Code of practice for in-situ determination of rock mass deformability using a flexible dilatometer, Part 1: volume change
1990 Edition

The flexible dilatometer probe must include a high-pressure flexible membrane mounted on a core that can withstand insertion and removal without damage. It should transmit at least 90% of the intended hydraulic pressure uniformly to the borehole wall, applying pressure evenly unlike rigid dilatometers. The probe must have precise positioning capability within ±5 cm using drill rods or cables and typically includes components such as the membrane, pressure transducer, readout devices, piston actuator, and high-pressure tubing to ensure accurate and reliable soil or rock characterization.

Calibration involves using cylinders with internal diameters matching the test borehole and lengths approximately equal to the active probe length, possessing known elastic properties. This process is performed prior to each test series, repeated weekly during testing, and after significant repairs such as membrane replacement. Temperature is monitored during calibration, and if it differs by more than 5°C from borehole conditions, recalibration is required. Accurate calibration is crucial to determine the system stiffness, ensuring that volume change measurements reflect the true behavior of the soil or rock mass, thereby guaranteeing test validity and repeatability.

Pressure corrections account for losses due to membrane stiffness by subtracting the membrane pressure loss from the observed pressure. Volume corrections involve adjusting the measured pump turns by removing the turns lost during probe seating and those caused by loading system inflation. Additionally, the hydraulic system must be sufficiently rigid to minimize errors; if volumetric changes inside the probe are monitored, hydraulic line expansion can be neglected. These corrections are essential to ensure accuracy in the determination of pressure and volume changes during testing.

First, plot the pressure versus volume change data and determine the slope (MT) representing the combined stiffness of the rock and apparatus. Then, calculate the shear modulus (Ga) using the relationship between this slope and the dilatometer geometry. The dilatometric modulus of elasticity (Ea) is computed using the formula Ea = 2(1 + ν) Ga, where ν is the Poisson’s ratio of the rock. This procedure yields an average deformability modulus of the rock mass, providing crucial input for geotechnical design.

The report must include comprehensive site-wide information such as drilling agency, method, equipment, detailed geotechnical logs including groundwater and rock type data, discontinuity characteristics within ±0.5 m of test sections as per IS 11315, and full calibration details including methods and deviations. For each test, tabulated raw and corrected readings with corresponding depths, graphical plots of pressure versus deformation, derived deformability parameters with methodological explanations, and logs showing parameter variations with depth or distance from the borehole collar are essential. This ensures thorough documentation for reliable interpretation and design application.