Method for subsurface sounding for soils, Part 3: Static cone penetration test
1976 Edition

The cone must be constructed from suitable steel with a hardened tip, having an apex angle of 60° ± 15 minutes, a base diameter of 35 ± 0.1 mm, and a cross-sectional area of 10 cm² to prevent soil ingress into moving parts. The friction jacket, positioned directly above the cone, is designed to measure frictional resistance separately and has dimensions specified in accordance with equipment standards, allowing the cone alone, the cone with friction jacket, and the entire assembly to be advanced in minimum 35 mm increments. The frictional resistance is calculated by the formula: Frictional Resistance = ((x - y) × b / a) + 100 m g, where x is total resistance, y is cone resistance, b is cone base area (10 cm²), a is friction jacket surface area (π × diameter × length), m is mass of the friction jacket, and g is gravitational acceleration.

Per IS 4968 Part 3 Clause 4.2, the cone resistance reading must be corrected by adding a dead weight correction that accounts for the mass of the cone, friction jacket, and sounding rods. The correction factor is computed as (m + n × m₁) multiplied by gravitational acceleration and scaled by the ram/plunger and cone base area ratio. The formula is: Corrected Cone Resistance = Gauge Reading + (m + n m₁) × (Ram Area / Cone Base Area) × g, where m is the mass of the cone and friction jacket, n is the number of rods, m₁ is the mass of each sounding rod (typically 1.5 kg), and g is the acceleration due to gravity (~9.81 m/s²). When the plunger area is 20 cm² and cone base area is 10 cm², the gauge reading is multiplied by 2 before adding the correction.

IS 4968 Part 3 recommends a steady penetration speed of approximately 1 centimeter per second during static cone penetration testing. This rate ensures reliable and consistent readings, as slower speeds down to 0.33 cm/s show minimal variation in cone resistance, whereas faster rates (2 to 3 cm/s) may influence readings—decreasing resistance in low-strength cohesive soils and increasing it by roughly 20% in non-cohesive soils. Maintaining this standard rate facilitates reproducibility and accuracy.

The equipment must be firmly anchored at the test site to provide the reaction force essential for pushing the cone assembly. The driving mechanism rack is first raised to its uppermost position, then the cone-friction jacket assembly is attached to the initial sounding rod and mantle tube. This assembly is positioned vertically over the test location using the mantle tube as a guide. The driving plunger is lowered to contact the protruding sounding rod. Penetration is then performed at a controlled rate of about 1 cm/s, advancing either the cone alone or the cone with friction jacket in minimum increments of 35 mm, or continuously if skin friction measurements are not required separately. This setup ensures accurate and standardized test execution.

Several limitations affect the interpretation of IS 4968 Part 3 testing outcomes. The static cone penetration test is unsuitable for gravelly soils, soils with Standard Penetration Test values exceeding 50, dense sands (due to anchorage challenges), and made-up or filled soils containing loose debris such as stones or brick fragments. Corrections must be applied for dead weight effects of the cone, friction jacket, and rods, and readings must be adjusted based on the plunger to cone base area ratio. Additionally, friction resistance measurement requires correction for the friction jacket’s mass and surface area. Results should always be tabulated and graphed alongside borehole logs for comprehensive soil profiling. These considerations ensure reliable data interpretation and mitigate errors arising from test limitations.