Code of practice for subsurface investigation for foundations
1979 Edition

IS 1892 recommends distinct sampling methods based on soil type: cohesive soils are best sampled using undisturbed techniques like tube or chunk samples to preserve structure, while disturbed samples can be collected via hand or auger methods. For cohesionless soils such as sands and gravels, undisturbed sampling is challenging below the water table and requires specialized samplers or shell samples; disturbed samples are obtained through wash or auger sampling. Soft clays should also be evaluated with in-situ vane shear tests to complement strength assessment. Sample quantities vary by test type, ensuring reliable laboratory analysis.

To maintain the integrity of undisturbed soil samples, IS 1892 advises removing approximately 2.5 cm from each end of the sample to discard disturbed soil, followed by applying multiple molten wax layers to create protective plugs. For porous samples, waxed paper should be placed before waxing. Tubes must be sealed tightly with lids or screw caps, secured with adhesive tape, and any longitudinal joints waxed and taped. Labels with sample identification should be placed inside and painted on the outside, indicating orientation. Samples are to be packed in cushioned wooden boxes and stored in cool, humid conditions if delays occur, with testing ideally conducted within two weeks.

IS 1892 details several drilling techniques suitable for different soil and rock conditions: auger boring for soft soils up to about 6 meters depth; wash boring employing jetting and chopping for cohesive and cohesionless soils with the aid of drilling mud or bentonite; rotary drilling for hardening soils and gravelly layers often combined with casing; percussion boring for breaking boulder-strewn and gravelly strata, albeit with potential disturbance to samples; and core drilling for extracting rock cores from hard formations. These methods provide flexibility to select appropriate approaches based on site conditions.

Water sampling as per IS 1892 involves collection either from trial pits or existing wells, which is straightforward, or from boreholes using suction pumps. Borehole sampling requires pumping out all water initially, waiting 24 hours for groundwater recharge, then pumping again to collect at least 5 liters from near the bottom, avoiding surface water. The sample is allowed to settle and the clear portion transferred into clean, airtight containers. Chemical analysis is then performed to assess corrosive substances such as sulphates and chlorides, which can inform foundation design considerations.

Dynamic penetration tests, primarily the Standard Penetration Test (SPT) defined by IS 2131-1963, involve driving a sampler into soil using a hammer of specified weight and drop height, measuring the number of blows needed to penetrate standard distances. The blow count (N-value) reflects soil density or consistency, aiding in estimating bearing capacity, especially for non-cohesive soils. IS 1892 also references related standards for dynamic and static cone penetrometer tests, providing a practical, standardized method to supplement borehole data and support foundation design through relative soil strength assessment.