1.Testing principles and core indicators
Q: What is the core physical process of the Pressure Test (PMT)?
Answer: The lateral pressure test is essentially a "transverse load test" conducted deep underground.
During the experiment, place the pressure measuring probe into the pre drilled hole to the specified depth. Inject high-pressure water (or gas) into the probe through the control box on the ground, causing the rubber film of the probe to expand outward and compress the hole wall.
As the pressure gradually increases, measure the radial displacement (volume change) of the soil on the borehole wall until the soil reaches a shear failure state.
Q: What parameters correspond to the three stages (elasticity, plasticity, and limit) in the lateral pressure curve?
The typical lateral pressure test curve (pressure $P $- volume change $V $) can be clearly divided into three sections:
Initial in-situ recovery section (starting point of elastic section): The probe initially expands, overcomes the collapse or unloading rebound of the hole wall, and re adheres to the soil wall. The corresponding pressure is the initial pressure P0
Linear deformation stage (elastic stage): The pressure and volume are linearly related, and the soil is in a quasi elastic deformation state. The slope of this curve is used to calculate the Mena pressure modulus Em, which reflects the stiffness of the soil.
Plastic yield and ultimate stage (failure stage): The curve begins to bend sharply to the right, indicating that the soil has undergone shear failure. The corresponding pressure when the volume tends to infinity (usually twice the initial pore volume) is the ultimate pressure PL, which is used to calculate the ultimate bearing capacity of the foundation.
2. On site drilling and drilling quality (key success factors)
Q: Why is it said that the quality of pore formation determines the success or failure of the lateral pressure test?
Answer: The lateral pressure test is extremely sensitive to the diameter of the borehole and the integrity of the borehole wall.
Excessive aperture (large hole): After the probe is inserted, the rubber membrane needs to expand a large volume to touch the hole wall, which may cause the instrument to reach the upper limit of the measurement range before entering the elastic stage, and the test is declared a failure.
Hole wall disturbance (scratching, hole collapse): If the disturbance is severe during drilling, or if the mud wall is not well protected and causes the hole wall to peel off, the probe will not squeeze the "undisturbed soil" but the crushed soil residue, and the measured modulus Em will be significantly lower.
Best practice: The diameter of the borehole is usually only about 2-4 mm larger than the outer diameter of the probe. It is recommended to use a thin-walled soil sampler to directly press into the hole in soft soil, or to use a specialized hole puncher.
3. Calibration and error control (must be done before testing)
Q: Why is it necessary to perform "resistance calibration" and "deformation calibration" before each on-site test?
The pressure gauge is connected to a probe deep underground through a long pipeline from the ground. If the system's own error is not deducted, the data will be completely incorrect:
Probe resistance calibration (correcting pressure loss): The rubber outer membrane itself has stiffness. When the probe expands in the ground air, a certain pressure also needs to be applied. The pressure required for the rubber membrane to expand itself must be subtracted from the final data in order to obtain the net pressure truly applied to the soil wall.
Pipeline and system deformation calibration (correcting volume loss): Under high pressure, the ground control box, pipelines that are tens of meters long, and the rigid skeleton of the probe itself will undergo slight elastic expansion. The probe must be locked into a rigid steel pipe on the ground for pressure testing, measuring the system's own volume expansion and deducting it from the test data.
4. Troubleshooting and on-site emergency response
Q: During the experiment, the volume reading (measuring cylinder water level) suddenly "skyrocketed in a straight line" and lost control. What is the reason?
Rubber film rupture (rupture amplitude): This is the most common fault on site. It may be due to the sharp gravel on the hole wall puncturing the rubber membrane, or the excessive pore size causing the rubber membrane to expand beyond its range. At this point, high-pressure water directly leaks into the borehole.
Emergency response: Immediately cut off the gas/water source and slowly lift the probe. When lifting in deep holes with mud walls, the speed should be slow to prevent the suction effect from causing large-scale collapse of the hole wall. Replace the rubber film with a new one and perform resistance calibration again.
Q: Is the pressure in the control box unable to rise, or does it quickly fall back after being pressurized?
Gas source inspection: Check whether the output pressure of the pressure reducing valve of the high-pressure nitrogen cylinder is greater than the predicted pressure required for the test.
Air/water leakage: Check if the quick connector of the ground coaxial high-pressure pipeline is locked. Due to frequent disassembly and assembly, the O-ring seal at the joint is very prone to wear and leakage.
