A general specific foundation pit monitoring plan is prepared by a professional monitoring unit; this plan only provides a brief description.

1.1 Monitoring Items

Statistical Table of Foundation Pit Monitoring Items

1.2 Layout of Monitoring Points

Plan Layout Diagram of Deep Foundation Pit Measuring Points for a Certain Utility Tunnel Project

Sectional Diagram of Measuring Point Layout for Soil Nail Wall Support

Sectional Diagram of Measuring Point Layout for Pile + Internal Bracing Support

Sectional Diagram of Measuring Point Layout for Double-Row Pile Support

Remarks: The above cross-sections are for illustration only; for detailed layout points, see the special plan for foundation pit monitoring.

1.3 Monitoring Methods

1.3.1Monitoring of Horizontal Displacement at Pit Top

1.3.2Monitoring of Settlement at Pit Top

To improve monitoring efficiency, the settlement monitoring points at the top of the foundation pit may directly use the displacement monitoring points for settlement monitoring, and no additional points need to be set.

1.3.3Monitoring of Deep Horizontal Displacement

The monitoring of deep horizontal displacement should adopt the method of embedding inclinometer tubes in the wall or soil and observing the horizontal displacement at each depth through an inclinometer.

The inclinometer tubes for observing the deep horizontal displacement of the soil shall be arranged according to the requirements for controlling deformation in foundation pit engineering. Generally, the deep horizontal displacement monitoring holes should be arranged at the center and representative positions around the foundation pit slope and retaining wall; the number and spacing shall depend on specific conditions, but at least 1 monitoring hole shall be set on each side; the length of the inclinometer tube is 20 m.

1.3.4Crack Monitoring

Crack investigation shall be carried out before foundation pit excavation, and observation marks shall be made. During foundation pit construction, cracks shall be investigated at any time; once cracks are found, they shall be recorded and observation marks shall be made for observation. The initial number of cracks shall be based on actual conditions. In accordance with the relevant provisions of the "Technical Code for Monitoring of Building Foundation Pit Engineering" (GB 50497-2009), the "General Technical Conditions for Vibrating Wire Sensors of Geotechnical Engineering Instruments" (GB/T 13606-2007), and the "Code for Deformation Measurement of Building and Structure" (JGJ8-2007), the layout of monitoring points shall be determined in combination with actual conditions for monitoring.

1.3.5Monitoring of Internal Forces in Support Structure

The internal forces of the support structure may be measured using strain gauges or stress gauges installed inside or on the surface of the structure.

Concrete members may be measured using rebar stress gauges or concrete strain gauges, etc.; steel members may be measured using axial force gauges or strain gauges, etc.

Internal force monitoring sensors should be embedded at least 1 week before foundation pit excavation, and the average value of stable test data obtained over 2 consecutive days before excavation shall be taken as the initial value.

1.3.6Earth Pressure Monitoring

Earth pressure monitoring should be measured using earth pressure gauges. The earth pressure gauges may be embedded in an embedded type or boundary type. After the earth pressure gauges are embedded, inspection and testing shall be carried out immediately; before foundation pit excavation, monitoring shall have been conducted for at least 1 week and a stable initial value shall be obtained.

1.3.7Pore Water Pressure Monitoring

Pore water pressure should be tested using vibrating wire or strain-type pore water pressure gauges. The pore water pressure gauges may be embedded by the pushing method, drilling method, etc. After the pore water pressure gauges are embedded, the initial value shall be measured, and measurement should be conducted daily for more than 1 week to obtain a stable initial value. The groundwater level near the location where the pore water pressure gauges are embedded shall be measured simultaneously with pore water pressure monitoring.

1.3.8Groundwater Level Monitoring

Groundwater level monitoring should be carried out by installing water level pipes in holes and measuring with a water level meter. The phreatic water level pipes shall be embedded before foundation pit construction, and the length of the filter pipe shall meet the measurement requirements; during confined water level monitoring, effective water isolation measures shall be taken between the measured aquifer and other aquifers. The water level pipes should be embedded at least 1 week before the foundation pit begins dewatering, and the water level should be continuously observed daily to obtain a stable initial value.

1.3.9Monitoring of Settlement of Surrounding Ground Surface, Buildings, Roads, and Pipelines around the Foundation Pit

The principle of settlement monitoring for the ground surface around the foundation pit is: buildings, roads, and pipelines within a range of 1.5 times the depth of the foundation pit all require settlement monitoring, and the layout spacing of monitoring points for buildings should be appropriately densified. Based on a foundation pit depth of about 10 m, building settlement monitoring points shall be arranged for buildings within 20 m around the foundation pit; the monitoring points shall be embedded in the vertical structures of the buildings, and their number shall depend on the building area and its distance from the foundation pit. Each building shall have 2 to 6 settlement monitoring points. Settlement monitoring points for main roads shall be arranged on the side of the road close to the foundation pit; the monitoring points should be set on both sides of the road at positions that do not affect traffic and are easy to preserve, with an observation point spacing of about 20 meters. To obtain more information on road settlement, the road observation points shall be staggered from the pipeline settlement monitoring points on both sides of the road. Underground pipelines shall be observed by sampling; the locations and number of points shall be implemented according to actual conditions, and a special monitoring plan shall be prepared.

1.3.10Monitoring Frequency

Foundation pit engineering monitoring runs through the entire construction process of foundation pit engineering and underground engineering. Monitoring generally starts before foundation pit construction and ends when underground engineering is completed. For monitoring of surrounding environments with special requirements, it should be continued as needed until deformation tends to stabilize before ending.

1.3.11Monitoring under Normal Conditions

According to the requirements of foundation pit monitoring specifications: the monitoring frequency of monitoring items shall take into account the safety level of the foundation pit engineering, different construction stages of the foundation pit and underground engineering, and changes in the surrounding environment and natural conditions. When monitoring values are relatively stable, the monitoring frequency may be appropriately reduced.

1.3.12Monitoring under Special Conditions

When any of the following situations occurs, monitoring shall be strengthened, monitoring frequency shall be increased, and monitoring results shall be promptly reported to the client and relevant units:

(1) Monitoring data reach the alarm value;

(2) The variation of monitoring data is relatively large or the rate accelerates;

(3) There are adverse geological conditions not discovered during investigation;

(4) Over-deep or over-long excavation or failure to install bracing in a timely manner, etc., not constructed according to design;

(5) Large amounts of water accumulation in and around the foundation pit, long-term continuous rainfall, or leakage of municipal pipelines;

(6) Sudden increase in ground load near the foundation pit or exceeding the design limit;

(7) Cracking of the support structure;

(8) Sudden large settlement or serious cracking of the surrounding ground surface;

1.4 Monitoring Technical Results

Monitoring result reports are generally divided into daily reports, weekly reports, monthly reports, and final result reports.

(1) Daily Report

The daily report refers to reporting the monitoring results of each day on a daily basis, mainly in the form of tables and curves. The specific contents include:

Project overview of the foundation pit, construction progress, monitoring items and measuring point layout, statistical table of monitoring results and typical time-history curves, analysis of monitoring results, conclusions, and recommendations.

(2) Weekly and Monthly Reports

The main contents of weekly and monthly reports include:

① Project overview of the monitored foundation pit;

② Construction progress;

③ Monitoring items and measuring point layout;

④ Statistics of phased monitoring results and typical time-history curves;

⑤ Analysis and prediction of monitoring results;

⑥ Conclusions and recommendations

(3) Contents of the Final Report

① Project overview;

② Monitoring purpose and content;

③ Monitoring items and measuring point layout;

④ Instrument models, specifications, and calibration data used;

⑤ Observation methods and data acquisition;

⑥ Monitoring result data and analysis and processing;

⑦ Full-process variation curves of monitoring values;

⑧ Evaluation of the effectiveness of advanced prediction.

1.5 Monitoring Data Processing and Analysis

1.5.1 Organization of Monitoring Data

The methods and contents of organizing, analyzing, and feeding back monitoring data generally include the collection, organization, analysis, feedback, and judgment and decision-making of monitoring data.

(1) Collection

Collection and recording of data obtained through on-site monitoring and other related materials. The instruments used in this monitoring project, such as levels, require manual reading and recording, and then the measured data are input into the computer; total stations automatically collect data and automatically transmit measured values to the database management system.

(2) Organization

After each observation, the original observation data shall be checked and organized immediately, including verification of original observation values, calculation of physical quantities, form filling and charting, elimination of abnormal values, preliminary analysis, and compilation, etc., and the verified data shall be input into the computer database management system.

(3) Analysis

Using comparison methods, graphing methods, and mathematical and physical models, analyze the magnitude, variation patterns, and development trends of each monitored physical quantity, so as to evaluate and make decisions on the safety status of the project and the measures to be taken.

During construction, curves of the variation of monitoring data with time—time-history curves (or scatter plots)—are generally drawn, and key construction procedures, etc., are marked on the time-history curves, so as to analyze the deformation and stress state of the engineering structure and guide design and construction. During on-site measurement, patrol records shall be made as required and measurement data shall be organized and analyzed in a timely manner. The time-history curves drawn are as follows:

a. Draw the time-history relationship curve of the cumulative value of the monitoring variable (P) versus time (t);

b. Draw the time-history relationship curve of the rate of change of the monitoring variable (△P) versus time (t).

(4) Prediction and Feedback

To ensure the quality of monitoring results and accelerate information feedback, all monitoring data are managed by computer. Each monitoring session must have monitoring results, and monitoring weekly reports shall be submitted promptly; monitoring monthly reports shall be submitted to relevant units on schedule, together with corresponding time-history curves of measuring point displacement, to evaluate the construction situation of the month and put forward construction recommendations.

Monitoring data generally vary with time and space, commonly referred to as time effects and space effects. They should be expressed promptly with variation curve relationship diagrams to make monitoring results "visualized," so that problems can be discovered and analyzed in a timely manner.

1.5.2 Management Levels and Countermeasures

Based on the analysis results of on-site measurement, corresponding monitoring management levels and countermeasures shall be formulated according to the monitoring control standards and warning value indicators, as shown in the table below.

Monitoring Management Levels and Countermeasures for This Project

1.6 Monitoring Information Feedback and Countermeasures

Flowchart of Monitoring Measurement Information Feedback