Foundation pit monitoring technology

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Based on the design of foundation pit is the pit position according to the elevation of basement and foundation excavation plane size, is the foundation of all engineering projects. So it is important to ensure the safety of foundation pit. The safety foundation determines the construction can be completed successfully, for foundation pit monitoring is to nip in the bud, to ensure the engineering safety.

1, horizontal displacement monitoring

Determination of horizontal displacement in specific directions can be used when the collimation method, small angle method, cast point method; visual monitoring points of the horizontal displacement distribution determination of monitoring points in any direction, using intersection method, free station method, polar coordinate method; when the reference point is far from the pit can be used. GPS measuring method or triangle, three edge and angle measurement and reference line method combined measurement method. When the monitoring accuracy requirements are relatively high, it can be used to monitor the accuracy of micro deformation measurement radar for automatic all-weather real-time monitoring. Horizontal displacement monitoring datum point should be buried in the depth of excavation is not affected by the construction of the stable area beyond 3 times the range, or the use of construction control points has been stable, should not be buried in the low-lying water, collapsibility, frost heave and swell shrink effect range; embedded benchmarks should be in accordance with relevant regulations, executive measure. It is advisable to set up a force to the observation pier; the use of precision optical alignment device, the error is not greater than 0.5mm.

2, vertical displacement monitoring

The vertical displacement monitoring can be used in the method of geometric or liquid static leveling. Heave (rebound) should be set by using standard geometric leveling monitoring rebound, and with the elevation of the auxiliary equipment transfer monitoring, elevation transfer of metal rod or ruler, of temperature, foot and tension correction, retaining wall (slope), the top wall surface and the column of the vertical displacement monitoring accuracy according to the vertical displacement of the alarm value determined.

3, deep horizontal displacement monitoring

The monitoring of the deep horizontal displacement of the retaining wall and the soil around the pit should be used in the wall or in the soil.

4, tilt monitoring

Buildings tilt monitoring shall determine the monitoring object relative to the bottom of the top horizontal displacement and elevation, were recorded and calculated the monitoring object tilt, tilt and tilt rate. According to the different observation conditions and requirements, selection of cast point method, horizontal angle method, intersection method, straight line method, differential settlement method etc..

5, crack monitoring

Crack monitoring should include the location, direction, length, width and degree of change of the crack. Crack monitoring number according to need to determine, the main or change of the larger cracks should be monitored. Crack monitoring can adopt the following methods: (1) the crack width monitoring, post gypsum cake, cracks in the draw parallel lines or stuck metal signs, using methods such as vernier caliper or micrometer measuring directly; also can use crack meter and paste installation gauge method, photography measurement method. (2) the crack depth measurement, when the crack depth is better than the use of chisel out method and one side contact ultrasonic method to monitor; the depth of the large cracks should be monitored by ultrasonic method. It should be recorded before the foundation pit excavation to monitor the distribution of the existing cracks in the location and quantity of the direction, length, width and depth of the case, the logo should have a clear end face or center. Crack width monitoring accuracy is not less than 0.1mm, length and depth monitoring accuracy is not less than 1mm.

6, supporting structure internal force monitoring

The internal force of the retaining structure can be measured by the strain gauge or the stress meter installed in the structure. For reinforced concrete support, it is appropriate to adopt the steel stress gauge (bar gauge) or concrete strain gauge for measurement; for the steel structure support, it is appropriate to use axis force meter for measuring. The internal force of retaining wall, pile and purlin should be in reinforced retaining wall, pile making, welding steel stress meter embedded method of measurement on the main reinforcement. The monitoring value of the internal force of the supporting structure should be considered, and the influence of the temperature variation should be taken into consideration, and the influence of concrete shrinkage, creep and crack development should be taken into consideration.

7, earth pressure monitoring

Earth pressure should be measured by earth pressure. The embedded boundary or burying soil pressure gauge (contact). Laying shall meet the following requirements: (1) the stress surface and pressure required for monitoring the vertical direction and close to the object being monitored; (2) membrane protective measures should be embedded in the process of soil pressure; (3) the drilling method is laid, backfill should be uniform and dense, and the surrounding rock and soil backfill materials should be consistent. (4) to complete burial record. Shall be inspected immediately after testing buried soil pressure gauge before excavation for at least 1 weeks after monitoring and obtain stable initial value.

8, pore water pressure monitoring

Pore water pressure should be buried by the steel string strain, such as pore water pressure or strain measurement using frequency meter. Pore water pressure meter should meet the following requirements: measuring range should meet the requirements of the range of the pressure and the water pressure and excess pore water pressure and 1.2 times; precision is not lower than S - 0.2%F, resolution is not less than 0.5%F - S. The indentation method, drilling method of buried pore water pressure gauge.

9, underground water level monitoring

Underground water level monitoring should be set up through the hole in the water level tube, using the method of water level meter and other methods of measurement. Underground water level monitoring accuracy is not less than 10mm.

10, bolt tension monitoring

Bolt pulling force measurement should adopt special steel anchor bolt dynamometer, the steel stress meter or strain gauge, when using reinforced beam should be respectively monitoring each bar force. bolt