Methods of test for soils, Part 34: Determination of density of soil in place by the rubber-balloon method
1972 Edition

The rubber-balloon approach, as specified in IS 2720 Part 34, is primarily suited for cohesive and fine-grained soils such as clays and silts where a small test cavity can be excavated without significant disturbance. It requires stable hole walls and is not recommended for coarse granular soils like sands or gravels, which may collapse or prevent full membrane expansion. This method relies on inflating a flexible membrane within the test cavity and works best where the soil maintains its shape without caving.

Calibration involves filling the apparatus with water and placing it on a flat, horizontal surface to record an initial volume reading. Subsequently, the apparatus is transferred to calibrated containers that simulate test hole volumes where the membrane is fully inflated to occupy the container, with air bubbles removed. Pressure is applied until volume readings stabilize, and surcharge weights are added if the apparatus tends to lift. The difference between final and initial readings gives the indicator volume. This process is repeated with containers of various sizes to ensure volume measurements are within ±1% accuracy.

First, the test hole surface must be prepared to be reasonably flat and free from loose soil. The calibrated apparatus is then positioned on the hole, and an initial volume reading is recorded under the same pressure and surcharge conditions as during calibration. Next, the flexible membrane is inflated carefully to fill the cavity, avoiding trapped air and applying the same surcharge weight and pressure. The volume reading after inflation minus the initial reading gives the hole volume, which is adjusted for water temperature effects. Finally, the membrane and water are withdrawn by vacuum to complete the measurement.

Water density varies with temperature, affecting the volume-to-weight relationship critical for accurate volume assessment. During testing, the temperature of water used must be recorded, and corresponding volume per gram values from the prescribed table are applied to correct the measurement. This adjustment ensures the calculated volume accurately reflects true conditions since water expands with temperature, and neglecting this can lead to errors in soil density determination.

The rubber-balloon method may cause soil deformation due to applied pressure and surcharge weights, especially in weak or unsupported soils, which can lead to inaccurate volume readings. It requires precise apparatus calibration and careful pressure control. Conversely, the sand replacement method is less sensitive to apparatus calibration, imposes minimal disturbance by directly filling the hole with sand, and is more suitable for loose or weak soils. While the balloon method can offer approximately 1% accuracy when properly conducted, the sand replacement technique is generally reliable and simpler but may be influenced by sand properties.