Question

Why soil sample could all not be moulded into the same number of shapes

Answer 1

It is necessary to adopt a formal system of soil description and classification in order to describe the various materials found in ground investigation. Such a system must be comprehensive (covering all but the rarest of deposits), meaningful in an engineering context (so that engineers will be able to understand and interpret) and yet relatively concise. It is important to distinguish between description and classification:
Description of soil is a statement describing the physical nature and state of the soil. It can be a description of a sample, or a soil in situ. It is arrived at using visual examination, simple tests, observation of site conditions, geological history, etc.

Soil classification is the separation of soil into classes or groups each having similar characteristics and potentially similar behaviour. A classification for engineering purposes should be based mainly on mechanical properties, e.g. permeability, stiffness, strength. The class to which a soil belongs can be used in its description.