data is the daily variable. According to the recognized data, if the daily variable is greater than 3mm and the cumulative variable is greater than 10mm, an alarm should be sent to the relevant parties