Methods of test for soils, Part 35: Measurement of negative pore water pressure
1974 Edition

The axis translation method is an indirect approach for determining negative pore water pressures (matric suction) in partially saturated soils when direct measurement is limited. This technique involves artificially increasing the pore air pressure above atmospheric levels, which shifts the pore water pressure into a positive range that can be measured accurately. The process requires careful de-airing of the system and specific valve operations to ensure precision. In summary, the applied air pressure is raised, causing the pore water pressure to increase above zero, enabling its indirect measurement; the negative pore water pressure is then calculated by subtracting the applied air pressure from the measured water pressure.

Preparation and calibration of the fine ceramic porous stone involve saturating it fully with de-aired water and removing any trapped air from the pore water pressure lines to prevent errors. The stone typically measures 32 mm in diameter and 10 mm in thickness, with an air entry value higher than the maximum negative pore water pressure expected. Calibration is conducted by applying air pressure increments—initially 0.5 kg/cm² and then reduced to 0.1 kg/cm² near the air entry threshold—holding each step for 30 minutes while monitoring water levels to precisely determine the air entry value, establishing the effective measurement range.

The essential apparatus for measuring negative pore water pressure includes a tensiometer featuring a porous ceramic cup saturated with water in contact with the soil, a water reservoir, and a pressure gauge or manometer to record the suction (negative pressure) in the water. Additional equipment consists of devices to saturate the porous cup before testing to ensure accurate contact and equilibrium between soil and water pressures. This setup enables direct measurement of matric suction as required by the standard.

Accuracy and leak prevention are ensured by thorough inspection and sealing of valves, joints, and connections to eliminate any leaks that might compromise readings. The testing environment must maintain a stable ambient temperature to avoid pressure fluctuations. The setup includes a balancing manometer to sustain positive pressure in most parts of the system, minimizing the volume of water under tension and reducing cavitation risk. Additionally, careful deaeration of water prevents air bubbles, and mercury levels are controlled to maintain a stable reference point, collectively ensuring precise and reliable measurements.

The measurable negative pore water pressure is constrained primarily by the tendency of water under tension to cavitate, which limits the maximum suction to approximately -100 to -150 kPa in laboratory settings. To mitigate this, the system minimizes the volume of water subjected to negative pressure and locates critical valves close to the soil sample. Water is thoroughly deaerated, and a balancing manometer maintains positive pressure in most of the apparatus. The measurement principle involves calculating suction as the difference between applied pore air pressure and the positive pore water pressure measured, confining tension to a small water volume to minimize cavitation and provide dependable results.