Subsurface investigation is essential for design with reliable safety. As discussed in last post, residual soils are competent but there have also exceptions which require careful investigation of the soil.
We have discussed about different sampling methods of soil and also discussed about difficulties in sampling of different types of soils. From the definition of residual soil, we know these soils hold their position even after weathering and weathered soil remains on the top of parent rock.
Thus finer particles remains on surface and the particle size increases gradually with depth and larger particles remain at the interface between soil and parent rock.
While drilling for collecting sample, difficulties arise as rock fragments are encountered after drilling up to few depth. These depths depend on weathering condition and types of rocks. Ordinary drilling is often cannot serve the purposes.
In case of ordinary drilling, a hole becomes abandoned, in many cases, due to rock fragments and it is required to drill another hole, few feet apart from previous one.
When residual soil is saprolites, very often rock coring techniques have to apply to collect samples. The samples are tested for ascertaining probable settlements and bearing capacities to design foundation or to determine number of piles. Sample recovery, for such soils, and laboratory testing procedures are almost similar as that of sedimentary deposits.