Subway Steel Support Testing

Metro steel support inspection mainly conducts scientific testing and analysis on the support axial force testing, weld quality non-destructive testing, support deflection and deformation monitoring, component geometric dimension and appearance inspection, connection node and ancillary component inspection and material property re-inspection, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Support axial force testing: This is the core item of the metro steel support inspection, aiming to monitor the axial loads borne by the support structures in real time. Through the measured data, whether the supports are in the normal stressed states can be judged, and their balancing effects on the earth pressures of the foundation pit side walls can be evaluated. It is the key basis of preventing the foundation pit deformation and support instability. Usually, the continuous or periodic monitoring needs to be carried out after the support installation, during the foundation pit excavation and at the key working condition changes.

Weld quality non-destructive testing: The comprehensive inspection is carried out on the welds at the connection parts such as between the steel support pipe sections and between the adjustable heads and the pipe bodies. The methods such as the ultrasonic testing (UT) and magnetic particle testing (MT) are mainly adopted to detect whether there are the defects such as the cracks, lack of fusion, blowholes and slag inclusions in the welds, ensuring the strengths and reliability of the connection nodes and preventing the overall structural damage caused by the weld failures.

Support deflection and deformation monitoring: The vertical and horizontal deformations of the steel supports under the long-term loads and surrounding construction disturbances are monitored. The equipment such as the total stations and levels are used to measure the deflection values at the support middles and their change trends. The excessive deflections will affect the axial load-bearing efficiencies of the supports, and may indicate the risks of the uneven settlement or lateral displacement of the foundation pit.

Component geometric dimension and appearance inspection: The key geometric dimensions of the steel supports such as the pipe diameters, wall thicknesses and lengths are re-verified, ensuring their consistency with the design drawings. At the same time, whether the steel support surfaces have the serious rust, mechanical damages, local buckling or torsional deformations is visually checked. These appearance defects will directly affect the load-bearing capacities and durability of the components.

Connection node and ancillary component inspection: The connection positions between the steel supports and the wales, the connection points of the diagonal braces, and the working states of the prestressing devices (such as the hydraulic jacks) are emphatically checked. Ensuring that the connection bolts are tightened without the looseness, the backing plates are without the deformation, and the prestressing devices are effective, guaranteeing the smoothness and stability of the load transfer paths.

Material property re-inspection: For the important steel support components entering the site, especially the batches used for the first time or with the quality doubts, the material property re-inspection needs to be carried out by sampling according to the provisions. The main testing items include the yield strength, tensile strength, elongation and impact toughness of the steel, ensuring that the material itself meets the designation standards such as Q235 and Q355 required by the design.

Testing Scope

Whole process of the foundation pit excavation: The testing covers the whole cycle from the installation of the first steel support to the complete removal of the last support. Including all the key construction stages such as the initial states of the support installation, after each layer of the earthwork excavation, when the support prestressing forces are applied or re-applied, the main structure construction and the support replacement, and the dynamic tracking monitoring is implemented.

All the steel support components: The testing objects include all the types of steel supports in the foundation pit, such as the opposite braces, corner braces, side trusses and diagonal braces, etc. Not only targeting the main load-bearing components, but also covering the secondary but important parts such as the connecting tie bars, ensuring the safety of the support system as a whole.

Stress concentration and key positions: Emphatically focusing on the stress concentration areas, such as the support midpoints (the maximum bending moment positions), the connection areas between the ends and the wales, the pipe section splicing welds and the telescopic sections of the adjustable heads, etc. These positions are prone to the first failures under the complex stresses, and are the top priority of the testing.

Surrounding environmental impact areas: For the foundation pits adjacent to the environmentally sensitive areas such as the important buildings, underground pipelines and metro tunnels, the testing scope of their steel supports needs to be expanded and densified. The effectiveness of the support systems in protecting the surrounding environments needs to be evaluated, and whether their deformations are within the controllable ranges needs to be monitored.

Special working conditions and abnormal periods: After the extreme weather such as the rainstorms, freezing and thawing and typhoons, or after the abnormal vibration loads such as the large vehicle traffic and heavy equipment hoisting around the foundation pits, the emergency or strengthened testing must be carried out on the steel supports, to evaluate the effects of the external factors on the support stability.

Components after the design changes or reinforcements: If the design changes occur during the construction, or the original supports are reinforced and strengthened (such as the additional supports and steel plate bonding), the steel supports after the changes or reinforcements must be re-included in the testing scope, and the effect verifications are carried out.

Testing Methods

Axial force gauge method (vibrating wire type/fiber Bragg grating type): The most commonly used axial force testing method. The axial force gauges (pressure transducers) are installed at the ends of the steel supports, and through the measurement of the changes of the steel string frequencies or grating wavelengths inside the transducers, the actual axial forces borne by the supports are converted and obtained. This method has the high precision and good stability, and can realize the automatic remote monitoring.

Surface strain gauge method: The resistance strain gauges are pasted or the surface strain gauges are installed at the specific positions on the steel support surfaces. Through the measurement of the micro-strains on the steel surfaces, combined with the material elastic moduli, the local stresses are converted. This method can be used to assist the verification of the axial force gauge data, or to detect the stress states of the positions where the axial force gauges cannot be installed.

Ultrasonic flaw detection method (UT): Using the principle that the high-frequency ultrasonic waves reflect when they encounter the defects during the propagation in the welds, the reflected waves are received through the probes to locate and evaluate the positions, sizes and properties of the internal defects in the welds. This method has the high detection sensitivity to the planar defects (such as the cracks and lack of fusion), and is the main means of the weld internal quality testing.

Magnetic particle flaw detection method (MT): Suitable for the detection of the surface and near-surface defects of the ferromagnetic materials. The magnetic fields are applied to the steel support weld areas and the magnetic particles are sprayed, and the magnetic marks will be displayed at the defects. This method has the simple equipment and convenient operation, is extremely effective for the surface crack testing, and is often used as the supplement of the ultrasonic flaw detection.

Total station/level measurement method: Used to monitor the deformations and deflections of the supports. The fixed monitoring points are arranged on the supports, and the high-precision total stations are used for the three-dimensional coordinate measurement, or the levels are used to measure the elevation changes. Through the data comparison of the different periods, the displacement and deflection deformation amounts of the supports in the horizontal and vertical directions are calculated.

Visual inspection and dimension measurement: The most basic and indispensable method. The experienced technical personnel use the tools such as the weld inspection gauges, vernier callipers, ultrasonic thickness gauges and steel tape measures to carry out the on-site inspection and recording of the support appearance defects, corrosion degrees and geometric dimensions, forming the intuitive quality evaluations.

Testing Standards

Domestic Standards:

1. GB 50497, Technical code for monitoring of building foundation pit engineering

2. GB 50911, Code for monitoring measurement of urban rail transit engineering

3. JGJ 120, Technical specification for retaining and protection of building foundation pit excavations

4. GB/T 11345, Non-destructive testing of welds — Ultrasonic testing — Techniques, testing levels and assessment

5. GB/T 228.1-2021, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

6. GB/T 229-2020, Metallic materials — Charpy pendulum impact test method

International Standards:

1. ISO 6892-1, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

2. ISO 148-1, Metallic materials — Charpy pendulum impact test — Part 1: Test method

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Vibrating wire axial force gauge and readout: The core axial force monitoring equipment. The axial force gauges are usually pie-shaped, with the built-in steel strings and temperature transducers, installed at the support ends. The matching portable readouts excite the steel string vibrations and measure their natural frequencies, thereby calculating the pressure values. This equipment has the strong anti-interference ability and is suitable for the long-term monitoring.

Total station: The high-precision angle and distance measurement instrument, used for the support deformation monitoring. By emitting the laser beams and receiving the reflected signals, the three-dimensional coordinates of the monitoring points can be automatically calculated. The modern intelligent total stations have the functions such as the automatic collimation, tracking and data processing, greatly improving the measurement efficiency and precision.

Digital ultrasonic flaw detector: The key equipment of the weld internal quality testing. The instrument generates the high-frequency electrical pulses to drive the probes to emit the ultrasonic waves, and receives, amplifies and displays the echo signals. The digital instruments have the functions such as the A-scan display, automatic defect positioning, recording and storage and report generation, making the testing results more objective and traceable.

Magnetic particle flaw detector: Including the yokes, rotating magnetic field flaw detectors and magnetic particle spraying devices. The yoke method equipment is light and portable, suitable for the local magnetization of the on-site welds; the rotating magnetic field instruments can realize the multi-directional magnetization and detect the defects in all the directions at one time. Combined with the fluorescent or non-fluorescent magnetic particles, the magnetic marks can be observed in the dark or light places.

Static strain testing system: Composed of the resistance strain gauges (or surface strain gauges), wires, static strain acquisition boxes and computer software. It can carry out the multi-point and long-term static strain monitoring, and the data are transmitted to the computers through the wired or wireless methods for the real-time display, storage and stress analysis.

Routine testing tool group: Including the weld inspection gauges (used to measure the weld reinforcement, widths and misalignment amounts), ultrasonic thickness gauges (used to measure the wall thickness corrosion thinning conditions of the steel pipes), vernier callipers, steel tape measures, magnifying glasses, flashlights and corrosion rating standard comparison plates, etc. These tools are the basic guarantees of carrying out the fast and qualitative on-site inspection.