Definition
A lateral pressure tensiometer is a field test instrument used to measure the stress-strain characteristics and strength parameters of materials such as soil or soft rock under lateral pressure. It evaluates the mechanical behavior of rock and soil by applying radial pressure to the borehole wall and measuring the corresponding deformation, and is one of the important tools in situ testing technology.
Principle
The core working principle of the side pressure tensiometer is based on the theory of axisymmetric cylindrical hole expansion in elastic mechanics. The main body of the instrument is an expandable cylindrical probe with a flexible membrane on the outside. During the test, the probe is placed in a pre-drilled hole, and a liquid or gas is injected into the probe cavity through a control system to expand the flexible film uniformly radially, applying gradually increasing lateral pressure to the hole wall. During this process, precision sensors synchronously record data on applied pressure and probe volume changes (or radial displacement). By analyzing the pressure-volume change curve, the deformation modulus of the soil, the ultimate pressure and other foundation parameters can be derived.
The basic mechanical relationship can be expressed as follows: in the elastic stage, the pressure increment Δp and the volume change ΔV approximately satisfy the linear relationship, which can be obtained by the formula E = 2(1+ν)V₀(Δp/ΔV) Estimate the lateral compression modulus E, where ν is the Poisson's ratio and V₀ is the initial volume.
Measurement method
The standard measurement process typically consists of three stages: pre-drilling pilot holes, probe placement, and pressurization testing. First, a drilling rig is used to form vertical holes with a diameter slightly larger than the diameter of the probe at the test point, minimizing disturbance to the surrounding soil. The side-pressure probe is then carefully lowered to a predetermined depth and made sure it is aligned with the hole wall. After starting the test, the control system applies pressure in stages at a constant rate, maintaining each pressure for a certain period of time until the deformation stabilizes, while recording stable volume increments. This process continues until the soil is plastically damaged or the upper limit of the instrument range is reached. Finally, by drawing and analyzing the complete lateral pressure curve (pressure-volume curve), the characteristic intervals such as elastic segments and plastic segments are identified, and then the relevant parameters are calculated.
Influencing factors
The accuracy of the measurement results is influenced by several factors. The drilling process is one of the key factors, and the mechanical disturbance during the hole formation process will change the original structure of the soil in the hole wall, which may lead to low modulus values. The contact between the probe and the hole wall also needs to be paid attention to, if there is a gap, the initial stage data will include the contact compaction process, which will interfere with the determination of the elastic segment. Test rate is also important, as pressurizing too quickly will increase pore water pressure, affecting effective stress measurements, while slowing down will prolong the test cycle. In addition, the anisotropy of the soil, the groundwater level, and the calibration status of the instrument all have a systemic impact on the data. Therefore, the corresponding standards and specifications must be strictly followed in operation to control these variables.
Applications
Side pressure tensiometer is widely used in the field of geotechnical engineering survey and foundation evaluation. In construction engineering, it provides the key parameters required for foundation bearing capacity and settlement prediction for the design of shallow or deep foundations. In traffic engineering, it is used to evaluate the stability and deformation characteristics of roadbed and tunnel surrounding rock. In water conservancy engineering, it can be used for the infiltration stability analysis of embankments and channel foundations. In addition, in the field of environmental geotechnical engineering, such as landfill liner system evaluation and mine tailings pond stability monitoring, side pressure testing can also provide valuable in-situ mechanical data. Its advantage is that it can directly test the soil as it is, reducing the error caused by sampling disturbances.
Selection considerations
Choosing the right side pressure tension meter requires comprehensive consideration of testing needs, site conditions, and instrument performance. First, the pressure range and volume change range of the instrument should be determined according to the estimated stiffness and strength range. For soft soil, a probe with high sensitivity and low starting pressure should be selected; For dense sand or soft rock, the instrument is required to have a higher pressure rating. Second, consider the test depth and match the probe and pressure transfer line of sufficient length. Ease of operation in the field is also a factor, including the portability of the instrument, the complexity of assembly, and the degree of automation of data acquisition. Finally, the long-term stability of the instrument, the ease of calibration and maintenance, and compliance with industry or international standards (e.g., ASTM, Eurocode) are all important decision-making factors.