Code of practice for site investigations for the foundation in gravel-boulder deposit
1981 Edition

IS 10042 suggests several field testing approaches for gravel-boulder soils including visual and manual assessment to estimate boulder and gravel content, standard penetration test (SPT) to evaluate in-situ soil strength and density, and dynamic cone penetration test (DCPT) for preliminary profiling. Laboratory grain size distribution analysis on disturbed samples using IS sieves separates particles greater than 4.75 mm (boulder/cobbles/gravel) from finer matrix material. Due to the coarse and heterogeneous nature of bouldery soils, tests primarily focus on penetration resistance and visual classification instead of undisturbed sampling.

The Dynamic Cone Penetration Test (DCPT) is utilized in boulder-gravel soils to identify subsurface layers and assess soil strength where aggregate sizes are up to approximately 100 mm. This test employs a cast iron cone with a 60° apex angle and 62.5 mm base diameter, dynamically driven into the soil. The resistance encountered, measured by the number of blows per penetration depth, reflects soil density and strength characteristics and helps determine allowable bearing pressure. DCPT is particularly valuable in coarse-grained soils where traditional SPT samplers are ineffective due to large aggregate sizes.

Sampling gravel-boulder soils is challenging because the large size of boulders (often exceeding 300 mm) prohibits obtaining undisturbed samples using conventional methods. The heterogeneity of the mixture with coarse boulders and finer gravels complicates representative sampling. Disturbed samples often do not preserve natural grain arrangements, affecting test reliability. IS 10042 addresses these challenges by mandating collection of only disturbed samples from technical pits at various depths, ensuring large sample quantities (around 1000 kg) for representativeness. Particles larger than 80 mm are separated and graded in the field, while smaller fractions are stored for laboratory grain size analysis using IS sieves with a 4.75 mm cutoff.

Allowable soil pressure on boulder-gravel foundations depends on the composition and configuration of the deposit. If filler material occupies the spaces between boulders, behavior is influenced by boulder packing, size, shape, and quantity. Alternatively, if boulders are embedded within a filler matrix, the filler governs the soil behavior. In the absence of filler, the soil exhibits high load capacity with low compressibility. For boulder content exceeding 30%, the soil typically undergoes an initial rapid compression followed by a stage of low compressibility as boulders carry the load, warranting design loads higher than the initial compression load to minimize settlement. For boulder content below 30%, conventional soil load test methods per IS 1888-1982 are applicable.

For load testing in soils rich with boulders, IS 10042 recommends using cast in-situ concrete footings or precast concrete blocks bonded with fresh mortar to ensure intimate contact with the soil. The footing dimensions should be large enough to mobilize group action of multiple boulders, with a minimum size equal to ten times the average grain size of boulders, but not less than 150 cm. Steel plates are generally discouraged due to difficulty in obtaining proper seating. Load is applied using a hydraulic jack with 100 tonnes capacity, and deformation is measured by four dial gauges with 0.001 mm accuracy. Load increments are maintained for at least one hour or until deformation rates drop below 0.02 mm per minute, continuing until 50 mm settlement or failure, with a minimum of eight increments.