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Foundation pit rebound monitoring

➊Overview

Foundation pit rebound observation is to measure the range of influence inside and outside the foundation pit caused by the removal of the self-weight load of the foundation pit soil after large-scale foundation excavation. The amount of internal soil rebound compared to before excavation provides information for improving foundation design and protecting the safety of adjacent buildings.

Pit bottom rebound monitoring is difficult to implement on site, but it can reflect the degree of deformation of the pit bottom after the foundation pit is excavated to the bottom. The size of the bulge directly reflects the settlement of the surface around the foundation pit, especially when the foundation pit site is soft soil. , Excessive foundation pit uplift will cause the entire foundation pit to become unstable and damaged. Therefore, pit bottom uplift (rebound) monitoring is very necessary for soft soil areas. However, due to limitations of the monitoring environment and methods, pit bottom uplift (rebound) monitoring is specified as a measurable item and should be monitored when there are special requirements.

Rebound observation can usually use two types of rebound monitoring markers and layered settlement markers. When the layered settlement ring is buried below the excavation surface of the foundation pit, the soil uplift monitored is the amount of soil rebound.

➋Layout location

The layout of springback monitoring points should be determined jointly with the design, construction or geotechnical engineering personnel. The minimum number of points should be used to measure the required springback amount of the longitudinal and cross-sections of the foundation pit. Generally, points are arranged along the vertical and horizontal axes of the foundation pit or at other important locations, and at the same time, monitoring points are arranged within a certain distance outside the foundation pit (1.5 to 2 times the depth of the foundation pit). And in areas that are relatively stable and not easily damaged during construction, control points and reference points for finding signs are selected and buried.

➌Measuring point burial

The burial of the rebound mark should be determined based on the foundation soil quality, excavation depth and construction conditions. Generally, a small-diameter drilling rig is used to drill the hole or the hole is dug manually for burial. The sign should be buried 20~30cm below the base surface and a protective tube should be added.

➍Observation method

According to the distribution of rebound monitoring points, the observation line should be reasonably organized. It should generally be laid out as a closed or attached line starting from and ending at the adjacent working base point, and has a graphic with verification conditions.

The error in the height difference of the measuring station for rebound observation is estimated based on the maximum allowable rebound amount. The error in the height difference between the farthest rebound monitoring point and the adjacent working base point should not be greater than μ√Q (Q is the corresponding weighted reciprocal), but the maximum should not exceed ±1.0mm.

For rebound observation after foundation pit excavation, the geometric level is generally used together with the auxiliary rod method or the plumb steel ruler method to measure the elevation of each mark under the base surface of the rebound mark. As shown in Figure 5 below:

a. Auxiliary rod method:

The auxiliary rod is generally made of a hollow metal pipe sealed at both ends. The top should be processed into a hemispherical shape, and a round box level should be placed on its side to ensure the verticality of the auxiliary rod. The length of the pole depends on the depth of the foundation pit. It is appropriate to put it into the hole and expose 20~40cm above the ground. The header of the rebound mark should also be processed into a hemispherical shape so that the ruler can be erected for observation. The length and expansion coefficient of the auxiliary rod must be measured before and after the test. The error in rod length measurement should be less than 1/2 of the error in rebound observation.

b. Steel ruler method:

After tightening the steel ruler with a tripod, pulley and weight on the ground, use level to read directly on the steel ruler. When a hook is used at the interface between the steel ruler head and the top of the rebound mark, the top of the mark should be processed into a hook shape. When using a weight hammer, it is advisable to set a magnet in the center of the bottom of the weight hammer to directly contact the top of the sign. When the pit depth and geological conditions are complex and it is impossible to accurately measure using a hook and magnet contact, use a special electromagnetic probe suspended on a steel ruler and If the top of the standard is in contact, the steel ruler used should be calibrated before and after the measurement. The error in the calibration should be less than the rebound observation error.

Foundation pit rebound monitoring point layout and rebound longitudinal and cross-sectional views