genetic and engineering classification of soils
geological classification of soils
soils are the disintegration products of rocks. when weathering affects the rock, it breaks into pieces and changes into various soil types. depending upon its origin, the soil can be alluvium (deposited by river), colluvium (deposited by gravity and surface water), and residual (product of in-situ weathering).
table 1 gives the common grading and grain roundness of the main geological soils.
table 1: grading and grain roundness of geological soils
| geologic soil |
well graded |
medium graded |
poorly graded |
grain roundness |
| alluvial terraces |
frequently |
rather rarely |
rarely |
well-rounded-rounded |
| alluvial fans |
frequently |
rather frequently |
rarely |
well-rounded-rounded |
| debris flow layers in alluvium |
rather rarely |
frequently |
frequently |
rounded-subrounded |
| glacio-fluviatile |
rather frequently |
frequently |
rather rarely |
rounded-subrounded |
| glacial deposits |
rarely |
rather frequently |
frequently |
subrounded-subangular |
| colluvium (slope debris) |
extremely rarely |
rarely |
very frequently |
angular |
| residual soils |
rarely |
rather rarely |
frequently |
subangular-angular |
the grain size of soil is identified in the field as follows:
1. boulder is larger than the palm size
2. cobble is approximately the palm size
3. pebble is about the size obtained by joining the thumb and fore finger
4. granule (gravel) is about the size of the little fingertip.
5. sand is smaller then 1/4 inch or sometimes less than 4.75 mm, but
it is visible to the unaided eye.
6. silt is invisible to the unaided eye, it feels soft with hand, but gritty
while ‘eating’
7. clay is soft and soapy, no gritty feeling while ‘eating’.
the soil can be classified as:
coarse-grained soil (boulder, cobble, pebble, granule); and
fine-grained: sand, silt, and clay.
the other properties of the soil are the following:
shape: ball-shaped (equant), disk-shaped (tabular), cigar-shaped (prolate)
roundness (lack of corners and edges): angular, subangular, subrounded,
rounded, well rounded
moisture content: dry, moist, wet
plasticity (possible to make 3 mm long ribbon): high, medium, low
grading: well graded, poorly graded
porosity: ratio of volume of voids to the total volume
permeability: rate of flow of water through the voids.
engineering classification of soils
for the purpose of engineering soil classification, it is necessary to classify the soils into coarse and fine types. In many places the Unified Soil Classification System (USCS) is applied. figure 2 gives a field method for classifying the coarse and fine soils according to USCS.
for this purpose, it is necessary to determine the ratio of grains visible to the unaided eye. use figure 3 for estimating the particles visible to the unaided eye.
it is also possible to classify the following grains:
· per cent of pebbles (4 mm - 64 mm), cobbles (64 mm - 256 mm)
and boulders (greater than 256 mm); and
· percent of gravel (2 mm - 4 mm).
by looking at the soil surface and by using figure 3, the test permits with some training to roughly estimate the per cent of material below 5-6 mm.
since the estimate is by eye (a surface is estimated) and not by weight, the test does not replace the classification obtained in sieving the different fractions, but it provides valuable information on the soil class.
fine grained soils and soil matrix
it is necessary to go into details of classification of fine soils, since their cohesion, angle of internal friction, and unit weight strongly depend upon the amount of fines. field tests on the matrix are therefore imperative.
field tests for estimate of dry strength, plastic limit and plasticity
the procedure is illustrated in figures 1, 2, and 3. this classification of fine material has also to be implemented for classifying the matrix of coarse soils.
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dry strength or breaking test: the test permits to estimate the cohesiveness of the soil and therefore its clay content.
after removing the particles larger than ~ 1-2 mm a pat of the soil is moulded to the consistency of a putty after having added some water. the pat is dried completely by sun, air or if available by oven. its strength is then tested by breaking and crumbling between the fingers (figure 1). the dry strength increases with increasing plasticity e.g. of its clay content.
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figure 1: field tests for fine soils and matrix
a low dry strength indicates silt, rock flour or silty sand/sandy silt. the sand feels however gritty when powdered. the dry pat can be powdered with slight finger pressure.
a medium dry strength indicates low to medium plastic inorganic clay. considerable finger pressure is required to powder the sample.
a high dry strength indicates highly plastic, inorganic clay. the dried sample can be broken but cannot be powdered by finger pressure.
remark: cohesion or high dry strength may be due to the presence of cementing material such as calcium carbonate or iron oxide.
dilatancy or shaking test
like the dry strength test, this test aids to have a view on the plasticity of the material.
after removing the coarse particles, a slightly moist pat of material is placed in the open palm of one hand and shacked horizontally by striking vigorously against the other hand several times (figure 1). a positive reaction consists of the appearance of water on the surface of the pat that becomes glossy. when the sample is squeezed between the fingers, the water and gloss disappear from the surface, the pat stiffens, and finally it cracks or crumbles.
a rapid reaction indicates a lack of plasticity such as in the case of a typical silt or very fine sand. a slow reaction indicates a slightly plastic silt or silty clay. no reaction indicates clay or organic material.
ribbon test
this test permits to have a view on the plastic limit.
a slightly moist pat of material, from which coarse grains have been removed, is rolled to obtain a thread on a board (figure 1, down). whenever the thread does not break into pieces the material is above the plastic limit (figure 1). the plastic limit is reached when the thread breaks (figure 1). the longer is the thread is without breaking, the higher is the plastic limit.
shine test
a slightly moist sample of soil is cut by a knife blade. a shiny surface indicates highly plastic clay. a dull surface indicates silt or clay of low plasticity.
figure 2: table for field identification of percentage
of coarse fragments
figure 3: ratio of coarse grains and shape of the grains
field test for estimating f and g of fine non-cohesive soils and soil matrix
after the completion of the field test and obtaining the Unified Soil type, it is necessary to estimate its angle of internal friction Φ and unit weight g (dry, wet, and saturated).
for this purpose, it is necessary to have an estimate of relative density of the soil or the matrix (in case of coarse soil). Table 1 gives the values of relative density of soils.
table 1: relative density estimate by penetration test
| term |
relative density |
penetration field test |
| very loose to loose |
0 -50% |
easily penetrated with ½ in. reinforcing rod push by hand |
| moderately dense |
50-70% |
easily penetrated with ½ in. reinforcing rod driven with 5 lb hammer |
| dense |
70-90% |
penetrated a foot with ½ in. reinforcing rod driven with 5 lb hammer |
| very dense |
90-100% |
penetrated a few inches with ½ in. reinforcing rod driven with 5 lb hammer |
after the completion of the relative density test, figure 4 is used for estimating the values of Φ and g.
figure 4: plots of f and g according to soil classification
and relative density
location: altitude:
mountain slope classification:
make a sketch profile of the region.
sketch of the soil slope:
sketch the soil slope, show dimensions, and the north direction. also colour it according to soil colour.
classification of soil according to unified soil classification:
use charts of Unified Soil Classification.
measurement of relative density:
use table for relative density. for testing the soil you need a ½ inch rod (about 1 foot long) and a hammer.
determination of friction angle f and unit weight g:
from the chart find out the values for:
dry unity weight, g: t/m3
saturated unit weight, gsat
soil depth:
less than 1 metre
from 1 to 3 metres
from 3 to 6 metres
more than 6 metres
sketch of the soil column:
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