Classification of soils - The Mechanics of Soils and Foundations - page 61

Table 5.2 Strength of clay soils estimated from observations in hand samples

Consistency Identification Undrained strength s u kPa

Very soft Extrudes between fingers

< 20

Soft Easily moulded in fingers 20-40

Firm Moulded by strong finger pressure 40-75

Stiff Cannot be moulded in fingers 75-100

Very stiff Can be indented by thumb nail > 150

Another measure of the consistency of a clay soil is its immediate strength. We will
see later that the (undrained) strength of a clay is related to the liquidity index, as
illustrated in Fig. 5.4(a). When the water content of a clay soil is at its liquid limit
the strength is close to 1.7 kPa and when the water content is at the plastic limit the
strength is close to 170 kPa. Rapid estimates of the strength of clays can be made in
hand samples using the criteria in Table 5.2.

5.8 Origins of soils
The mechanical behaviour of a soil is determined principally by its nature and its
current state, but these are governed, to some extent, by the manner of formation
of the soil which may be deposited, residual or compacted by machines. Detailed
discussions of the influence of the manner of formation of soils on their nature and
state are beyond the scope of this topic and are contained in topics on engineering
geology, but there are a few simple observations to be made:

1. Deposited soils. Soils may be deposited from water, ice or wind and the grading
and particle shape and texture are governed largely by the transporting agent.
Soils deposited from water or air are poorly graded because the ability of rivers
or wind to move different sizes depends on the velocity, while soils deposited
from ice (i.e. boulder clays) are well graded because ice can move all particle sizes
equally. Abrasion in moving water or air produces rounded and polished grains
while soil grains transported by ice generally retain their original angular shape
and rough texture. The mineralogy of transported soils is simply that of the parent
material, whichmay be rock fragments or weathered and eroded clay. The fabric of
deposited soil is usually bedded and layered, reflecting changes in the depositional
environment.
2. Residual soils. These are the products of weathering of rocks, or soils, in situ .
Their grading and mineralogy depend in part on the parent material but princi-
pally on the depth and type of weathering and on details of the drainage conditions.
Residual soils usually have low water contents and liquidity indices (or high rela-
tive density) and may be unsaturated. The fabric of immature residual soils often
reflects the fabric of the parent rock.
3. Compacted soils. Soils may be compacted into fills by rolling, vibration or impact.
They are usually unsaturated initially but may later become saturated. Often soils
are compacted in layers and may show horizontal structure.