If I understand your question correctly, there is no need to separate them in a boring log unless they are truly stratified and separate. Then they are separated by visual classification, often supplemented by index testing such as Percent Fines, Grain Size distribution, Atterberg limits, etc.
It is common for sands to be mixed with silt or with clay. This is part of the soil classification designation. For example, one popular method of soil classification is the unified soil classification system, wherein a sand mixed with silt would typically be classified as "SM" or more clearly, a silty sand. You can also mix sands with silty sands with a designation of SP-SM.
Refer to ASTM D2488 for the method of visual classification of soils.
Ron is absolutely right regarding ASTM D2487 (unified classification) and D2488 (Visual classification).
The best way to make a good soil profile of borehole logs is following:
- Dry small representative soil pieces (2-3 cm) from each run
- Arrange them after drying in the order of depth
- Divide visually a soil profile into layers (generally 4-5 for
30-60’) based on dry strength and structure ( look for a big
picture)
- Pick up representative samples from each layer
- Test the samples: gradation( D1140 and D422-NO HYDROMETER) and
Atterberg limits
- Repeat the same procedure with other boreholes but do not test
all samples but just pick up random samples to confirm a visual
description.
Never ever test boundary samples (close to boundary of layer) you will get some average between upper and lower layer. It does not help you later to make soil profile between boreholes.
In nature you cannot find clean sand or clay. You always find 1-3% of sand in clay or 1-3% silt/clay (fines) in sand. Even Dune sand has 2-5% of silt. Generally, there are different ratio between sand/gravel and silt/clay. Such as silty SAND, sandy CLAY or sandy silty CLAY ( see ASTM 2487).
